Villarrica (Chile) is one of the most active volcanoes in South America
having erupted about 60 times in the last 460 years. Although its
historical eruptive activity has been mainly effusive and weakly
explosive, it had strong explosive behaviour in postglacial times.
Chaimilla (< 3.1 ka) is one of the best exposed and widely dispersed
pyroclastic deposits, related to both fall and flow activity. The
deposit is dispersed over an area of 250 km2 and consists of 8 units
(A?H) which were grouped into four sequences. Stratigraphic data
suggest that the eruption had a relatively short duration and evolved
from i) an Opening phase, dispersing ash, lapilli clasts, accretionary
lapilli, blocks and bombs, to ii) a Pulsatory phase, originating
a series of magmatic explosions, to iii) a Collapsing phase, characterised
by unstable plumes which emplaced a series of pyroclastic density
currents intercalated with thin fallout layers and finally to iv)
a Climactic phase forming a more sustained plume which eventually
collapsed generating the final pyroclastic density currents. The
deposit (fall and flow) has a minimum cumulative volume of 0.6 km3,
with the main sustained phase being associated with a VEI 4 and the
flow units having a minimum estimated total volume of 0.04 km3. The
erupted material has a homogenous chemical composition but displays
a remarkable variability in both textural and physical properties.
The density distribution of juvenile products shows a clear bimodality
characterised by two main populations: P1 and P2. Population P1 consists
of highly vesicular clasts (modal density around 1000 kg m? 3) with
mostly sub-spherical bubbles and moderately crystallised groundmass
with large-sized microlites. Clasts from population P2 are poorly
vesicular (modal density around 1600 kg m? 3) with irregular to collapsed
bubbles and numerous smaller microlites. The variability of both
vesicularity and microlite characteristics suggests the involvement
of two magma batches with distinct pre-eruptive degassing and rising
histories. Our eruption conceptual model implies the arrival of new
magma (represented in the deposit by P1 clasts) into a small, outgassed
magma body which was accumulated at shallow level (mainly represented
by P2 clasts). A new Chaimilla-type eruption could significantly
affect the communities that have recently developed around Villarrica
volcano and subsist mainly on tourism and forestry. As a result,
a better understanding of the dynamics and evolution of the Chaimilla
eruption is necessary for the identification of potential hazard
scenarios at Villarrica volcano and, ultimately, for the risk mitigation
of this populated area of Southern Chile.