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Volcanologists have a tendency to drift westward in the United States because that's where the action is tectonically. North and Central America occupy the western portion of a big slab of the earth's crust known as the North American plate, which is shaped roughly like an inverted triangle. The bottom of the triangle is in the middle of the Atlantic Ocean halfway between South America and Africa. The top two corners are north of Siberia and northwest of Greenland. This piece of the earth's crust is constantly jockeying for position with the tectonic plates that surround it. In some places, like Iceland, the North American plate is pulling away from an adjoining plate, and molten material is welling up to fill the gap. In other places, like California, the North American plate is slipping past an adjoining plate, often getting stuck and then breaking free in earthquake-inducing jolts.
But the most dramatic and dangerous of these plate interactions occur in the Pacific Northwest. There, in a line from southern British Columbia to Northern California, a small piece of oceanic crust is being forced under the edge of the North American plate at the rate of a few inches per year.
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