The evolution of volcanic systems following sector collapse

Sector collapses affect volcanic edifices across all tectonic settings and involve a rapid redistribution of mass, comparable in scale to the largest magmatic eruptions. The eruptive behaviour of a volcano following sector collapse provides a test of theoretical relationships between surface loading and magma storage, which imply that collapse-driven unloading may lead to changes in eruption rate and erupted magma compositions. Large sector collapses are infrequent events globally, with all historical examples being relatively small in comparison to many of the events documented in the geological record. As a result, exploration of the impacts of sector collapse on eruptive behaviour requires detailed investigation of prehistoric collapses, but this is often hindered by poorly-resolved stratigraphic relationships and dating uncertainties. Nevertheless, observations from a number of volcanoes indicate sharp changes in activity following sector collapse. Here, a global synthesis of studies from individual volcanoes, in both arc and intraplate settings, is used to demonstrate a number of common processes in post-collapse volcanism. Multiple examples from large (>5 km ³) sector collapses in arc settings show that collapse may be followed by compositionally anomalous, large-volume and often effusive eruptions, interpreted to originate via disruption of a previously stable, upper-crustal reservoir. These anomalous eruptions highlight that magma compositions erupted during periods of typical (i.e. unperturbed by sector collapse) volcanism may not be representative of the range of compositions stored within a vertically extensive crustal reservoir. If eruptible magma is not present, upper-crustal reservoirs may rapidly solidify following collapse, without further eruption, allowing more mafic compositions to ascend to the surface with only limited upper-crustal modification, resulting in edifice regrowth at temporarily elevated eruption rates. Subsequent re-establishment of an upper-crustal reservoir further supports a relationship between surface loading and crustal storage, but long-term chemical and mineralogical differences between pre- and post-collapse evolved magmas imply that a newly-developed reservoir can overprint the influence of a preceding reservoir, forming a spatially and compositionally distinct plumbing system. These broad patterns are replicated in intraplate settings, despite differences in scale and melting processes; current evidence suggests that post-collapse evolution of intraplate volcanoes can be explained by unloading-induced destabilisation of the magma plumbing system, rather than increased melt production. What emerges from an apparently diverse set of observations is a systematic behaviour that strongly supports a coupling between edifice growth and magma ascent, storage and pressurisation. Eruption rates, erupted compositions, and the style of volcanism at any particular system may thus be modulated from the surface, and long-term shifts in surface behaviour may occur without any changes in the deep parts of magmatic systems. Observations of sharp post-collapse changes in erupted compositions, including the ascent of primitive mafic magmas, also require a crystal-dominated mid- to upper-crustal reservoir, consistent with recent models of crustal magmatic systems.