Eos, Vol. 87, No. 6, 7 February 2006
VOLUME 87
NUMBER 6
7 FEBRUARY 2006
EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION
Monitoring the Reawakening of
Canary Islands’Teide Volcano
PAGES 61, 65
Following more than 30 years of seismic
and volcanic quiescence, the Canary Islands
region located off the northwestern coast of
Africa started to show signs of seismovolcanic activity at the end of 2003 (Figure 1). In
spring 2004, there was a significant increase
in the number of seismic events (a mixture
of volcano-tectonic events and regional
earthquakes with pure volcanic events such
as tremors and long-period signals) located
inland on Tenerife Island.
The increase ofactivity in 2004 coincided
with an increase of fumarolic activity at the
Teide volcano on Tenerife Island, an increase
in the emission of carbon dioxide in the
northwestern part of the island, and changes
in the gravimetric field on the northern flank
of the volcano. After several seismic events
had been felt by the population, the first alert
level was declared by the civil protection division of the local government. This apparent
reawakening of Teide, which last erupted in
1909, provides an opportunity to study from
the initial stages the reactivation of this volcanic area and its related phenomena.
This article presents an automatic seismic
monitoring system, the Teide Information
Seismic Server (TISS), that is monitoring the
internal status of the volcano by means of
real-time seismic background noise analysis.
The system’s main goal is to detect precursors to a potentially dangerous eruptive episode at an early stage. The system, in operation at Teide volcano since November 2004,
has proven useful in monitoring changes in
the behavior of the volcano’s processes, such
as fumarole venting and seismic activity.These
external manifestations of the volcano’s processes have been preceded by changes in
the monitored parameters (see Figure 2).
them of the effusive type, where lava flows
freely without explosive power. Teide
(28.27ºN, 16.6ºW) is a complex stratovolcano,
the third-tallest volcano on Earth from base
to tip, reaching an altitude of 3717 meters
above sea level and approximately 7000
meters above the adjacent seabed. Teide’s
last explosive-type eruptions have been
dated as having occurred around 1500 years
ago. Future eruptions are considered likely
and will include the risk of highly dangerous
PAGES 61–72
pyroclastic flows similar to those on Mount
Pelée (Martinique) and Mount Vesuvius
(Italy). The Teide volcano’s explosive eruptions are the result of magma mixing processes in which basaltic eruptions act as a
triggering mechanism.
In 1990, the International Association of Volcanology and Chemistry of the Earth’s Interior
(IAVCEI) identified Teide as being worthy of
particular study in light of its history of large,
destructive eruptions and its proximity to populated areas. For the present high-risk level,
since 1992 Teide has been considered by the
IAVCEI as one of the European Laboratory
Volcanoes, thus receiving special consideration from the European Union concerning
research proposals.
A Brief History
Several eruptions have taken place in the
Canary Islands in the last 500 years, all of
BY A. GARCÍA, J.VILA, R. ORTIZ, R. MACIA,
R. SLEEMAN, J. M. MARRERO, N. SÁNCHEZ,
M. TÁRRAGA, AND A. M. CORREIG
Fig. 1. (top) Signs of activity at the Teide volcano (October 2004). Picture provided by the municipal government of Icod de los Vinos (Tenerife, Canary Islands, Spain). (bottom left) Epicentral
locations of events from January 2000 to June 2005. Red dots indicate events from May 2004 to
July 2005. (bottom right) Evolution of the cumulative seismic energy released by earthquakes
located inland on Tenerife (data provided by the Spanish Instituto Geográfico Nacional).
Eos, Vol. 87, No. 6, 7 February 2006
TEGETEIDE Project
The special topographic characteristics of
the Canary Islands, combined with the distribution of the islands’ more than two million
inhabitants, increase the level of volcanic
risk in this area. A Teide eruption similar to
the last dated explosive type eruption (with
a volcanic explosivity index (VEI) of 3) and
comparable with the last eruptions of Unzen
Volcano in Japan could affect up to 30,000
people; a VEI of 4 could affect more than
400,000 people.
The reported increase in seismic activity
that has been observed since spring 2004
could represent the beginning of a reactivation of Teide Volcano. An urgent call by the
Spanish government to the Spanish National
Commission on Seismic and Volcanic Risk
Evaluation to organize and manage the
available scientific resources to evaluate the
possibility of an imminent eruption has been
considered. As a result of this call, financial
support for research projects on this area
was provided, and in the spring of 2005, the
Spanish National Research Council (CSIC)
initiated the TEGETEIDE project (Geophysical and Geodetic Techniques for the Study
of the Teide-Pico Active Volcanic Area) .
This project will develop and implement
TISS, which is capable of characterizing the
predominant frequencies and the seismic
energy released that is observed in the background seismic noise at diverse frequency
bands. TISS is capable of monitoring in real
time changes of the status of the volcano by
collecting information in the time and the
spectral domains, using parameters in which
changes due to variations in the recorded
signals are easy to observe. The goal is to
detect changes before the expected episodes of activity.
TISS is based on a real-time quality-control
(RT/QC) analysis of continuously-collected
waveform data from seismic stations, and it
monitors changes in the behavior of the seismic signals.These changes reveal variations
in local site effects caused by changes in the
volcanic activity.The present TISS process
involves two different stages: The first stage,
termed ‘data preparation,’ and the second
stage, called ‘data analysis.’ The main tasks in
data preparation are to build the basic data
format and to provide visual Live Internet
Seismic Server-like outputs (http://www.liss.
org).The data analysis stage, which is devoted
to waveform analysis, selects fixed-length segments from the already established database,
assigns time stamps, and applies subroutines
of spectral analysis.
The current RT/QC analysis was developed in 2002 to test permanent stations at
the Observatori Fabra in Barcelona, Spain
[Llobet et al., 2003]. Its expansion to large
networks was accomplished in 2004 when
an adaptation of the QC procedure was
developed [Sleeman and Vila, 2006] to monitor all stations of the Virtual European Seismic Network [Van Eck et al., 2004]. Its offline application for monitoring changes in
background noise near active volcanoes is
Fig. 2.Variations in parameters provided by TISS before and after the appearance on 5 December
2004 of the fumarole emission in La Orotava Valley: (a) Daily lower envelope of all 60-minute
PSD curves; (b) Evolution of the amplitude; (c) Predominant frequency; (d) Integrated PSD
amplitude. Plots (b), (c), and (d) relates data in four frequency bands.
presented by Vila et al. [2005], who showed
that continuous monitoring of the background seismic noise levels may provide
signs of activity more than 40 days sooner
than classical seismological methods based
on earthquake analysis.
The adaptation of TISS for the Canary
Islands uses a power spectral density (PSD)
estimation to perform spectral analysis of
the continuously incoming data [Welsh,
1978]. TISS computes the PSD of 60-minute
segments, extracted from the pool of continuous data, and stores selected parameters as
time series. These time series are selected
frequencies of the PSD, integrated PSD in
various frequency bands (energy related
parameters), and absolute maximum PSD
amplitude and its corresponding frequency,
also in various frequency bands. Moreover,
the lower envelope of all the PSD curves
(minimum value for each frequency)
obtained after processing time intervals of
24 hours is tracked, thus giving an estimation
of all nontransient signals during one full
day of operation. This envelope contains the
noise that is always present at every frequency component [Vila et al., 2005] and is
continuously compared with previous days.
TISS provides details of the evolution of this
information by means of report plots.
The post-analysis derived time series have a
sampling rate of the order of one sample per
hour, determined by the length of the segments analyzed.These time series are then
suitable to be compared directly with many
other signals, such as deformation, temperature, and gas emission, that are sampled at sim-
ilar sampling rates, thus resulting in a multidisciplinary analysis.The sampling rates of the
post-analysis derived time series also allow a
fast and easy representation of the volcano’s
seismic activity.The time series also can be
used as new input for any supplementary
near-real-time analysis.This corresponds to the
third level of observatory automatic procedures [ESF-EVOP Working Group, 1994].
Evaluation of TISS During 2004
Since first becoming operational in November 2004,TISS has shown itself to be very useful for monitoring changes in the behavior of
the Teide volcano’s processes, such as fumaroles and seismic activity. For example, on 5
December 2004 a new fissure with fumarole
emission appeared in La Orotava valley, on
the northeastern flank of Teide volcano (see
Figure 2).This was preceded by a significant
increase in low-frequency seismic energy
nine days earlier, with a return to the normal
level a few hours after the fissure aperture.
