Deciphering the dynamics of the 2015 eruption of Calbuco volcano (Southern Andes of Chile) using textural analyses and numerical modeling

On April 22, 2015, Calbuco volcano (Southern Andes of Chile) started a new eruptive cycle after more than 40 years of inactivity. The eruptive sequence included 2 sub-Plinian eruptions in a 12-hour span, followed by a minor eruption 7 days later. Interestingly, the eruption was preceded by very weak seismic activity that was noticeable only 3 hours before the first eruption and produced tephra fall and pyroclastic density currents without the emission of lava, opposed to all previous historical eruptions. During the eruption, at least 3 different types of pyroclasts were emitted: a) Light-brown, vesicular scoria, b) Dark-grey, dense, crystalline juveniles and c) Whitish, highly-vesicular pumice. (a) and (b) had the same basaltic-andesite composition, while (c) was andesitic. (a) was the dominant type in the first sub-Plinian event, while a progressive increase in (b) was noted during the second eruption. (c) was accessory in both eruptions.

In this work we combine textural analysis of pyroclasts with a coupled numerical modeling of magma chamber dynamics and conduit ascent and preliminary geophysical data to analyze the mechanisms of the eruption. Our textural analysis included crystal and bubble size distribution, density, porosity and permeability estimations. Preliminary results indicate a higher mean size of bubbles and crystals, together with a higher permeability of (a) when compared with (b). Our data and numerical modeling suggest that the first eruption was triggered by a small intrusion of magma. The first eruption generated a decompression of the magma chamber, followed by vesiculation, triggering the second and larger sub-Plinian event. We believe that the low permeability of (b), which was the dominant pyroclast type at the end of the second eruption, reduced the escape of gas through the conduit and inhibited the explosive-effusive transition, preventing the effusion of lava flows or domes that was common in previous eruptions of Calbuco volcano.