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The Columbia River Basalt Group encompasses portions of 3 states.

The Columbia River Basalt Group is a large igneous province that lies across parts of the states of Washington, Oregon, and Idaho in the United States of America. During late Miocene and early Pliocene times, one of the largest flood basalts ever to appear on the earth's surface engulfed about 163,700 km² (63,000 mile²) of the Pacific Northwest, forming a large igneous province with an estimated volume of 174,300 km³. Eruptions were most vigorous from 17—14 million years ago, when over 99% of the basalt was released. Less extensive eruptions continued from 14—6 million years ago. ^[1]

These lava flows have been extensively exposed by the erosion resulting from the Missoula Floods, which laid bare many layers of the basalt flows at Wallula Gap, the lower Palouse River, the Columbia River Gorge and throughout the Channeled Scablands.

The oldest, the Imnaha Basalt, is exposed at Imnaha, Oregon.

Over a period of perhaps 10 to 15 million years lava flow after lava flow poured out, eventually accumulating to a thickness of more than 1.8 km (6,000 feet). As the molten rock came to the surface, the earth's crust gradually sank into the space left by the rising lava. The subsidence of the crust produced a large, slightly depressed lava plain now known as the Columbia Basin or Columbia River 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. In these ancient lake beds are found fossil leaf impressions, petrified wood, fossil insects, and bones of vertebrate animals. ^{[2][3] [4][5]}

In the Palouse River Canyon just downstream of Palouse Falls, the Sentinel Bluffs flows of the Grand Ronde Formation can be seen on the bottom, covered by the Ginkgo Flow of the Wanapum Basalt.

Prior to 17 million years ago, the Western Cascade Stratovolcanoes erupted with periodic regularity for over 20 million years, even as they do today. An abrupt transition to shield volcanic flooding occurred in the mid-Miocene. The ultimate cause of this volcanism is still up for debate, however, the most widely accepted idea is that a mantle plume or upwelling (similar to that associated with present day Hawaii) initiated widespread and voluminous basaltic volcanism, ~17 million years ago. While it has been postulated that the flood basalt volcanism was triggered by a meteorite impact in southeastern Oregon, no geologic evidence has been found (as of 11/25/2006) to support this hypothesis.

The flows can be divided into major categories: The Grand Ronde Basalt, the Wanapum Basalt, and the Saddle Mountains Basalt. The ages of the various lava flows have been dated by radiometric dating—particularly through measurement of the ratios of isotopes of potassium to argon.^[1]

Parts of the Grand Ronde, Wanapum & Saddle Mountain Basalts (in order from the bottom) are exposed at the Wallula Gap.

The oldest of the flows, from 17.4 million to 17.0 million years ago, is the Imnaha Basalt. The Imnaha flows welled up across northeastern Oregon. Although estimates are that this amounts to about 10% of the total flows, they have been buried under more recent flows, and are visible in few locations.^[5]

The next oldest of the flows, from 17 million to 15.6 million years ago, make up the Grand Ronde Basalt. Units (flow zones) within the Grand Ronde Basalt include the Meyer Ridge and the Sentinel Bluffs units. Geologists estimate that the Grand Ronde Basalt comprises about 85% of the total flow volume. It is characterized by a number of dikes called the Chief Joseph Dike Swarm through which the lava upwelling occurred (estimates range to up to 20,000 such dikes). Many of the dikes were fissures 5-10 meters wide, allowing for huge quantities of magma upwelling. Much of the lava flowed north into Washington as well as down the Columbia River channel to the Pacific Ocean; the tremendous flows created the Columbia River Plateau. The weight of this flow caused central Washington to sink, creating the broad Columbia Basin in Washington.^[1][5]

Three Devil's grade in Moses Coulee, Washington. The upper basalt is Roza Member, while the lower canyon exposes Frenchmen Springs Member basalt.

The Wanapum Basalt is comprised of the Eckler Mountain Member (15.6 million years ago), the Frenchmen Springs Member (15.5 million years ago), the Roza Member (14.9 million years ago) and the Priest Rapids Member (14.5 million years ago).^[1][6]

The Saddle Mountain Basalt is comprised of the Umatilla Member flows, the Wilbur Creek Member flows, the Asotin Member flows (13 million years ago), the Weissenfels Ridge Member flows, the Esquatzel Member flows, the Elephant Mountain Member flows (10.5 million years ago), the Bujford Member flows, the Ice Harbor Member flows (8.5 million years ago) and the Lower Monumental Member flows (6 million years ago).^[1]

• Grand Coulee
• Channeled scablands
• Wallula Gap
• Interior Plateau

• USGS Page on Columbia Plateau
• Geology of Lake Roosevelt National Recreation Area (source of much of this page)

1. ^ ^{a b c d e} Carson, Robert J. and Pogue, Kevin R. (1996). Flood Basalts and Glacier Floods:Roadside Geology of Parts of Walla Walla, Franklin, and Columbia Counties, Washington. Washington State Department of Natural Resources (Washington Division of Geology and Earth Resources Information Circular 90). ISBN none. 
2. ^ Alt, David. Glacial Lake Missoula & its Humongous Floods. Mountain Press Publishing Company. ISBN 0-87842-415-6. 
3. ^ Bjornstad, Bruce (2006). On the Trail of the Ice Age Floods: A Geological Guide to the Mid-Columbia Basin. Keokee Books; Sand Point, Idaho. ISBN 978-1-879628-27-4. 
4. ^ Portions of this article, including a figure, are adapted from works of the United States Government, which are in the public domain
5. ^ ^{a b c} Alt, David & Hyndman, Donald (1995). Northwest Exposures: a Geologic Story of the Northwest. Mountain Press Publishing Company. ISBN 0-87842-323-0. 
6. ^ Mueller, Marge & Ted (1997). Fire, Faults and Floods. University of Idaho Press, Moscow, Idaho. ISBN 0-89301-206-8. 

Exploring Ice Age Floods in the Pacific Northwest

| Glacial Lake Missoula | Missoula Floods | Channeled scablands | Grand Coulee | Dry Falls | Drumheller Channels | Columbia River Plateau | Wallula Gap | Touchet Formation | Lake Lewis | Columbia River Gorge | Columbia River Basalt Group | Palouse Falls | Sims Corner Eskers and Kames | Moses Coulee | Withrow Moraine | Crab Creek | Corfu Slide

Volcanoes of Oregon v • d • e
High Cascades		Mt Hood | Olallie Butte | Mt Jefferson | Three Fingered Jack | Hogg Rock | Hoodoo Butte | Hayrick Butte | Black Butte | Mt Washington | Belknap Crater | Black Crater | Three Sisters | Broken Top | Tumalo Mtn | Mt Bachelor | Maiden Peak | Diamond Peak | Howlock Mtn | Mt Thielsen | Mt Bailey | Mt Mazama / Crater Lake | Mt Scott | Union Peak | Pelican Butte | Mt McLoughlin | Aspen Butte
Western Cascades		Boring Lava Field (Mt Sylvania | Mt Tabor | Rocky Butte | Powell Butte | Larch Mtn)
Eastern Cascades		Pilot Butte | Lava Butte | Newberry Volcano | Yamsay Mtn
Basin and Range		Big Hole | Hole-in-the-Ground | Fort Rock | Diamond Craters	Columbia Plateau		Columbia River Basalt Group