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Mount Saint Helens Website

Why and when did Mount Saint Helens erupt.

More Info on MtStHelens.
Mount Saint Helens Eruption.
Effects of Mt St Helens.
MtSt Helens Ending.

Here you will learn about why and when Mount Saint Helens Erupted.

Mount Saint Helens erupted on 8:32 Sunday morning, May 18, 1980

Dormant since 1857, Mount St. Helens erupted on the fatal day, May 18, 1980 in one of the largest volcanic explosions in North American history.

After a fissure appeared along the north side of the mountain, a great portion of the rock facing fell, followed by a blast of stone, ash, and poison gas from the mountain. Landslides ensured that local forestation around the volcano which were hit by the volcanos debris nearly 20 mi (32 km) wiped out the the population around Mt St Helens and the vegetation.

The disaster took 60 lives, wiped out substantial populations of elk, deer, bear, and coyote, and destroyed miles of vegetation around Mount Saint Helens.

Two months preceding the historic explosion on May 18, 1980, more than 10,000 mostly small earthquakes had rumbled beneath the volcanic mountain. Normally these massive and strong seismic activity usually precedes to a volcanic eruption. But even the experts did not predict that a final large earthquake, measuring 5.1 on the Richter scale, would so suddenly set off the largest volcanic eruption in the continental United States in more than a century.

But Why did it Erupt?

What caused the eruption was that a body of magma moved into the shallow part of the volcano between the initial unrest (in March) and by May 18th had become so pressurized that the volcano failed and this magma was able to erupt. There was also an earthquake at this time which probably helped trigger the event. Here is a rough richter scale reading to show how much damage it would have caused.

Richter         Earthquake
Magnitudes      Effects

Less than 3.5   Generally not felt, but recorded.

3.5-5.4         Often felt, but rarely causes damage.

Under 6.0       At most slight damage to well-designed buildings.
Can cause major damage to poorly constructed buildings
over small regions.

6.1-6.9         Can be destructive in areas up to about 100 kilometers
across where people live.

7.0-7.9         Major earthquake. Can cause serious damage over larger areas.

8 or greater    Great earthquake. Can cause serious damage in areas several
hundred kilometers across.

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