Crustal-scale degassing and igneous mush re-organisation: a generic concept applied to episodic volcanism at the Soufrière Hills Volcano Montserrat

Abstract:

A complete theory of episodic volcanism is lacking. Melt generation related to large scale tectonic processes is likely continuous but surface volcanic activity is typically episodic; for most volcanoes short-lived eruptions alternate with long periods of dormancy. Many models of volcanic activity and geophysical unrest are framed by a conceptual model of shallow magma chamber recharge, in which various phenomena are attributed to magma transport from deeper levels. While many aspects of volcanism are explained by this concept it has little explanatory power for key aspects of volcanism, including time scales of dormancy, eruption duration and eruption magnitude. Extensive trans-crustal igneous systems develop beneath active volcanoes in which much of the system is in a mushy state in which buoyancy-driven segregation of melt and magmatic fluid occurs to form layers, which are inherently unstable. We postulate that such systems are prone to destabilisation in which segregating layers amalgamate to form ephemeral magma chambers and in which melts and magmatic fluids decouple. Periods of dormancy relate to slow processes of segregation while short periods of volcanic unrest and eruption relate to episodic and rapid processes of destabilisation of the mush system. In this conceptual framework volatiles rather than magma recharge plays the key role in the dynamics of the shallow parts of the magmatic systems. Magma ascent during episodes of destabilisation does not itself cause pressurisation because melts and crystals are near incompressible, while volatile exsolution and decompression results in major pressure changes that can lead to unrest and eruption. These concepts are applied to the interpretation of stratigraphic, geochronological, geophysical, geochemical, petrological and volcanological data of volcanic activity at the Soufrière Hills Volcano (SHV), Montserrat.