Radiometric dating of late Cenozoic igneous rocks of the central Oregon Cascade Range has shown that volcanic activity has been strongly episodic, with most lavas erupting during short intervals of 1 or 2 m.y. The strongest activity occurred during middle Miocene time concurrently with eruption of the Columbia River Basalt. Subsequent episodes followed at about 5-m.y. intervals. Preliminary data from other parts of the circum-Pacific area suggest that these same episodes occurred in unison over a large part of the Earth.

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