The Rhyolitic–Andesitic Eruptive History of Cotopaxi Volcano, Ecuador

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 flows on record, the Chillos Valley lahar. A thin pumice andesitic bimodal magmatism has occurred periodically lapilli fall represents the final rhyolitic outburst which during the past 0.5 Ma. The sequential eruption of rhyolitic occurred at 2,100 years BP. The pumices of these Holocene

(70–75% SiO2) and andesitic (56–62% SiO2) magmas from rhyolitic eruptions are chemically similar to those of older the same volcanic vent over short time spans and without rhyolites of the Barrancas series, with the exception of the significant intermingling is characteristic of Cotopaxi’s initial eruptive products of the Colorado Canyon series Holocene behavior. This study documents the eruptive whose chemistry is similar to that of the 211 ka ignimbrite history of Cotopaxi volcano, presenting its stratigraphy and of neighboring Chalupas volcano. Since the Colorado geologic field relations, along with the relevant mineralog- Canyon episode, andesitic magmatism has dominated ical and chemical nature of the eruptive products, in order Cotopaxi’s last 4,400 years, characterized by scoria bomb to determine the temporal and spatial relations of this and lithic-rich pyroclastic flows, infrequent lava flows that bimodal alternation. Cotopaxi’s history begins with the reached the base of the cone, andesitic lapilli and ash falls Barrancas rhyolite series, dominated by pumiceous ash that were carried chiefly to the W, and large debris flows. flows and regional ash falls between 0.4 and 0.5 Ma, which Andesitic magma emission rates are estimated at 1.65 km3 was followed by occasional andesitic activity, the most (DRE)/ka for the period from 4,200 to 2,100 years BP and important being the ample andesitic lava flows (∼4.1 km3) 1.85 km3 (DRE)/ka for the past 2,100 years, resulting in the that descended the N and NW sides of the edifice. present large stratocone. Following a ∼400 ka long repose without silicic activity, Cotopaxi began a new eruptive phase about 13 ka ago that Keywords Alternating rhyolitic–andesitic volcanism . consisted of seven rhyolitic episodes belonging to the Cotopaxi volcano . Holocene history . Northern Andes 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 Introduction volumes of 0.2–3.6 km3. Andesitic tephras and lavas are interbedded in the rhyolite sequence. The Colorado Canyon The almost simultaneous alternation of basic andesitic and episode (4,500 years BP) also witnessed dome and sector evolved rhyolitic magmatism from the same volcanic center collapses on Cotopaxi’s NE flank which, with associated is an interesting, if not perplexing problem. What mech- ash flows, generated one of the largest cohesive debris anism 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 interme- Editorial responsibility: J Stix : diate compositions? For many years Cotopaxi volcano, M. Hall (*) P. Mothes Ecuador, was considered to be a volcano of solely andesitic Instituto Geofísico, Escuela Politécnica Nacional, origin (Hall 1977; Barberi et al. 1995); here we show that Casilla, 1701-2759 Quito, Ecuador rhyolitic volcanism has played an important role in its his- e-mail: [email protected] tory starting at least 560 ka ago and again in the Holocene. Bull Volcanol

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 Fig. 1 Cotopaxi volcano’s 20-km-diameter cone is comprised mainly its past eruptions, in order to develop valid scenarios for of andesitic products of the past 4,000 years. The hills in the foreground future eruptions. are remnants of the F series ash flows and in the immediate foreground Figure 3 provides a brief introductory synopsis of blocks from the 1877 debris flows. Photo of the north face taken in 2004 Cotopaxi’s history to acquaint the reader to its overall activity prior to discussing it in greater detail.

The present study provides the geological framework and volcanic history of this alternating bimodal magmatism as Cotopaxi I recorded at Cotopaxi, a large well-known stratovolcano of the Northern Andes (Fig. 1). Emphasis is placed upon field Barrancas rhyolite series mapping of the deposits and the development of a comprehensive stratigraphic and chronological framework Along the lower S and SW flanks of the present Cotopaxi in order to clearly demonstrate the succession of rhyolitic edifice is exposed a thick older series of deposits made up and andesitic magmas. of rhyolitic ash flows, block-and-ash flows, tephra falls, and Cotopaxi volcano (Lat. 0°38′S; Long. 78°26′W) is associated volcaniclastic units, attaining a thickness of located on the Eastern Cordillera of the Ecuadorian Andes, >150 m, that generally dip to the S and SW, away from 60 km south of Quito and 35 km northeast of Latacunga, an arcuate alignment of rhyolitic domes and source vents. capital of Cotopaxi province. This 5,897 m high active This, the Barrancas series of Cotopaxi I, is best seen along the volcano is notable for its relief (2,000–3,000 m), conical Barrancas–Simarrones valley, but the nearby Burrohuaicu, form, massive size (22-km diameter), and its glacier-clad Saquimala, and San Lorenzo valleys display similar and steep flanks. Cotopaxi, along with other large active complementary sequences (Fig. 4). The age of the series is andesitic volcanoes, such as Tungurahua, Antisana, not well constrained, but Bigazzi et al. (1997)reported Cayambe, and Sangay, define the Eastern Cordillera in fission track ages of 0.42–0.56 Ma for several rhyolites. The Ecuador, some 35 km behind the dacite-dominated volcanic series is older than the 211 ka Chalupas ash flow and front that constitutes the Western Cordillera (Barberi et al. younger than the clastic Latacunga Fm., dated at 1.4–1.7 Ma 1988; Hall and Beate 1991). Between these two cordilleras (Lavenu et al. 1992). lies the densely populated InterAndean valley, a structural Its presumed source is a series of aligned, highly depression (Fig. 2). fractured, locally altered rhyolite domes and dikes associ- Early geological and petrological descriptions of Coto- ated with dome breccias and short obsidian lava flows paxi were given by La Condamine (1751), Humboldt (Fig. 4). The circumferential distribution of the rhyolitic (1837–1838), Reiss (1874), Sodiro (1877), Stübel (1897), vents around the SW side of Cotopaxi’s present edifice Wolf (1878, 1904), and Reiss and Stübel (1869–1902). suggests that these vents formed the outer segment of an Modern studies of the volcano and its hazards were carried older caldera that is mostly buried under younger deposits. out by Hradecka (unpublished data 1974), Miller et al. Associated pyroclastic sequences can be traced from the (1978), Hall (1987), Hall and Hillebrandt (1988), Mothes source area to the Barrancas valley where the most (1992), Barberi et al. (1995), Hall et al. (2000), and Mothes complete stratigraphy is seen (Figs. 5 and 6) (Inst. Nac. et al. (1998, 2004). Electrificación unpublished data 1983). The regional extent Cotopaxi has experienced at least 13 significant erup- of this sequence is unknown, as it is covered by younger tions since 1534, based upon tephrostratigraphy and deposits down valley. However, the significant thicknesses Bull Volcanol

Fig. 2 Regional map showing 78 35' 78 30' 78 25' 78 20' 78 15' the locations of Cotopaxi volca- 0 5 10 km Selva Alegre no and neighboring volcanic centers, as well as several towns in the InterAndean Valley. Note: Amaguaña Across the top are west longi- 4000 m tudes (e.g. 78° 35′ W) and along Pita RÌo the right side are south latitudes 3000 m

(e.g. 0° 50′ S) Clara Santa RÌo 0 25'

3000 m RÌo San Pedro

4000 m Pasochoa Volcano Bocatoma Machachi

4000 m

RÌo Salto Sincholahua

4000 m RÌo Pita Volcano

Huasi 0 35' 0 30' Rumiñahui Volcano Pan-AmericanBoliche Highwa Ingaloma4000 m Limpiopungo Mudadero RÌo Cutuchi 0 40' Pucahuaicu COTOPAXI RÌo Tamboyacu Queb. 5000 m VOLCANO Morurcu

RÌo Tambo 4000 m RÌo Cutuchi Queb. Saquimala C H A L U P A S Lasso Queb. 0 45' Queb. San Lorenzo Burro huaicu

Mulaló Simarrones 3000 m RÌo Barrancas --

4000 m C A L D E R A contour interval = 200 m 0 50'

(1–15 m) of the ash-flow and fall deposits suggest that slightly reworked by wind and rain, suggesting short repose many had wide distributions. intervals. Upwards the sequence continues with a pumice The Barrancas sequence begins with a 15-m-thick, pink- lapilli-fall unit (BF1) and is followed by a thick series of topped, pumiceous ash-flow deposit (unit BA) that contains five block-and-ash breccias (BF2) comprised of radially up to 5% small polylithic fragments, especially obsidian. fractured clasts and blocks, up to 1 m in diameter, of Lithic clasts are concentrated toward the base, while white obsidian, rhyolitic vitrophyres, and saccaroidal rhyolite, all pumice clasts abound toward the top. The ash flow is containing plagioclase, biotite, ± quartz, in a gray or red overlain in turn by a 20-m-thick sequence (units BB, BC, sandy matrix of glassy particles. The five units of crudely BD and BE) of interbedded ash-flow, surge, and air-fall bedded breccias, each defined by coarser blocks at its base, deposits of both pumice lapilli as well as obsidian and are the products of sequential dome collapses and related banded-rhyolite lapilli. Most of these units have been debris flows, traceable to an obsidian dome located 15 km Bull Volcanol

Fig. 3 Brief synthesis of the stratigraphic history of Coto- ...... paxi volcano, separated into ...... + + + + + + The present andesitic history of Cotopaxi volcano began about 4000 yr BP and Cotopaxi I, II A, and II B Late ...... has involved many tens of eruptions characterized by pumice and scoria tephra Holocene ...... falls, scoria, lithic, and pumice pyroclastic flows, frequent large debris flows, and eruptive periods ...... many blocky lava flows, all of which have contributed to the making of the Andesite + + + + + + Episode ...... present large stratocone. Minor rhyolitic activity occurred about 2100 yr BP. + + + + + + Historic eruption cycles, averaging one per century, comprise a significant COTOPAXI II B COTOPAXI ...... portion of the recent andesitic activity...... 4000 yr BP ...... The Colorado Canyon episode represents the climactic end of Cotopaxi's Colorado ...... Holocene rhyolitic activity, which involved phreatomagmatic explosions through Canyon ...... domes, a rhyolitic breccia flow, a major pumice lapilli fall, several ash flows, and Rhyolite + + + + + + finally a large sector collapse of the northeast flank of Cotopaxi's cone. This Episode + + + + + + activity triggered the Chillos Valley lahar, a gigantic cohesive debris flow...... Erupted DRE volumes were about 1.2 km 3 . 4500 yr BP + + + + + + COTOPAXI II A II COTOPAXI ------Following 400 ka without notable activity, Cotopaxi reactivated and F + + + + + + experienced a series of six rhyolitic episodes, here called the F series, that Rhyolite ------+ + + + + + consisted of pumiceous tephra falls, ash flows, a dome-collapse flow, and Series ------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 Cga Cangahua 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 and ------+ + + + + + enormous Chalupas ash flow unit (Chlp) (211 ka) erupted from the nearby Chalupas Chlp Chalupas caldera. The two Cangahua units form a regional mantle over the Units + + + + + + ------northern Ecuadorian Andes and apparently originated from the eolian reworking Cga of glacial loess and pumiceous ash from Chalupas and other rhyolitic eruptions. ~300 ka ? ...... O o O o O o O Detritial O o O o O o O An erosional period ensued, in which a detrital fan developed on the SW and W Fan and O o O o O o O sides of the edifice, made up of volcanic breccias, conglomerates, sands, ash Andesitic ...... layers, and a few pyroclastic flow units. Andesitic lava flows derived from the O o O o O o O Lavas Morurcu satellite vent, as well as from Cotopaxi itself, traveled to the SW. Other O o O o O o O mafic andesite flows traveled more than 40 km to the north down the Rio Pita. O o O o O o O + + + + + + ~420 ka Cotopaxi's early history involves the Barrancas rhyolite series, comprised of COTOPAXI I COTOPAXI Barrancas + + + + + + + + + + + + many biotite-bearing tephra falls, ash flows, dome growth and collapse, and Rhyolite + + + + + + associated block-and-ash flows, that occurred between 560 and 420 ka. They Series + + + + + + 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., ...... O o O o O o O 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.

