PLANETARY ENVIRONMENTAL CONTROLS ON MAGMA CHAMBER GROWTH

The location, shape and size of a magma reservoir within a volcanic edifice has a complex relationship with the growth history of the volcano. The depth below the surface to which magma can penetrate from the mantle is controlled by the possible presence of a super-lithostatic pressure in the source region and by the variation with depth of both the magma density and the components of lithostatic stress and strength. All of these are influenced by the planetary gravity, and lithostatic properties are also determined by the density structure and shape of the growing volcanic edifice and thus its constructional history. Magmas exsolve juvenile volatiles as a function of pressure, and those erupted at the surface produce vesicular lava flows or poorly compacted pyroclastic layers. By definition intrusions fail to reach the surface and so exsolve fewer volatiles and emplace denser rocks within the edifice than those erupted at the surface. Thus, a volcano that grows more by intrusion than extrusion, or that forms under a high planetary atmospheric pressure, should on average have denser rocks at a given depth below the surface. This places the depth at which rising magmas are neutrally buoyant closer to the surface and influences the nature (sheet-like or dike-like) of the intrusions.

We discuss how studying volcanoes on other planets throws light on these processes. On Venus the high atmospheric pressure greatly reduces gas exsolution from mafic magmas, and as a volcano grows the variation of ambient atmospheric pressure can have detectable consequences for volcano morphology. On Mars the lower gravity makes a large difference to the final vertical extents of magma reservoirs. Low mantle volatile contents characterised ancient eruptions on the Moon and may characterise current eruptions on Io. Given these complications, understanding why the Moon shows almost no evidence for shallow magma reservoirs whereas Io has numerous large calderas is a significant challenge.