The rhyolitic–andesitic eruptive history of Cotopaxi volcano

Transcription

Bull Volcanol
DOI 10.1007/s00445-007-0161-2
RESEARCH ARTICLE
The rhyolitic–andesitic eruptive history of Cotopaxi
volcano, Ecuador
Minard Hall & Patricia Mothes
Received: 27 October 2006 / Accepted: 7 June 2007
# Springer-Verlag 2007
Abstract At Cotopaxi volcano, Ecuador, rhyolitic and
andesitic bimodal magmatism has occurred periodically
during the past 0.5 Ma. The sequential eruption of rhyolitic
(70–75% SiO2) and andesitic (56–62% SiO2) magmas from
the same volcanic vent over short time spans and without
significant intermingling is characteristic of Cotopaxi’s
Holocene behavior. This study documents the eruptive
history of Cotopaxi volcano, presenting its stratigraphy and
geologic field relations, along with the relevant mineralogical and chemical nature of the eruptive products, in order
to determine the temporal and spatial relations of this
bimodal alternation. Cotopaxi’s history begins with the
Barrancas rhyolite series, dominated by pumiceous ash
flows and regional ash falls between 0.4 and 0.5 Ma, which
was followed by occasional andesitic activity, the most
important being the ample andesitic lava flows (∼4.1 km3)
that descended the N and NW sides of the edifice.
Following a ∼400 ka long repose without silicic activity,
Cotopaxi began a new eruptive phase about 13 ka ago that
consisted of seven rhyolitic episodes belonging to the
Holocene F and Colorado Canyon series; the onset of each
episode occurred at intervals of 300–3,600 years and each
produced ash flows and regional tephra falls with DRE
volumes of 0.2–3.6 km3. Andesitic tephras and lavas are
interbedded in the rhyolite sequence. The Colorado Canyon
episode (4,500 years BP) also witnessed dome and sector
collapses on Cotopaxi’s NE flank which, with associated
ash flows, generated one of the largest cohesive debris
Editorial responsibility: J Stix
M. Hall (*) : P. Mothes
Instituto Geofísico, Escuela Politécnica Nacional,
Casilla,
1701-2759 Quito, Ecuador
e-mail: mhall@igepn.edu.ec
flows on record, the Chillos Valley lahar. A thin pumice
lapilli fall represents the final rhyolitic outburst which
occurred at 2,100 years BP. The pumices of these Holocene
rhyolitic eruptions are chemically similar to those of older
rhyolites of the Barrancas series, with the exception of the
initial eruptive products of the Colorado Canyon series
whose chemistry is similar to that of the 211 ka ignimbrite
of neighboring Chalupas volcano. Since the Colorado
Canyon episode, andesitic magmatism has dominated
Cotopaxi’s last 4,400 years, characterized by scoria bomb
and lithic-rich pyroclastic flows, infrequent lava flows that
reached the base of the cone, andesitic lapilli and ash falls
that were carried chiefly to the W, and large debris flows.
Andesitic magma emission rates are estimated at 1.65 km3
(DRE)/ka for the period from 4,200 to 2,100 years BP and
1.85 km3 (DRE)/ka for the past 2,100 years, resulting in the
present large stratocone.
Keywords Alternating rhyolitic–andesitic volcanism .
Cotopaxi volcano . Holocene history . Northern Andes
Introduction
The almost simultaneous alternation of basic andesitic and
evolved rhyolitic magmatism from the same volcanic center
is an interesting, if not perplexing problem. What mechanism or process results in such a varied eruptive history?
Equally perplexing, how is such a clean alternation of these
two magma types maintained without generating intermediate compositions? For many years Cotopaxi volcano,
Ecuador, was considered to be a volcano of solely andesitic
origin (Hall 1977; Barberi et al. 1995); here we show that
rhyolitic volcanism has played an important role in its history starting at least 560 ka ago and again in the Holocene.
Bull Volcanol
Fig. 1 Cotopaxi volcano’s 20-km-diameter cone is comprised mainly
of andesitic products of the past 4,000 years. The hills in the foreground
are remnants of the F series ash flows and in the immediate foreground
blocks from the 1877 debris flows. Photo of the north face taken in 2004
The present study provides the geological framework and
volcanic history of this alternating bimodal magmatism as
recorded at Cotopaxi, a large well-known stratovolcano of
the Northern Andes (Fig. 1). Emphasis is placed upon field
mapping of the deposits and the development of a
comprehensive stratigraphic and chronological framework
in order to clearly demonstrate the succession of rhyolitic
and andesitic magmas.
Cotopaxi volcano (Lat. 0°38′S; Long. 78°26′W) is
located on the Eastern Cordillera of the Ecuadorian Andes,
60 km south of Quito and 35 km northeast of Latacunga,
capital of Cotopaxi province. This 5,897 m high active
volcano is notable for its relief (2,000–3,000 m), conical
form, massive size (22-km diameter), and its glacier-clad
steep flanks. Cotopaxi, along with other large active
andesitic volcanoes, such as Tungurahua, Antisana,
Cayambe, and Sangay, define the Eastern Cordillera in
Ecuador, some 35 km behind the dacite-dominated volcanic
front that constitutes the Western Cordillera (Barberi et al.
1988; Hall and Beate 1991). Between these two cordilleras
lies the densely populated InterAndean valley, a structural
depression (Fig. 2).
Early geological and petrological descriptions of Cotopaxi were given by La Condamine (1751), Humboldt
(1837–1838), Reiss (1874), Sodiro (1877), Stübel (1897),
Wolf (1878, 1904), and Reiss and Stübel (1869–1902).
Modern studies of the volcano and its hazards were carried
out by Hradecka (unpublished data 1974), Miller et al.
(1978), Hall (1987), Hall and Hillebrandt (1988), Mothes
(1992), Barberi et al. (1995), Hall et al. (2000), and Mothes
et al. (1998, 2004).
Cotopaxi has experienced at least 13 significant eruptions since 1534, based upon tephrostratigraphy and
historic accounts. They correspond to five cycles: 1532–
1534, 1742–1744, 1766–1768, 1853–1854, and 1877–
1880. These historic eruptions were all of andesitic
character and typically produced scoria pyroclastic flows,
ash and lapilli falls, blocky-lava flows, and far-reaching
debris flows. Fumaroles still exist within the summit crater,
along its inner and outer rims, and at the Yanasacha rock
face on its upper northern slope. Since late 2001 increased
levels of seismicity and fumarolic activity have been
observed and continue today. Cotopaxi has been monitored
instrumentally since 1977. Cotopaxi’s frequent eruptive
activity and the large growing population living around the
volcano and along the major rivers that head on the cone
stress the urgent need to carefully document the nature of
its past eruptions, in order to develop valid scenarios for
future eruptions.
Figure 3 provides a brief introductory synopsis of
Cotopaxi’s history to acquaint the reader to its overall
activity prior to discussing it in greater detail.
Cotopaxi I
Barrancas rhyolite series
Along the lower S and SW flanks of the present Cotopaxi
edifice is exposed a thick older series of deposits made up
of rhyolitic ash flows, block-and-ash flows, tephra falls, and
associated volcaniclastic units, attaining a thickness of
>150 m, that generally dip to the S and SW, away from
an arcuate alignment of rhyolitic domes and source vents.
This, the Barrancas series of Cotopaxi I, is best seen along the
Barrancas–Simarrones valley, but the nearby Burrohuaicu,
Saquimala, and San Lorenzo valleys display similar and
complementary sequences (Fig. 4). The age of the series is
not well constrained, but Bigazzi et al. (1997) reported
fission track ages of 0.42–0.56 Ma for several rhyolites. The
series is older than the 211 ka Chalupas ash flow and
younger than the clastic Latacunga Fm., dated at 1.4–1.7 Ma
(Lavenu et al. 1992).
Its presumed source is a series of aligned, highly
fractured, locally altered rhyolite domes and dikes associated with dome breccias and short obsidian lava flows
(Fig. 4). The circumferential distribution of the rhyolitic
vents around the SW side of Cotopaxi’s present edifice
suggests that these vents formed the outer segment of an
older caldera that is mostly buried under younger deposits.
Associated pyroclastic sequences can be traced from the
source area to the Barrancas valley where the most
complete stratigraphy is seen (Figs. 5 and 6) (Inst. Nac.
Electrificación unpublished data 1983). The regional extent
of this sequence is unknown, as it is covered by younger
deposits down valley. However, the significant thicknesses
Bull Volcanol
78 30'
78 25'
78 20'
78 15'
Selva Alegre
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Amaguaña
4000
00
m
m
30
0 25'
RÌo Santa Clara
10 km
5
RÌo Pita
78 35'
0
30
00
m
Fig. 2 Regional map showing
the locations of Cotopaxi volcano and neighboring volcanic
centers, as well as several towns
in the InterAndean Valley. Note:
Across the top are west longitudes (e.g. 78° 35′ W) and along
the right side are south latitudes
(e.g. 0° 50′ S)
4000 m
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4000
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contour interval = 200 m
(1–15 m) of the ash-flow and fall deposits suggest that
many had wide distributions.
The Barrancas sequence begins with a 15-m-thick, pinktopped, pumiceous ash-flow deposit (unit BA) that contains
up to 5% small polylithic fragments, especially obsidian.
Lithic clasts are concentrated toward the base, while white
pumice clasts abound toward the top. The ash flow is
overlain in turn by a 20-m-thick sequence (units BB, BC,
BD and BE) of interbedded ash-flow, surge, and air-fall
deposits of both pumice lapilli as well as obsidian and
banded-rhyolite lapilli. Most of these units have been
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slightly reworked by wind and rain, suggesting short repose
intervals. Upwards the sequence continues with a pumice
lapilli-fall unit (BF1) and is followed by a thick series of
five block-and-ash breccias (BF2) comprised of radially
fractured clasts and blocks, up to 1 m in diameter, of
obsidian, rhyolitic vitrophyres, and saccaroidal rhyolite, all
containing plagioclase, biotite, ± quartz, in a gray or red
sandy matrix of glassy particles. The five units of crudely
bedded breccias, each defined by coarser blocks at its base,
are the products of sequential dome collapses and related
debris flows, traceable to an obsidian dome located 15 km
COTOPAXI II A
Fig. 3 Brief synthesis of the
stratigraphic history of Cotopaxi volcano, separated into
Cotopaxi I, II A, and II B
eruptive periods
COTOPAXI II B
Bull Volcanol
Late
Holocene
Andesite
Episode
...........
++++++
...........
...........
...........
...........
++++++
...........
++++++
Colorado
Canyon
Rhyolite
Episode
...........
...........
...........
...........
...........
...........
...........
...........
++++++
++++++
...........
++++++
F
Rhyolite
Series
-------++++++
-------++++++
-------++++++
-------++++++
Cga
COTOPAXI I
Cangahua
-------and
++++++
Chalupas
Chlp
+
Units
- - +- +- -+ -+-+Cga
Detritial
Fan and
Andesitic
Lavas
OoOoOoO
...........
OoOoOoO
OoOoOoO
...........
OoOoOoO
OoOoOoO
OoOoOoO
Barrancas
Rhyolite
Series
++++++
++++++
++++++
++++++
++++++
++++++
++++++
...........
OoOoOoO
...........
--------
up the Barrancas valley. Similar rhyolitic domes and dikes
are also observed where the Saquimala and Burrohuaicu
canyons intersect the margin of the inferred caldera (Fig. 4).
Upsection there is a succession of seven similar pumice
lapilli-fall deposits (units BG1 to BG7), 1–4 m thick,
without obvious time breaks. These generally have a coarse
pumice lapilli base and trend to fine ash layers towards the
top, locally bearing accretionary lapilli. Obsidian and
vitrophyric sand often form thin lithic-fall intercalations
in the succession. A reworked pumice-rich ash-flow unit
(BG-3) also occurs in the sequence. Evidence of minor
reworking by rain is occasionally observed at the top of
these units. Some fine-grained ash-fall units have welldeveloped stratification, defined by variations in grain size
and sorting. Apparently this succession was generated by
The present andesitic history of Cotopaxi volcano began about 4000 yr BP and
has involved many tens of eruptions characterized by pumice and scoria tephra
falls, scoria, lithic, and pumice pyroclastic flows, frequent large debris flows, and
many blocky lava flows, all of which have contributed to the making of the
present large stratocone. Minor rhyolitic activity occurred about 2100 yr BP.
Historic eruption cycles, averaging one per century, comprise a significant
portion of the recent andesitic activity.
4000 yr BP
The Colorado Canyon episode represents the climactic end of Cotopaxi's
Holocene rhyolitic activity, which involved phreatomagmatic explosions through
domes, a rhyolitic breccia flow, a major pumice lapilli fall, several ash flows, and
finally a large sector collapse of the northeast flank of Cotopaxi's cone. This
activity triggered the Chillos Valley lahar, a gigantic cohesive debris flow.
Erupted DRE volumes were about 1.2 km 3 ..
4500 yr BP
Following 400 ka without notable activity, Cotopaxi reactivated and
experienced a series of six rhyolitic episodes, here called the F series, that
consisted of pumiceous tephra falls, ash flows, a dome-collapse flow, and
debris flows that took place between 13.2 and 4.5 ka. Minor andesitic activity
occurred in several episodes, manifested by scoria falls.
