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May 11, 2000

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

20th anniversary of the catastrophic eruption

May 18th marks the 20th anniversary of the catastrophic eruption of Mount St. Helens that laid waste to over 540 square km (200 sq mi) of forest, killing 57 people and countless wildlife. Hundreds of kilometers away in eastern Washington, as much as 5 cm (2 in) of ash fell, closing the interstate highway from Seattle to Spokane for a week and paralyzing air traffic. Local rivers crested at 6 m (20 ft) above their normal height as mudflows triggered by melted glacial ice ripped up bridges, roads, and houses.

Many of us at the Hawaiian Volcano Observatory have vivid memories of this eruption. When Mount St. Helens first began to show signs of life in 1980, the U.S. Geological Survey sent scientists to assess the activity. Most had already gained experience by monitoring eruptions at HVO. For others who had never dreamed of working on an active volcano, the eruption of Mount St. Helens was a dramatic turning point in our careers.

The first sign that magma was rising toward Mount St. Helens was an earthquake swarm beneath the volcano in mid-March 1980. A week later, a small steam explosion at the volcano's summit coated the downwind side of the snow-covered peak with ash. This was the first eruption in the conterminous U.S. since the 1914-1917 eruption of Mount Lassen, and it drew flocks of scientists, news media, and sightseers.

Using techniques that were developed to measure ground deformation on Kilauea, scientists discovered that the north flank of the cone was rapidly bulging outward as magma forced its way into the volcano. The infamous bulge grew about 25 m (85 ft) in 20 days, and the USGS warned that a major eruption was likely.

Area residents, however, had grown complacent about the harmless ash plumes and clamored to be allowed back into the restricted zone. Public officials finally relented and, on Saturday, May 17, allowed people to visit their cabins on Spirit Lake. Those people will be forever grateful that they were ordered to leave the area by nightfall.

At 8:32 the next morning, an earthquake triggered a landslide of the unstable bulge, and within seconds the entire north flank of the mountain was in motion. As the mountainside collapsed, it uncapped the shallow magma body within the cone. The sudden release of pressure on the gas-rich magma caused it to explode violently, unleashing a lateral blast of hot gas and ash. The blast swept northward at nearly the speed of sound, flattening forests like jackstraws as far as 30 km (19 mi) from the volcano.

Meanwhile, the collapse of the north flank had coalesced into the largest debris avalanche in recorded history, surging 23 km (13 mi) down the North Fork of the Toutle River at speeds of 300 km per hour (180 mph) and burying the valley under 45 m (150 ft) of a hummocky mixture of rock, mud, and glacial ice. Later in the day, pyroclastic flows of hot gas, ash, and pumice boiled from the newly formed crater and fanned out over the debris avalanche at the base of the volcano.

In the first six months after the cataclysmic eruption of May 18, a series of much smaller explosive eruptions produced small pyroclastic flows. A lava dome formed in the crater as short sticky lava flows piled up over the vent. Sixteen dome-building eruptions over the next 6 years added to the dome until it was 40 times the size of the King Dome in Seattle. The volcano has been quiet since 1986, except for a few small steam explosions that deposited thin layers of ash in the crater.

To hear more about Mt. St. Helens, come to "After Dark in the Park" at the Kilauea Visitor Center at 7:00 p.m. on Tuesday, May 16. Two geologists from the Hawaiian Volcano Observatory will present a slide show, "Mount St. Helens: Personal Remembrances of a Mighty Eruption."

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through a network of tubes toward the coast. Lava is visible at times on Pulama pali, and surface flows are active in the coastal flats near the Royal Gardens subdivision private access road. Lava is entering the ocean near the site of Waha`ula, in Hawaii Volcanoes National Park.

Readers are reminded that the ocean-entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The steam clouds are highly acidic and laced with glass particles.

There were no felt earthquakes this week on the island.

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Updated: 11 May 2000