The Distribution and Origin of Radon, CO2 and SO2 Gases at
Arenal Volcano, Costa Rica
par
Glyn Williams-Jones
Département de géologie
Faculté des Arts et des Sciences
Mémoire présenté à la Faculté des études supérieures
en vue de l’obtention du grade de
Maître ès sciences (M.Sc.)
Avril 1996
Université de Montréal
 Glyn Williams-Jones - MCMXCVII
Université de Montréal
Faculté des études supérieures
Ce mémoire intitulé
The Distribution and Origin of Radon, CO2 and SO2 Gases at
Arenal Volcano, Costa Rica
présenté par:
Glyn Williams-Jones
a été évalué par un jury composé des personnes suivantes:
Président-rapporteur: Dr. Walter E. Trzcienski
Membre du jury:
Dr. Hélène Gaonac’h
Membre du jury:
Dr. John Stix
Frontispiece
Arenal
To all those that made this thesis possible.
ii
Abstract
Volcanic gases are one of several important indicators used to better understand
and forecast volcanic activity.
However, direct sampling of these gases is often
dangerous or impossible due to the high level of activity and the common inaccessibility
of the crater areas of many volcanoes. Indirect methods such as the study of soil gases or
the use of remote sensing techniques are thus required. Soil gases such as radon and
carbon dioxide have been shown to correlate well with variations in volcanic activity.
Similarly, the remote sensing of gases such as sulphur dioxide has proven significant in
the geochemical characterisation of both passively and actively degassing volcanoes.
Techniques such as these can now provide important clues to the behaviour and future
activity of the volcano.
This thesis investigates the degassing of Arenal volcano. A small stratovolcano
in northwestern Costa Rica, Arenal is one of the most active volcanoes in Central
America, having been in continuous eruption since its reactivation in July 1968.
Estimates, using petrologic and remote sensing techniques, are made of the quantity of
SO2 emitted from Arenal since 1968 and are related to a degassing model for the
volcano. Observed spatial and temporal patterns of soil and plume gases are correlated
to eruptive and seismic activity, and the origin and transport of these gases at Arenal is
discussed. Measurements of seismicity, radon, CO2 and SO2 gas were made as (1) the
results could be compared to other volcanoes where similar measurements have been
made, (2) it was comparatively simple to measure radon, CO2, and SO2, and (3) these
gases are believed to respond to changes in activity and the stress-state of the volcano.
iii
Resumen
Los gases volcánicos son uno de los varios indicadores importantes usados para
entender mejor y pronosticar la actividad volcánica. Sin embargo, el muestreo directo de
éstos gases es frecuentemente peligroso o imposible a causa del alto nivel de actividad y
la usual inaccesibilidad del área de cráter de muchos volcanes. Los métodos indirectos
tal como el estudio de gases de suelo o el uso de técnicas de teledetección son
necesarios. Se observó que los gases de suelo tal como radon y el dióxido de carbono
son apropiados para correlacionar bien las variaciones de la actividad volcánica.
Igualmente, los gases estudiados a distancia tal como: dióxido de azufre, ha probado ser
importante en la caracterización geoquímica pasiva y activa de volcanes con emisiones
gaseosas.
Técnicas como éstas pueden proveer ahora, pistas importantes del
comportamiento y actividad futura del volcán.
Esta tésis investiga la emisión gaseosa del volcán Arenal, un pequeño
estratovolcán en el noroeste de Costa Rica. Arenal, es uno de los volcanes más activos
en Centroamérica, con erupción continua desde su reactivación en Julio de 1968. La
estimación, por medio de técnicas petrológicas y de teledetección, se hizo teniendo en
cuenta la cantidad de gas SO2 emitidas del volcán desde 1968, y son relativas a un
modelo de emanaciones gaseosas para Arenal. Los modelos espaciales y temporales de
los gases de pluma y suelo observados, se correlacionan con la actividad sísmica y
eruptiva. También, el orígen y el transporte de éstos gases en Arenal se discute en esta
tésis.
Las medidas de sismicidad y medidas particulares de los gases radon, CO2 y SO2
se hicieron con motivo de que: (1) los resultados podrían compararse a otros volcanes
iv
donde medidas similares son disponibles, (2) los gases radon, CO2, y SO2 son
comparativamente simples de medir, y (3) se piensa que éstos gases responden a
cambios en la actividad volcánica y el estado de tensión del volcán.
v
Résumé
L’Arenal est un stratovolcan situé à 10.463°N 84.703°O dans le nord-ouest du
Costa Rica en Amérique Centrale. Arenal est le volcan costaricien le plus petit, avec un
volume de 15 km3, mais aussi le volcan le plus actif du pays. Il consiste d’un édifice
avec des flancs pentus formé de deux cratères sommitaux (C and D), et il est verdoyant
sur les flancs nord, sud, et est. Un grand champ de laves jeunes, mis en place depuis
1968, couvre le flanc ouest. Arenal est situé entre deux massifs, la Cordillera de
Guanacaste au sud-est et la Cordillera Central au nord-ouest. L’ensemble de ces deux
cordillères forment la chaîne volcanique qui comprend l’arc du Costa Rica. Environ
trois kilomètres au sud de l’Arenal on retrouve le volcan dormant de Cerro Chato.
Le matin du 29 juillet, 1968, après 10 heures d’activité sismique intense, l’Arenal
est entré en éruption de façon explosive, il a continué son activité éruptive durant une
période de trois jours, tuant ainsi 78 personnes et dévastant une région de 12 kilomètres
carrés sur le flanc ouest. L’explosion initiale était suivie par des colonnes d’éruptions
pliniennes, des coulées pyroclastiques et des bombes et blocs éjectés de façon balistique.
Trois nouveaux cratères (A, B, C) ont été formés durant cette période, avec une
orientation approximative est-ouest sur le flanc ouest du volcan. Un autre épisode
explosif a commencé le 17 juin 1975, avec l’emplacement d’une coulée pyroclastique de
cendres et blocs le long de la vallée Rio Tabacon. Ce dépôt provient de la formation des
nuées ardentes produites par des avalanches d’une coulée de lave provenant du cratère C.
Jusqu’en juin 1984, l’Arenal a continué son activité fumerolienne forte avec l’extrusion
de lave bloqueuse de type aa. Cette date marque une augmentation de l’activité à
Arenal, avec le début d’éruptions de cendres et de grandes coulées pyroclastiques. Cette
vi
activité se change en phase éruptive strombolienne produisant des tephras et des laves de
composition basaltique-andésitique. De nos jours le volcan Arenal est encore dans cette
phase strombolienne.
Les objectifs de ce mémoire sont, premièrement l’évaluation de la distribution
des gaz de sol sur les flancs du volcan Arenal, deuxièmement l’estimation de la quantité
du gaz SO2 dégagé par le volcan Arenal depuis 1968, en utilisant des techniques
pétrologique et télédétectée, troisièmement la tentative de trouver des patrons spatial et
temporel observés dans les gaz de sol et les fumerolles durant l’activité éruptive et
sismique, et finalement une discussion sur l’origine et le transport des gaz pour le volcan
Arenal. Pour réussir à ces objectifs, des mesures de sismicité, et des mesures de gaz de
radon, de CO2 et de SO2 ont été effectuées avec l’idée de comparer les résultats à
d’autres volcans où des mesures semblables ont été faites. De plus, il était relativement
simple de mesurer le radon, le CO2, et le SO2, et il est suggéré que ces gaz répondent
bien aux changements de l’activité volcanique et à l’état de stress du volcan.
Des mesures des concentrations de radon et CO2 ont montré des maxima
seulement près des failles possibles et sur les flancs inférieurs du volcan. Les données
de δ13C ont aussi été les plus lourdes sur les flancs inférieurs et près de ces failles
possibles. Il y a peu d’expression de la structure en surface du volcan Arenal parce qu’il
est couvert de laves récentes. Le niveau d’activité élevé de l’Arenal rend difficile les
corrélations entre l’activité sismique et les fluctuations des gaz de sol. Par contre, ces
fluctuations peuvent être expliquées par des variations dans la pression atmosphérique.
Ces observations impliquent que les concentrations des gaz de sol sont influencées
principalement par le niveau de développement du sol. Ensuite, le dégazage diffus des
vii
gaz magmatiques profonds sur les flancs supérieurs des volcans est négligeable dû à la
faible perméabilité du sol causée par le couvert des roches volcaniques jeunes.
Finalement, l’augmentation du dégazage magmatique sur les flancs inférieurs est le
résultat d’une augmentation de la fracturation des laves plus âgées.
Les gaz de sol mesurés pour deux autres stratovolcans actifs associés à la
subduction (le Poás, Costa Rica et le Galeras, Colombie) ressemblent bien à ce qui a été
observé à l’Arenal. Les concentrations de radon étaient maximales seulement près des
zones de failles, des zones d’activité sismique, près des cratères et fumerolles, et sur les
flancs inférieurs de ces volcans. Les valeurs les plus negatives de δ13C ce trouvaient
près des fumerolles à l’intérieur des cratères actifs, près des failles et sur les flancs
inférieurs. Ces observations impliquent que les failles majeures peuvent canaliser les
gaz profonds vers la surface seulement s’ils ont une expression superficielle.
Des
volcans, comme ceux étudiés ici, réagissent comme des bouchons dans la croûte
continentale, limitant le dégazage aux fumerolles, failles, et flancs inférieurs fracturés.
L’utilisation de la télédétection des gaz pour l’étude des volcans actifs est encore
jeune. Le seul gaz qui peut être télédétecté de façon routinière est le dioxyde de soufre,
en utilisant la spectroscopie de corrélation dans la région ultraviolette du spectre
électromagnétique.
Par l’intégration de ces données avec d’autres données
géophysiques, les changements temporaux dans les flux de SO2 peuvent suggérer des
comportements et activités dans l’avenir du volcan.
Arenal a dégagé un minimum de 1.31 x 106 tonnes métriques de SO2 depuis sa
réactivation en 1968.
Les émissions du volcan ont continué avec une production
moyenne quotidienne de 130 ± 60 t/d SO2. Par sa forte activité, Arenal montre des
viii
cycles de diminution du flux de SO2 et de sismicité avant les éruptions. Suite à ces
événements, le flux de SO2 et l’activité sismique augmente. Ces fluctuations montrent
aussi une corrélation distincte avec les marées terrestres. En effet, lors du maximum de
marée, nous observons une diminution de l’activité éruptive, ce qui coïncide avec
l’augmentation des événements tremor.
Arenal a probablement une chambre
magmatique profonde ou mi-croûte se comportant comme un système ouvert.
Par
contre, le système ouvert est périodiquement bloqué près de la surface par la
cristallisation du magma dans le conduit et le développement subséquent d’une zone
étanche. Ceci cause la surpression du conduit et la destruction explosive éventuelle de
cette zone.
Les gaz volcaniques sont un outil pour mieux comprendre et prédire l’activité
volcanique. Donc, la télédétection des gaz de sol ou de la colonne est critique dans la
caractérisation géochimique, car elle est une méthode sécuritaire et efficace pour les
volcans actifs qui sont souvent inaccessibles.
ix
Acknowledgements
I would like to thank my supervisor, Prof. John Stix, for his constant enthusiasm,
encouragement and support, as well as his ability to put up with my terrible jokes.
Thanks to Martin Heiligmann for stimulating discussion and constructive criticism, his
help in Costa Rica, and having to submit (graciously, most often) to really early
mornings...wackey wackey! Many thanks to Isabel Lépine for help with translating, final
production, and many other things.
Thanks also to the many other friends at the
Université de Montréal for accepting la Tête Carrée into their midst over these many
years, Jean-Marc Séguin, Chantal Bilodeau, Stephan Lamarche, Alain Legault, Kazuko
Saruwatari, Sandrine Caderon, André Lafferière, and Alex Beaulieu. I’m grateful to
Jorge Barquero, Erik Fernández, Eliazar Duarte, and Eduardo Malavassi of OVSICORI
for welcoming the gringo with open arms - Pura Vida! Thanks also to Vilma Barboza
(OVSICORI) for help with 1995 and 1996 seismic data. Special thanks to Ray Hoff and
Andrew Sheppard (Environment Canada) for the use of their COSPEC in 1996 and Niki
Stevens (Reading Univ.) and Mark Davis (Open Univ.) for their enthusiastic help during
the second field season in Costa Rica. Also many thanks to Neil Arner and Barbara
Sherwood Lollar at the University of Toronto, without whom my isotope work would
have been a nightmare! Thanks to Hazel Rymer (Open Univ.) for tidal gravity data
which turned out to be crucial, to Glen Poirier (McGill Univ.) for help and patience with
the microprobe analyses, and to Bill Melson (Smithsonian Inst.) for seismic data from
1995.
This research was supported by grants to John Stix from the Natural Sciences and
Engineering Research Council of Canada (NSERC), les Fonds pour la Formation de
x
Chercheurs et l’Aide à la Recherche (FCAR), and the Université de Montréal. Thanks
finally and most of all, to my parents and brother, I would not have made it here without
you.
xi
Table Of Contents
ABSTRACT __________________________________________________________ ii
RESUMEN __________________________________________________________ iii
RÉSUMÉ_____________________________________________________________v
ACKNOWLEDGEMENTS ________________________________________________ ix
LIST OF FIGURES ___________________________________________________ xiii
LIST OF TABLES ___________________________________________________ xvii
PREFACE _________________________________________________________ xviii
GENERAL INTRODUCTION ___________________________________________1
INTRODUCTION _______________________________________________________2
OBJECTIVES _________________________________________________________3
LOCATION ___________________________________________________________3
GEOLOGICAL SETTING _________________________________________________5
REGIONAL GEOLOGY ___________________________________________ 5
LOCAL GEOLOGY ______________________________________________ 9
VOLCANIC ACTIVITY _________________________________________________11
VOLCANIC ACTIVITY PRIOR TO 1968 ______________________________ 11
VOLCANIC ACTIVITY DURING AND AFTER 1968 _____________________ 11
REFERENCES ________________________________________________________16
CHAPTER I __________________________________________________________20
ABSTRACT __________________________________________________________21
INTRODUCTION ______________________________________________________22
FACTORS AFFECTING SOIL GAS CONCENTRATIONS AND DISTRIBUTIONS________23
METHODOLOGY _____________________________________________________24
STATION LOCATIONS __________________________________________ 24
SOIL GAS MEASUREMENTS _____________________________________ 24
SEISMIC MEASUREMENTS ______________________________________ 31
RESULTS ___________________________________________________________31
RADON _____________________________________________________ 31
RADON EMANATING POTENTIAL _________________________________ 32
CARBON DIOXIDE _____________________________________________ 38
CARBON ISOTOPES IN CO2 SOIL GAS ______________________________ 38
CARBON DIOXIDE FLUX ________________________________________ 49
SOIL GAS TIME SERIES _________________________________________ 52
RADON ___________________________________________________ 52
CARBON DIOXIDE ___________________________________________ 54
SEISMICITY __________________________________________________ 55
ATMOSPHERIC VARIATIONS _____________________________________ 58
DISCUSSION _________________________________________________________58
ORIGIN OF RADON AND CARBON DIOXIDE _________________________ 58
xii
SOIL GAS, SOIL DEVELOPMENT, AND ELEVATION ___________________ 62
RELATIONSHIP TO PRESSURE CHANGE ____________________________ 67
RELATIONSHIP TO SEISMICITY ___________________________________ 69
CONCLUSIONS _______________________________________________________70
REFERENCES ________________________________________________________72
CHAPTER II _________________________________________________________74
ABSTRACT __________________________________________________________75
INTRODUCTION ______________________________________________________76
METHODOLOGY _____________________________________________________76
RESULTS ___________________________________________________________81
DISCUSSION _________________________________________________________82
CONCLUSIONS _______________________________________________________85
REFERENCES ________________________________________________________86
CHAPTER III ________________________________________________________88
ABSTRACT __________________________________________________________89
INTRODUCTION ______________________________________________________90
METHODOLOGY _____________________________________________________94
SO2 FLUX ___________________________________________________ 94
SO2 FLUX ERRORS ____________________________________________ 98
SEISMICITY _________________________________________________ 102
RESULTS __________________________________________________________102
SO2 FLUX __________________________________________________ 102
SO2 BUDGETS AT ARENAL _____________________________________ 111
COSPEC/PLUME TRACKER ESTIMATES __________________________ 111
PETROLOGICAL ESTIMATES ___________________________________ 111
SEISMIC DATA ______________________________________________ 114
DISCUSSION ________________________________________________________119
CONDUIT OPENING AND CLOSING _______________________________ 119
THE SULPHUR BUDGET OF ARENAL ______________________________ 122
THE OPEN NATURE OF ARENAL _________________________________ 124
CONCLUSIONS ______________________________________________________125
REFERENCES _______________________________________________________127
CONCLUSIONS _____________________________________________________131
GENERAL CONCLUSIONS _____________________________________________132
RECOMMENDATIONS FOR FUTURE WORK________________________________134
APPENDIX _________________________________________________________136
xiii
List Of Figures
Figure I-1
Geographic map of Costa Rica showing the location of Arenal
volcano
4
Topographic map of Arenal volcano showing the lava fields
emplaced since 1968
6
Geological map of Costa Rica. After Mora (1983), Alvarado
(1984), and Borgia (1988)
7
Figure I-4
Geological map of Arenal volcano. After Borgia (1988)
10
Figure I-5
Blocks from the blocky ash flow emplaced on the western flank
of Arenal in July 1968. A 1992 blocky-aa lava flow is seen in
the background
13
White cloud on left of image represents a small pyroclastic flow
caused by the collapse of part of the blocky aa flow (a levee is
in evidence to the upper right of the cloud). View is of the
western flank
15
A small strombolian eruption towards the western flank. Crater
C is to left, crater D, to the right. View from the Arenal
Volcano Observatory, south of the volcano
15
Figure I-2
Figure I-3
Figure I-6
Figure I-7
Figure 1.1
Topographic map of Arenal volcano showing the location of Rn
and CO2 soil gas stations (stars). The locations of seismic stations
are shown (red triangles). Contours are every 100 m
26
Figure 1.2
PVC tube (left) used in measuring radon and an aluminium tube
(right) used in the measurement of CO2 soil gas. Both tubes are
buried to a depth of ~75 cm
27
(a) Average Rn (pCi/l) versus elevation of stations (metres).
There is an approximately negative linear trend. (b) Topographic
map of Arenal volcano showing concentrations of Rn (pCi/l)
soil gas. There is a tendency towards increasing radon with
distance from the summit
35
Figure 1.3
Figure 1.4
Radon (pCi/l, Table 1.4) versus radon emanating potential (RnERaC
in pCi/kg, Table 1.2). Stations with elevated radon generally have
elevated RnERaC
37
Figure 1.5
(a) Average CO2 (%) versus elevation of stations (m). There is a
xiv
general trend of increasing concentration with decrease in elevation.
(b) Topographic map of Arenal volcano showing the concentrations
of CO2 (%) soil gas. There is a tendency towards increasing CO2
with distance from the summit
42
Figure 1.6
(a) Average Rn (pCi/l) versus average CO2 (%). Note the generally
positive correlation for the N, S and W lines. Stations from the NE
and E lines are anomalous. (b) Average Rn (pCi/l) normalised by
RnERaC (pCi/kg) versus average CO2 (%). Stations from the NE
line are anomalous
43
Figure 1.7
(a) Average δ13C (‰) in CO2 soil gas versus elevation of stations
(metres). Note the negative correlation (r = -0.73). (b) Topographic
map of Arenal volcano showing concentrations of δ13C (‰) in CO2
soil gas. Note the tendency towards heavier δ13C with distance
from the summit
45
Figure 1.8
(a) δ13C (‰) in CO2 soil gas versus CO2 (%). Note the positive
correlation. (b) Average δ13C (‰) in CO2 soil gas versus average
Rn (pCi/l). (c) Average δ13C (‰) in CO2 soil gas versus average
Rn (pCi/l) normalised by RnERaC (pCi/kg). Stations from the NE
line are anomalous
47
Figure 1.9
(a) δ13C (‰) in CO2 soil gas versus organic δ13C (‰) in soil.
Samples are for the NE line which are stations furthest from the
summit
50
Figure 1.10
(a) CO2 (%) soil gas versus CO2 flux (mg/m2⋅min). Note the
grouping of points where samples with elevated CO2 have low CO2
flux. (b) δ13C (‰) versus CO2 flux (mg/m2⋅min). Note that stations
with low flux have heavier δ13C
51
Figure 1.11
Plot of radon and CO2 concentrations versus time for the E, S, N,
and W lines. Solid lines and symbols are radon values, dashed and
clear symbols are CO2 concentrations. Note the common peaks on
March 22 and April 6, 1995
53
Figure 1.12
(a) Number of volcanic eruptions per day and (b) total hours of
tremor per day between March 23 and April 16, 1995
Figure 1.13
57
Fluctuations in radon concentration (pCi/l) and number of eruptions
versus time. Note the increase in average number of eruptions
coinciding with a peak in Rn concentration (April 6, 1995). The
histogram represents the average number of eruptions between Rn
xv
Figure 1.14
measurement periods
59
Daily atmospheric pressure (mbar) fluctuations measured at the
Fortuna base station (~250 m). Note the relative minima on
April 6, 1995
61
Figure 1.15
View from the western flank of Arenal towards Lago Arenal. Large
boulders and blocks are part of the 1968 pyroclastic deposit. The
remains of the once dense jungle (a few trees) are visible
64
Figure 2.1
Topographic map of Arenal volcano showing average (a) radon
concentrations in pCi/l, (b) CO2 concentrations in volume %, and
77
(c) δ13C values expressed as ‰. Contour interval of 100 m
Figure 2.2
Topographic map of Poás volcano showing average (a) radon
concentrations in pCi/l, (b) CO2 concentrations in volume %, and
78
(c) δ13C values expressed as ‰. Contour interval of 500 m
Figure 2.3
Topographic map of Galeras volcano showing average (a) radon
concentrations in pCi/l, (b) CO2 concentrations in volume %, and
79
(c) δ13C values expressed as ‰. Contour interval of 400 m
Figure 3.1
Geographic map of Costa Rica showing the location of
Arenal volcano
91
Figure 3.2
Topographic map of Arenal volcano showing the locations of
seismometer stations (red triangle) and tilt stations (green house).
