Journal of Volcanology and Geothermal Research 91Ž. 1999 1±21 www.elsevier.comrlocaterjvolgeores Tungurahua Volcano, Ecuador: structure, eruptive history and hazards Minard L. Hall a,1, Claude Robin b,), Bernardo Beate c, Patricia Mothes a,1, Michel Monzier a,d,2 a Instituto Geofõsico, Escuela Politecnica Nacional, P.O. Box 1701-2759, Quito, Ecuador b Institut de Recherches Pour le DeÂÕeloppement() IRD, ex-ORSTOM , UR 6, OPGC, 5 Rue Kessler, 63038, Clermont-Ferrand, France c Departamento de Geologõa, Facultad de Geologõa, Minas y Petroleos, Escuela Politecnica Nacional, P.O. Box 1701-2759, Quito, Ecuador d Institut de Recherches pour le DeÂÕeloppement() IRD, ex-ORSTOM , UR 6, A.P. 17-11-6596, Quito, Ecuador Accepted 25 March 1999 Abstract Tungurahua, one of Ecuador's most active volcanoes, is made up of three volcanic edifices. Tungurahua I was a 14-km-wide andesitic stratocone which experienced at least one sector collapse followed by the extrusion of a dacite lava series. Tungurahua II, mainly composed of acid andesite lava flows younger than 14,000 years BP, was partly destroyed by the last collapse event, 2955"90 years ago, which left a large amphitheater and produced a ;8-km3 debris deposit. The avalanche collided with the high ridge immediately to the west of the cone and was diverted to the northwest and southwest for ;15 km. A large lahar formed during this event, which was followed in turn by dacite extrusion. Southwestward, the damming of the Chambo valley by the avalanche deposit resulted in a ;10-km-long lake, which was subsequently breached, generating another catastrophic debris flow. The eruptive activity of the present volcanoŽ. Tungurahua III has rebuilt the cone to about 50% of its pre-collapse size by the emission of ;3km3 of volcanic products. Two periods of construction are recognized in Tungurahua's III history. From ;2300 to ;1400 years BP, high rates of lava extrusion and pyroclastic flows occurred. During this period, the magma composition did not evolve significantly, remaining essentially basic andesite. During the last ;1300 years, eruptive episodes take place roughly once per century and generally begin with lapilli fall and pyroclastic flow activity of varied compositionŽ. andesiteqdacite , and end with more basic andesite lava flows or crater plugs. This pattern is observed in the three historic eruptions of 1773, 1886 and 1916±1918. Given good age control and volumetric considerations, Tungurahua III growth's rate is estimated at ;1.5=106 m3ryear over the last 2300 years. Although an infrequent event, a sector collapse and associated lahars constitute a strong hazard of this volcano. Given the ;3000 m relief and steep slopes of the present cone, a future collapse, even of small volume, could cover an area similar to that affected by the ;3000-year-old avalanche. The more frequent eruptive episodes of each century, ) Corresponding author. E-mail: [email protected] 1 E-mail: [email protected]. 2 E-mail: [email protected], [email protected]. 0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0377-0273Ž. 99 00047-5 2 M.L. Hall et al.rJournal of Volcanology and Geothermal Research 91() 1999 1±21 characterized by pyroclastic flows, lavas, lahars, as well as tephra falls, directly threaten 25,000 people and the Agoyan hydroelectric dam located at the foot of the volcano. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Andesite magmatism; Debris avalanche and dam; Magma flux rate; Hazards; Tungurahua Volcano; Ecuador 1. Introduction Ecuador, some 35 km behind the volcanic front that lies on the Western Cordillera. X Tungurahua VolcanoŽ Lat. 01828 S; Long. Early geological and petrographical descriptions X 78827 W. is located on the Eastern Cordillera of the of Tungurahua were given by ReissŽ. 1874 , StubelÈ Ecuadorian Andes, 120 km south of Quito and 33 Ž.1886 , Wolf Ž. 1892 and Reiss and StubelÈ Ž. 1904 . km southeast of Ambato, the capital of Tungurahua More modern studies of the volcano and its hazards ProvinceŽ. Fig. 1 . This 5023-m-high active volcano were carried out by SalazarŽ. 1977 , Hall and Vera is notable for its extreme reliefŽ. 3200 m , steep sides, ŽŽ.1985, Hall and Hillebrandt 1988 and Almeida and and small summit glacier. Along with other large RamonŽ. 