The magmatic and eruptive response of arc volcanoes to deglaciation: Insights from southern Chile

In tectonic settings where decompression melting drives magmatism there is compelling evidence that changes in ice- or water-loading across glacial/interglacial cycles modulate activity. In contrast, the response of subduction-related volcanoes remains unclear. A high-resolution post-glacial eruption record from a large Chilean stratovolcano, Mocho-Choshuenco, provides new insight into the arc magmatic response to ice load removal. Following deglaciation, we identify three distinct phases of activity characterized by different eruptive fluxes, sizes, and magma compositions. Phase 1 (13 – 8.2 ka) was dominated by large dacitic/rhyolitic explosive eruptions. During Phase 2 (7.3 – 2.9 ka) eruptive fluxes were lower and dominated by moderate-scale basaltic-andesite eruptions. Since 2.4 ka (Phase 3) eruptive fluxes have been elevated and of more intermediate magmas. We suggest that this time-varying behavior reflects changes in magma storage timescales, modulated by the changing crustal stress field. During glaciation, magma stalls and differentiates to form large, evolved crustal reservoirs. Following glacial unloading, much of the stored magma erupts (Phase 1). Subsequently, less-differentiated magma infiltrates the shallow crust (Phase 2). As storage timescales increase, volcanism returns to more evolved compositions (Phase 3). Data from other Chilean volcanoes show a similar tripartite pattern of evacuation, relaxation and recovery, and this could be a general feature of previously-glaciated arc volcanoes