During those nine days, the scientific team
remained on alert.
TISS archives the original time series data
in standard formats such as SEED (Standard
for the Exchange of the Earthquake Data) or
GSE (Group of Scientific Experts) and is
thus a complete and open system that allows
the data to be processed by means of standard seismological software packages.The
software package runs under Linux, and all
software used can be obtained through GNU
and other free software sites.There are no
intrinsic limitations for the number of chan-
Eos, Vol. 87, No. 6, 7 February 2006
nels or computations.The performance analysis of the system reveals that a 1.2 GHz Pentium III personal computer can handle more
than 400 channels at 100 samples per second.
Sources of data and programming facilities
can be found on the TISS Web pages (http://
www.sistegeteide.info/tiss and http://sismic.
am.ub.es/tiss). Examples of RT/QC implementations are available at http://sismic.am.ub.es/
ISEV/ and http://www.orfeus-eu.org/data-info/
dataquality.htm.
Acknowledgments
TEGETEIDE (Tenerife) is a project funded
by the Spanish Ministry of Education and
Science (contract CGL2003-21643-E). TISS is
hosted by the Departament d’Astronomia
i Meteorologia of the Universitat de Barcelona, Spain, and by the municipal government of Icod de los Vinos (Tenerife). Software developments are performed together
with the Institut d’Estudis Catalans and the
Observatories and Research Facilities for
European Seismology.
References
ESF-EVOP Working Group (1994), Automated Systems
for Volcano Monitoring, Eur.Volcanol. Proj. Monogr.,
vol. 2, 20 pp., Eur. Sci. Found., Strasbourg, France.
Llobet,A., J.Vila, O. Fors, and R. Macià (2003), Linux
based LISS systems:The ISEV and SISLook examples, ORFEUS Electron. Newsl., 5(1), 5.
Sleeman, R., and J.Vila (2006),Towards an automated
uniform quality control of the Virtual European
Broadband Seismograph Network (VEBSN), ORFEUS Electron. Newsl.,. In Press.
Van Eck,T., C.Trabant, B. Dost,W. Hanka, and
D. Giardini (2004), Setting up a virtual broadband
seimograph network across Europe, Eos Trans.AGU,
85(13), 125, 129.
Vila, J., R. Macià, D. Kumar, R. Ortiz, and A. M. Correig
(2005), Real time analysis of the unrest of active volcanoes using variations of the base level noise seismic spectrum, J.Volcanol. Geotherm. Res., in press.
Welsh, P. D. (1978),The use of fast Fourier transform for
the estimation of power spectra: A method based
Adam Paulsen, a Pioneer
in Auroral Research
PAGES 61, 66
The 20 to 30 years following the first International Polar Year in 1882–1883 was a period
of quickly advancing knowledge and understanding of auroral phenomena.This was the
time when hypotheses of aurora being due to,
for example, reflections of fires from the interior of the Earth or sunlight from ice particles
were abandoned and replaced by the mechanism of precipitating electrons.
One of the auroral researchers at that time
was the Dane Adam Frederik Wivet Paulsen
(1833–1907). However, when reading literature about auroral history, his ideas and
work do not seem to have attracted much
interest outside his own and neighboring
countries. For example, in his sweeping historical account Majestic Lights: The Aurora in
Science, History, and the Arts [1980], author
Robert Eather only referred to Paulsen in a
couple of lines.
Eather did not mention any of Paulsen’s
original and still valid ideas, namely, that electrical currents flowing parallel to the Earth’s
magnetic field exist and move upward within
the auroral bands, that the aurora is produced by cathode rays (electrons), and that
the source of the cathode rays is located in
the upper regions of the Earth’s atmosphere.
The purpose of this article is to draw attention to Adam Paulsen and his contributions
to the understanding of the aurora.
Paulsen and the 1882–83 Expedition
to Greenland
Paulsen was born on 2 January 1833 in
Nyborg, Denmark. He had planned to be an
BY T. S. JØRGENSEN AND O. RASMUSSEN
Fig. 1 Adam Paulsen, age 67. The pencil
drawing (25 x 30 cm) was made by Harold
Moltke on 10 February 1900 during Paulsen’s
expedition to Akureuyi, Iceland.
officer in the Danish army, and from 1849
to 1851 he participated in a war against
Germany. However, an interest in physics
changed his career path. Adam Paulsen
earned a master’s degree in physics in
1866 from the University of Copenhagen,
Denmark, and two years later he was
awarded a gold medal by the same for a
prize essay on the different theories of
galvanism (electricity produced by
chemical processes).
After completing his university studies,
Paulsen became a high school teacher in
Copenhagen. In 1880, at the age of 47, he
quit teaching after he was asked by the
director of the Danish Meteorological Institute to head a Danish expedition to Green-
on time averaging over short modified periodograms, IEEE Trans.Audio Electroacoust.,AU-15, 70–73.
Author Information
Alicia García, Ramon Ortiz, Jose M. Marrero,
Nieves Sánchez, and Marta Tárraga, Departamento
de Volcanología, National Museum of Natural Sciences–CSIC, Madrid; Josep Vila and Antoni M. Correig, Departament d’Astronomia i Meteorologia,
Universitat de Barcelona and Laboratori d’Estudis
Geofísics “Eduard Fontserè,” Institut d’Estudis Catalans, Barcelona, Spain; Ramon Macià, Departament
de Matemàtica Aplicada 2, Universitat Politècnica
de Catalunya, Barcelona, Spain; Reinoud Sleeman,
Koninklijk Nederlands Meteorologisch Instituut, De
Bilt, Netherlands.
For further information, contact J.Vila; E-mail:
[email protected]
land during the upcoming Polar Year. Denmark, as part of an international consortium
that had pooled their resources to study the
Arctic, was to conduct meteorological, geomagnetic, and auroral observations (including the shape, color, strength, movement, and
position of aurorae). Paulsen, along with five
other men, sailed from Copenhagen aboard
the Royal Greenland Trade Department’s
small three-masted sailing ship Ceres on 17
May 1882, bound for Godthaab (64º10’N,
51º40’W), a small town on the west coast of
Greenland. After a four-week voyage they
arrived at their destination.
The location was found favorable for auroral observations. Godthaab is located just on
the poleward side of the belt of maximum
auroral activity, an area advantageous for the
study of the periodic variations of auorae.
The team also planned to investigate relationships between the aurora and magnetic
disturbances, and to attempt to measure the
height above the ground of the aurora,
because such knowledge might help to disclose its causal mechanisms.
The height measurements were not successful, because the distance of about five
kilometers between two observation sites on
the ground was much too short for a good
quality triangulation to be made. However,
the study of the relation between aurora and
magnetic disturbances inspired Paulsen to
suggest that field-aligned currents exist. This
was a milestone in auroral research.
The expedition team returned to Denmark
in the autumn of 1883, and the following
year Paulsen became the director of the
Danish Meteorological Institute, a position
he held until his death in 1907.
Paulsen’s Discoveries
In a comprehensive report about observations during the International Polar Year and
the following years in Greenland, Paulsen
[1893] presented, among other discoveries,
observations indicating the existence and
Eos, Vol. 87, No. 43, 24 October 2006
Recent Unrest at Canary Islands’
Teide Volcano?
PAGE 462, 465
When a volcano that has been dormant
for many centuries begins to show possible
signs of reawakening, scientists and civil
authorities rightly should be concerned
about the possibility that the volcanic unrest
might culminate in renewed eruptive activity.
Such was the situation for Teide volcano,
located on Tenerife in the Canary Islands,
when a mild seismic swarm during April–
July 2004 garnered much attention and
caused public concern. However, that attention completely ignored the fact that the
seismic recordings of the swarm were due to
a much improved monitoring system rather
than due to an actual event of alarming
magnitude or extent.
It is important in any effective program of
volcano-risk mitigation that the response to
an apparent change in the status of a volcano should include the immediate implementation or augmentation of monitoring
studies to better anticipate possible outcomes
of the volcanic unrest. Equally important,
emergency-management officials, using available scientific information and judgment, must
take appropriate precautionary measures—
including information of the populations at
potential risk—while not creating unjust
anxiety or alarm.