Craters A and B are now buried by lava flows emplaced since
1968. Contours are every 100 m
92
Figure 3.3
Plume Tracker and COSPEC ultraviolet spectrometers. The rightangle light tube of the Plume Tracker is visible in the top picture,
while the bottom picture shows the control panel for the COSPEC
IV. Black and red cables connect to an analogue chart recorder
and portable computer
95
Figure 3.4
Tilt station maintained by the Departamento de Geología of the
Instituto Costaricense de Electricidad (ICE), located on the upper
western flank of Arenal volcano. A portable seismometer was
buried approximately 50 cm below the surface, to the right of the
door of the green hut
103
Figure 3.5
SO2 flux versus time for (a) February 28, (b) March 4, (c) March 5,
1995 at Arenal. Eruptions are shown by inverted arrows
109
xvi
Figure 3.6
SO2 flux and eruption amplitude and SO2 flux and eruption duration
versus time for (a),(b) February 29; (c),(d) March 1; (e),(f) March 5;
(g),(h) March 6; (i),(j) March 8, 1996. Amplitude is in digital units,
duration in seconds, and SO2 flux in metric tonnes per day
110
Figure 3.7
Fluctuation of gravity due to Earth tides between (a) February 29
to (i) March 8, 1996. Gravity data is in microgals. Note that tremor
generally begins at or just past high tide on March 5, 6, and 8
118
xvii
List Of Tables
Table 1.1
Radon soil gas from Arenal volcano for 1995-1995
34
Table 1.2
Radon emanating potential for soils and lava/debris at Arenal
volcano
36
Table 1.3
CO2 soil gas from Arenal volcano for 1995-1996
39
Table 1.4
Average CO2, radon and δ13C from Arenal volcano for 19951996
41
Table 1.5
δ13C in CO2 soil gas from Arenal volcano for 1995-1996
44
Table 1.6
Organic δ13C in soil from the NE line of Arenal volcano
48
Table 1.7
CO2 flux from Arenal volcano in 1995
48
Table 1.8
Seismic data from March 23 to April 16, 1995 at Arenal volcano
56
Table 1.9
Correlation coefficients for Rn vs. CO2 and Rn and CO2 vs.
pressure at Arenal volcano
68
Average Rn, CO2, and δ13C for Arenal, Poás, and Galeras
volcanoes
80
Table 2.1
Table 3.1
SO2 concentrations of gas calibration cells for SO2 remote sensing
instruments
97
Table 3.2
Error calculation for SO2 measurements
Table 3.3
Windspeed measurements taken 4 km west (elev. ~550 m) of Arenal
volcano
101
Table 3.4
Daily SO2 flux for Arenal volcano from 1982 to 1996
104
Table 3.5
Average daily SO2 flux for Arenal volcano with and without
eruptions
106
99
Table 3.6
Chemical analyses of melt inclusions and matrix glasses from the
1968 surge deposit and 1984 and 1992 lavas at Arenal volcano
112
Table 3.7
117
Predicted daily maximums of tidal gravity at Arenal volcano
xviii
Preface
This thesis consists of five chapters, the third and fourth of which are in
manuscript form, and are intended for submission to refereed journals.
My thesis
advisor, Dr. John Stix, is second author on both manuscripts. His role in the preparation
of the manuscripts consisted of critical evaluation of the data and my interpretations
presented therein, as well as editorial suggestions regarding organisation of the text.
GENERAL INTRODUCTION
General Introduction
______________________
2
Introduction
V
OLCANIC gases are one of several important indicators used to better
understand and forecast volcanic activity. However, direct sampling of these
gases is often dangerous or impossible due to the high level of activity and the common
inaccessibility of the crater areas of many volcanoes. Indirect methods such as the study
of soil gases or the use of remote sensing techniques are thus required. Soil gases such
as radon or carbon dioxide have been shown to correlate well with variations in volcanic
activity (cf. Allard et al., 1991; Brantley and Koepenick, 1995; Heiligmann et al., 1997).
Similarly, the remote sensing of gases such as sulphur dioxide has proven significant in
the geochemical characterisation of both passively and actively degassing volcanoes (cf.
Casadevall et al., 1981; Stoiber et al., 1986; Zapata et al., 1997). Techniques such as
these can now provide important clues to the behaviour and future activity of a volcano.
Arenal volcano, a small stratovolcano in northwestern Costa Rica, is one of the
most active volcanoes in Central America, having been in continuous eruption since its
reactivation in July 1968. Until the onset of this new activity, very little was known
about the geology or volcanic activity of Arenal.
The geology of Arenal was
comprehensively described and mapped by Malavassi (1979). This work was followed
by that of Alvarado (1984), and Borgia et al. (1988) who continued the study of the
structural, stratigraphic and petrological evolution of the Arenal-Chato system.
Minakami et al. (1969), Melson and Saenz (1968, 1973, 1977), Fudali and Melson
(1972), and Saenz (1977) investigated the 1968 eruptive activity and estimated many of
the physical characteristics (e.g., volume, energy and released pressure), while Matumoto
and Umana (1976) and Van der Bilt et al. (1976) described the 1975 eruptive events.
General Introduction
______________________
3
Bennett and Raccichini (1977), Borgia et al. (1983), and Cigolini et al. (1984) studied
the dynamics and structure of lava flows while Wadge (1983) and Reagan et al. (1987)
investigated the origins of lava extruded between 1968 and 1986 and determined that
significant chemical changes in the magma had occurred since 1968. Casadevall et al.
(1984) made some of the first COSPEC sulphur dioxide measurements and particle
studies in Arenal’s volcanic plume.
Objectives
This thesis was undertaken to (1) evaluate soil gas distribution on the flanks of
Arenal volcano, (2) estimate, using petrologic and remote sensing techniques, the
quantity of SO2 emitted from Arenal since 1968, (3) attempt to correlate the observed
spatial and temporal patterns of soil and plume gases to eruptive and seismic activity,
and (4) discuss the origin and the transport of these gases at Arenal. In order to achieve
these objectives, I measured radon, CO2 and SO2 gas because (1) the results can be
compared to other volcanoes where similar measurements have been made, (2) it is
comparatively simple to measure radon, CO2, and SO2, and (3) these gases are believed
to respond to changes in volcanic activity and the stress-state of the volcano.
Location
Arenal is a stratovolcano located at 10.463°N 84.703°W in northwestern Costa
Rica, 90 km northwest of the capital, San José (Figure I-1). The volcano is a steep sided
0
10o00'
Kilometres
50
100
84o00'
Puerto
Limón
PANAMA
CARIBBEAN
SEA
Golfito
San Isidro
SAN JOSÉ
Arenal
volcano
Puntarenas
NORTH PACIFIC
OCEAN
Nicoya
Liberia
Cabo Gracias
a Dios
NICARAGUA
General Introduction
_____________________
4
Figure I-1: Geographic map of Costa Rica showing the location of A r e n a l
volcano.
General Introduction
______________________
5
edifice vegetated to the north, east and south. The volcano consists of two summit
craters (C and D). A large field of young (post-1968) lava covers the western flank
(Figure I-2). Arenal is situated between two massifs, the Cordillera de Guanacaste (SE)
and the Cordillera Central (NW) which together form the volcanic chain that makes up
the Costa Rican Arc (Figure I-3, Stoiber and Carr, 1973).
Approximately three
kilometres south of Arenal lies the small truncated and dormant volcano, Cerro Chato
(Figure I-2).
Geological Setting
Regional Geology
Arenal volcano is situated in the Central American volcanic chain, on the
boundary between the northern and central Costa Rican segments (Stoiber and Carr,
1973).
The Cocos Plate is being subducted under the Caribbean Plate along the
Mesoamerican trench northeast of Arenal (Figure I-3).
Costa Rica consists of six
principal geological provinces paralleling the Mesoamerican trench: 1) the Cretaceous to
Middle Tertiary ophiolitic suite; 2) Tertiary basins; 3) Tertiary volcanic ranges; 4) Active
Quaternary volcanic ranges; 5) Intra-arc basins; and 6) the Caribbean coastal plain
(Mora, 1983; Alvarado, 1984; Borgia et al., 1988).
An ophiolitic suite is found in the Nicoya Complex, which is comprised of
cherts, graywackes, tholeiitic pillow lavas and basaltic agglomerates. It is intruded by
gabbroic, diabasic, and dioritic rocks. The Tertiary basin is composed of sediments of
mainly marine origin, intercalated with volcaniclastic deposits. The Tertiary volcanic
General Introduction
_____________________
Figure I-2: Topographic map of Arenal volcano showing the lava fields
emplaced since 1968.
6
7
General Introduction
_____________________
]
Orosi
Rincón de
la Vieja
Miravalles
Tenorio
Arenal-Chato
volcanic system
Poás
Caribbean
Plate
Barva
Turrialba
Irazú
Mi
ddl
eA
Cocos Plate
0
50
me
rica
Tre
n
ch
100
Kilometres
Arenal-Chato volcanic system
Historically active volcanoes
Major volcanoes
Cinder cones
Contacts betwen major geological
provinces
Submerged contacts
Cretaceous to Middle Tertiary ophiolitic suite
Tertiary basins
Tertiary volcanic range
Active Quaternary volcanic range
Intra-arc basins
Caribbean coastal plain
Figure I-3: Geological map of Costa Rica. After Mora (1983), Alvarado (1984)
and Borgia (1988).
General Introduction
______________________
8
range is made up of a block-faulted horst, the Sierra de Tilaran y Abangares, extending
southeast into the Montes del Aguate.
Composed of andesitic and basaltic flows,
volcanic agglomerates and tuffs, this range is part of the late Miocene-early Pliocene
Aguacate Volcanic Group (Dengo, 1962; Malavassi, 1979; Weyl, 1980).
The
Quaternary Volcanic ranges comprise the Cordillera de Guanacaste to the northwest and
Cordillera Central to the southeast.
The Cordillera de Guanacaste contains five
stratovolcanoes (Orosi, Rincón de la Vieja, Miravalles, Tenorio and Arenal), eruptive
products of which have compositions that vary from basaltic andesite (Melson and
Saenz, 1973) to andesite (Dengo, 1962; Pichler and Weyl, 1973). Large-scale rhyolitic
and dacitic tuffs crop out on the southwest flank of the Cordillera and overlie part of the
Aguacate Volcanic Group and Nicoya Complex (Dengo, 1962). The Cordillera Central
consists of four stratovolcanoes (Poás, Barva, Irazu, and Turrialba), deposits of which
have compositions that vary from basalt to dacite and andesite (Pichler and Weyl, 1973).
Extensive mudflows and volcanic ash deposits are exposed on the southwest side of the
Cordillera (Williams, 1952).
Intra-arc basins comprise the Arenal graben to the
northwest and the Valle Central to the southeast. The Valle Central basement consists of
slightly folded Oligocene and Miocene marine sediments, overlain by tuffs, lavas and
ignimbrite sheets of the Cordillera Central (Weyl, 1980). Finally, the Caribbean coastal
plain is a sedimentary basin of Early Tertiary age composed of river alluvium and lahar
deposits from the volcanoes of the Cordillera Central. Some small Quaternary cinder
cones also are found in the coastal plains near Tortuguero (Weyl, 1980).
General Introduction
______________________
9
Local Geology
Arenal has a volume of only 15 km3 (Carr, 1984) and is the smallest but most
active of seven historically active Costa Rican volcanoes. The tectonic setting of the
volcano is disputed, with some authors suggesting that Arenal overlies a tear in the
subducting Cocos plate (Carr et al., 1979; Carr, 1984; Burbach et al., 1984) and others
believing there is a smooth transition in the orientation of the Wadati-Benioff zone,
thought to lie 150 km below Arenal (Guendel et al., 1984; Reagan et al., 1987). The
small truncated and dormant volcano, Cerro Chato, lies approximately three kilometres
southeast of Arenal (Figure I-4). Arenal is most likely directly tapping a lower to midcrustal magma chamber, possibly located at a discontinuity which lies at a depth of 22
km (Matumoto et al., 1977; Wadge, 1983; Reagan et al., 1987).
Three stages of differing magma compositions at Arenal are believed to coincide
with variations in eruptive activity. Stage-1 zoned magmas likely resided in the magma
chamber prior to the 1968 eruption. A new magma intruded into the chamber resulting
in the ejection of the stage-1 magma in July 1968. It subsequently mixed with the more
mafic parts of stage 1 to produce stage-2 magmas. Stage-3 magmas (mid-1974 to
present) are the product of continued mixing and fractional crystallisation along the
walls of the conduit and chamber. Each change in stage appears to correlate with a
variation in the cumulative volume of extruded material (Reagan et al., 1987).
The rocks around Arenal range in age from Pliocene-Pleistocene to Holocene and
are divided into five lithologies: 1) undivided Pliocene-Pleistocene volcanics, 2) Chato
lava flows, 3) Arenal lava flows, 4) undivided tephra from Arenal and Chato, and 5)
Figure I-4: Geological map of Arenal volcano. After Borgia (1988).
0
1
2
A1
A2
uPPv
A4
uPPv
uTAC
sd
Volcan
Chato
A3
A4
utAC1 2 3 5
Modified after Borgia et al. (1988)
A4
Volcan
Arenal
B C D
uTAC
A
E
LA
A2 utAC
utAC1 9 7 5
utAC1 9 6 8
al
ren
A
Rio
Kilometres
Lago
Arenal
uPPv
Crater
Rivers
Contact
Lakes
Sedimentary deposits
utAC - undivided tephra from
Arenal and Chato
utACdate - 1235, 1968 & 1975
pyroclastic flows
Pyroclastic flow
Undivided PliocenePleistocene volcanics
uPPv
> 1550 B.C.
Chato lavas
A1 - post 1968
A2 - 1700 to 1800 A.D.
A3, A4 - ~ 1080 A.D.
LA - 220 B.C.
Arenal lavas
General Introduction
_____________________
11
General Introduction
______________________
11
sedimentary deposits (Figure I-4, Malavassi, 1979; Borgia et al., 1988). The oldest rocks
are from the Venado Formation, consisting of Miocene continental shelf deposits
(Malavassi and Madrigal, 1970; Obando, 1986).
These are overlain by Miocene-
Pliocene deposits from the Aguacate Volcanic Group (Dengo, 1968), which are in turn
overlain by local Holocene alluvium deposits. These alluvium deposits are typically
found in the Lago Arenal area and along the margins of major rivers in the region
(Malavassi, 1979).
Volcanic Activity
Volcanic Activity Prior to 1968
Prior to 1965, no research had been conducted on Arenal volcano, and it was not
even mentioned in the Catalogue of Active Volcanoes of the World (Mooser et al., 1959).
However, based on tephrochronology and radiocarbon dating, Arenal previously has
erupted in 1750 ± 50 A.D., 1525 ± 20 A.D., 1080 ± 50 A.D., 220 ± 75 B.C., and 900 ±
150 B.C. (Simkin and Siebert, 1994). Arenal formed from the base of Chato’s edifice,
with pyroclastic and lava flows being deposited mainly to the northeast between the
Monterrey Hills to the north and Chato to the southeast. To the west, Lago Arenal was
formed by the damming effect of the volcanic deposits on the local drainage system.
Since the construction of a hydroelectric dam, the area of the lake has increased
dramatically to within 10 km of the summit of Arenal (Figure I-4).
Volcanic Activity During and After 1968
Premonitory manifestations of volcanic activity began in 1965 with the release of
General Introduction
______________________
12
colourless gas on the northeast flank of Arenal, the drying out of the Cedeño lagoon,
formation of a new hot spring, and an increase in the temperature of Rio Tabacón (Avila,
1978; Alvarado and Barquero, 1987; Barquero et al., 1992). On the morning of July 29,
1968, after 10 hours of intense seismic activity, Arenal erupted explosively and
continued to erupt over a period of three days, killing 78 people and devastating an area
12 square kilometres. The activity started with a lethal lateral blast that levelled the
densely forested western flank and destroyed the village of Pueblo Nuevo, 6 km west of
the summit. The initial blast was followed by plinian eruption columns, pyroclastic
flows, and ballistically ejected blocks and bombs (Figure I-5). Three new craters (A, B,
C) were formed during this time, with an approximately east-west orientation on the
western flank of the volcano (Figure I-4). The largest, crater C (1100 m), was the source
of all the major explosions (Melson and Saenz, 1973). These events were followed by
three days of relative calm consisting of minor ash and fumarolic activity. A fumarolic
phase began August 10 and continued to September 14. Activity was noted at all of the
craters, with the most intense activity at crater C. From September 14 to September 19,
renewed explosions consisting of low-energy, low-volume ejection of scoriaceous to
pumiceous andesite were observed (Melson and Saenz, 1973). This was followed by a
period of lava effusion from crater A (Sept. 19, 1968 to the end of 1973) consisting of
blocky lava which descended into the Quebrada Tabacón valley. Another explosive
phase started on June 17, 1975, with the emplacement of a blocky ash flow along the Rio
Tabacon valley. This deposit resulted from the formation of nuées ardentes produced by
avalanching of a lava flow being extruded from crater C (Figure I-4, Malavassi, 1979).
General Introduction
_____________________
Figure I-5: Blocks from the blocky ash flow emplaced on the western flank
of Arenal in July 1968. A 1992 blocky-aa lava flow is seen in the
background.
13
General Introduction
______________________
14
This eruptive phase was followed by upslope migration of crater C to within close
proximity of crater D (Van der Bilt et al., 1976; Matumoto and Umana, 1976; Malavassi,
1979; Cigolini et al., 1984). Strong fumarolic activity and extrusion of blocky aa lava
followed, continuing until June 1984. This date marked an increase in activity at Arenal,
with the beginning of eruptions consisting of ash and large pyroclastics (Figure I-6).
This activity led to a Strombolian eruptive phase which produced basaltic-andesite
tephra and lava and continues at the time of the writing of this thesis (April 1997,
Figures I-4, I-7).
General Introduction
_____________________
Figure I-6: White cloud on left of image represents a small pyroclastic flow
caused by the collapse of part of the blocky aa flow (a levee is in evidence to
the upper right of the cloud). View is of the western flank.
Figure I-7: A small strombolian eruption towards the western flank. Crater
C is to the left, crater D to the right. View from the Arenal Volcano
Observatory, south of the volcano.
15
General Introduction
______________________
16
References
Alvarado, G. E., 1984. Aspectos petrologicos-geologicos de los volcanes y unidades
lavicas del Cenozoico Superior de Costa Rica. Thesis, Esc. Centroam. Geol.,
Univ. Costa Rica, San Jose, Costa Rica, 183 pp.
Alvarado, G. E. and Barquero, R., 1987. Las señales sismicas del volcan Arenal
(Costa Rica) y su relacion con las fases eruptivas (1968-1986). Cienc. Tec., 2:
19-35.
Avila, G., 1978. Investigación y vigilancia del volcan Arenal, Alajuela, Costa Rica.
Instituto Costaricense de Electricidad (ICE), Informe interno, San José, Costa
Rica, 40 pp.
Barquero, R., Alvarado, G. E., and Matumoto, T., 1992. Arenal Volcano (Costa Rica)
premonitory seismicity. In: R. Gasparini, R. Scarpa, and K. Aki (Editors),
Volcanic Seismology. Springer-Verlag, New York, pp. 84-96.
Bennett, F. D. and Raccichini, S., 1977. Las erupciones del Volcan Arenal, Costa Rica.
Rev. Geograf. Amer. Cent., 5-6: 7-35.
Borgia, A., Casertano, L., and Cigolini, C., 1983. Estructura y dinámica de los flujos
de lava del Arenal. Bol. Vulcanol., 14: 79-80.
Borgia, A., Poore, C., Carr, M. J., Melson, W. G., and Alvarado G.E., 1988.
Structural, stratigraphic, and petrographic aspects of the Arenal-Chato volcanic
system, Costa Rica: Evolution of a young stratovolcanic province. Bull.
Volcanol., 50: 86-105.
Burbach, G. V., Frohlich, C., Pennington, W. D., and Matumoto, T., 1984.
Seismicity and tectonics of the subducted Cocos plate. J. Geophys. Res., 89:
7719-7735.
Brantley, S. L. and Koepenick, K. W., 1995. Measured carbon dioxide emissions from
Oldoinyo Lengai and the skewed distribution of passive volcanic fluxes.
Geology, 23: 933-936.
Carr, M. J., 1984. Symmetrical and segmented variation of physical and geochemical
characteristics of the Central American Volcanic Front. J. Volcanol. Geotherm.
Res., 20: 231-252.
Carr, M. J., Rose Jr., W. I., and Mayfield, D. G., 1979. Relation of compositions to
volcano size and structure in El Salvador. J. Volcanol. Geotherm. Res., 5: 387401.
Casadevall, T. J., Johnson, D. A., Harris, D. A., Rose Jr., W. I., Malinconico Jr., L.
General Introduction
______________________
17
L., Stoiber, R. E., Bornhorst, T. J., Williams, S. N., Woodruff, L., and
Thompson, J. M., 1981. SO2 emission rates at Mount St. Helens from March 29
through December, 1980. In: P. W. Lipman and D. R. Mullineaux (Editors), The
1980 Eruptions of Mount St. Helens. U.S. Geol. Surv. Prof. Pap., 1250, pp. 193200.
Casadevall, T. J., Rose Jr., W. I., Fuller, W. H., Hunt, W. H., Hart, M. A., Moyers,
J. L., Woods, D. C., Chuan, R. L., and Friend, J. P., 1984. Sulfur dioxide and
particles in quiescent volcanic plumes from Poás, Arenal, and Colima volcanoes,
Costa Rica and Mexico. J. Geophys. Res., 89: 9633-9641.
Cigolini, C., Borgia, A., and Casertano, L., 1984. Intra-crater activity, aa-block lava,
viscosity and flow dynamics: Arenal Volcano, Costa Rica. J. Volcanol.
Geotherm. Res., 20: 155-176.
Dengo, G., 1968. Estructura geologica, historia tectonica y morfologia de la America
Central. Centro Regional de Ayuda Tecnica (AID), Mexico DF, Mexico, 52 pp.
Dengo, G., 1962. Estudio Geológico de la Region de Guanacaste. Instituto Geografico
Nacional, San Jose, Costa Rica, 112 pp.
Fudali, R. F. and Melson, W. G., 1972. Ejecta velocities, magma chamber pressure and
kinetic energy associated with the eruption of Arenal volcano. Bull. Volcanol.,
35: 383-401.
Guendel, F., McNally, K. C., Lower, J., Malavassi, E., and Saenz, R., 1984. New
evidence regarding subduction mechanisms near southern terminus of the Middle
America Trench, Costa Rica. EOS Trans. Am. Geophys. Union, 65: 998.
Heiligmann, M., Stix, J., Williams-Jones, G., Sherwood Lollar, B., and Garzón V.,
G., 1997. Distal degassing of radon and carbon dioxide on Galeras volcano,
Colombia. J. Volcanol. Geotherm. Res., 77: 267-284.
