Portal:Disasters/Selected anniversary/May 2008

The 1980 eruption of Mount St. Helens was a major volcanic eruption. The eruption was the most significant to occur in the contiguous 48 U.S. states in recorded history (VEI = 5, 0.3 cu mi, 1.2 km3 of material erupted), in terms of power and volume of material released, since the 1915 eruption of California's Lassen Peak. The eruption was preceded by a two-month series of earthquakes and steam-venting episodes, caused by an injection of magma at shallow depth below the mountain that created a huge bulge and a fracture system on Mount St. Helens' north slope. An earthquake at 8:32 a.m. on May 18, 1980, caused the entire weakened north face to slide away, suddenly exposing the partly molten, gas- and steam-rich rock in the volcano to lower pressure. The rock responded by exploding into a very hot mix of pulverized lava and older rock that sped toward Spirit Lake so fast that it quickly passed the avalanching north face.

A volcanic ash column rose high into the atmosphere and deposited ash in 11 U.S. states. At the same time, snow, ice, and several entire glaciers on the mountain melted, forming a series of large lahars (volcanic mudslides) that reached as far as the Columbia River, nearly fifty miles (eighty kilometers) to the south. Less severe outbursts continued into the next day only to be followed by other large but not as destructive eruptions later in 1980. By the time the ash settled, 57 people (including innkeeper Harry Truman and geologist David A. Johnston. Hundreds of square miles were reduced to wasteland, over a billion U.S. dollars in damage had occurred, and the face of Mount St. Helens was scarred with a huge crater on its north side. At the time of the eruption, the summit of the volcano was owned by the Burlington Northern Railroad, but afterward the land passed to the United States Forest Service. The area was later preserved, as it was, in the Mount St. Helens National Volcanic Monument.