Volcano Watch — Huge eruption of Mount St. Helens recalled

May 18 marks the anniversary of the 1980 eruption of Mount St. Helens that laid waste to over 200 square miles of forest and killed 57 people, as well as thousands of wild animals and birds.

The first warning of the impending catastrophe was an earthquake swarm beneath the volcano in late 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 people, and sightseers.

U.S. Geological Survey scientists were among the first to arrive, and they immediately began to monitor the new activity and to try to predict what would happen next. Most of these geologists and geophysicists were alumni of the Hawaiian Volcano Observatory who had worked for 2-5 years in Hawai`i before returning to permanent posts on the mainland.

Using techniques that were developed to measure ground deformation on Kīlauea, scientists discovered that the north flank of the cone was rapidly bulging as magma forced its way into the volcano. The infamous bulge grew about 85 feet in 20 days, and the USGS warned that a major eruption was likely. However, area residents had grown complacent about the harmless ash plumes and clamored to be allowed back into the restricted zone. Public officials finally gave in and allowed people to enter the restricted area on Saturday, May 17, to visit their cabins on Spirit Lake. Those people will be forever grateful that they were required 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 mountain side 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, creating a lateral blast of hot gas and ash. The blast swept northward at close to the speed of sound, flattening forests as far as 19 miles from the volcano.

Meanwhile, the collapse of the north flank had coalesced into the largest debris avalanche in recorded history, surging 13 miles down the North Fork of the Toutle River at speeds of 180 miles an hour, burying the valley 150 feet deep in 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. Mudflows began in midmorning and continued the next night as local rivers crested at 20 feet above their normal height. As much as two inches of ash fell in towns in eastern Washington, paralyzing ground and air transportation. The interstate highway from Seattle to Spokane was closed for a week, and ash removal from airport runways went on for several months.

In the first half year after the cataclysmic eruption of May 18, the volcano produced a series of explosive eruptions accompanied by small pyroclastic flows. The last of these occurred in October 1980. A composite lavadome began to form in the crater during the October eruption, and a series of 16 dome-building eruptions followed through October 1986. Each of these eruptions added a new lobe onto the dome, which is now nearly 3,500 feet in diameter by 1,150 feet high. Since 1986, the volcano has been quiet, except for a few small steam explosions that deposited thin layers of ash in the crater.

Since 1980, geologists have taken the potential threat of other volcanoes in the Cascade Range very seriously. The USGS has initiated monitoring programs that will enable us to detect the first signs of reawakening at these volcanoes, which include Mount Baker and Mount Rainier in Washington, Mount Hood and Crater Lake in Oregon, and Mount Shasta and Mount Lassen in California. The staff at the Hawaiian Volcano Observatory has participated in these studies, as well as in monitoring work at Alaskan volcanoes.