Malavassi, E., 1979. Geology and petrology of Arenal Volcano, Costa Rica. M.Sc.
Thesis, Department of Geology and Geophysics, University of Hawaii at Manoa,
U.S.A., 111 pp.
Malavassi, V. and Madrigal, R., 1970. Reconocimiento geólogico de la zona norte de
Costa Rica. Direccion Geologia, Minas y Petrolio, Informe Tecnico y Nota
Geologica, Costa Rica, 9 (38): 12 pp.
Matumoto, T., Ohtake, M., Latham, G., and Umana, J., 1977. Crustal structure in
southern Central America. Bull. Seismol. Soc. Am., 67: 121-134.
Matumoto, T. and Umana, J. E., 1976. Informe sobre la erupción del volcan Arenal
occurida el 17 de Junio de 1975. Rev. Geofis. Inst. Panama Geogr. Hist., 5: 299315.
General Introduction
______________________
18
Melson, W. G. and Saenz, R., 1968. The 1968 eruption of Arenal Volcano: preliminary
summary of field and laboratory studies. Smithsonian Center for Short-Lived
Phenomena, Report 7-1968, 35 pp.
Melson, W. G. and Saenz, R., 1977. Las erupciones del Volcan Arenal, Costa Rica, en
Julio de 1968. Rev. Geograf. Amer. Cent., 5-6: 55-148.
Melson, W. G. and Saenz, R., 1973. Volume, energy and cyclicity of eruptions of
Arenal Volcano, Costa Rica. Bull. Volcanol., 37: 416-437.
Minakami, T., Utibori, S., and Hiraga, S., 1969. The 1968 eruption of Volcano
Arenal, Costa Rica. Tokyo Univ. Earthquake Res. Inst. Bull., 47: 783-802.
Mooser, F., Meyer-Abich, H., and McBirney, A. R., 1959. Catalogue of the Active
Volcanoes of the World including Solfatara Fields, Part 6, Central America.
International Association of Volcanology, Napoli, Italy, 114 pp.
Mora, S., 1983. Una revisión y actualización de la clasificación morfotectónica de Costa
Rica. Bol. Vulcanol., 13: 18-36.
Obando, A. L. G., 1986. Estratigrafia de la formación Venado y rocas sobreyacentes
(Micocebo Reciente) provincia de Alajuela, Costa Rica. Rev. Geograf. Amer.
Cent., 5: 73-104.
Pichler, H. and Weyl, R., 1973. Petrochemical aspects of Central American
magmatism. Geol. Rund., 62: 357-396.
Reagan, M. K., Gill, J. B., Malavassi, E., and Garcia, M. O., 1987. Changes in
magma composition at Arenal Volcano, Costa Rica, 1968-1985: Real-time
monitoring of open-system differentiation. Bull. Volcanol., 49: 415-434.
Saenz, R., 1977. Erupción del volcan Arenal en 1968. Rev. Geograf. Amer. Cent., 5-6:
149-188.
Simkin, T. and Siebert, L., 1994. Volcanoes of the World. Geoscience Press, Tucson,
349 pp.
Stoiber, R. E. and Carr, M. J., 1973. Quaternary volcanic and tectonic segmentation of
Central America. Bull. Volcanol., 37: 304-325.
Stoiber, R. E., Williams, S. N., and Huebert, B. J., 1986. Sulfur and halogen gases at
Masaya caldera complex, Nicaragua: Total flux and variations with time. J.
Geophys. Res., 91: 12215-12231.
Van der Bilt, H., Pangiagua, S., and Avila, G., 1976. Informe de la actividad del
volcan Arenal iniciada el 17 de Junio de 1975. Rev. Geofis. Inst. Panama Geogr.
Hist., 5: 295-298.
General Introduction
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19
Wadge, G., 1983. The magma budget of Volcán Arenal, Costa Rica from 1968 to 1980.
J. Volcanol. Geotherm. Res., 19: 281-302.
Weyl, R., 1980. Geology of Central America. Gebrüder Borntraeger, Berlin, 371 pp.
Zapata, J. A., Calvache V., M. L., Cortés J., G. P., Fischer, T. P., Garzon V., G.,
Gómez M., D., Narvaez M., L., Ordoñez V., M., Ortega E., A., Stix, J.,
Torres C., R., and Williams, S. N., 1997. SO2 fluxes from Galeras Volcano,
Colombia, 1989-1995: Progressive degassing and conduit obstruction of a
Decade Volcano. J. Volcanol. Geotherm. Res., 77: 195-208.
CHAPTER I
DIFFUSE DEGASSING AT ARENAL VOLCANO,
COSTA RICA
___________________
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
21
Abstract
Radon, CO2, and δ13C in soil gas have been measured at Arenal volcano, Costa
Rica. Rn and CO2 concentrations ranged from 1.2 to 69 pCi/l and from 0.01 to 9.62%,
respectively. Soil gases reach maxima near possible faults and on the lower flanks of the
volcanoes. The δ13C values, which varied between -10.7 ‰ and -30.8 ‰, were heaviest
on the lower slopes and close to possible fault lines. There are few surface-penetrating
structures on Arenal, as they are covered by young lavas. Arenal’s high level of activity
makes correlations between seismic activity and soil gas fluctuation difficult, if not
impossible. Rather, these fluctuations may be explained by variations in atmospheric
pressure. The trends of increasing soil gas concentrations and heavier isotope values
with distance from the summit suggest that (1) the soil gas concentrations are strongly
influenced by the level of development of the soil; (2) diffuse degassing of deep,
magmatic gas on the upper flanks of the volcanoes is negligible due to low permeability
from the cover of young volcanic rocks; and (3) increased magmatic degassing on the
lower flanks is the result of greater fracturing in the older lavas. Volcanoes such as
Arenal act as plugs in the continental crust, limiting degassing to fumaroles, faults, and
the fractured lower flanks.
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
22
Introduction
T
HE high level of activity at volcanoes such as Arenal and the general
inaccessibility of the crater area make the direct sampling of gas from crater
fumaroles highly problematic. By contrast, soil gases may be sampled safely at a certain
distance from the crater over extended periods of time. Many studies have shown the
association of soil gas variations and geologic activity (e.g., Thomas et al., 1986;
Baubron et al., 1991). Thus, in this chapter I present the results of a study of soil gas
CO2 and 222Rn concentrations and carbon isotope analyses of CO2 from Arenal volcano,
Costa Rica. The purpose of this chapter is to (1) evaluate the distribution of soil gas on
the flanks of Arenal, (2) correlate the observed spatial and temporal variations with
volcanic activity, and (3) investigate the origin and transport of these gases at Arenal.
222
Radon is a radioactive noble gas with a half-life of 3.82 days which may be
emitted from any rock, soil or water that contains uranium or radium (Tilsley, 1992). It
is produced from the 238U decay series, with 226Ra being its immediate parental isotope.
Another isotope of radon,
220
Rn, also referred to as thoron with a half-life of 55 s, is
produced from the Th decay chain.
226
Ra, which is soluble in water, may be transported
significant distances before decay. This may result in the emplacement of large amounts
of radium near the surface, which can significantly increase
(Tilsley, 1992).
222
222
Rn concentrations
Rn also may be transported significant distances by water, by
advection and by mobile gases such as CO2 (Ozima and Podosek, 1983).
The importance of CO2 degassing on volcanoes was first recognised by
Carbonnelle and Zettwoog (1982) and Carbonnelle et al. (1985). After H2O, CO2 is the
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
23
most abundant species in volcanic gas. Its low solubility in silicate melts at low to
medium pressure allows for early exsolution, making CO2 a potentially good tracer gas
for the study of sub-surface magma degassing (Baubron et al., 1991). In the process of
diffuse magma degassing, the preferential removal of reactive gas species (e.g., SO2)
from the gas phase allows CO2 and inert gases (e.g., He) to reach the surface (Allard et
al., 1991). Volcanic CO2 has many different sources, among them the mantle, organic
processes, thermal metamorphism of carbonate rocks, and the mixing of gas from these
different sources (Irwin and Barnes, 1980).
Factors Affecting Soil Gas Concentrations and Distributions
Soil gas concentrations may be affected by processes that cause the development
of stress regimes in the ground or change the pore spaces and volume of cracks and
fissures. Processes such as (1) climatic variations (wind, rain, temperature, and soil
humidity), (2) atmospheric pressure variations, (3) deformation of a volcanic edifice, (4)
volcanic and volcanic seismic activity, and (5) tectonic seismic activity all may have
significant effects on soil gas concentrations (Heiligmann, 1997).
The nature and development of the soil at various elevations also may have a
significant effect on the radon, CO2 concentrations, and CO2 flux values. At higher
elevations, the soil often consists mainly of unconsolidated pyroclastic material, while
the soils become progressively more developed at lower elevations, i.e., the amount of
clay and organic material increases. This can affect concentrations and fluxes, since the
more developed soils are better able to retain moisture which leads to increased sealing
of the ground and the subsequent build-up of gas. The less consolidated material will
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
24
likely dry out faster after periods of precipitation, having efficiently removed the
humidity by percolation. This allows the gas to escape relatively easily. The more
organic-rich soils also may lead to increased concentrations of CO2 due to the bacterial
production of CO2 and decomposition of organic material (Hinkle, 1990).
Methodology
Station Locations
Due to Arenal’s relatively small size, most of the volcano is easily accessible by
car and by foot. The upper flanks, however, are generally inaccessible due to the
extremely high level of activity of the volcano. Four radial lines of 20 stations were
installed on the north, south, west and east flanks of the volcano, while another five
stations were installed around part of the base on the north and northeast sides. Labelled
for their geographic locations, the stations N-1 to N-5, S-1 to S-4, W-1 to W-5, E-1 to E5, and NE-1 to NE-5 are shown on Figure 1.1. The stations, which range in elevation
from 338 m to 855 m, were sampled for CO2 and 222Rn on a weekly basis over a period
of two months in 1995. The N, S, E, and W stations were sampled for carbon isotopes
twice during 1995, while the N, E, and NE lines were again sampled during the 1996
field season.
Soil Gas Measurements
222
Rn was measured using the E-Perm technique developed by Rad-Elec Inc.
(Kotrappa et al., 1988; Kotrappa and Stieff, 1992) which consists of an electrostatically
charged Teflon disk (an electret) attached to an ion chamber of known volume. The disk
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
25
is placed at the bottom of a one metre long, 7.62 cm diameter PVC tube buried ~75 cm
in the ground for a period of approximately one week (Figure 1.2). The radioactive
decay of radon in the chamber ionises the air, producing negative ions which contact the
positively charged Teflon disk, resulting in a decrease of the electret voltage. Based on
the voltage drop, chamber volume, exposure time and ambient pressure, the
concentration of Rn may be calculated:
 Vi − V f
[ Rn ] = 
 CF ⋅ T

 − ( 0.120 ⋅ M )

(1.1)
where [Rn] is the concentration of radon in pCi/l; Vi and Vf are the initial and final
voltages, respectively; T is the period of exposure in days; 0.120 is a calibration constant
in units of pCi/l per µRad/hour; and M is the ambient gamma radiation in µR/h. For a
blue short-term electret in an L chamber, CF is a calibration factor calculated by:
 Vi + V f 
CF = 0.2613 + 0.0001386 ⋅ 

 2 
(1.2)
where 0.2613 and 0.0001386 are calibration constants in units of pCi/l per µR/h (Rad
Elec Inc., 1993).
Instrumental error can be attributed to three sources. The first (%E1) is due to
imperfections in the instruments such as uncertainties in chamber volume and electret
thicknesses.
This error has been experimentally measured at approximately 5%
(Kotrappa et al., 1990). The second source of error (%E2) is the uncertainty in voltage
measurement which amounts to an error of 1.4 volts between the initial and final
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
Figure 1.1: Topographic map of Arenal volcano showing the location of Rn
and CO 2 soil gas stations (stars). The locations of seismic stations are shown
(red triangles). Contours ae every 100 m.
25
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
Figure 1.2: PVC tube (left) used in measuring radon and an aluminium tube
(right) used in the measurement of CO 2 soil gas. Both tubes are buried ~75
cm.
27
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
28
measurements (the square root of the sum of the squares of a 1 volt error per reading),
thus:
 100 ⋅ 14
. 
% E2 = 

 Vi − V f 
(1.3)
The third source of error (%E3) is due to the uncertainty in background gamma
radiation. These uncertainties have been estimated at 0.1-0.2 pCi/l; thus:
 100 ⋅ 01
. 
%E3 = 

 [ Rn] 
(1.4)
The total error can be determined by taking the square root of the sum of the squares of
the three sources of error:
ET = % E12 + % E 2 2 + % E 32
The
226
(1.5)
Radium emanating potential (RnERaC) of the ground was measured by
taking soil samples from the bottom (~75 cm) of the
222
Rn holes. These samples were
kept in tightly sealed plastic bags in order to retain the soil humidity until they could be
analysed. In the laboratory, 20 to 30 g of soil were placed in a ceramic petri dish and
subsequently exposed to a short-term E-PERM electret in a sealed 3.74 L glass bottle for
a period of 11 days. The radon emanating potential of
226
Ra then was calculated using
the following formula (Rad Elec Inc., 1993):




3.74 ⋅ ([ Rn] / M )
RnERaC = 
⋅ (1000)
(1 − Exp(−01813
.
⋅ T )) 
1 −



01813
.
⋅T
where 3.74 is the volume (L) of the glass jar; [Rn] is the
(1.6)
222
Rn concentration (pCi/l); M
is the mass of soil (g) and T is the exposure time (days). The RnERaC, expressed in
pCi/kg, represents the ability of the soil to produce radon gas. Thus, rather than a
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
29
measure of 226Ra concentration, the RnERaC is a measure of radon-emitting 226Ra. This
radon is typically produced near the edges of the soil grains, escaping into the pore
spaces and subsequently travelling to the electret where it is measured.
For CO2 soil gas measurements, a metre-long, one centimetre-diameter
aluminium tube, with 5-6 small perforations cut along the bottom 10 cm, was placed
next to the PVC tube to the same depth. CO2 in the soil diffused into the tube, where it
was measured periodically by an infrared gas analyser (ADC LFG-20 Landfill Gas
Analyser). CO2 and CH4 were analysed by non-dispersive infrared absorption, while O2
was measured by an electrochemical cell. The ADC LFG-20 Landfill Gas Analyser has
measurement ranges for CO2 and CH4 of 0-10% and 10-100%, with corresponding
precisions of 0.5% and 3%, respectively. Only the first measurement range (0-10%) was
used due to the comparatively low concentrations of CO2 in the soil. O2 measurements
have a precision of ±0.4% on a scale of 0-25%. CO2 concentrations were corrected for
altitude using the following formula:
([CO2])·(Cf)
(1.7)
where [CO2] is the volume percent CO2 measured at the site and Cf is the correction
factor calculated using:
Cf = 1 + (∆Elev.)⋅(2.678 x 10-4)
(1.8)
where ∆Elev. is the difference in elevation between the station and the base station in
Fortuna (~250 m), and 2.678 x 10-4 is a factor based on the linear regression of
measurements of gas standards of known concentration taken at different elevations
(Heiligmann et al., 1997).
Diffuse CO2 flux was measured using a technique developed by Moore and
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
30
Roulet (1991). The technique consists of burying a 15 L chamber, with the chamber
opening towards the soil, ~75 cm in the ground. Tygon tubing pierces the top of the
chamber and allows for the measurement, by connection to the infrared gas analyser, of
CO2 concentrations over a period of 3-4 hours. Measurements were taken every 15
minutes for the first 1.5 hours and subsequently every 30 minutes for the remaining
period. By plotting CO2 concentration versus time and taking the slope of the initial
linear segment of the curve, a flux measurement (mg/m2·min) was calculated using the
following formula:
 1 
 ∆CO2 
1
 ⋅ (10 −6 ppm) ⋅ 
Flux = 
 ⋅ M CO2 ⋅ (1000 mg g ) ⋅ (V ) ⋅   ⋅ (δ air )
 A
 ∆t 
 M air 
(
)
(1.9)
where ∆CO2 is the difference between initial and final CO2 concentrations (ppm); ∆t is
the elapsed time (min); Mair is the molecular weight of air (28.964 g/mole); MCO2 is the
molecular weight of CO2 (44 g/mole); V is the chamber volume (14.9 L); A is the area of
the chamber bottom (0.0519 m2); and δair is the density of air (g/L) calculated using
[(P/T)⋅(0.3483677)] where P is the pressure in mbar and T is the temperature in K.
Carbon isotope (13C/12C) analyses of soil gases were performed at the University
of Toronto using a Finnigan MAT 252 gas source mass spectrometer (MS) linked to a
Varian 3400 gas chromatograph (GC) equipped with a capillary column (GC-C-IRMS).
The interface of the GC and MS consists of a combustion oven containing Cu-Ni-Pt
trimetal. CO2 in the samples was separated by the GC at 27ºC. Three aliquots of each
sample were injected into the GC-C-IRMS system during δ13C analysis, with the average
δ13C for these three analyses reported. Accuracy and reproducibility of the isotopic
analyses are both < 0.1‰. δ13C analyses of the organic soil component were made of
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
31
samples from stations NE-1 to NE-5. The samples were initially sun-dried in the field
and transported in aluminium foil containers to the laboratory.
They subsequently
underwent hand picking and removal of organic and lithic material. The soils were then
heated in an oven at 60°C for 2-3 days to remove any remaining humidity. They were
then crushed and decarbonated in 1 N HCl. The decarbonated samples were then rinsed
with deionised water and dried at 60°C. A subsample of 50-500 mg of soil was placed
in quartz tubes containing pellets of Cu and CuO catalyst. These were heated at 850°C
for 1 hour, followed by another hour at 600-650°C, and subsequently allowed to cool.
The resulting gas was cryogenically purified on a vacuum line and analysed on a
Finnigan MAT 252 mass spectrometer. All carbon isotope ratios are expressed as ‰
(per mil) difference from a Peedee Belemnite (PDB) standard.
Seismic Measurements
Seismic data of were collected in 1995 from a permanent seismographic station,
located 4 km east of summit, which continuously monitors the volcano (Figure 1.1).
This station has a short-period vertical seismometer (1 Hz) that is telemetered by
telephone line to the Observatorio Volcanológico y Sismológico de Costa Rica
(OVSICORI) of the Universidad Nacional in Heredia. Seismic data were limited in
coverage due to technical difficulties with the instrument in 1995.
Results
Radon
Observed 222Rn concentrations on Arenal range from 1.2 to 69 pCi/l with an error
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
32
from <1 to 4 pCi/l (Table 1.1). There is also a general inverse correlation between Rn
and altitude (Figure 1.3a). On the northern line, for example, station N-1 has an average
222
Rn value of 6 pCi/l while station N-5, the furthest from the summit, has an average
value of 36 pCi/l (Figure 1.3b). The western and southern lines also have values which
increase outwards from the summit, with high stations showing low concentrations (e.g.,
W-1: 8 pCi/l; S-1: 8 pCi/l) and lower stations showing comparatively high values (e.g.,
W-5: 27 pCi/l; S-4: 32 pCi/l). The eastern line, however, is anomalous because it does
not show the tendency towards increasing radon with decreased elevation. Stations on
the northeastern line, which are the furthest from the crater and generally oriented
concentrically with the volcano, have the highest 222Rn values (e.g., NE-2: 62 pCi/l; NE5: 57 pCi/l).
Radon Emanating Potential
The
226
Ra emanating potentials (RnERaC) for the 25 stations range from 60 to
288 pCi/kg. Generally, the RnERaC values are highest at stations with high radon (e.g.,
E-2, E-3 and E-4) and lowest at stations with low radon concentrations (e.g., W-1, N-1,
and N-3) (Table 1.2, Figure 1.4). The radon emanating potential also was measured for
crushed samples (35 mesh or 500 µm) of a 1968 pyroclastic debris flow and a 1992 lava
with a resulting RnERaC varying between 66 and 140 pCi/kg (Table 1.2).
