The dynamics of magma storage and ascent beneath an active arc volcano (Villarrica, Chile)

Volcanic arcs, formed along convergent plate margins, produce most of Earth's subaerial volcanic activity and are responsible for the largest and most devastating historical eruptions. At arcs, magma generated in the mantle above the subducting slab ascends through the lithosphere, where it is stored and chemically processed before feeding eruptions at the surface, typically from large stratovolcanoes. Understanding the factors that govern when and how this stored magma ultimately leaves its crustal reservoir(s) to be erupted, and what size that eruption is likely to be, remains a central goal of volcanology and should facilitate more accurate forecasts of future eruptions. This project will employ geochemical and petrological methods to constrain pre-eruptive processes, storage conditions and/or origins of magmas erupted from an arc volcano in southern Chile (Villarrica) that has produced numerous post-glacial eruptions of varying magnitude. Villarrica is one of Chile's most active volcanoes and forms part of the southern Volcanic Zone (SVZ) group of volcanoes. Volcanism along the SVZ has been strongly influenced by tectonics and Villarrica lies along a fracture system associated with a regional strike-slip fault system, the Liquine Ofqui fault zone. The volcano has also experienced extensive late-Pleistocene glaciation and its summit region remains permanently ice-covered. Postglacial activity at Villarrica has varied from small-volume eruptive events to large ignimbrite forming pyroclastic eruptions. Despite this range in eruption magnitude, it is notable that the majority of these events involve relatively mafic magmas, typically basaltic to basaltic-andesitic in composition. Large explosive eruptions of Villarrica do not therefore involve magmas that have undergone significant extents of storage and/or fractionation compared to those feeding smaller lower-intensity eruptions. It is also notable that while smaller eruptions typically vary in composition from each other, large volume eruptions tend to be relatively compositionally homogenous, implying efficient mixing of the discrete melt batches that constitute the magma system feeding the more common smaller eruptions. In addition, geodetic studies show that maximum extents of ground deformation around Villarrica, inferred to be linked to magma movement, are offset from the centre of the volcano by several kms, implying that at least some of the magmas feeding the volcano move laterally through the crust.

This research will involve a comparative study of the conditions of magma generation and storage associated with eruptions of Villarrica that vary in size between small cone forming events to large pyroclastic eruptions. You will undertake fieldwork in Chile to sample deposits from several eruptions, selected to cover as wide a range of eruption magnitudes as possible. After petrographic characterization of these samples, you will embark on an analytical programme designed to constrain the origin of these melts as well as the condition of pre-eruptive storage.