Degassing at the Soufriere Hills Volcano, Montserrat, Recorded in Matrix Glass Compositions

Compositions of matrix glasses from the current eruption ofSoufrière Hills Volcano indicate that decompression-drivencrystallization results in 20–70 wt % groundmass crystallizationduring eruption and variable degassing. Variations in crystallinityand volatile contents (water and chlorine) of matrix glassesare attributed to variations in extrusion rates and residencetimes in the lava dome. Residual water contents in pumice clasts(0·2–0·6 wt %) indicate minimum pressuresof 1·1–3·7 MPa in 1997 Vulcanian explosions.Residual water contents of 1·6 wt % in a ballistic blockejected in sub-Plinian explosive activity on 17 September 1996imply larger pressure drops (