~20 - 13 ka
A long repose period followed with the regional deposition of the thick lower and
upper Cangahua units (Cga) which are fine-grained ashy tuffs, as well as by the
enormous Chalupas ash flow unit (Chlp) (211 ka) erupted from the nearby
Chalupas caldera. The two Cangahua units form a regional mantle over the
northern Ecuadorian Andes and apparently originated from the eolian reworking
of glacial loess and pumiceous ash from Chalupas and other rhyolitic eruptions.
~300 ka ?
An erosional period ensued, in which a detrital fan developed on the SW and W
sides of the edifice, made up of volcanic breccias, conglomerates, sands, ash
layers, and a few pyroclastic flow units. Andesitic lava flows derived from the
Morurcu satellite vent, as well as from Cotopaxi itself, traveled to the SW. Other
mafic andesite flows traveled more than 40 km to the north down the Rio Pita.
~420 ka
Cotopaxi's early history involves the Barrancas rhyolite series, comprised of
many biotite-bearing tephra falls, ash flows, dome growth and collapse, and
associated block-and-ash flows, that occurred between 560 and 420 ka. They
erupted from rhyolitic domes and dikes aligned along an arcuate fracture zone
on the SW and S sides of the present edifice.
~560 ka
The Cotopaxi sequence lies unconformably on the Pleistocene Latacunga Fm.,
a thick detrital package of conglomerates, sandstones, reworked volcanic ash,
and occasional lava flows, that underlies most of the InterAndean Valley.
Interbedded lavas have ages of 1.4 and 1.7 Ma.
continuous explosions whose fall deposits accumulated
upon the ash-flow fan dipping gently to the SW.
The series continues upwards with two massive ignimbrites (the 6-m-thick BH1-2 and the 15-m-thick BH3 units)
with a thin obsidian-rich lapilli-fall layer interbedded
between the flows. These units can be traced at least
3 km down valley to the SW and possibly as far as northern
Latacunga (17 km). The BH3 salmon-colored ash-flow unit
is unconformably overlain by a 6-m-thick volcaniclastic
bed that carries pinkish-gray porphyritic rhyolite clasts,
derived from the Sta. Barbara dome, the morphologically
youngest dome.
In the lower Burrohuaico valley are exposed younger
units of the Barrancas series. There, the BH3 ash-flow unit
is only 10 m thick and is overlain by two fine lapilli falls,
Bull Volcanol
Fig. 4 Geologic map of early
Cotopaxi history
0º20'
5
0
Chalupas Ash Flow (211 ka)
10 km
N
?
COTOPAXI I
0º25'
Detrital Fan and Andesitic
Lavas (418-460 ka ?)
PASOCHOA
C
VOLCANO
CA
Morurcu vent
0º30'
Detrital fan and
associated lavas
unconformity
approximate limits of
Chalupas Ash Flow
RUMI
M ÑAHUI
Ñ
VOLCANO
O
0º35'
unconformity
?
INFERRED
COTOPAXI
CALDERA
0º40'
Morurcu Vent
CHALUPAS
Note that the older Barrancas
and clastic fan units are placed
graphically over the Chalupas
Ash Flow so that they are visible.
Sta. Barbara
Dome
CALDERA
0º45'
Barrancas
Section
78º35'
units BJ and BK1, 4 and 6 m thick respectively, that are
composed of very white, microvesicular pumice. Separating
these two fall layers is a 15-m-thick detrital bed of
obsidian-rich breccias, conglomerates, and sands, which is
overlain by a 20-cm-thick paleosol, implying a significant
repose period prior to the eruption of BK1. A 6-m-thick
ash-flow unit (BK2) ends the Barrancas sequence and is
truncated by an erosional unconformity.
Juvenile clasts of this series are almost entirely of white
pumice lapilli and ash, bearing a constant mineralogy of
clear plagioclase, biotite, magnetite, ± quartz and Kfeldspar, although traces of amphibole and hypersthene
appear in some younger units. Obsidian fragments, ranging
in color from clear to gray to very black, at times strongly
banded, are the dominant lithic components, although light
gray vitrophyres as well as aphanitic and saccaroidal
approximate limits
of Chalupas caldera
78º30'
78º25'
78º20'
78º15'
rhyolites are also observed. Fragments of black slate from
the metamorphic basement and black andesite are occasionally found. The pumices of the series are composed of 73–
76% SiO2 and 2.5–3.7% K2O. (Note: SiO2 and K2O values
were obtained from complete major-element analyses (wt.
percent) that were normalized on an anhydrous basis and
are presented in the text to aid the reader in characterizing
many of the magmatic units; the chemistry of the Cotopaxi
rocks will be discussed in a forthcoming article.)
In conclusion, the Barrancas series is the result of
prolonged explosive and effusive activity of rhyolitic
affinity that occurred approximately 420–560 ka ago,
associated with the rhyolitic domes and dikes that define
an 8-km-long, arcuate structure, interpreted as an old
caldera rim that encircles the S and SW sides of the present
Cotopaxi edifice. It was characterized by dome emplace-
Bull Volcanol
meters
Upper Cangahua: fine eolian sediments
= Cotopaxi repose period. Age at top
estimated at approx. 20 ka
8-10
Chalupas
Ash Flow
Fig. 5 Cotopaxi I composite
stratigraphic column. In this
and all following columns, the
presence and thickness of each
unit may vary locally
+++
10- + + + +
15 + + +
.3 l l l l l
organic-rich paleosol
Lower Cangahua: fine eolian sediments
= Cotopaxi repose period
7-10
Detrital Fan
and Lavas
Chalupas pum ash flow (211±14 ka)
and basal plinian pum lap AF
unconformity
Detrital Package: mainly coarse sediments of andesitic affinity;
crudely bedded breccias; interbedded andesitic AF and lavas
from Morurcu and Cotopaxi edifices that traveled down the Pita,
Cutuchi, Saquimala rivers. Presumed age >211 and <420 ka.
100
to
300
++++
6
BK2
unconformity
white pum ash flow: matrix-rich, rhyolitic
+++
6
.2
15
4
BK1
lllll
BJ
Barrancas Rhyolite Series
white pumice lap AF with little stratification
detrital: bedded sands, cangahua lenses;
underlain by unconformity
+++
BH3
++++
.2
unconformity with organic-rich paleosol
detrital sequence: bedded sands, conglomerates,
lithic breccias
6
15
white pum lap plinian AF: well-bedded at top and base;
w/ black andesitic and slate clasts, no obsidian.
+++
rosy pum ash flow: with obsidian and banded rhyolite
clasts.
obsidian and crystal-rich AF
Rw ash flow: slightly stratified, fine ash and pum lenses
3
BH2
3
+++
BH1
pum ash flow:
2
BG7
fine ash AF: with accretionary lapilli
4
BG6
G6
white plinian pum AF: with large white pumice clasts
and altered lithic clasts; Rw at top
1.2
BG5
obsidian-rich lithic AF: many discrete layers
1.5
BG4
rosy pum lap AF: stratified at top
+ + +
8 + + +
Abbreviations employed:
AF = ash- or lapilli-fall deposit
DF = debris flow or lahar deposit
lap = lapilli-size clasts
lith = lithic fragment
LV = lava flow
PF = pyroclastic flow deposit
pum = pumice clasts
Rw = reworked material
scor = scoria clasts
xl = crystals
Rw pum ash flow and AF: slightly stratified sequence
of pum ash and clasts; with accretionary lapilli.
BG3
.3
2
BG2
BG1
Rw AF with two small obsidian-rich fall layers
grey pum and lithic lap AF: poorly sorted
BF2
dome collapse flows - five pulses:- mainly
obsidian and rhyolite clasts; andesite clasts
toward base; clasts up to 1 m in size in
well-compacted grey sandy matrix
plinian pum AF: in three layers
20-30
.6
4
BF1
+++
BE
white pum ash flow: with black lithic clasts
7
4
+++
15
1
1
+ +
BD
+ ++
BC2
BC1
Rw pum ash flow
plinian pum lap AF: well-stratified, pulses
plinian pum lap AF: few lithics
ment, preceded or accompanied by frequent large pumicerich eruptions, smaller ash-rich explosions, and both
phreatic and phreatomagmatic explosions that left thin
obsidian-rich fall layers. Major pyroclastic flows of both
column collapse and dome collapse origin were generated.
15
10-20
DF: andesitic clasts
+ ++
BB
+++
pink ash flow: with
plinian pum lap AF at base
+++
pink ash flow: clast-rich at base,
pumice-rich at top.
BA
+++
volcaniclastic sequence of
Latacunga Fm. (1.4 - 1.7 Ma)
The former resulted in ash flows that traveled ≥17 km down
valley and had a combined volume of about 19 km3, while
the latter generated obsidian-rich block-and-ash flows, the
largest extending more than 15 km down the Simarrones–
Barrancas valley. Pumice lapilli-fall units up to 1 m thick
Bull Volcanol
Fig. 6 Here in the Barrancas valley the Barrancas rhyolite series
contains ash-flow units (BA-BE; see Fig. 5) at the outcrop’s base,
followed by obsidian dome-collapse deposits (BF2), and overlain by
rhyolitic ash-fall and flow units (BG–BH). Note that the series was
greatly eroded prior to the deposition of the Cangahua and Chalupas
units (see text)
are exposed as far away as 30 km to the SSW, implying that
the rhyolitic ash falls of this series probably involved a total
bulk volume of ∼13 km3. Also noteworthy is that black
andesite clasts were observed in several detrital or debris
flow deposits, but never as a primary volcanic unit; it
would suggest that andesitic magmatism had occurred
earlier. The total bulk volume of the Barrancas series is
estimated at 32 km3 (see Table 2 concerning volume and
DRE calculations).
Detrital fan and andesitic lavas
An erosional period followed the Barrancas series that
resulted in a 300–400 m-thick detrital package composed of
fluvial, glacial, and debris flow deposits on Cotopaxi’s SW
side and a similar clastic sequence to the NE of Cotopaxi.
Occasional volcanic manifestations transpired during this
period, resulting in interbedded ash-fall layers, andesitic
lavas, and lava-flow collapse breccias. The detrital package
forms a wide depositional fan whose apex is centered on the
SW side of the present Cotopaxi edifice and whose crude
stratification dips away gently to the SW, S, and SE (Fig. 4).
It contains massive, poorly stratified beds, 10–20 m thick,
with angular blocks up to 3–4 m across, but average only
10–50 cm in size, supported in a sandy matrix. Many
blocks are gray andesites from Morurcu peak. No radiometric dates exist for these units, however they lie stratigraphically between the Barrancas series (420–560 ka) and
the Chalupas ash flow (211 ka). Prior to the deposition of
the subsequent Cangahua Fm. and the Chalupas flows, the
clastic fan suffered notable erosion with the formation of
30- to 50-m-deep valleys (Fig. 6).
Morurcu peak, a volcanic neck remnant (4,850 m) of a
glacially eroded satellite vent, is located at the southern foot
of today’s cone near the inferred caldera rim of Cotopaxi I
(Fig. 4). Several of its silicic andesitic lava flows are
traceable 8 km down valley and occur interbedded in the
detrital package, thus linking Morurcu’s activity with this
erosional period. The lavas are medium gray, slightly
porphyritic andesites with microcrystalline, aphanitic, or
glassy matrices (SiO2 =60–62%; K2O=1.5–1.6%). Phenocrysts include small square crystals of plagioclase and few
hypersthene and amphibole crystals. The lava’s volume is
estimated at 0.11 km3.
To the NE of Cotopaxi along the Rio Pita canyon near
Bocatoma (Fig. 2), a similar clastic sequence is exposed,
comprised of fluvial and debris flow conglomerates,
andesitic lava flows, and a distinctive yellow-tan wellbedded series of andesitic ash and scoria of both primary
fall and reworked origin. The character of this sequence
and its similar stratigraphic position under the lower
Cangahua and Chalupas units suggest a close affinity to
the detrital fan of Cotopaxi’s SW side. Intercalated in the
upper part of this clastic sequence are five mafic andesite
lava flows, known as the Rio Pita lavas. These lavas
flooded the wide upper Pita and Salto river valleys, before
flowing down the narrow lower Rio Pita valley. One can be
traced northwards to the town of Selva Alegre, more than
40 km from source, and other lobes traveled 27 and 32 km.
Another flow is traced westwards down the upper Cutuchi
valley for >15 km (Fig. 4). Their volumes range from 0.84
to 1.27 km3 for the younger flows and from 0.24 to
0.47 km3 for the older flows, for a combined total volume
of 4.1 km3.
These lavas are dark gray to black, porphyritic, mafic
andesites that vary somewhat in their mineralogy. All
include 3–20% plagioclase phenocrysts, generally 0.3–
0.5 mm in size, but the youngest flow (Bocatoma lava)
contains large tabular plagioclase phenocrysts (30%), up to
1.5 cm in length, that are semi-aligned in the matrix. Mafic
phenocrysts include 10–20% hypersthene (3–10 mm), small
augites (5–10%), and occasional olivine in a glassy to
microcrystalline matrix. Chemically these rocks are varied:
the older lavas are more mafic (SiO2 =56%; K2O=0.98%),
while the younger lavas are more silicic (SiO2 =58–62%;
K2O=1.3–2.2%). They are texturally, mineralogically, and
chemically different than the Morurcu lavas, but follow the
general chemical trend of basic Cotopaxi lavas.
