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Between 17 and 13 million years ago, immense floods of basalt repeatedly inundated what is now eastern Washington and Oregon, flowing west in some cases all the way to the Pacific Ocean.
These are the Columbia River Basalt flows, and they are the largest flood basalts in North America.
As the last of the Kula Plate decayed and the Farallon Plate advanced back into this area from the south, it brought with it the next and final chapter in regional geologic evolution: the Cascade Episode.
This episode takes its name from the Cascade Arc, which spread up the coast with the Farallon Plate over Eocene time to reach the Pacific Northwest by about 37 million years ago. Over the last 37 million years, the Cascade Arc has been erupting a chain of volcanoes along the trace of the modern Cascade Range in Washington and British Columbia.
One of these is the eastward-migrating Yellowstone ìHot Spotî which traverses through this area between 17 and 13 million years ago, before continuing to burn its way across the Snake River Plain and onto its present location at Yellowstone.
Continued folding and slipping along these northwest - southeast trending features continues today.
The west-northwest trending Seattle Fault is one of these features, active over the past several hundred years.
The last remnant of the Farallon Plate, now called the Juan de Fuca Plate after a Spanish explorer, began subducting along the western margin of Washington.
Melting of the Juan de Fuca Plate at depth intruded magma into the continental margin to form the Cascade Arc.