up the Barrancas valley. Similar rhyolitic domes and dikes continuous explosions whose fall deposits accumulated are also observed where the Saquimala and Burrohuaicu upon the ash-flow fan dipping gently to the SW. canyons intersect the margin of the inferred caldera (Fig. 4). The series continues upwards with two massive ignim- Upsection there is a succession of seven similar pumice brites (the 6-m-thick BH1-2 and the 15-m-thick BH3 units) lapilli-fall deposits (units BG1 to BG7), 1–4 m thick, with a thin obsidian-rich lapilli-fall layer interbedded without obvious time breaks. These generally have a coarse between the flows. These units can be traced at least pumice lapilli base and trend to fine ash layers towards the 3 km down valley to the SW and possibly as far as northern top, locally bearing accretionary lapilli. Obsidian and Latacunga (17 km). The BH3 salmon-colored ash-flow unit vitrophyric sand often form thin lithic-fall intercalations is unconformably overlain by a 6-m-thick volcaniclastic in the succession. A reworked pumice-rich ash-flow unit bed that carries pinkish-gray porphyritic rhyolite clasts, (BG-3) also occurs in the sequence. Evidence of minor derived from the Sta. Barbara dome, the morphologically reworking by rain is occasionally observed at the top of youngest dome. these units. Some fine-grained ash-fall units have well- In the lower Burrohuaico valley are exposed younger developed stratification, defined by variations in grain size units of the Barrancas series. There, the BH3 ash-flow unit and sorting. Apparently this succession was generated by is only 10 m thick and is overlain by two fine lapilli falls, Bull Volcanol

Fig. 4 Geologic map of early 0º20' Cotopaxi history

0 5 10 km Chalupas Ash Flow (211 ka) N

? 0º25' COTOPAXI I Detrital Fan and Andesitic Lavas (418-460 ka ?) PASOCHOAC VOLCANOCA

Morurcu vent 0º30' Detrital fan and associated lavas approximate limits of unconformity Chalupas Ash Flow

RUMIM ÑAHUI unconformity 0º35' VOLCANOO

INFERRED 0º40' COTOPAXI CALDERA

Morurcu Vent

Note that the older Barrancas CHALUPAS and clastic fan units are placed graphically over the Chalupas Sta. Barbara Ash Flow so that they are visible. CALDERA Dome 0º45'

Barrancas approximate limits Section of Chalupas caldera

78º35' 78º30' 78º25' 78º20' 78º15'

units BJ and BK1, 4 and 6 m thick respectively, that are rhyolites are also observed. Fragments of black slate from composed of very white, microvesicular pumice. Separating the metamorphic basement and black andesite are occasion- these two fall layers is a 15-m-thick detrital bed of ally found. The pumices of the series are composed of 73– obsidian-rich breccias, conglomerates, and sands, which is 76% SiO2 and 2.5–3.7% K2O. (Note: SiO2 and K2O values overlain by a 20-cm-thick paleosol, implying a significant were obtained from complete major-element analyses (wt. repose period prior to the eruption of BK1. A 6-m-thick percent) that were normalized on an anhydrous basis and ash-flow unit (BK2) ends the Barrancas sequence and is are presented in the text to aid the reader in characterizing truncated by an erosional unconformity. many of the magmatic units; the chemistry of the Cotopaxi Juvenile clasts of this series are almost entirely of white rocks will be discussed in a forthcoming article.) pumice lapilli and ash, bearing a constant mineralogy of In conclusion, the Barrancas series is the result of clear plagioclase, biotite, magnetite, ± quartz and K- prolonged explosive and effusive activity of rhyolitic feldspar, although traces of amphibole and hypersthene affinity that occurred approximately 420–560 ka ago, appear in some younger units. Obsidian fragments, ranging associated with the rhyolitic domes and dikes that define in color from clear to gray to very black, at times strongly an 8-km-long, arcuate structure, interpreted as an old banded, are the dominant lithic components, although light caldera rim that encircles the S and SW sides of the present gray vitrophyres as well as aphanitic and saccaroidal Cotopaxi edifice. It was characterized by dome emplace- Bull Volcanol

Fig. 5 Cotopaxi I composite meters stratigraphic column. In this Upper Cangahua: fine eolian sediments 8-10 and all following columns, the = Cotopaxi repose period. Age at top presence and thickness of each estimated at approx. 20 ka + + + unit may vary locally Chalupas pum ash flow (211±14 ka) 10- + + + + and basal plinian pum lap AF 15 + + + Chalupas Ash Flow .3 l l l l l organic-rich paleosol 7-10 Lower Cangahua: fine eolian sediments = Cotopaxi repose period unconformity Detrital Package: mainly coarse sediments of andesitic affinity; 100 crudely bedded breccias; interbedded andesitic AF and lavas to from Morurcu and Cotopaxi edifices that traveled down the Pita, 300 Cutuchi, Saquimala rivers. Presumed age >211 and <420 ka. unconformity Detrital Fan and Lavas + + + + 6 BK2 white pum ash flow: matrix-rich, rhyolitic + + + white pum lap plinian AF: well-bedded at top and base; 6 BK1 w/ black andesitic and slate clasts, no obsidian.

.2 l l l l l unconformity with organic-rich paleosol 15 detrital sequence: bedded sands, conglomerates, lithic breccias

4 BJ white pumice lap AF with little stratification detrital: bedded sands, cangahua lenses; 6 underlain by unconformity + + + rosy pum ash flow: with obsidian and banded rhyolite 15 BH3 clasts. + + + + .2 obsidian and crystal-rich AF + + + Rw ash flow: slightly stratified, fine ash and pum lenses 3 BH2 BH1 pum ash flow: 3 + + + 2 BG7 fine ash AF: with accretionary lapilli Abbreviations employed: white plinian pum AF: with large white pumice clasts AF = ash- or lapilli-fall deposit 4 BG6 G 6 and altered lithic clasts; Rw at top DF = debris flow or lahar deposit lap = lapilli-size clasts 1.2 BG5 obsidian-rich lithic AF: many discrete layers lith = lithic fragment 1.5 BG4 rosy pum lap AF: stratified at top LV = lava flow + + + PF = pyroclastic flow deposit 8 + + + Rw pum ash flow and AF: slightly stratified sequence pum = pumice clasts of pum ash and clasts; with accretionary lapilli. Rw = reworked material BG3 scor = scoria clasts .3 Rw AF with two small obsidian-rich fall layers Barrancas Rhyolite Series BG2 xl = crystals 2 BG1 grey pum and lithic lap AF: poorly sorted dome collapse flows - five pulses:- mainly 20-30 obsidian and rhyolite clasts; andesite clasts BF2 toward base; clasts up to 1 m in size in well-compacted grey sandy matrix .6 BF1 plinian pum AF: in three layers 7 DF: andesitic clasts + + + + + + 4 BB pink ash flow: with 4 BE white pum ash flow: with black lithic clasts + + + plinian pum lap AF at base + + + + + + + + pink ash flow: clast-rich at base, 15 BA 15 BD Rw pum ash flow + + + pumice-rich at top. + + + 1 BC2 plinian pum lap AF: well-stratified, pulses 10-20 volcaniclastic sequence of 1 BC1 plinian pum lap AF: few lithics Latacunga Fm. (1.4 - 1.7 Ma)

ment, preceded or accompanied by frequent large pumice- The former resulted in ash flows that traveled ≥17 km down rich eruptions, smaller ash-rich explosions, and both valley and had a combined volume of about 19 km3, while phreatic and phreatomagmatic explosions that left thin the latter generated obsidian-rich block-and-ash flows, the obsidian-rich fall layers. Major pyroclastic flows of both largest extending more than 15 km down the Simarrones– column collapse and dome collapse origin were generated. Barrancas valley. Pumice lapilli-fall units up to 1 m thick Bull Volcanol

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%). Pheno- crysts 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 Fig. 6 Here in the Barrancas valley the Barrancas rhyolite series Bocatoma (Fig. 2), a similar clastic sequence is exposed, contains ash-flow units (BA-BE; see Fig. 5) at the outcrop’s base, comprised of fluvial and debris flow conglomerates, followed by obsidian dome-collapse deposits (BF2), and overlain by andesitic lava flows, and a distinctive yellow-tan well- – rhyolitic ash-fall and flow units (BG BH). Note that the series was bedded series of andesitic ash and scoria of both primary greatly eroded prior to the deposition of the Cangahua and Chalupas units (see text) 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 are exposed as far away as 30 km to the SSW, implying that upper part of this clastic sequence are five mafic andesite the rhyolitic ash falls of this series probably involved a total lava flows, known as the Rio Pita lavas. These lavas bulk volume of ∼13 km3. Also noteworthy is that black flooded the wide upper Pita and Salto river valleys, before andesite clasts were observed in several detrital or debris flowing down the narrow lower Rio Pita valley. One can be flow deposits, but never as a primary volcanic unit; it traced northwards to the town of Selva Alegre, more than would suggest that andesitic magmatism had occurred 40 km from source, and other lobes traveled 27 and 32 km. earlier. The total bulk volume of the Barrancas series is Another flow is traced westwards down the upper Cutuchi estimated at 32 km3 (see Table 2 concerning volume and valley for >15 km (Fig. 4). Their volumes range from 0.84 DRE calculations). 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 Detrital fan and andesitic lavas of 4.1 km3. These lavas are dark gray to black, porphyritic, mafic An erosional period followed the Barrancas series that andesites that vary somewhat in their mineralogy. All resulted in a 300–400 m-thick detrital package composed of include 3–20% plagioclase phenocrysts, generally 0.3– fluvial, glacial, and debris flow deposits on Cotopaxi’sSW 0.5 mm in size, but the youngest flow (Bocatoma lava) side and a similar clastic sequence to the NE of Cotopaxi. contains large tabular plagioclase phenocrysts (30%), up to Occasional volcanic manifestations transpired during this 1.5 cm in length, that are semi-aligned in the matrix. Mafic period, resulting in interbedded ash-fall layers, andesitic phenocrysts include 10–20% hypersthene (3–10 mm), small lavas, and lava-flow collapse breccias. The detrital package augites (5–10%), and occasional olivine in a glassy to forms a wide depositional fan whose apex is centered on the microcrystalline matrix. Chemically these rocks are varied:

SW side of the present Cotopaxi edifice and whose crude the older lavas are more mafic (SiO2=56%; K2O=0.98%), stratification dips away gently to the SW, S, and SE (Fig. 4). while the younger lavas are more silicic (SiO2=58–62%; It contains massive, poorly stratified beds, 10–20 m thick, K2O=1.3–2.2%). They are texturally, mineralogically, and with angular blocks up to 3–4 m across, but average only chemically different than the Morurcu lavas, but follow the 10–50 cm in size, supported in a sandy matrix. Many general chemical trend of basic Cotopaxi lavas. blocks are gray andesites from Morurcu peak. No radio- metric dates exist for these units, however they lie strati- graphically between the Barrancas series (420–560 ka) and Regional deposition of the Cangahua the Chalupas ash flow (211 ka). Prior to the deposition of and Chalupas units the subsequent Cangahua Fm. and the Chalupas flows, the clastic fan suffered notable erosion with the formation of Following the erosional period, the northern Ecuadorian 30- to 50-m-deep valleys (Fig. 6). Andes experienced the protracted deposition of the Cangahua Bull Volcanol

