Soufrière Hills in Montserrat has been erupting since 1995.
The extended eruption of a lava dome at Soufrière Hills Volcano that began in 1995 generated large amounts of fine ash by (1) explosive events from the dome; and (2) frequent collapse of unstable parts of the growing dome that generated pyroclastic flows and associated plumes of ash. A detailed study of ash from both types of events determined that the sub-10 micron fraction of ash from the pyroclastic flows consisted of 10-24 percent crystalline silica, the highest yet documented for a historical eruption (Baxter and others, 1999). In contrast, the sub-10 micron fraction of ash from the explosive events consisted of 3-6 percent crystalline silica. The free silica minerals are produced within the lava dome over a period of many days or weeks.
Monitoring of the concentration of airborne respirable dust and ash around the volcano beginning in August 1997 showed that concentrations of ash have regularly exceeded 50 micrograms/m3 per 24-hour rolling average in areas subject to frequent ashfall. The exposures to cristobalite sometimes reached the 0.05 mg/m3 averaged over an 8-hour workday. Also, the monitoring consistently showed increased concentrations of airborne dust whenever there was human activity.
The eruptions of Soufrière Hills during 1997 produced chemical contamination of rainwater and surface water. Water sampling in January 1997 indicated highly acidic water with high concentrations of sulphates, chloride and fluorides. Similar results were recorded until June 1997 although all fell within World Health Organization recommended levels for all measured components (see Smithsonian Institution Global Volcanism Program).