Q:

How many times has Mt. St. Helens erupted?

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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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  • Q:

    How did Mount St. Helens form?

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    Mount St. Helens is a cinder cone volcano that formed through the gradual accumulation of cinders and ash at the base of the mountain. Unlike a shield volcano, such as Mauna Loa in Hawaii, cinder cones can rise sharply from the surrounding terrain and maintain a steep, angular profile throughout their existence.

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  • Q:

    Which plates caused Mount St. Helens to erupt?

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    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:

    When is Mount St. Helens going to erupt again?

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    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:

    What type of lava was produced in the most recent Mount St. Helens eruption?

    A:

    The most recent volcanic eruption of Mount St. Helens in Washington State, which began in October, 2004, produced hot, but solid, extrusions of smooth-sided lava called "spines." The extrusion of the solid lava spines continued until January 2008, producing more than 120 million cubic yards of solidified lava. Unlike the previous and catastrophic eruption, which occurred in 1980 and produced thick lava flows, the eruptions occurring between 2004 and 2008 produced only solid lava extrusions and resulted in no loss of life or property.

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