Regional deposition of the Cangahua
and Chalupas units
Following the erosional period, the northern Ecuadorian
Andes experienced the protracted deposition of the Cangahua
Bull Volcanol
Fm., a 25–30-m-thick, fine-grained, tan-colored, indurated
volcanic tuff consisting chiefly of eolian-reworked volcanic
ash and glacial loess (Hall and Mothes 1997). No Cotopaxi
eruptive products are found in this unit, implying a cessation
in Cotopaxi activity. The timing and duration of the
Cangahua Fm. are not well constrained. Studies in the Quito
Basin suggest that the top of the formation may be about
20 ka (Hall and Mothes 1997), while its base is older than
the Chalupas ash flow (211 ka) but younger than the
Barrancas series.
Interrupting the Cangahua depositional period, Chalupas
volcano, lying immediately SE of Cotopaxi (Fig. 4),
became active, resulting in a voluminous rhyolitic ash flow
and the formation of a 20-km-wide caldera (Beate 1989).
Briefly, the Chalupas activity consisted of a basal ash flow
of limited extent, followed by a regional plinian pumice
fall, and finally a far-reaching ignimbrite, called the
Chalupas ash flow, whose bulk volume is estimated at
80–100 km3. Its thick deposit is traceable north and south
along the InterAndean valley for tens of kilometers, as well
as eastwards into the Amazon Basin and westwards onto
the coastal plain. An 40Ar/39Ar age of 211±14 ka was
recently assigned to the Chalupas ash flow (L. Hammersly
personal communication 2005).
Chalupas’s rhyolites share many characteristics with
Cotopaxi’s old and young rhyolites, which warrants a brief
comparison. Typically the Chalupas pumice is very fibrous,
light to medium gray in color, and contains 1–2%
phenocrysts of biotite, plagioclase, and iron oxides. In
comparison, all of Cotopaxi’s rhyolitic pumices tend to be
white, non-fibrous, slightly richer in the same minerals, and
bear quartz. Obsidian and rhyolite fragments are the
dominant lithics in Cotopaxi’s ignimbrites, while at
Chalupas small scarce fragments of dark gray to black
aphyric andesite and dense rhyolite are more common, and
never obsidian. The Chalupas and Cotopaxi pumices have
similar, but distinguishable chemical compositions, differing most notably in the greater K2O and incompatible
element contents of the Chalupas pumices (K2O>4.0% vs.
2.5–3.6%, respectively). However, we have found that the
pumices of Cotopaxi’s 4,500 years BP eruption are
chemically identical to those of the Chalupas ignimbrite,
suggesting a genetic relation between their magmas, to be
discussed later in the Colorado Canyon episode section.
Cotopaxi II A
F rhyolite series
Following the long quiescence of the Cangahua and
Chalupas depositional period, major rhyolitic eruptions of
the F rhyolite series mark Cotopaxi’s reawakening. These
eruptions spanned about 8,700 years of periodic eruptive
activity, starting weakly at about 13,200 years BP,
intensifying around 9,600 and 5,800 years BP, and
subsequently dwindling around 4,500 years BP. However,
this rhyolitic activity was preceded and sporadically
accompanied by small andesitic eruptions, evidenced by
thin scoria lapilli-fall layers found dispersed in this series.
The rhyolitic activity was followed by widespread andesitic
magmatism that suddenly intensified around 4,000 years
BP and has dominated Cotopaxi’s subsequent activity to the
present (Note: all 14C year BP dates employed here are the
uncalibrated 14C ages reported by the lab; the precise date,
error, and reference of each age date are given in the text or
in the respective stratigraphic column.).
For convenience the F series activity is subdivided into
five major rhyolitic eruptive episodes, which are based
upon >100 studied stratigraphic sections located within a
radius of 30 km of the volcano, the more complete sections
being found to the W. A composite section is given in
Fig. 7 and the corresponding geologic map in Fig. 8. In
general the cumulative thickness of this series varies from
10 to 40 m. Most tephra units share similar characteristics;
distinction between units was attained by studying pumice
textures and mineral separates from crushed pumice clasts.
The succession is best defined chronologically where
rhyolitic ash-fall layers are interbedded with 14C-dated peat
layers in the Rio Tamboyacu area and in other regional peat
sequences. The F series is described below from its base
upward.
Episode F-1 (13,200–9,600 years BP)
Episode F-1 begins with several thin lapilli-fall layers
composed of two pyroxene andesitic scoria (SiO2 =56–
57%; K2O=1.1–1.3%) or pumice (SiO2 =61%; K2O=1.7%)
that rest upon the eroded top of the Cangahua unit. These
layers crop out mainly on the W-NW side of the cone,
especially in Pucahuaicu canyon (Fig. 2). This initial
andesitic activity postdates the 13,200 years BP rhyolitic
event mentioned above, but predates the first major
rhyolitic ash fall (F-1) and its underlying peat bed at
Boliche, dated at 9,640±69 14C years BP (J. Garrison
personal communication 2001).
In the Boliche area, episode F-1 begins locally with a
15-cm-thick grayish-white pumice lapilli-fall layer (F-1),
which is made up of 90% white microvesicular pumice
(SiO2 =75%; K2O=3.1%) that includes plagioclase and
quartz, lesser amounts of biotite, magnetite, and hypersthene, plus ∼5% gray to clear obsidian and gray to black
aphyric rhyolite clasts. Most notable is the presence of
about 5% gray- and white-streaked pumice clasts, suggesting magma intermingling. The pumice lapilli unit is
Bull Volcanol
Fig. 7 F rhyolite series of Cotopaxi II A composite stratigraphic column; abbreviations as in
Fig. 5. Note sources of radiocarbon dates
Colorado Canyon Series
meters
.4
.4
2
6
top paleosol dated at 4420±80* and 4670±70
14 C
series of scoria and lithic lap AF with interbedded paleosols
DF: with abundant pumiceous ash, derived from
underlying ash flow unit
very white pum dacitic ash flow: top eroded
7
lava: basic andesite
thin scoria AF
grayish-tan lithic-and-pum lap AF: marker bed, unit GF
series of thin scoria + lithic or pumice + lithic andesitic AF
lava: basic andesite, SE flank
DF: with abundant dacitic ash and clasts
rosy-gray block-and-ash breccia: of dense dacite
clasts in pinkish-gray sandy lithic matrix
.4
co-ignimbritic AF
2.5
2
15
4500 yr BP
yr BP*
Episode 5
5800 yr BP
white pum ash flow: with obsidian and dacite
clasts; dated 5830±80 14 C yr BP***
series of 5 thin pum lap AF
w/ rhyolitic lithics and obsidian clasts
1-4
1
.1
1.5
.2
.2
2
1
3
3
.2
2
.06
1
.25
1
.04
4
white pum lap plinian AF: with abundant dense
rhyolite clasts at base
Episode 4
thin paleosol: dated 5940±30 14C yr BP****
small pum ash flow
5900 yr BP
white pum lap plinian AF: lithics at top
series of rhyolitic and andesitic lap AF,
small ash flows, surges, and co-ignimbritic AF:
with interbedded paleosols, peat, and lithic AF
Episode 3
brown scoria and lithic-rich lap AF: marker bed
peat: dated 6300±70 14C yr BP***
series of white pum AF: obsidian, rhyolite, and
oxidized clasts
white co-ignimbritic AF
white pum ash flow: obsidian and rhyolite clasts
white pum lap plinian AF: many pulses; streakedpumice at base; abundant obsidian, oxidized lithics
6300 yr BP
Episode 2
crystal-rich AF and rosy tan surge units
organic paleosol: dated 7770±7014C yr BP****
eolian Rw PF: lithic and xl-rich layers
paleosol
pumiceous ash flow
rhyolitic and streaked-pumice lapilli AF
7700 yr BP
peat: dated 9,640±69 ***** and 10,075±50 14C yr BP******
greyish-white fine ash AF: biotite-rich
andesite scoria and rhyolite ash AF units
peat: dated 13,200±60* and 13,550±20 14C yr BP***
glacial till -- (est. age = 20-13 ka)
base of section: underlain by Upper Cangahua
or Chalupas ash flow units
followed by rosy-tan, fine-grained ash-fall beds, possibly
related to a surge unit, and then a medium gray, wellstratified, coarse sandy ash-fall layer, containing many
obsidian grains, which is observed chiefly in the upper
Cutuchi river drainage but also further to the WSW. The
Episode 1
13,200 yr BP
Uncalibrated radiocarbon
dates from:
* = Smyth (1991)
** = Barberi et al. (1995)
*** = Mothes and Hall (1998)
**** = C. Robin (pers. com. 1999)
***** = Garrison (pers. com. 2001)
****** = Clapperton et al. (1997)
rhyolitic units have a total bulk volume of less than
0.002 km3. A peat layer underlying the earliest rhyolitic
units in the upper Tambo canyon of Cotopaxi’s SE side
gave dates of 13,200±60 and 13,550±20 14C years BP
(Smyth 1991).
Bull Volcanol
Fig. 8 Geologic map of the F
rhyolite series
5
0
COTOPAXI II A
10 km
?
N
F Rhyolite Series* (13 - 4.5 ka)
Episode 5 ash flow
Episode 4
Dome-collapse flow
Ash flows
F- 4 ash-fall limits
Rio
Pita
> 20 cm
PASOCHOA
VOLCANO
C
Episode 2 ash flows
The few debris flows of this
Series are not shown.
* Units of Episodes 1 and 3
are too limited to be shown
F Series ash-fall units,
5 to 15 m-thick,
cover the InterAndean Valley
west of Cotopaxi's
edifice.
SINCHOLAHUA
VOLCANO
RUMI
MIÑAHUI
Ñ
VOLCANO
OL
Episode 2 ash flow
underlies Episode 4
i
ash flow
ch
tu
u
C
o
Ri
x Lorna
Loma
INFERRED
COTOPAXI
CALDERA
Rio T
amboy
acu
?
?
?
Lasso
anca
Rio Barr
?
?
On top of a 15–20-cm-thick soil, dated at 7,770±70 14C
years BP (C. Robin personal communication 1999),
episode 2 begins with a rosy-tan, cross-bedded, fine ash
surge unit, followed by a light gray, crystal-rich, sandy ashfall bed. They are succeeded immediately by a series of ash
falls and ash flows pertaining to one of the largest
eruptions of the F period. A well-stratified pumice lapillifall unit (F-2), up to 3 m thick near the volcano, initiates
this series. Glossy white, microvesicular pumice lapilli
(SiO2 =74–76%; K2O=2.7–2.9%) is the principal component and carries plagioclase, 1–3% biotite, 1% hypersthene
and magnetite, and ± quartz. About 1% of the pumice clasts
of its basal layer are gray- and white-streaked, suggesting
s
Chalupas
Caldera
magma mingling. This aspect, as well as the <5% lithic
components comprised of oxidized accessory fragments,
gray aphyric rhyolites, obsidian, and meta-sediment clasts,
aid in identifying the F-2 layer. This plinian fall unit has a
regional distribution whose chief axis lies to the WSW of
the volcano (Fig. 9a), however, a thin layer of this ash is
traceable to the ENE as well. Its total bulk volume is
estimated at about 7.9 km3.
Overlying the F-2 lapilli-fall unit is a 2–3 m-thick, white
ash-flow deposit seen in the Cutuchi, San Lorenzo, and
Saquimala canyons, which consists of 80% pumiceous
matrix and 20% small pumice, obsidian, aphyric rhyolite,
and altered lithic clasts. The pumice is very similar to that
of the underlying lapilli-fall bed, except that it carries less
biotite, while the unit bears more lithic grains. Because this
Bull Volcanol
a
0¼ 15' S ___
5
Pifo
Quito
Alluriquin
4
Sucus
?
?
5 cm
0¼ 30' S ___
10 cm
25 cm
29
100 cm
230
50 cm
18
17
200 cm
25
Sigchos
10 14
10
?
17
V. Quilotoa
V. Antisana
5
10
12
10
24 Maquimallanda
8
300 cm
154
19
0¼ 45' S ___
10 Aloag
V. Pasochoa
15
10
25
90 Machachi
114
30 V. Illiniza
400
8
22 13
145
45
230 110 V. Cotopaxi
13 8
28
70
70
28
20 Chalupas 3
38
75
16
54 40 60 28
20
15
41 32
14
Morro
24 15 10
18
7
8
10
Latacunga
1¼ 00' S ___
7
20
10
30
8
Cunchibamba
km
l
79¼ 00' W
0¼ 15' S ___
l
78¼ 45' W
l
78¼ 30' W
l
78¼ 15' W
Pifo
Quito
b
l
78¼ 00' W
Alluriquin
13
Sucus
?
10 cm
15 cm
20 cm
18
16
8
15
?
12
Sigchos
0¼ 45' S ___
9
11
6 23
17
105
85
10
V. Antisana
20
22 15
30 20
200
70
200 cm
50
28
38
14
Chalupas
70 71 50
52
24 65 20
20
10
23
8 10
15 Morro
10
?
?
V. Quilotoa
88
15
5
8
Latacunga
1¼ 00' S ___
5
10
20
30
km
6
Cunchibamba
l
79¼ 00' W
l
78¼ 45' W
Maquimallanda
40
100 cm 85
200 V. Cotopaxi 45
64
15
12
14
24
7 7
17
24
24
50 cm 70
20
15 19
70 15
57
46
10
15
V. Pasochoa
Machachi
20
0¼ 30' S ___
clasts. These clasts are uniform in size and shape, suggesting that they may be the products of phreatomagmatic
explosions in domes, possibly associated with glacial ice.
In the Tamboyacu area, episode 2 is represented by a thin
sequence of fine to coarse sand-size pumice-rich ash-fall,
co-ignimbritic ash-fall, and interbedded peat/soil layers.