Fm., a 25–30-m-thick, fine-grained, tan-colored, indurated the F rhyolite series mark Cotopaxi’s reawakening. These volcanic tuff consisting chiefly of eolian-reworked volcanic eruptions spanned about 8,700 years of periodic eruptive ash and glacial loess (Hall and Mothes 1997). No Cotopaxi activity, starting weakly at about 13,200 years BP, eruptive products are found in this unit, implying a cessation intensifying around 9,600 and 5,800 years BP, and in Cotopaxi activity. The timing and duration of the subsequently dwindling around 4,500 years BP. However, Cangahua Fm. are not well constrained. Studies in the Quito this rhyolitic activity was preceded and sporadically Basin suggest that the top of the formation may be about accompanied by small andesitic eruptions, evidenced by 20 ka (Hall and Mothes 1997), while its base is older than thin scoria lapilli-fall layers found dispersed in this series. the Chalupas ash flow (211 ka) but younger than the The rhyolitic activity was followed by widespread andesitic Barrancas series. magmatism that suddenly intensified around 4,000 years Interrupting the Cangahua depositional period, Chalupas BP and has dominated Cotopaxi’s subsequent activity to the volcano, lying immediately SE of Cotopaxi (Fig. 4), present (Note: all 14C year BP dates employed here are the became active, resulting in a voluminous rhyolitic ash flow uncalibrated 14C ages reported by the lab; the precise date, and the formation of a 20-km-wide caldera (Beate 1989). error, and reference of each age date are given in the text or Briefly, the Chalupas activity consisted of a basal ash flow in the respective stratigraphic column.). of limited extent, followed by a regional plinian pumice For convenience the F series activity is subdivided into fall, and finally a far-reaching ignimbrite, called the five major rhyolitic eruptive episodes, which are based Chalupas ash flow, whose bulk volume is estimated at upon >100 studied stratigraphic sections located within a 80–100 km3. Its thick deposit is traceable north and south radius of 30 km of the volcano, the more complete sections along the InterAndean valley for tens of kilometers, as well being found to the W. A composite section is given in as eastwards into the Amazon Basin and westwards onto Fig. 7 and the corresponding geologic map in Fig. 8.In the coastal plain. An 40Ar/39Ar age of 211±14 ka was general the cumulative thickness of this series varies from recently assigned to the Chalupas ash flow (L. Hammersly 10 to 40 m. Most tephra units share similar characteristics; personal communication 2005). distinction between units was attained by studying pumice Chalupas’s rhyolites share many characteristics with textures and mineral separates from crushed pumice clasts. Cotopaxi’s old and young rhyolites, which warrants a brief The succession is best defined chronologically where comparison. Typically the Chalupas pumice is very fibrous, rhyolitic ash-fall layers are interbedded with 14C-dated peat light to medium gray in color, and contains 1–2% layers in the Rio Tamboyacu area and in other regional peat phenocrysts of biotite, plagioclase, and iron oxides. In sequences. The F series is described below from its base comparison, all of Cotopaxi’s rhyolitic pumices tend to be upward. white, non-fibrous, slightly richer in the same minerals, and bear quartz. Obsidian and rhyolite fragments are the Episode F-1 (13,200–9,600 years BP) dominant lithics in Cotopaxi’s ignimbrites, while at Chalupas small scarce fragments of dark gray to black Episode F-1 begins with several thin lapilli-fall layers aphyric andesite and dense rhyolite are more common, and composed of two pyroxene andesitic scoria (SiO2=56– never obsidian. The Chalupas and Cotopaxi pumices have 57%; K2O=1.1–1.3%) or pumice (SiO2=61%; K2O=1.7%) similar, but distinguishable chemical compositions, differ- that rest upon the eroded top of the Cangahua unit. These ing most notably in the greater K2O and incompatible layers crop out mainly on the W-NW side of the cone, element contents of the Chalupas pumices (K2O>4.0% vs. especially in Pucahuaicu canyon (Fig. 2). This initial 2.5–3.6%, respectively). However, we have found that the andesitic activity postdates the 13,200 years BP rhyolitic pumices of Cotopaxi’s 4,500 years BP eruption are event mentioned above, but predates the first major chemically identical to those of the Chalupas ignimbrite, rhyolitic ash fall (F-1) and its underlying peat bed at suggesting a genetic relation between their magmas, to be Boliche, dated at 9,640±69 14C years BP (J. Garrison discussed later in the Colorado Canyon episode section. 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

Cotopaxi II A (SiO2=75%; K2O=3.1%) that includes plagioclase and quartz, lesser amounts of biotite, magnetite, and hyper- F rhyolite series sthene, plus ∼5% gray to clear obsidian and gray to black aphyric rhyolite clasts. Most notable is the presence of Following the long quiescence of the Cangahua and about 5% gray- and white-streaked pumice clasts, suggest- Chalupas depositional period, major rhyolitic eruptions of ing magma intermingling. The pumice lapilli unit is Bull Volcanol

Fig. 7 F rhyolite series of Coto- Colorado Canyon Series paxi II A composite stratigraph- 4500 yr BP meters top paleosol dated at 4420±80* and 4670±70 14 C yr BP* ic column; abbreviations as in .4 Fig. 5. Note sources of radio- .4 series of scoria and lithic lap AF with interbedded paleosols carbon dates DF: with abundant pumiceous ash, derived from 2 underlying ash flow unit 6 very white pum dacitic ash flow: top eroded lava: basic andesite thin scoria AF 2.5 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 Episode 5 2 DF: with abundant dacitic ash and clasts 5800 yr BP rosy-gray block-and-ash breccia: of dense dacite 7 clasts in pinkish-gray sandy lithic matrix .4 co-ignimbritic AF

15 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

white pum lap plinian AF: with abundant dense 1-4 rhyolite clasts at base Episode 4 thin paleosol: dated 5940±30 14 C yr BP**** 5900 yr BP 1 small pum ash flow .1 white pum lap plinian AF: lithics at top series of rhyolitic and andesitic lap AF, small ash flows, surges, and co-ignimbritic AF: 1.5 with interbedded paleosols, peat, and lithic AF

.2 brown scoria and lithic-rich lap AF: marker bed Episode 3 .2 peat: dated 6300±70 14 C yr BP*** 6300 yr BP series of white pum AF: obsidian, rhyolite, and 2 oxidized clasts 1 white co-ignimbritic AF 3 white pum ash flow: obsidian and rhyolite clasts white pum lap plinian AF: many pulses; streaked- 3 pumice at base; abundant obsidian, oxidized lithics Episode 2 crystal-rich AF and rosy tan surge units .2 organic paleosol: dated 7770±70 14 C yr BP**** 7700 yr BP 2 eolian Rw PF: lithic and xl-rich layers .06 paleosol 1 pumiceous ash flow .25 rhyolitic and streaked-pumice lapilli AF 1 peat: dated 9,640±69 ***** and 10,075±50 14 C yr BP****** Episode 1 .04 greyish-white fine ash AF: biotite-rich andesite scoria and rhyolite ash AF units 13,200 yr BP peat: dated 13,200±60* and 13,550±20 14 C yr BP*** 4 glacial till -- (est. age = 20-13 ka) Uncalibrated radiocarbon dates from base of section: underlain by Upper Cangahua : or Chalupas ash flow units * = Smyth (1991) ** = Barberi et al. (1995) *** = Mothes and Hall (1998) **** = C. Robin (pers. com. 1999) ***** = Garrison (pers. com. 2001) ****** = Clapperton et al. (1997)

followed by rosy-tan, fine-grained ash-fall beds, possibly rhyolitic units have a total bulk volume of less than related to a surge unit, and then a medium gray, well- 0.002 km3. A peat layer underlying the earliest rhyolitic stratified, coarse sandy ash-fall layer, containing many units in the upper Tambo canyon of Cotopaxi’s SE side obsidian grains, which is observed chiefly in the upper gave dates of 13,200±60 and 13,550±20 14C years BP Cutuchi river drainage but also further to the WSW. The (Smyth 1991). Bull Volcanol

Fig. 8 Geologic map of the F rhyolite series

0 5 10 km COTOPAXI II A N ? F Rhyolite Series* (13 - 4.5 ka) > 20 cm Episode 5 ash flow Episode 4 Dome-collapse flow Ash flows F- 4 ash-fall limits PASOCHOA VOLCANOC Rio Pita 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 Inter- SINCHOLAHUA Andean Valley VOLCANO west of Cotopaxi's edifice. RUMIMIÑAHUI VOLCANOOL x Lorna Loma

Episode 2 ash flow underlies Episode 4 ash flow INFERRED COTOPAXI Rio Cutuchi CALDERA Rio Tamboyacu

? ? ?

Lasso Chalupas ? ancas Rio Barr

? Caldera

Episode F-2 (7,770–6,300 years BP) magma mingling. This aspect, as well as the <5% lithic components comprised of oxidized accessory fragments, On top of a 15–20-cm-thick soil, dated at 7,770±70 14C gray aphyric rhyolites, obsidian, and meta-sediment clasts, years BP (C. Robin personal communication 1999), aid in identifying the F-2 layer. This plinian fall unit has a episode 2 begins with a rosy-tan, cross-bedded, fine ash regional distribution whose chief axis lies to the WSW of surge unit, followed by a light gray, crystal-rich, sandy ash- the volcano (Fig. 9a), however, a thin layer of this ash is fall bed. They are succeeded immediately by a series of ash traceable to the ENE as well. Its total bulk volume is falls and ash flows pertaining to one of the largest estimated at about 7.9 km3. eruptions of the F period. A well-stratified pumice lapilli- Overlying the F-2 lapilli-fall unit is a 2–3 m-thick, white fall unit (F-2), up to 3 m thick near the volcano, initiates ash-flow deposit seen in the Cutuchi, San Lorenzo, and this series. Glossy white, microvesicular pumice lapilli Saquimala canyons, which consists of 80% pumiceous

(SiO2=74–76%; K2O=2.7–2.9%) is the principal compo- matrix and 20% small pumice, obsidian, aphyric rhyolite, nent and carries plagioclase, 1–3% biotite, 1% hypersthene and altered lithic clasts. The pumice is very similar to that and magnetite, and ± quartz. About 1% of the pumice clasts of the underlying lapilli-fall bed, except that it carries less of its basal layer are gray- and white-streaked, suggesting biotite, while the unit bears more lithic grains. Because this Bull Volcanol

5 clasts. These clasts are uniform in size and shape, suggest- 0¼ 15' S ___ a Quito Pifo ing that they may be the products of phreatomagmatic Alluriquin 4 ? Sucus ? explosions in domes, possibly associated with glacial ice. 5 cm In the Tamboyacu area, episode 2 is represented by a thin 10 Aloag 0¼ 30' S ___ V. Pasochoa 15 10 10 cm 25 V. Antisana sequence of fine to coarse sand-size pumice-rich ash-fall, 90 Machachi 25 cm 114 10 5 29 100 cm 200 cm 12 230 50 cm 10 co-ignimbritic ash-fall, and interbedded peat/soil layers. 18 Maquimallanda 154 300 cm 24 17 30 V. Illiniza 8 22 400 8 Most ash-fall beds are similar in texture and mineralogy to 19 25 13 145 V. Cotopaxi 45 230 110 8 Sigchos 28 13 0¼ 45' S ___ 70 70 28 3 ? 10 14 75 38 20 Chalupas the F-2 lapilli-fall unit of the W side, however they are each 10 28 16 54 40 60 41 20 15 17 32 14 interbedded with peat layers, 5–60 cm thick, whose presence 24 15 10 Morro V. Quilotoa 18 7 implies variable repose intervals between eruptions. Al- 8 10 1¼ 00' S ___ Latacunga though most of the ash-fall units are poorly represented to the 7 E, the F-2 plinian fall was widespread and its deposits are

