Mount St. Helens Eruption

The subject of volcanoes is a hot topic in the Pacific Northwest as a result of the March 1980 eruption of Mount St. Helens. Curious how much more attention a volcanic eruption in the Lower 48 gets compared to more spectacular ones such as we get on the Alaska Peninsula, in the Aleutians, or elsewhere around the Pacific's "ring of fire."

Of course, the Mount St. Helens eruption might become rather spectacular and dangerous before it's all over since a number of substantial eruptions have occurred there during the past 30,000 years. Actually, Mount St. Helens is the most active volcano in the conterminous United States; its last event ended in 1856.

There is a record of St. Helens eruptive events in flow deposits on the southeast side of the mountain. Erupting ash and rock debris with entrapped hot air has created hot dry flows--called pyroclastic flows--that have carried large rocks downslope and occasionally blocked rivers near the volcano. Another type of flow from Mount St. Helens is the layer flow of mud and rock mobilized by water created on the mountain slopes from rain induced by the eruption or from melting ice and snow. Altogether, pyroclastic and layer flows from Mount St. Helens have a volume of between three and four cubic kilometers, compared to the 27-cubic kilometer volume of the mountain itself.

Both pyroclastic and layer flows are hazardous to life and property because they can move large masses of rock and soil debris downslope for long distances and in a short time. Past flows have reached as far as 55 km (35 miles) downstream from Mount St. Helens.

Other dangers include the fall of airborne ash and rock ejected from the volcano and the rapid flow of hot gas clouds down the volcano's flanks, the so-called nuee ardentes or glowing avalanches. It was a nuee ardente that killed 30,000 people in a few minutes' time at the town of St. Pierre, Martinique, in 1902. Only two people there survived.