Q:

How many times has Mt. St. Helens erupted?

A:

Quick Answer

Mount St. Helens has erupted numerous times in its 300,000-year history, most notably on May 18, 1980. While much of its early history remains a mystery, geologic studies have concluded that Mount St. Helens is the most active volcano in the Pacific Northwest.

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Full Answer

In determining the activity of Mount St. Helens, geologists study ash layers left behind by eruptions throughout its history. The resulting volcanic deposits are then sorted by relative age and broken down into stages marked by periods of dormancy. While the 1980 eruption was devastating, the largest eruption of Mount St. Helens was around the year 1850 B.C., which was four times as powerful as the eruption in 1980.

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Related Questions

  • Q:

    What damage did Mount St. Helens cause?

    A:

    The eruption of Mount St. Helens on May 18, 1980, caused the destruction of 27 bridges, over 200 homes and 4.7 billion boards of timber. In addition, 15 miles of railway and 185 miles of highway were damaged or destroyed. Fifty-seven people also lost their lives during the eruption, and another four deaths occurred as an indirect result of the eruption.

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  • Q:

    When is Mount St. Helens going to erupt again?

    A:

    While it is not yet possible to predict volcanic eruptions decades in advance, the U.S. Geological Survey’s Cascades Volcano Observatory continually monitors Mount St. Helens to gather data for more accurate short-term predictive models. According to “Future Eruptions at Mount St. Helens,” this volcano is the most likely to erupt in the Cascade Mountains of Washington again in the coming decades.

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  • Q:

    Which plates caused Mount St. Helens to erupt?

    A:

    Mount St. Helens' 1980 eruption was related to complex interactions between the Pacific Plate, the North American Plate and the much smaller Juan de Fuca Plate. It was the first major eruption to take place in the 48 contiguous U.S. states since Mount Lassen erupted in 1915.

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  • Q:

    What type of plate boundary is Mount St. Helens?

    A:

    Mount St. Helens resides within the Cascade Mountain range, which was formed by a subduction plate boundary. However, this particular boundary is a relatively specialized type, in that three different tectonic plates converge at this location.

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