Columbia plateau basalt
The Columbia Plateau Basalt is an extensive flood basalt in the United States that covers parts of the states of Washington , Oregon, and Idaho .
During the Middle and Late Miocene to the Early Pliocene , vast amounts of basalt consumed more than 163,700 km² of the Pacific Northwest , forming an igneous large province with an estimated volume of 174,300 km³. The eruptions were most violent 17 to 14 million years ago when over 99% of the basalt was extracted. Recent studies limit the phase of the main activity to the period 16.7 to 15.9 million years ago. The eruptions that occurred 14 to 6 million years ago were less extensive.
The lava flows were later eroded multiple times by the water masses of the Missoula Floods , which exposed many layers of basalt castings at Wallula Gap , the lower Palouse River , Columbia River Gorge, and Channeled Scablands .
The Columbia plateau basalt is considered to be a possible link to the chilcotin plateau basalt in central southern British Columbia , Canada .
The formation of the Columbia Plateau basalt
Over a period of maybe 10 to 15 million years lava flow poured over lava flow, accumulating to a thickness of more than 1,800 meters. As the molten rock flowed out on the surface, the earth slowly subsided over the emptying magma reservoir . The subsidence of the earth's crust created a large, slightly bowl-shaped lava plain, now known as the Columbia Basin or Columbia River Plateau . The predecessor of today's Columbia River was forced into its current course by the northward advancing lava flood. As the lava spread over the area, it dammed up the river valleys in many places, creating dammings and lakes. The remains of these lakes now contain fossil leaf prints, petrified wood , fossil insects and the petrified bones of vertebrates , including those of the Blue Lake Rhino .
The transition to flood volcanism
Until more than 17 million years ago, the eruptions of the volcanoes of the Cascade Range had been occurring with constant regularity for more than 20 million years, as is the case today. In the middle Miocene there was an abrupt transition to the effusion of flood basalts. The ultimate reason for this type of volcanism is still debated. The most widely accepted idea is that, as in the case of Hawaii, a mantle diapir or ascent of the upper mantle brought about extensive and voluminous basaltic volcanism.
The basalt casts can be divided into three main divisions: the Grande Ronde Basalt , the Wanapum Basalt and the Saddle Mountains Basalt . The ages of the various lava effusions were determined by radiometric dating , primarily the potassium-argon method .
Imnaha basalt
The oldest part of the basalt, 17.4 to 17 million years old, is the Imnaha basalt . The Imnaha lava flows poured over northeastern Oregon. Although it is estimated that they make up ten percent of the total amount of flood basalts, they were buried under the subsequent effusions and are only visible in a few places.
Grande Ronde Basalt
The next oldest of the lava effusions is the Grande Ronde Basalt , which is 17 to 15.6 million years old . Subunits or flow zones within the Grande Ronde Basalt are the Meyer Ridge Unit and the Sentinel Bluffs Unit . Geologists estimate that the lavas of the Grande Ronde Basalt account for up to 85% of the total volume. The Grande Ronde Basalts have the Chief Joseph Dike Swarm , a swarm of Dykes through which the lava rose, and the number of which is estimated at up to 20,000. Some of these ascents were fissures five to ten meters wide through which an enormous amount of lava could escape. Much of the lava flowed north into what is now Washington, and through the Columbia River valley to the Pacific Ocean . These huge lava flows created the actual Columbia Plateau. The basalts and shoals of the Grande Ronde basalt can be seen well in the craggy 600-meter walls of Joseph Canyon on Oregon Route 3 . Parts of the Grande Ronde , Wanapum and Saddle Mountain Basalts are exposed at Wallula Gap (in the picture from bottom to top).
Wanapum basalt
The Wanapum Basalt consists of the units of the Eckler Mountain Member (15.6 million years old), the Frenchmen Springs Member (15.5 million years old), the Roza Member (14.9 million years old) and the Priest Rapids Member ( 14.5 million years old).
Saddle Mountains basalt
The Saddle Mountains Basalt , particularly well observed in the Saddle Mountains , consists of several lava effusions: Umatilla Member , Wilbur Creek Member , Asotin Member (13 million years old), Weissenfels Ridge Member , Esquatzel Member , Elephant Mountain Member (10.5 million Years old), Bujford Member , Ice Harbor Member (8.5 million years old), and Lower Monumental Member (6 million years old).
Individual evidence
- ↑ Jennifer Kasbohm, Blair Schoene: Rapid eruption of the Columbia River flood basalt and correlation with the mid-Miocene climate optimum . (PDF) In: Science Advances . 4, No. 9, September 2018. doi : 10.1126 / sciadv.aat8223 .
- ^ A b c d e Robert J. Carson, Kevin R. Pogue: Flood Basalts and Glacier Floods: Roadside Geology of Parts of Walla Walla, Franklin, and Columbia Counties, Washington . In: Washington Division of Geology and Earth Resources Information Circular . tape 90 . Washington State Department of Natural Resources, 1996.
- ↑ Stephen P. Reidel: A Lava Flow without a Source: The Cohasset Flow and Its Compositional Members . In: The Journal of Geology . tape 113 , January 2005, p. 1-21 .
- ↑ RE Ernst, KL Buchan: Igneous rock associations in Canada 3. Large Igneous Provinces (LIPs) in Canada and adjacent regions: 3 ( Memento of the original from August 9, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ David Alt: Glacial Lake Missoula & its Humongous Floods . Mountain Press Publishing Company, ISBN 0-87842-415-6 .
- ↑ Bruce Bjornstad: On the Trail of the Ice Age Floods: A Geological Guide to the Mid-Columbia Basin . Keokee Books, Sand Point, Idaho 2006, ISBN 978-1-879628-27-4 .
- ^ A b c David Alt, Donald Hyndman: Northwest Exposures: a Geologic Story of the Northwest . Mountain Press Publishing Company, 1995, ISBN 0-87842-323-0 .
- ^ Walter M. Chappell, J. Wyatt Durham, Donald E. Savage: Mold of a rhinoceros in basalt, Lower Grand Coulee, Washington. In: Bulletin of the Geological Society of America. 62, 1951, pp. 907-918.
- ^ Marge and Ted Mueller: Fire, Faults and Floods . University of Idaho Press, Moscow, Idaho 1997, ISBN 0-89301-206-8 .
