The Spectacular Eruption of Mount St. Helens

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 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

flank of the mountain bulged, and the summit rose by 500 feet.