School of Earth Sciences

Shallow sub-volcanic structures of the Ecuadorian

Andes: Volcan Cotopaxi and the Chalupas caldera

Supervisor: J. Gottsmann, University of Bristol; Project partner: Geophysical Institute Ecuador, Quito, Ecuador

Density anomalies in volcanic areas can be related to subvolcanic structures such as magmatic plumbing systems, volcano-tectonic structures and major changes in subsurface lithologies. Gravimetric measurements quantify such anomalies and can be used to obtain information on the subsurface architecture from the generation of spatial density distribution models.

High precision gravity data for Ecuador is sparse. This project aims to significantly add to the knowledge base on the shallow crustal structure of this tectonically and volcanically active area in a region dominated by a large negative Bouguer anomaly representing the low-density root of the Andes and a crustal thickness of 50-70 km.

The project involves the collection of Bouguer anomaly data at two volcanic centres in Ecuador's Sierra province which includes the Andean Cordillera and its foothills. The research will place constraints on the shallow subsurface structures and their spatial variations at two related volcanic centres:

1. Volcan Cotopaxi (5897 m) is one of the largest and most hazardous stratovolcanoes in the Northern Andes with an andesitic and rhyolitic magmatic history. It is formed by a large symmetrical cone with 3000 m of relief and a 22 km basal diameter. Its upper flanks are covered by a glacier. Periodic alternations of rhyolitic and andesitic eruptions dominated the the last eruptive period that started about 13 ka ago.

2. With neighbouring Volcan Cotopaxi built to its NW, the Chalupa caldera (15 x 20 km, average elevation of 3600 m) is one of several large silicic centres in Ecuador with a complex geological history including major ignimbrite eruptions, post-collapse resurgence and post-collapse volcanism building the Quilindana stratocone (4878 m). In addition, the caldera has been affected by tectonic activity and is hydrothermally active with potential for geothermal exploitation.

There is evidence that magma erupted from Cotopaxi recycled the Chaplupas pluton and one must hence infer that at least part of Cotopaxi's plumbing system intersects that of the Chalupas caldera.

Key objectives of the project include:

• determination of collapse morphology, depth to the metamorphic basement and assessment of the role of tectonics in the generation of the Chalupa caldera
• imaging of the shallow structure and plumbing system of the Chalupa caldera
• imaging of the shallow plumbing system of Volcan Cotopaxi in relation to the subvolcanic architecture of the Chalupas caldera
• improving the knowledge base on the 3D morphology of the large negative Bouguer anomaly of the Ecuadorian Sierra which Cotopaxi and the Chalupas caldera form part of
• comparison of the subsurface structure at the two centres with the subvolcanic architecture of the Altiplano Puna region of southern Bolivia which forms part of a large regional low density anomaly

The project is suitable for a numerate graduate of geology or geophysics with strong interests in applied geophysics and who does not mind challenging work conditions in the field. The candidate is expected to conduct two gravimetric field campaigns in Ecuador over the course of the project, to post-process, analyse and to interpret the data using mathematical inversion tools to construct models of the subvolcanic density distribution. An at least basic command of the Spanish language would be advantageous. Training will be provided in the collection, post-processing, and interpretation of gravity data, the use of forward and inverse modelling techniques as well as gravimetric and elevation data timeseries analysis.

Members of Bristol's Volcanology research group enjoy their lunch during a gravity and GPS survey in the Bolivian Altiplano

The student will join a team of international researchers funded by the European Commission and the Natural Environment Research Council with focus on Andean volcanism and magmatism and will benefit from a series of related outreach and impact activities including participation in workshops and summerschools in Europe and Latin America.

Further reading:

Feininger T. and Seguin M.K.: Simple Bouguer gravity anomaly field and the inferred crustal structure of continental Ecuador. Geology, v.11, p.40 - 44, 1983

Garrison, J., Davidson, J., Turner, S., Reid, M., 2003. Recycling of the Chalupas pluton at Cotopaxi volcano, NVZ, Ecuador: evidence from 238U-230Th disequilibria. Geophy Res Lett Abst 5:11998

Hall M. & Mothes, P., RHYOLITIC CALDERAS AND CENTERS CLUSTERED WITHIN THE ACTIVE ANDESITIC BELT OF ECUADOR’S EASTERN CORDILLERA, IOP Conf. Series: Earth and Environmental Science 3 (2008) 012007 doi:101088/1755-1307/3/1/0120072008

Hall, M., Mothes, P., 2007. The Rhyolitic-Andesitic Eruptive History of Cotopaxi Volcano, Ecuador. Bull. Volcan. DOI 10.1007/s00445-007-0161-2.

Gottsmann, J., Camacho, A.G., Marti, J., Wooller, L., Fernández, J., Garcia, A., and Rymer, H., 2008, Shallow structure beneath the Central Volcanic Complex of Tenerife from new gravity data: Implications for its evolution and recent reactivation: Physics of the Earth and Planetary Interior, v. 68 p. 212–230.