Decompression-driven crystallization warms pathway for volcanic eruptions

The reason may be counter-intuitive, but the more magma crystallizes, the hotter it gets and the more likely a volcano will erupt, according to a team of scientists that includes a University of Oregon geologist. The knowledge likely will aid monitoring of conditions at Mount St. Helens and other volcanic hot spots around the world.

Reporting in the Sept. 7 issue of the journal Nature, the researchers show that rapid crystallization of magma within one to two kilometers of the surface (about one-half to one mile) causes magma to heat up to as much as 100 degrees Celsius (212 degrees Fahrenheit).

“While this sort of heating has been expected in theory, we are the first to show that we can measure it,” said Katharine Cashman, a professor of geologic sciences at the University of Oregon. “These results have important consequences for models of magma ascent beneath volcanoes, as increasing the melt temperatures causes the melt viscosity to decrease so that it can flow more easily, like heating up a jar of honey to allow the honey to flow out of the jar.”

Explosive volcanic eruptions are fueled by the escape of volcanic gases from magma stored in underground reservoirs and pipes several kilometers below the surface. Predicting such eruptions requires a real-time knowledge of just where the magma is at any one time and what it is doing.

“This work is now being used to gauge the direction of the volcanic activity currently happening at Mount St. Helens and could be applied to any active volcano for which monitoring and petrological records are available,” said Jon Blundy, professor of earth sciences at the University of Bristol (United Kingdom), in a news release.

Cashman and Blundy have now collaborated since 1998, when Blundy took a sabbatical at the University of Oregon, on four published studies on Mount St. Helens, located 53 miles northeast of Portland, Ore. Cashman has studied the volcano and similar ones elsewhere for more than a decade.

The latest study was a follow-up to one Blundy and Cashman published in Geology last year (October 2005), in which they used small pockets of melt that get trapped in crystals as they expand to demonstrate that the crystals grow by decompression as magma rises toward the surface. That paper also showed that these crystals grow rapidly, in months rather than years. The new study refined their conclusions in Geology by using experimental calibrations to show the rapid heating as magma nears the surface.

“This may sound counter-intuitive, but think about the need to add heat to something to melt it,” Cashman said.

In this follow-up study to last year's report, the researchers were able to reconstruct changes in pressure, temperature and crystallization that occur in magma before an eruption. They showed that as pressure decreases, crystallinity increases; the more magma crystallizes, the hotter it gets.

The finding that a drop in pressure rather than a loss of heat to surrounding rocks, as previously thought, means that there are more possibilities for eruption dynamics, the researchers concluded.

If ascending magma is able to heat itself up simply by crystallizing, they report, it may provide an important trigger for eruption without the need to invoke an extraneous heat source such as a shot of hotter magma from deep below the surface. The new findings also suggest the possibility that volcanic crystals grow in response to decompression by heating on an unexpectedly short timescale of several years, a period during which volcanoes can be more effectively monitored.