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May 13, 1999

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Some volcanoes can wreak havoc without erupting

May 18 marks the anniversary of the 1980 eruption of Mount St. Helens, which laid waste to over 520 square kilometers (200 square miles) of forest and killed 57 people. This was the first eruption in the conterminous United States since the 1914-1917 eruption of Mount Lassen, and it immediately focused attention on the potential threat posed by other volcanoes in the Cascade Range. The U.S. Geological Survey began taking the pulse of these hibernating giants on a regular basis by monitoring seismicity, ground deformation and gas emissions. You might be surprised to learn, however, that the volcano they are most concerned about doesn't even have to erupt to produce a catastrophe that could dwarf that of May 18, 1980.

The snow-clad cone of Mount Rainier looms nearly 4,392 m (14,410 ft) above Tacoma, Washington, and the lowlands of Puget Sound. The highest volcano in the Cascades, Rainier has a long history of sending lahars--mixtures of water, rock, and sand-sweeping down the river valleys leading away from the cone. These deadly flows, which contain so much rock that they look like fast-moving rivers of concrete, can rip up boulders, trees, houses, and bridges and carry them far downstream. Father from the volcano, as a lahar loses speed and dumps its load of debris, it entombs everything in its path in mud. Valley deposits around Mount Rainier record a major lahar at least every 500-1,000 years. Some of these lahar deposits have been dated by the ancient forests that were overrun and buried in place.

While we expect to see precursory activity before an eruption, a lahar can occur with no warning. The largest lahars are triggered by massive collapse of the steep flank of a volcano. The slopes of a stratocone, such as Mount Rainier, are inherently unstable, because hot, acidic water percolates through the volcano and slowly alters the layers of lava and ash into a soft, clay-rich rock. This process has been likened to the progressive weakening of a house infested by termites. A catastrophic flank collapse may be set in motion by a major earthquake, by an eruption or intrusion of magma, by heavy rainfall that saturates the slope, or for no apparent reason except the pull of gravity. Lahars also form during explosive eruptions when ash flows melt snow and ice-and Mount Rainier has plenty of both.

More than 100,000 people reside on old lahar deposits in the river valleys leading from Mount Rainier, making the risk to life and property substantial in the event of a large lahar, especially one formed by flank collapse. The U.S. Geological Survey, together with local agencies, has installed a lahar-detection system in two river valleys as part of a pilot project. The system uses acoustic flow monitors that detect ground vibrations created by an oncoming lahar and send out an automatic alert. Once the alert is received by county emergency officials, the time for evacuation may be as little as 30 minutes. Lahars can travel at speeds up to 80 kph (50 mph), quickly reaching populated areas at the foot of the volcano. Public education and advance planning are essential to get people out of the path of an oncoming lahar in such a short time.

To learn more about the "perilous beauty" of Mount Rainier, come to Hawai`i Volcanoes National Park next Tuesday evening (May 20) for a slide show at 7:00 p.m. by Steve Brantley of the Hawaiian Volcano Observatory. Steve's talk is part of the "After Dark in the Park" lecture series held at the Kilauea Visitor Center auditorium. If you can't make the talk, check out the USGS Volcano Hazards Program website at http://volcanoes.usgs.gov.

Eruption Update

Eruptive activity from the east rift zone of Kilauea Volcano is back to normal. Lava is erupting at Pu`u `O`o and flowing through the old network of tubes to the coast at Kamokuna. The ocean entry restarted on May 7. The public is reminded that the ocean-entry area is extremely hazardous, with explosions accompanying unpredictable collapses of new land. The steam clouds are highly acidic and laced with glass particles.

There were three earthquakes reported felt for the week ending on May 13. Early Saturday morning at 4:59 a.m. on May 8, a resident of Hilo felt an earthquake that was located 18 km (10.8 mi) southeast of the summit of Kilauea. The temblor originated from a depth of 6.5 km (3.9 mi) and had a magnitude of 3.4. Residents of Ka`u reported feeling two earthquakes (magnitudes 3.2 and 3.8) on Monday morning, May 10. The 09:28 a.m. and 11:15 a.m. earthquakes were aftershocks of the magnitude-5.6 Wood Valley earthquake of April 16. Both aftershocks originated from a depth of 10 km (6 mi).

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Updated: 20 May 1999