Southeastern Washington is covered with volcanic deposits from the last 20 million years or so. The reddish-brown areas are the Columbia River Basalt, a gigantic lava pile marking the path of the Yellowstone hotspot.
Western Washington, the edge of the North American plate, has been sliding over oceanic plates like the Pacific, Gorda, and Juna de Fuca plates. The coastline rises and falls from that subduction activity, and the friction of the plates produces rare, very large earthquakes. The pale blue and green areas near shore are young sedimentary rocks, laid down by streams or deposited during high stands of sea level. The subducted rocks heat up and release upwellings of magma that emerge as arcs of volcanoes, shown by the brown and tan areas of the Cascade Range and Olympic Mountains.
In the more distant past, islands and microcontinents have been carried from the west against the continental edge. 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 covered northern Washington 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 them down elsewhere in the cream-colored regions, accounting for the streaky patterns on the map. That region is the famous Channeled Scablands. Glaciers also left thick beds of unconsolidated sediments (yellow-olive) filling the basin where Seattle sits.
This map was compiled in 2002 by Eric Schuster for the state department of natural resources. I have prepared a 1200x1200 pixel version (500 KB) and a 1600x1600 pixel version (800 KB), both of which have keys to the rock units.
Explore Washington geology in more detail at the Washington State Geological Information Portal.
More Washington resources on About.com:
Washington Geography, State Symbols & Facts
Washington National Parks
Washington State Parks, Winter
Washington State Parks, Spring
Washington Scenic Roads