Journal of Volcanology and Geothermal Research 117 (2002) 129^153
www.elsevier.com/locate/jvolgeores
Emplacement of pyroclastic £ows during
the 1998^1999 eruption of Volca¤n de Colima, Me¤xico
R. Saucedo a; , J.L. Mac|¤as a , M.I. Bursik b , J.C. Mora a , J.C. Gavilanes c ,
A. Cortes c
a
c
Instituto de Geof|¤sica, UNAM, Coyoaca¤n, 04510 Me¤xico, D.F., Mexico
b
Geology Department, SUNY at Bu¡alo, Bu¡alo, NY 14260, USA
Observatorio Vulcanolo¤gico, Universidad de Colima, Colima 28000, Col., Mexico
Received 30 December 2000; accepted 24 August 2001
Abstract
After three years of quiescence, Volca¤n de Colima reawakened with increasing seismic and rock fall activity that
reached its peak on November 20, 1998, when a new lava dome forced its way to the volcano’s summit. The new lava
rapidly reached the S^SW edge of the summit area, beginning the generation of Merapi-type pyroclastic flows that
traveled down La Lumbre, and the El Cordoban Western and Eastern ravines, reaching distances of 3, 4.5, and 3 km,
respectively. On December 1, 1998, the lava flow split into three fronts that in early 1999 had reached 2.8, 3.1, and 2.5
km in length, advancing down the El Cordoban ravines. The lava flow fronts disaggregated into blocks forming
pyroclastic flows. One of the best examples occurred on December 10, 1998. As the lava flow ceased moving in early
1999, activity became more explosive. Strong blasts were recorded on February 10, May 10, and July 17, 1999. The
last event developed a 10-km-high eruptive column from which a pyroclastic flow developed from the base, traveling
3.3 km SW from the summit into the San Antonio^Montegrande ravines. Regardless of the mechanism of pyroclasticflow generation, each flow immediately segregated into a basal avalanche that moved as a granular flow and an upper
ash cloud in which particles were sustained in turbulent suspension. When the basal avalanche lost velocity and
eventually stopped, the upper ash cloud continued to move independently as a dilute pyroclastic flow that produced a
massive pyroclastic-flow deposit and an upper dune-bedded surge deposit. The dilute pyroclastic flow scorched and
toppled maguey plants and trees, and sandblasted vegetation in the direction of the flow. At the end of the dilute
pyroclastic-flow path, the suspended particles lifted off in a cloud from which a terminal ash fall was deposited. The
basal avalanche emplaced block-and-ash flow deposits (up to 8 m thick) that filled the main ravines and consisted of
several flow units. Each flow unit was massive, monolithologic, matrix-supported, and had a clast-supported steep
front (ca. 1.5 to 2 m thick) composed of boulders up to 1.7 m in diameter. The juvenile lithic clasts had an average
density of 1800 kg/m3 . The dilute pyroclastic flow emplaced overbank deposits, found on valley margins or beyond
the tip of block-and-ash flow deposits. They consist from bottom to top of a massive medium to coarse sand-size flow
layer (2^4 cm thick), a dune-bedded surge layer (2^10 cm thick), and a massive silt-size layer (0.5 cm thick). The total
estimated volume of the pyroclastic-flow deposits produced during the 1998^1999 eruption is 24U105 m3 . B 2002
Elsevier Science B.V. All rights reserved.
* Corresponding author. Present address: Instituto de Geolog|¤a/Fac. de Ingenier|¤a UASLP Av. Manuel Nava # 5, Zona Universitaria 78240 San Luis Potos|¤, S.L.P., Mexico. Tel.: +5 (444) 817-1039; Fax: +5 (444) 811-1741.
E-mail address: [email protected] (R. Saucedo).
0377-0273 / 02 / $ ^ see front matter B 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 7 - 0 2 7 3 ( 0 2 ) 0 0 2 4 1 - X
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Keywords: Merapi and Soufriere type block-and-ash £ows; detached dilute pyroclastic £ow; Colima; Mexico
1. Introduction
Volcanic eruptions produced by the disruption
of an andesitic to dacitic dome have been some of
the most common events observed and studied by
volcanologists during the last century. The best
known examples worldwide were the eruptions
of Soufriere Hills, Montserrat, in 1995^2001
(Cole et al., 1998; Calder et al., 1999); Unzen,
Japan, in 1991^1995 (Yamamoto et al., 1993; Fujii and Nakada, 1999; Miyabuchi, 1999; Takahashi and Tsujimoto, 2000); Colima, Mexico, in
1991 and 1998^1999 (Rodr|¤guez-Elizarrara¤s et
al., 1991; Saucedo, 2001); La Soufriere de Guadaloupe, Martinique, in 1976 (Sheridan, 1980);
Merapi, Indonesia, in 1984 (Boudon et al.,
1993); Santa Mar|¤a, Guatemala, in 1906 (Rose,
1972a); Mt. Pele¤e, Martinique, in 1902 (Lacroix,
1904). The main trigger for the generation of this
type of event is the intrusion of a new batch
of magma under a volcanic edi¢ce previously
blocked by an andesitic-dacitic central dome. According to Sato et al. (1992), disruption of the
new lava to produce pyroclastic materials is a
function of the relationship between the excess
pore pressure and the tensile strength of the
lava. Minor amounts of erupted magma can destabilize the central lava dome, causing its partial
collapse and forming hot, dense basal avalanches
and an upper light ash cloud (Merapi-type), both
parts moving downslope due to gravity until they
lose energy or lift o¡ due to buoyancy. If larger
amounts of magma are intruded beneath the summit dome, more powerful volcanic events can be
produced either by explosions directed to a speci¢c sector of the volcano (Pele¤an-type events) or
by explosions a¡ecting the entire central dome,
forming radially moving pyroclastic £ows (Sato
et al., 1992).
