Columbia River Basalt Group (Wikipedia)

This article incorporates some material from the USGS article Columbia River Basalt Group Stretches from Oregon to Idaho.

excerpt

The Columbia River Basalt Group is the youngest, smallest and one of the best-preserved continental flood basalt province on Earth, covering over 210,000 km2 (81,000 sq mi) mainly eastern Oregon and Washington, western Idaho, and part of northern Nevada. The basalt group includes the Steens and Picture Gorge basalt formations.

Mount Pilchuck (summitpost.org) There are many factors which make Mount Pilchuck one of Washington’s favorite and most sought after hiking peaks. Second only to Mount Si, it’s estimated that Pilchuck sees 28,000 visitors each year. The number one draw for the mountain is the old fire lookout on the summit, which was constructed in 1918 by the US Forest Service. It was renovated by the Everett Mountaineers in 1977. At the peak use of fire lookouts, there were nearly 700 Washington State, but now there are only a handful remaining. Most were burned by lightning strikes.

Columbia River Basalt Group Stretches from Oregon to Idaho (usgs.gov) The thick, layered lava flows of the CRBG erupted as flood basalts, which originate as some of the most highly effusive eruptions in the world. The CRBG sequence a classic example of flood basalt activity that erupted more than 350 lava flows from about 16.7 Ma to 5.5 Ma. The eruptions originated from a series of generally north-northwest-trending linear fissures, ranging from tens to hundreds of kilometers in length, located along the Washington/Oregon/Idaho border. The magma that fed these massive eruptions may have come from a plume-like upwelling from the mantle called a hot spot. Since the time of the CRBG eruptions, the North American plate has moved in a west-southwestwardly motion, and that hot spot is now believed to reside beneath Yellowstone volcano in northwest Wyoming.