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Tiny crystals hold the key to Augustine Volcano's dramatic 2006 eruption

Samples of extremely small crystal clots, each polished to the thickness of a human hair or thinner, have revealed information about the process triggering the major 2006 eruption of Alaska's Augustine Volcano.

Graduate student researcher Valerie Wasser at the University of Alaska Fairbanks Geophysical Institute determined that the addition of hot new magma into Augustine's reservoir of cooler, older magma increased the pressure enough to trigger the 2006 eruption.

Wasser's analysis of Augustine crystal clots was published May 29 in .

Crystal clots are two or more adjoined crystals formed in magma. New magma can partially melt crystals, leaving what are called dissolution surfaces. A cycle of melt followed by crystal growth can leave a series of these boundary lines.

"When crystals grow, they form something like tree rings," Wasser said. "Since pressure and temperature changed, these new 'tree rings' on the crystals will have a different composition than the previous rings."

Wasser concluded that the influx of new magma increased pressure in the chamber by 7.7 megapascals, an increase equivalent to about 125 times that of an average home pressure cooker. She reached that conclusion by discovering an approximately 3.9% increase of calcium in plagioclase, a mineral often used by volcanologists to interpret magma composition, cooling history and water content.

That amount of pressure can fracture rocks and provide pathways for magma to flow upward.

"It's difficult with Augustine to say exactly what the tensile strength of rocks would have been, but it's generally understood to be between one and 10 megapascals," Wasser said. "We're at the upper end of this range, so it is very likely that this pressure increase was large enough to break rocks."

And that, the research paper suggests, was likely to have caused the upward flow of magma and possibly been enough to break the surface as an eruption.

Wasser's method can be used to produce post-eruption estimates of pre-eruption pressure changes at other arc volcanoes, those that form where one tectonic plate sinks beneath another. The method complements geophysical modeling and increases the understanding of magmatic processes.

In January 2006, a series of explosive eruptions from Augustine sent ash plumes as high as 45,000 feet, disrupted air traffic and deposited ash 70 miles away in Homer.

It was the most significant eruption at Augustine, about 176 miles southwest of Anchorage on Augustine Island in Cook Inlet, since 1986. It hasn't erupted since 2006.

Wasser analyzed 10 crystal clots from an Augustine bread-crust bomb, a volcanic rock formed when a blob of viscous lava is ejected into the atmosphere. Its outer surface cools and solidifies, while expanding gases of the still-hot interior crack the crust, creating a surface texture resembling a loaf of bread.

Each clot consisted of plagioclase and orthopyroxene crystals. Plagioclase crystals are more susceptible to change, so they provided better evidence of the pressure increase.

Wasser's bread-crust bomb includes plagioclase crystals with a variety of growth patterns, indicating they came from various locations in the magma chamber.

"In geophysical methods, such as geodesy, scientists look at how Earth's surface moves and calculate back what that means for the magma pressurizing in the subsurface," Wasser said. "Here, instead of looking at the effects that the pressure has on the surface, we're looking directly at what the increasing pressure meant to the crystals in the magma itself."

Co-authors include research associate professor Pavel Izbekov; research associate professor Taryn Lopez; and former graduate student researcher Jamshid A. Moshrefzadeh, now with the Alaska Division of Geological and Geophysical Surveys.

All are with the Alaska Volcano Observatory, a joint program of the UAF Geophysical Institute, the Alaska Division of Geological and Geophysical Surveys and the U.S. Geological Survey.

Nathan Graham, manager of the Geophysical Institute's Advanced Instrumentation Laboratory, is also a co-author.