Most ash-fall beds are similar in texture and mineralogy to
the F-2 lapilli-fall unit of the W side, however they are each
interbedded with peat layers, 5–60 cm thick, whose presence
implies variable repose intervals between eruptions. Although most of the ash-fall units are poorly represented to the
E, the F-2 plinian fall was widespread and its deposits are
observed 50 km to the NE and 25 km to the E of Cotopaxi,
suggesting that the eruptive cloud was high enough to be
affected by E-trending, >15-km-high stratospheric winds.
The combined bulk volume of this episode is estimated at
8.6 km3. At Tamboyacu a 10–20-cm-thick soil formed on
top of the F-2 series and an associated peat horizon
provided a date of 6,300±70 14C years BP.
l
78¼ 30' W
l
78¼ 15' W
l
78¼ 00' W
Fig. 9 Isopach maps of the a F-2 and b F-4 rhyolitic ash falls,
showing the typical westerly distribution of most ash falls. Thickness
in centimeters
unit is almost everywhere buried by the episode F-4
ignimbrite, its distribution is poorly known. However, it
appears to have covered approximately the same area on
Cotopaxi’s lower western flanks as that of the F-4 ash flow.
In other areas, such as at Boliche and Tamboyacu, the ash
flow is correlated to a yellowish-white, pumiceous sandy
ash unit with faint stratification that grades upwards to a
fine ash. Its bulk volume is estimated to be 0.7 km3.
The F-2 ash-flow deposits are followed by 15–20 ash-fall
beds, each 5–15 cm thick and well-stratified, which have
the same composition as the initial plinian phase and
apparently represent a series of discrete explosions during
the waning phases of this episode. These beds are best
exposed at Boliche and Pucahuaicu where the episode 2
sequence ends in a series of discrete, well-sorted lapilli-fall
layers, each consisting of 50% white, slightly fibrous
pumice and 50% angular gray aphyric rhyolite and obsidian
Overlying the 6,300 years BP peat, episode 3 begins with
an air-fall unit of andesitic scoria and lithic lapilli seen only
on Cotopaxi’s west flank. Its base consists of a 10-cm-thick
layer of slightly vesiculated, dark scoriaceous clasts that
have streaks of white pumiceous material, again suggesting
a magma mingling onset. The unit becomes richer in scoria
upwards and ends with a dark andesitic scoria. The
presence of possible mixed magma clasts in several discrete
units of this episode would again suggest the close
proximity in time and space of two magma types.
Subsequently there is a modest series of rhyolitic ashfall, ash-flow, surges, and co-ignimbritic ash-fall deposits
which are found only locally around the volcano. A plinian
lapilli-fall deposit (F-3), composed of white pumice
(SiO2 =75%; K2O=2.7%) and lithic lapilli, is observed
toward the top of the sequence at Tamboyacu, but it is not
prominent on the W side of the volcano. Unlike previous
rhyolitic fall deposits, its white microvesicular pumice has
more biotite (3–5%), which forms booklets up to 5 mm in
diameter. In addition, this pumice includes plagioclase and
a trace of magnetite and quartz, as well as a small amount
of black aphyric rhyolite lava and grey obsidian grains.
Generally the base of this unit is dominated by pumice,
while lithic fragments increase upwards. Overall the
proportion of hydrothermally altered clasts is less than
1%. On the E flank the F-3 lapilli-fall bed is followed by a
series of poorly sorted ash-fall layers and a small ash-flow
unit of similar composition. This flow traveled a short
distance down the Rio Tamboyacu into the Chalupas
caldera. The F-3 ash-fall bed is traceable 28 km to the
NE, where a soil dated at 5,940±30 14C years BP formed
Bull Volcanol
on top of ash-fall layers of this series (M. Monzier personal
communication 1998). The bulk volume of rhyolitic
material is estimated at 1.18 km3.
The largest eruptive events of the F series occurred
during episode F-4, and included a regional plinian lapilli
fall, large ash flows, and a block-and-ash flow. The basal
plinian fall deposit consists of a 1–4-m-thick pumice
lapilli layer, whose white microvesicular pumice (SiO2 =
75%; K2O=2.8%) carries plagioclase, biotite, magnetite,
±quartz, and small clasts of gray aphyric, sugary, or
aphanitic rhyolite lava and minor obsidian. The lower 10%
of the unit is greatly enriched in gray rhyolite lithics,
suggesting an explosion through a dome or the conduit.
The plinian fall unit has a widespread distribution and can
be traced >45 km to the E as well as 60 km to the W
(Fig. 9b), implying that the eruption cloud attained
stratospheric heights. It has a bulk volume of 5.3 km3.
Five thin rhyolite pumice lapilli-fall beds, containing
abundant obsidian and rhyolitic lava fragments, overlie
the F-4 plinian layer.
The sequence continues upwards with the F-4 ash-flow
unit, representing the largest ash flow of the F series. It
flowed eastwards into the Chalupas caldera for >20 km,
northwards down the Pita river valley to near Selva Alegre
(40 km), and southwestwards down the upper Cutuchi
drainage for at least 21 km, reaching Lasso (Figs. 8 and
10). Deposits of smaller ash flows are observed in the
Barrancas valley.
In the Salto and Pita river valleys the F-4 ash flow left a
10–15 m thick, inversely graded deposit that is creamy
white and matrix-rich (∼75–85%), and carries white microvesicular pumice clasts (<15%) with phenocrysts of
plagioclase, quartz, ∼5% amphibole, ∼1% biotite, magnetite, and ±hypersthene. The pumice is 68–71% SiO2 and
2.3–2.5% K2O. The deposits have 5–10% lithic fragments
primarily made up of gray or banded aphyric rhyolite lava
and gray to black obsidian. Hydrothermally altered fragments are more abundant toward the base. Clasts with dark
gray and white swirled streaks are also present, again
suggesting the possible intermingling of different magmas.
Also notable is the first appearance of amphibole, associated with less biotite. A xenocryst of melted quartz from the
gneissic basement was found in this ash flow (L. Hammersley
personal communication 2003). A carbonized tree root taken
from the soil directly underlying the F-4 ash flow in Cutuchi
canyon gave a date of 5,830±80 14C years BP. A 40-cmthick fine-grained co-ignimbritic ash is frequently observed
on top of the ash-flow layer. The F-4 ignimbrites have an
estimated bulk volume of about 2.8 km3.
A monolithologic rhyolitic block-and-ash-flow deposit,
having a volume of 0.24 km3, ends the episode 4 eruptive
sequence. It traveled 22 km down the most northern
Cutuchi drainage, apparently having come from a dome
collapse high up on the NNW side of the cone. Near its
terminus the >7-m-thick, homogeneous, rosy-gray deposit
is composed of 20–40% angular gray clasts up to 14 cm in
diameter, dispersed in a pinkish-gray lithic sandy matrix
(60–80%). These rhyolite clasts and sparse pumiceous
dome clasts have identical chemical compositions (SiO2 =
69%; K2O=2.4%). The vitrophyric rhyolite contains few
plagioclase, hypersthene, and oxyhornblende phenocrysts
in a glassy matrix. Apparently dome formation and
destruction were important events in this late stage of the
F series.
A khaki-colored 1–2-m-thick ash-rich debris-flow deposit, carrying pumice, rhyolite, and obsidian clasts (10%)
in a fine-sandy matrix (90%), overlies the F-4 block-andash-flow unit. It is found only in the Cutuchi, Pucahuaicu,
and Saquimala valleys, down which it traveled more than
50 km. Presumably it formed by the erosion of both the
F-4 ash-flow and block-and-ash-flow deposits. Episode
F-4 has a total bulk volume of ∼8.3 km3.
Fig. 10 In San Lorenzo valley the F-4 ignimbrite and F-5 andesitic
ash-fall units of the F series are closely associated, implying a rapid
compositional change in magma type. The Chillos Valley lahar of the
Colorado Canyon rhyolite episode was followed in turn by the late
Holocene andesitic activity
Following episode 4, there was a short repose in activity, as
suggested by the 5 m of relief developed locally upon the
youngest unconsolidated F-4 units. Episode 5 begins with
considerable andesitic activity, is followed by a dacitic
outburst of limited extent, and ends with minor, sporadic
andesitic activity.
Bull Volcanol
The initial andesitic activity is represented by a series
of scoria and pumice lapilli falls, blocky lava flows, and
associated debris flows, a sequence in which only a few
thin soils occur. The fall deposits are thin scoria > lithic
and pumice > lithic lapilli units in which the vesiculated
products have compositions that increase from about 58
to 62% SiO2 and 1.4 to 1.8% K2O, progressing from the
older to the younger beds. These units are mainly
observed on the cone and its lower flanks, the exception
being a widely distributed lapilli-fall deposit (unit GF)
that serves as a local marker bed (Fig. 7). The GF bed is
2.5 m thick at the SE foot of the cone and is traceable to
the E, NE, NW, and W of the volcano, covering
∼3,700 km2; its bulk volume is about 0.96 km3. It is an
extremely well-sorted lapilli deposit containing two clast
types: a slightly vesicular tan pumice (SiO2 = 62%; K2O =
1.8%) and a medium gray, dense andesitic lava (SiO2
∼59%; K2O ∼1.6%), both being plagioclase-phyric. The
pumice becomes less vesicular and less abundant upwards
in the unit, and contains equal amounts (∼5–7%) of augite
and hypersthene, 3% magnetite, and 10% plagioclase
phenocrysts.
Andesitic lavas associated with these tephra units occur
along the SE flank of the volcano. The lavas are short,
blocky flows that extend 2–3 km downhill toward the SE
and E foot of the cone. The pinkish-grey lavas (57% SiO2
and 1.2% K2O) bear characteristic green and white clots,
comprised of plagioclase (60%), augite (20%), and hypersthene (10–20%) crystals, scattered in a rosy-grey, aphanitic
groundmass. The several flows have a combined area of
∼9 km2 and a volume of ∼0.14 km3, clearly indicating an
important emission of andesitic magma.
Shortly after this andesitic activity, a dacitic ash flow
was generated which traveled 10–14 km down the Rio Pita
valley and a similar distance down the Rio Cutuchi valley.
This unit is unique, because its pumice has the only
intermediate chemical composition (SiO2 =66%; K2O=
1.7%) in the recent history of Cotopaxi. Near Ingaloma
its very white deposit rests upon a thin soil that overlies
the GF marker bed, suggesting that a short repose
followed the GF airfall. This ash-flow unit is composed
of highly vesiculated white pumice that has phenocrysts of
plagioclase, 5% amphibole, 2% magnetite, 2% hypersthene, a trace of biotite, and few lithic fragments. This
mineral assemblage is similar to that of the slightly older
F-4 ash flow, but contains more hypersthene and less
biotite, and the pumice is less silicic. The ignimbrite
covers approximately 56 km2 and has a bulk volume of
∼0.56 km3. In the upper Rio Pita drainage the deposits of a
dacitic ash-rich debris flow are associated with this unit,
probably generated by the transformation of the ash flow to
a lahar, as the result of melting of Cotopaxi’s glacial cover
by the ash flow.
The duration of episode 5 is constrained by the final
dates of episode 4 (5,830±80 14C years BP) and the two
dates of 4,420±80 14C years BP and 4,670±70 14C years
BP (Smyth 1991) obtained from soils that overlie the F-5
sequence. In summary, episode 5 began with intense
andesitic magmatism, notable for its large volume
(∼1.1 km3), and yet finished with another outburst of
relatively silicic magma.
Discussion of the F rhyolite series
The Holocene F series represents the first major activity of
Cotopaxi following a >400 ka long repose interval. Its five
rhyolitic eruptive episodes lasting 8,700 years involved
plinian falls of regional distribution, numerous ash flows
and surges some traveling >40 km from the crater, domecollapse flows with ubiquitous fragments of destroyed
domes showing up in most eruptive products, and ash-rich
debris flows. Consistently the rhyolites contain plagioclase,
biotite, magnetite, ±hypersthene, and ±quartz, and only in
the F-4 and F-5 ash flows does amphibole appear. Notably,
amphibole replaces biotite as the dominant mafic mineral in
the transition from the F-4 plinian fall to the F-4 ash flow
units, implying the deeper sampling of a vertically zoned
rhyolite chamber or conduit. The steady decrease in both
SiO2 and K2O in the rhyolites from F-1 to F-5 also suggests
a progressive depletion of the higher parts of a zoned
rhyolitic body. On the other hand, andesitic magmas (SiO2 =
57%–60%) erupted repeatedly during this series, clearly
indicating that andesitic magma already existed or was rising
from depth. Progressive magma mingling as suggested by
the presence of streaked pumices in the basal units of several
F episodes may also be a plausible mechanism to explain the
observed chemical trends. Garrison et al. (2006) found
chemical and isotopic evidence of repeated andesitic
injections during the last 8,000 years. If so, an origin
involving the injection and mixing of limited amounts of
andesitic magma into a resident rhyolitic magma body might
be valid, especially for the later F episodes.
The size of Cotopaxi’s edifice at this time remains
unclear. The radial distribution of the F series ignimbrites to
the S, SE, W, and N implies that these flows came from a
central vent. The 22 km long run-out of the F-4 block-andash flow, if it was an unfluidized, gravitational flow formed
by dome collapse, would suggest that it came from an
elevation of about 5,400 m, based upon energy-line arguments (Siebert 1984). Conversely, the near total absence of
lahar units within the F series might suggest a cone of low
elevation with limited glacier cover. Possibly, a high cone
did exist at this time, albeit with a variable snow and ice
cover.