10 20 30 8 observed 50 km to the NE and 25 km to the E of Cotopaxi, km Cunchibamba l l l l l suggesting that the eruptive cloud was high enough to be 79¼ 00' W 78¼ 45' W 78¼ 30' W 78¼ 15' W 78¼ 00' W affected by E-trending, >15-km-high stratospheric winds. The combined bulk volume of this episode is estimated at 3 Pifo – 0¼ 15' S ___ b Quito 8.6 km . At Tamboyacu a 10 20-cm-thick soil formed on

Alluriquin top of the F-2 series and an associated peat horizon 13 Sucus provided a date of 6,300±70 14C years BP. ? 10 cm 10 20 V. Pasochoa 15 0¼ 30' S ___ 12 V. Antisana Machachi 10 15 cm 20 14 – 24 22 15 Episode F-3 (6,300 5,940 years BP) 20 cm 7 7 30 20 17 18 24 16 24 Maquimallanda 50 14 8 50 cm 70 100 cm 85 40 12 20 45 28 Overlying the 6,300 years BP peat, episode 3 begins with 15 15 19 105 200 V. Cotopaxi ? Sigchos 200 38 ? 70 Chalupas 0¼ 45' S ___ 70 15 200 cm 50 17 57 85 70 71 52 an air-fall unit of andesitic scoria and lithic lapilli seen only 9 46 64 65 20 11 24 20 10 10 6 23 15 8 23 15 ’ 10 Morro ? on Cotopaxi s west flank. Its base consists of a 10-cm-thick V. Quilotoa 8 8 15 layer of slightly vesiculated, dark scoriaceous clasts that 5 Latacunga 8 1¼ 00' S ___ have streaks of white pumiceous material, again suggesting 5

10 20 30 a magma mingling onset. The unit becomes richer in scoria km 6 upwards and ends with a dark andesitic scoria. The Cunchibamba l l l l l presence of possible mixed magma clasts in several discrete 79¼ 00' W 78¼ 45' W 78¼ 30' W 78¼ 15' W 78¼ 00' W units of this episode would again suggest the close Fig. 9 Isopach maps of the a F-2 and b F-4 rhyolitic ash falls, proximity in time and space of two magma types. showing the typical westerly distribution of most ash falls. Thickness Subsequently there is a modest series of rhyolitic ash- in centimeters fall, 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 unit is almost everywhere buried by the episode F-4 (SiO2=75%; K2O=2.7%) and lithic lapilli, is observed ignimbrite, its distribution is poorly known. However, it toward the top of the sequence at Tamboyacu, but it is not appears to have covered approximately the same area on prominent on the W side of the volcano. Unlike previous Cotopaxi’s lower western flanks as that of the F-4 ash flow. rhyolitic fall deposits, its white microvesicular pumice has In other areas, such as at Boliche and Tamboyacu, the ash more biotite (3–5%), which forms booklets up to 5 mm in flow is correlated to a yellowish-white, pumiceous sandy diameter. In addition, this pumice includes plagioclase and ash unit with faint stratification that grades upwards to a a trace of magnetite and quartz, as well as a small amount fine ash. Its bulk volume is estimated to be 0.7 km3. of black aphyric rhyolite lava and grey obsidian grains. The F-2 ash-flow deposits are followed by 15–20 ash-fall Generally the base of this unit is dominated by pumice, beds, each 5–15 cm thick and well-stratified, which have while lithic fragments increase upwards. Overall the the same composition as the initial plinian phase and proportion of hydrothermally altered clasts is less than apparently represent a series of discrete explosions during 1%. On the E flank the F-3 lapilli-fall bed is followed by a the waning phases of this episode. These beds are best series of poorly sorted ash-fall layers and a small ash-flow exposed at Boliche and Pucahuaicu where the episode 2 unit of similar composition. This flow traveled a short sequence ends in a series of discrete, well-sorted lapilli-fall distance down the Rio Tamboyacu into the Chalupas layers, each consisting of 50% white, slightly fibrous caldera. The F-3 ash-fall bed is traceable 28 km to the pumice and 50% angular gray aphyric rhyolite and obsidian 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 In the Salto and Pita river valleys the F-4 ash flow left a communication 1998). The bulk volume of rhyolitic 10–15 m thick, inversely graded deposit that is creamy material is estimated at 1.18 km3. white and matrix-rich (∼75–85%), and carries white microvesicular pumice clasts (<15%) with phenocrysts of Episode F-4 (5,940–5,830 years BP) plagioclase, quartz, ∼5% amphibole, ∼1% biotite, magne-

tite, and ±hypersthene. The pumice is 68–71% SiO2 and The largest eruptive events of the F series occurred 2.3–2.5% K2O. The deposits have 5–10% lithic fragments during episode F-4, and included a regional plinian lapilli primarily made up of gray or banded aphyric rhyolite lava fall, large ash flows, and a block-and-ash flow. The basal and gray to black obsidian. Hydrothermally altered frag- plinian fall deposit consists of a 1–4-m-thick pumice ments are more abundant toward the base. Clasts with dark lapilli layer, whose white microvesicular pumice (SiO2= gray and white swirled streaks are also present, again 75%; K2O=2.8%) carries plagioclase, biotite, magnetite, suggesting the possible intermingling of different magmas. ±quartz, and small clasts of gray aphyric, sugary, or Also notable is the first appearance of amphibole, associ- aphanitic rhyolite lava and minor obsidian. The lower 10% ated with less biotite. A xenocryst of melted quartz from the of the unit is greatly enriched in gray rhyolite lithics, gneissic basement was found in this ash flow (L. Hammersley suggesting an explosion through a dome or the conduit. personal communication 2003). A carbonized tree root taken The plinian fall unit has a widespread distribution and can from the soil directly underlying the F-4 ash flow in Cutuchi be traced >45 km to the E as well as 60 km to the W canyon gave a date of 5,830±80 14C years BP. A 40-cm- (Fig. 9b), implying that the eruption cloud attained thick fine-grained co-ignimbritic ash is frequently observed stratospheric heights. It has a bulk volume of 5.3 km3. on top of the ash-flow layer. The F-4 ignimbrites have an Five thin rhyolite pumice lapilli-fall beds, containing estimated bulk volume of about 2.8 km3. abundant obsidian and rhyolitic lava fragments, overlie A monolithologic rhyolitic block-and-ash-flow deposit, the F-4 plinian layer. having a volume of 0.24 km3, ends the episode 4 eruptive The sequence continues upwards with the F-4 ash-flow sequence. It traveled 22 km down the most northern unit, representing the largest ash flow of the F series. It Cutuchi drainage, apparently having come from a dome flowed eastwards into the Chalupas caldera for >20 km, collapse high up on the NNW side of the cone. Near its northwards down the Pita river valley to near Selva Alegre terminus the >7-m-thick, homogeneous, rosy-gray deposit (40 km), and southwestwards down the upper Cutuchi is composed of 20–40% angular gray clasts up to 14 cm in drainage for at least 21 km, reaching Lasso (Figs. 8 and diameter, dispersed in a pinkish-gray lithic sandy matrix 10). Deposits of smaller ash flows are observed in the (60–80%). These rhyolite clasts and sparse pumiceous

Barrancas valley. 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-and- ash-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.

Episode F-5 (5,830–4,500 years BP)

Following episode 4, there was a short repose in activity, as suggested by the 5 m of relief developed locally upon the Fig. 10 In San Lorenzo valley the F-4 ignimbrite and F-5 andesitic youngest unconsolidated F-4 units. Episode 5 begins with ash-fall units of the F series are closely associated, implying a rapid considerable andesitic activity, is followed by a dacitic compositional change in magma type. The Chillos Valley lahar of the Colorado Canyon rhyolite episode was followed in turn by the late outburst of limited extent, and ends with minor, sporadic Holocene andesitic activity andesitic activity. Bull Volcanol

The initial andesitic activity is represented by a series The duration of episode 5 is constrained by the final of scoria and pumice lapilli falls, blocky lava flows, and dates of episode 4 (5,830±80 14C years BP) and the two associated debris flows, a sequence in which only a few dates of 4,420±80 14C years BP and 4,670±70 14C years thin soils occur. The fall deposits are thin scoria > lithic BP (Smyth 1991) obtained from soils that overlie the F-5 and pumice > lithic lapilli units in which the vesiculated sequence. In summary, episode 5 began with intense products have compositions that increase from about 58 andesitic magmatism, notable for its large volume 3 to 62% SiO2 and 1.4 to 1.8% K2O, progressing from the (∼1.1 km ), and yet finished with another outburst of older to the younger beds. These units are mainly relatively silicic magma. observed on the cone and its lower flanks, the exception being a widely distributed lapilli-fall deposit (unit GF) Discussion of the F rhyolite series 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 Holocene F series represents the first major activity of the E, NE, NW, and W of the volcano, covering Cotopaxi following a >400 ka long repose interval. Its five ∼3,700 km2; its bulk volume is about 0.96 km3.Itisan rhyolitic eruptive episodes lasting 8,700 years involved extremely well-sorted lapilli deposit containing two clast plinian falls of regional distribution, numerous ash flows types: a slightly vesicular tan pumice (SiO2=62%; K2O= and surges some traveling >40 km from the crater, dome- 1.8%) and a medium gray, dense andesitic lava (SiO2 collapse flows with ubiquitous fragments of destroyed ∼59%; K2O ∼1.6%), both being plagioclase-phyric. The domes showing up in most eruptive products, and ash-rich pumice becomes less vesicular and less abundant upwards debris flows. Consistently the rhyolites contain plagioclase, in the unit, and contains equal amounts (∼5–7%)ofaugite biotite, magnetite, ±hypersthene, and ±quartz, and only in and hypersthene, 3% magnetite, and 10% plagioclase the F-4 and F-5 ash flows does amphibole appear. Notably, phenocrysts. amphibole replaces biotite as the dominant mafic mineral in Andesitic lavas associated with these tephra units occur the transition from the F-4 plinian fall to the F-4 ash flow along the SE flank of the volcano. The lavas are short, units, implying the deeper sampling of a vertically zoned blocky flows that extend 2–3 km downhill toward the SE rhyolite chamber or conduit. The steady decrease in both and E foot of the cone. The pinkish-grey lavas (57% SiO2 SiO2 and K2O in the rhyolites from F-1 to F-5 also suggests and 1.2% K2O) bear characteristic green and white clots, a progressive depletion of the higher parts of a zoned comprised of plagioclase (60%), augite (20%), and hyper- rhyolitic body. On the other hand, andesitic magmas (SiO2= sthene (10–20%) crystals, scattered in a rosy-grey, aphanitic 57%–60%) erupted repeatedly during this series, clearly groundmass. The several flows have a combined area of indicating that andesitic magma already existed or was rising ∼9km2 and a volume of ∼0.14 km3, clearly indicating an from depth. Progressive magma mingling as suggested by important emission of andesitic magma. the presence of streaked pumices in the basal units of several Shortly after this andesitic activity, a dacitic ash flow F episodes may also be a plausible mechanism to explain the was generated which traveled 10–14 km down the Rio Pita observed chemical trends. Garrison et al. (2006)found valley and a similar distance down the Rio Cutuchi valley. chemical and isotopic evidence of repeated andesitic This unit is unique, because its pumice has the only injections during the last 8,000 years. If so, an origin intermediate chemical composition (SiO2=66%; K2O= involving the injection and mixing of limited amounts of 1.7%) in the recent history of Cotopaxi. Near Ingaloma andesitic magma into a resident rhyolitic magma body might its very white deposit rests upon a thin soil that overlies be valid, especially for the later F episodes. the GF marker bed, suggesting that a short repose The size of Cotopaxi’s edifice at this time remains followed the GF airfall. This ash-flow unit is composed unclear. The radial distribution of the F series ignimbrites to of highly vesiculated white pumice that has phenocrysts of the S, SE, W, and N implies that these flows came from a plagioclase, 5% amphibole, 2% magnetite, 2% hyper- central vent. The 22 km long run-out of the F-4 block-and- sthene, a trace of biotite, and few lithic fragments. This ash flow, if it was an unfluidized, gravitational flow formed mineral assemblage is similar to that of the slightly older by dome collapse, would suggest that it came from an F-4 ash flow, but contains more hypersthene and less elevation of about 5,400 m, based upon energy-line argu- biotite, and the pumice is less silicic. The ignimbrite ments (Siebert 1984). Conversely, the near total absence of covers approximately 56 km2 and has a bulk volume of lahar units within the F series might suggest a cone of low ∼0.56 km3. In the upper Rio Pita drainage the deposits of a elevation with limited glacier cover. Possibly, a high cone dacitic ash-rich debris flow are associated with this unit, did exist at this time, albeit with a variable snow and ice probably generated by the transformation of the ash flow to cover. a lahar, as the result of melting of Cotopaxi’s glacial cover The two principal plinian fall units of the F series (F-2 by the ash flow. and F-4) have smooth lobate isopachs with major fall axes Bull Volcanol trending almost due W (Fig. 9). Both units maintain (145 km), which gave soil ages of 5,350±110 and 7,510± thicknesses up to 10 cm as far away as Sigchos and 80 14C years BP (Athens and Ward 1999), and are thought Pucayacu, 50 and 70 km to the W, respectively. However, to be the equivalent of the F-2 and F-4 fall units. 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 Colorado Canyon rhyolite episode Quito, 60 km to the N, where ash formed a 1.2-m-thick accumulation. Its presence is confirmed by the mineral and Upon the soil dated at 4,420±80 and 4,670±70 14C years textural nature of the pumice, a date of about 5,800 14C BP, a new rhyolitic eruptive cycle of regional importance years BP, and by its chemistry (A. Alvarado personal began, herein called the Colorado Canyon rhyolite episode, communication 1998). Additionally, the fine-grained frac- named for the good exposures in that valley at the northern tion of the plinian fall was carried by stratospheric winds foot of the volcano. The sequential order of eruptive events, far to the NE and has been identified in stratigraphic whose deposits are best observed around the N and NE sequences in the Papallacta (50 km) and Oyacachi (70 km) base of the cone, is presented below in order of decreasing areas. Two rhyolitic ash beds were found as far E as Coca age (Figs. 11 and 12). This order varies locally, however,