This article briefly reviews the circumstances
and unfortunate societal consequences of
the scientific, governmental, and media response
to the seemingly heightened seismic activity
on Tenerife in 2004. This article describes a
series of misinterpretations of geophysical
and geological data that led to raised levels
of alarm on the island, although being of little actual volcanological significance.
into a stratigraphic sequence, particularly
those eruptions that have occurred in the
past 10,000 years (the Holocene).
The results of this study indicated that the
central differentiated (phonolitic) Teide volcanic complex was the direct consequence
of the activity of the rifts. The rifts developed
a progressively steepening and unstable volcano at the center of Tenerife, and about
180,000–200,000 years ago rift activity triggered a massive landslide that generated an
impressive horseshoe-shaped collapse
embayment. The embayment’s headwall is
the 17 by 10 kilometer Las Cañadas caldera.
Subsequent rift and central activity progressively filled the embayment, finally building
up the 3718-meter-high Teide stratovolcano,
which is nested in the collapse embayment.
The main phase of construction of Teide
concluded 30,000 years ago. Since then, the
stratovolcano has erupted just once: the
eighth-century summit eruption, during
which the vent system and lavas increased
the elevation of the volcano from about
3600 to 3718 meters.
This recent eruptive history is in stark contrast to the much higher explosivity, with frequent plinian eruptions, of the precollapse
activity of the Cañadas volcano (>200,000
years ago). In the past 30,000 years, eruptions have occurred at a rate of only four to
six per millennium, with a predominance
(70%) of very low hazard, basaltic eruptions
from fissures and cones on the rift zones,
and the remaining eruptions from phonolitic
lava domes with only localized explosive
activity (e.g., Mña Blanca, ~2000 years ago)
at the basal perimeter of the main stratovol-
Volcanological and Structural Evolution
of Mount Teide
At 3718 meters, Mount Teide volcano is
the third-highest volcanic structure on the
planet and the highest peak in the Atlantic
Ocean. Conversely to the intensely studied
and geologically well-understood Hawaiian
volcanoes, many crucial geological aspects
of Teide volcano were insufficiently addressed
until quite recently. As an example, until 2001,
only a single radiometric age was available
for the most recent activity of the Teide volcanic complex.
A joint Spanish-French project in 2001–2005
produced a detailed geological map of Teide
and its northwest and northeast rifts, and
provided 28 new radiocarbon ages and 26
K/Ar (potassium/argon) ages. This project
completely changed the understanding of
the geological and structural evolution of
the Teide volcanic complex, allowing the
majority of the eruptions to be separated
Fig. 1. Seismicity within the island of Tenerife: (a) From 1 January 1985 to 31 December 1999; values listed are the local ‘Richter’ magnitudes; (b) From 1 January 2000 to 1 April 2005 (modified
from Instituto Geográfico Nacional, Spain); MbLg is the body wave magnitude using the Lg wave
(a surface wave). Note that the MbLg values in (b) are essentially equivalent to the local magnitudes in (a), suggesting that seismic intensity between these two time periods has remained
constant.
Eos, Vol. 87, No. 43, 24 October 2006
cano. The recent eruptive record, combined
with the available petrological and radiometric
data, provides a rather optimistic outlook on
major volcanic hazards related to Teide and
its rift zones, posing only very localized threats
to the one million inhabitants of Tenerife
and the 4.5 million annual visitors to Teide
National Park.
Unrest at Teide Volcano?
The prediction of an explosive eruption in
2004 was based on reports of significantly
increased earthquake activity and volcanic
gas emissions. However, no major eruption
followed and questions remain whether the
available evidence was used in a correct and
sensible way.
From 22 April to 28 July 2004, about 50
low-magnitude (M = 1–3) earthquakes were
recorded in Tenerife, with most of the epicenters localized at the northwest rift zone,
in the area of the Icod Valley. Only three
were felt by residents. Earthquakes of this
type are normal in volcanic oceanic islands
(e.g., Hawaiian Islands, Reunión). In addition,
low-magnitude earthquakes (generally M <
3) have been reported in all of the Canary
Islands (Figure 1). In May 1998, a bigger
earthquake (M = 5.3) hit Tenerife, but television
and newspaper sources notified the public
that the seismic activity was not hazardous,
and the event was promptly forgotten. Prior
to 1998, smaller earthquakes were frequently
recorded throughout the archipelago, but no
volcanic activity took place since the 1971
Teneguía eruption on La Palma island.
In 2004, however, the local and international mass media were bombarded with
persistent reports of unrest at Teide volcano
with little to no mention of more conservative views, and predictions were made of an
imminent, large-scale explosive eruption that
was dubbed ‘El Volcán de Octubre’ (the
‘October Volcano’). The tourist island of
Tenerife was renamed ‘Terrorife’ in the international press [Christie, 2004], and residents
along the northern coast towns began sleeping
fully dressed and panic-buying food and
other household supplies.
Instead of providing clarifying and reassuring press communiqués, the authorities
decided to raise the level of alert on the
basis of a reported increase in seismicity
and the emission of volcanic gases. At no
time was there universally accepted scientific evidence of volcanic activity on the
island, but the ‘volcanic crisis’ alert was officially maintained until February 2005, with
frequent reports in the media of enormous
emissions of volcanic gases, boiling of the
island aquifer, movements of magma underneath the volcano, and impending explosive
eruptions of Teide forecast for October 2004.
Was there any convincing evidence for
this at all? The official institution responsible
for monitoring seismic and volcanic activity
in Spain—the Instituto Geográfico Nacional
(IGN)—improved the seismic network of
Tenerife in 2000. According to the IGN, the
Fig. 2. (a) Long-term variation in CO2 emission in Tenerife [Martín, 1999]. (b) Short-term variation of radon-222 (222Rn) and carbon dioxide (CO2) in a deep borehole in Las Cañadas caldera
[from Soler et al., 2004]. (c) Frequent spectacular ‘plumes’ in the summit area of Teide known
locally as ‘la Toca del Teide,’ or ‘Teide’s headdress’ (Photo by J. C. Carracedo). (d) Model of the
formation of clouds at the summit of Teide volcano by local orographic convergence (see http://
www.acanmet.org.es/menu_archivos/documentos/FTLCTeide.doc).
addition of new seismic stations immediately
led to low-magnitude events within the island
being recorded for the first time since surveillance work began in 1985 (Figure 1).
The focal depths of the majority of these
events were not determined as part of the
routine seismic monitoring, and therefore
one of the most powerful constraints in
locating the source of the seismicity was
lacking. An initial interpretation of the seismicity suggested dyke emplacement at a
depth of three to four kilometers. However,
the majority of the epicenters were located
far from the rift zone, in an area in the Icod
Valley that satellite interferometry has shown
to have subsided by up to 10 centimeters
[Fernández et al., 2003]. Groundwater has
been continuously and intensively extracted
in this area since the 1960s, depleting the aquifer and causing the ground to sink. Microfaulting associated with the subsidence may
well have caused seismicity. It is noteworthy
that according to the interferometry study,
this is the only area of recent ground deformation in Tenerife. The putative eruptive
regions of Teide and Las Cañadas are completely stable.
But if the source of the seismicity is nonvolcanic, what about the reported enormous increase in gas emission? Continuous,
real-time monitoring of gas emissions at
Teide and the rifts [Martín, 1999] provided
evidence of the nearly constant total gas
emission of the volcanic system (Figure 2a).
Significant diurnal and seasonal variations
in gas emission rates are observed [Soler et
al., 2004] and linked to systematic changes
in barometric pressure (Figure 2b). Therefore, if discrete measurements are taken
and reported, apparent ‘significant’
increases in gas emissions can be obtained,
which simply correspond to barometric
pressure changes.
Nevertheless, a recent article [García
et al., 2006] insists on the reawakening of Teide
based on the aforementioned seismicity and
volcanic gas emissions. In support of the prediction of increasing volcanic activity, these
authors have cited two apparently new features: fumaroles at the summit crater of
Teide and a new fumarole inside the Orotava Valley. Spectacular ‘plumes’ in the summit area of Teide (known locally as ‘la Toca
del Teide,’ or ‘Teide’s headdress’) are caused
by strong winds and other atmospheric conditions as well as by increased fumarolic
activity related to barometric pressure changes,
and have frequently been cited over the centuries in ships’ logs (Figures 2c and 2d). The
new ‘fumarole’ of the Orotava Valley, in turn,
which gave a clear meteoric water isotopic
signature, is located 50 meters from an
unlined 40-meter-deep well used for the disposal of high-temperature wastes from a
nearby cheese factory, suggesting a rather
more simple explanation for the origin of
this particular vapor exhalation site [Carracedo et al., 2006].
