Sneaky Mount Spurr

"Boy, Alaska is sure a tough place," my friend wisecracked into our long-distance phone conversation. "First you beat up on Exxon, now you're going after robots." For sheer brute force, any of our active volcanoes outpowers the average megacorporation, so her comparison was hardly fair. Spurr just threw rocks at Dante until the hapless robot fell over, that's all. Remember that old line--It's not nice to fool Mother Nature?

Not that Mt. Spurr was either fooled or even close to giving up all its secrets, despite Dante's slow but steady work. And Alaskans--those in Anchorage and on the Kenai especially--have good reason to wish the volcano wasn't so secretive. Just two years ago, on August 18, 1992, Spurr erupted so much gritty ash into the air over Southcentral Alaska that automatic streetlights turned on hours before sunset--and that was only one of its 1992 eruptions. An ash-spewing volcano upsets all manner of human activities, from flying to breathing; a restless volcano does not make a restful neighbor.

One of Spurr's secrets may have been deciphered by scientists of the U.S. Geological Survey. Geochemist Terry Gerlach and volcanologist Michael Doukas have written about it in a new USGS bulletin, "The 1992 Eruptions of Crater Peak Vent, Spurr Volcano, Alaska."

Often, before a volcano erupts, it signals the forthcoming event by emitting quantities of sulfur-dioxide gas. This gas is among those held in suspension within the molten magma under the volcano. As the magma rises closer to the earth's surface, it is under less pressure from the weight of the overlying rock. The reduced pressure lets the gasses escape, just as carbon dioxide fizzes out of a bottle of seltzer once the top is opened.

Changes in sulfur-dioxide emissions have been a useful gauge for volcanologists to check the status of an active volcano. Mount Pinatubo in the Philippines, for example, gushed out thousands of tons of sulfur dioxide to warn of its impending eruption.

Spurr doesn't play by those rules. It erupted three times in 1992, but its sulfur-dioxide emissions before those eruptions were too weak to be detected with the volcanologists' measuring devices. This sneaky trick might seem especially unfair to the scientists, because the magma powering Spurr's activity is well endowed with the gas. During the course of the eruptions, the volcano released between 200,000 and 400,000 metric tons of sulfur-dioxide. This gas flow shut down again soon after the eruptive activity ceased.

Gerlach and Doukas think the key to this gas-emission pattern is water. Spurr's crater is a wet place. Normally the crater holds a deep lake, and a glacier wraps around Crater Peak. During the 1992 activity, glacial meltwater poured from the crater walls. The USGS researchers think this water percolated down through the volcano's porous and cracked rock lining, trapping and combining with the sulfur dioxide to form hydrogen-sulfide gas and sulfuric acid. This, they suspect, is masking the actual amount of sulfur dioxide Spurr emitted before erupting.

If they're right, they've also found a possible way to predict Spurr's increasing activity by measuring its output of another gas. Carbon dioxide is also emitted from magma, and carbon dioxide is not so easily trapped by water. The magma contains only about a tenth as much carbon dioxide as it does sulfur dioxide, but that can still be a huge quantity; just after the 16-17 September eruption in 1992, the scientists measured a daily outflow of 11,000 metric tons of carbon dioxide from Crater Peak.

Though Gerlach and Doukas think they have mustered good evidence for their views, they are hunting for more proof. The powers that be in the USGS have coughed up funds for more sensitive carbon-dioxide monitors to keep an eye on Cook Inlet's volcanoes. And just possibly, the dauntless but no longer dentless Dante found something to help them out.