December 6, 1996

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

The Montserrat Dome

Last Thursday, scientists at HVO's sister observatory in Vancouver, Washington (the USGS Cascades Volcano Observatory, or CVO) received a distress call from concerned officials on the small Caribbean Island of Montserrat. Heightened seismicity and continued growth of a monstrous lava dome in the crater of Soufriere Hills create the potential for a giant landslide and explosive release of the immense pressure building in the volcano.

The Montserrat government first requested assistance from CVO's international crisis response team in July of 1995 when Soufriere Hills vented a puff of ash and steam. In cooperation with local scientists, the CVO team installed an array of seismometers and ground deformation equipment, very similar to our network here in Hawaii, to monitor the reawakening of this long-dormant volcano.

Ash and steam emissions continued until November of 1995, when the volcano started to extrude a thick plug of sticky lava. Over the next several months, lava piling up over the vent created a steep-sided dome in the summit crater. Gravitational instability cause parts of the dome to periodically collapse and shoot a burning landslide of rock fragments and ash through a gap in the crater wall, down the Tar River valley on the east flank of the volcano, and on to the sea.

The most destructive collapse occurred two months ago, on September 17, after an intense swarm of shallow earthquakes. A huge part of the dome broke lose and roared down the valley. The sudden removal of rock "uncorked" the pressurized magma system below. A lateral blast ejected hot boulders to the NE and spawned a vertical ash column over 40,000 feet high. Inhabitants reported "falling stones and fire from the volcano" and buildings were "pushed as if by impact of a very strong wind." Airplane pilots encountering the ash cloud reported "smoke in the cockpit", engine problems, and pitted windshields. The explosive activity was over in 48 minutes, leaving downed power lines, broken buildings, and a blanket of ash over the southern half of the island. There were no reported casualties.

At present, dome growth continues and the situation is becoming increasingly hazardous. The now 600 feet thick dome is pressing on the southern wall of the crater. Scientists in Montserrat say that if the wall gives way it may catastrophically "expose hot, gas-rich magma from low down in the dome, and cause a lateral blast to the south of the volcano" with "little or no warning." The near 7000 residents have gathered on the northern end of the island. Travel to the south is restricted, including access to the town of Plymouth, the seat of the Montserrat government. Volcanologists from CVO are expected to leave for Montserrat this week.

Kilauea Eruption Status

Meanwhile, Hawai`i's restless volcano, Kilauea, experienced its own "collapse" last week as nearly 25 acres of new land created by the current eruption fell into the sea near Lae'apuki in Hawaii Volcanoes National Park (see Hawaii Tribune Herald December 3, 1996). Collapses of this type have occurred many times over the course of this eruption. At 1:40 pm on December 5, residents in East Hawaii were rocked gently by a M3.1 earthquake located to the SE of Kilauea's summit.