It would thus seem that the prediction of an
imminent volcanic eruption at Teide was and
is lacking hard scientific evidence, and its dramatic presentation by the media was not only
Eos, Vol. 87, No. 43, 24 October 2006
unnecessarily damaging to the tourism-based
economy of the island industry, but also
caused undue anxiety and hardships for citizens coping with the alarmist forecasts (Note,
none of the authors is in any form associated
with the tourism industry anywhere).
Effective and reliable communications
must be established among scientists, government officials, the news media, and the
populace affected, and must be tried and
tested before an actual crisis. This is an
essential step in restoring the credibility of
the scientists and civil authorities, so that
they, working together, will be much better
positioned to respond adequately to a potential
genuine volcanic crisis at Teide.
Acknowledgments
Insightful reviews by Robert I. Tilling and
Chris Stillman greatly improved this contribution and are gratefully acknowledged.
References
Author Information
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Badiola, A. Hansen, R. Paris, H. Guillou, and S. Scaillet (2006), Análisis de los riesgos geológicos en
el Archipiélago Canario: Origen, características,
probabilidades y tratamiento, Anu. Estud. Atl., 51,
513–574.
Christie, M. (2004), Welcome to Terrorife, Daily Record,
16 June.
Fernández, J., T. T.Yu, G. Rodríguez-Velasco, and
J. González-Matesanz (2003), New geodetic monitoring system in the volcanic island of Tenerife,
Canaries, Spain: Combination of InSAR and GPS
techniques, J.Volcanol. Geotherm. Res., 124, 241–253.
García, A., J.Vila, R. Ortiz, R. Macia, R. Sleeman, J. M.
Marrero, N. Sánchez, M. Tárraga, and M. Correig
(2006), Monitoring the reawakening of Canary
Island´s Teide volcano, Eos Trans. AGU, 87(6), 61, 65.
Martín, M. C. (1999),Variación espacio-temporal del
nivel de emisión de radón en una zona volcánica
activa: Tenerife (Islas Canarias), Ph.D. dissertation,
270 pp., Univ. of La Laguna, Laguna, Spain.
Soler,V., et al. (2004), High CO2 levels in boreholes
at El Teide volcano complex (Tenerife, Canary
Islands): Implications for volcanic activity monitoring, Pure Appl. Geophys., 161, 1519–1532.
Juan Carlos Carracedo, Estación Volcanológica
de Canarias, Consejo Superior de Investigaciones
Científicas (CSIC), La Laguna, Tenerife, Spain;
E-mail: [email protected]; Valentin R. Troll,
Department of Geology, Trinity College, Dublin,
Ireland; Francisco J. Pérez Torrado, Departamento
Física-Geología, Universidad de Las Palmas de
Gran Canaria, Spain; Eduardo Rodríguez Badiola,
Departamento de Geología, Museo Nacional de
Ciencias Naturales, CSIC, Madrid, Spain; Alex Hansen Machín, Departamento de Geografía, Universidad de Las Palmas de Gran Canaria; Raphael Paris,
Maison de la Recherche, Centre Nationale de
Recherche Scientifique (CNRS), Clermont-Ferrand,
France; and Hervé Guillou and Stéphane Scaillet,
Laboratoire des Sciences du Climat et de
l’Environnement, Comision Energie Atomique,
CNRS, Paris, France.
news
Refocusing NASA Planetary Science Funding
PAGE 463
NASA should invest more money in data
analysis for its planetary science missions,
even if it means delaying or canceling a
future mission, members of the science committee of the NASA Advisory Council (NAC)
suggested at a 12 October meeting.
Science committee member Mark Robinson,
director of the Center for Planetary Sciences
at Northwestern University in Chicago, Ill.,
said that large amounts of data from NASA
planetary science missions are accumulating,
and the funds for data analysis often are
inadequate to properly analyze all of it. For
example, there is a large amount of currently
unanalyzed Mars data that could be used in
planning for the Mars Science Laboratory
(expected to launch in 2009), and particularly for site selection. This includes data
from the recently arrived Mars Reconnaissance Orbiter, which is likely to send more
data to Earth in its first year than has been
collected by all previous Mars missions
combined.
Science committee member Alan Stern,
executive director of the Space Science and
Engineering Division of the Southwest Research
Institute in San Antonio, Tex., explained that
missions need to have more research and
analysis (R&A) funds attached to them from
the beginning. As a comparison, in the field
of astrophysics, researchers who are awarded
observing time with the Hubble Space Telescope also are provided funds for analyzing
the data and publishing results. In planetary
missions, the data usually are certified and
archived, but very little of it is properly analyzed and transformed into scientific results,
he said.
“This is really an issue of getting our value
for the dollar. We spend hundreds of millions,
sometimes billions, [of dollars] on these
planetary missions, and very little of the data
is ever looked at,” Stern said.“I think that it is
the sense of the planetary [science] community that skipping a future mission to solve
this problem would be worth it,” he said.
Mary Cleave, associate administrator for
NASA’s Science Mission Directorate, said that
the agency has been trying to maintain an
opportunity for launching a mission to Mars
every 26 months. The agency could skip one
of these launch opportunities and put more
money into R&A, but it needs guidance from
the NAC, she said.
NAC Chair Harrison Schmidtt asked Robinson to draw up a formal recommendation
emphasizing the need for NASA to provide
funds for data analysis for these missions.
The council could consider the recommendation at its next meeting in February 2007.
Another issue that could be addressed by
the NAC in February, following initial discussion at the October meeting, is the future
availability of rockets for small- and mediumsized science missions.
Science committee member Neil DeGrasse
Tyson, director of the Hayden Planetarium at
the American Museum of Natural History in
New York, N.Y., noted that the marketplace
for these rockets has diminished, and that
this could threaten the ability of NASA to
obtain them on time and in needed quantities. He suggested that the NASA administrator should discuss with other federal agencies
the potential for using other types of rockets
as launch vehicles for NASA science missions.
Science Committee Chair Edward David,
Jr., president of EED, Inc., and a former presidential science advisor, said that “it is important that the whole NAC continues to monitor
the situation and make suggestions how to
address the shortage of access.” Schmidtt noted
that there are many other potential launch
systems available and in use by other federal
agencies, particularly the U.S. Department of
Defense.
David also told the NAC that the science
committee needed a member with expertise
in Earth science, citing the departure of former
committee chair Charles Kennel. Schmidtt
said that he hoped such an appointment
would be made by the NASA administrator
within the next few weeks. Kennel was one
of three science committee members who
resigned in August and were replaced the
next month (see Eos 87(40), 2006).
—SARAH ZIELINSKI, Staff Writer
Eos, Vol. 88, No. 28, 10 July 2007
forum
Communication Between Professionals
During Volcanic Emergencies
PAGe 288
Successful communication between scientists, officials, media, and the public is imperative during a volcanic crisis. Misunderstanding
can lead to confusion and distrust, and it ultimately can transform an emergency into a
disaster.
Experience developed during volcanic crises in the Caribbean has helped identify ‘good
practice’ guidelines for communication by scientists during volcanic emergencies (see
“Communication during volcanic emergencies: An operations manual for the Caribbean,
by C. Solana et al., Benfield Greig Haz. Res.
Cent., Univ. Coll. London, 2001; available at
http://www.bghrc.com). Key actions include
the following:
• Ensuring effective communication and
appropriate understanding of information
within and between groups managing the crisis.
• Disseminating only one clear and consistent message—the one established by the officials in charge.
• Building trust among participants and the
public through unanimous support of official
decisions, maintaining the credibility of all
groups and individuals involved in the crisis,
and being responsible and honest with media
by keeping them informed of all facts and any
changes in the situation.
However, during the 2004–2005 seismic crisis
on the Canary Island of Tenerife (see “Monitoring the reawakening of Canary Islands’ Teide
volcano,” by A. García et al., Eos Trans AGU,
87(6), 61, 2006), scientists did not always follow
this protocol, and confusion and occasional
panic within the population ensued (see
“Recent unrest at Canary Islands’ Teide volcano?,” by J. C. Carracedo et al., Eos Trans. AGU,
87(43), 2006). The island is volcanically active,
and the most recent eruption took place in
1909. In 2004, scientists working on the island
interpreted observed changes in geochemistry
and seismicity as being of volcanic origin,
which led to the creation of an official scientific committee to forecast the likely evolution
of the crisis and to assess different possible scenarios.
