Montserrat Volcano's Renewed Activity Aids In Unraveling Mechanism Of Dome Collapse

"Although it is not always the case, this volcano is continuing its eruption in the same cyclic, pulsing way as before," says Dr. Barry Voight, professor of geosciences.

The Soufriere Hills Volcano explosively erupted in September 1996 and on June 25, August 3, September 21 and December 26, 1997. In each case the eruption closely followed the collapse of the lava dome.

Lava domes are viscous, sticky masses of magma that pile up high above the vent. Such lava domes can fail in two ways. In some cases, domes can collapse simply because, with sufficient dome growth, the weight of the thick, steeply-sloping lava finally exceeds its interior strength, and the lava mass then breaks apart. Domes can also fail when gases accumulate under the dome, and gas pressures diffuse throughout the dome, weaken it, and promote failure.

The first type of failure commonly causes lava block avalanches, but the second type of failure can generate violent storms of hot ash and gas that can travel many miles and destroy everything in their paths.

"Studies of lava domes on active volcanoes made us realize that domes fail explosively a large percentage of the time," says Voight. "With the Soufriere Hills volcano again going through cycles of pressurization and dome forming, we again have an opportunity to monitor seismic activity and pressurization and refine our mathematical model of gas-pressurized dome failure."

Voight and Dr. Derek Elsworth, professor of geoenvironmental engineering, reported on their model in a recent issue of Geophysical Research Letters. The researchers created gas-diffusion models to calculate gas over pressures in lava domes. These modeled gas pressures were then used in stability analyses to show that gas diffusion causes deep-seated instability in the dome.

Pressure builds up near the vent of the Soufriere Hills volcano and other andesite volcanoes because as the magma rises and pressure reduces, dissolved water bubbles out of the melt and the magma becomes much more viscous. This viscous lava obstructs the path and gas pressures build up.

The diffusion models explain how the gases in the magma wend their way through tight cracks and connected pores to distribute pressure in the dome.

"The mechanism we are suggesting explains why some dome failures do not occur at the first pulse of activity," says Voight. "It takes time for the gas pressures to distribute through the dome, and this can cause an explosive release many hours or days after the first pulse of new lava is detected."

The researchers will continue to work with the Montserrat volcano observatory to interpret the behavior of the dome using seismicity and deformation meters.

"In 1997, about once a day the observatory monitored the concentration of sulfur dioxide gas coming from the volcano," says Voight. "We need to monitor this gas much more frequently, because the lava is being squeezed out in pulses that occur several times daily, and the gas escapes in quantities that are synchronous with the lava output. We need to link up the monitoring effort to this time scale, to capture the variation of gas released over any given day. This quantity is a direct measure of the gas pressure."

Sulfur dioxide monitoring can be done remotely, which is advantageous as the original tilt meters on the shoulder of the crater rim have been destroyed by eruption.

Another area that the researchers would like to explore is the behavior of the magma itself.

"We do not know as much as we need to know about the characteristic behavior of materials that are partly liquid and partly crystalline," says Voight. "These semi-liquids are crystalline enough so that they can actually crack and while they can flow, are mostly solids. The strength and creep properties of these materials are needed for stability assessments, but they are very poorly known."

Andesite magma, the type of lava found at Montserrat and many other volcanoes at convergent plate tectonic boundaries, can be 60 to 80 percent crystalline, while basalt magma, typical of Hawaii, is mostly liquid. Also, andesite magma within a half mile beneath the surface can be a thousand times more viscous than it was three miles down because of bubbling off of water dissolved in the melt crystallization as the magma rises.

The mechanism that Voight and Elsworth suggest for the Soufriere Hills volcano can be applied to lava domes on any andesite volcano that is subject to pressurized gas. Andesite volcanoes are the most common type found on Earth, and pose the greatest hazards to large populations around the Pacific Rim and Mediterranean areas. As to the future of Montserrat, the researchers are less certain.

"The volcano was formerly on a 30-year cycle of seismic crises, related to underground magma activity that never quite reached the surface," says Voight. "But now the volcano seems to have an open system and we cannot yet tell when it will stop or once it stops, when it is likely to start again."