While volcanologists can see the dome of the Soufriere Hills Volcano onthe island of Montserrat grow and collapse, it takes instrumentation todelve beneath the surface. Now, Penn State geologists, using tiltmetermeasurements, have investigated a shallow area under the dome and whatthey found was not quite what they expected.
"The Soufriere Hills Volcano has been building a lava dome,collapsing and rebuilding a dome since 1995, when it first erupted,"says Dr. Christina Widiwijayanti, postdoctoral researcher ingeosciences, working with Dr. Barry Voight, professor of geosciences."We are working with data collected from tiltmeters in 1997 to try tounderstand the volcano's behavior and what is happening inside."
Voight had placed several tiltmeters around the crater rim of thevolcano in 1996-97, but no more than two were ever working at once andduring the important June 25, 1997 dome collapse, only one wasoperational. However, from a record the previous month, two tiltmetersrecorded the cycle of pressurization and depressurization that tookplace under the dome on a 3 to 30-hour cycle.
A tiltmeter, like a carpenter's level, measures the local angularmovement of the Earth. With synchronized data from two tiltmeters, theresearchers, who included Dr. Amanda Clarke a former Penn Stategraduate student who is now an assistant professor at Arizona StateUniversity, and Dr. Derek Elsworth, professor of energy andgeo-environmental engineering, could determine the depth of the sourceregion causing the tilting near the dome. They reported their work in arecent issue of Geophysical Research Letters.
"But, what we really would like to know is the configuration of thepressurized area, its shape and size, as well as position," saysWidiwijayanti. "We know the size and shape of the conduit system thatdelivers the lava, but our results suggest that a more extensive regionis involved in the pressurization."
The researchers found the pressure to be centered about a half milebelow the dome or nearly 2.5 miles above the magma chamber feeding thesurface flow of lava. The magma tube or conduit in this area is about100 feet in diameter, but, using tiltmeter data collected during thecollapse, the researchers found that the region undergoingpressurization and depressurization is between about 700 and 1100 feetin diameter. The researchers used a sphere and a cylinder to model thepressurized area. The known size of the dome collapse could be used tocalibrate the source pressure.
"When the dome collapses, the area should be rebounding, going up, butthe tiltmeter shows that it goes down" said Widiwijayanti. "There mustbe something related to depressurizing the system in the volcano thatdoes this."
The researchers believe that the region around the conduit isfractured, with the pore spaces filled by hot water and gas. "When thevolcano conduit at depth is under pressure, super-heated steam andother gases can leak out of the conduit and raise the pressure in thefractured rock over a broad region. That is what we think we are seeingas the pressurized zone," says Voight.
The 1997 dome collapse, with 8.5 million cubic yards of lava and talus,was not the largest at the Soufriere Hills Volcano, although 19 peoplewere killed by it and the event rewrote the political history ofMontserrat. In July 2003 the dome collapse produced 275 million cubicyards, the largest on Earth in historic time.
The 2003 collapse was recorded using new and more varied equipmentinstalled by the CALIPSO project (Caribbean Andesite Lava IslandPrecision Seismo-geodetic Observatory), funded by the National ScienceFoundation and the U.K. Environment Research Council. Voight is projectdirector of the consortium, which involves a number of institutions inthe U.S. and U.K. While researchers recorded the 1997 data before theinitiation of CALIPSO, the analysis of both data sets is part of theproject.
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