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It is the Juan de Fuca plate, Pacific and the North America plates. The Juan de Fuca plate went under the North American plate and is melted into material in the mantle. This is called a destructive plate margin. This material then comes up through the volcano, and solidifies, making a cone shaped volcano. It often blocks itself up with residue magma, so eruptions are violent as much builds up. This happened with Mount St. Helens.

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7y ago

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