Petrologic determination of ascent rates for the 1995-1997 Soufriere Hills Volcano andesitic magma

The recent eruption of the Soufriere Hills Volcano in Montserrat (July, 1995, to present; October, 1997) has produced a hornblende-bearing, andesitic lava dome. It is possible to petrologically estimate changes in ascent rates of amphibole-bearing magmas. For certain rates of decompression, a breakdown rim of fine-grained, anhydrous reaction products forms where amphibole is in contact with melt. The thickness of the rim varies with ascent rate. Most of the amphibole phenocrysts in the magma storage region lack breakdown rims. About 10% have 200-400 mu m-thick, coarse-grained breakdown rims that are interpreted to be relicts of a past heating event. Study of a time series of new dome andesites showed that ascent rate increased from December, 1995 (similar to 0.001 m/s), to July, 1996 (similar to 0.008 m/s), while eruptive style remained extrusive. Ascent rate increased to >0.012 m/s in August, 1996, and the first major explosive eruption occurred on 17-18 September, 1996.