Fig. 11 Geologic map of the Colorado Canyon rhyolite ac- Chillos Valley tivity of Cotopaxi II A and the Glaciers and moraines subsequent andesitic lava out- 0 5 10 km N (approx. limits) pourings of Cotopaxi II B COTOPAXI II B Post - Colorado Canyon Andesitic Activity B lavas (1880 - 1195 yBP) A lavas (4060 - 2000 yBP) PASOCHOA approximate VOLCANO COTOPAXI II A limits of Colorado Canyon Rhyolite debris flows Episode (4500 yBP) Chillos Valley debris flow Avalanche hummocks and deposits mainly Outcrops - ash flows 1-2 rhyolitic Rhyolite breccia flow hummocks Limits of both main ash flow and avalanche deposits Plinian ash fall limits

mainly RUMIÑAHUI andesitic VOLCANO hummocks on Sincholahua slopes

San Agustin lava flows Cotopaxi II B approximate andesitic limits of cone debris flows Estimated limit of glaciers between 6 and 4 ka ago, based upon glacier-disturbed Colorado Canyon tephra stratigraphy Yanasacha Peñas Blancas Bull Volcanol

top of Colorado Canyon Series ently came from domes of the F series, given their similar meters paleosol: dated at 4170±110 ** and 3950±70 14 C yr BP* chemical composition. No vesiculated juvenile material 3 rosy scoria bomb flow (4460±140 14 C yr BP**); .1 series of three scoria and lithic lap AF occurs in the deposit, nor andesitic or hydrothermally 3 ash flow 3: similar to ash flow 2 altered fragments. The outcrop distribution implies that Chillos Valley Lahar: rosy-tan pumiceous ash-rich the flow traveled northwards at least 8 km from the base of 2 cohesive lahar; dated at 4500 14 C yr BP*** the cone (Fig. 11). This deposit is covered locally by a thin, 3 ash flow 2: similar to lower ash flow, but with more periclinally deposited layer of gray silt-size ash with obsidian and few andesite clasts abundant biotite, presumably elutriated from the flow. The 50- sector-collapse breccia: hummock formation, being 100 andesite clast-rich to northeast; rhyolite clast-rich to thickness of the breccia deposit is highly variable, but is to north; overlain by <115 cm of lithic powder generally 7–20 m thick along its axis. Its volume is about 6 3 .1 three thin sandy layers of aphyric rhyolite: blast-like deposits 46×10 m . This unit is considered to be the result of a ash flow 1: rosy-tannish white pum ash flow with andesite, violent collapse of older domes, possibly triggered by the < 25 obsidian, and rhyolite clasts preceding phreatomagmatic explosion. 1 pum surge beds 1-2 white pum lap plinian AF dome-collapse breccia of obsidian and rhyolite clasts in Plinian airfall and ash-flow series 1 < 20 obsidian-rich sandy matrix; overlain by a thin co-ignimbritic ash .02 white sand-size pum AF .02 obsidian-rich sandy AF: phreatomagmatic explosion Following the breccia flow the eruption of the main plinian > 4 debris flow with black andesite clasts pumice lapilli fall left a 1 m-thick uniform deposit, .4 paleosol: dated 4670±70* and 4420±80 14 C yr BP* composed of white, highly-vesicular, fibrous pumice con- top of F Rhyolite Series taining plagioclase, quartz, 1% magnetite, and <4% biotite

Fig. 12 Colorado Canyon composite stratigraphic column; abbre- booklets, 2–3 mm in size. The pumice (SiO2=75%; K2O= viations and sources of radiocarbon dates as in Figs. 5 and 7 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 due to chaotic or inverted stratigraphic relations, suggesting obsidian. The well-sorted fall deposit is best exposed at the that many events are contemporaneous or closely related in mouth of Colorado Canyon, where it is 2 m thick and time. This is true for the rhyolite breccia flow, the overlies an andesitic lahar bed. Here the fall unit becomes avalanche, the ash flows, and the debris flow. finer-grained upwards, and accretionary lapilli are observed at the top. This unit is also seen to the W and NW of the Phreatomagmatic eruption and the ensuing rhyolite volcano, especially at Pucahuaicu and Boliche. It has been breccia flow 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. Overlying the thin soils that conclude the F series is a thin Directly overlying the plinian fall deposit is the princi- layer of obsidian-rich sand, followed by a white pumice fall pal pyroclastic flow unit of this episode, here labeled ash unit. This 2-cm-thick bed of rosy beige-colored obsidian flow 1 (Fig. 13). It begins with a series of pumiceous surge sand grains (<1 mm) is only observed in a few outcrops to layers totaling about 1 m in thickness and grades upwards the N and NNE of the volcano. The overlying pumice fall into the main ash-flow deposit that is up to 25 m thick. It is bed is notable for the small, uniform grain size (∼3 mm) of a rosy tan-colored, matrix-rich (∼70%) unit, containing its pumice. The obsidian-sand unit and pumice fall bed are clasts of white, well-vesiculated pumice (25%), and small considered to be of phreatomagmatic origin, triggered by a grains (3–5%) of black and gray obsidian, gray aphyric short-lived leakage of the main rhyolitic magma. These rhyolites, and andesite. The pumice contains plagioclase, directly underlie and probably immediately preceded the quartz, ∼3% biotite, magnetite, and a trace of hypersthene. rhyolitic breccia. One xenolith of altered gneiss was found in the basal part

Shortly after this explosive onset a polylithic rhyolite of the ash flow. The pumice (SiO2=75%; K2O=4.4%) is breccia flow was emplaced. Its deposit is a light gray, chemically similar to that of the preceding plinian fall, as poorly sorted, homogeneous unit, comprised of ∼30% well as to that of the Chalupas ash flow. It is everywhere angular lithic clasts up to 25 cm in size dispersed in a inversely graded with pumice clasts increasing upwards, matrix (∼70%) of gray lithic-and-obsidian-rich sand of fine- suggesting that the flow was notably fluidized. to-medium grain size with a notable lack of smaller fines. This ash flow swept over the northern and northeastern Black and gray obsidian, black perlite, red-and-gray-banded flanks of the volcano (Fig. 11). Its deposits are found at the rhyolite (SiO2=74%; K2O=2.8%) and light gray aphyric to northern foot of the cone, on the sides of Ingaloma ridge, in biotite-phyric rhyolite comprise these clasts which appar- the Mudadero area to the NE, northwards across the wide Bull Volcanol

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 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 lower slopes of Sincholagua volcano to an elevation of cycles J and JJ of the late Holocene andesite series were dammed by about 4,100 m (Figs. 2 and 11). There, hummocks were left the eroded edge of the Colorado Canyon ash flow fan. A post-lahar on the floor of glacial valleys and upon lateral moraines, scoria bomb flow of 1877 is seen in the foreground 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 Rio Pita and Salto valleys, and finally down the Rio Pita Pasochoa volcano. These hummocks are comprised canyon for 25 km. This unit has not been found in the mainly of unaltered, light gray aphyric rhyolitic clasts Cutuchi drainage, implying that its source was on the NNE (but no obsidian) and relatively few andesitic fragments, side of the edifice. This ash flow left a 10–20-m-thick all clearly belonging to a Cotopaxi source. Chemical ignimbritic mantle over the Rio Pita and Salto valleys, as analyses of many avalanche clasts clearly denote two shown by the distribution and heights of remnant hills, compositional populations: (1) andesites similar to the late which are often mistaken for avalanche hummocks. F series andesites, and (2) a rhyolitic grouping very 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.

