Why are there so many volcanoes in the Pacific Northwest and Alaska?

Veniaminof is a large stratovolcano that suffered a huge caldera-forming eruption about 3,700 years ago. The caldera is almost 6 miles (10 km) across and filled with ice and snow. All the recent eruptions have taken place from a cinder cone that pokes up through the ice in the caldera. This photo shows active lava flows on January 23, 1984. Flows from November 1983 are covered with snow. Photo courtesy of U.S. Geological Survey.

The distribution of volcanoes in the northwest and Alaska is the result of plate tectonics. In the northwest, the oceanic Farallon Plate is being pushed beneath (subducted) the continental margin of the North American Plate. When the subducted plate comes in contact with the hot asthenosphere beneath the continental plate conditions are right for the rocks in the asthenosphere to melt. The melt, called magma, rises to the surface to build volcanoes. Since the subduction zone is a long curvi-linear feature it produces a similar line of volcanoes, called an arc, on the continent. Alaskan volcanoes are the result of the subduction of the Pacific plate under the North American plate.

Steve Mattox

Subduction
If new oceanic lithosphere is created at mid-ocean ridges, where does it go? Geologists had the answer to this question before Vine and Matthews presented their hypothesis. In 1935, K. Wadati, a Japanese seismologist, showed that earthquakes occurred at greater depths towards the interior of the Asian continent. Earthquakes beneath the Pacific Ocean occurred at shallow depths. Earthquakes beneath Siberia and China occurred at greater depths. After World War II, H. Benioff observed the same distribution of earthquakes but could not offer a plausible explanation.

The movement of oceanic lithosphere away from mid-ocean ridges provides an explanation. Convection cells in the mantle help carry the lithosphere away from the ridge. The lithosphere arrives at the edge of a continent, where it is subducted or sinks into the asthenosphere. Thus, oceanic lithosphere is created at mid-ocean ridges and consumed at subduction zones, areas where the lithosphere sinks into the asthenosphere. Earthquakes are generated in the rigid plate as it is subducted into the mantle. The dip of the plate under the continent accounts for the distribution of the earthquakes.

To learn more about plate tectonics click here.

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