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Dacites not responsible for rapid uplift at Uturuncu volcano

High temperature experiments reveal previous eruptions were characterized by shallow magma storage, a scenario incompatible with the depth of the current anomaly

Cerru Uturuncu, Bolivia, is a continental arc volcano located in the Central Andes. Recent satellite observations of ground deformation in the area have measured uplift of 1 – 2cm per year. This has been accompanied by persistent seismic activity and indications are strong that Uturuncu may be entering a period of unrest and possible magma build up. Inverse modelling of the deformation has indicated a large diameter anomaly at 11 – 17km beneath the volcano.

A new study by Muir and co-workers at the University of Bristol conducted high temperature experiments on the two types of lava primarily from the volcano: rhyolite and dacite. The aim of their work to determine if previous episodes of magma storage are consistent with the depth of the anomaly causing the current deformation, and whether future eruptions would likely be effusive (in continuation of past activity at Uturuncu) or larger-scale explosive events.

BSE image of an experimentally synthesized dacitic lava.

BSE image of an experimentally synthesized dacitic lava.

The natural mineral assemblages in both types of lava were replicated by experiments at 870ºC at pressures equivalent to 2 – 6 km depth, a similar crustal level to the location of recent earthquakes recorded at the volcano. This experimental evidence precludes the role of dacites and rhyolites in producing the observed anomaly beneath Uturuncu. Instead, the authors propose a model where dacitic magmas are formed from fractional crystallisation in an underlying, deeper magma body before stalling in the shallow crust prior to their effusive eruption.

Muir DD, Blundy JD, Rust AC, & Hickey J (2014) ‘Experimental constraints on dacite pre-eruptive magma storage conditions beneath Uturuncu volcano’. Journal of Petrology, 55(4), 749-767.