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Japan's Unzen Volcano is Volcanic Mystery

Volcanoes often wake in one of two moods. Some explode, launching rocks the size of minivans and shooting ash miles into the atmosphere. Others stretch, belch a few gases, and ooze molten rock. A new project in Japan might help scientists predict which way a volcano will behave when it stirs from a long slumber.

Explosive eruptions grab more attention than effusive eruptions, during which lava flows from a vent without much of a pyrotechnics show. One of Alaska's most famous explosive eruptions was Mt. Katmai in 1912, which in two days spewed more than 30 cubic kilometers of pumice and grit into the air. If Katmai erupted with the same vigor tomorrow, the plume of ash would prevent planes from flying over the north Pacific, and the ash would muck up power plants, vehicles, and life in general across the southern half of the state.

Volcanoes that erupt in a non-explosive fashion don't affect people over as large an area, but they can be deadly. Unzen Volcano in southwest Japan, which erupted from 1991 to 1995, is a good example. While Katmai went off like a nuclear bomb, Unzen was more like a leaky water pipe. During its recent four-year eruption, Unzen oozed lava at the rate of 2 cubic meters-about the volume of an office desk-every second of every day. The lava flow created a mountain of unstable hot rocks. Thousands of avalanches tumbled down the slopes of Unzen Volcano during its four-year eruption, killing 44 people and burying 10 square kilometers of the downslope city of Shimabara.

Unzen Volcano also was the site of Japan's worst volcano disaster. In 1792, a dome of old lava on Unzen's flank collapsed following a small lava eruption at the summit. Much of the dome tumbled into the nearby ocean, causing a tsunami that killed 15,000 people at the volcano's base and on the opposite shore.

Scientists today have a unique opportunity to examine Unzen's innards, which are still hot from the recent eruptions. John Eichelberger, of the Alaska Volcano Observatory at the Geophysical Institute, is part of a team of international scientists who will drill a hole deep into Unzen Volcano. Researchers have drilled into volcanoes before-Eichelberger and colleagues tapped a cold volcano in California some years back-but no one has before reached the temperatures hoped for at Unzen. If things go well, the researchers' drill bit will penetrate more than one mile beneath Unzen, encountering temperatures of about 600 degrees C (1100 degrees F).

From its perch on Unzen's flank, the scientists' drill will intersect the conduit that carries lava to the surface. This conduit is one of the more mysterious features of volcanoes. For some reason, volcanoes use the same channel for repeated eruptions, even though the chambers harden when the internal molten rock from a previous eruption, called magma, cools. This channel may hold clues as to why a volcano blows, like Katmai in 1912, or flows, like Unzen in 1991-95.

In both kinds of eruptions, magma foams as it rises. Differences in the two types of eruptions may be that during a non-explosive eruption, gases within magma escape as the magma rises, by flowing from bubble to bubble in the foam and then through cracks around the conduit. In an explosive eruption, gases don't escape, and the magma foam continues to build pressure until the explosion.

Knowing more about a volcano's plumbing also may help scientists predict eruptions with more accuracy. The pre-eruption bulging of volcanoes and swarms of small earthquakes used to predict eruptions both seem to originate at the conduit.