Independently of the magnitude of these three
types of events, the pyroclastic £ows produced are
very similar in nature. The pyroclastic £ows tend
to quickly separate into a dense basal avalanche
of hot debris (forming a block-and-ash £ow
deposit; Cas and Wright, 1987; Druitt, 1998)
and an overriding dilute ash cloud (Druitt, 1998;
Freundt and Bursik, 1998; Mellors et al., 1988;
Denlinger, 1987; Valentine, 1987; Fisher and
Heiken, 1982), producing two di¡erent £ow regimes (Fink and Kie¡er, 1993). During single pyroclastic-£ow events, the deposits left by the basal
avalanche and the upper ash cloud are both preserved, but during multiple £ow events the upper
ash cloud deposit may be eroded by subsequent
£ows (Fujii and Nakada, 1999). Therefore, to
understand the nature of the transport and emplacement mechanisms of these £ows, it is necessary to study pyroclastic deposits emplaced by
single well-observed events.
The generation of pyroclastic £ows at Volca¤n
de Colima has been the focus of several studies
during the last century, beginning with the pioneering work of Waitz (1915) following the 1913
eruption. In 1975, after 62 years, Volca¤n de Colima produced pyroclastic £ows during lava
emplacement, ¢rst described by Thorpe et al.
(1977). Rodr|¤guez-Elizarrara¤s (1995) in his geological study presented a preliminary distribution
of pyroclastic-£ow deposits around the volcano.
Sheridan and Mac|¤as (1995) presented a hazard
map for several types of pyroclastic £ows based
on probabilities derived from observations and
data from these past events. However, modern
studies of pyroclastic £ows began after the April
16, 1991, eruption of Volca¤n de Colima documented by Rodr|¤guez-Elizarrara¤s et al. (1991)
and extensively studied by Saucedo (2001).
The 1998^1999 eruption of Volca¤n de Colima
presented an excellent opportunity to study andesitic pyroclastic £ows, because excellent video
footage, photographic material, and ¢eld observations were available. The main goal of this paper
is to reconcile the stratigraphic and depositional
features found in the 1998^1999 pyroclastic-£ow
deposits with the documented observational information for each event. These data provide a
strong basis for reconsideration of the generation,
transport, and sedimentation models for pyroclastic £ows worldwide. We also demonstrate that
the facies relationships found in the Colima
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131
Fig. 1. (a) Location of Volca¤n de Colima at the western end of the Trans Mexican Volcanic Belt, abbreviations are: Ce = Ceboruco, CVC = Colima Volcanic Complex, Ta = Tanc|¤taro, Pa = Paricut|¤n, Jo = Jocotitla¤n, NT = Nevado de Toluca, MC = Mexico
City, Iz = Iztacc|¤huatl, Po = Popocate¤petl, Ma = Malinche, CP = Cofre de Perote, PO = Pico de Orizaba, and TVF = Tuxtlas Volcanic Field. (b) The inset shows the location of Volca¤n de Colima in the Colima Volcanic Complex, the dotted lines represent
main rivers.
deposits correlate with models presented elsewhere.
In this paper we use the term pyroclastic £ow to
refer to an actively moving £ow. Because the pyroclastic £ows observed at Volca¤n de Colima and
elsewhere rapidly segregated into a basal dense
part and an upper turbulent part, we refer to
the ¢rst as the basal avalanche and to the second
as the ash cloud. We refer to the deposit left by
the basal avalanche as a block-and-ash £ow deposit.
2. Summary of the 1998^2000 eruption
After three years of inactivity, following an explosive blast on July 21, 1994, Volca¤n de Colima
(19‡31PN; 103‡37PW; 3860 masl) started to show
signs of increasing seismic unrest during the
months of March, June, and November, 1997
(Fig. 1). In November, 1997, a N^NW fracture
system was ¢rst recognized at the summit dome
(Corte¤s and Gavilanes, 1998). On July 6, 1998,
RESCO (the seismic network for Colima State)
reported a seismic swarm that ended with a small
explosion at the summit (Smithsonian Institution,
1998a). Taran et al. (2000) and Taran et al. (2002this volume) reported consistent geochemical variations indicating an upcoming eruption: speci¢cally, increases of deuterium and HCO3
3 in high
temperature summit fumaroles 18 months prior to
the event, and an increase in boron in all springs
just three months before the eruption. COSPEC
measurements showed undetectable amounts of
SO2 until October 30, 1998, when a ¢gure of
408 t/day was recorded (Smithsonian Institution,
1998c). On November 18, the COSPEC crew reported 1610 t/day, followed on November 19 by
harmonic tremor and associated with a number of
rock falls (Smithsonian Institution, 1998c). On
November 20, 1998, a new dome had already
formed in the SW part of the Colima summit. It
was 30U50 m in diameter and 15 m high with a
volume of 3.8U105 m3 (Smithsonian Institution,
1998b; Navarro et al., 2002-this volume) (Fig.
