Eruption volume, periodicity, and caldera area: Relationships and inferences on development of compositional zonation in silicic magma chambers

Abstract

In order to put constraints on the mechanisms of compositional zonation in magma chambers, data were collected on caldera areas, ash-flow volumes and repose times between zoned ash flows for a number of magmatic systems of different age, composition, size and tectonic environment. First-order correlations between volume, repose time and stratification rate are apparent. Chemical zonation is developed to partially compensate for the internal production of entropy due to heat conduction through magma chamber thermal boundary layers. Repose times are proportional to eruption volumes and are consistent with convection-aided diffusion processes such as Soret diffusion or double-diffusive convection. Volume and area relations suggest that small-volume systems tend to be more conical than cylindrical in shape. An important factor controlling magma chamber evolution appears to be the ratio of magma chamber surface area to magma chamber volume, other factors remaining the same. Small magma chambers appear to stratify in shorter periods of time and at faster rates than large-volume systems.