Calderas: Structure, Unrest, Magma Transfer and Eruptions

Abstract Calderas are the largest, most destructive and least predictable type of volcano, as they often experience dramatic unrest commonly not followed by eruption. Among the crucial problems related to caldera eruption forecasting is to understand what signals may reveal, whether unrest will culminate in eruption and, in case, determine the likely location for the eruptive vent, the time to eruption and its expected size. Here we summarize the recent studies on calderas addressing these issues, also providing working hypotheses to focus future caldera research. In particular, we consider the structure of calderas, showing how the subsidence/diameter ratio and the ratio between subsidence stresses (excavation stresses) and other stresses (tectonic, edifice spreading) may influence the location of any eruptive vent. With this regard, we discuss the end-member calderas with substantial (as Fernandina, Galapagos) and minor (as Campi Flegrei, Italy) topographic depression and edifice size, explaining their observed vent patterns deriving from their stress state. Using such approach, advanced 3D modeling combined with detailed knowledge of the surface and subsurface structure may allow improved forecast of the location of future vents also at other calderas. As for the shorter-term forecast of unrest culminating into an eruption, available data suggest that future studies should focus on the factors and processes controlling the eruptibility of the magma intruded during unrest.