Geology of Oregon - Columbia Plateau

Columbia Plateau

The Columbia Plateau province is enveloped by one of the world's largest accumulations of lava. Over 500,000 km2 (190,000 sq mi) of the Earth's surface is covered by it. The topography here is dominated by geologically young lava flows that inundated the countryside with amazing speed, all within the last 17 million years.

Over 170,000 km3 (41,000 cu mi) of basaltic lava, known as the Columbia River basalts, covers the western part of the province. These tremendous flows erupted between 17-6 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.

The Snake River Plain stretches across Oregon, through northern Nevada, southern Idaho, and ends at the Yellowstone Plateau in Wyoming. Looking like a great spoon scooped out the Earth surface, the smooth topography of this province forms a striking contrast with the strong mountainous fabric around it.

The Snake River Plain lies in a distinct depression. At the western end, the base has dropped down along normal faults, forming a graben structure. Although there is extensive faulting at the eastern end, the structure is not as clear.

Like the Columbia River region, volcanic eruptions dominate the story of the Snake River Plain in the eastern part of the Columbia Plateau Province. The earliest Snake River Plain eruptions began about 15 million years ago, just as the tremendous early eruptions of Columbia River Basalt were ending. But most of the Snake River Plain volcanic rock is less than a few million years old, Pliocene age (5-1.6 million years ago) and younger.

In the west, the Columbia River Basalts are just that:almost exclusively black basalt. Not so in the Snake River Plain, where relatively quiet eruptions of soupy black basalt lava flows alternated with tremendous explosive eruptions of rhyolite, a light-colored volcanic rock.

Cinder cones dot the landscape of the Snake River Plain. Some are aligned along vents, the fissures that fed flows and cone-building eruptions. Calderas, great pits formed by explosive volcanism, and low shield volcanoes, and rhyolite hills are also part of the landscape here, but many are obscured by later lava flows.

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.

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. Geologists know that 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.

When the hot plume arrives at the base of the lithosphere, some of the lighter rock of the lithosphere rapidly melts. It is this molten lithosphere that becomes the basalt lavas that gush onto the surface to form the Columbia River and Snake River Plain basalts.

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 probably quite stationary, but the North American plate is moving over it, creating a superb record of the rate and direction of plate motion.