Inferring the Processes of Gas Release at Active Volcanoes from Remote Observations
Hayer, Catherine1; Wadge, G.1; Edmonds, M.2; Christopher, T.3
1University of Reading, UNITED KINGDOM; 2University of Cambridge, UNITED KINGDOM; 3Montserrat Volcano Observatory, MONTSERRAT
Volatiles are released by magmas rising through volcanoes. The vapour can exist in bubbles within the magma or as a gas in fractured networks. Different volatile species have different solubilities and their release is sensitive to the temperature-pressure history of the magma. The transport history of the emitted gas is largely dependent on the permeability of the conduits carrying the magma, which is in turn modulated by the internal and external forcing events that make up the eruptive history of the volcano. Here we use spectrometer observations, from space and from the ground, of the emission rate of sulphur dioxide from the Soufriere Hills Volcano, Montserrat over the last ten years, to infer what processes have operated. Sulphur dioxide loadings, mainly daily, from the OMI, IASI and SEVIRI spaceborne instruments are presented, together with ground-based DOAS measurements. Deep-seated mingling of basalt and andesite magmas feeds the release the 17-year long average emission of sulphur dioxide at a rate of about 500 t/d. Multi-year modulation of this rate is driven by some deep-seated, unknown mechanism that does not correspond obviously to the intermittent phases of lava extrusion. A multi-week cycle driven by dyke storage of magma appears to emit gas at higher rates in the early part of the cycle. A sub-daily cycle of stick-slip magma motion within the top 1-2 km produces a peak of gas release just after slip starts, but this is poorly sampled. Events such as major collapses of the lava dome, Vulcanian explosions and volcano-tectonic events can all release large pulses of gas to the atmosphere over a few minutes to days. The ability to measure the variations in sulphur dioxide flux is limited by the repeat time of the satellites and confused by the variable sensitivities of the instruments to the emplacement altitude of the gas. The latter varies from major Vulcanian explosions to passive degassing from a dome with a chilled carapace.