The two principal plinian fall units of the F series (F-2
and F-4) have smooth lobate isopachs with major fall axes
Bull Volcanol
(145 km), which gave soil ages of 5,350±110 and 7,510±
80 14C years BP (Athens and Ward 1999), and are thought
to be the equivalent of the F-2 and F-4 fall units.
trending almost due W (Fig. 9). Both units maintain
thicknesses up to 10 cm as far away as Sigchos and
Pucayacu, 50 and 70 km to the W, respectively. However,
the F-4 fall unit had a notable distribution to the NW as
well; it is seen along the Pan American Highway as far as
Quito, 60 km to the N, where ash formed a 1.2-m-thick
accumulation. Its presence is confirmed by the mineral and
textural nature of the pumice, a date of about 5,800 14C
years BP, and by its chemistry (A. Alvarado personal
communication 1998). Additionally, the fine-grained fraction of the plinian fall was carried by stratospheric winds
far to the NE and has been identified in stratigraphic
sequences in the Papallacta (50 km) and Oyacachi (70 km)
areas. Two rhyolitic ash beds were found as far E as Coca
Fig. 11 Geologic map of the
Colorado Canyon rhyolite activity of Cotopaxi II A and the
subsequent andesitic lava outpourings of Cotopaxi II B
Colorado Canyon rhyolite episode
Upon the soil dated at 4,420±80 and 4,670±70 14C years
BP, a new rhyolitic eruptive cycle of regional importance
began, herein called the Colorado Canyon rhyolite episode,
named for the good exposures in that valley at the northern
foot of the volcano. The sequential order of eruptive events,
whose deposits are best observed around the N and NE
base of the cone, is presented below in order of decreasing
age (Figs. 11 and 12). This order varies locally, however,
Chillos
0
5
Valley
Glaciers and moraines
(approx. limits)
10 km
N
COTOPAXI II B
Post - Colorado Canyon
Andesitic Activity
B lavas (1880 - 1195 yBP)
A lavas (4060 - 2000 yBP)
approximate
limits of
debris flows
PASOCHOA
VOLCANO
COTOPAXI II A
Colorado Canyon Rhyolite
Episode (4500 yBP)
Chillos Valley debris flow
Avalanche hummocks
and deposits
Outcrops - ash flows 1-2
mainly
rhyolitic
hummocks
Rhyolite breccia flow
Limits of both main ash flow
and avalanche deposits
Plinian ash fall limits
mainly
andesitic
hummocks
on Sincholahua
slopes
RUMIÑAHUI
VOLCANO
San Agustin
lava flows
Cotopaxi II B
andesitic
cone
Estimated limit of glaciers
between 6 and 4 ka ago,
based upon glacier-disturbed
tephra stratigraphy
approximate
limits of
debris flows
Colorado Canyon
Yanasacha
Peñas Blancas
Bull Volcanol
top of Colorado Canyon Series
meters
3
.1
paleosol: dated at 4170±110 ** and 3950±70 14C yr BP*
rosy scoria bomb flow (4460±140 14C yr BP**);
series of three scoria and lithic lap AF
3
ash flow 3: similar to ash flow 2
2
Chillos Valley Lahar: rosy-tan pumiceous ash-rich
cohesive lahar; dated at 4500 14 C yr BP***
3
50100
.1
< 25
1
1-2
< 20
.02
.02
>4
.4
ash flow 2: similar to lower ash flow, but with more
obsidian and few andesite clasts
sector-collapse breccia: hummock formation, being
andesite clast-rich to northeast; rhyolite clast-rich to
to north; overlain by <115 cm of lithic powder
three thin sandy layers of aphyric rhyolite: blast-like deposits
ash flow 1: rosy-tannish white pum ash flow with andesite,
obsidian, and rhyolite clasts
pum surge beds
white pum lap plinian AF
dome-collapse breccia of obsidian and rhyolite clasts in
obsidian-rich sandy matrix; overlain by a thin co-ignimbritic ash
white sand-size pum AF
obsidian-rich sandy AF: phreatomagmatic explosion
debris flow with black andesite clasts
paleosol: dated 4670±70* and 4420±80 14C yr BP*
top of F Rhyolite Series
Fig. 12 Colorado Canyon composite stratigraphic column; abbreviations and sources of radiocarbon dates as in Figs. 5 and 7
due to chaotic or inverted stratigraphic relations, suggesting
that many events are contemporaneous or closely related in
time. This is true for the rhyolite breccia flow, the
avalanche, the ash flows, and the debris flow.
Phreatomagmatic eruption and the ensuing rhyolite
breccia flow
Overlying the thin soils that conclude the F series is a thin
layer of obsidian-rich sand, followed by a white pumice fall
unit. This 2-cm-thick bed of rosy beige-colored obsidian
sand grains (<1 mm) is only observed in a few outcrops to
the N and NNE of the volcano. The overlying pumice fall
bed is notable for the small, uniform grain size (∼3 mm) of
its pumice. The obsidian-sand unit and pumice fall bed are
considered to be of phreatomagmatic origin, triggered by a
short-lived leakage of the main rhyolitic magma. These
directly underlie and probably immediately preceded the
rhyolitic breccia.
Shortly after this explosive onset a polylithic rhyolite
breccia flow was emplaced. Its deposit is a light gray,
poorly sorted, homogeneous unit, comprised of ∼30%
angular lithic clasts up to 25 cm in size dispersed in a
matrix (∼70%) of gray lithic-and-obsidian-rich sand of fineto-medium grain size with a notable lack of smaller fines.
Black and gray obsidian, black perlite, red-and-gray-banded
rhyolite (SiO2 =74%; K2O=2.8%) and light gray aphyric to
biotite-phyric rhyolite comprise these clasts which appar-
ently came from domes of the F series, given their similar
chemical composition. No vesiculated juvenile material
occurs in the deposit, nor andesitic or hydrothermally
altered fragments. The outcrop distribution implies that
the flow traveled northwards at least 8 km from the base of
the cone (Fig. 11). This deposit is covered locally by a thin,
periclinally deposited layer of gray silt-size ash with
abundant biotite, presumably elutriated from the flow. The
thickness of the breccia deposit is highly variable, but is
generally 7–20 m thick along its axis. Its volume is about
46×106 m3. This unit is considered to be the result of a
violent collapse of older domes, possibly triggered by the
preceding phreatomagmatic explosion.
Plinian airfall and ash-flow series 1
Following the breccia flow the eruption of the main plinian
pumice lapilli fall left a 1 m-thick uniform deposit,
composed of white, highly-vesicular, fibrous pumice containing plagioclase, quartz, 1% magnetite, and <4% biotite
booklets, 2–3 mm in size. The pumice (SiO2 =75%; K2O=
4.3%) is higher in K2O and other incompatible elements
with respect to F series pumices, but is similar to that of the
Chalupas ash flow. In addition, the pumice carries <5%
lithic fragments of light gray aphyric rhyolite and some
obsidian. The well-sorted fall deposit is best exposed at the
mouth of Colorado Canyon, where it is 2 m thick and
overlies an andesitic lahar bed. Here the fall unit becomes
finer-grained upwards, and accretionary lapilli are observed
at the top. This unit is also seen to the W and NW of the
volcano, especially at Pucahuaicu and Boliche. It has been
traced as far as 25 km to the W, but is not seen to the E, S,
or N of the cone; its bulk volume is estimated at 0.47 km3.
Directly overlying the plinian fall deposit is the principal pyroclastic flow unit of this episode, here labeled ash
flow 1 (Fig. 13). It begins with a series of pumiceous surge
layers totaling about 1 m in thickness and grades upwards
into the main ash-flow deposit that is up to 25 m thick. It is
a rosy tan-colored, matrix-rich (∼70%) unit, containing
clasts of white, well-vesiculated pumice (25%), and small
grains (3–5%) of black and gray obsidian, gray aphyric
rhyolites, and andesite. The pumice contains plagioclase,
quartz, ∼3% biotite, magnetite, and a trace of hypersthene.
One xenolith of altered gneiss was found in the basal part
of the ash flow. The pumice (SiO2 =75%; K2O=4.4%) is
chemically similar to that of the preceding plinian fall, as
well as to that of the Chalupas ash flow. It is everywhere
inversely graded with pumice clasts increasing upwards,
suggesting that the flow was notably fluidized.
This ash flow swept over the northern and northeastern
flanks of the volcano (Fig. 11). Its deposits are found at the
northern foot of the cone, on the sides of Ingaloma ridge, in
the Mudadero area to the NE, northwards across the wide
Bull Volcanol
Fig. 13 In Colorado Canyon the plinian pumice lapilli-fall layer and
the overlying ash-flow 1 unit are exposed. Two andesite lava flows of
cycles J and JJ of the late Holocene andesite series were dammed by
the eroded edge of the Colorado Canyon ash flow fan. A post-lahar
scoria bomb flow of 1877 is seen in the foreground
Rio Pita and Salto valleys, and finally down the Rio Pita
canyon for 25 km. This unit has not been found in the
Cutuchi drainage, implying that its source was on the NNE
side of the edifice. This ash flow left a 10–20-m-thick
ignimbritic mantle over the Rio Pita and Salto valleys, as
shown by the distribution and heights of remnant hills,
which are often mistaken for avalanche hummocks.
Significantly this unit is not observed in the hummocky
terrain left by the following sector collapse, clearly
implying that this ash flow predates the collapse event. It
covers about 194 km2 and has a volume of 1.9 km3.
by subsequent lahar and fluvial activity, or covered by
younger tephra units, yet the distribution and maximum
limits of the avalanche are readily discernible (Smyth and
Clapperton 1986). The present deposits cover an area of
138 km2, and their original volume is estimated at 2.1 km3.
Distinct lithologic facies within the avalanche deposit are
seen in different hummocks and areas. Close to the NNE
foot of the cone, clast-supported andesite breccias and
megablocks constitute the largest and tallest hummocks,
some 170 m high and <500 m in diameter (Fig. 14). The
dominant rock is a light gray, two-pyroxene andesite,
similar to that generated at the end of the F series period;
however, older andesites (some altered) are occasionally
found, as well as gray rhyolites. Smaller hills of andesite
blocks and fragments are found spread out in great
abundance between 025° and 065°, the farthest being about
11 km to the NE across the Rio Pita valley and up onto the
lower slopes of Sincholagua volcano to an elevation of
about 4,100 m (Figs. 2 and 11). There, hummocks were left
on the floor of glacial valleys and upon lateral moraines,
assigned to the late Last Glaciation Maximum (18–14 ka;
Clapperton 1993). A fine dust layer, up to 115 cm thick,
locally overlies the deposit.
Twenty kilometers down the Rio Salto valley, many
small hummocks are found on the southern slopes of
Pasochoa volcano. These hummocks are comprised
mainly of unaltered, light gray aphyric rhyolitic clasts
(but no obsidian) and relatively few andesitic fragments,
all clearly belonging to a Cotopaxi source. Chemical
analyses of many avalanche clasts clearly denote two
compositional populations: (1) andesites similar to the late
F series andesites, and (2) a rhyolitic grouping very
Sector collapse avalanche
Subsequently a sector collapse was generated on the NE
side of Cotopaxi. Today, there is no obvious amphitheater
on this side of the cone, possibly due to its infilling by
younger eruption products and glaciers. However, a shallow
elongate depression lies below the Yanasacha rock face
near the volcano’s summit and may be the remnant scar of
the slide. A recent radar survey across this glacier-filled
depression found it to be 120 m deep (J. Ramirez
unpublished data 2006), and thus a possible source area
for the avalanche. At the NE foot of the volcano an extensive
avalanche hummock field spreads out northeastwards from
the cone in a fan that arcs from about 355° N to 065° NE, with
its main axis lying at approximately 025° NE. Many
hummocks have been eroded from the center of the valley
Fig. 14 Two of the largest avalanche hummocks lie at the base of the
northeast face of Cotopaxi. The inferred source area for the 4,500 years
BP sector collapse lies below Yanasacha. The taller hummock is
170 m high. Cotopaxi’s summit is 10 km distant in this photo
Bull Volcanol
similar to the late F series rhyolites; both rock groups
apparently co-existed high up on the cone.
The maximum height of Cotopaxi’s cone at that time
can be estimated by employing the energy-line concept
for unfluidized gravity slides (Siebert 1984). As such, the
andesitic portions of the debris avalanche that were
directed to the NE might have come from an elevation of
∼4,900 m. Similarly, the rhyolitic portions that have
longer run-outs to the N might have descended from
elevations of 5.4–5.9 km, if the avalanche was not
fluidized.
Along the SE side of Ingaloma, the temporal sequence
of events is best exposed. Overlying ash flow 1 but
underlying the andesitic avalanche deposit, a thin powdery layer is found that is coarsest at the base and finergrained at the top. It consists of small (1–2 mm) angular
particles of aphyric rhyolite, dispersed in a light gray
powdery matrix of pulverized rock. This thin deposit may
represent a blast event that immediately preceded the
avalanche.
The erosive ability of the avalanche flow during
emplacement was variable. In places the inferred blast
deposit was left intact, while in many other areas the flow
was highly erosive and scraped away pre-existing materials,
at times gouging deeply into the older Colorado Canyon
units and the Chalupas ash-flow deposit. On the SE side of
Ingaloma, the avalanche clearly plowed into pre-existing
ash-flow and rhyolite-breccia deposits. Nearby, a veneer of
andesitic avalanche material rests upon the scoured surface
of ash-flow 1 unit and is covered in turn by the later Chillos
valley lahar deposit (Fig. 15).