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 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 its main axis lying at approximately 025° NE. Many BP sector collapse lies below Yanasacha. The taller hummock is hummocks have been eroded from the center of the valley 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 Ash-flow series 2 and 3 and the Chillos valley lahar apparently co-existed high up on the cone. The maximum height of Cotopaxi’sconeatthattime Following the sector collapse, a second group of ash flows, can be estimated by employing the energy-line concept the so-called ash-flow series 2, were erupted and their for unfluidized gravity slides (Siebert 1984). As such, the deposits are found at the E, NE, and N foot of the cone and andesitic portions of the debris avalanche that were down the Rio Salto valley. They also descended the upper directed to the NE might have come from an elevation of Cutuchi drainage and almost reached Lasso, about 17 km ∼4,900 m. Similarly, the rhyolitic portions that have away. They form a rosy tan-colored, matrix-rich (92%) longer run-outs to the N might have descended from deposit, several meters thick, that carries white micro- elevations of 5.4–5.9 km, if the avalanche was not vesicular pumice clasts (8%), bearing plagioclase, 2% fluidized. biotite and magnetite, ± quartz, as well as 1% aphyric Along the SE side of Ingaloma, the temporal sequence rhyolite and obsidian clasts and few andesite fragments. of events is best exposed. Overlying ash flow 1 but The pumice composition (SiO2=72%; K2O=2.5%) is most underlying the andesitic avalanche deposit, a thin pow- similar to that of the F series pumices and significantly dery layer is found that is coarsest at the base and finer- different than that of the plinian lapilli-fall and ash-flow 1 grained at the top. It consists of small (1–2mm)angular units. Some deposits contain pods of andesitic avalanche particles of aphyric rhyolite, dispersed in a light gray debris and rhyolitic dome-collapse breccia. Its volume is powdery matrix of pulverized rock. This thin deposit may roughly estimated at ∼1.5 km3. represent a blast event that immediately preceded the Synchronous with ash-flow series 2, as its lahar deposits avalanche. both underlie and overlie these ash-flow units, is a debris- The erosive ability of the avalanche flow during flow deposit of gigantic proportions. This is the Chillos emplacement was variable. In places the inferred blast valley lahar (CVL) (Mothes et al. 1998) which has a rosy deposit was left intact, while in many other areas the flow tan pumiceous matrix (80–90%), made up of ash, pumice, was highly erosive and scraped away pre-existing materials, and lithic grains. The remaining 10–20% are lithic clasts at times gouging deeply into the older Colorado Canyon >1 cm in size, comprised of perlitic obsidian, black and units and the Chalupas ash-flow deposit. On the SE side of gray obsidian, red-and-black-banded rhyolites, gray aphyric Ingaloma, the avalanche clearly plowed into pre-existing rhyolites, and black plagioclase-phyric andesites. This ash- ash-flow and rhyolite-breccia deposits. Nearby, a veneer of rich debris flow left a homogeneous, 1–2-m-thick deposit andesitic avalanche material rests upon the scoured surface that mantles the extended floodplains of the Pita, San of ash-flow 1 unit and is covered in turn by the later Chillos Pedro, and Guayllabamba river systems, from the volcano valley lahar deposit (Fig. 15). 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 Fig. 15 Partial sequence of the Colorado Canyon series exposed on lithic lapilli-fall deposits and a scoria pyroclastic flow dated the west side of Ingaloma. The deposits of ash flow 1 lie at the base 14 with the overlying avalanche debris and the Chillos Valley cohesive at 4,460±140 C years BP, which are overlain by a soil debris flow above 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), this magma might have had a greater volatile content and providing closing dates for Cotopaxi’s last large rhyolitic thus greater mobility, which may have favored its escape event. from the Chalupas body.

Discussion of the Colorado Canyon episode

The Colorado Canyon rhyolite episode represents a Cotopaxi II B continuation of the F series activity, presented here as a separate episode to emphasize its varied sequence. This Late Holocene to present andesite episode episode began with a phreatomagmatic explosion through pre-existing domes or conduits probably belonging to the F From the end of the Colorado Canyon episode 4,000 years series, given the abundant rhyolitic lava and obsidian ago until the present, Cotopaxi has experienced a contin- fragments and the lack of obvious juvenile products in the uous series of periodic eruptions, all of which have resulting breccia flow. With the explosion the conduit involved andesitic magma. The one exception corresponds became unplugged, leading to the probable decompression to a thin rhyolitic ash estimated to be about 2,100 years BP of the rhyolitic magma and the formation of a large plinian which is chemically similar to ash flows 2 and 3 of the eruption that resulted in the ensuing pumice lapilli fall and Colorado Canyon series. During this andesitic episode there ash flow 1. These eruptive events may have destabilized have been at least 18 eruptive cycles, amounting to at least Cotopaxi’s NE flank, triggering its sector collapse. The 32 eruptions of at least moderate size (VEI=3). The cycles, main part of the debris avalanche, composed chiefly of labeled H through P in the stratigraphic column (Fig. 16), andesitic components, traveled northeastwards toward are defined by repose intervals such as soil horizons or Sincholahua volcano, whereas rhyolitic-rich portions trav- initial plinian airfall deposits. Each cycle is characterized by eled northwards down the Rio Salto valley to Pasochoa a similar eruptive pattern, involving plinian scoria or volcano. Smyth and Clapperton (1986), appreciating the pumice tephra falls, scoria or pumice pyroclastic flows, apparently great mobility of the avalanche despite its small blocky-lava flows, and widespread debris flows. All size, concluded that its mobility could be explained by (1) eruptive products of this episode are two pyroxene an explosive initiation, (2) the 2 km vertical drop from the andesites with 57–62% SiO2,1.2–1.7% K2O, and a cone, or (3) an increased fluidity due to the probable phenocryst assemblage that includes plagioclase, hyper- generation of considerable steam in the process. Appar- sthene, augite, magnetite, and olivine. Generally the ently all three factors may have been important in hypersthene is 2–3 times more abundant than augite. producing the observed result. The passage of both the Olivine is occasionally present in trace amounts, but avalanche and the hot ash flows resulted in the rapid increases up to 2% in the youngest rocks. melting of the glaciers and the formation of the gigantic A detailed stratigraphic column of this andesitic episode Chillos valley lahar, possibly the largest cohesive-type is presented in Fig. 16 and the pertinent geologic maps in debris flow of ash-flow origin as yet recognized (Mothes Figs. 17 and 18. This composite column is based upon et al. 1998). The total volume of juvenile rhyolitic several hundred stratigraphic sections measured and studied material extruded during the Colorado Canyon episode is on every side of the cone, in the adjacent river valleys, and estimated at ∼4.0 km3. in the InterAndean Valley, downwind from the volcano, The early Colorado Canyon magma can not be the resulting in a much more detailed compilation of recent continuation of the rhyolitic magma stream belonging to the activity than that presented in previous studies (cf. Barberi last erupted magma (F-5), which was rich in amphibole and et al. 1995). Their 14C dates, as well as those from other poorer in silica, but must represent a new rhyolitic sources, have been used in our final column, where they injection. The pumices of this early magma are strongly mostly fall in a logical, sequential order. The chief enriched in incompatible elements, very similar to those of characteristics of this andesitic episode are discussed below. the Chalupas ash flow, whereas the later pumices of ash flows 2 and 3 and associated air-fall deposits are more akin Pyroclastic flows to those of the Barrancas and F series (ref. comparison with the Chalupas unit presented earlier). As such, Garrison et Almost every eruptive cycle of this episode has been al. (2003) concluded that this evolved rhyolitic magma may accompanied by pyroclastic flows whose deposits are found have originated by the re-melting of a possible Chalupas on the cone or on the depositional fans that surround the pluton and its subsequent emission at Cotopaxi. Also, cone. Most have runouts of 6–9 km from the crater and the Colorado Canyon’s early pumices are more highly vesicu- farthest traveled 19 km. Their distribution implies that most lated and fibrous than previous pumices, suggesting that originated from the summit area. Bull Volcanol

Fig. 16 Late Holocene to meters Present composite stratigraph- present soil .04 scoria and lithic lap AF: 1880 ic column of Cotopaxi II B; .03 brown fine ash AF: post-1877 debris flow abbreviations and sources of .2 scoria and lithic lap AF: 1877 AD 1.5 radiocarbon dates as in Figs. 5 2nd scoria bomb PF: 1877 AD 3 debris flow: 1877 AD and 7 P 1.5 scoria bomb PF: 1877 AD .05 black lithic sandy AF: 1854 AD? white pum lap AF: -PeÒas Blancas unit PB lava flow: W flank -1853 ash-rich debris flow w/ PeÒas Blancas pum .15 tan pum lap AF:--1853 AD? several paleosol horizons w/ scoria AF layers 1.3 14 1 debris flow: 1768 AD (2050±80; 2170±100; 2310±90 C yr BP)** JK .10 tan-gray coarse sandy ash AF lava flows and lava flow collapse PF: N flank .15 black sandy ash AF .3 scoria lap AF w/ altered lithics .10 tan dense pum lap AF: 1768 AD 12 2nd lava type A: at Colorado Canyon 1 debris flow w/ red oxidized clasts -1766 AD JJ .07 tan pum lap AF: 1766 AD debris flow w/ scoria bombs .3 rosy grey pum lap AF M 1 debris flow: 1744 AD .10 pum and scoria lap AF: lithics at base -1744 AD .2 ashy paleosol debris flow w/ colonial ceramics - 1743 AD J 1st lava type A: at Colorado Canyon .20 black scoria lap AF: 1743 AD .4 yellow tan pum lap AF and PF co-ignim. ash: Ninahuilca-2269±16 14 C yr BP**** debris flow: Dec. 1742 AD .1 lava flow collapse PF .20 tan pum lap AF w/ colonial ceramics at Callo debris flow w/ colonial tiles: June 1742 AD I2 lava type A: at San Agustin .10 paleosol: repose of 1532 to 1742 AD scoria bomb PF debris flow: 1534 AD .3 tan pum lap AF scoria bomb PF and AF: 1534 AD .2 paleosol MZ lava flow: Yanasacha flow two hot debris flows w/ scoria bombs scoria lap and bomb AF: mixed magma clasts .15 lava type A: at Tamboyacu, NE flank debris flow: 1532 AD I1 .15 scoria lap AF and PF: 1532 AD .3 tan pum lap AF w/ gray lithics .1 co-ignimbritic ash: Cuicocha~2900 yBP -? .20 paleosol w/ Inca artifacts: 1470-1532 AD .25 two yellow tan pum lap AF .30 Quilotoa AF: 840±50; 785±50; 900±150 14 C yr BP*** .15 paleosol .4 buff white pum lap AF w/ grey lithics .15 tan pum sandy AF hot debris flows w/ scoria bombs Y .15 tan pum lap AF H debris flow lava type A: E flank, overran hummocks 14 .20 tan pum lap AF .2 paleosol: dated 4170±110** 3950±70 C yr BP* .10 lithic and pum lap AF; glacial clay at base co-ignim. ash: Ninahuilca-4440±35 14 C yr BP**** 14 scoria bomb PF rosy scoria bomb PF: - 4460±140 C yr BP**

2.5 gray pum lap AF w/ lithic band at top underlain by: X Colorado Canyon Rhyolite Series debris flow .10 tan pum and lithic lap AF debris flow 14 .15 glacial clay and paleosol: 820 ±80 C yr BP** ash cloud surge L2 debris flow 1.5 rosy-tan pum and bomb PF .10 paleosol: dated 1180±80 14 C yr BP** debris flow lava flow: at Refugio L1 1.0 scoria bomb AF and PF .2 paleosol: dated 1210±80 14 C yr BP** .10 tan pum lapilli AF KB2 lava type B: at Tamboyacu, N flank

.25 paleosol: dated 1770±110 14 C yr BP** hot debris flow KB1 lava type B: at Burrohuaicu, Limpiopungo 2 scoria bomb and lapilli AF 1.5 scoria bomb and lava collapse PF: N and W flanks debris flow 14 .2 paleosol: dated 1880±160 C yr BP** .10 tan co-ignimbritic ash AF KA2 debris flow: N,NE, E flanks tan pum lap AF paleosol KA1 .6 tan pum lap AF w/ oxidized clasts 8 rosy tan pum PF

Three types of flows are recognized. The most common adorn the exterior of the flows (Figs. 13 and 19). The are scoria-bomb flows that descended the narrow canyons deposit’s interior consists of scoria and andesitic lithic of the cone and extended out onto the alluvial fans as clasts (∼40%) in a matrix (∼60%) of silt-, sand-, and gravel- narrow, elongate flows having low levees. Their deposits sized fragments of the same material. The composition of consist mainly of vesicular, cauliflower-head-shaped clasts the scoria tends to be 57–58% SiO2 and 1.2–1.4% K2O. of reddish-black vuggy scoria, up to 1 m in diameter, that The longest run-outs of these flows are typically 6–12 km. Bull Volcanol

Fig. 17 Geologic map of the late Holocene andesite episode (<1,195 years BP) 0 5 10 km N COTOPAXI II B

