Friday, April 29, 2005
Small magma chamber might create trouble
“We can look at changes on the surface and infer what is happening at depth, but we don’t know the physical dimensions,” said Glen Mattioli, professor of geosciences at the University of Arkansas.
Mattioli and colleagues from Penn State University, Duke University, Cornell University, the Carnegie Institution in Washington, D.C., and from England, have extensively studied the Soufriere Hills Volcano on the island of Montserrat in the Caribbean, which has been erupting sporadically since 1995. To date, they have focused their attention on the processes occurring beneath the earth’s surface both before and during eruption episodes – building knowledge that may help build better forecast models for areas with active volcanoes.
Based on years of data, the researchers believe they have detected a magma chamber that lies beneath the volcano. Determining the size and contents of the magma chamber would go a long way toward helping researchers determine the future behavior of the volcano. A small magma chamber, for instance, may produce shorter or fewer eruptions than a large magma chamber.
The researchers have obtained funding from the National Science Foundation (NSF) and the Natural Environmental Research Council to coordinate sensors already in place on the island with sensors used at sea to create an image of the magma chamber, estimated to be more than five kilometers beneath the surface. The project builds on the NSF-supported CALIPSO (Caribbean Andesitic Lava Island Precision Seismo-geodetic Observatory) project, which funded the installation of four high-sensitivity borehole seismic stations around the Soufriere Hills Volcano to measure ground deformation with strainmeters, tiltmeters, and cGPS. The stations enable researchers to take detailed measurements of volcanic activity.
The new experiment, dubbed SEA-CALIPSO (for Seismic Experiment with Air-gun source) will use air guns and a string of sensors off the back of a research ship combined with sensors on land to try to image the magma chamber. The air guns will create seismic waves that will reverberate through the earth. As the seismic waves propagate, a certain percentage of the energy created is reflected and can be measured by the sensors. The direction of that energy is related to the properties of the material the waves pass through, and based on the direction, scientists can determine if the material is gas, liquid or a combination of liquids and solids. The researchers also can use the speed of the waves to determine information about temperatures, which can help pinpoint whether or not the material in the magma chamber is liquid, solid, gas or a combination of the three. The researchers map the speed and direction of the various waves to get an image of the shape and contents of any underground chambers.
The experiment will require a large number of sensors in a small amount of space. “We’re looking for something that’s pretty small,” Mattioli said. Estimates of the size of the magma chamber are between one kilometer to tens of kilometers in diameter.
The prolonged eruption on Montserrat, which began in 1995, drove away more than half of the island's 11,000 inhabitants, killed the tourism industry and buried the airport in a pyroclastic flow. The island has become a living observatory for researchers who want to learn more about active volcanoes. They work closely with researchers at the Montserrat Volcano Observatory to help improve their ability to assess hazards and forecast events for the benefit of public safety.