"This type of volcanic eruption has not been studied very much because it is unsteady," says Amanda B. Clarke, graduate student in geosciences. "Unlike eruptions like Pinatubo's in 1991 that have sustained, steady eruptions and have been widely studied, vulcanian eruptions are an unsteady phenomenon."
What is unusual about the Soufriere Hills volcano is that vulcanologists have been documenting its eruptions since 1995. Information on the internal structure and rock composition, the conduit size and the deformation and seismic patterns exists for most of the eruptions.
"We have combined field data with a numerical model to refine a model of vulcanian eruptions and create a simulation of this type of eruption," Clarke told attendees at the fall meeting of the American Geophysical Society today (Dec. 12). "Testing the models of short duration vulcanian explosions against the 1997 explosions at the Soufriere Hills volcano revealed that the leakage of water vapor through the surrounding rocks greatly affects the energy of the explosion and that the overhang in a fountain collapse was not the source of the pyroclastic flows."
The model, developed by Augusto Neri, University of Pisa, is an adaptation of original models developed decades ago by Los Alamos National Laboratory to analyze nuclear explosions. The model now simulates the explosion of a volcano.
Clarke, working with Neri and Dr. Barry Voight, Penn State professor of geosciences, was able to duplicate the observations. The output data matched the actual explosions of Soufriere Hills. In this way she validated the model, but also showed, by varying the amount of water vapor that leaked out the sides of the conduit, that this escape of water vapor is an important factor in the rate and force of explosions.
Pressure builds up near the vent of the Soufriere Hills volcano and other andesite volcanoes because as magma rises and pressure reduces, water dissolved in the magma bubbles out of the melt and crystals form. The magma becomes much more viscous. The viscous lava obstructs the path and gas pressure builds up. Eventually the pressure ruptures the cap and an explosion occurs.
"Volatile leakage is really key to the timing and force of the explosion," says Clarke. "In the model, the amount of leakage dictated the type of explosion that occurred."
Running the model with only one size particle in the flow did not work, according to Clarke. "Eventually we used three particle sizes, and the model produced results that match the real events," she says.
The particle sizes used included the mean particle size of 2 millimeters, the largest size the model could handle – 5 millimeters and a very fine particle. The particles forced out of the volcano are controlled by the force of the blast, rising heated air and gravity. Most of the particles initially rise with the explosion, but then curl over away from the cone and form an overhang. Eventually, gravity causes these particles to fall, either as a rain of particles or as part of the pyroclastic flows.
"The simulation clearly shows the formation of the overhang and then the fountain collapse," says Clarke. "However, it also shows that the pyroclastic flow that moves rapidly away from the volcano and that causes the most damage to people and property, comes from the center of the volcanic plume and not from the overhang."
Soufriere Hills continues as an active volcano, dome building and dome collapsing. At least for now, the dangerous vulcanian eruptions have stopped. Researchers, however, continue to monitor the volcano, collecting data to refine their models and improve understanding of these erratic eruptions.