At the beginning of the crisis, the flow of
information was constrained to the monitoring scientists and civil authorities. When the
general population felt three earthquakes, the
scientific committee deemed it necessary to
communicate the nature of the crisis to the
public. As the media pressed for news, scientists involved in the official committee, and
some who were not involved, volunteered
their views on potential scenarios, including
improbable catastrophic ones that made
headlines. This information contradicted the
official ‘call to calm’ and caused confusion
among the public. Although the head of the
official committee tried to clarify the situation,
the official scientific team was continually criticized by scientists not involved in the management of the crisis, some of whom had
rejected the offer to become a part of the official team.
While the crisis in Tenerife did not result in
an eruption, it is vital to learn from it lessons
about scientific communications and relations,
specifically during volcanic crises. While volcanic areas host many competent experts, during times of crisis responsible scientists should
never volunteer their personal forecasts and
opinions to the media and should support the
decisions taken by the civil authorities and the
official scientific group. Differences of opinion
among scientists are expected, and dialogue
and discussion should be encouraged. However, this should be conducted out of the public eye. Before a crisis, encouraging open dialogue between the parties involved to agree to
a protocol of communication can prevent a
future recurrence of the situation encountered
on Tenerife.
—Carmen Solana and Claire Spiller, School
of Earth and Environmental Sciences, University of
Portsmouth, Portsmouth, UK; E-mail: Carmen.solana@
port.ac.uk
FEATURE
Feature
Seismicity and gas emissions on Tenerife: a
real cause for alarm?
Juan Carlos
Carracedo1 &
Valentin R. Troll2
In recent months the media has been drawing attention to the possibility of
a dangerous eruption of the Teide Volcano on Tenerife in the Canary Islands.
Before accepting this prediction, which may well be detrimental to the
tourist-based economy of the island, it would be wise to examine the
evidence on which it is based.
Teide has long been recognized as a significant
volcano. At 3718 m above sea level, it is the third
highest volcanic structure on the planet (Fig. 1), after
Mauna Loa and Mauna Kea in the Hawaiian Islands,
and early drew the attention of renowned scientists
such as Leopold von Buch and Alexander von
Humboldt. Regrettably this interest was not sustained
into the twentieth century; whilst Hawaii attracted
intense study, geological investigations of Teide’s
history languished. For example, hundreds of
radiocarbon age determinations have allowed for a
precise reconstruction of the Hawaiian volcanic
history, but until 2001 only one single radiometric
age was available for Teide volcano. With no regular
recordings, evidence of historic seismic and volcanic
activity has only been by word of mouth.
The situation changed in 2001 when a five-year,
joint Spanish–French research project began; a
project which produced a detailed geological map of
Teide and the NW and NE rifts (Fig. 2), petrographic
analysis of numerous samples, and radiometric
studies, which produced 28 new radiocarbon ages
and 26 new K/Ar ages.
This
project
completely
changed
our
understanding of the geological and structural
evolution of the Teide volcanic complex, and
identified the sequence of eruptions that had occurred
in the last 10 000 years. For example, one eruption of
historic significance was that of 24 August 1492,
which Christopher Columbus reported as ‘a big fire in
the sierra of Tenerife... similar to those of Mount Etna
in Sicily’. It seems it was not a summit eruption of
Teide; a radiocarbon age from an eruption of the Boca
Cangrejo cinder cone (Crab’s Mouth cone), located in
the NW Rift, is in close agreement with this historic
date. It had been believed that the ‘Columbus
138
Estación Volcanológica de
Canarias, IPNA-CSIC, La
Laguna, Tenerife, Spain,
[email protected];
2
Department of Geology,
Trinity College Dublin,
Dublin 2, Ireland,
[email protected]
1
eruption’ was the final summit eruption of Teide, but
the new radiometric data now shows that eruption to
have been much older, 1140 ± 60 years before
present, i.e. in the eighth century AD.
One of the interesting features this study has
revealed is that the Teide central volcanic complex
was a direct consequence of the activity of the rifts.
The rifts developed a progressively steepening,
unstable volcano at the centre of the island, and
about 180–200 000 years ago triggered a massive
landslide that generated the impressive horseshoeshaped collapse embayment, whose headwall is the
Fig. 1. View of Teide volcano
from the north east.
© Blackwell Publishing Ltd, Geology Today, Vol. 22, No. 4, July–August 2006
FEATURE
Fig. 2. Geological map of Teide
and the caldera, showing in
colour the most recent eruptions
(last 30 000 years).
famous 17 × 10 km Las Cañadas caldera (Fig. 3).
Following this, further rift and central activity
progressively filled the embayment and built up the
3718 m-high Teide stratovolcano, nested in the
collapse embayment (Fig. 4). The main phase of
construction ended 30 000 years ago, since when
the stratovolcano has erupted only once. That
eruption was the eighth century AD summit eruption,
which raised the volcano from a height of 3600 m to
its present 3718 m.
This recent eruptive history is in stark contrast to
the pre-collapse activity of the Cañadas volcano,
which was much more explosive, with frequent
plinian eruptions that generated extensive ash flows
and ash falls. In the last 30 000 years, eruptions
have occurred at a rate of only 4–6 per millennium,
the majority (70 per cent) being low-hazard basaltic
eruptions from fissure and cones on the rift zones,
and the others producing phonolitic lava-domes with
only very localized explosive activity (Fig. 5), as for
example Mña Blanca, around 2000 years old, which
is situated at the foot of the main stratavolcano.
It can be seen from the results of the study that in
the present phase of the island’s volcanic activity
there appears to be little risk of major volcanic
hazards, with any eruptions likely to pose only very
localized threats to the inhabitants of Tenerife or to
visitors to the Teide National Park (about 4.5 million
annually). Why then are there dire predictions of
volcanic catastrophe (Fig. 6)?
Fig. 3. Western view of Las
Cañadas caldera.
© Blackwell Publishing Ltd, Geology Today, Vol. 22, No. 4, July–August 2006
139
FEATURE
Unrest at Teide volcano?
The prediction of an explosive eruption seems to be
based on reports of significantly increased earthquake
activity and volcanic gas emissions. But is this
evidence correct, and does it imply an imminent
violent eruption?
From 22 April to 28 July 2004, about 50 lowmagnitude (1 to 3) earthquakes were recorded in
Tenerife, with most of the epicentres localized on the
NW rift zone in the area of the Icod Valley. Only three
were actually felt by residents. This frequency is
nothing special, such earthquakes are normal in
volcanic oceanic islands; about 10 000 are recorded
annually in Hawaii, of which only 1200 are greater
than magnitude 3. Low magnitude earthquakes
(generally less than magnitude 3) have been recorded
in all the Canary Islands. In May 1998 a bigger
quake (magnitude 5.3) hit Tenerife, but a hazarddisclaiming statement was
published in the media
and the event promptly
forgotten.
So why, in 2004, were
the local and international
mass media bombarded
with persistent reports of
unrest at Teide volcano,
and predictions made of an
imminent,
large-scale
explosive eruption that
became dubbed ‘El Volcán
de
Octubre’
(October
Volcano)? The publicity
had such a bad effect that
Tenerife was nick-named
‘Terrorife’ and residents
Fig. 4. Teide volcano nested in
the Las Cañadas caldera. Image
by NASA.
along the northern coast in the towns of Icod,
Garachico, etc., began sleeping fully dressed and
panic-buying food and other household supplies.
Some even sold their houses! Evacuation instructions
were given in schools and hospitals, and gas masks,
power generators and other supplies were stockpiled
in public buildings.
The situation was made even worse by the
authorities who, instead of clarifying the situation,
raised the level of alert apparently only on the basis of
what was reported as an increase in seismicity and
the emission of volcanic gases. However, at no time
was there any universally accepted scientific
evidence of actual volcanic activity on the island. The
‘volcanic crisis’ alert was officially maintained until
February 2005, with frequent reports in the media of
enormous emissions of volcanic gases, boiling of the
island aquifer, presumably from raised volcanic
temperatures, movements of magma underneath the
volcano and an explosive eruption forecast for
October 2004.
Was there really any convincing evidence for
any of this?
