January GSA Today cover (IMAGE)

Geological Society of America

January GSA Today cover

Caption

The break in slope is coincident with a discontinuity representing a regional three- to five-million-year hiatus separating two distinct styles of volcanic activity in the Cascadia backarc region. The lower slope is composed of high-K calc-alkaline lavas derived from melting of a hydrated mantle source from 30 to 20 Ma. The steeper upper slope is composed of thin flows of the tholeiitic Steens Basalt (early lavas of the Columbia River Basalt Group) derived from melting of a dry mantle source with a plume component at ca. 17 Ma.

Credit

Victor Camp and Ray Wells

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Credit Victor Camp and Ray Wells

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