Fig. 15 Partial sequence of the Colorado Canyon series exposed on
the west side of Ingaloma. The deposits of ash flow 1 lie at the base
with the overlying avalanche debris and the Chillos Valley cohesive
debris flow above
Ash-flow series 2 and 3 and the Chillos valley lahar
Following the sector collapse, a second group of ash flows,
the so-called ash-flow series 2, were erupted and their
deposits are found at the E, NE, and N foot of the cone and
down the Rio Salto valley. They also descended the upper
Cutuchi drainage and almost reached Lasso, about 17 km
away. They form a rosy tan-colored, matrix-rich (92%)
deposit, several meters thick, that carries white microvesicular pumice clasts (8%), bearing plagioclase, 2%
biotite and magnetite, ± quartz, as well as 1% aphyric
rhyolite and obsidian clasts and few andesite fragments.
The pumice composition (SiO2 =72%; K2O=2.5%) is most
similar to that of the F series pumices and significantly
different than that of the plinian lapilli-fall and ash-flow 1
units. Some deposits contain pods of andesitic avalanche
debris and rhyolitic dome-collapse breccia. Its volume is
roughly estimated at ∼1.5 km3.
Synchronous with ash-flow series 2, as its lahar deposits
both underlie and overlie these ash-flow units, is a debrisflow deposit of gigantic proportions. This is the Chillos
valley lahar (CVL) (Mothes et al. 1998) which has a rosy
tan pumiceous matrix (80–90%), made up of ash, pumice,
and lithic grains. The remaining 10–20% are lithic clasts
>1 cm in size, comprised of perlitic obsidian, black and
gray obsidian, red-and-black-banded rhyolites, gray aphyric
rhyolites, and black plagioclase-phyric andesites. This ashrich debris flow left a homogeneous, 1–2-m-thick deposit
that mantles the extended floodplains of the Pita, San
Pedro, and Guayllabamba river systems, from the volcano
to the Pacific Ocean, 326 km distant. Furthermore, CVL
deposits are found in the Tamboyacu drainage eastwards, as
well as in the Cutuchi drainages to the SW. Its total volume
is estimated at 3.8 km3. This debris flow was apparently
generated by the transformation of hot ash flows 1 and 2
into a cohesive lahar by mixing with water derived from the
rapid melting of Cotopaxi’s glacier cap. Two radiocarbon
dates on logs caught up in the flow gave ages around
4,500 years BP (N. Banks personal communication 1988).
Everywhere it is observed, ash flow 3 overlies both ash
flow 2 and the CVL deposits. It is a yellowish tan-colored,
matrix-rich deposit containing white pumice and 1% vitric
grains, perlitic fragments, and red-and-gray-banded rhyolitic clasts. The pumice carries a trace of amphibole, in
addition to plagioclase, 1% biotite, 3% hypersthene, and
2% magnetite. It is significantly smaller in volume and
extent than the previous ash flows with a distribution
restricted to the N and NE sides of the cone. Chemically,
the pumice (SiO2 =75%; K2O=2.8%) is most similar to that
of ash flow 2. It is overlain by a series of three scoria and
lithic lapilli-fall deposits and a scoria pyroclastic flow dated
at 4,460±140 14C years BP, which are overlain by a soil
dated at 4,170±110 14C years BP (both dates from Barberi
Bull Volcanol
et al. 1995) and 3,950±70 14C years BP (Smyth 1991),
providing closing dates for Cotopaxi’s last large rhyolitic
event.
this magma might have had a greater volatile content and
thus greater mobility, which may have favored its escape
from the Chalupas body.
Discussion of the Colorado Canyon episode
The Colorado Canyon rhyolite episode represents a
continuation of the F series activity, presented here as a
separate episode to emphasize its varied sequence. This
episode began with a phreatomagmatic explosion through
pre-existing domes or conduits probably belonging to the F
series, given the abundant rhyolitic lava and obsidian
fragments and the lack of obvious juvenile products in the
resulting breccia flow. With the explosion the conduit
became unplugged, leading to the probable decompression
of the rhyolitic magma and the formation of a large plinian
eruption that resulted in the ensuing pumice lapilli fall and
ash flow 1. These eruptive events may have destabilized
Cotopaxi’s NE flank, triggering its sector collapse. The
main part of the debris avalanche, composed chiefly of
andesitic components, traveled northeastwards toward
Sincholahua volcano, whereas rhyolitic-rich portions traveled northwards down the Rio Salto valley to Pasochoa
volcano. Smyth and Clapperton (1986), appreciating the
apparently great mobility of the avalanche despite its small
size, concluded that its mobility could be explained by (1)
an explosive initiation, (2) the 2 km vertical drop from the
cone, or (3) an increased fluidity due to the probable
generation of considerable steam in the process. Apparently all three factors may have been important in
producing the observed result. The passage of both the
avalanche and the hot ash flows resulted in the rapid
melting of the glaciers and the formation of the gigantic
Chillos valley lahar, possibly the largest cohesive-type
debris flow of ash-flow origin as yet recognized (Mothes
et al. 1998). The total volume of juvenile rhyolitic
material extruded during the Colorado Canyon episode is
estimated at ∼4.0 km3.
The early Colorado Canyon magma can not be the
continuation of the rhyolitic magma stream belonging to the
last erupted magma (F-5), which was rich in amphibole and
poorer in silica, but must represent a new rhyolitic
injection. The pumices of this early magma are strongly
enriched in incompatible elements, very similar to those of
the Chalupas ash flow, whereas the later pumices of ash
flows 2 and 3 and associated air-fall deposits are more akin
to those of the Barrancas and F series (ref. comparison with
the Chalupas unit presented earlier). As such, Garrison et
al. (2003) concluded that this evolved rhyolitic magma may
have originated by the re-melting of a possible Chalupas
pluton and its subsequent emission at Cotopaxi. Also,
Colorado Canyon’s early pumices are more highly vesiculated and fibrous than previous pumices, suggesting that
Cotopaxi II B
Late Holocene to present andesite episode
From the end of the Colorado Canyon episode 4,000 years
ago until the present, Cotopaxi has experienced a continuous series of periodic eruptions, all of which have
involved andesitic magma. The one exception corresponds
to a thin rhyolitic ash estimated to be about 2,100 years BP
which is chemically similar to ash flows 2 and 3 of the
Colorado Canyon series. During this andesitic episode there
have been at least 18 eruptive cycles, amounting to at least
32 eruptions of at least moderate size (VEI=3). The cycles,
labeled H through P in the stratigraphic column (Fig. 16),
are defined by repose intervals such as soil horizons or
initial plinian airfall deposits. Each cycle is characterized by
a similar eruptive pattern, involving plinian scoria or
pumice tephra falls, scoria or pumice pyroclastic flows,
blocky-lava flows, and widespread debris flows. All
eruptive products of this episode are two pyroxene
andesites with 57–62% SiO2, 1.2–1.7% K2O, and a
phenocryst assemblage that includes plagioclase, hypersthene, augite, magnetite, and olivine. Generally the
hypersthene is 2–3 times more abundant than augite.
Olivine is occasionally present in trace amounts, but
increases up to 2% in the youngest rocks.
A detailed stratigraphic column of this andesitic episode
is presented in Fig. 16 and the pertinent geologic maps in
Figs. 17 and 18. This composite column is based upon
several hundred stratigraphic sections measured and studied
on every side of the cone, in the adjacent river valleys, and
in the InterAndean Valley, downwind from the volcano,
resulting in a much more detailed compilation of recent
activity than that presented in previous studies (cf. Barberi
et al. 1995). Their 14C dates, as well as those from other
sources, have been used in our final column, where they
mostly fall in a logical, sequential order. The chief
characteristics of this andesitic episode are discussed below.
Pyroclastic flows
Almost every eruptive cycle of this episode has been
accompanied by pyroclastic flows whose deposits are found
on the cone or on the depositional fans that surround the
cone. Most have runouts of 6–9 km from the crater and the
farthest traveled 19 km. Their distribution implies that most
originated from the summit area.
Bull Volcanol
Fig. 16 Late Holocene to
Present composite stratigraphic column of Cotopaxi II B;
abbreviations and sources of
radiocarbon dates as in Figs. 5
and 7
meters
.04
.03
.2
1.5
3
P
1.5
.05
.15
1
.10
.15
.10
.20
debris flow: Dec. 1742 AD
tan pum lap AF w/ colonial ceramics at Callo
.10
.10
MZ
.15
Y
.15
.20
.30
.15
.15
.15
.20
.10
2.5
X
.10
.15
L2
1.5
.10
L1
1.0
.3
.25
paleosol: dated 1770±110 14C yr BP**
hot debris flow
lava flows and lava flow collapse PF: N flank
scoria lap AF w/ altered lithics
2nd lava type A: at Colorado Canyon
.3
debris flow w/ scoria bombs
rosy grey pum lap AF
ashy paleosol
.2
J
debris flow
tan pum and lithic lap AF
debris flow
glacial clay and paleosol: 820 ±80 14C yr BP**
ash cloud surge
debris flow
rosy-tan pum and bomb PF
debris flow
lava flow: at Refugio
scoria bomb AF and PF
several paleosol horizons w/ scoria AF layers
(2050±80; 2170±100; 2310±90 14C yr BP)**
12
JJ
gray pum lap AF w/ lithic band at top
tan pum lapilli AF
lava type B: at Tamboyacu, N flank
.4
.1
1st lava type A: at Colorado Canyon
yellow tan pum lap AF and PF
co-ignim. ash: Ninahuilca-2269±16 14C yr BP****
lava flow collapse PF
lava type A: at San Agustin
.3
.2
.3
.1
.25
.4
scoria bomb PF
tan pum lap AF
paleosol
two hot debris flows w/ scoria bombs
lava type A: at Tamboyacu, NE flank
tan pum lap AF w/ gray lithics
co-ignimbritic ash: Cuicocha~2900 yBP -?
two yellow tan pum lap AF
buff white pum lap AF w/ grey lithics
hot debris flows w/ scoria bombs
.2
lava type A: E flank, overran hummocks
paleosol: dated 4170±110** 3950±70 14 C yr BP*
co-ignim. ash: Ninahuilca-4440±35 14C yr BP****
rosy scoria bomb PF: - 4460±140 14C yr BP**
underlain by:
Colorado Canyon Rhyolite Series
lava type B: at Burrohuaicu, Limpiopungo
KB1
2
1.5
KA1
JK
I2
debris flow w/ colonial tiles: June 1742 AD
paleosol: repose of 1532 to 1742 AD
debris flow: 1534 AD
scoria bomb PF and AF: 1534 AD
lava flow: Yanasacha flow
scoria lap and bomb AF: mixed magma clasts
I1
scoria lap AF and PF: 1532 AD
paleosol w/ Inca artifacts: 1470-1532 AD
Quilotoa AF: 840±50; 785±50; 900±150 14 C yr BP***
paleosol
tan pum sandy AF
H
tan pum lap AF
debris flow
tan pum lap AF
lithic and pum lap AF; glacial clay at base
scoria bomb PF
.2
.10
KB2
KA2
1.3
tan-gray coarse sandy ash AF
black sandy ash AF
tan dense pum lap AF: 1768 AD
.20
1
white pum lap AF: -PeÒas Blancas unit
ash-rich debris flow w/ PeÒas Blancas pum
PB
tan pum lap AF:--1853 AD?
debris flow w/ red oxidized clasts -1766 AD
tan pum lap AF: 1766 AD
pum and scoria lap AF: lithics at base -1744 AD
debris flow w/ colonial ceramics - 1743 AD
black scoria lap AF: 1743 AD
1
.07
M
present soil
scoria and lithic lap AF: 1880
brown fine ash AF: post-1877 debris flow
scoria and lithic lap AF: 1877 AD
2nd scoria bomb PF: 1877 AD
scoria bomb PF: 1877 AD
black lithic sandy AF: 1854 AD?
lava flow: W flank -1853
.2
.10
.6
8
scoria bomb and lapilli AF
scoria bomb and lava collapse PF: N and W flanks
debris flow
tan co-ignimbritic ash AF
debris flow: N,NE, E flanks
tan pum lap AF
paleosol
tan pum lap AF w/ oxidized clasts
rosy tan pum PF
Three types of flows are recognized. The most common
are scoria-bomb flows that descended the narrow canyons
of the cone and extended out onto the alluvial fans as
narrow, elongate flows having low levees. Their deposits
consist mainly of vesicular, cauliflower-head-shaped clasts
of reddish-black vuggy scoria, up to 1 m in diameter, that
adorn the exterior of the flows (Figs. 13 and 19). The
deposit’s interior consists of scoria and andesitic lithic
clasts (∼40%) in a matrix (∼60%) of silt-, sand-, and gravelsized fragments of the same material. The composition of
the scoria tends to be 57–58% SiO2 and 1.2–1.4% K2O.
The longest run-outs of these flows are typically 6–12 km.
