Plate tectonic map of the Western United States and nearby Pacific Ocean. Volcanoes shown by red triangles. Arrows show direction of plate movement. Teeth (black triangles) are on the overriding tectonic plate. Mid-ocean ridges (spreading centers) shown as thin white rectangles.
Simplified cross-section of the subduction zone that produces the volcanoes
of the Cascades.
The volcanoes of the western United States are the result of subduction of the Juan de Fuca and Gorga plates beneath the edge of the North American Plate. The oceanic plates are made at mid-ocean ridges (spreading centers) and are denser than the North American Plate. The North American Plate is made of continental crust and lithospheric mantle. Water from the oceanic plate helps to melt the hot asthenospheric mantle beneath the North American Plate. Magma rises through the plate to supply volcanoes. Submarine volcanoes help to build the Juan de Fuca and Gorga plates.
Based on diagrams in Foxworthy and Hill (1982).
Foxworthy, B.L., and Hill, M., 1982, Volcanic eruptions of 1980 at Mount St. Helens: The First 100 Days: U.S. Geological Survey Professional Paper 1249, 124 p.
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