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
Table 1.1 Radon soil gas from Arenal volcano for 1995-1996
Station
E-1
E-1
E-1
E-1
E-1
E-1
E-2
E-2
E-2
E-2
E-2
E-2
E-3
E-3
E-3
E-3
E-3
E-3
E-4
E-4
E-4
E-4
E-4
E-4
E-5
E-5
E-5
E-5
E-5
E-5
N-1
N-1
N-1
N-1
N-1
N-2
N-2
N-2
N-2
N-2
N-3
N-3
Date
Rn
Error
(d/m/y) (pCi/L) (pCi/L)
06/03/95
15
2
13/03/95
19
1
21/03/95
21
1
26/03/95
22
1
05/04/95
23
1
14/04/95
20
1
06/03/95
48
3
13/03/95
48
2
21/03/95
60
3
26/03/95
49
3
05/04/95
67
4
14/04/95
58
3
06/03/95
37
3
13/03/95
39
2
21/03/95
41
2
26/03/95
41
2
05/04/95
61
3
14/04/95
34
2
06/03/95
40
3
13/03/95
34
2
21/03/95
38
2
26/03/95
29
2
05/04/95
44
2
14/04/95
32
2
06/03/95
27
3
13/03/95
20
1
21/03/95
34
2
26/03/95
30
2
05/04/95
38
2
14/04/95
52
3
01/03/95
7
1
06/03/95
2
1
13/03/95
2
21/03/95
6
1
05/04/95
12
1
01/03/95
8
2
13/03/95
3
21/03/95
6
1
05/04/95
8
1
14/04/95
3
01/03/95
4
1
06/03/95
2
1
Station
N-3
N-3
N-3
N-3
N-4
N-4
N-4
N-4
N-4
N-4
N-4
N-5
N-5
N-5
N-5
N-5
N-5
N-5
NE-1
NE-1
NE-2
NE-2
NE-3
NE-3
NE-4
NE-4
NE-5
NE-5
S-1
S-1
S-1
S-1
S-1
S-1
S-1
S-1
S-2
S-2
S-2
S-2
S-2
S-2
Date
13/03/95
21/03/95
26/03/95
05/04/95
01/03/95
06/03/95
13/03/95
21/03/95
26/03/95
05/04/95
14/04/95
01/03/95
06/03/95
13/03/95
21/03/95
26/03/95
05/04/95
14/04/95
09/03/96
09/03/96
09/03/96
09/03/96
04/03/96
04/03/96
08/03/96
08/03/96
08/03/96
08/03/96
26/02/95
03/03/95
07/03/95
12/03/95
20/03/95
25/03/95
03/04/95
12/04/95
26/02/95
03/03/95
07/03/95
12/03/95
20/03/95
25/03/95
Rn
Error
(pCi/L) (pCi/L)
2
4
1
4
39
2
37
2
40
2
62
3
33
2
45
2
31
2
33
2
35
2
35
2
39
2
35
2
45
2
31
2
17
1
18
1
57
3
69
4
30
2
44
2
48
2
47
2
56
3
58
3
15
2
7
1
6
1
5
1
10
1
7
1
13
1
4
29
2
34
2
42
2
37
2
44
2
45
2
33
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
Table 1.1 Continued
Station
S-2
S-2
S-3
S-3
S-3
S-3
S-3
S-3
S-3
S-3
S-4
S-4
S-4
S-4
S-4
S-4
S-4
S-4
W-1
W-1
W-1
W-1
W-1.5
W-2
W-2
W-2
W-2
W-2
W-2
W-2
W-3
W-3
W-3
W-3
W-3
W-3
W-3
W-3
W-4
W-4
W-4
Date
04/04/95
12/04/95
26/02/95
03/03/95
07/03/95
12/03/95
20/03/95
25/03/95
04/04/95
12/04/95
26/02/95
03/03/95
07/03/95
12/03/95
20/03/95
25/03/95
04/04/95
12/04/95
26/02/95
03/03/95
07/03/95
12/03/95
04/04/95
26/02/95
03/03/95
07/03/95
12/03/95
20/03/95
25/03/95
12/04/95
26/02/95
03/03/95
07/03/95
12/03/95
20/03/95
25/03/95
04/04/95
12/04/95
26/02/95
03/03/95
07/03/95
Rn
Error
(pCi/L) (pCi/L)
47
2
49
3
23
2
17
1
19
1
18
1
20
1
18
1
21
1
15
1
28
2
28
2
34
2
31
2
32
2
32
2
38
2
35
2
10
1
7
1
11
1
4
1
10
1
11
1
7
1
7
1
6
1
9
1
6
1
6
11
1
7
1
4
1
5
1
5
1
4
1
8
1
4
1
9
1
7
1
6
1
Samples were taken at ~0.75 m depth
Station
W-4
W-4
W-4
W-4
W-4
W-5
W-5
W-5
W-5
W-5
W-5
W-5
W-5
Date
12/03/95
20/03/95
25/03/95
04/04/95
12/04/95
26/02/95
04/03/95
07/03/95
12/03/95
20/03/95
25/03/95
03/04/95
12/04/95
Rn
Error
(pCi/L) (pCi/L)
5
1
7
1
7
1
11
1
54
3
24
2
23
1
25
2
23
1
33
2
23
1
26
1
40
2
34
35
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
A
Average Rn vs Altitude
70
60
Rn (pCi\l)
50
E
40
N
NE
S
30
W
20
10
0
300
400
500
600
700
800
900
Altitude (m)
B
Figure 1.3: (a) Average Rn (pCi/l) versus elevation of stations (metres).
There is an approximately negative linear trend. (B) Topographic map of
Arenal volcano showing concentrations of Rn (pCi/l) soil gas. There is a
tendency towards increasing radon with distance from the summit.
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
Table 1.2 Radon emanating potential for soils and lava
/debris flow at Arenal volcano
Station
E-1
E-2
E-3
E-4
E-5
N-1
N-2
N-3
N-4
N-5
NE-1
NE-2
NE-3
NE-4
NE-5
S-1
S-2
S-3
S-4
S-4
W-1
W-2
W-3
W-4
W-5
Date
29/01/96
25/06/96
08/08/96
25/06/96
29/01/96
08/08/96
25/06/96
25/06/96
29/01/96
29/01/96
08/08/96
08/08/96
08/08/96
08/08/96
08/08/96
08/08/96
29/01/96
25/06/96
29/01/96
08/08/96
25/06/96
25/06/96
29/01/96
25/06/96
29/01/96
Rn* RnERaC
(pCi/L) (pCi/kg)
0.60
196
0.73
237
0.80
263
0.88
288
0.62
203
0.23
74
0.45
123
0.36
119
0.61
192
0.64
205
0.29
92
0.31
101
0.32
103
0.19
60
0.39
130
0.32
104
0.46
142
0.46
151
0.57
180
0.39
127
0.38
109
1.02
280
0.53
156
0.51
155
0.57
180
Lava/Debris flow (35 mesh)
1968a 04/09/96 0.24
1968b 04/09/96 0.29
1992b 04/09/96 0.23
1992c 23/09/96 0.42
71
83
66
140
*Radon concentrations are measured in the laboratory during
RnERaC measurements and are not related to field measurements.
Samples were taken at ~0.75 m depth
36
37
RnERaC (pCi/kg)
0
10
20
30
40
50
60
70
0
50
100
150
Rn vs RnERaC
200
250
300
W
S
NE
N
E
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
Rn (pCi/L)
Figure 1.4: Radon (pCi/l, Table 1.4) versus radon emanating potential
(RnERaC in pCi/kg, Table 1.2). Stations with elevated radon generally have
elevated RnERaC.
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
38
Carbon Dioxide
CO2 concentrations vary from 0.01 to 9.6% (Table 1.3). On the northern line, for
example, station N-1 has an average CO2 concentration of 0.60% while station N-5, the
furthest from the summit, has an average 3.4% (Table 1.4). The western and southern
lines also have CO2 values which increase away from the summit, with high stations
showing low values (e.g., W-1: 0.04%; S-1: 0.96%) and stations on lower flanks
showing comparatively high values (e.g., W-5: 7.9%; S-4: 2.6%). There is a general
trend towards increasing concentration with decreased elevation (Figure 1.5a). Stations
on the northeastern line furthest from the crater have the highest CO2 concentrations,
with values ranging from 2.9 % at NE-2 to 9.6 % at NE-1. The stations on this line are
all approximately at the same elevation, with a difference of only 170 m between the
highest and lowest stations (Table 1.3, Figure 1.5b). A plot of average Rn versus
average CO2, shows a positive correlation for the N, S, and W lines (Figure 1.6). Lines
E and NE are anomalous, with the NE stations showing a strong negative correlation. If
one normalises radon by the RnERaC, in order to remove the effect of soil-produced Rn,
the negative correlation is degraded but nonetheless present (Figure 1.6b). By contrast,
line E shows a weak positive correlation.
Carbon Isotopes in CO2 Soil Gas
The δ13C values of CO2 vary from -10.7 to -30.8‰ (Table 1.5). There appears to
be a strong inverse relationship between elevation and δ13C (correlation coeff. r = -0.73,
Figure 1.7a).
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
Table 1.3 CO2 soil gas from Arenal volcano for 1995-1996
Station
E-1
E-1
E-1
E-1
E-1
E-1
E-1
E-2
E-2
E-2
E-2
E-2
E-2
E-2
E-3
E-3
E-3
E-3
E-3
E-3
E-3
E-4
E-4
E-4
E-4
E-4
E-4
E-4
E-5
E-5
E-5
E-5
E-5
E-5
E-5
N-1
N-1
N-1
N-1
N-1
N-1
N-1
N-2
Date
CO2 (%)
06/03/95 2.45
13/03/95 2.63
21/03/95 2.81
26/03/95 2.85
05/04/95 3.48
14/04/95 3.08
04/03/96 2.29
06/03/95 2.10
13/03/95 2.06
21/03/95 2.13
26/03/95 2.06
05/04/95 2.44
14/04/95 2.06
04/03/96 2.22
06/03/95 6.15
13/03/95 5.98
21/03/95 6.17
26/03/95 5.70
05/04/95 7.06
14/04/95 6.76
04/03/96 7.25
06/03/95 4.62
13/03/95 4.25
21/03/95 4.34
26/03/95 3.73
05/04/95 5.26
14/04/95 4.40
04/03/96 5.44
06/03/95 7.03
13/03/95 6.83
21/03/95 7.19
26/03/95 7.06
05/04/95 8.17
14/04/95 7.19
04/03/96 7.64
01/03/95 0.57
06/03/95 0.48
13/03/95 0.57
21/03/95 0.80
26/03/95 0.78
05/04/95 0.85
04/03/96 0.15
01/03/95 0.60
Station
N-2
N-2
N-2
N-2
N-2
N-2
N-2
N-3
N-3
N-3
N-3
N-3
N-3
N-3
N-3
N-4
N-4
N-4
N-4
N-4
N-4
N-4
N-4
N-5
N-5
N-5
N-5
N-5
N-5
N-5
NE-1
NE-2
NE-3
NE-4
NE-5
S-1
S-1
S-1
S-1
S-1
S-1
S-1
S-1
Date
CO2 (%)
06/03/95 0.69
13/03/95 0.73
21/03/95 0.82
26/03/95 0.81
05/04/95 0.96
14/04/95 0.73
04/03/96 0.49
01/03/95 1.68
06/03/95 1.66
13/03/95 1.68
21/03/95 1.95
26/03/95 1.86
05/04/95 2.44
14/04/95 1.51
04/03/96 1.17
01/03/95 6.35
06/03/95 6.39
13/03/95 6.52
21/03/95 7.38
26/03/95 7.54
05/04/95 7.99
14/04/95 6.77
04/03/96 8.27
01/03/95 3.40
06/03/95 3.26
13/03/95 2.96
21/03/95 3.37
26/03/95 3.41
05/04/95 4.05
14/04/95 3.14
09/03/96 9.62
09/03/96 2.86
04/03/96 7.53
08/03/96 5.57
08/03/96 4.74
20/02/95 0.92
26/02/95 0.88
03/03/95 1.06
07/03/95 1.20
12/03/95 0.80
25/03/95 0.70
03/04/95 1.07
12/04/95 1.07
39
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
Table 1.3
Continued
Station
S-2
S-2
S-2
S-2
S-2
S-2
S-2
S-2
S-3
S-3
S-3
S-3
S-3
S-3
S-3
S-3
S-4
S-4
S-4
S-4
S-4
S-4
S-4
S-4
W-1
W-1
W-1
W-1
W-1.5
W-1.5
W-2
W-2
W-2
W-2
W-2
W-2
W-2
W-2
W-3
W-3
W-3
W-3
W-3
Date
CO2 (%)
26/02/95 4.48
03/03/95 5.36
07/03/95 6.22
12/03/95 6.88
20/03/95 8.18
25/03/95 7.64
04/04/95 8.88
12/04/95 8.18
26/02/95 1.73
03/03/95 1.94
07/03/95 2.08
12/03/95 1.66
20/03/95 2.01
25/03/95 2.04
04/04/95 2.10
12/04/95 2.04
26/02/95 2.01
03/03/95 2.37
07/03/95 2.48
12/03/95 2.61
20/03/95 2.83
25/03/95 2.70
04/04/95 2.83
12/04/95 3.06
26/02/95 0.01
03/03/95 0.03
07/03/95 0.06
12/03/95 0.06
25/03/95 0.06
04/04/95 0.10
26/02/95 1.18
03/03/95 1.35
07/03/95 1.26
12/03/95 1.06
20/03/95 1.21
25/03/95 0.78
04/04/95 1.36
12/04/95 0.82
26/02/95 0.52
03/03/95 0.50
07/03/95 0.41
12/03/95 0.59
20/03/95 0.76
Station
W-3
W-3
W-3
W-4
W-4
W-4
W-4
W-4
W-4
W-4
W-4
W-5
W-5
W-5
W-5
W-5
W-5
W-5
W-5
Date
CO2 (%)
25/03/95 0.67
04/04/95 0.59
12/04/95 0.72
26/02/95 0.62
03/03/95 0.88
07/03/95 0.76
12/03/95 0.80
20/03/95 1.12
25/03/95 1.01
04/04/95 1.23
12/04/95 1.18
26/02/95 7.67
04/03/95 7.57
07/03/95 7.23
12/03/95 7.38
20/03/95 8.07
25/03/95 7.73
03/04/95 8.60
12/04/95 8.59
Error less than ± 0.05%. Samples were taken at ~0.75 m depth
40
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
Table 1.4 Average CO2, Radon and d
volcano for 1995 & 1996.
Station
E-1
E-2
E-3
E-4
E-5
N-1
N-2
N-3
N-4
N-5
NE-1
NE-2
NE-3
NE-4
NE-5
S-1
S-2
S-3
S-4
W-1
W-1.5
W-2
W-3
W-4
W-5
13
C from Arenal
13
Altitude Avg. CO2 Avg. Rn Avg. d C
(m)
(%)
(pCi/L)
(‰)
695
2.80
20
-25.23
600
2.15
55
-24.49
490
6.44
42
-19.76
405
4.58
36
-17.78
360
7.30
33
-14.00
855
0.60
6
-25.58
730
0.73
16
-26.10
655
1.75
3
-23.73
645
7.15
41
-14.18
505
3.37
36
-23.92
338
9.62
18
-10.78
360
2.86
63
-22.74
360
7.53
37
-14.95
508
5.57
47
-14.56
500
4.74
57
-25.40
830
0.96
8
-30.39
740
6.98
41
-26.09
595
1.95
19
-25.73
730
2.61
32
-25.64
805
0.04
8
770
0.08
10
710
1.13
7
-25.32
645
0.61
6
-21.86
605
0.95
13
-20.92
575
7.85
27
-20.73
41
42
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
A
Average CO2 vs. Altitude
10
9
8
7
CO2 (%)
6
E
N
5
NE
S
4
W
3
2
1
0
300
400
500
600
700
800
900
Altitude (m)
B
Figure 1.5: (a) Average CO 2 (%) versus elevation of stations (m). There is a
general trend of increasing concentration with decrease in elevation. (B).
Topographic map of Arenal volcano showing the concentrations of CO2 (%) soil
gas. There is a tendency towards increasing CO2 with distance from the summit.
43
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
A
Average Rn vs. Average CO2
70
60
Rn (pCi/l)
50
E
N
NE
S
W
40
30
20
10
0
B
0
1
2
3
4
5
CO2 (%)
6
7
8
9
10
Rn/RnERaC vs. CO2
0.6
Rn/RnERaC
0.5
0.4
E
N
NE
S
W
0.3
0.2
0.1
0
0
1
2
3
4
5
CO2 (%)
6
7
8
9
10
Figure 1.6: (a) Average Rn (pCi/l) versus average CO 2 (%). Note the generally
positive correlation for the N, S and W lines. Stations from the NE and E lines are
anomalous. (b) Average Rn (pCi/l) normalised by RnERaC (pCi/kg) versus average CO2
(%). Stations from the NE line are anomalous.
44
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
Table 1.5 d
Station
E-1
E-1
E-1
E-2
E-2
E-2
E-3
E-3
E-3
E-4
E-4
E-4
E-5
E-5
E-5
N-1
N-1
N-2
N-2
N-2
N-3
N-3
N-3
N-4
N-4
N-4
N-5
N-5
NE-1
NE-2
NE-3
NE-4
NE-5
S-1
S-1
S-2
S-2
S-3
S-3
13
C in CO2 soil gas from Arenal volcano for 1995-1996
Date
(d/m/y)
13/03/95
14/04/95
04/03/96
13/03/95
14/04/95
04/03/96
13/03/95
14/04/95
04/03/96
13/03/95
14/04/95
04/03/96
13/03/95
14/04/95
04/03/96
13/03/95
05/04/95
13/03/95
14/04/95
04/03/96
13/03/95
14/04/95
04/03/96
13/03/95
14/04/95
04/03/96
13/03/95
14/04/95
09/03/96
09/03/96
04/03/96
08/03/96
08/03/96
12/03/95
12/04/95
12/03/95
12/04/95
12/03/95
12/04/95
CO2
(%)
2.35
2.75
2.05
1.88
1.88
2.03
5.62
6.35
6.81
4.08
4.22
5.22
6.63
6.98
7.42
0.49
0.73
0.65
0.65
0.43
1.52
1.36
1.06
5.90
6.12
7.48
2.77
2.94
9.40
2.78
7.31
5.21
4.44
0.69
0.93
6.08
7.23
1.52
1.87
d
13
C
(‰)
-25.56
-25.43
-24.69
-24.53
-24.61
-24.33
-19.70
-19.46
-20.11
-18.26
-17.51
-17.57
-13.79
-13.39
-14.82
-25.71
-25.46
-26.56
-25.61
-26.14
-24.05
-23.11
-24.05
-14.34
-14.31
-13.90
-23.83
-24.01
-10.78
-22.74
-14.95
-14.56
-25.40
-30.78
-30.00
-26.08
-26.11
-25.71
-25.75
Std. Dev.
(‰)
0.06
0.04
0.08
0.08
0.03
0.05
0.06
0.08
0.09
0.03
0.07
0.07
0.06
0.03
0.04
0.18
0.22
0.10
0.11
0.28
0.10
0.06
0.22
0.04
0.04
0.03
0.20
0.04
0.03
0.04
0.14
0.13
0.07
0.11
0.19
0.09
0.09
0.09
0.00
Station
S-4
S-4
W-2
W-2
W-3
W-3
W-4
W-4
W-5
W-5
Date
(d/m/y)
12/03/95
12/04/95
12/03/95
12/04/95
12/03/95
12/04/95
12/03/95
12/04/95
12/03/95
12/04/95
CO2
(%)
2.31
2.71
0.94
0.73
0.53
0.65
0.73
1.08
6.79
7.90
d
13
C
(‰)
-25.82
-25.45
-25.78
-24.85
-22.38
-21.35
-21.28
-20.57
-20.69
-20.76
Std. Dev.
(‰)
0.22
0.02
0.13
0.38
0.30
0.07
0.11
0.02
0.09
0.06
45
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
A
Average d C vs. Altitude
1 3
-10
d 13 C (‰)
-15
-20
E
N
NE
S
W
-25
-30
-35
300
400
500
600
700
800
900
Altitude (m)
B
Figure 1.7: (a) Average d C (per mil) in CO2 soil gas versus elevation of stations
(m). Note the negative correlation (r = -0.73). (b) Topographic map of Arenal
volcano showing the concentrations of d 13C (per mil) in CO2 soil gas. Note the
13
tendency towards heavier d C with distance from the summit.
13
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
46
There is also a good positive correlation (r = 0.76) between δ13C and CO2 concentrations
(Figure 1.8a).
By contrast, only lines E and NE show a negative correlation the
correlation between δ13C and Rn (Figure 1.8b). This is also the case for the plot of δ13C
against Rn normalised by RnERaC, where line NE has a good negative correlation and
line E, a weak positive correlation (Figure 1.8c).
On the eastern line, station E-1 has an average δ13C value of -25.5‰ while
station E-5, the furthest from the summit, has an average -13.6‰ (Table 1.4, Figure
1.7b). The western and southern lines also have increased values outwards from the
summit, with high stations showing light values (e.g., W-2: -25.3‰; S-1: -30.4‰) and
stations on the lower flanks showing heavier values (e.g., W-5: -20.7‰). The northern
line also shows a trend of increasing δ13C with distance from the active crater, with the
lower stations having slightly heavier values (e.g., N-3: -23.7‰; N-5: -23.9‰) than the
higher stations (e.g., N-1: -25.6‰; N-2: -26.1‰). Station N-4, however, is anomalously
heavy with a δ13C value of -14.3‰.
Radon and CO2 concentrations are also
anomalously high at N-4. Stations on the northeastern line are furthest from the crater
and have highly variable δ13C, with values ranging from -25.4‰ at NE-5 to -10.8‰ at
NE-1.
δ13C measurements were made of organic matter in soil samples from four of the
five stations on the northeastern line. Carbonated (i.e., carbonate-bearing) samples had
values ranging from -23.2‰ at NE-1 to -26.2 for NE-5. Decarbonated samples (i.e.,
carbonates were removed from the sample) showed very similar values, with -22.9‰ at
NE-1 to -26.3‰ for NE-5. There was an average difference of only -0.067‰ between
δ13C for the carbonated and decarbonated samples (Table 1.6). There also appears to be
47
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
A
Average d C vs. CO2
13
-10
3
d 1C
(‰)
-15
-20
E
N
NE
S
W
-25
-30
-35
B
0
1
2
3
4
5
CO2 (%)
6
7
8
9
10
Average d C vs. Average Rn
13
-10
d 1 3C (‰)
-15
E
N
NE
S
W
-20
-25
-30
-35
0
10
20
30
40
50
60
70
Rn (pCi/l)
C
d C vs. Rn/RnERaC
13
-10.00
d 1 3C (‰)
-14.00
E
N
NE
S
W
-18.00
-22.00
-26.00
-30.00
0
0.1
0.2
0.3
Rn/RnERaC
0.4
0.5
0.6
Figure 1.8: (a) d C (per mil) versus CO 2 (%). Note the positive correlation. (b)
13
13
Average d C (per mil) in CO2 soil gas versus average Rn (pCi/l). (c) Averaged C (per
mil) in CO2 soil gas versus average Rn (pCi/l) normalised by RnERaC (pCi/kg). Stations
from the NE line are anomalous.
13
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
Table 1.6 Organic d
Station
NE-1
NE-2
NE-3
NE-5
13
C in soil from the NE line of Arenal volcano
Carbonated Std. Dev. Decarbonated Std. Dev. D (Carb-Decarb)
d 13C (‰)
d 13C (‰)
(‰)
(‰)
(‰)
-23.20
0.07
-22.90
0.01
-0.31
-24.98
0.00
-24.84
0.07
-0.15
-24.79
0.01
-24.88
0.06
0.09
-26.23
0.07
-26.32
0.01
0.09
all values ± 0.2 ‰ vs. PDB
Table 1.7
Station
E-2
E-3
E-4
E-5
N-2
N-4
S-1
S-2
S-3
W-2
W-3
W-5
W-5
CO2 flux from Arenal volcano in
1995
Date
CO2
CO2 Flux
(d/m/y)
11/04/95
10/04/95
10/04/95
09/04/95
15/04/95
16/04/95
06/04/95
07/04/95
08/04/95
29/03/95
29/03/95
28/03/95
28/03/95
(%)
2.03
6.50
5.00
8.30
0.55
5.60
0.72
8.00
1.80
0.84
0.69
7.15
7.15
(mg/m ·min)
0.243
0.012
0.024
0.008
0.660
0.030
0.285
0.041
0.092
0.061
0.212
0.064
0.017
2
48
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
49
a correlation between δ13C in CO2 soil gas and soil organic δ13C (Figure 1.9). For
example, station NE-5 has both the lightest soil gas δ13C (-25.4‰) and lightest soil
organic δ13C (-26.2‰). Stations NE-2 and NE-3 have relatively intermediate values for
this line (e.g., NE-2 soil gas δ13C: -22.7‰, NE-2 organic δ13C: -24.9‰). Station NE-1
has the heaviest soil gas δ13C (-10.8‰) and heaviest soil organic δ13C (-23.2‰).
