Geologic Map of Washington
Washington State Department of Natural Resources
Washington is a rugged, glaciated, volcanic patchwork on the edge of the North American continental plate that has been vigorously active for the last 100 million years. Its geology can be discussed in four tidy pieces.
The southeastern quadrant of the state is covered with volcanic deposits from geologically recent times, meaning within the last 20 million years or so. The reddish-brown areas are the Columbia River Basalt, a gigantic pile of lava that marks the path of the Yellowstone hotspot during Miocene times. Today the hotspot underlies the Idaho-Wyoming-Montana tri-state area, in Yellowstone National Park.
Western Washington, on the edge of the North American plate, has been sliding over oceanic plates like the Pacific, Gorda, and Juna de Fuca plates, which have been subducted beneath it. The coastline rises and falls from that activity, and the friction between the plates takes the form of rare, very large earthquakes. The pale blue and green areas near the shore are young sedimentary rocks, laid down by streams from the east or deposited during high stands of sea level. The subducted rocks, as they do elsewhere on Earth, heat up and release upwellings of magma that emerge as arcs of volcanoes. The brown and tan areas are volcanic rocks of the Cascade Range and Olympic Mountains that arose from arc volcanism.
In the more distant past, islands and microcontinents have been carried from the west onto the continental edge, which has scraped them off the underlying oceanic plate. These crustal flakes were plastered onto the western edge of Washington and are known to geologists as exotic terranes—large bodies of rock with consistent, unified histories that differ from the terranes that surround them. Northern Washington shows them well. The purple, green, magenta, and gray areas are terranes of Paleozoic and Mesozoic age that began their existence thousands of kilometers to the south and west. Light-pink areas are more recent intrusions of granitic rocks.
The Pleistocene ice ages affected Washington strongly, covering the north part of the state deep in glaciers. The ice dammed some of the rivers that flow through here, creating large lakes. When the dams burst, gigantic floods burst across the whole southeastern part of the state. The floods stripped sediments off of the underlying basalt and laid the sediment down elsewhere in the cream-colored regions, accounting for the streaky patterns on the map. That region is the famous Channeled Scablands. In the west, the glaciers left thick beds of unconsolidated sediments (yellow-olive) filling the basin where Seattle sits.
This is a reduced image of a map compiled in 2002 by Eric Schuster for the state department of natural resources. See the 1200x1200 pixel version (500 KB) for full legibility. And the 1600x1600 pixel version (800 KB) is suitable for printing.
Explore Washington geology in more detail at the Washington State Geological Information Portal.
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