Feb. 7, 2002
Vol. 21 No. 9

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    American Geophysical Union honors Anderson with Bowen Award for magma research

    By Steve Koppes
    News Office

    Chicago volcanologist Alfred Anderson has received the N.L. Bowen Award from the American Geophysical Union for developing a method for reconstructing the subsurface evolution of magma from volcanic glass.

    “When a big eruption goes, it delivers to the surface pieces of magma that were stored in the ground beforehand at various depths and lateral positions as well,” said Anderson, Professor in Geophysical Sciences and the College. “The big challenge is to reconstruct the pre-eruptive body of magma that was in the ground.”

    Anderson would like to better understand the inner workings of a volcano, whose roots can extend many miles below the surface. He asks basic questions about the content of water, carbon dioxide and other volatiles in the system. For example, is the water that is dissolved in magma intrinsic to the magma? Is it derived from the crust as the magma makes its way toward the surface? Or does the water actually come from deep below the crust?

    As a practical matter, he is forced to take a top-down approach to such problems. Fortunately, volcanoes preserve a record of their development in small pieces of glass called melt inclusions, which are encased in volcanic crystals. Anderson analyzes the water and gas trapped in these melt inclusions, which measure just a small fraction of an inch in diameter, for clues regarding the pre-eruptive behavior of magma.

    “Nature has given us few precious tools with which to look backward in time in the recon-struction of geological processes,” said Charles Bacon of the U.S. Geological Survey in Menlo Park, Calif., in Anderson’s award citation. “A special genius is often required to recognize one of those tools. In the case of melt inclusions as indicators of pre-eruption volatile contents of magmas, we have Fred Anderson to thank.”

    Anderson’s use of a technique called Fourier Transform Infrared Spectroscopy sparked a revolution in melt inclusion studies of volcanic rocks, according to Bacon. “The analytical method perfected by Fred and associates is the industry standard today,” Bacon said in the award citation.

    Anderson also has shown his colleagues how to coax the maximum amount of data out of their samples. “He has led the field of melt inclusion research because he has noticed things, measured them and extracted information about process others would have missed,” Bacon said.

    One of Anderson’s findings runs against a popularly held view about the subsurface behavior of volcanoes. Volcanologists have assumed that the volatile constituents of magma are dissolved before eruption. But Anderson’s measurements indicate that volatiles such as water and carbon dioxide exsolve and form bubbles far sooner in the eruption process than anyone would have guessed.

    Normally, one would expect the gas bubbles to rise through the magma and escape out the top. “Gas has enormous escape tendency. It’s really impressive,” Anderson said. So how did the gas get stuck in the magma?

    “I don’t have an answer for that, but the retort I give is, well, what about the crystals?” One would expect the heavier crystals to sink out of the magma, “yet nobody questions the reasonable presence of crystals in the magma when it’s erupting,” Anderson said.

    Anderson has studied volcanoes from Hawaii to Iceland. Soon he will begin a project in New Zealand, but his most recent work has focused on a large volcanic rock formation called the Bishop tuff in California’s Long Valley, home of the Mammoth Mountain ski resort.

    “There are big puzzles still that we haven’t solved about how the various parts were related to each other prior to the eruption,” he said. “They’re just out there on the ground, and it’s up to us to put them back together again.”