2a).
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Fig. 2. (a) View of the dark gray lava dome on top of Volca¤n de Colima on November 22, 1998, and intense fumarolic activity
around it, (b) Advancing lava £ow on steep slopes of the volcano with small blocks collapsing at the front on November 22,
1998. Photographs by A. Corte¤s.
On November 21, 1998, a lava £ow started to
move down the SSW side of the volcano. Blocks
collapsing from the £ow margins produced the
¢rst Merapi-type pyroclastic £ows (Fig. 2b). On
November 22, the lava £ow was 150 m long and
three days later it was 370 m long, advancing
toward the western, central, and eastern branches
of El Cordoban ravine. The following day col-
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133
lapsing parts of the lava £ow front generated
large pyroclastic £ows down these ravines (Fig.
3). The largest pyroclastic £ow followed El Cordoban West ravine, reaching a maximum distance
of 4.5 km (H/L = 0.42) (Fig. 4). A shower of ¢ne
ash was reported in association with the emplacement of these £ows, up to 12 km from the volcano (Smithsonian Institution, 1998c). On December 1, 1998, the lava £ow had divided into
three advancing fronts (Smithsonian Institution,
1998d). By early 1999, these lava £ows were 2.8,
3.1, and 2.5 km long, and were advancing down
the three branches of El Cordoban ravine, with a
total estimated volume of lava of 39U106 m3
(Navarro et al., 2002-this volume). On December
8, 1998, several pyroclastic-£ow deposits were observed at El Cordoban East ravine (Fig. 5). In
December, 1998, and January, 1999, COSPEC
measurements recorded 4930 t/day SO2 (Smithsonian Institution, 1998d, 1999a). On February 9
and 10, 1999, seismic activity increased, ending
with a strong explosion at the summit dome
(Smithsonian Institution, 1999b). The noise produced by the explosion was heard in the cities of
Colima and Ciudad Guzman, located 32 and 25
km from the volcano, respectively. The event
formed an ash-laden column that rose 3^4 km
above the crater. Ballistic projectiles launched by
the explosion £ew distances of 3.5 to 4 km in a
N^NE direction. Their impacts formed craters up
to 2 m in diameter (Smithsonian Institution,
1999b). Apparently, this event emplaced pyroclastic £ows that reached the San Antonio^Montegrande ravines (Smithsonian Institution, 1999a).
Activity continued with sporadic explosions at
the summit dome until May 10, when another
loud explosion occurred and was heard at the
City of Colima. The explosion was followed by
Fig. 3. Aerial views from the east of a pyroclastic £ow that
occurred on November 22, 1998. The collapse of blocks
from the advancing lava £ow formed a tan cloud that started
moving downslope, and from which rolling and saltating
blocks moved ahead of the cloud (a); seconds later, the pyroclastic £ow developed a dense basal layer and an overriding turbulent cloud (b,c). Strong degassing took place at the
summit dome. Photographic set by A. Corte¤s.
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Fig. 4. Maximum extent of block-and-ash £ow, surge, and lava £ow deposits. Numbered dots indicate the sites studied. Contour
interval is 100 m.
the formation of an ash column that rose 6.5 km
above the crater (Smithsonian Institution, 1999c),
and ballistic projectiles were launched up to 4.5
km from the summit, causing forest ¢res (Smithsonian Institution, 1999c). An eyewitness located
at ca. 8 km from the summit reported pyroclastic
£ows heading toward La Lumbre and El Cordoban ravines (Smithsonian Institution, 1999c). Seismic activity and discrete explosions continued until May 26, followed by a repose period of a few
days. On June 3, 1999, an inspection £ight revealed that part of the new lava dome was destroyed and a new crater about 180U200 m in
diameter and about 70 m deep had formed
(Smithsonian Institution, 1999e).
On July 17, 1999, after 13 h of a seismic swarm
consisting of high-frequency earthquakes, Volca¤n
de Colima produced the strongest explosion of the
1998^1999 crisis. At 12:41 local time, the inhab-
itants of Colima heard the volcano rumbling,
after which an eruptive column ascended to
more than 10 km above the volcano. This column
later showered areas more than 30 km away from
the volcano with ash. At 13 km from the volcano,
the ash layer had a thickness between 3 and 5 mm
(Smithsonian Institution, 1999d). Pyroclastic
£ows were also produced by this explosion and
moved down ravines on the W, SW, and S £anks
of the volcano up to distances of 3.5 km, although
some reports indicated that vegetation within La
Lumbre ravine was burned up to distances of 5.5
km (Smithsonian Institution, 1999d). The explosion left a crater 230 m in diameter and 70^80 m
deep, enlarging the previous May 10 crater. After
July 19, 1999, the monitoring parameters returned
to the levels recorded prior to the eruption
(Smithsonian Institution, 1999d).