Carbon Dioxide Flux
CO2 flux values range from 0 to 0.66 mg/m2⋅min (Table 1.7). Due to time
constraints in the field, only partial flux coverage of the volcano is available. When CO2
concentration is plotted versus CO2 flux, two groups of values are apparent (Figure
1.10). The first group consists of stations comparatively high on the volcano (N-2, W-2,
W-3, S-1, S-3, E-2) with high and variable flux values ranging from 0.06 to 0.66
mg/m2⋅min. The second group consists of lower lying stations (E-3, E-4, E-5, N-4, S-2)
with low flux values ranging from 0.0008 to 0.064 mg/m2⋅min. The stations on the
upper part of the volcano also have lower CO2 concentrations (0.55 to 2.0%), lower Rn
and lighter δ13C, while stations at lower elevation have higher CO2 (5.0 to 8.3%), higher
Rn and heavier δ13C. It has been shown that the isotopic composition of soil CO2 is
strongly influenced by soil respiration rates (Cerling et al., 1991).
This becomes
apparent if one plots δ13C versus CO2 flux. It is also apparent that there is a wide range
of δ13C (-26.1 to -14.2 ‰) at stations with low CO2 flux (0-0.01 mg/m2⋅min) The
stations with the heaviest δ13C have the lowest flux (Figure 1.10b).
The elevated CO2 at lower elevations may be due in part to the decomposition of
50
d C in CO2 (‰)
-27
-26
-25
-24
-23
-22
-26
-24
-22
-20
d
13
13
-18
C in soil vs. d
13
-16
C in CO2
-14
-12
-10
Decarbonated
Carbonated
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
d C in soil (‰)
13
Figure 1.9: (a) d 13C (per mil) in CO2 soil gas versus organic d 13C (per mil) in soil.
Samples are for the NE line stations furthest from the summit.
51
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
A
CO2 vs CO2 Flux
9
8
7
CO2 (%)
6
E
5
N
4
S
W
3
2
1
0
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
2
CO2 Flux (mg/m min)
B
d 13C vs CO2 Flux
-10
-15
E
-20
d 13C (‰)
N
S
W
-25
-30
-35
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
2
CO2 Flux (mg/m min)
2
Figure 1.10: (a) CO2 (%) soil gas versus CO2 flux (mg/m min). Note the grouping of
13
points where samples with elevated CO2 have low CO2 flux. (b) d C (per mil) versus CO2
2
13
flux (mg/m min). Note that stations with low flux have heavier d C.
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
52
organic material as well as to bacterial production (Hinkle, 1990). One would therefore
expect lighter carbon isotope values for the lower stations. The inverse is true, with the
lower stations having the heaviest values. The organic component can therefore not be
the main cause of the elevated CO2 levels.
Soil Gas Time Series
Radon and CO2 soil gas samples were plotted over time in order to investigate
fluctuations which may correlate with seismic or climatic variations.
Radon
Line E radon values are somewhat variable over time, with an overall increase
from March 7, 1995 to April 6, 1995 (e.g., E-2: 48 to 67 pCi/l). Except for E-5, the
values subsequently drop off (e.g., E-2: 58 pCi/l, Figure 1.11a). Line S radon values are
also quite variable, with only S-2 and S-4 showing a progressive increase in
concentration (e.g., S-4: 28 to 38 pCi/l) to a maximum on April 4 and subsequent decline
to 35 pCi/l on April 12, 1995 (Table 1.1, Figure 1.11b). Stations S-1 and S-3 do not
increase progressively, but rather fluctuate at fairly low levels. As with line E, stations
N-3 and N-4 radon values show a prominent peak at March 22 and a smaller peak on
April 6. Stations N-3 and N-4 also show the most variability of the northern line (Figure
1.11c). Radon concentrations at stations W-4 and W-5 show a marginal increase over
time.
However, concentrations for stations W-1 through W-4 are approaching the
reliable limits of detection (~ 10 pCi/l), and thus any fluctuations may be instrumental in
nature (Figure 1.11d).
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
53
FIGURE 1.11
Plot of radon and CO2 concentrations versus time for the E, S, N, and W lines. Solid
lines and symbols are radon values, dashed and clear symbols are CO2 concentrations.
Note the common peaks on March 22 and April 6, 1995.
Chapter I - Diffuse Degassing at Arenal Volcano
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54
There are clearly some similarities in the fluctuation with time of radon values
amongst the four lines of stations. Lines E and S show the April 6 peak, while the peak
on March 22 is less evident. The inverse is true for the N line, where the April 6 peak is
weak or missing, while the March 22 peak is clearer. The W line is the least useful line
due to the comparatively low values for all stations except W-5.
Carbon Dioxide
As with radon, line E CO2 values show a small increase to April 6 and
subsequent decline (Table 1.3, Figure 1. 11a). CO2 values for S-1 and S-3 vary little,
whereas station S-2 shows a marked increase in concentration (e.g., 4.5% to 8.9%) on
April 4 and subsequent decline (e.g., 8.2%) on April 12, 1995 (Figure 1. 11b). Line N
CO2 values show little variation, although there is a small but visible peak on April 6
(Figure 1. 11c). Stations N-1 and N-2 are less clear due to their low concentrations.
Line W CO2 values are comparatively low (W-1: 0.01% to W-4: 1.2%) except for the
most distal station W-5, which shows a steady increase from 7.6% on February 2 to
8.6% on April 12, 1995 (Figure 1. 11d).
As with the radon time series, there are some similarities in the fluctuation of
CO2 concentrations between the four lines of stations. Line E, N, and station S-2 show
the April 6 peak clearly while that on March 22 is less evident. Little can be said for the
W line as the majority of the stations show comparatively low concentrations. All four
lines show very little fluctuation in the CO2 concentrations at the higher stations, which
may in part be due to their low concentrations.
Chapter I - Diffuse Degassing at Arenal Volcano
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55
Seismicity
Due to technical difficulties with the seismometers maintained by OVSICORI,
seismic data is incomplete for the sampling period in question, with data available only
from March 23, 1995 to April 16, 1995 (Table 1.8). Arenal’s high level of activity can
be seen from the number of daily eruptions which range from 4 to 26, with an average of
13 (Figure 1.12a). The highest number of eruptions (26) was recorded on April 16,
while the fewest eruptions (4) occurred on April 13. The total hours of daily tremor
varied from 0 to 21 hours, with an average of 12 hrs (Figure 1.12b). The maximum daily
tremor (21 hrs) was recorded on April 9, while no tremor was recorded on April 13. The
largest eruption for this period (amplitude: 123 digital units) occurred on April 10, while
the smallest eruption (amplitude: 4 digital units) occurred on March 29. The maximum
eruption duration of 210 s was noted for an eruption on April 15, while a minimum
duration of 14 s was recorded for an eruption on April 10 (W. Melson, personal
communication, 1995).
Although both the daily number of eruptions and daily tremor vary greatly over
the period of measurement, there may be a possible correlation with soil gas fluctuations.
For April 6, 1995, there was a relatively small number of eruptions and relatively
elevated tremor (Figure 1.12). This may be significant, as both radon and CO2 soil gases
on line E peak on this date (Figure 1.11a).
The subsequent decline in soil gas
concentrations after April 6 could relate to the increase in the number of daily eruptions
from April 8 to April 11, 1995 (Figure 1.12a). As radon measurements are made on a
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
Table 1.8 Seismic data from March 23 to April 16, 1995 at Arenal
volcano.
Date
23/03/95
24/03/95
25/03/95
26/03/95
27/03/95
28/03/95
29/03/95
30/03/95
31/03/95
01/04/95
02/04/95
03/04/95
04/04/95
05/04/95
06/04/95
07/04/95
08/04/95
09/04/95
10/04/95
11/04/95
12/04/95
13/04/95
14/04/95
15/04/95
16/04/95
Number of Hours of Avg. Amplitude Avg. Duration
Eruptions Tremor (digital units)
(s)
5
5.00
52
38
7
19.00
39
34
8
12.00
30
30
18
13.75
32
34
16
10.50
23
35
15
8.50
30
33
22
9.50
25
31
10
13.00
32
30
17
9.00
18
35
13
14.50
21
30
9
15.50
21
24
12
17.50
28
26
20
9.50
37
32
13
14.50
51
34
9
14.50
41
30
10
20.50
33
34
7
16.50
36
33
15
21.00
39
32
18
3.25
41
34
23
11.00
34
34
18
2.50
20
30
4
0.00
10
23
8
15.00
10
37
11
13.00
40
61
26
13.00
27
54
Data from V. Barboza, Departamento de Sismologia, OVSICORI.
56
0
16/04/95
15/04/95
14/04/95
13/04/95
12/04/95
11/04/95
10/04/95
09/04/95
08/04/95
07/04/95
06/04/95
05/04/95
04/04/95
03/04/95
02/04/95
B
01/04/95
31/03/95
30/03/95
29/03/95
28/03/95
27/03/95
26/03/95
25/03/95
24/03/95
16/04/95
15/04/95
14/04/95
13/04/95
12/04/95
11/04/95
10/04/95
09/04/95
08/04/95
07/04/95
06/04/95
05/04/95
04/04/95
03/04/95
02/04/95
01/04/95
31/03/95
30/03/95
29/03/95
28/03/95
27/03/95
26/03/95
25/03/95
24/03/95
23/03/95
0
23/03/95
Hours of Tremor per day
Number of Eruptions
A
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
57
Eruptions per day
March 23 to April 16, 1995
30
25
20
15
10
5
Date
Hours of Tremor per day
March 23 to April 16, 1995
25
20
15
10
5
Date
Figure 1.12: a) Number of volcanic eruptions per day and b) total hours of tremor
per day between March 23 and April 16, 1995.
Chapter I - Diffuse Degassing at Arenal Volcano
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58
weekly basis, the average number of daily eruptions over the same period was taken in
order to facilitate comparisons. Thus for line E, which most clearly shows the April 6
peak, it becomes apparent that there is an increased number of eruptions for the period
before April 6 (Figure 1.13).
Atmospheric Variations
Over a two month period in 1995, atmospheric pressure measurements were
made twice daily at a base station in Fortuna (elev.: ~250 m). There was a gradual but
variable decrease in atmospheric pressure from 1018 mbar on February 22, 1995 to a low
of 1005 mbar on April 10, 1995, and subsequent increase to 1012 mbar on April 16,
1995 (Figure 1.14).
There should be only a minimal temperature effect, as there was very little
temperature variation during these two months and only limited variation in Costa Rica
in general. Due to Arenal’s distance from large populated centres, there are no rainfall
data available. However, the sampling period (March to April, 1995 and FebruaryMarch, 1996) was towards the end of the Costa Rican dry season.
Discussion
Origin of Radon and Carbon Dioxide
Radon is unlikely to originate from a magma chamber, since its half-life of 3.8
days is too short to allow for travel from even a shallow magma chamber or conduit.
Radon may escape diffusely through soil from a mean depth of only 2 m, making it
59
E-5
Number of
Eruptions
E-4
E-3
E-2
E-1
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
16/04/95
0
0
Date
02/03/95
10
20
30
40
50
60
70
07/03/95
12/03/95
17/03/95
22/03/95
27/03/95
01/04/95
Rn and Number of Eruptions vs. Time
Line E
06/04/95
11/04/95
2
4
6
8
10
12
14
16
18
20
Number of Eruptions
Rn (pCi/L)
Figure 1.13: Fluctuations in radon concentration (pCi/l) and number of
eruptions versus time. Note the increase in average number of eruptions
coinciding with a peak in Rn concentration (April 6, 1995). The histogram
represents the average number of eruptions between Rn measurements periods.
Chapter I - Diffuse Degassing at Arenal Volcano
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59
FIGURE 1.13
Fluctuations in radon concentration (pCi/l) and number of eruptions versus time. Note
the increase in average number of eruptions coinciding with a peak in Rn concentration
(April 6, 1995). The histogram represents the average number of eruptions between Rn
measurement periods.
Chapter I - Diffuse Degassing at Arenal Volcano
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60
difficult, if not impossible, to migrate from a conduit or chamber in such a short time
period (Graustein and Turekian, 1990; Appleby and Oldfield, 1992). Radon may also
travel significant distances by convection processes in hydrothermal systems or along
structures (Connor et al., 1996). However, there is little structure or hydrothermal
activity on Arenal, making convective transport of radon unlikely. Increased radon flux
may be found near structural weaknesses such as faults, since the distance over which
radon can travel may be enhanced by advective transport along a pressure gradient
(Tilsley, 1992). However, unlike larger and older edifices such as Poás and Galeras
(Charland et al., 1997; Heiligmann et al., 1997), there are few faults on Arenal that show
surface expression (Malavassi, 1979; Borgia et al., 1988). This is because they are
covered by recent lava flows, reducing the structural influence.
The E line may,
however, be affected by a fault structures on the southern and eastern flanks of the
volcano (Figure 1.1). It is possible that this fault, mapped by Malavassi (1979) and
Borgia et al. (1988), runs between Arenal and Cerro Chato. Such a structure, should it
exist, might explain the anomalous behaviour of line E with respect to the other lines.
Other than the NE line, E stations have the highest radon and CO2 and heaviest δ13C on
the volcano.
On the rest of the volcano, high radon values are typically associated with soil
development at lower elevations, as shown by the high radon emanating potentials of the
soil (Figure 1.4). For example, station S-1 has an RnERaC of 104 pCi/kg and an average
radon concentration of 8 pCi/l, while S-4 has an RnERaC of 180 pCi/kg and an average
radon value of 32 pCi/l (Tables 1.1, 1.2). Similar trends are observed on the N line
61
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
15/04/95
13/04/95
11/04/95
09/04/95
07/04/95
05/04/95
Dailly Atmospheric Pressure variations
03/04/95
01/04/95
30/03/95
27/03/95
23/03/95
20/03/95
14/03/95
12/03/95
08/03/95
06/03/95
04/03/95
02/03/95
28/02/95
26/02/95
24/02/95
1000
1005
1010
1015
22/02/95
1020
Date
25/03/95
Pressure (mbar)
Figure 1.14: Daily atmospheric pressure (mbar) fluctuations measured at the
Fortuna base station (~250 m). Note the relative minima on April 6, 1995.
Chapter I - Diffuse Degassing at Arenal Volcano
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62
where N-1 has an RnERaC of 74 pCi/kg and radon concentration of 6 pCi/l, whereas N-5
has an RnERaC of 205 pCi/kg and an average radon value of 36 pCi/l (Figure 1.4). Thus
the radon emanating potential of the soil may be partly responsible for the radon values
measured here.
However, the degree of soil development cannot explain the general correlation
between radon and carbon isotopes. Lines N, S, and W show a general tendency towards
increasing radon concentrations with heavier δ13C values (Figure 1.8b). The eastern line
is again anomalous, as is the northeastern line. These relations also are observed for a
plot of radon versus CO2, where lines E and NE are anomalous, while the remaining
lines show a general increase of CO2 concentrations with increasing radon (Figure 1.6a).
These observations suggest that variations in radon, CO2 and δ13C may be caused by a
similar mechanism.
Soil Gas, Soil Development, and Elevation
As stated above, levels of 222Rn, CO2, and δ13C generally appear to increase with
distance from the summit (Figures 1.3, 1.5, and 1.7). The apparent correlation with
altitude is likely due to the direct correlation between soil type with elevation. As has
been described previously, the upper flanks of the volcano consist mainly of
unconsolidated pyroclastic material while those on the lower flanks have more
developed clay-rich soil. Thus, although the highest station is only half-way up the flank
(855m) it nevertheless represents the soil type of the uppermost, inaccessible flanks
(>855 m) of the volcano.
Line W, which is situated in the devastated zone of the 1968 eruption, shows an
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
increase in
222
63
Rn and CO2 concentrations and δ13C with distance from the crater.
Stations W-1 through W-4 have extremely low
222
Rn and CO2 concentrations, which
may also be due to the fact that they are all situated in loose unconsolidated 1968
pyroclastic deposits (Figure 1.15). This may allow for the possibility of wind breathing
which occurs when wind increases the pressure on the ground, forcing air into the soil,
and diluting the soil gas. It is clearly the case for W-1 where CO2 concentrations are
essentially atmospheric. Only W-5, with a partially developed soil due to its proximity
to Lago Arenal, shows elevated levels of 222Rn and CO2.
Radon, carbon dioxide, and carbon isotopes for the southern line also appear to
follow this trend, in a general sense at least. There is, however, a polarity in the values,
with stations S-1 and S-3 having lower
222
Rn and CO2 concentrations than S-2 and S-4
(Table 1.4). This again may be explained by the degree of soil development, i.e., the
degree of permeability and porosity of the substrate.
Station S-1 is situated in
unconsolidated volcanic debris while S-3 is in a gravel-rich soil in close proximity to a
stream. S-2 and S-4, on the other hand, are situated in better-developed, organic and
clay-rich soils. As discussed above, this type of soil acts as a low permeability layer,
which allows for the concentration of radon and CO2 below the surface. The organic
material also may enhance CO2 output from bacterial and organic decay processes.
Similarly, the
222
Rn and CO2 concentrations and δ13C for the N line exhibit the
highest values in the more distal stations N-3 to N-5. N-4 is clearly anomalous, with
high CO2 (7.2%), radon (41 pCi/l), and heavy δ13C (-14.0‰), implying the possible
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
Figure 1.15: View from the western flank of Arenal towards Lago Arenal. Large
boulders and blocks are part of the 1968 pyroclastic deposit. The remains of the
once dense jungle (a few trees) are visible.
64
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
65
presence of a structural weakness such as fault. This would facilitate the transport of
more
226
Ra close to the surface and allow for higher radon concentrations. Similarly,
magmatic or deep CO2 could travel along the structure and concentrate near the surface.
A structural weakness below the surface may also explain the anomalous
behaviour of line E.
Eastern stations have some of the highest radon and CO2
concentrations and heaviest δ13C on the volcano. The eastern side of the volcano has
been affected only slightly by recent volcanic activity and is consequently densely
forested. The soil has therefore had more time to develop. This also may allow for the
greater flow of soil gases from depth with a more magmatic component.
It is unclear, however, why the NE line behaves anomalously for plots of δ13C vs.
Rn and RnERaC.
In both cases, the NE line shows a good negative correlation.
Although there is no obvious pattern or trend within the line, it should be noted that in
contrast to the four other lines, the NE line does not radiate down the flank of the
volcano. It is possible that, with the increased distance from the crater and subsequent
increase in fracturing, there may be significant local variations in levels of
fracturing/faulting which are not apparent on the surface.
Although stations high on the edifice have elevated CO2 fluxes, their CO2
concentrations are comparatively low (Figure 1.5). The inverse is true for stations on the
lower flanks. Again this also may be explained by pore closure in the soil due to
elevated humidity, as well as soil development and porosity/permeability of the
substrate. Poorly developed soils such as the unconsolidated pyroclastics of the western
flanks will be susceptible to wind breathing (D. Thomas, personal communication to M.
Heiligmann, 1996). Increased pressure on the ground, resulting from wind blowing
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
66
around the summit, may force air into the soil. This will create local circulation cells
and result in increased CO2 flux, while at the same time diluting the subsurface CO2. In
the unconsolidated pyroclastics of W-3, for example, there is elevated CO2 flux (0.21
mg/m2⋅min) but low CO2 concentration (0.69%). On the other hand, W-5 has a very low
flux (0.017 mg/m2⋅min) but comparatively high CO2 concentration (7.15%, Table 1.7,
Figure 1.10).
There is an apparent negative correlation for both CO2 and Rn concentrations
with elevation (r CO2: -0.66; Rn: -0.58, Figures 1.3a, 1.5a) . This may be explained by
the relationship between elevation and soil development, where the elevated stations
(except for the eastern line) are situated in unconsolidated volcanic debris while stations
lower on the volcano show better developed clay- and organic-bearing soils.
Allard et al. (1991) have suggested that Mt. Etna is covered by a dome of
magmatic CO2 from crater and diffuse flank degassing, especially concentrated near
faults and radial dykes. The heavy δ13C values (~-3.0‰ to ~0‰) along these structures
and at lower elevations are interpreted as due to the influence of marine carbonates
(Allard et al., 1991). However, the country rock surrounding Arenal is composed of
undivided Pliocene-Pleistocene volcanics with no surface exposure of carbonate rocks in
evidence (Borgia et al., 1988). Arenal δ13C values are highly negative near the summit,
with heavier values in the more distal areas of the volcano. CO2 and Rn concentrations
also are elevated in the more developed soils of the lower flanks. Only in these more
distal regions does there appear to be a significant magmatic component (e.g., NE-1:
δ13C = -10.8‰, Figure 1.7b). Thus, rather than a dome of magmatic CO2, it appears
more likely that the recent lavas of Arenal act as impermeable layers which plug
Chapter I - Diffuse Degassing at Arenal Volcano
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67
magmatic CO2.
Relationship to Pressure Change
As shown above, there is a gradual decrease in atmospheric pressure over time
(Figure 1.14). If this pressure variation is correlated with the variations of CO2 over
time, 68% of stations show a negative correlation (r > -0.5, Table 1.9). W-1, for
example, has a correlation coefficient of -0.95 with pressure, S-2 a -0.85 correlation
coefficient, and E-1 a -0.81 correlation. Line E also shows a good correlation between
CO2 and Rn versus time (r = 0.4 to 0.92), with most stations peaking on April 6, 1995.
Similar radon - pressure trends are visible on line E, with correlation coefficients (except
E-4) ranging from -0.74 to -0.53.
