Textural and geophysical characterization of explosive basaltic activity at Villarrica volcano

Abstract

Villarrica volcano (Chile) is one of the most active volcanoes in South America. Its activity is currently characterized by continuous degassing from a summit lava lake/vent punctuated by explosive events. During November 2004 a multidisciplinary experiment was deployed for a 10-d period to define the style of emission and infer shallow conduit dynamics at this basaltic center. This involved collection of thermal, seismic and infrasonic data to describe the background activity confined inside the crater, and use of samples to texturally and chemically characterize the ejecta from more energetic explosions able to attain the crater rim. The background activity was characterized by gas bursting with a frequency of 9 events per minute. This involved emission of gas puffs fed by bubble bursting, with larger bursts emplacing sheets of magma onto the lower crater walls. The ejecta population from the more energetic events was characterized by the coexistence of both scoriae and golden pumice. These two types of clasts have different textures but identical glass compositions, suggesting that they underwent different conduit histories. The golden pumice is interpreted as the expanding inner part of a short-lived jet fed by a rapidly ascending, magma batch. The scoria forms the outer portion of the jet and comprises degassed material entrained during passage of the fresh batch through material residing in the upper-most portion of the conduit. We thus have a largely degassed upper column that feeds persistent bubble bursting, through which fresh batches occasionally rise to feed events of relatively higher energy.