Firstly, consider the seismicity. Prior to 2000 there
were only two seismic stations on Tenerife, both at
the far north-eastern end of the island. In that year a
third station was deployed inside the Caldera de Las
Cañadas, and immediately low-magnitude events
within the island began to be recorded for the first
time since surveillance began in 1985. Unfortunately
the focal depths of these events were not determined,
thus depriving observers of one of the most powerful
constraints on locating the source of the seismicity.
In the initial reports, the activity was interpreted as
being due to dyke emplacement at a depth of 3–4 km.
However, it is possible that a number of the events
may have had a non-volcanic origin, such as traffic
or explosions made during the excavation of water
tunnels for groundwater mining.
The majority of the epicentres were located far
from the rift zone, in an area in the Icod Valley which
satellite interferometry has shown to have subsided
by up to 10 cm. Groundwater has been continuously
and intensively extracted in this area since the
1960s, depleting the aquifer and causing the ground
to sink. Microfaulting associated with the subsidence
may well have caused seismicity. It is noteworthy
that according to the interferometry study this is the
only area of recent ground deformation in Tenerife.
The putative eruptive region of Teide and Las
Cañadas is completely stable.
Fig. 5. Geological map of Teide’s peripheral phonolitic domes.
140
© Blackwell Publishing Ltd, Geology Today, Vol. 22, No. 4, July–August 2006
FEATURE
What is the scientific evidence for the
reported increase in gas emissions?
Continuous real-time monitoring of gas emissions at
Teide and in the rifts, reported in 1999, gave
evidence of a nearly constant total gas emission from
the volcanic system. On a day-to-day basis, local
emissions may vary and significant diurnal and
seasonal variations in gas emission rates do
apparently relate directly to systematic changes in
barometric pressure. If spot measurements are taken
in periods of low barometric pressure, they can show
an apparent ‘significant’ increase in emissions. In
2004, shortly before the predicted October eruption,
a visit to the summit crater of Teide, (in which both of
us took part), did not reveal any fumarolic activity at
all.
In support of the prediction of increasing volcanic
activity, observers have cited two apparently new
features; fumaroles at the summit crater of Teide and
a new fumarole inside the Orotrava Valley. In fact
spectacular ‘plumes’ are commonly seen in the
summit area of Teide and are known locally as ‘la
Toca del Teide’ or ‘Teide’s head-dress’ (Fig. 7). These
are caused by atmospheric conditions as well as by
increases in fumarolic activity related to barometric
pressure changes and have frequently been cited over
the centuries in ship’s logs. They are in no way
Fig. 6. ‘Scientists’ predictions’
in the local press.
unusual for the volcano. The new ‘fumarole’ in the
Orotava Valley probably has a quite different
explanation. Its water isotope signature is clearly
phreatic, not volcanic, and it is located 50 m from an
unlined 40 m-deep well used for the disposal of hightemperature wastes from a nearby cheese factory.
It would thus seem that the prediction of an
imminent volcanic eruption at Teide is based on
distinctly flimsy and dubious evidence, and its
dramatic presentation by the media is not only
unnecessarily damaging to the tourism-based
economy of the island, but also to the credibility of
responsible scientists, the true value of which will be
crucial for the correct management of any future
genuine volcanic crisis.
Suggestions for further reading
Carracedo, J.C. 1994. The Canary Islands: an
example of structural control on the growth of
large oceanic island volcanoes. Journal of
Volcanological and Geothermal Research, v.60,
pp.225–242.
Carracedo, J.C. 1999. Growth, structure, instability
and collapse of Canarian volcanoes and
comparisons with Hawaiian volcanoes Journal of
Volcanological and Geothermal Research, Special
Issue, v.94, pp.1–19.
Carracedo, J.C. & Day, S.J. 2002. Geological Guide of
the Canary Islands. Classic Geology in Europe. Terra
Publishing, London.
Guillou, H., Carracedo, J.C., Paris, R. & Pérez
Torrado, F.J. 2004. Implications for the early
shield-stage evolution of Tenerife from K/Ar ages
and magnetic stratigraphy. Earth and Planetary
Science Letters, v.222, pp.599–614.
Walter T.R., Troll V.R., Caileau B., Schmincke H.-U.,
Amelung F. & van den Bogaard, P. 2005. Rift
zone reorganization through flank instability on
ocean islands – an example from Tenerife, Canary
Islands. Bulletin of Volcanology, v.65, pp.281–291.
Fig. 7. A. Frequent spectacular
‘plumes’ in the summit area are
locally known as ‘Teide’s headdress’ and are usually clouds
caused by strong winds and
pressure-dependant changes in
fumarole activity and are not
unusual for the volcano (picture
by J.C. Carracedo). B. Model of
cloud formation at Teide
volcano (Alvarez and Hernandez
2006, Canary Meteorological
Service, see http://
www.acanmet.org.es/
menu_archivos/documentos/
FTLCTeide.doc).
© Blackwell Publishing Ltd, Geology Today, Vol. 22, No. 4, July–August 2006
141
Click
Here
GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L20311, doi:10.1029/2006GL027523, 2006
for
Full
Article
New evidence for the reawakening of Teide volcano
J. Gottsmann,1,2 L. Wooller,3 J. Martı́,1 J. Fernández,4 A. G. Camacho,4 P. J. Gonzalez,4,5
A. Garcia,6 and H. Rymer3
Received 17 July 2006; revised 13 September 2006; accepted 19 September 2006; published 25 October 2006.
[1] Geophysical signals accompanying the reactivation of
a volcano after a period of quiescence must be evaluated as
potential precursors to impending eruption. Here we report
on the reactivation of the central volcanic complex of
Tenerife, Spain, in spring 2004 and present gravity change
maps constructed by time-lapse microgravity measurements
taken between May 2004 and July 2005. The gravity
changes indicate that the recent reactivation after almost a
century of inactivity was accompanied by a sub-surface
mass addition, yet we did not detect widespread surface
deformation. We find that the causative source was evolving
in space and time and infer fluid migration at depth as
the most likely cause for mass increase. Our results
demonstrate that, even in the absence of previous baseline
data and ground deformation, microgravity measurements
early in developing crises provide crucial insight into the
dynamic changes beneath a volcano. Citation: Gottsmann, J.,
L. Wooller, J. Martı́, J. Fernández, A. G. Camacho, P. J. Gonzalez,
A. Garcia, and H. Rymer (2006), New evidence for the
reawakening of Teide volcano, Geophys. Res. Lett., 33, L20311,
doi:10.1029/2006GL027523.
1. Introduction
[2] Anomalous geophysical signals at dormant volcanoes, or those undergoing a period of quiescence, need to
be evaluated as potential precursors to reawakening and
possible eruption [White, 1996]. There are several recent
examples of volcanic re-activation after long repose intervals culminating in explosive eruption [Nakada and Fujii,
1993; Robertson et al., 2000], but non-eruptive behaviour is
equally documented [De Natale et al., 1991; Newhall and
Dzurisin, 1988]. The dilemma scientists are confronted with
is how to assess future behaviour and to forecast the
likelihood of an eruption at a reawakening volcano, when
critical geophysical data from previous activity is missing
due to long repose periods. In Spring 2004, almost a century
after the last eruption on the island, a significant increase in
the number of seismic events located inland on the volcanic
island of Tenerife (Figure 1) marked the reawakening of the
1
Institute of Earth Sciences ‘‘Jaume Almera’’, Consejo Superior de
Investigaciones Cientificas, Barcelona, Spain.
2
Department of Earth Sciences, University of Bristol, Bristol, UK.
3
Department of Earth Sciences, Open University, Milton Keynes,
UK.
4
Institute of Astronomy and Geodesy, Consejo Superior de Investigaciones Cientificas, Universidad Complutense de Madrid, Madrid, Spain.
5
Environmental Research Division, Institute of Technology and
Renewable Energies, Tenerife, Spain.
6
Department of Volcanology, Museo Nacional de Ciencias Naturales,
Consejo Superior de Investigaciones Cientificas, Madrid, Spain.
Copyright 2006 by the American Geophysical Union.
0094-8276/06/2006GL027523$05.00
central volcanic complex (CVC), the third-highest volcanic
complex on Earth rising almost 7000 m from the surrounding seafloor [Garcı́a et al., 2006]. The increase in onshore
seismicity, including five felt earthquakes, coincided with
both an increase in diffuse emission of carbon dioxide along
a zone known as the Santiago Rift [Pérez et al., 2005] and
increased fumarolic activity at the summit of the 3718 m
high Teide volcano [Garcı́a et al., 2006].