A possible explanation for these variations with pressure may be seen by again
looking at the degree of soil development. For example, as seen above, line S is
polarised with respect to
222
Rn and CO2 concentrations, with S-2 and S-4 stations
showing higher soil gas values than S-1 and S-3 (Table 1.4). A similar polarisation
becomes apparent when looking at pressure correlations. Stations S-2 and S-4, situated
in well developed soils, show high correlations for CO2 (S-2: -0.85, S-4: -0.82) and Rn
with pressure (S-2: -0.93, S-4: -0.81). S-1 and S-3, on the other hand, are situated in
loose unconsolidated volcanics and gravel-rich soil and have quite low correlations for
CO2 (S-1: -0.06) and Rn (S-1: 0.37, S-3: 0.38) with pressure (Table 1.9). Thus, as the
Chapter I - Diffuse Degassing at Arenal Volcano
___________________________________________
Table 1.9
Correlation coefficients for Rn vs. CO2 and Rn and CO2
vs. pressure at Arenal volcano
Station Rn vs CO2 Rn vs P CO2 vs P
E-1
0.85
-0.74
-0.81
E-2
0.79
-0.53
-0.49
E-3
0.55
-0.57
-0.42
E-4
0.92
0.07
-0.14
E-5
0.42
-0.56
-0.66
N-1
0.78
-0.88
-0.76
N-2
0.13
-0.14
-0.70
N-3
0.48
0.09
-0.54
N-4
0.36
0.25
-0.79
N-5
0.82
-0.39
-0.63
S-1
0.37
-0.06
S-2
0.82
-0.93
-0.85
S-3
0.65
0.38
-0.73
S-4
0.78
-0.81
-0.82
W-1
0.30
0.10
-0.95
W-2
0.44
0.81
0.37
W-3
-0.59
0.68
-0.36
W-4
-0.72
-0.30
-0.73
W-5
-0.29
-0.26
-0.43
68
Chapter I - Diffuse Degassing at Arenal Volcano
_____________________________________________
69
atmospheric pressure drops, the concentrations of radon and carbon dioxide in the better
developed soils of S-2 and S-4 increase.
This suggests that elevated atmospheric
pressure may enhance the sealing capacity of these already impermeable soils and allow
for increased concentrations of soil gas beneath the surface. When the pressure drops,
this sealing effect is reduced and more soil gases are allowed to escape (Schery and
Petschek, 1983). On the other hand, there is little if any correlation with atmospheric
pressure for stations in the unconsolidated volcanics/gravels (Table 1.9). This may be
due to the fact that the soil gas concentrations are so low as to be barely affected by any
variation in atmospheric pressure and/or by the poor sealing of the unconsolidated
materials.
Relationship to Seismicity
Due to only a partial coverage by seismic data, there is very little that can be said
with respect to correlation with soil gas fluctuations over time. There may be a partial
correlation with radon fluctuations. The peak in radon concentrations seen on April 6,
1995, for the eastern line appears to coincide with an average increase in the number of
eruptions prior to April 6. This is followed by a decrease in radon concentrations, as
well as a decrease in the average number of eruptions (Figure 1.13). However, due to
the limited seismic coverage, this correlation should not be taken as being representative.
Furthermore, as has been discussed above, the eastern line is anomalous with respect to
the other lines on the volcano, and thus this correlation may not be characteristic of the
other stations.
Chapter I - Diffuse Degassing at Arenal Volcano
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70
Conclusions
This study of CO2 and radon soil gas at Arenal volcano suggests that soil gases
may allow for a better understanding of volcanic behaviour.
Some important
conclusions from this work are the following:
1. Correlations between soil gas concentrations and seismic data are difficult, due in part
to limited seismic coverage, but more importantly due to the high level of activity at
Arenal.
2. Variations with time for radon and CO2 soil gases are due in large part to changes in
atmospheric pressure over time.
3. Rn and CO2 soil gases from the upper flanks of Arenal are unlikely to originate from
deep magmatic gases, but rather emanate from shallow surface sources, as the radon
half-life is too short and transport process too slow.
4. The diffuse soil gases are generally unable to penetrate the young lavas which cover
and seal the upper flanks of Arenal. Only on the lower flanks, where young lavas do not
crop out, is there significant gas flow from depth. This is shown by the increased CO2
concentrations and heavier δ13C values at distance from the crater.
5. The degree of soil development and the porosity/permeability of the substrate also
strongly influences, the concentrations of CO2 and radon soil gas at Arenal.
Unconsolidated volcanic soils on the upper flanks of the volcano have relatively low
RnERaC and subsequently low radon values. These soils are also more apt to rapidly
dissipate any precipitation, thus limiting sealing effects. This is in contrast to the more
clay- and organic-rich soils of the lower flanks, which retain humidity and increase
sealing. This results in a lower permeability in the better developed soils and permits the
Chapter I - Diffuse Degassing at Arenal Volcano
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71
accumulation of soil gas below the surface.
6. Arenal acts as a volcanic plug, sealing shallow levels of the continental crust and
limiting deep gas flux to faults and the fractured lower flanks of the volcano.
Chapter I - Diffuse Degassing at Arenal Volcano
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72
References
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diffuse emissions of CO2 from Mount Etna. Nature, 351: 387-391.
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Carbonnelle, J., Dajlevic, D., Le Bronec, J., More, P., Obert, J.-C., and Zettwoog,
P., 1985. Etna: composantes sommitales et pariétales des émissions de gaz
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PIRPSEV-CNRS
Cerling, T. E., Solomon, D. K., Quade, J., and Bowman, J. R., 1991. On the isotopic
composition of carbon in soil carbon dioxide. Geochim. Cosmochim. Acta, 55:
3403-3405.
Charland, A., Stix, J., Barquero, J., Fernández, E., Williams-Jones, G., Barboza,
V., and Sherwood Lollar, B., 1997. Controls on diffuse degassing of radon and
CO2 at Poás volcano, Costa Rica. Bull. Volcanol., in review.
Connor, C., Hill, B., LaFemina, P., Navarro, M., and Conway, M., 1996. Soil 222Rn
during the initial phase of the June-August 1995 eruption of Cerro Negro,
Nicaragua. Journal of Volcanology and Geothermal Research, 73: 119-127.
Graustein, W. C. and Turekian, K., 1990. Radon fluxes from soils to the atmosphere
measured by 210Pb-226Ra disequilibrium in soils. J. Geophys. Res., 17: 841-844.
Heiligmann, M., 1997. Soil gases at Galeras volcano, Colombia, and their utility in
eruption prediction. M.Sc. Thesis, Département de Géologie, Université de
Montréal, Montréal, Canada, 114 pp.
Chapter I - Diffuse Degassing at Arenal Volcano
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73
Heiligmann, M., Stix, J., Williams-Jones, G., Sherwood Lollar, B., and Garzón V.,
G., 1997. Distal degassing of radon and carbon dioxide on Galeras volcano,
Colombia. J. Volcanol. Geotherm. Res., 77: 267-284.
Hinkle, M. E., 1990. Factors affecting concentrations of helium and carbon dioxide in
soil gases. In: E. M. Durance (Editor), Geochemistry of Gaseous Elements and
Compounds. Theophrastus Publications SA, Athens, pp. 421-447.
Irwin, P. W. and Barnes, I., 1980. Tectonic relations of carbon dioxide discharge and
earthquakes. J. Geophys. Res., 85: 3115-3121.
Kotrappa, P., Dempsey, J. C., Hickey, J. R., and Stieff, L. R., 1988. An electret
passive environmental 222Rn monitor based on ionization measurement. Health
Phys., 54: 47-56.
Kotrappa, P. and Stieff, L. R., 1992. Elevation correction factors for E-PERM radon
monitors. Health Phys., 62: 82-86.
Kotrappa, P., Dempsey, J. C., Rasey, R. W., and Stieff, L. R., 1990. A practical EPerm (electret passive environmental monitor) system for indoor radon
measurement. Health Phys., 58: 461-467.
Malavassi, E., 1979. Geology and petrology of Arenal Volcano, Costa Rica. M.Sc.
Thesis, Department of Geology and Geophysics, University of Hawaii at Manoa,
U.S.A, 111 pp.
Moore, T. R. and Roulet, N. T., 1991. A comparison of dynamic and static chambers
for methane emission measurements from subarctic fens. Atmosphere-Oceans,
29: 102-109.
Ozima, M. and Podosek, F. A., 1983. Noble Gas Geochemistry. Cambridge University
Press, Cambridge, Australia, 367 pp.
Rad Elec Inc., 1993. E-PERMR system manual. Rad Elec Inc., Virginia
Schery, S. D. and Petschek, A. G., 1983. Exhalation of radon and thoron: the question
of the effect of thermal gradients in soil. Earth Planet. Sci. Lett., 64: 56-60.
Thomas, D. M., Cuff, K. E., and Cox, M. E., 1986. The association between ground
gas radon variations and geologic activity in Hawaii. J. Geophys. Res., 91:
12186-12198.
Tilsley, J. E., 1992. Radon: Sources, hazards and control. Geosci. Can., 19: 163-167.
CHAPTER II
A MODEL OF DIFFUSE DEGASSING AT THREE SUBDUCTION-RELATED
VOLCANOES
GLYN WILLIAMS-JONES
AND
JOHN STIX
Département de géologie,
Université de Montréal,
Montréal, Québec,
Canada
Chapter II - Diffuse Degassing on Arenal, Poás, & Galeras
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75
Abstract
Radon, CO2, and δ13C in soil gas have been measured at three active subductionrelated stratovolcanoes (Arenal and Poás, Costa Rica; Galeras, Colombia). Rn values
reach maxima only near fault zones, areas of seismic activity, and on the lower flanks of
the volcanoes. The heaviest δ13C values were found near fumaroles in the active craters,
close to faults, and on the lower slopes of the volcanoes. These observations suggest
that (1) major faults can channel deep gases to the surface only if the faults have surface
expression; (2) diffuse degassing of deep, magmatic gas on the upper flanks of the
volcanoes is negligible due to low permeability from the cover of young volcanic rocks;
and (3) increased magmatic degassing on the lower flanks is the result of greater
fracturing in the older lavas. These results are in contrast to findings for Mount Etna
where a broad dome of magmatic CO2 has been recognised over much of the edifice
(Allard et al., 1991).
Volcanoes, such as those studied here, act as plugs in the
continental crust, limiting degassing to fumaroles, faults, and the fractured lower flanks.
Chapter II - Diffuse Degassing on Arenal, Poás, & Galeras
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76
Introduction
A
RENAL (1,657 m) is a ~5 km-diameter stratovolcano situated in Costa Rica
(10.463°N, 84.703°W) 90 km northwest of the capital San José and has been in
continuous activity since 1968, with frequent lava flows and strombolian-vulcanian
eruptions (VEI = 3, Figure 2.1). Poás (2708 m) is a stratovolcano ~14 km in diameter
located approximately 35 km northwest of San José (10.20°N, 84.233°W) and is
bordered by the Rio Desague and the Rio Toro faults (Figure 2.2). The central crater,
Laguna Caliente, has been active since the late 19th century. Galeras (4200 m) is a ~25
km-diameter stratovolcano in southern Colombia (1.22°N, 77.37°W). The volcanic
complex is intersected by the regional Romeral-Buesaca fault system which trends
northeast-southwest (Figure 2.3). Its most recent activity has been marked by explosive
eruptions in May 1989, lava dome emplacement in late 1991, and six vulcanian
eruptions in 1992-1993 (Stix et al., 1993).
Methodology
Diffuse degassing of Rn and CO2 was studied at 25 representative stations on
Arenal, 16 stations on Poás, and 30 stations on Galeras, between 1994 and 1996 (Table
2.1) (Heiligmann et al., 1997; Charland et al., 1997). Radon soil gas was sampled using
the E-PERM technique (Kotrappa et al., 1988), CO2 in soil gas was measured using an
ADC LFG-20 Landfill infrared gas analyser, and carbon stable isotopes were measured
in CO2, collected with 25 ml vials, by gas chromatograph combustion - isotope ratio
mass spectrometry (GC-C-IRMS). Rn data had average reproducibility of ~6%, CO2
Chapter II - Diffuse Degassing on Arenal, Poás, & Galeras
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77
FIGURE 2.1
Topographic map of Arenal volcano showing average (a) radon concentrations in pCi/l,
(b) CO2 concentrations in volume %, and (c) δ13C values expressed as ‰. Contour
interval of 100 m.
Chapter II - Diffuse Degassing on Arenal, Poás, & Galeras
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78
FIGURE 2.2
Topographic map of Poás volcano showing average (a) radon concentrations in pCi/l,
(b) CO2 concentrations in volume %, and (c) δ13C values expressed as ‰. Contour
interval of 500 m.
Chapter II - Diffuse Degassing on Arenal, Poás, & Galeras
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79
FIGURE 2.3
Topographic map of Galeras volcano showing average (a) radon concentrations in pCi/l,
CO2 concentrations in volume %, and (c) δ13C values expressed as ‰. Contour interval
of 400 m.
Chapter II - Diffuse Degassing on Arenal, Poás, & Galeras
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80
Chapter II - Diffuse Degassing on Arenal, Poás, & Galeras
_____________________________________________________
81
measurements had instrumental precisions of between 0.5% and 3% depending on
concentration, and δ13C reproducibility was better than 0.1‰.
Results
Radon values differed substantially among the three volcanoes, from 3 to 60
pCi/l for Arenal (Figure 2.1a), 7 to 1150 pCi/l for Poás (Figure 2.2a), and 10 to 1380
pCi/l for Galeras (Figure 2.3a). The values vary with the elevation and structure of the
volcanoes. At Arenal, radon concentrations increase towards the lower flanks (up to 60
pCi/l; no data is available for the crater area). Poás displays low values (7 to 130 pCi/l)
in the vicinity of the active crater and higher values on its flanks (360 pCi/l) and near
faults (60 to 1150 pCi/l). At Galeras, values also are high near faults and zones where
swarms of high-frequency earthquakes were recorded in 1995 and 1996 (520 to 1380
pCi/l). Fairly high radon concentrations also were observed near fumaroles on the outer
flanks of the active cone within the caldera (~330 pCi/l).
CO2 concentrations vary from 0.04 to 10.2% at Arenal (Figure 2.1b), <0.1 to
16% at Poás (Figure 2.2b), and 0.0 to 12.6% at Galeras (Figure 2.3b). On Arenal, the
concentrations are low on the upper flanks (0.04 to 2.8%) and higher on the lower flanks
(1.1 to 9.6%). At Poás, low CO2 values are found in the summit area (0.01 to 3.4%) and
higher values are found along faults (3.2 to 16%). This contrasts with Galeras where
CO2 concentrations are more variable and commonly higher on the volcano (up to
12.6%) than near faults (1.2 to 3.2%).
δ13C values range from -10.8 to -30.4‰ at Arenal (Figure 2.1c), -6.2 to -26.0‰
at Poás (Figure 2.2c), and -8.5 to -23.2‰ at Galeras (Figure 2.3c), respectively. On
Chapter II - Diffuse Degassing on Arenal, Poás, & Galeras
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82
Arenal, δ13C values are generally heaviest on the lower flanks (-10.8 to -25.7‰) and
lighter at higher elevations (-25.5 to -30.4‰). As with CO2 concentrations, δ13C values
at Poás are generally lower in the summit area (-19.5 to -24.7‰) and higher along faults
and on the lower flanks. The heaviest δ13C values are found near the Rio Toro fault zone
(-12.7 to -18.4‰) and at the Dome 2 station (-6.2‰) which is situated near fumaroles in
the active crater (Table 2.1) (Charland et al., 1997). The other summit stations at Poás
have δ13C values lighter than -19.5‰, suggesting that the magmatic component is absent
or insignificant. At Galeras δ13C in CO2 heavier than -15‰ is recorded only near active
crater fumaroles (e.g., Chavas: -7.9‰), faults (e.g., SHC: -15.1‰), areas of seismicity
(e.g., Sismo5: -8.5‰) (Heiligmann et al., 1997). All other areas on Galeras have no
significant deep CO2 component.
Discussion
Radon values vary by up to two orders of magnitude from the crater/summit area
to the active faults on Galeras and Poás. Although high radon concentrations on these
volcanoes are associated with faults and areas of seismic activity, the slow transport
velocities and short half-life of radon suggest that deep radon probably does not reach
the surface directly.
Rather, increased concentrations are likely due to (1) faster
advective transport of radon produced near the surface, (2) greater availability of radon
at shallow levels due to superficial fracturing (Thomas et al., 1986), and (3) gases and
aqueous solutions rising through faults and depositing minerals rich in potassium and Rn
precursors near the surface (Nishimura and Katsura, 1990).
It has been suggested that Mt. Etna is covered by a dome of magmatic CO2 from
Chapter II - Diffuse Degassing on Arenal, Poás, & Galeras
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83
crater and diffuse flank degassing, especially concentrated near faults and radial dykes
(Allard et al., 1991).
The heavy δ13C values along these structures and at lower
elevations are interpreted as due to the influence of marine carbonates superimposed on
magmatic δ13C values (Allard et al., 1991). However, our observations made on Arenal,
Poás and Galeras, indicate that there is no significant magmatic CO2 being diffusely
released on the flanks of these volcanoes. Instead, deep CO2 is released from the active
craters, in areas of seismic activity, and along fault zones that intersect the edifices and
have surface expression, i.e., are not covered by recent volcanic rocks. Recent work on
Oldoinyo Lengai (Brantley and Koepenick, 1995), however, shows that ~75% of the CO2
flux comes from 7 crater vents, with less than 2% of the total flux from the flanks
(Koepenick et al., 1996).
Using SO2 fluxes measured by COSPEC and appropriate CO2/SO2 ratios
(Galeras SO2 0.0042 x 1012 mol/yr (Zapata et al., 1997), molar CO2/SO2 2.9 (Fischer et
al., 1997); Poás SO2 0.00034 x 1012 mol/yr (Rowe et al., 1995), molar CO2/SO2 2.2
(Williams et al., 1992); Arenal SO2 0.000729 x 1012 mol/yr, molar CO2/SO2 4.2
(Williams et al., 1992)), we calculate crater CO2 fluxes of 0.0084 x 1012, 0.00075 x 1012,
and 0.0031 x 1012 mol/yr for Galeras, Poás, and Arenal, respectively. These values are
2-3 orders of magnitude smaller than at Mt. Etna (Allard et al., 1991; Brantley and
Koepenick, 1995).
Why does Arenal have significantly lower Rn values than Galeras and Poás? We
propose three possible explanations. (1) Arenal is less than 3000 years old (Borgia et al.,
1988), while Poás is ~1 Ma (Prosser and Carr, 1987) and Galeras is >1.1 Ma (Calvache
V. et al., 1997). Owing to its youth, Arenal is less fractured and faulted than Poás and
Chapter II - Diffuse Degassing on Arenal, Poás, & Galeras
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84
Galeras, resulting in less surface area for radon production at Arenal. (2) The older
fractured edifices of Galeras and Poás also tend to have more mature hydrothermal
systems (Fischer et al., 1997; Rowe et al., 1995) which may increase leaching of the
rock, thereby flushing radon into a carrier gas such as CO2 and promoting its transport to
the surface (Yoshikawa et al., 1990). Aqueous fluids also may help in transporting
radon-parent elements such as Ra and U. Significantly, soil gas near Chavas fumarole
on Galeras shows elevated levels of both Rn and magmatic CO2 (~330 pCi/l Rn, 16.2%
CO2, δ13C -7.9‰). (3) Large regional structures also may have an effect by controlling
surficial radon distributions. Central America is divided into seven tectonic segments
mirrored by different styles of volcanism (Stoiber and Carr, 1973). Arenal is situated
~80 km from the nearest segment break, while Poás lies ~40 km northwest of a segment
break running through the Irazu-Turrialba volcanic complexes. Poás also is associated
with major structures such as those manifested by its north-south alignment of cones and
the Rio Torro and Rio Sarapiqui faults. Galeras lies on a major tectonic break, the
Guairapungo Fracture, which runs northwest-southeast through the northern Andes, and
is intersected by the north-south trending Interandean Valley (Hall and Wood, 1985).
These major structures may thus facilitate the transport and remobilisation of the
parental isotope radium from wallrock and hydrothermal sources and result in elevated
Rn concentrations.
Chapter II - Diffuse Degassing on Arenal, Poás, & Galeras
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85
Conclusions
The three mechanisms discussed above all may affect flank degassing to variable
degrees. The radon and δ13C evidence presented here suggest that diffuse gases are
unable to penetrate young lavas on the flanks of these volcanoes. We propose that
volcanoes such as Arenal, Poás, and Galeras act as volcanic plugs which seal shallow
levels of the continental crust, limiting deep gas flux to fumaroles, faults, and fractured
lower flanks of the volcanoes.
Paradoxically, volcanoes which are responsible for
significant output of magmatic gas through the central conduit also act as barriers to gas
flow on their upper flanks. We envisage a concentric zoning of gas flow: an inner zone
in the active crater where strong degassing occurs, an intermediate zone on the upper
flanks where gas flow is impeded, and an outer, fractured zone where deep gas can again
reach the surface.
Chapter II - Diffuse Degassing on Arenal, Poás, & Galeras
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86
References
Allard, P., Carbonnelle, J., Dajlevic, D., Le Bronec, J., Morel, P., Robe, M. C.,
Maurenas, J. M., Faivre-Pierret, R., and Martin, D., 1991. Eruptive and
diffuse emissions of CO2 from Mount Etna. Nature, 351: 387-391.
Borgia, A., Poore, C., Carr, M. J., Melson, W. G., and Alvarado G.E., 1988.
Structural, stratigraphic, and petrographic aspects of the Arenal-Chato volcanic
system, Costa Rica: Evolution of a young stratovolcanic province. Bull.
Volcanol., 50: 86-105.
Brantley, S. L. and Koepenick, K. W., 1995. Measured carbon dioxide emissions from
Oldoinyo Lengai and the skewed distribution of passive volcanic fluxes.
Geology, 23: 933-936.
Calvache V., M. L., Cortés J., G. P., and Williams, S. N., 1997. Stratigraphy and
chronology of Galeras Volcanic Complex, Colombia. J. Volcanol. Geotherm.
Res., 77: 5-20.
Charland, A., Stix, J., Barquero, J., Fernández, E., Williams-Jones, G., Barboza,
V., and Sherwood Lollar, B., 1997. Controls on diffuse degassing of radon and
CO2 at Poás volcano, Costa Rica. Bull. Volcanol., in review.
Fischer, T. P., Sturchio, N. C., Stix, J., Arehart, G. B., Counce, D., and Williams, S.
N., 1997. The chemical and isotopic composition of fumarolic gases and spring
discharges from Galeras Volcano, Colombia. J. Volcanol. Geotherm. Res., 77:
229-254.
Hall, M. L. and Wood, C. A., 1985. Volcano-tectonic segmentation of the northern
Andes. Geology, 13: 203-207.
Heiligmann, M., Stix, J., Williams-Jones, G., Sherwood Lollar, B., and Garzón V.,
G., 1997. Distal degassing of radon and carbon dioxide on Galeras volcano,
Colombia. J. Volcanol. Geotherm. Res., 77: 267-284.
Koepenick, K. W., Brantley, S. L., Thompson, J. M., Rowe, G. L., Nyblade, A. A.,
and Moshy, C., 1996. Volatile emissions from the crater and flank of Oldoinyo
Lengai volcano, Tanzania. J. Geophys. Res., 101: 13819-13830.
