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Except for volcanoes in the Adel Mountains and near the Black Hills, the first 10 m.y. of the Cenozoic was a time of igneous quiescence. Basaltic volcanism in the eugeosyncline west of the present-day Cascade volcanic arc began in early Eocene time, was most intense between 54 and 44 m.y. ago, and tapered off slowly, with injection of basaltic and alkali trachyte dikes continuing until the Oligocene.

In Oligocene time the eugeosynclinal rocks became welded to the continental margin, shorelines shifted westward, and volcanic activity west of the Cascade arc virtually ceased.

Igneous activity began about 55 m.y. ago over a broad region in Washington, northern Idaho, and Montana, and during the early Eocene this activity swept southward across all the Northwestern United States. Volcanism, plutonism, regional high heat flow with associated geothermal circulation systems, block faulting, and ductile deformation at depth associated with tectonic denudation as a consequence of diapiric rise of plutonic rocks occurred synchronously during an intense culmination between 50 and 43 m.y. ago (Challis episode). Volcanism and deformation then died out abruptly as the locus of igneous eruption centers shifted south into Nevada-Utah and west into the Cascade arc, where volcanic activity persisted through the rest of the Cenozoic.

Following widespread tectonic and igneous quiescence between about 38 and 18 m.y. ago, volcanic activity suddenly commenced over a large region with rapid eruption of Columbia River and related basalts. Volcanism, with bimodal chemistry, began during this episode (Columbia, 13 to 16 m.y. ago) in southwestern Idaho and over all of southeastern Oregon. Between 13 m.y. ago and today, bimodal igneous activity migrated eastward across Idaho to produce the Snake River Plain-Yellowstone volcanic field. In Oregon, at the same time, siliceous volcanic centers retreated westward. As a consequence rhyolitic volcanic centers today are most active only in Yellowstone and close to the Cascade Range. Striking synchroneity is shown by pulses of more intense igneous activity in the Cascade and Snake River Plain-Yellowstone regions.

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