Control of the aspect ratio of the chamber roof on caldera formation during silicic eruptions

Several factors may control the development of a silicic caldera during an eruption: here we match previous studies of caldera forming eruptions with analogue experiments to consider the role of the aspect ratio (thickness/width) of the magma chamber roof. These data suggest that large silicic caldera formation is controlled by: (1) the availability of magma; (2) initial explosive eruption through a conduit connecting the reservoir and the surface; (3) fast depressurization of the reservoir; and (4) chamber roof collapse, with newly-formed fractures connecting the reservoir with the surface and creation of a continuous ring conduit feeding annular vents, capable of sustaining the eruption. The occurrence of the latter condition may depend upon the aspect ratio of the chamber roof: lower ratios (< 1.6) form coherent collapse, ensuring connection from the chamber to the surface; higher ratios (> 1.6) form incoherent collapse, which may hinder the continuation of the eruption after an initial partial emptying of the chamber. The aspect ratio of the chamber roof should therefore be considered as a further factor, capable of controlling the formation of large silicic calderas.