Kotrappa, P., Dempsey, J. C., Hickey, J. R., and Stieff, L. R., 1988. An electret
passive environmental 222Rn monitor based on ionization measurement. Health
Phys., 54: 47-56.
Nishimura, S. and Katsura, I., 1990. Radon in soil gas: Applications in exploration
and earthquake prediction. In: E. M. Durance (Editor), Geochemistry of Gaseous
Elements and Compounds. Theophrastus Publications SA, Athens, pp. 497-533.
Chapter II - Diffuse Degassing on Arenal, Poás, & Galeras
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87
Prosser, J. T. and Carr, M. J., 1987. Poás volcano, Costa Rica: Geology of the summit
region and spatial and temporal variations among the most recent lavas. J.
Volcanol. Geotherm. Res., 33: 131-146.
Rowe Jr., G. L., Brantley, S. L., Fernández, J. F., and Borgia, A., 1995. The
chemical and hydrologic structure of Poás Volcano, Costa Rica. J. Volcanol.
Geotherm. Res., 64: 233-267.
Stix, J., Zapata G., J. A., Calvache V., M., Cortes J., G. P., Gómez M., D., Narvaez
M., L., Ordoñez V., M., Ortega E., A., Torres C., R., and Williams, S. N.,
1993. A model of degassing at Galeras Volcano, Colombia, 1988-1993. Geology,
21: 963-967.
Stoiber, R. E. and Carr, M. J., 1973. Quaternary volcanic and tectonic segmentation of
Central America. Bull. Volcanol., 37: 304-325.
Thomas, D. M., Cuff, K. E., and Cox, M. E., 1986. The association between ground
gas radon variations and geologic activity in Hawaii. J. Geophys. Res., 91:
12186-12198.
Williams, S. N., Schaefer, S. J., Calvache V., M. L., and Lopez, D., 1992. Global
carbon dioxide emission to the atmosphere by volcanoes. Geochim. Cosmochim.
Acta, 56: 1765-1770.
Yoshikawa, H., Endo, K., and Nakahara, H., 1990. 220Rn and 222Rn in volcanic gas.
In: E. M. Durance (Eds.), Geochemistry of Gaseous Elements and Compounds.
Theophrastus Publications SA, Athens, pp. 149-161.
Zapata, J. A., Calvache V., M. L., Cortés J., G. P., Fischer, T. P., Garzon V., G.,
Gómez M., D., Narvaez M., L., Ordoñez V., M., Ortega E., A., Stix, J.,
Torres C., R., and Williams, S. N., 1997. SO2 fluxes from Galeras Volcano,
Colombia, 1989-1995: Progressive degassing and conduit obstruction of a
Decade Volcano. J. Volcanol. Geotherm. Res., 77: 195-208.
CHAPTER III
A MODEL OF DEGASSING AND SEISMICITY AT ARENAL VOLCANO,
COSTA RICA
_________________________
GLYN WILLIAMS-JONES
AND
JOHN STIX
Département de géologie,
Université de Montréal,
Montréal, Québec,
Canada
Chapter III - SO2 and Seismicity: A Degassing Model
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89
Abstract
Arenal volcano is the most active volcano in Costa Rica and has emitted a
minimum of 1.3 Mt of SO2 and an estimated 4.5 x 108 m3 of lava since its lethal
reactivation in July 1968. Gas emissions from the volcano have been both by passive
degassing and explosive eruptions, with passive degassing being dominant. Based on
COSPEC measurements made during 1982, 1995, and 1996, the average daily output is
128 ± 62 t/d SO2 emitted from Arenal. Arenal is extremely active and shows repeated
cycles of decreasing SO2 flux and tremor prior to eruptions. Following eruptions, SO2
flux and tremor levels increase. These fluctuations show a distinct correlation with
Earth tides, with decreased explosive activity and increased tremor coinciding with the
peak of high tide. The cyclic nature of explosive activity also may be caused by
corresponding fluctuations in the extrusion rate of lava. At high extrusion rates, the lava
of a non-explosive vent may overflow into an explosive vent, temporarily blocking the
conduit. Arenal is likely tapping a deep to mid crustal magma chamber and, unlike
many volcanoes, there is little difference between petrological (0.61 Mt since 1968) and
COSPEC SO2 estimates (1.3 Mt), suggesting that Arenal is being continuously supplied
by fresh magma. However, the open system is periodically blocked near the surface due
to crystallisation of magma in the conduit and/or minor variations in extrusion rate. This
results in the development of a seal, leading to the overpressurisation of the conduit and
eventual explosive destruction of the seal.
Chapter III - SO2 and Seismicity: A Degassing Model
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90
Introduction
A
RENAL is a 1,657 m high conical stratovolcano situated in northern Costa Rica
(10.463°N, 84.703°W), 90 km northwest of the capital San José (Figure 3.1). It
has been in continuous activity since July 29, 1968, when it reactivated with a plinian
explosive phase. Aa to blocky lava flows of basaltic andesite have been extruded nearly
continuously since September 19, 1968 (Cigolini et al., 1984). Arenal progressed into a
second major eruptive phase in June 1975, which included numerous Merapi- and
Soufrière-type nuées ardentes. The volcano entered an intense strombolian phase in
1984, with increased eruptions of ash, lapilli, and blocks which continue to the present
day. The frequency of eruptions was observed to be approximately 30 minutes in 1984
(Van der Laat and Carr, 1989), while a similar eruptive frequency was noted by us in
1995 and 1996. Activity at Arenal has been accompanied by gas emissions since 1968.
Current activity includes continued lava extrusion and numerous ash emissions, some
ascending to over 1 km above the active crater C (Figure 3.2), with small infrequent
pyroclastic flows travelling down the northwest flanks (Fernández et al., 1996a). Bombs
and blocks have been ejected ballistically to 1,100 m elevation (Fernández et al., 1996b).
Field observations indicate that crater C is likely divided into a least 2-3 vents from
which lava, gas and ash are emitted separately. The summit (crater C) continues to grow
at a rate of ~5 m/yr (Fernández et al., 1996a).
Arenal is the smallest but most active of seven historically active Costa Rican
volcanoes. The volcano, which has a volume of only 15 km3 (Carr, 1984), is situated
between two massifs, the Cordillera de Guanacaste (SE) and the Cordillera Central
Chapter III - SO2 and Seismicity: A Degassing Model
_________________________________________________
FIGURE 3.1
Geographic map of Costa Rica showing the location of Arenal volcano.
91
Chapter III - SO2 and Seismicity: A Degassing Model
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92
FIGURE 3.2
Topographic map of Arenal volcano showing the locations of seismometer stations (red
triangle) and inclinometer stations (green house). Craters A and B are now buried by
lava flows emplaced since 1968. Contours are every 100 m.
Chapter III - SO2 and Seismicity: A Degassing Model
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93
(NW) which form the volcanic chain of the Costa Rican Arc (Stoiber and Carr, 1973). A
small truncated and likely extinct volcano, Cerro Chato, lies approximately three
kilometres south of Arenal (Figure 3.2). Arenal is most likely tapping a lower to midcrustal magma chamber possibly located at a discontinuity 22 km below the surface
(Matumoto et al., 1977; Wadge, 1983; Reagan et al., 1987).
Three stages of differing magma compositions at Arenal are believed to coincide
with variations in eruptive activity (Reagan et al., 1987). Stage-1 zoned magmas likely
resided in the magma chamber prior to the 1968 eruption. A new magma intruded into
the chamber in July 1968, resulting in the plinian eruption and ejection of the stage-1
magma. It subsequently mixed with the more mafic parts of stage 1 to produce stage-2
magmas. Stage-3 magmas (mid-1974 to present) are the product of continued mixing
and fractional crystallisation along the walls of the conduit and chamber. Each change in
stage appears to correlate with a variation in the cumulative volume of extruded material
(Reagan et al., 1987).
The extended duration and high level of activity of Arenal are enigmatic and
require further study. The periodic explosions and fluctuations in tremor and SO2 flux
raise numerous questions as to the eruptive mechanisms which control the volcano.
Arenal’s near-continuous extrusion of lava, in conjunction with explosive activity, also
raises the question of how magma travels to the surface. Due to the high level of activity
at the volcano and the inaccessibility of the crater area, we used remote sensing
techniques to study volcanic gases at Arenal.
Ultraviolet correlation spectrometry
(COSPEC) has been used to study volcanoes since the early 1970’s and is ideal for the
measurement of SO2 at active volcanoes such as Arenal. In this article, we present
Chapter III - SO2 and Seismicity: A Degassing Model
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94
results from two field seasons on Arenal, in which 151 SO2 flux measurements were
made, currently the largest data set for this volcano. These data are compared with
seismological measurements which we made in 1996 in order to develop a degassing
model that can explain the cyclic variations in seismicity and SO2 flux seen at Arenal.
The problems of SO2 flux measurements also are discussed. COSPEC and petrological
estimates of the total SO2 emitted since 1968, as well as annual rates of CO2 and SO2
flux, are used to show the impact that young volcanoes such as Arenal have on the
troposphere.
Methodology
SO2 Flux
The concentration of SO2 in the volcanic plume was measured using a Plume
Tracker (1995) and a COSPEC (1996). Solar ultraviolet radiation passes through a
Cassegrain telescope, connected to the instrument, and is focused onto a diffraction
grating which separates the individual wavelengths (Figure 3.3). This radiation then
passes through a correlator disc that isolates the UV radiation values into wavelengths
where there is positive and negative absorption by SO2. This UV radiation is then
measured by a photomultiplier which converts the amount of radiation into voltage. The
ratio of these two sets of radiation (positively and negatively absorbed) is known when
there is no SO2 present and compared to the drop in voltage when SO2 is present. The
difference in the ratio is proportional to the amount of SO2 in the field of view of the
instrument. Calibrations are made by placing gas cells with known concentrations of
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FIGURE 3.3
Plume Tracker and COSPEC ultraviolet spectrometers. The right-angle light tube of the
Plume Tracker is visible in the top picture, while the bottom picture shows the control
panel for the COSPEC IV. Black and red cables connect to an analogue chart recorder
and portable computer.
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SO2 into the optical path of the instrument (Stoiber et al., 1983).
The instruments were connected to a portable computer and chart recorder and
transported beneath the volcanic plume, during which time the SO2 signal was digitally
recorded every 1-2 s (Figure 3.3). The instruments were driven below the column at an
approximately constant speed (e.g., 20 kph) and approximately perpendicular to the
column. A gas-cell calibration was made before and after each traverse. Comparatively
elevated SO2 concentrations in the plume necessitated the use of high-concentration
calibration cells (300 ppm⋅m and 339.2 ppm⋅m for Plume Tracker and COSPEC,
respectively, Table 3.1).
The digital data were then processed and graphed using
commercial spreadsheet software. From the graph, the beginning and end of the plume
transect may be deduced, and consequently the flux may be calculated. A chart recorder
was used to gather analogue data as a backup and in instances where digital data were
lacking (e.g., during battery changes of the computer). Windspeed measurements were
typically made in the morning prior to the start of SO2 flux measurements (~1000 hrs
local time), at midday, and in the afternoon at the end of flux measurements (~1600 hrs).
There is a relatively steady easterly trade wind that blows over Arenal and westward out
over Lago Arenal. Thus, windspeeds were measured on the western flank of the volcano
at an elevation of ~550 m using a handheld anemometer. An individual traverse below
the gas plume was divided into segments in order to correct for the deviation from
perpendicularity of the traverse with respect to the column. The SO2 flux for each
segment was calculated and summed to determine the total SO2 flux for a given traverse.
The SO2 flux in metric tonnes per day (F) was calculated using the following equation:
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F = (cosθ)·(dcol)·(νwind)·(0.00023)·([SO2]col)
98
(3.1)
where θ (º) is the deviation from perpendicularity of the segment of road with respect to
the gas column; dcol is the width (m) of a particular segment determined using the vehicle
speed, time and distance travelled in the column; νwind is the average windspeed (ms-1)
measured at ground level 2-3 times per day; 0.00023 is a factor to convert ppm⋅m3⋅s-1
into metric tonnes per day; and [SO2]col is the path-length concentration of SO2 (ppm·m)
in the column. The concentration of SO2 was calculated from:
[ SO ]
2 col
 Pavg 
 ⋅ ( Ccal )
=
 Pcal 
(3.2)
where Pcal is the peak height of the calibration gas cell in arbitrary units; Pavg is the
average peak height for the segment; and Ccal is the concentration of the calibration gas
cell in ppm⋅m (Table 3.1).
SO2 Flux Errors
Variations in measured SO2 fluxes may be due, in part, to fluctuations in
windspeed and direction, changes in cloud cover, and change in sun angle, resulting in
variable amounts of solar ultraviolet radiation. The opacity of an eruptive plume also
varies due to changes in ash content which will increase the absorption of ultraviolet
radiation. Instrumental uncertainties include instrument calibration (± 2%), digital chart
reading error (± 2%), varying car speed (± 5%), and windspeed measurement (± 0-60%)
(Table 3.2, Casadevall et al., 1981; Stoiber et al., 1983).
The error in windspeed measurement is due to the fact that measurements were
made at the base of Arenal (elev.: ~550 m) and thus do not represent windspeeds at the
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summit of the volcano where degassing is taking place. As there is approximately 1-1.5
km difference in altitude between the ground and the plume, the plume velocity may be
up to 1.5 to 3 times greater than that measured on the ground (Willett and Sanders,
1959). Windspeed measurements also were affected by instrumentation errors. Wind
measurements made in 1995 have an average standard deviation of ~30%, as the digital
anemometer that was used gave readings that varied continually with local wind gusts
(Table 3.3). The operator was thus forced to estimate the average range of speeds at the
time of measurement.
The 1996 wind measurements used a fully mechanical
anemometer that allowed for the integration of windspeed over an interval of one
minute, thus eliminating the need for estimates on the part of the operator.
Consequently, the standard deviation was ~14%, or about half that of the previous year
(Table 3.3). Total average errors of ~31% and ~15% for the SO2 flux were calculated
for 1995 and 1996, respectively (Table 3.2). The errors are similar to those calculated by
Stoiber et al. (1983).
Generally, 10 to 15 traverses were made per day, with individual traverses lasting
about 20 minutes. Plume Tracker/COSPEC measurements were typically made to the
west and southwest of the volcano, at a distance of between 4 and 4.5 km from the
crater. Plume widths varied between 2 and 6 km but were typically 2 to 3 km. In order
to minimise the errors arising from variable amounts of ultraviolet radiation,
measurements were made only when the sun was at a relatively high angle, from ~0900
hrs until ~1600 hrs local time.
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Seismicity
Seismic data were collected using a Personal Seismograph PS-1 having a
frequency range of 0.2 to 30 Hz. In the low-gain mode used in this study, the instrument
was able to detect a vertical acceleration of 0.93 µg and ground displacement of 231 nm
at 1 Hz. At 40 Hz, a vertical acceleration of 2.92 µg and ground displacement of 0.45
nm was detectable. The PS-1 was placed at ~750 m elevation on the western flank of the
volcano (Figure 3.2) near an inclinometer maintained by the Departemento de Geologia
of the Instituto Costaricense de Electricidad (ICE) (Figure 3.4). The seismometer was
buried approximately 50 cm below the surface to reduce the effect of wind, and was
connected to a portable computer for the collection of digital data. The seismic data later
were analysed using commercial software packages.
Results
SO2 Flux
Sulphur dioxide flux measurements were collected during the months of
February-April 1995 and February-March 1996 (Table 3.4). The 1995 data consisted of
11 days of Plume Tracker measurements, with average SO2 flux of 109 ± 61 tonnes/day
(Table 3.5). The overall SO2 flux varied between 47 ± 23 and 202 ± 107 t/d and
between 22 (a single measurement) and 193 ± 120 t/d when eruption-related SO2 was
excluded.
Eruption-related SO2, which is the additional sulphur dioxide from an
explosive eruption, was distinguished from passive degassing of the volcano in order to
study trends that were not influenced by eruptions. A maximum explosive value for the
1995
field
season
of
367
t/d
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FIGURE 3.4
Inclinometer station maintained by the Departemento de Geologia of the Instituto
Costaricense de Electricidad (ICE), located on the upper western flank of Arenal
volcano. A portable seismometer was buried approximately 50 cm below the surface, to
the right of the door of the green hut.
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was measured on March 30, 1995, while a minimum of 18 t/d also was seen on the same
day.
The 1996 field season consisted of 6 days of COSPEC measurements, with an
average SO2 flux of 162 ± 58 t/d (Table 3.5). The overall SO2 flux varied between 110 ±
46 and 259 ± 83 t/d, and between 87 ± 17 and 186 ± 39 t/d when eruption-related SO2
was excluded. Standard deviations varied between 14 and 86 t/d. A maximum value for
the 1996 field season of 360 t/d was measured on March 8, while a minimum of 41 t/d
was seen on March 5 (Table 3.4). Maximum values for both years are quite similar, with
the average daily flux for 1996 (162 t/d) only slightly higher than the 1995 flux (109 t/d).
Similarly, the non-eruptive passive SO2 flux is also marginally higher in 1996 (128 t/d)
than in 1995 (95 t/d), however any differences are well within the error of the
measurements. However, if one takes into account the fact that windspeed at the plume
height may be as much as 3 times greater than at ground level, a maximum average value
of 327 t/d and 486 t/d is obtained for 1995 and 1996, respectively. Maximum noneruptive passive SO2 flux was therefore 285 t/d (1995) and 384 t/d (1996). The 19951996 data are similar to the 8 measurements made by Casadevall et al. (1984) in 1982
which had average SO2 flux of 198 ± 41 t/d and are 2-3 times greater than the flux of
~50 t/d measured by Stoiber et al. (1982) in 1982 (Tables 3.4, 3.5).
Due to the high level and frequency of eruptive activity at Arenal, it is difficult to
obtain the statistical data necessary to prove systematic decreases in SO2 levels prior to
an eruption. There are, however, some instances where SO2 levels appear to decrease
progressively prior to an explosive eruption. On February 28, 1995, SO2 flux dropped
from 96 ± 30 t/d to 60 ± 19 t/d before an eruption at 1025 hrs local time. The flux
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subsequently rose to 203 ± 63 t/d immediately after the eruption (Figure 3.5a). Although
this flux clearly contains an eruptive component, the marked contrast before and after the
eruption is nevertheless significant as it suggests that sealing of the conduit is taking
place. On March 4, SO2 flux decreased from 147 ± 46 t/d prior to an eruption at 1243
hrs to 34± 11 t/d after the eruption. In the afternoon of the same day, SO2 fluxes
dropped from 124 ± 38 t/d immediately after an eruption at 1444 hrs to 59 ± 18 t/d
(Figure 3.5b). On March 5, SO2 flux increased from 55 ± 17 t/d before an eruption at
1204 hrs to 114 ± 35 t/d after the eruption. In the same afternoon, SO2 rose to 152 ± 47
t/d from 40 ± 12 t/d (Figure 3.5c). No eruption was noted at that time, but the increase
in SO2 flux may be explained by a discrete gas release that went unrecorded.
Similar fluctuations in SO2 flux were seen during the 1996 field season. In the
afternoon of March 5, for example, SO2 flux decreased from 117 ± 17 t/d to 41 ± 6 t/d
prior to an eruption at 1417 hrs. The flux subsequently increased to 103 ± 15 t/d after
the eruption (Figure 3.6e,f). March 8 also showed these cyclical fluctuations. SO2 was
seen to drop from 360 ± 54 t/d (likely from an unrecorded eruption) to 114 ± 17 t/d
before an eruption at 1219 hrs and subsequently climbed to 242 ± 36 t/d immediately
afterwards (Figure 3.6i,j). Although more data is necessary to fully characterise the
eruptive nature of the volcano, these repetitive fluctuations raise the possibility that
Arenal may undergo cyclical opening and closing of the conduit, resulting in variable
pressurisation and leading eventually to explosive eruptions.
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FIGURE 3.5
SO2 flux versus time for a) February 28, b) March 4, c) March 5, 1995 at Arenal.
Eruptions are shown by inverted arrows.
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FIGURE 3.6
SO2 flux and eruption amplitude and SO2 flux and eruption duration versus time for a,b)
February 29; c,d) March 1; e,f) March 5; g,h) March 6; i,j) March 8, 1996. Amplitude is
in digital units, duration in seconds, and SO2 flux in metric tonnes per day.
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SO2 Budgets at Arenal
COSPEC/Plume Tracker Estimates
An estimate for the total SO2 emission from Arenal may be made by taking the
average of measured SO2 flux and extrapolating back to 1968. There is very little
published SO2 flux data for Arenal, with only 8 airborne COSPEC measurements made
in February 1982 (Casadevall et al., 1984) and some ground-based measurements in
November, 1982 (Stoiber et al., 1982). At the time of these measurements, the gas
originated mainly from fumaroles near crater C (Cheminée et al., 1981; Casadevall et al.,
1984). 151 measurements were made by us at Arenal between 1995 and 1996, resulting
in an average of 128 ± 59 t/d of SO2 gas, likely originating from crater C (Table 3.5). If
one takes a weighted average of the three data sets, a mean daily flux of 128 ± 62 t/d SO2
is calculated. This leads to an estimate of ~1.31 Mt SO2 emitted since 1968, which is a
lower limit for the following reasons: (1) significant quantities of SO2 likely were
emitted explosively during the initial 1968 eruptive episode; (2) our windspeeds are
clearly minimums due to the difference in altitude between the column height and place
of measurement; (3) these data also neglect sulphur released in the form of H2S.
