Volcano Watch — Update on Soufriere Hills, Montserrat

Last Friday, July 18, marked the two-year anniversary of the ongoing eruption of Soufriere Hills volcano on the small Caribbean island of Montserrat. The sulfurous gases, drifting ash clouds, and avalanches of hot rock that periodically burgeon from the volcano have inflicted social and economic hardships that have become increasingly difficult for islanders to bear.

The volcano's most destructive act was staged last month with the collapse of part of the massive dome of lavathat had been accumulating at the vent since November 1995. The ensuing avalanche of incandescent boulders and ash inundated the northern flank of the volcano, destroying homes and claiming lives. Margaret Mangan, HVO volcanologist currently in Montserrat assisting the team of Caribbean and United Kingdom scientists of the Montserrat Volcano Observatory, reports on the recent activity.

On Wednesday, June 25, a large section of the 240-m (800-ft) -high lava dome growing at the summit of Soufriere Hills volcano collapsed. In an instant, lethal surges of hot rock, called pyroclastic flows, swept over the north flank of the volcano toward the sea. A roiling column of ash shot up to 9,000 m (30,000 ft) within minutes. A thick rain of gray mud started to fall amid streaks of heat lightning, and a sickening, surreal darkness closed over the observatory. The pyroclastic flows and mud rain stormed the island for 25 minutes, destroying 150 homes and claiming eight lives. Eleven people remain missing.

In the days that followed, frequent but smaller dome collapses fed pyroclastic flows down both the northern and western flanks of the volcano. Flows invaded the capital city of Plymouth, whose 5,000 residents had been evacuated several months ago in anticipation of such an event. The entire city was choked with a searing cloud of ash. Fortunately, the flows stayed within the Gages-Fort Ghaut river valley that runs from the summit of the volcano through central Plymouth. Only houses on the banks of Fort Ghaut were damaged.

The most intense pyroclastic-flow activity began to subside after July 4. Presently, the volcano suffers only minor rockfalls and dilute ash emissions. The dome itself was obscured by clouds, steam, and ash for several weeks running. The curtain lifted for a short while on July 17, however, revealing a new mass of hot lava nested in the gouge left by the June 25 collapse.

The pattern of dome growth and collapse seen at Soufriere Hills is typical of many explosive volcanic eruptions. Previous such eruptions in the Caribbean and elsewhere in the world indicate

that these cycles of growth and collapse can persist for 5 to 10 years. As the volcano enters its third year of eruption, the people of Montserrat bear up with remarkable fortitude in the face of continuing disruption and loss.

Volcano Activity Update

The big news at Kīlauea this past week was the renewed entry of lava into the ocean on the night of July 12, the first time since January 27 that liquid rock and liquid water have tried to mix. The flow, fed from a perched lava pond on the south side of Pu`u `O`o, entered the ocean about 500 m west of Waha`ula Heiau in Hawaii Volcanoes National Park. The flow front was wide, ranging from 300 m to 500 m, and an unstable lava delta was constructed 30-40 m beyond the old coastline. The flow into the sea nearly stopped on Thursday, July 17, because of blockages in the tube system that caused breakouts on the surface. At the time of this writing on Friday, there are numerous surface flows and an active ocean entry. The public is reminded that the area is extremely hazardous with frequent collapses of the lava delta accompanied by explosions.