Magma flow instability and cyclic activity at Soufriere Hills Volcano, Montserrat, British West Indies

Dome growth at the Soufriere Hills volcano (1996 to 1998) was frequently ac companied by repetitive cycles of earthquakes, ground deformation, degassin g, and explosive eruptions. The cycles reflected unsteady conduit flow of v olatile-charged magma resulting from gas exsolution, rheological stiffening , and pressurization. The cycles, over hours to days, initiated when degass ed stiff magma retarded flow in the upper conduit. Conduit pressure built w ith gas exsolution, causing shallow seismicity and edifice inflation. Magma and gas were then expelled and the edifice deflated. The repeat time-scale is controlled by magma ascent rates, degassing, and microlite crystallizat ion kinetics. Cyclic behavior allows short-term forecasting of timing, and of eruption style related to explosivity potential.