Bernardo Beate

Tungurahua Volcano, Ecuador : structure, eruptive history and hazards

Abstract Tungurahua, one of Ecuadors 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 volcano (Tungurahua III) has rebuilt the cone to about 50% of its pre-collapse size by the emission of ∼3 km3 of volcanic products. Two periods of construction are recognized in Tungurahuas 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 composition (andesite+dacite), 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 growths rate is estimated at ∼1.5×106 m3/year 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, 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.

Mio–Pliocene adakite generation related to flat subduction in southern Ecuador: the Quimsacocha volcanic center

New geochemical and geochronological data on the Miocene^Pliocene Quimsacocha volcanic center (QVC) have led to the recognition of adakitic lavas generated by slab melting related to the flat slab subduction in southern Ecuador and northern Peru. The QVC, located in the presently inactive southern part of the Ecuadorian arc, was built up during three distinctive volcanic phases. The first phase generated a basal edifice with mainly andesitic lava flows, while the second phase is characterized by the emplacement of cryptodomes, domes and related outflow breccias comprised of andesites and some dacites. The last phase released rhyolitic ignimbrites associated with the formation of a large caldera, which was later partly filled by dacitic^rhyolitic domes. Geochemical data for the QVC indicate higher Al2O3, TiO2 ,N a 2O, Zr and Sr contents and lower Fe2O3*, MgO, Y, MREE and HREE abundances, compared to other eruptive rocks of the Plio^Quaternary volcanic front of Ecuador. Such geochemical features, as well as the frequent presence of an associated epithermal gold deposit, are characteristic of the involvement of slab melts, also known as adakites [1,2], in the generation of these magmas. After a calc-alkaline arc magmatism phase, slab horizontalization ^ in response to the subduction of a buoyant oceanic plateau ^ results in increased involvement of a slab melting component in the magmas produced. However, pristine adakites were generated and emplaced during a relatively short period, as indicated by zircon fission-track ages. Then volcanic activity stopped and a volcanic gap formed. The identification of these adakites, their location and age support a model of slab melting associated with flat slab subduction [M.A. Gutscher et al., Geology 28 (2000) 535^538]. fl 2001 Elsevier Science B.V. All rights reserved.

A Capstone Course in Ecuador: The Andes/Galapagos Volcanology Field Camp Program.

ABSTRACT We developed and implemented the Galapagos Volcanology Field Camp, a 3 week, 3 credit hour course for upper-level university students with a major course of study in geology. The course is offered by the South Dakota School of Mines and Technology, is open to any student, and is usually populated by students from many universities across the U.S. The course offers the essential skills of field geology, such as lithologic description, unit identification and correlation, stratigraphic logging, and geologic mapping, taught exclusively in an environment of volcanic rocks. Beginning in the subduction setting of the Andes, students are introduced to the regional volcanic and tectonic history. The course visits volcanoes that have produced andesitic, dacitic, and rhyolitic products. Students study lava flows, pyroclastic flows, ignimbrites, lahar flows, and debris avalanche deposits. Students are also introduced to the volcanic hazards and monitoring efforts in Ecuador. During the second phase of the course, the group flies to the Galápagos Islands to examine the basaltic features of hotspot volcanism. Students study recent lavas erupted from well-developed shield volcanoes, lava tunnels and collapse features, and the history of uplift, subsidence, and sea-level variation. Through design and implementation of this program, a sequence of exercises has been found that build a robust curriculum while fitting into an itinerary that is logistically feasible. Study of volcanic deposits in this classic setting leads to strong student learning, with 84% of students who have participated scoring higher than 80% on activities designed to assess the stated learning objectives.