The total volume of the pyroclastic deposits
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135
Fig. 5. View from the south of Volca¤n de Colima. In the background, the eastern lava £ow was descending the £anks. In the
foreground, geologists are sampling the block-and-ash £ow deposits in the El Cordoban East ravine. Photograph taken on December 8, 1998, by R. Saucedo.
generated during the 1998^1999 activity was
2.4U106 m3 , calculated by measuring the area
covered by the deposits times the average thickness measured in the ¢eld (see descriptions be-
low), or estimated deposit thicknesses for unvisited ravines (Table 1). This volume estimate
considers that in El Cordoban West and East ravines the pyroclastic deposits were partly covered
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VOLGEO 2477 16-9-02
Fig. 6. Panoramic view from La Yerbabuena village to the east of the December 10, 1998, pyroclastic £ow. A large block fallen from the SE part of the dome
formed a small-volume pyroclastic £ow (a,b) that accelerated, developing a semirounded front (c,d). At the break in slope, an ash cloud started billowing backward (e,f) while the £ow entered El Cordoban East ravine, arresting motion moments later with the formation of a second ash cloud (g,h). Total elapsed time
of travel was 132 s.
137
Fig. 6 (Continued).
R. Saucedo et al. / Journal of Volcanology and Geothermal Research 117 (2002) 129^153
VOLGEO 2477 16-9-02
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R. Saucedo et al. / Journal of Volcanology and Geothermal Research 117 (2002) 129^153
Fig. 7. Composite stratigraphic column of the November^December 1998, pyroclastic-£ow deposits exposed along El Cordoban
East ravine. (a) Channel deposits, (b) overbank deposits. The dates in the columns represent the time periods of emplacement.
by the lava £ows. We considered these buried
deposits in the calculation.
3. December 10, 1998, pyroclastic £ows
On December 10, 1998, we visited La Yerbabuena village on the lower southwestern £ank of
the volcano, from which we witnessed the formation of a pyroclastic £ow generated from cascading blocks on the outer edge of the summit dome
and probably also from external parts of the eastern lava £ow front located a few hundred meters
below the summit. The pyroclastic £ow moved
downslope parallel to the eastern edge of the
lava £ow. After a few seconds, the pyroclastic£ow front consisted of cascading blocks that
moved by rolling, sliding, and saltating, followed
by a more coherent ash-rich £ow separated into
two main parts: a basal avalanche and an upper
turbulent ash cloud left behind by the £ow and
blown towards the summit (Fig. 6a,b). As the
£ow approached the break in slope (2 km from
the source at ca. 2500 masl) it developed a round
protruding front followed by a thin basal avalanche and a slowly thickening turbulent upper
ash cloud (Fig. 6c,d). As the £ow proceeded
through the break in slope ( s 30‡ to 20‡) a small
turbulent cloud started to develop (Fig. 6e,f). Immediately afterward the £ow thickened and surmounted the ridge between the San Antonio and
Cordoban East ravines; video images at this stage
showed meter-size blocks being ejected beyond
the £ow front, prior to being captured into the
El Cordoban East ravine. As the £ow continued
to feed material through the break in slope, the
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rising ash cloud increased noticeably in thickness.
After 132 s the £ow came to a complete rest inside
the ravine, and the less dense part of the ash
cloud ¢nally lift o¡ (Fig. 6g,h).
139
dium sand-size matrix, whereas at site 2 they had
a clast-supported texture with rare matrix material (Figs. 8b and 9a,b). The block-and-ash
£ow deposits ¢lled El Cordoban East ravine
and La Lumbre up to 3 km from the summit
(H/L = 0.47) with an approximate volume of
8.1U105 m3 .
3.1. Description of deposits
We were unable to visit El Cordoban East ravine until January 22, 1999, due to the volcano’s
intense activity. During this visit the eastern lava
£ow front had almost reached the break in slope
at 2500 m in elevation and no major changes
within the ravine were noted with respect to those
observed in November^December, 1998 (Fig. 4,
sites 1 to 5), except that the path traveled by the
December 10, 1998, block and ash £ow was delineated by well-exposed deposits.
3.1.2. Overbank deposits
These deposits were found outside the ravine
on the valley margins or beyond the tip of the
block-and-ash £ow deposits (Fig. 7b). Near site
5 on the western margin of El Cordoban East
ravine we found an excellent stratigraphic record.
