Columbia River Basalt Group

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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 errosion 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.

[edit] Formation of the Columbia River Basalt Group

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]}

[edit] Catastrophic beginnings

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. It has been postulated taht the transition was triggered by impact of a larger meteorite in southeast Oregon, creating a crater 160 km (100 miles) in diameter. The upwelling of lava changed the volcanic discharge regime for the Northwest, resulting in millions of years of flood basalt discharges. ^[5]

Although small when compared with the 600 km by 450 km across Shiva crater, which led to the similar Deccan Traps the phenomena is analogous. The Deccan Traps igneous province is 2,000 m thick and cover an area of 500,000 km² while the Columbia River Basalt Group is 1,800 km deep and covers an area of 160,000 km². The Oregon event was small enough that it did not result in major extinctions like those from the Shiva crater, which produced the Cretaceous-Tertiary extinction event; none-the-less the event had major regional impacts.^[1]

The flows can be devided 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.

[edit] Imnaha Basalt

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]

[edit] Grand Ronde Basalt

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]

[edit] Wanapum Basalt

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]

[edit] Saddle Mountains Basalt

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]

[edit] See also

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

[edit] External links

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

[edit] References

1. ^ ^{a b c d e f} 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 d} 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