Periodic sulphur dioxide degassing from the Soufrière Hills Volcano related to deep magma supply

Soufrière Hills Volcano produced prodigious quantities of sulphur dioxide (SO 2 ) gas throughout 1995–2013. An unprecedented, detailed record of SO 2 flux shows that high SO 2 fluxes were sustained through eruptive pauses and for two years after the end of lava extrusion and are decoupled from lava extrusion rates. Lava extrusion rates have exhibited strong 1- to 2-year cyclicity. Wavelet analysis demonstrates periodicities of c. 5 months and c. 2 years within the SO 2 time series, as well as the shorter cycles identified previously. The latter period is similar to the wavelength of cycles in lava extrusion, albeit non-systematically offset. The periodicities are consistent with pressure changes accompanying deformation in a coupled magma reservoir system whereby double periodic behaviour may arise from limited connectivity between two reservoirs. During periods of lava extrusion SO 2 is released together with the lava (yielding the c. 2 year period), albeit with some offset. In contrast, when magma cannot flow because of its yield strength, SO 2 is released independently from lava (yielding the c. 5 month period). Our results have implications for eruption forecasting. It seems likely that, when deep supply of magma ceases, gas fluxes will cease to be periodic.