Over 170,000 cubic kilometers (105,633 mi) of basaltic lava, known as the Columbia River Basalts, covers the western part of the province. These tremendous flows erupted between 17 million - 6 million years ago. Most of the lava flooded out in the first 1.5 million years - an extraordinarily short time for such an outpouring of molten rock.
Over 300 high-volume individual lava flows have been identified, along with countless smaller flows. Numerous linear vents, some over 150 km (93 mi) long, show where lava erupted near the eastern edge of the Columbia River Basalts, but older vents were probably buried by younger flows.
In an effort to determine why there was an unusually large outpouring of lava so far from a plate border, scientists established hardening (cooling) dates for the lavas. They found that the youngest rocks were grouped around the Yellowstone plateau, with lavas increasing in age to the west. This evidence suggests that a concentrated heat source is melting rocks at the base of the lithosphere underneath the Columbia Plateau province.
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. 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.