Geology
Cobb Seamount lies 270 mi (430 km) off the coast of Washington, in the 8,500 ft (2,600 m)-deep Cascadia Basin. Potassium-argon dating of basalts retrieved from the volcano show that it is 27 ± 6 million years in age. Cobb Seamount's slopes average 12° in grade, and are indented by four prominent terraces at various depths; this unique morphology suggests that it was formed in the Oligocene period, and was then terraced over time by wave erosion. Cobb Seamount's summit itself sits atop a 100 m (328 ft) deep terrace. The volcano's pinnacle is generally flat, and is defined by a pocketed area approximately 880 m (2,887 ft) by 577 m (1,893 ft) in size.
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