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The SpectacularEruptionofMountSt. Helens
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A The eruption in May 1980 of Mount St. Helens, Washington State, astounded the world with
its violence. A gigantic explosion tore much of the volcano's summit to fragments; the energy
released was equal to that of 500 of the nuclear bombs that destroyed Hiroshima in 1945.
B The event occurred along the boundary of two of the moving plates that make up the Earth's
crust. They meet at the junction of the North American continent and the Pacific Ocean. One
edge of the continental North American plate over-rides the oceanic Juan de Fuca micro-plate,
producing the volcanic Cascade range that includes Mounts Baker, Rainier and Hood, and Lassen Peak as well as Mount St. Helens.
C Until Mount St. Helens began to stir, only Mount Baker and Lassen Peak had shown signs of
life during the 20th century. According to geological
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evidence found by the United States
Geological Survey, there had been two major eruptions of Mount St. Helens in the recent
(geologically speaking)past: around 1900 B.C., and about A.D. 1500. Since the arrival of
Europeans in the region, it had experienced a single period of spasmodic activity, between 1831
and 1857. Then, for more than a century, Mount St. Helens lay dormant.
D By 1979, the Geological Survey, alerted by signs of renewed activity, had been monitoring
the volcano for 18 months. It warned the local population against being deceived by the
mountain's outward calm, and forecast that an eruption would take place before the end of the
century. The inhabitants of the area did not have to wait that long. On March 27, 1980,a few
clouds of smoke formed above the summit , and slight tremors were felt. On the 28th, larger and
darker clouds,. consisting of gas and ashes,. emerged and climbed as high as 20,000 feet. In April
a slight lull ensued, but the volcanologists remained pessimistic. The, in early May, the northern
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flank of the mountain bulged, and the summit rose by 500 feet.
E Steps were taken to evacuate the population. Most- campers, hikers, timbercuttersleft the
slopes of the mountain. Eighty-four-year-old Harry Truman, a holiday lodge owner who had lived
there for more than 50 years, refused to be evacuated, in spite of official and public, including an
entire class of school children, wrote to him, begging him to leave. He never did.
F On May 18, at 8.32 in the morning, Mount St. Helens blew its top. literally. Suddenly, it was
1300 feet shorter than it had been before its growth had begun. Over half a cubic mile of rock had
disintegrated . At the same moment, an earthquake with an intensity of 5 on the Richter scale was
recorded. It triggered an avalanche of snow and ice. mixed with hot rock-the entire north face of
the mountain had fallen away. A wave of scorching volcanic gas and rock fragments shot
horizontally from the volcano's riven flank, at an inescapable 200 miles per hour. As the sliding
ice and snow melted, it touched off devastating
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