From bottom to top were a gray, massive, medium to coarse sand-size layer (4 cm thick), a
beige dune-bedded sand-size layer (1 cm thick),
and a tan, massive, well-sorted ¢ne sand to silt
layer (0.5 cm thick) (Fig. 10a). The maguey plants
and trees were scorched and toppled in the direction of £ow (Fig. 10b). The maguey plants had
sand plastered on their upslope sides.
3.1.1. Block-and-ash £ow deposits
At least ¢ve recognizable lobes of block-andash £ow deposits ¢lled the eastern El Cordoban
ravine, making the total superimposed thickness
around 8 m (Fig. 7a,b). Each £ow unit consisted
of dark-gray andesite and rare red-altered clasts
set in a medium to coarse sand-size matrix; these
deposits are considered as essentially monolithologic. Each £ow unit had a clast-supported steep
front (ca. 1.5^2 m thick) composed of boulders up
to 1.7 m in diameter. The lobes were bounded by
marginal levees up to 0.7 m thick, commonly
¢nes-poor and supported by boulder-size clasts
with an average diameter of 60 cm (Fig. 8a,b).
The surfaces of the deposits were poorly sorted
with pebble to boulder size clasts suspended in a
coarse-sand matrix. However, the surfaces showed
local variations from place to place ; for instance,
at site 1, the fragments were supported by a me-
3.1.3. Interpretation of deposits and genetic
implications
El Cordoban East ravine is ¢lled with a sequence of at least ¢ve superimposed block-andash £ow units emplaced between November 21
and December 8, 1998 (Fig. 7a). The uppermost
£ow unit within the channel facies (block-and-ash
£ow deposit) and the overbank deposits (Fig. 7b)
that we cross-correlated through direct observations, video ¢lming, and study of the runout paths
are considered associated with the December 10,
1998, event. This conclusion is supported by the
fact that no other pyroclastic £ows were reported
after this date in the El Cordoban East ravine.
Table 1
Volumes of the pyroclastic-£ow deposits produced during the 1998^1999 eruption of Volca¤n de Colima
Ravine
Runout
(km)
H
(km)
H/L
Area
(km2 )
Volume
(105 m3 )
Cordoban West
Cordoban East
San Antonio^Montegrande
La Lumbre
Total
4.5
3.0
3.3
3.0
1.9
1.42
1.48
1.44
0.42
0.47
0.45
0.48
0.14
0.09
0.231
0.09
8.0
4.5
7.9
3.6
24
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4. July 17, 1999 pyroclastic £ows
Starting on February 10, 1999, the activity at
the summit dome shifted dramatically to a more
explosive style, with progressively larger explosions on February 10, May 10, and July 17,
1999 (Fig. 11). These explosive events were accompanied by pyroclastic £ows that increased in
magnitude and volume through time, reaching a
climax on July 17, 1999 (Smithsonian Institution,
1999d).
The July 17 explosion launched projectiles with
diameters of 9 cm as far as 3 km from the summit, after which an ash-laden plume rose convectively to about 10 km above the summit, where
¢ne particles were dispersed by prevailing winds.
A few seconds after its initiation, parts of the
buoyant column collapsed toward the southern
sector of the volcano, forming a Soufriere-type
pyroclastic £ow (Sato et al., 1992). The pyroclastic £ow moved along the San Antonio^Montegrande ravines, forming a 3^4 km high ash cloud
(Fig. 11), and reaching a maximum distance of 3.3
km from the summit (Figs. 4 and 12a).
The basal avalanche of the £ow removed foliage of 6-m-high trees and sandblasted the lower
2 m. The associated upper ash cloud moved
across topographic barriers and a¡ected the ridges
and marginal portions of the ravines. In these
areas, trees with trunks 30 cm in circumference
were bent, broken, or uprooted in the direction
of £ow (Fig. 12b). The £ow had variable temperatures since in some places it scorched thin roots
and agave leaves, while in other places vegetation
remained unburned.
4.1. Description of deposits
We visited the San Antonio and Montegrande
ravines on February 15 and March 11, 2000. The
ravines were completely ¢lled by pyroclastic deposits between the elevations of 2400 and 2500 m,
141
making it di⁄cult to recognize the ridge between
them (Fig. 12a). Pyroclastic-£ow deposits (described below) extended beyond the lower end
of the ravines (Fig. 4).
4.1.1. Block-and-ash £ow deposit
The surface of the deposit did not show recognizable £ow lobes, from which we infer that these
ravines were ¢lled by a single large deposit, as was
con¢rmed by limited erosion through the deposit
surface. It is a gray, massive monolithologic deposit with thickness from 1.2 m (site 13) to 4^5 m
(site 14, Fig. 13a). The deposit is generally massive, occasionally showing concentrations of lithic
clasts (average diameter of 70 cm) towards the
top, supported by a medium to coarse sand-size
matrix. The average density of the juvenile lithic
clasts was 1800 kg/m3 . The block-and-ash £ow
deposit ¢lled the San Antonio^Montegrande
ravines, reaching 3.3 km from the source
(H/L = 0.45), with a total estimated volume of
7.9U105 m3 .