Río Pita Late Holocene Andesite Episode Río Santa Clara Santa Río (< 1195 yr BP) Glaciers and moraines (approx. limits) Historic debris flows PASOCHOA Scoria flows VOLCANOC Pumice flows Andesitic lavas -younger flows -older flows Typical limits of a large andesitic ash fall deposit > 5 cm

SINCHOLAHUA

Río Pita VOLCANO

RUMIUM ÑAHUIUI VOLCANO

Río Tamboyacu Río Cutuchi

Río San Lorenzo

Río Tambo Río Saquimala

Río Burrohuaicu

Río Barrancas

Given that the bombs have a vesiculated exterior, these those of the scoria-bomb flows, but with somewhat shorter flows apparently originated by gas-rich magma spilling out run-outs. Their dark deposits are comprised chiefly of or by being thrown out of the crater or by the low-level angular andesitic lava clasts up to 10 cm in size and collapse of the eruption column. Their long run-outs subordinate amounts of black scoria. Many andesite clasts suggest that they were partially fluidized, since simple are vesiculated on only one side, the remaining sides having gravitational flows descending Cotopaxi’sslopesare glassy or microcrystalline textures, implying that they were calculated to reach only 4–5 km. In the eye-witnessed derived from lava lakes or lava flow fronts. Scoria occurs as eruption of 1877, Wolf (1878) reported that ‘a dark foam- small bombs, but more often as fractured, vesiculated clasts. like cloud boiled over the rim of the crater and descended The matrix (∼50–60%) is comprised chiefly of sand- and all sides of the cone, much like the boiling over of a pot of gravel-size particles of dark andesite and scoria, the latter cooking rice’ in reference to the scoria-bomb pyroclastic having a chemical composition similar to that of the scoria of flows whose deposits are still pristine today. the scoria-bomb flows. These flows likely owe their origin to Another type of flow, second in abundance, are the lava- (1) the collapse of lava flow fronts, or (2) explosions through and-scoria-clast flows. These have distributions similar to lava lakes, domes, or conduit plugs of solidified magma. Bull Volcanol

Fig. 18 Geologic map of the products of the 26 June 1877 eruption 0 5 10 km N

COTOPAXI II B Río Santa Clara Santa Río Río Pita Glaciers and moraines

26 June 1877 eruption PASOCHOAO Debris flows VOLCANOOLC Scoria flows: synchronous and post-debris flows > 10 cm isopach limit for 1877 scoria lapilli fall

SINCHOLAHUA VOLCANO

RUMIM ÑAHUI VOLCANOO

Río Tamboyacu

Río Cutuchi

Río Tambo

Río Saquimala Río San Loren Río Burrohuaicu

Río Barrancas

A third type of pyroclastic flow, much less common, are fluidized. These flows likely originated by partial or full pumice flows, whose deposits consist of khaki-colored, column collapse. Invariably they were associated with well-vesiculated pumice clasts with red vuggy interiors (up prominent plinian airfalls that often left a widespread, 1– to 80%) and of small gray or black andesite fragments (5– 2 m-thick, relatively well-sorted pumice lapilli bed. 10%) in a rosy tan-colored, ash-rich matrix (20–80%). The pumice of these flows is more silicic (60–62% SiO2; 1.7% Debris flows K2O). Like the other flows, these descended gullies and canyons until they reached the foot of the cone, where they Debris flow deposits are found everywhere around the crossed the alluvial fans, as narrow, elongate flows having volcano, generally associated with pyroclastic flow deposits subdued convex cross-sections. The longest flow is 19 km. in almost every eruptive cycle. Large debris flow fans make The pumice clasts often form interflow segregations, up the northern, western, and eastern bases of the volcano suggesting separate pulses, or are concentrated at the top and from there their floodplain deposits can be traced down of the deposit, suggesting that the flows were somewhat 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

Blocky lava flows have occurred in most eruptive cycles. Fig. 19 Scoria bomb pyroclastic flows with 2 m high levees They form narrow (<1 km wide), lengthy flows on the immediately followed the devastating debris flows of the 26 June cone’s flanks that generally stop or spread out laterally 1877 eruption seen here on the west side of Ingaloma. Distance upon reaching the alluvial fan at the base of the cone. between black points: ∼250 m 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 deposits are dark gray, clast-supported, well-packed brec- eight andesitic lavas flowed down the W, N, NE, E, and SE cias, usually several meters thick, which may include flanks of the volcano between 4,000 and 2,100 years BP. blocks up to 8 m in size, although 2–20 cm-sized clasts Large flows occurred on the NE and N flanks, where they predominate. Most clasts are gray to black andesitic lavas; overran the avalanche-hummock fields and flowed around few scoria fragments are observed, probably having been both sides of Ingaloma ridge (Fig. 11). These flows have a ground up during transport. The matrix (20–40%) contains combined area of 25 km2 and a volume of about 0.75 km3. silt to sand-size particles of the same rock types. At Subsequently in the I-1 cycle, the Tamboyacu lava flow distances greater than 50–70 km from the volcano their (volume=0.09 km3) descended the ESE side of the cone. deposits take on the textural character of hyper-concentrat- The largest lava flow of the episode, the more mafic San ed stream flow deposits (Mothes et al. 2004). Agustin lavas, traveled about 17 km and spread out over the Descriptions of the 1877 eruption and the resulting cone’s lower western slopes during the I-2 cycle; it has an debris flows in the Latacunga valley clearly indicate the area of 42 km2 and a volume of 1.3 km3. During each of the power, velocity, and nature of these flows (Wolf 1878; subsequent J, JJ, and JK cycles, smaller flows occurred Sodiro 1877). While not considered to have been a large along the N foot of the cone. Petrographically the A-type flow as compared to previous historic lahars, its distribution lavas of the H to JK cycles are identical, being micro- and extent are nonetheless impressive, having inflicted porphyritic andesites (SiO2 =57–59%; K2O=1.2–1.5%) widespread destruction in the Chillos and Latacunga with a microcrystalline to somewhat vitreous groundmass valleys and along its 326 km-path to the sea (Fig. 18). and with a light to medium gray color with a rosy tone. Most Cotopaxi debris flows were probably generated in a Phenocrysts include plagioclase, hypersthene, augite, and similar fashion, as the result of instantaneous melting of the iron oxides, often grouped together in small green and glaciers by scoria pyroclastic flows, as described by Wolf white clots; olivine is rarely present. The scoria pyroclastic (1878). Jordan (1983) determined that the glacial cover in the flows of the H to JK cycles often contain numerous angular 1970s was about 21 km2;todayitisabout12km2 (E. Cadier lava clasts suggesting that they might have originated from personal communication 2007). The mid-Holocene glacial collapsing lava flow fronts. cover was undoubtedly greater (Fig. 11). During the KB1 and KB2 cycles (1,880 to 1,195 years Another type of debris-flow deposit is often observed, BP) emissions of the B-type lavas produced small isolated also associated with scoria-bomb pyroclastic flows. These flows on both the W and E sides of the cone, as well as in form vertical cliffs, 6–10 m high, that draw attention the upper Burrohuaicu canyon. The larger flows descended because of their brilliant reddish-color and crude vertical the NW slopes and spilled out upon the W end of columnar jointing. Their lithology, texture, and poor sorting Limpiopungo plain and near Ingaloma; their total area and are similar to most debris flow deposits, although these are volume are estimated at 24 km2 and 0.87 km3. These are matrix-rich and include scoria bombs; they lack the molten dark gray to black, porphyritic andesites (SiO2=59–60%; Bull Volcanol

K2O=1.4–1.5%) with an aphanitic to glassy groundmass succession (e.g. in the L and M cycles). Within individual and conspicuously large, square plagioclase phenocrysts cycles other air-fall layers often exist, but they tend to be with a dark core. They are similar to the older A-type lavas, finer-grained and thinner. Tan-colored pumice lapilli fall although the latter have more mafic minerals and a more layers are more common in the early cycles (I through mafic composition. KA2), but beginning with the KB cycle both pumice and Belonging to the L, X, MZ, M, and P cycles is a series of scoria fall units are found as associated, but discrete beds, similar but smaller lava flows, each only 3–4 km long, that and the pumice takes on a darker tan color. Following the are found on the lower NW, N, NE, SE, and SW flanks of M cycle, the lapilli air-fall units occasionally carry lapilli

Cotopaxi, many of which were mapped by Wolf (1878). of both pumice (59.3% SiO2;1.3%K2O) and scoria 3 The largest is the Yanasacha lava (0.05 km ), which (56.9% SiO2;1.2%K2O) in the same bed, suggestive of a descended the NNW flank for 5.5 km and overran the mixed magma origin. Limpiopungo KB flows during the MZ cycle (∼1532– Many of the plinian air-fall beds have recognizable 1534 A.D.). Stübel (in Wolf 1878) reported that the characteristics that make them useful marker units (e.g. Manzana-huaicu and Pucahuaicu lava flows of the P cycle lithic types and contents, vertical grading, associated correspond to a flank eruption on Cotopaxi’s middle W side stratigraphy). In addition, dated ash-fall units from in 1853. Wolf implies that flows also descended the ESE Quilotoa and Ninahuilca volcanoes aid in dating the side (Chiri-machay flow), the NE side (Diaz-chaiana flow), sequence. Given the prevailing winds from the E and and the N side (Tauri-pamba flow), but we have found SE, tephra is mainly carried to the W, SW, and NW from that these flows are all scoria-bomb pyroclastic flows and the volcano, where the most complete sequences are not lava flows. In general these lavas tend to be dark gray found. to black, microporphyritic andesites (SiO2 =56–61%; On the S flank of Ruminahui volcano, underlying the K2O=1.3–1.4%) with a glassy matrix. Small plagioclase, KA unit, is found the Peñas Blancas airfall, a biotite-rich, hypersthene, augite, iron oxides, and ±olivine comprise white pumice lapilli-fall layer containing minor amounts the <10% microphenocryst content. In the MZ cycle of black lithic fragments (Fig. 16). The mineralogy and olivine makes its first permanent appearance in Cotopaxi chemistry of its pumice (SiO2=73%; K2O=2.8%) corre- rocks; the crystals are few but often the largest of the late closely with that of the pumice from the late Colorado phenocrysts. Milky white quartz xenocrysts up to several Canyon ash flows, suggesting a genetic linkage. The centimeters long often occur in these flows. presence of well-sorted, lapilli-size pumice clasts implies In summary, the largest lava outpourings (∼2.1 km3)of a nearby source. Downstream, a pumice- and ash-rich this episode occurred between ∼4,000 and ∼2,100 years debris-flow deposit occurs at the same stratigraphic level BP (cycles H-JK). Similar mineralogy and chemistry of andisapparentlyrelatedtothisrhyoliticevent.Inthe these lavas imply that all were derived from the same Lasso area and at other localities around the cone, the magma injection, whose ascent may have triggered the Peñas Blancas event is represented by a fine white ash Colorado Canyon rhyolitic outbreak. More silicic ande- with biotite that overlies the thick soil layer of the JK sites then erupted around 1,880–1,770 years BP and cycle dated at about 2,100 years BP. This event apparently continued until the MZ cycle (1532 A.D.). Finally, historic corresponds to a small leakage from Cotopaxi’s rhyolitic eruptions after 1532 A.D. tend to have more mafic magma chamber. andesites carrying plagioclase, olivine, hypersthene, augite, and iron oxides, indicating an injection of more basic Discussion of the late Holocene andesite episode magma. Each of the 18 eruptive cycles of the late Holocene eruptive Tephra falls activity was characterized by a repeated pattern of events, generally consisting of a plinian lapilli fall early in the IsopachmapsofmanyofthelateHolocenetephrafalls sequence, followed by pyroclastic flows, and often one or have been prepared which show good correlations with more lava flows and associated lava-collapse flows. Debris those of Barberi et al. (1995), especially for air-fall units flows occurred frequently in the eruptive sequence. In J, KB, L, X, and M; the interested reader is referred to that general there is a uniform distribution of flowage deposits publication to see these maps. around the cone, implying that most flowage events came All cycles were initiated or accompanied by coarse from the summit crater or nearby. lapilli fall deposits, often 10–20 cm thick, that are The uncalibrated 14C dates employed in Fig. 16 show comprised of either tannish-grey microvesicular pumice good agreement among themselves as well as a good or brownish-black, coarse-vesicular scoria clasts. At times chronological fit to the column. The 810 years BP age for both types of clasts followed one another in quick the Quilotoa ash fall is considered to be well established, Bull Volcanol