5
0
ra
nta Cla
Río Sa
late Holocene andesite episode
(<1,195 years BP)
10 km
N
Río
Pita
Bull Volcanol
COTOPAXI II B
Late Holocene Andesite Episode
(< 1195 yr BP)
(approx. limits)
Historic debris flows
PASOCHOA
VOLCANO
C
Scoria flows
Pumice flows
Andesitic lavas
-younger flows
-older flows
Typical limits of a large
andesitic ash fall deposit
cm
Río
Pita
>5
SINCHOLAHUA
VOLCANO
Rí
oC
utu
ch
i
RUMI
UM Ñ
ÑAHUI
UI
VOLCANO
o
Rí
an
S
Río
Río
zo
ren
Lo
S
oyac
u
bo
la
ima
u
aq
Tamb
Río
Tam
Río Burrohu
aicu
ncas
arra
Río B
Given that the bombs have a vesiculated exterior, these
flows apparently originated by gas-rich magma spilling out
or by being thrown out of the crater or by the low-level
collapse of the eruption column. Their long run-outs
suggest that they were partially fluidized, since simple
gravitational flows descending Cotopaxi’s slopes are
calculated to reach only 4–5 km. In the eye-witnessed
eruption of 1877, Wolf (1878) reported that ‘a dark foamlike cloud boiled over the rim of the crater and descended
all sides of the cone, much like the boiling over of a pot of
cooking rice’ in reference to the scoria-bomb pyroclastic
flows whose deposits are still pristine today.
Another type of flow, second in abundance, are the lavaand-scoria-clast flows. These have distributions similar to
those of the scoria-bomb flows, but with somewhat shorter
run-outs. Their dark deposits are comprised chiefly of
angular andesitic lava clasts up to 10 cm in size and
subordinate amounts of black scoria. Many andesite clasts
are vesiculated on only one side, the remaining sides having
glassy or microcrystalline textures, implying that they were
derived from lava lakes or lava flow fronts. Scoria occurs as
small bombs, but more often as fractured, vesiculated clasts.
The matrix (∼50–60%) is comprised chiefly of sand- and
gravel-size particles of dark andesite and scoria, the latter
having a chemical composition similar to that of the scoria of
the scoria-bomb flows. These flows likely owe their origin to
(1) the collapse of lava flow fronts, or (2) explosions through
lava lakes, domes, or conduit plugs of solidified magma.
Bull Volcanol
Pita
10 km
Río
5
0
N
Río Santa Clar
a
products of the 26 June 1877
eruption
COTOPAXI II B
26 June 1877 eruption
Debris flows
PASOCHOA
O
VOLCANO
OLC
Scoria flows: synchronous
and post-debris flows
> 10 cm isopach limit
for 1877 scoria lapilli fall
SINCHOLAHUA
VOLCANO
ch
i
RUMI
M ÑAHUI
Ñ
VOLCANO
O
Rí
o
Cu
tu
Río
o
Rí
Sa
n
r
Lo
en
zo
Rí
o
S
u
aq
im
al
a
R
ío
m
Ta
Tamb
oyac
u
bo
Río Bu
rroh
uaicu
Río Barrancas
A third type of pyroclastic flow, much less common, are
pumice flows, whose deposits consist of khaki-colored,
well-vesiculated pumice clasts with red vuggy interiors (up
to 80%) and of small gray or black andesite fragments (5–
10%) in a rosy tan-colored, ash-rich matrix (20–80%). The
pumice of these flows is more silicic (60–62% SiO2; 1.7%
K2O). Like the other flows, these descended gullies and
canyons until they reached the foot of the cone, where they
crossed the alluvial fans, as narrow, elongate flows having
subdued convex cross-sections. The longest flow is 19 km.
The pumice clasts often form interflow segregations,
suggesting separate pulses, or are concentrated at the top
of the deposit, suggesting that the flows were somewhat
fluidized. These flows likely originated by partial or full
column collapse. Invariably they were associated with
prominent plinian airfalls that often left a widespread, 1–
2 m-thick, relatively well-sorted pumice lapilli bed.
Debris flows
Debris flow deposits are found everywhere around the
volcano, generally associated with pyroclastic flow deposits
in almost every eruptive cycle. Large debris flow fans make
up the northern, western, and eastern bases of the volcano
and from there their floodplain deposits can be traced down
river for many tens of kilometers. Near the volcano the
Bull Volcanol
textures of welded air-fall tuffs. These deposits are
thoroughly cemented, due to the formation of iron oxides
in the fine-grained matrix and on the surface of the clasts.
Presumably these lahars were deposited in a relatively hot
state, which facilitated their greater oxidation. At Cotopaxi
their presence may be due to (1) shorter run-outs, implying
less heat loss in transit, and (2) thicker accumulations,
implying greater heat retention; both factors would lead to
longer exposures to hot oxidizing conditions. Conceivably,
these deposits are those of hot scoria-bomb pyroclastic
flows, and not debris flows, that overran water-rich tephra.
Lava flows
Fig. 19 Scoria bomb pyroclastic flows with 2 m high levees
immediately followed the devastating debris flows of the 26 June
1877 eruption seen here on the west side of Ingaloma. Distance
between black points: ∼250 m
deposits are dark gray, clast-supported, well-packed breccias, usually several meters thick, which may include
blocks up to 8 m in size, although 2–20 cm-sized clasts
predominate. Most clasts are gray to black andesitic lavas;
few scoria fragments are observed, probably having been
ground up during transport. The matrix (20–40%) contains
silt to sand-size particles of the same rock types. At
distances greater than 50–70 km from the volcano their
deposits take on the textural character of hyper-concentrated stream flow deposits (Mothes et al. 2004).
Descriptions of the 1877 eruption and the resulting
debris flows in the Latacunga valley clearly indicate the
power, velocity, and nature of these flows (Wolf 1878;
Sodiro 1877). While not considered to have been a large
flow as compared to previous historic lahars, its distribution
and extent are nonetheless impressive, having inflicted
widespread destruction in the Chillos and Latacunga
valleys and along its 326 km-path to the sea (Fig. 18).
Most Cotopaxi debris flows were probably generated in a
similar fashion, as the result of instantaneous melting of the
glaciers by scoria pyroclastic flows, as described by Wolf
(1878). Jordan (1983) determined that the glacial cover in the
1970s was about 21 km2; today it is about 12 km2 (E. Cadier
personal communication 2007). The mid-Holocene glacial
cover was undoubtedly greater (Fig. 11).
Another type of debris-flow deposit is often observed,
also associated with scoria-bomb pyroclastic flows. These
form vertical cliffs, 6–10 m high, that draw attention
because of their brilliant reddish-color and crude vertical
columnar jointing. Their lithology, texture, and poor sorting
are similar to most debris flow deposits, although these are
matrix-rich and include scoria bombs; they lack the molten
Blocky lava flows have occurred in most eruptive cycles.
They form narrow (<1 km wide), lengthy flows on the
cone’s flanks that generally stop or spread out laterally
upon reaching the alluvial fan at the base of the cone.
Typically, the flows are 5–40 m high, 4–8 km in length, and
covered with jumbled angular blocks of lava.
Following the Colorado Canyon episode, a series of
eight andesitic lavas flowed down the W, N, NE, E, and SE
flanks of the volcano between 4,000 and 2,100 years BP.
Large flows occurred on the NE and N flanks, where they
overran the avalanche-hummock fields and flowed around
both sides of Ingaloma ridge (Fig. 11). These flows have a
combined area of 25 km2 and a volume of about 0.75 km3.
Subsequently in the I-1 cycle, the Tamboyacu lava flow
(volume=0.09 km3) descended the ESE side of the cone.
The largest lava flow of the episode, the more mafic San
Agustin lavas, traveled about 17 km and spread out over the
cone’s lower western slopes during the I-2 cycle; it has an
area of 42 km2 and a volume of 1.3 km3. During each of the
subsequent J, JJ, and JK cycles, smaller flows occurred
along the N foot of the cone. Petrographically the A-type
lavas of the H to JK cycles are identical, being microporphyritic andesites (SiO2 =57–59%; K2O=1.2–1.5%)
with a microcrystalline to somewhat vitreous groundmass
and with a light to medium gray color with a rosy tone.
Phenocrysts include plagioclase, hypersthene, augite, and
iron oxides, often grouped together in small green and
white clots; olivine is rarely present. The scoria pyroclastic
flows of the H to JK cycles often contain numerous angular
lava clasts suggesting that they might have originated from
collapsing lava flow fronts.
During the KB1 and KB2 cycles (1,880 to 1,195 years
BP) emissions of the B-type lavas produced small isolated
flows on both the W and E sides of the cone, as well as in
the upper Burrohuaicu canyon. The larger flows descended
the NW slopes and spilled out upon the W end of
Limpiopungo plain and near Ingaloma; their total area and
volume are estimated at 24 km2 and 0.87 km3. These are
dark gray to black, porphyritic andesites (SiO2 =59–60%;
Bull Volcanol
K2O=1.4–1.5%) with an aphanitic to glassy groundmass
and conspicuously large, square plagioclase phenocrysts
with a dark core. They are similar to the older A-type lavas,
although the latter have more mafic minerals and a more
mafic composition.
Belonging to the L, X, MZ, M, and P cycles is a series of
similar but smaller lava flows, each only 3–4 km long, that
are found on the lower NW, N, NE, SE, and SW flanks of
Cotopaxi, many of which were mapped by Wolf (1878).
The largest is the Yanasacha lava (0.05 km3), which
descended the NNW flank for 5.5 km and overran the
Limpiopungo KB flows during the MZ cycle (∼1532–
1534 A.D.). Stübel (in Wolf 1878) reported that the
Manzana-huaicu and Pucahuaicu lava flows of the P cycle
correspond to a flank eruption on Cotopaxi’s middle W side
in 1853. Wolf implies that flows also descended the ESE
side (Chiri-machay flow), the NE side (Diaz-chaiana flow),
and the N side (Tauri-pamba flow), but we have found
that these flows are all scoria-bomb pyroclastic flows and
not lava flows. In general these lavas tend to be dark gray
to black, microporphyritic andesites (SiO2 = 56–61%;
K2O=1.3–1.4%) with a glassy matrix. Small plagioclase,
hypersthene, augite, iron oxides, and ±olivine comprise
the <10% microphenocryst content. In the MZ cycle
olivine makes its first permanent appearance in Cotopaxi
rocks; the crystals are few but often the largest of the
phenocrysts. Milky white quartz xenocrysts up to several
centimeters long often occur in these flows.
In summary, the largest lava outpourings (∼2.1 km3) of
this episode occurred between ∼4,000 and ∼2,100 years
BP (cycles H-JK). Similar mineralogy and chemistry of
these lavas imply that all were derived from the same
magma injection, whose ascent may have triggered the
Colorado Canyon rhyolitic outbreak. More silicic andesites then erupted around 1,880–1,770 years BP and
continued until the MZ cycle (1532 A.D.). Finally, historic
eruptions after 1532 A.D. tend to have more mafic
andesites carrying plagioclase, olivine, hypersthene, augite, and iron oxides, indicating an injection of more basic
magma.
Tephra falls
Isopach maps of many of the late Holocene tephra falls
have been prepared which show good correlations with
those of Barberi et al. (1995), especially for air-fall units
J, KB, L, X, and M; the interested reader is referred to that
publication to see these maps.
All cycles were initiated or accompanied by coarse
lapilli fall deposits, often 10–20 cm thick, that are
comprised of either tannish-grey microvesicular pumice
or brownish-black, coarse-vesicular scoria clasts. At times
both types of clasts followed one another in quick
succession (e.g. in the L and M cycles). Within individual
cycles other air-fall layers often exist, but they tend to be
finer-grained and thinner. Tan-colored pumice lapilli fall
layers are more common in the early cycles (I through
KA2), but beginning with the KB cycle both pumice and
scoria fall units are found as associated, but discrete beds,
and the pumice takes on a darker tan color. Following the
M cycle, the lapilli air-fall units occasionally carry lapilli
of both pumice (59.3% SiO2; 1.3% K2O) and scoria
(56.9% SiO2; 1.2% K2O) in the same bed, suggestive of a
mixed magma origin.
Many of the plinian air-fall beds have recognizable
characteristics that make them useful marker units (e.g.
lithic types and contents, vertical grading, associated
stratigraphy). In addition, dated ash-fall units from
Quilotoa and Ninahuilca volcanoes aid in dating the
sequence. Given the prevailing winds from the E and
SE, tephra is mainly carried to the W, SW, and NW from
the volcano, where the most complete sequences are
found.
On the S flank of Ruminahui volcano, underlying the
KA unit, is found the Peñas Blancas airfall, a biotite-rich,
white pumice lapilli-fall layer containing minor amounts
of black lithic fragments (Fig. 16). The mineralogy and
chemistry of its pumice (SiO2 =73%; K2O=2.8%) correlate closely with that of the pumice from the late Colorado
Canyon ash flows, suggesting a genetic linkage. The
presence of well-sorted, lapilli-size pumice clasts implies
a nearby source. Downstream, a pumice- and ash-rich
debris-flow deposit occurs at the same stratigraphic level
and is apparently related to this rhyolitic event. In the
Lasso area and at other localities around the cone, the
Peñas Blancas event is represented by a fine white ash
with biotite that overlies the thick soil layer of the JK
cycle dated at about 2,100 years BP. This event apparently
corresponds to a small leakage from Cotopaxi’s rhyolitic
magma chamber.
Discussion of the late Holocene andesite episode
Each of the 18 eruptive cycles of the late Holocene eruptive
activity was characterized by a repeated pattern of events,
generally consisting of a plinian lapilli fall early in the
sequence, followed by pyroclastic flows, and often one or
more lava flows and associated lava-collapse flows. Debris
flows occurred frequently in the eruptive sequence. In
general there is a uniform distribution of flowage deposits
around the cone, implying that most flowage events came
from the summit crater or nearby.
