Lecture%20%20#8%20Geol%203950%20notes%202010%20CR%20Stern -...

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Lecture #8 notes; Geology 3950, Spring 2010; CR Stern May 1980 eruption of Mt St Helens volcano Mt St Helens in southwest Washington State is one of approximately 15 stratovolcanoes that form the Cascade volcanic arc, which extends from northern California into southernmost Canada (Figure 1 below). Other important volcanoes in this arc include Mt Shasta and Lassen in northern California, Crater Lake (formally Mt Mazama) and Hood in Oregon, Mt Rainer and Adams in Washington, and Mt Garibaldi in Canada. The volcanoes in the Cascade volcanic arc result from the slow subduction of the small Juan de Fuca oceanic plate into the Cascadia subduction zone below the northwest US, the same tectonic activity that produces the risk of large subduction earthquakes in this part of the country (Fig 2 below).
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Subduction produces volcanism because water-rich oceanic sediments and crust is tectonically carried down into the mantle. When this water is released from the subducting lithosphere plate, it rises into the overlying mantle and fluxes melting and magma formation, since water lowers the melting temperature of mantle rock (Figure 3) Note that there is no volcanism associated with the San Andreas Strike-slip plate boundary between the Pacific and North American plates, and that there is no volcanism associated with the thrust faults below Los Angeles, as both these fault systems are only in the crust, and magmas are derived by tectonic disturbances in the mantle. Volcanoes do occur in association with the extension taking place in the Basin and Range and Rio Grande rift in the area between eastern California and western Colorado, and also in association with the Yellowstone hot spot in Wyoming, but the Cascade arc of
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Lecture%20%20#8%20Geol%203950%20notes%202010%20CR%20Stern -...

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