Table 1 Character and number of late Holocene eruptive events

Eruption Date No. Plinian Other Series of Lava Series of Series of VEI cycles eruption eruption airfalls pyroclastic flows collapse debris flows lava flows flows

Totals 43 32 23 >18 4 >32 >10 P 1880 AD 1 1 2–3 1877 2 1 1 >10 ≥7?4 1853–54 3 1 1 1 1 2 3–4 M 1768 1 1 5 1 ? MANY 1 ? 4 1766 1 1 1 MANY 3 1744 1 1 1 1 2 series 4 1743 1 1 ? 1 3–4 1742 3 2 1 1 2 series 4 MZ 1532–34 AD 2 2 1 2 2 1 3–4 Y ∼900 years 43 11 1 3–4 BP X 1000 years 22 11 2 Q 4 BP L-2 1180 years 11 1 1 3–4 BP L-1 1210 years 11 1 2 1 4 BP KB-2 ∼1770 years 1 1 ? 1 MANY 4 BP KB-1 <1880 years 11 1 1 2 1 >4 BP KA-2 1880– 11 1 1 4 2200 BP KA-1 1880– 11 1 ? 4 2200 years BP PENAS BLANCAS 11 1 ∼2100 years BP JK 2200 years 3? 1 2–31 1 3 ? 4 BP JJ 3? 1 2–31 1 1 4 J 2350 years 11 14 BP I-3 1 1 1 1 1 MANY 4 I-2 1 1 1 3 1 4 I-1 4 ? 1 2–324 H ∼4060 years 33 2 4 BP

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.

14 since it is based upon an AMS and three C dates on 1532 A.D. eruption, and that of 208 years between 1534 and carbonized wood. The post-Quilotoa units are stratigraph- 1742 A.D.. The onsets of the most recent eruptions have ically correlated to written historic records, where possible. occurred at intervals of 24, 85, and 128 years. From Fig. 16 one can calculate the frequency of eruptive The character of past andesitic eruptions is shown in cycles. The early cycles (I and J) roughly average 300 to Table 1, a pattern of associated eruptive events which is 400 years/cycle, whereas the intermediate cycles (KA to Y) likely to repeat itself in future eruptions; consequently, average 100–150 years/cycle. During historic times there mitigation efforts should take this pattern into account in have been significant periods of repose, such as that of their planning and prevention activities. In Table 2 we 390 years between the Quilotoa ash (1140 A.D.)andthe 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 DRE volume SubTotal DRE volume (km3) (km3) (km3)

Barrancas rhyolite series Ash flows ∼500,000 300.0 19.000 5.70 Tephra falls ∼500,000 550.0 13.200 3.96 9.66

Rio Pita andesite lavas ∼450,000 165.0 4.100 4.10 4.10

F rhyolite series F-1 Tephra falls 9,600 8.0 0.002 0.00 F-2 Tephra fallsa 7,800 12100.0 7.900 2.37 2.37 Ash flows 7,800 140.0 0.700 0.21 0.21 F-3 Tephra fallsa 6,300 3800.0 1.180 0.35 0.35 F-4 Tephra fallsa 5,800 12000.0 5.300 1.59 1.59 Ash flows 5,800 280.0 2.800 0.84 0.84 Block-and-Ash flows 5,800 16.0 0.240 0.24 0.24 F-5 GF Plinian fall (andesitic)a 5,800 3700.0 0.960 0.36 0.36 Andesitic lavas 5,500 9.0 0.140 0.14 0.14 Ash flow (rhyolitic) 5,500 56.0 0.560 0.17 0.17

Total DRE Volume of F Series=6.27 km3

Colorado Canyon rhyolite episode Rhyolitic breccia flow 4,500 9.0 0.046 0.05 Plinian falla 4,500 3360.0 0.468 0.14 0.140 Ash flow 1 4,500 194.0 1.940 0.58 0.580 Sector collapse avalanche debris 4,500 138.0 2.100 2.10 Ash flow 2 4,500 150? 1.5? 0.45 0.450 Chillos Valley lahar 4,500 3.800 0.00 Ash flow 3 (too small to measure) 4,500 N/D 0.00

Total DRE Vol. juvenile Colorado Canyon S. = 1.17 km3

Late Holocene andesite episode H Lavas between hummocks 4,200 25.0 0.750 0.75 0.75 I1 Tamboyacu lavas 4,000 3.0 0.090 0.09 2 Plinian falls 4,000 1,450.0 1.450 0.54 0.63 I2 San Agustin lava 3,000 42.0 1.300 1.30 2 scoria pyroclastic flows 3,000 23.0 0.045 0.02 Plinian falla 3,000 2600.0 0.576 0.21 1.53 J 1st lava 2,350 3.0 0.090 0.09 Plinian falla 2,350 2,300.0 0.372 0.14 0.23 JJ 2nd lava 2,200 3.0 0.090 0.09 Plinian fall 2,200 1,100.0 0.320 0.12 0.21 JK Plinian scoria fall 2,000 200.0 0.060 0.02 Lava flow 2,000 6.0 0.090 0.09 0.11 Ka1 Pumiceous ash flow 1,900 27.0 0.080 0.03 Plinian falla 1,900 2,600.0 0.480 0.18 0.21 Ka2 Plinian fall 1,800 1,430.0 0.430 0.16 0.16 Kb1 Pyroclastic scoria flows 1,770 23.0 0.070 0.03 Plinian scoria fall 1,770 245.0 0.370 0.14 Lava B: Burrohuaicu and 1,770 24.0 0.870 0.87 1.04 Limpiopungo Kb2 Lava B: Tamboyacu, N. 1,200 4.0 0.060 0.06 flank Plinian falla 1,200 2,300.0 0.312 0.12 0.18 Bull Volcanol

Table 2 (continued)

Unit-product Age (years BP) Area (km2) Bulk volume DRE volume SubTotal DRE volume (km3) (km3) (km3)

L1 Scoria fall and pyroclastic 1,100 145.0 0.290 0.11 flow Lava flow at Refugio 1,100 1.5 0.030 0.03 0.14 L2 Pyroclastic flow 1,000 14.5 0.044 0.02 0.02 X Pre-climatic fall 900 800.0 0.080 0.03 Main Plinian falla 900 4,100.0 1.044 0.39 Pyroclastic flow 900 35.0 0.070 0.03 0.44 Y 4 Pumiceous falls 600 1,600.0 0.960 0.36 0.36 MZ 2 scoria tephra falls 472 1,500.0 0.450 0.17 Yanasacha lava flow 472 2.7 0.054 0.05 Scoria fall and pyroclastic flow 470 145.0 0.290 0.11 0.33 M Pumice plinian falla 264 2,020.0 0.564 0.21 Scoria plinian fall 264 1,600.0 0.317 0.12 3 pumice falls 260 1,600.0 0.480 0.18 2 sandy ash falls 238 850.0 0.200 0.07 0.58 P Pumice tephra fall 153 1,600.0 0.240 0.09 Lava-W flank 150 4.0 0.120 0.12 Scoria pyroclastic flows -many 129 0.130 0.05 2 Scoria tephra falls 128 1,125.0 0.450 0.17 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 later by intermittent andesitic activity in the same SW of andesitic magmatism versus time in Fig. 20. Average quadrant of the edifice, and later still by major andesitic emission rates were 1.65 km3 (DRE)/1000 years andesitic lava outpourings (∼4.1 km3) on the cone’sN from 4,200 to 2,100 years BP and 1.85 km3 (DRE)/ and NW sides. 1,000 years for the past 2,100 years, showing an increase 3. After a repose of ∼400 ka without silicic magmatism, with time. At nearby Tungurahua volcano, its magma during which the neighboring volcano, Chalupas, had a emission rate was estimated at 1.5 km3 per 1000 years (Hall 100 km3 rhyolitic eruption at 211 ka, Cotopaxi became et al. 1999).

Conclusions rate of andesitic magmatism DRE volume vs. time 1. Cotopaxi’s Holocene activity offers a rare look at recent 10 bimodal magmatism where rhyolitic and andesitic 8 magmas erupt in quick succession, from the same conduit, displaying no or very limited intermingling. 6 The latest bimodal eruption series occurred only 4,500 4 and 2,100 years ago, clearly indicating that this activity kilometers 2

is on-going. Volume DRE in cubic 2. Early Cotopaxi history (∼420–560 ka) was character- -7000 -6000 -5000 -4000 -3000 -2000 -1000 0 ized by rhyolitic eruptions comprised of dome years before present emplacements and collapses, glassy dikes, small to Fig. 20 Cotopaxi’s accumulated rate of andesitic magmatism versus moderate-sized rhyolitic ash flows and plinian tephra time. Curve corresponds to an average rate of about 1.7 km3 of andesitic falls (DRE Vol. ∼9.7 km3). This was followed much magma per 1,000 years, which has increased during the past 500 years Bull Volcanol

active and entertained eight rhyolitic outbursts (F series, 7. Cotopaxi’s frequent activity, the high probability of Colorado Canyon series, and the Peñas Blanco tephra generating large debris flows, and the growing popula- fall) between 13,200 and 2,100 years BP. The rhyolite tion living within 40 km of the cone and along the emission rate for the F series and Canyon Colorado major rivers that head on the volcano, are all factors episodes totaled ∼6.9 km3 (DRE) over the 8700 year that clearly emphasize the high hazard and risk status of duration. Andesitic tephra falls accompanied most of the future eruptions at this volcano and the need to utilize rhyolitic events, testifying to a simultaneous bimodal past eruptive behavior as a guide to properly prepare magmatism. During the course of the F rhyolitic for future eruptions. emissions, the andesitic output slowly increased in volume, while the rhyolites became slightly less-silicic in composition. The lack of eruptive products with Acknowledgments The authors kindly thank Silvana Hidalgo for compositions intermediate between andesite and rhyolite helping to prepare the geologic maps, Joseph Cotten, the Institut de Recherche pour le Dévéloppement (IRD) of France, Dennis Geist, and the rare examples of possible intermingled magmas Robert Tilling, and the U.S. Geological Survey for chemical analyses suggest that over 11,000 years there was no contact of these rocks. We thank Judy Fierstein and J-C Thouret for their between these two magma types or at best only a very constructive reviews and many suggestions. Finally we thank the limited intermingling. Periodic magma withdrawal from Instituto Geofísico of the Escuela Politécnica Nacional in Quito for their continued support. 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 References 211 ka-Chalupas magma, while the subsequent rhyolites are similar to the less-evolved rhyolites of the Athens JS, Ward JV (1999) The Late Quaternary of the western – — Amazon: climate, vegetation and humans. Antiquity 73:287 302 older Barrancas F series. The appearance of a Barberi F, Coltelli M, Ferrara G, Innocentii F, Navaro J, Santacroce Chalupas-like magma in the Colorado Canyon series R (1988) Plio-Quaternary volcanism in Ecuador. 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