2. Integrated Geodetic Network on Tenerife
[3] As a reaction to the developing crises, we installed
the first joint ground deformation/microgravity network on
the island in early May 2004, two weeks after the start of
increased seismicity. The network consists of 14 benchmarks, which were positioned to provide coverage of a
rather large area (>500 km2) of the CVC, including the Pico
Viejo-Pico Teide complex (PV-PT), the Las Cañadas caldera
(LCC) as well as the Santiago Rift (SR) (Figures 1 and 2).
The network was designed to meet rapid response requirements, i.e., the network can be fully occupied to a precision
of less than 0.01 mGals of individual gravity readings and
less than 0.04 m in positioning errors within six working
days despite the frequently rugged terrain. The first reoccupation of the network was performed in July 2004,
followed by campaigns in April 2005 and July 2005.
Benchmark locations and cumulative ground deformation
and gravity changes between May 2004 and July 2005 are
given in Table 1 in the auxiliary material.1 All results are
given with respect to a reference located south of the LCC
(benchmark LAJA). Within the average precision of benchmark elevation measurements (±0.03 m), using two dualfrequency GPS receivers during each campaign, we did not
observe widespread ground deformation. However, between
May 2004 and July 2005, four benchmarks, two located in
the eastern sector of the LCC (MAJU and RAJA), one
marking the northern-most end of the network and also the
lowest elevation (766 m; CLV1) and finally a benchmark
located on an isolated rock spur on the western LCC rim
(UCAN, supporting online material) did show ground uplift
above measurement precision. Residual gravity changes
(corrected for the theoretical Free-Air effect), observed
during the May– July 2004, May 2004 – April 2005, and
May 2004– July 2005 periods are listed in the supporting
online material and shown in Figure 2.
3. Results
[4] The observed gravity changes do not fit a simple
symmetrical pattern as observed, for example, during cal1
Auxiliary materials are available at ftp://ftp.agu.org/apend/gl/
2006gl027523.
L20311
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GOTTSMANN ET AL.: NEW EVIDENCE FOR REAWAKENING TEIDE VOLCANO
L20311
July 2005 (Figure 2c). During the same time, a marked
gravity decrease was recorded at the intersection of the
Orotava Valley (OV) and the LCC (Figure 2c).
[5] Except for two benchmarks (MAJU and RAJA)
where observed gravity changes can be explained by freeair effects (gravity changes due to elevation changes), mass/
density changes in the sub-surface appear to cause the major
part of the perturbation of the gravity field.
4. Effect of Water Table Fluctuations
Figure 1. Perspective view of Tenerife island located in
the Canarian Archipelago off the coast of northwest Africa
(inset), using a colour-coded digital elevation model
(DEM; elevation in meters). Highest point is Teide volcano
(3718 m a.s.l.) located at 28.27°N and 16.60°W. Black
open squares indicate epicentres of seismic events recorded
between May 2004 and July 2005 by the National
Geographic Institute (available at http://www.ign.es).
Black rectangle identifies the area covered by the joint
GPS/gravity network. LCC indicates the location of the
Las Cañadas caldera.
dera unrest at the Campi Flegrei [Gottsmann et al., 2003] or
at Long Valley [Battaglia et al., 2003]. The spatial distribution of gravity changes across the area under investigation is asymmetrical. The smallest gravity changes were
observed in the central and eastern depression of the LCC,
where cumulative changes over the 14-month period where
only slightly higher than the precision level (±0.015 mGal
on average; 1 mGal = 10 mm/s2). A marked positive gravity
anomaly, with a maximum amplitude of around 0.04 mGal,
developed in the Northwest of the covered area between
May and July 2004, while a negative anomaly was found to
the east, centered on station MIRA. The gravity increase
noted between the first two campaigns (benchmarks C774
and CLV1) was followed by a decrease sometime between
July 2004 and April 2005. During the same period, a N-S
trending positive anomaly appears northwest of the PV-PT
summit area between, reaching the western part of the LCC
(Figures 2a – 2b). In addition, gravity increased significantly
along the northern slopes of Pico Teide, including benchmarks TORR and FUEN located close to the La Orotava
valley between July 2004 and April 2005, adding to the
impression of a spatio-temporal evolution of the causative
source. It is interesting to note that on 5 December 2004 a
new fissure with fumarole emission appeared in the Orotava
valley (further information available at http://www.iter.es).
A gas plume emanating from the summit fumaroles of Pico
Teide was particularly noticeable during October 2004
[Garcı́a et al., 2006], between surveys 2 and 3. In summary,
significant gravity changes occurred mainly across the
northern flanks of the PV-PT and along a ca. 6 km wide
zone along the western side of the volcanic complex into
the westernmost parts of the LCC between May 2004 and
[6] Data from two drill holes, located in the eastern half
of the LCC (Figure 2d), provide information on water
table fluctuations during the period of interest. A drop of
ca. 5 cm/month between surveys 1 and 4 was recorded in
one drill hole located close to benchmarks 3RDB and
MAJU, which is similar to the average monthly drop in
water level due anthropogenic extraction over the past
3 years [Farrujia et al., 2004]. Water levels decreased by
22 cm/month on average between February 2000 and
January 2004 in a drill hole located close to benchmark
MIRA. The gravity decrease of 0.025 mGal recorded
between May 2004 and July 2005 at benchmark MIRA,
located at the intersection of the Las Cañadas caldera and
the Orotava valley, can be explained by a net water
table decrease (dh) of 3 m, consistent with this earlier
trend, assuming a permeable rock void space (8) of 20%
and a water density (r) of 1000 kg/m3 (Dgw = 2pGrfdh)
[Battaglia et al., 2003]. Following the same rationale,
gravity changes at 3RDB and MAJU are corrected by
0.008 mGal to account for the recorded water table fall
in the nearby borehole. Hence, any gravity change observed
within the central and eastern parts of the LCC (3RDB,
MAJU, RAJA, MIRA) can be fully attributed to changes in
(shallow) groundwater levels and we treat the net mass
change as zero for this area in the computation of overall
mass changes in the following sections (Figure 2d).
[7] Outside the LCC, comprehensive monitoring data on
groundwater level is lacking and correction for groundwater
level variations is difficult. Groundwater is collected and
extracted along several hundred (sub)horizontal tunnels
(galerias) protruding into the upper slopes of the CVC
[Marrero et al., 2005]. Since 1925, a decrease of several
hundred meters in the groundwater level has been noted for
the area covered by the northern and western slopes of the
CVC (available at http://www.aguastenerife.org). We therefore consider it very unlikely that the gravity increase noted
in the north and west of the CVC is related to an increase in
the groundwater table, and hence infer deeper processes to
be the most probable cause of gravity change in this region.
5. Interpretation
[8] The coincidence of earthquake epicenter concentration (a mixture of volcano-tectonic events and regional
earthquakes with pure volcanic events such as tremors
and long-period signals) in the area of gravity increase over
the same time period (Figure 2d), suggests that both signals
are related to the same or linked phenomena. Unfortunately,
precise data on earthquake hypocentres are not available,
but a semi-qualitative analysis suggests a depth of several
kilometres (R. Ortiz, personal communication, 2005). The
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Figure 2. Residual gravity changes between (a) May and July 2004, (b) May 2004 and April 2005, and (c) May 2004 and
July 2005. (d) Same as Figure 2c but corrected for the effect of water table changes. Gravity changes are draped over a
DEM of the central volcanic complex (CVC) of Tenerife. Black line in Figure 2a delineates the Las Cañadas caldera (LCC)
wall. Benchmark locations (crosses) and identification are shown as well as the prominent topographic features of the
Santiago Rift, Teide volcano, and the Orotava Valley (OV). Uncertainty in gravity changes are on average ±0.015 mGal
(1 mGal = 10 mm/s2). In Figure 2c the area to the east of the CVC, where a gravity decrease was detected, coincides with the
intersection of the Las Cañadas caldera with the collapse scar of the Orotava valley. This zone represents a major
hydrological outlet of the caldera. In Figure 2d stars represent epicentres of seismic events recorded between May 2004 and
July 2005. Both gravity increase and seismicity appear to be spatially and temporally correlated. Line A-B represents datum
for profile shown in Figure 4.
spatial coverage of the benchmarks does not allow the
wavelength of the May 2004 – July 2005 gravity anomaly
to be assessed precisely. In particular, the lower limit of the
wavelength along the northern slopes of the PV-PT complex
cannot be unambiguously retrieved on the basis of the
available data. The maximum wavelength of the gravity
anomaly is on the order of 17 km if defined by both
observed and interpolated (kriging) data (Figure 2d) on
the northern slopes of the PV-PT complex, which implies
a maximum source depth of between 2.5 to 5.2 km below
the surface, assuming simple axisymmetrical source geometries [Telford et al., 1990]. This would place the source to
within the depth of the shallow magma reservoirs beneath
the PV-PT complex believed to host chemically evolved
magma [Ablay et al., 1998]. However, since the positive
anomaly is only defined by four benchmarks (CLV1, C774,
CRUC, and TORR) its actual wavelength could be smaller
than 17 km and the source depth could be shallower than
inferred above. Furthermore, ambiguities remain on the
actual amplitude of the anomaly, which is defined only by
data observed at CRUC. The continuation of the positive
anomaly in the western part of the LCC (Figure 2c) shows a
shorter wavelength indicating a shallow (few km deep)
source.
