Evidence of magma-water interaction during the 13,800 years BP explosive cycle of the Licán Ignimbrite, Villarrica volcano (southern Chile)

Abstract

Villarrica is an active stratovolcano located in the Southern Andean Volcanic Zone. About 13,800 years BP (conventional radiocarbon ages), this volcano experienced major explosive eruptions which resulted in the emission of a sequence of pyroclastic flows, known as the 'Licán Ignimbrite', the bulk volume of which is estimated in -10 km³ (non-DRE, Dense Rock Equivalent). The deposits mainly consist of massive pyroclastic flows and stratified pyroclastic surges. Typical flow fades showscoriaceous bombs, dense juvenile blocks, lithics and scoria lapilli immersed in a dark-grey to brownish matrix, whereas surges expose lapilli-sized scoria in a fine, light-brown or yellow-green matrix. Juvenile clasts range from 55 to 58 wt% Si0₂ in composition. This paper describes the general architecture of the Licán Ignimbrite deposits and, based on SEM (Scanning Electron Microscope) observations and lithologic data, emphasizes the role of fragmentation due to magma-water interaction during the eruption. The results indicate that gas expansion was an important process. However, field characteristics, surface textures of ashes, enrichment of lithics towards the top of the sequence and variable palagonitization of matrix glass show the intervention of water since the initial stages of the eruption and its increasing influence during the later phases.