4.1.2. Overbank deposits
A ¢ne-grained layer blanketed the margins of
the El Cordoban East, San Antonio, and Montegrande ravines. At site 15, this layer consisted of
two subunits: (a) a basal, tan, massive deposit (2
cm thick) composed of sand to silt size particles
with weak layering and a lower erosive contact
and (b) a light-gray cross-bedded deposit with irregular thickness (9 and 18 cm, sites 13 and 15,
respectively) composed of medium sand to siltsize particles with a lower erosive contact (Fig.
13b).
In San Antonio and Montegrande ravines, the
stratigraphic relationship of these two layers with
respect to the block-and-ash £ow deposit is ambiguous. In proximal areas (site 15, Fig. 14a),
these layers overlie the block-and-ash £ow deposit, whereas in distal areas they underlie it (site 13,
Figs. 14b and 15a,b).
Fig. 8. (a) View to the northeast of the path of the December 10, 1998, pyroclastic-£ow deposits at El Cordoban East ravine.
The arrows point to the ¢nes-poor levees, and (b) view from the north of the block-and-ash £ow deposit fronts (black arrow).
Sites 1 and 2 are those shown in Fig. 9.
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Fig. 9. Surface view of two di¡erent block-and-ash £ow deposits, (a) blocks set in a ¢ne matrix at site 1, and (b) clast-supported
texture of the deposit with sparse matrix at site 2. In both cases, the ruler segments represent 1 m.
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143
Fig. 10. Image of the western margin of El Cordoban East ravine. (a) Aspect of the deposit composed of a basal, gray, massive
pyroclastic £ow (pf), a dune-bedded surge (s), and a ¢ne-grained ash fall (af). Knife is 16 cm long. (b) Maguey plant bent in the
direction of the £ow and sand accumulated at its base. Black arrows indicate the direction of the £ow.
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Fig. 11. Panoramic view from the southeast of the July 17, 1999, eruptive column. The column rose 10 km above the crater
(background), after which a small pyroclastic £ow developed from its base traveling toward the San Antonio^Montegrande ravines as a smaller dark gray cloud (foreground). Photograph by Oscar Huerta.
5. Granulometry
Eighteen dry-sieve sedimentological analyses
were carried out with a set of sieves ranging
from 35 to 4P at one-phi intervals. The December
10, 1998, block-and-ash £ow deposits have bimodal grain-size distributions with typical modes at
34 and 1^2P, without major variations among
£ow units (Fig. 7a). The distribution curves of
the overbank deposits (Fig. 7b) have better sorting (MdP = 2.80, cP = 1.70 for the massive pyroclastic-£ow deposit; MdP = 2.85, cP = 1 for the
dune-bedded pyroclastic-surge deposit; MdP =
2.70, cP = 1.10 for the ash fall layer) relative to
the block-and-ash £ow deposits (MdP = 30.05 to
0.40, cP = 2.63 to 3.45).
The July 17 1999 deposits have similar granulometric parameters to the November^December
1998 deposits. The distribution curves of blockand-ash £ow deposits are bimodal with modes
at 34P and 1P but are coarse-grained and more
poorly sorted (MdP = 31.75, cP = 3.18 to 3.20;
Fig. 14a). The overbank deposits have a unimodal
asymmetric curve towards the ¢ne fractions with
one mode at 4P for the overlying surge deposits
(MdP = 2.45, cP = 1.28 and 1.30) and with well-
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145
Fig. 12. (a) Panoramic view from the northwest of the block-and-ash £ow deposits ponded in the San Antonio^Montegrande ravines. The dashed white line shows the limit of the deposits. (b) Detailed view of a beheaded tree (30 cm in diameter), with
branches bent in the direction of the pyroclastic £ow at site 15. Shovel is 69 cm long.
VOLGEO 2477 16-9-02
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R. Saucedo et al. / Journal of Volcanology and Geothermal Research 117 (2002) 129^153
Fig. 13. (a) Vertical exposure of the July 17, 1999, block-and-ash £ow deposit along the San Antonio^Montegrande ravines, in
which blocks are embedded in a sand size matrix at site 14. (b) View of the overbank deposits at site 15. The ¢ne-grained surge
deposit (s) with embedded roots rests on top of a thin pyroclastic-£ow deposit (pf) and a soil. The pf is bracketed by white
dashed lines.
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147
Fig. 14. Stratigraphic correlation of the July 17, 1999, deposits in proximal (14 and 15) and distal sites (13). Notice the reversed
stratigraphic position of the ¢ne-grained surge deposit.
de¢ned modes in the 3^4 P fraction for the underlying surge layer (MdP = 2.45, cP = 1.20; Fig.
14b).
The cumulative curves of these deposits clearly
suggest that overbank deposits were derived from
the block-and-ash-£ow deposits found in the main
ravines (Fig. 16A,B) by elutriation of ¢ne particles. In particular, the November^December
1998 deposits show a clear trend from coarser,
poorly sorted to ¢ner, better-sorted deposits going
from the block-and-ash £ow to the overbank deposits (Fig. 16A,B). The slope of the cumulative
grain-size distribution curves, as well as the details
within the curves are similar to those presented
for the 1991 deposits of Unzen Volcano, Japan
(Fujii and Nakada, 1999; Miyabuchi, 1999), Merapi, Indonesia (Boudon et al., 1993), Mount
St. Helens, USA (Mellors et al., 1988), and Fuego, Guatemala (Davies et al., 1978; Rose et al.,
1977).