[9] Due to the spatial separation of benchmarks an
assessment of sub-surface mass addition is greatly biased
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Figure 3. (a) Predicted and (b) residuals between observed and predicted gravity changes (mGal) for the period May 2004
to July 2005. Predicted values are derived from inversion for an infinite horizontal cylinder as an approximation of the zone
undergoing a mass/density increase at the northern and western slopes of the PV-PT complex. Observed gravity changes
were corrected for the effect of water table fluctuations in the central and eastern part of the LCC prior to inversion. Red
colours indicate that the model is predicting higher gravity changes than observed, blue colours indicate the opposite. Green
colours indicate match between predictions and observations.
on the selection of the area affected by gravity increases. We
define a maximum area by a kriging-based interpolation of
the gravity changes between May 2004 and July 2005 in the
northern and western parts of the CVC. A Gaussian Quadrature integration over this area gives a mass addition of
1.1*1011 kg, with lower and upper 95% confidence bounds
of 8.4*109 kg and 2.0*1011 kg, respectively. These values
should be regarded as maximum values.
[10] In theory, subsurface volume changes derived from
ground deformation data can be correlated to sub-surface
mass changes from gravity data to infer the density of the
causative source. However, in the absence of significant
surface deformation, the source density cannot be determined directly and the nature of the source remains ambiguous. However, three scenarios are worth considering when
assessing causative processes for the observed gravity
increase: (1) arrival of new magma at depth, (2) migration
of hydrothermal fluids, and (3) a hybrid of both. Volcanic
eruptions of the CVC over the past few centuries were
dominantly fed by basic and intermediate magmas in the
form of fissure eruptions along the Santiago Rift [Ablay and
Marti, 2000], implying shallow dyke emplacement along
this NW-SE trending extension zone. The observed gravity
increase between May 2004 and April 2005 (Figure 2)
appears to denote a zone at a 45° angle to the strike of
the rift. The wavelength of the anomaly in the western and
central parts of the LCC (Figure 2d) is not consistent
with shallow dyke emplacement to perhaps within a few
tens or hundred meters depth. There is also no other
direct geophysical or geochemical evidence in support of
magma emplacement in the form of a shallow dyke over the
14-month observation time. However, dyke emplacement at
greater depth (a few km below the surface) into the Santiago
Rift (with partial contribution to the gravity increases at
benchmarks CLAV1, C774 and CRUC), perhaps recharging
an existing reservoir, cannot be unambiguously excluded
for the period May– July 2004, coinciding with the peak in
the number of earthquakes recorded by the National Geographic Institute (available at http://www.ign.es). Dykes
along the Santiago Rift are on average less than 1 m wide.
Ground deformation caused by an individual dyke of this
size a few km below the surface would be below the
precision of our GPS measurements. Thus, a magma injection into a conjugated fault system, perhaps at some angle to
the Santiago Rift, cannot be unambiguously ruled out as
the trigger for the reawakening of the volcanic complex in
May 2004. There is, however, little evidence to support the
idea that the mass increase observed during campaigns 2
and 3 is caused solely by magma movement.
[11] An alternative explanation for the observed gravity
increase is fluid migration through the CVC. Volcanotectonic events detected in the seismic record [Garcı́a et
al., 2006; Tárraga et al., 2006] may have triggered the
release and upward migration of hydrothermal fluids from a
deep magma reservoir. Alternatively, fluid migration may
have resulted from (1) the perturbation of an existing deep
hydrothermal reservoir and resultant upward movement of
fluids due to magma injection or (2) from pressurising
seawater saturated rocks.
[12] Migration of hydrothermal fluids through a permeable medium causes little surface deformation, but the
filling of pore space increases the bulk density of the
material resulting in a gravity increase at the ground surface.
To explore this scenario, and as a first order approximation,
we performed a inversion of the gravity change recorded
between May 2004 and July 2005 along the northern and
western slopes of the PV-PT complex for a source represented by a N-S striking infinite cylindrical horizontal body
[Telford et al., 1990]. The approximation of an infinite body
is valid as long as the radius of the cylinder is far smaller
than its length. The model results depend linearly on density
change but non-linearly on both the radius and depth of the
body. Using a global optimization iterative method [Sen and
Stoffa, 1995] with various initial values for depth and
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Figure 4. Cross-section through the CVC along the profile
A-B shown in Figure 2d, including a conceptual model of
events between May 2004 and July 2005. (1) Likely
injection of magma during peak of onshore seismic activity
two weeks before installation of network (May 2004).
(2) Release of fluids or perturbation of existing hydrothermal system causing migration of fluids from NW to SE
(May – July 2004; Figure 2a) and later (July 2004 – April
2005 and further into July 2005) along a N-S striking zone
(Figures 2b– 2d). (3) Upward migration of fluids along the
upper surface of the high density/low permeability Boca
Tauce magmatic body situated beneath the western caldera
[Ablay and Kearey, 2000]. (4) Fluid migration into an
overlying aquifer, located at a depth greater than 900 m
beneath the LCC floor and thought to feed the PT summit
fumaroles [Araña et al., 2000], can explain the increased
fumarolic activity of Teide in 2004. The western caldera
boundary fault may act as a pathway for fluids to shallower
depth.
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fluids inside the complex triggering the observed gravity
changes.
[14] We demonstrate that time-lapse microgravity monitoring of active volcanoes can provide vital insights into
their sub-surface dynamics, particularly where structural
complexities and heterogeneous mechanical properties of
the subsurface do not obey a simple linearly elastic relationship of stress generation and resultant ground deformation [Dvorak and Dzurisin, 1997]. Arrival of a small batch
of magma at depth and the release and upward migration of
hot fluids may be a common trigger of reactivation after
long repose periods and may be quantifiable by perturbations in the gravity field but may not be accompanied by
ground deformation. Quantification of sub-surface mass/
density changes must be regarded as essential for the
detection of potential pre-eruptive signals at reawakening
volcanoes before ground deformation or other geophysical
signals become quantifiable [Rymer, 1994].
[15] Acknowledgments. This research is part of the TEGETEIDE
project funded by the Spanish Ministry of Education and Science (MEC)
under contract CGL2003-21643-E. J.G. also acknowledges support from a
‘‘Ramon y Cajal’’ grant by the MEC and a Royal Society University
Fellowship. L.W. acknowledges the support of an Open University Research Development Fund fellowship. J.F., A.G.C., and P.J.G. also acknowledge support from EU projects ALERTA (MAC/2.3/C56) of the
INTERREG III B Program and from Spanish MEC projects REN200212406-E/RIES and GEOMOD (CGL2005-05500-C02). The authors thank
the Consejo Insular de Aguas de Tenerife (CIATFE) and I. Farrujia for
granting access to water table data on Tenerife and N. Perez for providing
cGPS data.
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radius, we find convergence of the inversion results at a
depth of 1990 ± 120 m below the surface using residual
gravity data from all benchmarks. While depth is insensitive
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which is fully permeable, filling this void space with
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radius is around 80 ± 20 m. Although the fit to the data is
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anomaly in the eastern part of the LCC cannot be satisfyingly modelled. For this area, we conclude on either a local
effect or, more likely, an error in the GPS measurements
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geophysical information, we conclude that migration of
hydrothermal fluids along a permeable N-S striking zone
is the most likely cause of the observed perturbation of the
gravity field. A conceptual model of mass migration covering the 14-month observation period is shown in Figure 4.
6. Conclusions
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