1991 . According to the catalogue of active active volcanoes such as Cotopaxi, Sangay, Antisana volcanoesŽ. Simkin and Siebert, 1994 , this volcano and Cayambe, it defines the eastern volcanic row in has experienced 17 eruptions since its discovery in Fig. 1. Location of Tungurahua Volcano. The names of the 20 main Quaternary volcanic edifices are given. M.L. Hall et al.rJournal of Volcanology and Geothermal Research 91() 1999 1±21 3 1532, 10 of them being uncertain. Four major erup- ish and only available locallyŽ. INECEL, 1989 , on tive episodes are recognized by historical reports: in fieldwork carried out in 1995±1997, and the study of 1641±1646, 1773±1781, 1886±1888 and 1916± about 50 stratigraphic sections, 11 of which are 1918, the former needing to be confirmed. In total, presented here. A petrological study based on 90 they amount to seven major emissions of lava, pyro- whole-rock analyses will be the focus of another clastic flows and fallout tephra. The development paper. Here, only the broad petrographic nature of and consequences of the two largest historic erup- the volcanic units is presented to help the reader in tions, those of 1886 and 1916±1918, were described understanding the volcanic history. in considerable detail by A. MartinezŽ. 1886 and N. MartinezŽ. 1932 . Additionally, crater explosions oc- curred in 1797, coinciding with the earthquake that 3. Overall structure destroyed nearby Riobamba. Increased fumarolic ac- tivity was reported in 1857, and in 1873, 1877, 1883 The Tungurahua volcanic complex was con- and 1885, apparently preceding the eruption of 1886. structed upon the Paleozoic±Cretaceous regional After the 1916±1918 eruptive period, important fu- metamorphic basement that underlies the Eastern marolic activity continued until 1925. Fumaroles still CordilleraŽ. Litherland and Eguez,È 1993 . It has a exist along the northern edge of the present summit broad conical form with a basal diameter of 14 km. craterŽ. 150±200 m in diameter and 50 m deep and The surrounding topography varies between 2000 hot springs occur around the base of the volcano. and 3000 m in elevation and the principal rivers that encircle the coneÐthe Chambo, Puela and Pastaza riversÐdescend from 2400 to 1600 m elevation 2. Focus of the study Ž.Fig. 2 . The complex consists of three successive edifices, Because of its numerous eruptions, the widespread the two former ones have been partially destroyed by ash deposits around the cone, and the frequent gener- large sector collapses:Ž. i the northern, eastern and ation of pyroclastic flows and lahars, Tungurahua is southern flanks belong to the older edifice of Tungu- considered to be a dangerous volcano which threat- rahua I, which is represented today by large, out- ens the tourist town of Banos,Ä as well as other small wardly-dipping surfaces that are not graded to the villagesŽ. Fig. 2 . A few kilometers east of Banos,Ä the present cone and are greatly incised by deep canyons large Pastaza river is contained by the Agoyan dam Ž.Ž.Fig. 3 ; ii the intermediate cone of Tungurahua II, and used for hydroelectric power generation. Future now only represented by a series of lava flows on the eruptions of Tungurahua threaten this dam and reser- upper southern flank; andŽ. iii the young edifice, a voir, the second most important in Ecuador. near-symmetrical cone with steep 30±358 slopes, Unfortunately, historical reports concerning Tun- occupies the western third of the older volcanic gurahua's early eruptions are commonly brief and complex, nearly filling the avalanche amphitheater unclear, making it difficult to correlate eruptive carved into the west flank by the last sector collapse events with the tephra stratigraphy, especially with- event. Incipient erosion has cut gorges only 10±40 m out 14C dates and petrological support. As an exam- deep into this cone. ple, pyroclastic flow deposits attributed to the 1641 AD eruption by Almeida and RamonŽ. 1991 were dated by us at more than 1000 years BP. Moreover, 4. Main stages of development EgredŽ. 1989 questions the authenticity of the 1641 eruption. The aim of the present paper is to provide 4.1. Older Tungurahua() Tungurahua I an overall view of the recent volcanic development of this little known volcano and will focus on the Immediately south of BanosÄ the northern remnant Holocene and Historic eruptive activity, including flanks of Tungurahua I consist of a ;400-m-thick the last sector collapse. This study is based on work concordant series of basic andesitic and andesitic carried out before 1989 which was written in Span- lava flowsŽ. 55.0±58.6 wt.% SiO2 ; see Table 1 and 4 M.L. Hall et al.rJournal of Volcanology and Geothermal Research 91() 1999 1±21 Fig. 2. Physiographic map of Tungurahua Volcano with location of sites and sections referenced in text. Bold line with teeth: avalanche caldera scar. QAsQuebrada Achupashal; QCsQuebrada Confesionario; QMsQuebrada Mandur; QMTsQuebrada Motilones; QLPs Quebrada La Piramide; QPUsQuebrada Palma Urcu; QRsQuebrada Rea. Continuous bold linessroads. interbedded tephra, which form the old Pondoa and the west side of the cone. On the upper canyon wall Runtun surfaces, now bisected by the Vazcun valley of the Ulba valley, a thick unit of andesitic breccias Ž.Fig. 3 . In general, these rocks are two pyroxene having a chaotic internal structure typical of debris andesites with or without olivine. This first stage of avalanche deposits overlies the lava seriesŽ.