During late
Miocene and early
Pliocene times, a
flood basalt engulfed about of the Pacific Northwest, forming a
large igneous province. Over a period of perhaps 10 to 15 million years, lava flow after lava flow poured out, ultimately accumulating to a thickness of more than 6,000 feet (1.8 km). The subsidence of the crust produced a large, slightly depressed lava plateau. The ancient
Columbia River was forced into its present course by the northwesterly advancing lava. The lava, as it flowed over the area, first filled the stream valleys, forming dams that in turn caused impoundments or lakes. Entities found in these lake beds include
fossil leaf impressions,
petrified wood, fossil insects, and bones of vertebrate animals. Evidence suggests that some concentrated heat source is melting rock beneath the Columbia Plateau Province at the base of the lithosphere (the layer of crust and
upper mantle that forms Earth's moving tectonic plates). In an effort to figure out why this area, far from a plate boundary, had such an enormous outpouring of lava, scientists established hardening dates for many of the individual lava flows. They found that the youngest volcanic rocks were clustered near the Yellowstone Plateau and that the farther west they went, the older the lavas.{{USGS|url=https://geomaps.wr.usgs.gov/parks/province/columplat.html Although scientists are still gathering evidence, a probable explanation is that a
hot spot, an extremely hot plume of deep
mantle material, is rising to the surface beneath the Columbia Plateau Province. Beneath
Hawaii and
Iceland, a temperature instability develops (for reasons not yet well understood) at the
boundary between the core and mantle. The concentrated heat triggers a plume hundreds of kilometers in diameter that ascends directly through to the surface of the Earth. The track of this hot spot starts in the west and sweeps up to
Yellowstone National Park. The steaming fumaroles and explosive
geysers are ample evidence of a concentration of heat beneath the surface. The hotspot is stationary, but the North American plate is moving over it, creating a superb record of the rate and direction of plate motion. ==Flora==