6. Modes of emplacement of the 1998^1999
pyroclastic-£ows at Volca¤n de Colima
During its 1998^1999 eruptive activity, Volca¤n
de Colima generated pyroclastic £ows through
two di¡erent mechanisms: (1) Merapi-type, generated from the front of a lava £ow descending a
steep slope, such as on November 21, 1998 (Fig.
3); and (2) Soufriere-type, generated by collapse
from the base of an eruption column, such as on
July 17, 1999. Despite the di¡erent origins, the
pyroclastic £ows behaved the same and produced
similar types of deposits. Therefore, in the following discussion, we refer to the transport and emplacement processes of the Colima pyroclastic
£ows regardless of their generating mechanism.
Merapi-type pyroclastic £ows can be initiated
by the destabilization of external parts of a
dome or the front of a lava £ow. Blocks start to
roll and bounce down the steep slopes of the vol-
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cano, breaking into smaller fragments that almost
immediately create an upper turbulent ash cloud
and give the £ow the two-layer structure of a
basal dense avalanche and an overriding ash
cloud. On steep slopes, some blocks move faster
than the £ow front, as was observed during the
December 10, 1998, pyroclastic £ows (Fig. 6a^h).
These observations, along with the clast-supported, unsorted nature of the proximal deposits
of the 1991 Colima eruption (Saucedo, 2001), the
percussion marks on clasts from similar deposits
elsewhere (Sieh and Bursik, 1986; Grunewald et
al., 2000), and laboratory experiments (Drake,
1990; Takahashi and Tsujimoto, 2000), suggest
that the basal avalanche of the pyroclastic £ows
at Colima and elsewhere move as granular £ows
(inertial grain £ows) under the action of gravity
and particle interaction, wherein the gas escaping
from blocks is minimal (Davies et al., 1978; Nairn
and Self, 1978; Boudon et al., 1993; Yamamoto
et al., 1993; Fujii and Nakada, 1999; Mellors et
al., 1988; Takahashi and Tsujimoto, 2000).
As the £ow continues to move down slope, it
forms a bulbous, turbulent front from which
clasts are occasionally ejected ahead of the £ow,
then re-ingested. Once the £ow passes through a
break in slope ( s 30‡ to 20‡), a turbulent, vigorously convecting ash cloud develops, possibly as
a consequence of entrainment within a hydraulic
or granular jump (Woods and Bursik, 1994), or
due to the fact that the basal avalanche becomes
more rigidly constrained within channels. In either case, a speed reduction causes a decrease in
the ability of the £ow to carry large particles, as
has been documented at other volcanoes (e.g.
Cole and Scarpati, 1993; Freundt and Schmincke,
1985; Levine and Kie¡er, 1991; Mac|¤as et al.,
1998).
Beyond the break in slope, the pyroclastic £ow
of December 10, 1998, was able to partially surmount the valley walls before entering the channel. Once in the channel, it moved about 1 km
149
before stopping, after which ¢ne material lifted o¡
as an ash cloud (Fig. 6a^h). It is possible that the
continued breakage of particles and release of gas
within this £ow aided propagation beyond the
break in slope, because these processes sustain
an upward gas current capable of keeping large
particles moving within a ¢ne-grained matrix
(Saucedo, 2001).
Beyond the break in slope, basal avalanches
often decelerate, while the upper ash cloud continues to move at a nearly constant rate (Nairn
and Self, 1978; Denlinger, 1987; Palladino and
Valentine, 1995; Freundt and Bursik, 1998) and
detaches, becoming an independent dilute pyroclastic £ow (Fisher and Heiken, 1982; Fisher,
1995; Mellors et al., 1988; Fujii and Nakada,
1999; Takahashi and Tsujimoto, 2000). This process explains the stratigraphic sequence of the July
17, 1999, pyroclastic-£ow deposit, in which the
surge layer derived from the ash cloud overlies
the block-and-ash £ow deposit in the proximal
facies, while beyond the break in slope, in the
distal facies, the surge layer underlies the blockand-ash £ow deposit.
Recent conceptualization of ignimbrites (Branney and Kokelaar, 1992) suggests that pyroclastic£ow deposits, such as is seen in the overbank
deposits at Colima, are the result of high rates
of sedimentation in a depositional system that
gives little evidence of the nature of the £ow itself.
If this conceptualization can be applied to the
segregated £ows of Colima, then it means that
the stratigraphic sequence emplaced by the dilute
pyroclastic £ow re£ects a transition from higher
to lower sedimentation rates (waning £ow). The
change in sedimentation rate can result from a
progressive dilution with time, or a change in
mean grain size, similar to the concept of strati¢ed £ows in pyroclastic surges (Valentine, 1987).
