SAN FRANCISCO, CALIFORNIA--A bulge of new magma continues to grow into a bizarre, hazardous shape within the recently awakened crater of Mount St. Helens. The steaming lava dome--engorging at the rate of a truckload per day--already has swollen nearly 300 meters above the crater floor in just 2 months. Volcanologists worry that the angular pile will collapse, unleashing a major ash cloud and a hazardous flow of hot rock and mud into surrounding valleys.
Mount St. Helens is the most active volcano in the Pacific Northwest. When it exploded in 1980, it killed 57 people. For the next 6 years, smaller explosive eruptions built a rounded, rocky dome of hardened lava within the new crater. After 18 years of relative calm, the mountain rumbled to life again this year with swarms of earthquakes and a new mound of rock that poked into view in early October.
The dome already has grown taller than its older neighbor, and its impressive growth rate of 5 cubic meters per second shows no sign of subsiding, researchers reported here 15 December at a meeting of the American Geophysical Union. But the dome's odd shape has startled scientists even more. It's as elongated and sharp as "the overturned keel of a large ship," says volcanologist Dan Dzurisin of the U.S. Geological Survey (USGS) Cascades Volcano Observatory in Vancouver, Washington. "It's something extraordinary, and no one anticipated it."
The thick and pasty magma allows parts of the pile to remain almost vertical, says USGS volcanologist John Pallister. During hasty 5-minute helicopter landings inside the crater to collect freshly crystallized rock, Pallister has noted molten rock glowing at temperatures as high as 775°C just 10 meters beneath the dome's jagged skin. The fragile edifice could tumble if it gets much taller, says USGS volcanologist Cynthia Gardner. The exposed molten rock could then explode as it releases pressurized gas. "Our primary concern is an ash cloud that could rise tens of thousands of feet above the crater," Gardner says, threatening nearby aircraft. Another risk is that the lava would melt ice and snow, triggering a flow of liquefied ash. A large dam in the valley below probably will protect nearby towns, Gardner notes.
A dome collapse doesn't appear imminent, but scientists are watching for moderate earthquakes that could disrupt the pile, says seismologist Steve Malone of the University of Washington in Seattle. Tiny earthquakes of less than magnitude 1.5 now jiggle the crater once every minute. That's not nearly strong enough to cause rockslides, Malone says.