Feb. 19, 1999 University Park, Pa. -- A study of the Soufriere Hills Volcano provides important clues to short-term prediction of and the mechanisms behind cyclic eruptions of the most common type of volcanoes, according to an international team of volcanologists.
"We had precisely the right equipment, in the right place, and at the right time, to monitor the changeover from a steady magma flow to one that was not steady but cyclical," says Dr. Barry Voight, professor of geosciences at Penn State and a senior scientist appointed by the British Geological Survey to work at the Montserrat Volcano Observatory. "No one before had documented these cyclic events nearly so well, or had monitored the additional background data necessary to understand the mechanisms behind them."
In today's (Feb. 19) issue of the journal Science, the researchers note that their analysis of the Soufriere Hills Volcano, Montserrat, British West Indies, is applicable to other andesite volcanos, the predominant type of explosive volcano worldwide. The researchers monitored the seismic and deformation behavior of the mountain in real time, allowing both an improved understanding of the volcanic system and enabling prediction of when eruptions might occur and what areas they were likely to affect.
There were two types of dangerous eruptions at Montserrat. For the scorching-hot, block-and-ash hurricane-type eruption, caused by collapse of a growing lava mound over the volcano vent, the team could identify the time when their occurrence was probable, within a few hours, says Voight.
"We could also identify the directions they would travel. However, we could not reliably say for a given cycle if, in fact, a major collapse, with an exceptionally-long-running ash hurricane, would occur," he notes.
"But we could predict say 11 hours before hand, that if a collapse-generated ash hurricane were to occur, it would occur at a certain time and would probably move in a general direction."
For the second type of eruption that involved large vertical explosions with nine-mile high eruption columns and ash hurricanes simultaneously in a number of river valleys, it became possible to forecast with some confidence each impending explosion. In general, this forecasting ability aided civil officials to define zoning and to carry out evacuation, and many lives were saved, says Voight of Penn State.
"The scientists could make predictions because the volcano had switched over to repetitive cyclic activity," says Voight.
The magma inside the volcano contained water that was boiling off and trying to escape as the hot mass rose. When some of the water left the magma melt, the melt began to crystallize. Partially crystallized magma is much more viscous than uncrystallized magma. As a result, the thick, sticky magma plugged the upper part of the volcano's conduit. Then, pressure in magma underneath the plug built up, causing ground swelling and earthquakes, and eventually pushed the magma plug out of the way. Magma was then rapidly ejected and this commonly caused collapse of the surface lava mound and ash hurricanes.
Researchers monitored the cycles of sticking and slipping, using state-of-the-art monitoring equipment and software provided by the U.S. Geologic Survey, the BGS and others, that allowed real-time data collection and analysis. The scientists could analyze events within minutes of their occurrence. Tilt meters high on the volcano indicated how much and where the pressure was building.
As the lava dome grew, some large landslide collapses thinned areas of the dome, quickly reduced the external pressure inside and under the dome and made them more likely to be the site of vertical or horizontal explosions of hot ash and gases, says Voight. From these deep uncorked pockets, magma with fine bubbles of pressurized gas would explode outward causing hot ash hurricanes. These pyroclastic flows moved down toward the sea at speeds as fast as 70 m.p.h. The capital town of Plymouth was destroyed by these flows.
"It was a sad moment to watch Plymouth burn," says Voight.
The pyroclastic flows happened right after the pressure peak in the stick slip cycle. Knowing the cyclic timing of the magma, the researchers could identify when eruptions might occur. Because the researchers also knew where the lava-dome deformations and slope failures were occurring, they could define the flow direction.
The explosive eruptions in August 1997 happened because the magma corking the conduit became thin from a previous dome collapse and the underlying high pressure buildup popped the cork. The resulting explosions rose vertically as much as nine miles. Ballistic blocks a yard wide were shot out over a mile and ash hurricanes flowed in all sectors around the volcano, running to the sea.
"Because we were able to predict these eruptions, we were able to put teams in the field to document the explosive events by video, still photography and surveying. We are learning a lot from this data which is still being worked on," says Voight. He noted that this explosion destroyed his tilt meters.
At least for now, the Soufriere Hills Volcano is relatively quiet. The last very large eruption occurred on Dec. 26, 1997, in where the south side of the whole volcano collapsed, and an explosive blast completely destroyed the two towns on that side of the island. The communities had been evacuated and no lives were lost. It was extremely fortunate that the evacuation was maintained, because no one could have survived the blast, says Voight.
Magma is no longer rising to build the dome, but the volcano remains dangerous. The lava is still very hot -- 1300 degrees F -- and its surface is unstable. Occasional gravity collapses still cause ash hurricanes that can run to the sea. However, it appears that this activity is winding down.
The researchers have found clear links among the seismic and deformation data from the volcano, the volcano's behavior and the way that gas and ash eruptions occur.
"Understanding these links advances our ability to interpret our data in terms of the physical processes and helps us to forecast the timing and, to a usable extent, the eruptive style of the volcano," says Voight. "These results, which can be of use elsewhere, improve our ability to mitigate the very dangerous effects of explosive volcanism."
The research team consisted of scientists from British Universities, the British Geological Survey, the Seismic Research Unit of the University of the West Indies, U.S.G.S. and Penn State. The members were Voight, R.S.J. Sparks, A.D. Miller, R.C.Stewart, R.P. Hoblitt, A. Clarke, J. Ewart, W. Aspinall, B. Baptie, T. H. Druitt, R. Herd, P. Jackson, A.M. Lejeune, A.B. Lockhart, S.C. Loughlin, R. Luckett, L. Lynch, G.E. Norton, R. Robertson, I.M. Watson and S.R. Young, all working through the Montserrat Volcano Observatory, Montserrat, British West Indies.
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