The Rhyolitic–Andesitic Eruptive History of Cotopaxi Volcano, Ecuador
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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 y 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.