The uncalibrated 14C dates employed in Fig. 16 show
good agreement among themselves as well as a good
chronological fit to the column. The 810 years BP age for
the Quilotoa ash fall is considered to be well established,
Bull Volcanol
Table 1 Character and number of late Holocene eruptive events
Eruption
cycles
Totals
P
Date
1880 AD
1877
1853–54
M
1768
1766
1744
1743
1742
MZ
1532–34 AD
Y
∼900 years
BP
X
1000 years
BP
L-2
1180 years
BP
L-1
1210 years
BP
KB-2
∼1770 years
BP
KB-1
<1880 years
BP
KA-2
1880–
2200 BP
KA-1
1880–
2200 years
BP
PENAS BLANCAS
∼2100 years BP
JK
2200 years
BP
JJ
J
2350 years
BP
I-3
I-2
I-1
H
∼4060 years
BP
No.
eruption
Plinian
eruption
Other
airfalls
Series of
pyroclastic flows
Lava
collapse
flows
Series of
debris flows
Series of
lava flows
43
1
2
3
1
1
1
1
3
2
4
32
1
1
1
1
1
1
1
2
2
3
23
>18
4
>32
>10
1
1
5
≥7
1
MANY
MANY
2 series
1
2 series
2
1
?
2
1?
1
1
1
>10
1
1
1
1
?
1
2
1
2
2
1
1
2
Q4
1
1
1
1
3–4
1
1
1
2
1
4
1
1
?
1
MANY
4
1
1
1
2
1
>4
1
1
1
1
4
1
1
1
?
4
1
1
3?
1
2–3
1
3?
1
1
1
2–3
1
1
1
4?
3
1
1
1
3
1
?
1
1
VEI
2–3
4
3–4
4
3
4
3–4
4
3–4
3–4
1
1
2–3
2
1
1
1
3
?
4
1
1
1
4
4
1
3
2
MANY
1
4
4
4
4
Note that “series” of flows refers to a large number of flowage events that may have descended all sides of the volcano. VEI values are
approximate.
since it is based upon an AMS and three 14C dates on
carbonized wood. The post-Quilotoa units are stratigraphically correlated to written historic records, where possible.
From Fig. 16 one can calculate the frequency of eruptive
cycles. The early cycles (I and J) roughly average 300 to
400 years/cycle, whereas the intermediate cycles (KA to Y)
average 100–150 years/cycle. During historic times there
have been significant periods of repose, such as that of
390 years between the Quilotoa ash (1140 A.D.) and the
1532 A.D. eruption, and that of 208 years between 1534 and
1742 A.D.. The onsets of the most recent eruptions have
occurred at intervals of 24, 85, and 128 years.
The character of past andesitic eruptions is shown in
Table 1, a pattern of associated eruptive events which is
likely to repeat itself in future eruptions; consequently,
mitigation efforts should take this pattern into account in
their planning and prevention activities. In Table 2 we
estimate the total bulk volume of andesitic magma emission
Bull Volcanol
Table 2 Volume of magma erupted over time-cotopaxi volcano
Unit-product
Age (years BP)
Area (km2)
Bulk volume
(km3)
DRE volume
(km3)
SubTotal DRE volume
(km3)
Barrancas rhyolite series
Ash flows
Tephra falls
∼500,000
∼500,000
300.0
550.0
19.000
13.200
5.70
3.96
9.66
Rio Pita andesite lavas
∼450,000
165.0
4.100
4.10
4.10
9,600
7,800
7,800
6,300
5,800
5,800
5,800
5,800
5,500
5,500
8.0
12100.0
140.0
3800.0
12000.0
280.0
16.0
3700.0
9.0
56.0
0.002
7.900
0.700
1.180
5.300
2.800
0.240
0.960
0.140
0.560
0.00
2.37
0.21
0.35
1.59
0.84
0.24
0.36
0.14
0.17
2.37
0.21
0.35
1.59
0.84
0.24
0.36
0.14
0.17
F rhyolite series
F-1 Tephra falls
F-2 Tephra fallsa
Ash flows
F-3 Tephra fallsa
F-4 Tephra fallsa
Ash flows
Block-and-Ash flows
F-5 GF Plinian fall (andesitic)a
Andesitic lavas
Ash flow (rhyolitic)
Total DRE Volume of F Series=6.27 km3
Colorado Canyon rhyolite episode
Rhyolitic breccia flow
Plinian falla
Ash flow 1
Sector collapse avalanche debris
Ash flow 2
Chillos Valley lahar
Ash flow 3 (too small to measure)
4,500
4,500
4,500
4,500
4,500
4,500
4,500
9.0
3360.0
194.0
138.0
150?
0.046
0.468
1.940
2.100
1.5?
3.800
N/D
0.05
0.14
0.58
2.10
0.45
0.00
0.00
4,200
4,000
4,000
3,000
3,000
3,000
2,350
2,350
2,200
2,200
2,000
2,000
1,900
1,900
1,800
1,770
1,770
1,770
25.0
3.0
1,450.0
42.0
23.0
2600.0
3.0
2,300.0
3.0
1,100.0
200.0
6.0
27.0
2,600.0
1,430.0
23.0
245.0
24.0
0.750
0.090
1.450
1.300
0.045
0.576
0.090
0.372
0.090
0.320
0.060
0.090
0.080
0.480
0.430
0.070
0.370
0.870
0.75
0.09
0.54
1.30
0.02
0.21
0.09
0.14
0.09
0.12
0.02
0.09
0.03
0.18
0.16
0.03
0.14
0.87
1,200
4.0
0.060
0.06
1,200
2,300.0
0.312
0.12
0.140
0.580
0.450
Total DRE Vol. juvenile Colorado Canyon S. = 1.17 km3
Late Holocene andesite episode
H Lavas between hummocks
I1 Tamboyacu lavas
2 Plinian falls
I2 San Agustin lava
2 scoria pyroclastic flows
Plinian falla
J 1st lava
Plinian falla
JJ 2nd lava
Plinian fall
JK Plinian scoria fall
Lava flow
Ka1 Pumiceous ash flow
Plinian falla
Ka2 Plinian fall
Kb1 Pyroclastic scoria flows
Plinian scoria fall
Lava B: Burrohuaicu and
Limpiopungo
Kb2 Lava B: Tamboyacu, N.
flank
Plinian falla
0.75
0.63
1.53
0.23
0.21
0.11
0.21
0.16
1.04
0.18
Bull Volcanol
Table 2 (continued)
Unit-product
L1 Scoria fall and pyroclastic
flow
Lava flow at Refugio
L2 Pyroclastic flow
X Pre-climatic fall
Main Plinian falla
Pyroclastic flow
Y 4 Pumiceous falls
MZ 2 scoria tephra falls
Yanasacha lava flow
Scoria fall and pyroclastic flow
M Pumice plinian falla
Scoria plinian fall
3 pumice falls
2 sandy ash falls
P Pumice tephra fall
Lava-W flank
Scoria pyroclastic flows -many
2 Scoria tephra falls
Age (years BP)
Area (km2)
Bulk volume
(km3)
DRE volume
(km3)
1,100
145.0
0.290
0.11
1,100
1,000
900
900
900
600
472
472
470
264
264
260
238
153
150
129
128
1.5
14.5
800.0
4,100.0
35.0
1,600.0
1,500.0
2.7
145.0
2,020.0
1,600.0
1,600.0
850.0
1,600.0
4.0
0.030
0.044
0.080
1.044
0.070
0.960
0.450
0.054
0.290
0.564
0.317
0.480
0.200
0.240
0.120
0.130
0.450
0.03
0.02
0.03
0.39
0.03
0.36
0.17
0.05
0.11
0.21
0.12
0.18
0.07
0.09
0.12
0.05
0.17
1,125.0
SubTotal DRE volume
(km3)
0.14
0.02
0.44
0.36
0.33
0.58
0.43
DRE Volume of Andesite Episode = 7.34 km3
Total ERUPTED DRE Volume in last 0.5 Ma = 28.54 km3
Most bulk volumes were calculated by rounding-out the unit’s known area multiplied by its average thickness. DRE conversions were made using
0.90 g/cm3 for andesitic scoria and 0.73 g/cm3 for rhyolitic pumice.
a
Volumes of important tephra fall units were calculated using the methods of Fierstein and Nathenson 1992, extrapolated to 0 cm thickness.
to be 14.82 km3 for the past 5800 years and present the rate
of andesitic magmatism versus time in Fig. 20. Average
andesitic emission rates were 1.65 km3 (DRE)/1000 years
from 4,200 to 2,100 years BP and 1.85 km3 (DRE)/
1,000 years for the past 2,100 years, showing an increase
with time. At nearby Tungurahua volcano, its magma
emission rate was estimated at 1.5 km3 per 1000 years (Hall
et al. 1999).
later by intermittent andesitic activity in the same SW
quadrant of the edifice, and later still by major
andesitic lava outpourings (∼4.1 km3) on the cone’s N
and NW sides.
3. After a repose of ∼400 ka without silicic magmatism,
during which the neighboring volcano, Chalupas, had a
100 km3 rhyolitic eruption at 211 ka, Cotopaxi became
Conclusions
Volume DRE in cubic
kilometers
1. Cotopaxi’s Holocene activity offers a rare look at recent
bimodal magmatism where rhyolitic and andesitic
magmas erupt in quick succession, from the same
conduit, displaying no or very limited intermingling.
The latest bimodal eruption series occurred only 4,500
and 2,100 years ago, clearly indicating that this activity
is on-going.
2. Early Cotopaxi history (∼420–560 ka) was characterized by rhyolitic eruptions comprised of dome
emplacements and collapses, glassy dikes, small to
moderate-sized rhyolitic ash flows and plinian tephra
falls (DRE Vol. ∼9.7 km3). This was followed much
rate of andesitic magmatism
DRE volume vs. time
10
8
6
4
2
-7000
-6000
-5000
-4000
-3000
-2000
-1000
0
years before present
Fig. 20 Cotopaxi’s accumulated rate of andesitic magmatism versus
time. Curve corresponds to an average rate of about 1.7 km3 of andesitic
magma per 1,000 years, which has increased during the past 500 years
Bull Volcanol
active and entertained eight rhyolitic outbursts (F series,
Colorado Canyon series, and the Peñas Blanco tephra
fall) between 13,200 and 2,100 years BP. The rhyolite
emission rate for the F series and Canyon Colorado
episodes totaled ∼6.9 km3 (DRE) over the 8700 year
duration. Andesitic tephra falls accompanied most of the
rhyolitic events, testifying to a simultaneous bimodal
magmatism. During the course of the F rhyolitic
emissions, the andesitic output slowly increased in
volume, while the rhyolites became slightly less-silicic
in composition. The lack of eruptive products with
compositions intermediate between andesite and rhyolite
and the rare examples of possible intermingled magmas
suggest that over 11,000 years there was no contact
between these two magma types or at best only a very
limited intermingling. Periodic magma withdrawal from
a compositionally zoned rhyolitic magma body may best
explain this succession.
4. Initial rhyolites of the 4,500 years BP Colorado Canyon
eruptive episode have a close chemical affinity to the
211 ka-Chalupas magma, while the subsequent rhyolites are similar to the less-evolved rhyolites of the
older Barrancas—F series. The appearance of a
Chalupas-like magma in the Colorado Canyon series
might be explained by (1) an injection of a new
rhyolitic magma, (2) limited re-melting of Barrancas
rhyolites, (3) the escape of remnant Chalupas magmas,
and (4) limited re-melting of a Chalupas pluton
(Garrison et al. 2003). Chemical studies in progress
should resolve the origin of the rhyolites.
5. The eruptive conduit of Cotopaxi’s magmatic activity
has remained stationary during its known history. The
ash flow distributions of both the Barrancas and early
F series are radial to the central conduit under today’s
crater, further confirmed by the centered apices of
subsequent andesitic and rhyolitic events. The long
run-outs of the F-4 dome collapse flow and the
Colorado Canyon debris avalanche imply that both
originated from a high volcanic edifice. The presence
of both andesite and rhyolite clasts in the sector
collapse avalanche debris indicates that both rock
types co-existed high up on a presumed stratocone
and that their magmas likely erupted from the same
central conduit.
6. Following the Colorado Canyon events, major andesitic
lava outpourings (Vol. ∼2.9 km3) began, initiating the
present andesitic episode 4400 years ago, which has
been characterized by VEI=3–4 events approximately
every 100–150 years. The andesitic magma emission
rate was ∼1.65 km3 (DRE)/1,000 years from 4,200 to
2,100 years BP and ∼1.85 km3 (DRE)/1,000 year from
2,100 years BP to the present, showing a significant
increase in recent times.
7. Cotopaxi’s frequent activity, the high probability of
generating large debris flows, and the growing population living within 40 km of the cone and along the
major rivers that head on the volcano, are all factors
that clearly emphasize the high hazard and risk status of
future eruptions at this volcano and the need to utilize
past eruptive behavior as a guide to properly prepare
for future eruptions.
Acknowledgments The authors kindly thank Silvana Hidalgo for
helping to prepare the geologic maps, Joseph Cotten, the Institut de
Recherche pour le Dévéloppement (IRD) of France, Dennis Geist,
Robert Tilling, and the U.S. Geological Survey for chemical analyses
of these rocks. We thank Judy Fierstein and J-C Thouret for their
constructive reviews and many suggestions. Finally we thank the
Instituto Geofísico of the Escuela Politécnica Nacional in Quito for
their continued support.
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The rhyolitic–andesitic eruptive history of Cotopaxi volcano

Transcription

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