Within the Colima £ows, the basal avalanche
traveled 1 km beyond the break in slope, ¢lling
the channels with up to 8 m of material. The
Fig. 15. (a) Aspect of the July 17, 1999, deposits at site 13, composed of a basal dark-gray surge layer (s) and an upper blockand-ash £ow deposit (baf) that toppled vegetation in the direction of £ow. Observe the large block on top of the deposit. Ruler
is 1 m long. (b) Detailed view of the lower surge unit, which developed weak cross-strati¢cation (s), and was eroded by the overlying block and ash £ow deposit (baf). The white handle on the knife is 12 cm long.
VOLGEO 2477 16-9-02
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Fig. 16. (A) Cumulative grain-size distribution curves of the December 10, 1998, deposits and (B) the July 17, 1999, deposits in
proximal (continuous line) and distal sites (dotted line). In both diagrams, the curves for the ¢ne-grained deposits are consistent
with derivation from the coarse block-and-ash £ow deposits by elutriation of ¢nes.
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R. Saucedo et al. / Journal of Volcanology and Geothermal Research 117 (2002) 129^153
surfaces of these deposits consist of superimposed
£ow lobes, each with steep £ow fronts, marginal
¢nes-poor levees, and centimeter-size dispersed
blocks (Boudon et al., 1993; Davies et al., 1978;
Nairn and Self, 1978; Rodr|¤guez-Elizarrara¤s et
al., 1991), resembling the morphology of debris
£ows (Johnson, 1984). Structures such as reverse
grading of lithic fragments and incipient fumarolic pipes on the surface of the deposit suggest
deposition from £uidized £ows (type 1 of Wilson,
1980). Despite the presence of evidence for £uidization in the Colima pyroclastic £ows, this does
not mean that £uidization was important to £ow
mobility. Hayashi and Self (1992) considered that
£uidization is not an important transport mechanism in pyroclastic £ows because they found that
mobility of dry and hot avalanches and small volume pyroclastic £ows is adequately explained by a
dependence on the original volume, as was later
con¢rmed by Dade and Huppert (1998).
The detached, dilute pyroclastic £ows, as opposed to the basal avalanches themselves, were
able to reach high up valley walls, where vegetation was locally burned or scorched and bent in
the direction of £ow. The deposits left by the
detached dilute pyroclastic £ows consisted of a
sandy, massive layer (pyroclastic £ow), a crossbedded layer (ash cloud surge), and a massive,
silty layer (air fall). The evidence that the dilute
pyroclastic £ows were able to rise high on valley
walls and produce this stratigraphy, unlike the
basal avalanche, indicates that these £ows consisted of a waning suspension of particles that
left pyroclastic £ow and surge deposits, and
from which a billowing ash cloud arose after the
£ow stopped to ultimately deposit the ash fall.
7. Conclusions
The 1998^1999 pyroclastic £ows of Volca¤n de
Colima were generated from the collapse of external parts of the new lava dome and active lava
£ow fronts (Merapi type), and from partial collapse of the lower parts of an eruptive column
(Soufriere type). These varieties of pyroclastic
£ows were extensively videotaped and photographed, and their deposits and e¡ects were
151
studied in the ¢eld. We propose that block and
ash £ows produced by either Merapi or Soufriere
type events, move in a similar fashion and that
the mobility of such £ows is largely dependent on
the initial volume and the slope on which the
£ows move (Hayashi and Self, 1992; Dade and
Huppert, 1998).
The Colima pyroclastic £ows of 1998^1999 immediately separated into a basal avalanche and an
upper turbulent ash cloud. The basal avalanches
moved as a granular £ow on steep slopes. If suf¢ciently voluminous, they continued to move on
gentler slope where there is evidence for extensive
clast break-up, and £uidization either during or
following propagation. Whenever the basal avalanche decelerates and eventually stops, the superjacent turbulent ash cloud is able to move independently as a newly formed dilute pyroclastic
£ow (Fujii and Nakada, 1999; Takahashi and
Tsujimoto, 2000). The dilute pyroclastic £ow is
able to surmount topographic barriers, emplacing a massive, ¢ne-grained pyroclastic-£ow deposit and a dune-bedded, ash-cloud-surge deposit.
When the ash cloud stops or traverses a topographic obstacle, a rising cloud of ash forms, depositing a terminal ash fall.
Acknowledgements
This project was supported in part by grants
from CONACYT (27993T to J.L.M. and
32312T to J.M. Esp|¤ndola), and NSF (EAR
9725361 to M.I.B.). We are indebted to Jorge
Mart|¤nez from the University of Colima who
joined us in ¢eldwork and who ¢lmed the behavior of Colima’s pyroclastic £ows. P. Julio, C. Ortiz, R. Castro, A. Noriega, and V. Cruz also accompanied us during our ¢eldwork. M. Urzua
and J. Velasco provided logistical support. Careful reviews by M. Ort, P. Cole, Y. Miyabuchi, and
J. Luhr helped clarify the presentation of several
of the ideas proposed in this paper.
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