Petrological Estimates
Estimates of total SO2 released also were made using melt inclusion data from
samples of the 1968 surge deposit and a 1992 lava flow (Table 3.6). We assumed that
the non-degassed pre-eruption melt was the only source of sulphur, and that melt
inclusions trapped in plagioclase and pyroxene crystals represent the non-degassed
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sulphur content of the magma. The coexisting matrix glass was assumed to represent
sulphur contents of the degassed melt after eruption. The difference, which is the
quantity of SO2 released, can be determined from the difference in the sulphur contents
of the melt inclusions and matrix glass. This petrological SO2 emission (ESO2 in Mt) can
be calculated for Arenal basaltic andesites using the following equation (Gerlach and
McGee, 1994):
ESO2 = (2 x 10-15) ⋅∆Sm⋅ ρm⋅ φm ⋅V
(3.3)
where the constant is a conversion factor for S (ppm) into SO2 (Mt); ∆Sm is the S lost
from the melt during eruption in ppm, determined by the difference (332 ppm) in mean S
contents of 7 melt inclusions (356 ppm) and 8 matrix glasses (<24 ppm); ρm is the
basaltic andesite melt density, assumed to be 2700 kg⋅m-3; φm is the melt volume fraction
of 0.5, estimated from thin sections of 1968, 1984 and 1992 surge deposit/lava; and V is
the volume of magma extruded between 1968 and 1996. As there is currently no
available data for extrusion rates after 1985, a rough estimate of the total volume of lava
extruded to date may be obtained by assuming that the current rate of lava extrusion is
similar to that between 1973 and 1985 (9.3 x 106 m3yr-1; Wadge, 1983; Reagan et al.,
1987). Thus, by adding the volume of lava extruded from 1968 to 1985 (3.5 x 108 m3) to
the volume of lava extruded from 1985 to 1996, (9.3 x 106 m3yr-1 times 11 years = 1.0 x
108 m3), we estimate that a total lava volume of 4.5 x 108 m3 has been extruded since
1968. The resulting ESO2 is approximately 0.40 Mt of SO2, approximately 3.3 times less
that the 1.31 Mt estimated using COSPEC data.. If one assumes that the maximum
sulphur in melt inclusions (avg.: 671 ppm S; px92-15d, px92-15e, Table 3.6) represents
samples from least degassed magma, the petrologically estimated mass of SO2 emitted
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since 1968 is 0.815 Mt. This is only 1.6 times less than the mass estimated from
COSPEC data.
Even using maximum windspeeds, three times that of ground
measurements, a total of 3.92 Mt is obtained, still only 4.8 times less than petrological
estimates.
Using SO2 fluxes measured by COSPEC and appropriate CO2/SO2 ratios (Arenal
SO2 128 t/d, CO2/SO2 6.4, Williams et al. (1992)), we calculate crater CO2 fluxes of 820
t/d for Arenal. This flux is 2-3 orders of magnitude smaller than at Mt. Etna (Allard et
al., 1991; Brantley and Koepenick, 1995).
Seismic Data
Seismic measurements were made in conjunction with COSPEC measurements
during 6 days in 1996. A total of 63 eruptions were measured over the 6 day period
between ~1000 hrs and ~1600 hrs local time. The durations of these events varied
between ~5 s and ~90 s, while amplitudes ranged from 120 to 4400 digital units.
Numerous periods of tremor also were recorded between eruptions, with durations
ranging from 14 s up to a period of continuous tremor lasting >6600 s on March 2, 1996.
FFT analyses of the tremor signal revealed that frequencies typically varied between 2
and 5 Hz. Harmonic tremor with frequencies of ~4.5 Hz were also common. These
tremors fall in the class of intermediate frequency tremor which is believed to be
associated with strong degassing. Low frequency tremor at <3 Hz also has been noted
and may represent conduit resonance, gas fluctuation, or degassing (Barquero et al.,
1992).
The relationship between tremor and eruptive events varies somewhat over the
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period of measurement; nevertheless, some interesting correlations are apparent. A
relative decrease in seismicity was seen before eruptions on February 29 (1450, 1537 hrs
local time, Figure 3.6a,b) and March 1, 1996 (1520, 1529 hrs, Figure 3.6c,d). Many
eruptions also were followed by tremor events on February 29 (1043, 1257, 1342, 1450
hrs).
Chugs are locomotive-like sounds caused by repetitive gas emissions (Melson,
1989). Such chugs frequently were accompanied by harmonic tremor, likely indicating
the presence of an open conduit (Barboza and Melson, 1990). We also have noted this
correlation, frequently in the early part of a day. For example, on March 2, a general
decrease or dissipation of both chugs and tremor was observed prior to eruptions at
1129, 1159 and 1214 hrs. This was also the case on February 29 (1450, 1537 hrs) and
March 1 (1520 and 1529 hrs). March 1 was also noted for the comparatively low level
of explosions and the comparatively high number of chugs and tremor events (Figure
3.6c,d).
We have noted that there was a higher frequency of eruptions in the mornings
compared to the afternoons. This also was remarked upon briefly by Barboza and
Melson (1990). For example, on March 5, there were 11 eruptions between 1000 and
1300 hrs, yet only 3 between 1300 and 1650 hrs. The morning eruptions on March 5
also were characterised by pairs of eruptions (8 of the 11 morning eruptions) with less
than 10 minutes separating them (Figure 3.6e,f).
The amplitudes of the morning
eruptions (except for March 6) also were greater and more variable than those of the
afternoon. The subsequent decrease in afternoon eruption occurrences and amplitudes
generally coincided with the peak in predicted tidal gravity values. The predicted tidal
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116
gravity showed an increase from 83.6 µgals at 0930 on February 29 to 165.7 µgals at
1230 on March 6 (Table 3.7, Figures 3.6, 3.7). Maximum gravity values then decreased
to 155.6 µgals at 1330 on March 8, the end of field measurements. It should be noted
that the gravity values were calculated on the half hour.
This variation coincides with changes in the presence and nature of tremor
(Barboza and Melson, 1990). Prior to ~1326 hrs on March 5, there was very little
chugging or tremor activity; however, after this point significant tremor commenced and
lasted throughout the afternoon until the cessation of measurements at ~1600 hrs.
Similarly, on March 6 and 8, tremor started after ~1328 and ~1357 hrs, respectively, and
was accompanied by a decreased frequency of eruptions. The afternoon tremor on
March 5 and 6 also began just after the onset of high tide (Figure 3.7f,g), whereas the
tremor of March 8, began immediately after the peak of high tide (Figure 3.7i).
The presence of small-amplitude, high-frequency (15-17 Hz) events prior to
eruptions was observed on March 6. This coincided with the highest daily SO2 average
(259 ± 83 t/d) of the field season. Excluding the eruption-related SO2 flux, March 6 still
has the second highest daily value of 162 t/d (single value). Such high-frequency events
may be related to rock fracturing (Anderson, 1978). The high-frequency events and high
SO2 fluxes together suggest that there may have been increased intrusion of
comparatively gas-rich magma into the conduit.
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FIGURE 3.7
Fluctuation of gravity due to Earth tides between a) February 29 to i) March 8, 1996.
Gravity data is in microgals. Note that tremor generally begins at or just past high tide of
March 5, 6, and 8.
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Discussion
Conduit Opening and Closing
The seismic data presented above appear to point to the repetitive opening and
closing of the shallow conduit system. The frequent periods of chugs and tremor, as well
as Arenal’s continuous activity since 1968, indicate that the volcano is generally
behaving as an open system. However, on a daily scale, Arenal appears to go through
cyclic changes between a closed and an open system.
The higher frequency of eruptions in the morning (e.g., February 29, March 5, 6,
and 8, 1996) and the observed sequence of paired eruptions in the morning (e.g., March
5 and 6, 1996) suggest that Arenal behaves as a comparatively closed system at these
times. The relative quiescence of seismic activity before eruptions on the mornings of
February 29, March 1, and March 6 may further indicate closure of the conduit prior to
the eruptions (Figure 3.6). The paired eruptions of March 5 also suggest that the system
is relatively closed. The first eruption of the pair may partially open the conduit with
comparatively little release of gas pressure. The second, larger-amplitude event will then
destructively open the conduit. These paired eruptions (seen on February 29, March 1, 2,
5, and 6) disappear with decreased eruptive activity in the afternoon, which coincides
with the onset of tremor. In contrast to these days, March 1 shows a distinct lack of
eruptive activity and greater tremor and chugging. This suggests that the volcano was
behaving as an open system throughout the day on March 1.
The decrease in frequency of eruption and increase in tremor activity also
coincide with the arrival of high tide (Figure 3.7). The correlation between volcanic
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activity and Earth tides has been recognised for some time (Eggers and Decker, 1969;
Hamilton, 1973; Johnston and Mauk, 1972; Mauk and Johnston, 1973; Golombek et al.,
1978; Mauk, 1979), as has a correlation between Earth tides and volcanic SO2 emissions
(Stoiber et al., 1986; Connor et al., 1988). The increased lunar attraction causes the flow
of water towards the locus of maximum attraction, resulting in high tide.
This
mechanism also may affect volcanic activity. Barboza and Melson (1990) suggest that
the inverse correlation between tremor activity and eruptions may be due to the rise of
magma in the conduit. As the lunar attraction increases to a maximum, comparatively
hot fresh magma may be pulled towards the surface, resulting in a decrease in surface
crystallisation and consequent decrease in sealing of the conduit. Thus, there is less
pressurisation of the conduit and a resulting decrease in explosive eruptions. Similarly,
degassing will occur more easily, evidenced by the increased chugs and tremor that we
observed in the afternoons. Stoiber et al. (1986) noted that at Masaya caldera, bursts of
gas from the lava lake were twice as likely to occur during the maxima or minimum of
Earth tides. The close relationship between the reduced frequency of eruptions, the
beginning of tremor and the high tide, for three days at least, suggests that Arenal’s
eruptive activity may be sensitive to relatively small changes in the confining pressure.
In a generally open system, these small changes may be sufficient to shift the activity
from a relatively closed system to a more open one or vice-versa.
There may also be a relationship between magma supply/extrusion rates and
periods between explosive eruptions. It was noted that changes in the extrusive activity
from the northern vent of crater C could affect the explosive activity of the southern vent
(M. Davis, personal communication, 1996). If extrusion rates are high, lava may be
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forced into the southern conduit, causing a temporary blockage. This would allow for
overpressurisation and explosive eruption.
Our COSPEC measurements (e.g., March 4, 1995, March 8, 1996) show
fluctuations in SO2 flux, suggesting that the conduit may undergo cycles of gradual
sealing leading to overpressurisation and eventual eruption. Progressive decreases in
SO2 flux suggest a decrease in the conduit opening and resulting increase in conduit
pressure. In many instances, this increase in pressure continues until a critical limit is
reached, at which point the closure is destroyed by an explosive event. This often may
involve the release of relatively large ash columns and the ballistic ejection of bombs
and blocks.
There are, however, instances where there is gas release without ash
emission.
Similar cycles of progressive decrease in SO2 flux have also been noted at
volcanoes such as Galeras (Zapata et al., 1997) and Nevado del Ruiz (Williams et al.,
1990). During the 1992-1993 eruptive period at Galeras, three of the largest eruptions
were preceded by low levels of degassing and immediately followed by intense longperiod seismicity and relatively high SO2 fluxes. Between 1985 and 1987, there were at
least three cases where the SO2 fluxes at Nevado de Ruiz decreased significantly prior to
eruptions (Williams et al., 1990). Arenal’s high level of activity also may allow cycles
of sealing and pressurisation of the conduit leading to eruptions, but on an extremely
short time scale. In contrast to volcanoes such as Galeras or Popocatepétl, sealing and
overpressurisation on Arenal generally occur over a matter of minutes to hours.
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The Sulphur Budget of Arenal
The SO2 (0.047 Tg⋅yr-1) and CO2 estimates (0.301 Tg⋅yr-1) from Arenal are only a
relatively small fraction (SO2: 0.40%, CO2: 3.9%) of the annual global output of erupting
volcanoes (Stoiber et al., 1987; Williams et al., 1992). Galeras, with at least 1.9 Mt SO2
emitted over a seven year period, represents approximately 1.4% of annual volcanic
output, while CO2 estimates of 2.6 Mt at Galeras represent only 0.57% of total annual
volcanic CO2 flux (Zapata et al., 1997; Williams et al., 1992). Over 1,000 COSPEC
measurements from Mount St. Helens between 1980 and 1988 give a total SO2 emission
of 2 Mt (Gerlach and McGee, 1994). The 1963 eruption of Gunung Agung (Bali,
Indonesia) also released significant amounts into the atmosphere, with an estimated 2.5
Mt of SO2 emitted (Self and King, 1996). Although Arenal’s annual SO2 output is
small, its total output is nevertheless quite similar to those of Mt. St. Helens, Galeras,
Redoubt, and Gunung Agung. but merely over a longer time scale. On a short time
scale, small volcanoes such as Arenal may not have a significant impact on global
volcanic output. However, over long periods of time, continuously active volcanoes
such as Arenal may emit significant quantities of SO2 into the troposphere. These are
comparable to volcanoes which exhibit vigorous degassing but over shorter time scales.
The difference in SO2 estimates between Plume Tracker/COSPEC and
petrological methods necessitates a source for this excess sulphur. Various mechanisms
have been proposed to explain these discrepancies. At El Chichón, for example, excess
sulphur may have been derived from magmatic anhydrite and an S-rich vapour phase
(Luhr et al., 1984). At Nevado del Ruiz, anhydrite (Fournelle, 1990) and sulphur-rich
vapour phases (Sigurdsson et al., 1990) also were shown to be important. Excess
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sulphur also may derive from the syn-eruptive degassing of sulphur in the melt of a nonerupted convecting magma. Convection cells would allow for the continued upward
cycling of undegassed magma and consequent downward movement of degassed magma
(Casadevall et al., 1983; Andres et al., 1991; Kazahaya et al., 1994). Excess sulphur also
may arise from the degassing of mixed or commingled intrusions of basaltic magma.
The exsolution and upward migration of less soluble species such as CO2 also may
transport more soluble species such as SO2 to the surface (Andres et al., 1991). SO2 also
might be directly absorbed by the hydrothermal system (Williams et al., 1990).
Extensive hydrothermal systems also may act to seal a volcano and allow for the
accumulation of an independent vapour phase. This could then lead to the release of a
large sulphur-rich gas bubble. This is unlikely at Arenal, however, as it does not have an
extensively developed hydrothermal system.
Unlike Láscar and Lonquimay volcanoes in Chile (Andres et al., 1991) where
petrological estimates were 50-100 times less than COSPEC estimates, petrological
estimates for Arenal are only 1.6 to 4.8 times less than COSPEC values. The melt
inclusions analysed in our study may be samples of melt that had already undergone
some degassing. Thus, these melt inclusions may not represent pristine undegassed
melt. Whatever the cause of excess sulphur, it is of much less importance at Arenal.
This small difference does, however, suggest that Arenal is not being supplied by an
isolated, slowly degassing body of magma. Rather, it is more likely that Arenal is an
open system which is being continuously intruded by fresh magma. There may also be
convection in the conduit, allowing for the continued upward cycling of undegassed
magma.
In contrast, volcanoes such as Láscar, Lonquimay, and Pinatubo have
Chapter III - SO2 and Seismicity: A Degassing Model
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undergone extensive sulphur degassing from the melt within an isolated slowly cooling
magma chamber, with the result that there are significant discrepancies between
COSPEC and petrological emission estimates.
The Open Nature of Arenal
According to Stoiber et al. (1986), an overall steady decrease in volcanic SO2
flux with time suggests that a single batch of magma is progressively degassing, without
the influx of new gas-rich magma. This appears to be the case for Masaya caldera in
Nicaragua, Galeras and Nevado del Ruiz in Colombia, and Mount St. Helens in the
United States, which show rate decay constants of 0.04 yr-1, 0.27 yr-1, 0.38 yr-1 and 1.41
yr-1, respectively (Stoiber et al., 1986; Zapata et al., 1997; Williams et al., 1990). In
contrast to Galeras which clearly has acted as a relatively closed system (Zapata, 1997),
SO2 fluxes at Arenal actually have increased slightly over time (Table 3.4). Unlike
Galeras or even Masaya, which receive episodic (decades) supply of small shallow
magma batches, Arenal appears to have a continuous input of magma from depth.
Rather than being supplied from a stagnant shallow chamber or small body of magma
that progressively degasses, magma beneath Arenal may reside in a chamber which itself
is open to replenishment. Based on observed geochemical changes in extruded lavas,
modelling by Reagan et al. (1987) concluded that the Arenal magma has undergone three
compositional stages prior to and after the 1968 reactivation. Changes in composition of
stage 3 magmas (1974-present) also indicate continued influx of magma along with
crystal removal. The chamber is probably located in the middle to lower crust (Reagan
et al., 1987), from which a series of conduits or fractures, opened by magmatic pressure,
Chapter III - SO2 and Seismicity: A Degassing Model
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125
rise to a kilometre beneath the volcano. The final kilometre to few hundred metres
consist of a conduit or conduits which open and close on short time scales, that result in
frequent strombolian eruptions and extrusion of lava (Wadge, 1983).
Extrusion of lava also appears to be independent of these strombolian eruptive
cycles, which suggests that there may be a complex system of conduits or fractures just
below the surface. Low frequency volcanic tremors (2-4 Hz) seen before and after
eruptions may represent the oscillation of magma or an organ-pipe effect in these
conduits (Matumoto and Umana, 1976; Wadge, 1983; Alvarado et al., 1988; Barquero et
al., 1992).
Conclusions
Due to the high level of activity at Arenal, collection of a large set of SO2 fluxes
between eruptions was difficult. However, it is nevertheless possible to observe cyclical
variations in SO2 fluxes before and after eruptions. When one compares the gas flux to
the seismic data showing declines in tremor prior to eruptions, it becomes apparent that
there is a repetitive cycle of activity. Correlations between seismic activity and Earth
tides suggest that an extremely open system such as Arenal may be quite sensitive to
minor variations in confining pressures, changing from a relatively closed system to a
comparatively open system over the space of minutes to hours. The small difference
between petrological and COSPEC SO2 flux suggests that Arenal is being continuously
supplied by fresh magma.
The cycle of explosive eruptions may be explained by
repeated closure of the conduit(s) due to crystallisation of the magma, leading to
overpressure and explosive destruction of the magma cap. While Arenal may have only
Chapter III - SO2 and Seismicity: A Degassing Model
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126
a small influence in terms of the annual global volcanic input of SO2 and CO2 to the
atmosphere, the continuous activity of the volcano has nevertheless contributed at least
1.3 Mt of SO2 to the troposphere since 1968, comparable to volcanoes such as Mt. St.
Helens, Nevado del Ruiz, and Galeras. Arenal’s high level of activity allows for the
study of multiple cycles of conduit opening and closing and thus is an excellent tool for
better understanding the manner in which an open-system volcano degasses.
Chapter III - SO2 and Seismicity: A Degassing Model
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127
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CONCLUSIONS
Conclusions
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General Conclusions
HIS study of seismicity, CO2 and radon soil gas, and SO2 at Arenal has provided a
T
better understanding of volcanic behaviour. Some important conclusions from
this work are as follows:
1. Correlations between soil gas concentrations and seismic data are difficult to
establish, due in part to limited seismic coverage, but more importantly, due to the high
level of activity at Arenal.
2. Temporal variations in radon and CO2 soil gas concentrations are due in large part to
changes in atmospheric pressure over time.
3. Rn and CO2 soil gases from the upper flanks of Arenal are unlikely to originate from
deep levels, but rather come from shallow surface sources, as the radon half-life is too
short and the transport process too slow.
4. The diffuse soil gases are generally unable to penetrate the young lavas which cover
and seal the upper flanks of Arenal. Only on the lower flanks, where young lavas do not
crop out, is there any gas flow from deeper levels. This is evident from the increased
CO2 concentrations and heavier δ13C values at greater distances from the crater.
5. The degree of soil development and permeability of the substrate also strongly
influences the concentrations of CO2 and radon soil gas at Arenal. Unconsolidated
volcanic soils on the upper flanks of the volcano have relatively low RnERaC and
consequently low radon values. These soils are also more apt to rapidly dissipate any
precipitation, thus limiting sealing effects. This is in contrast to the more clay- and
organic-rich soils of the lower flanks, which retain humidity and increase sealing. This
results in a lower permeability in the better developed soils and permits the accumulation
Conclusions
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133
of soil gas below the surface.
6. Volcanoes such as Arenal, Poás, and Galeras act as volcanic plugs which seal shallow
levels of the continental crust, limiting deep gas flux to fumaroles, faults, and fractured
lower flanks of the volcanoes.
7. The high level of activity at Arenal makes collection of a large set of SO2 flux
measurements between eruptions difficult. It was, nevertheless, possible to see cyclical
variations in SO2 fluxes before and after eruptions. When one compares the SO2 results
to the seismic data showing declines in tremor prior to eruptions, it becomes apparent
that there is a repetitive cycle of activity.
8. Correlations between seismic activity and Earth tides suggest that an extremely open
system such as Arenal may be quite sensitive to minor variations in confining pressures,
changing from a relatively closed system to one that is open over the space of minutes to
hours.
9. The small difference between petrological and COSPEC SO2 output suggests that
Arenal is being continuously supplied by fresh magma. The cycle of explosive eruptions
may be explained by repeated sealing of the conduit(s) due to cooling of the magma.
Subsequent gas pressure increases may then lead to explosive destruction of the magma
cap.
10. Arenal exerts a small influence in terms of annual global volcanic input of SO2 and
CO2 to the atmosphere. However, the continuous and high activity of the volcano has
nevertheless contributed at least 1.3 Mt of SO2 to the troposphere since 1968. Arenal’s
high level of activity permits us to the study multiple cycles of conduit closing and
overpressurisation and is thus an excellent means for better understanding the manner by
Conclusions
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134
which an open-system volcano degasses.
Recommendations for Future Work
During this study, only the lower part of the edifice was surveyed by soil gas
measurements. Should current activity decrease to a point where access to the upper
flanks is possible, systematic sampling in closer proximity to the crater would greatly
improve the degassing hypothesis. Stable carbon isotopes should be studied in more
detail at Arenal, specifically in proximity to areas of structural weakness and with
increased distance from the summit. This also will aid in clarifying the relationship
between the capping effect of young lavas on the upper flanks and their subsequent
breakdown and fracturing on the lower flanks.
Year-round monitoring of SO2 flux in conjunction with the emplacement of a
more extensive seismic network on the volcano are necessary. This would allow for
more detailed analysis of the correlations between flux and seismic fluctuations.
Volcanic seismic activity may cause soil gas anomalies on Arenal. Additional research
is necessary to better understand the short term fluctuations due to seismic fluctuations.
Should explosive activity decrease substantially, emplacement of a small
meteorological station near the summit would greatly increase the accuracy of windspeed
measurements and thus decrease the uncertainty of SO2 flux measurements. While the
current strombolian activity continues, a small helium balloon and/or theodolite should
be used to better constrain the actual windspeed of the plume.
A more detailed study of recent extrusion rates and volumes of lava emplaced
since 1985 is required. In conjunction with more detailed analyses of concentrations of
Conclusions
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135
sulphur in melt inclusions, this allow will for better petrological estimates of the total
SO2 emitted since reactivation of the volcano.
This will also lead to a better
understanding of the presence or lack of excess sulphur in open-system volcanoes.
APPENDIX
Appendix A
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Appendix A
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A-2
Appendix A
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A-3
Appendix A
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A-4
Appendix A
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A-5
Appendix A
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A-7
Appendix A
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Appendix A
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A-9