Acknowledgments

1. Columbia River Flood Basalts Sami Butler — GEO 208, Green River Community College Origins The obvious origins of the basalts are, well, obvious. They erupted from massive North/Northwest trending fissures who’s feeder dikes litter the East and Southeast part of the Plateau. The dike swarms include the Monument swarm (which fed only the isolated Picture Gorge basalt) the Grande Ronde swarm, the Cornucopia swarm, and the largest, the Chief Joseph swarm. The mystery lies in the mechanism and circumstance for eruption. There are several ideas for the origin of the Columbia River Basalt Group. The most popular theories are the Yellowstone Hotspot, or mantle plume theory, and the Back-arc spreading theory. Introduction The Columbia River Basalt Group is a group of vast flood basalts that make up the North American Columbia Plateau. They erupted in the Miocene epoch. The series includes the Imnaha, Grande Ronde, Picture Gorge, Wanapum, and Saddle Mountain basalts. They erupted to create the Clearwater, Weiser, and St. Maries embayments. Other Ideas Though the mantle plume theory and the back-arc spreading theory are the most popular ideas of how the Columbia River Basalt group came into existence, there are other suggestions as well. Some evidence might suggest that a narrow, wedge-shaped area North of the Blue Mountains was underlain by oceanic crust. The main focus of the eruptions lie at the apex of this wedge. Another idea is that the eruptions had to do with the plate overriding the East Pacific Rise. This mid-ocean ridge disappears at the Gulf of California and supposedly continues North, its axis extending into Eastern Oregon. The idea of an asteroid impact was also popular, though discounted now because of the lack of evidence. Conclusions The origins of the Columbia Plateau flood basalts is still a mystery. The remains of the monumental floods are all we have to try and decipher what caused the eruptions in the Miocene. The mantle plume could very well have been the source. If so, will it ever uplift and send basalt flooding across the land again? Back-arc spreading is very real, but is it the answer to this puzzle? Could the supposed back-arc basin of the Columbia Plateau ever become active again, as long as the arc of the Cascades is still active? In another few million years, we’ll let you know. Fig. 5. Saddle Mountain basalt (Elephant Mountain member) east of Ice Harbor. The Back-arc Spreading Theory Another very popular theory is back-arc spreading. (fig 6.) The tectonic setting for the Columbia River Basalt Group eruptions is North-South compression and East-West extension as terranes accreted Northward. At the time, the Cascade mountains were being created as the Juan De Fuca plate was subducting underneath the North American plate. Stretching occurred east of the Cascades inducing faulting behind the arc which the subduction was creating. This was apparently sufficient for shallow magma to erupt. Not much is known about back-arc spreading. However, at places like the Lau Basin (between the islands of Fiji and Tonga) and the Mariana Trough, back-arc spreading is occurring today; proof that it does happen. The roadblock with back-arc spreading is the confined time period of the basalt eruptions, and the fact that it is not known why back-arc spreading really occurs in the first place. It seems that the back-arc basin should still be as active as the arc that it lies behind. Fig. 1. Stratigraphy of the Columbia River Basalt Group. (modified after Swanson and others, 1979c) Asterisks denote basalt units that have not been found in Idaho. R=reversed, N=Normal, T=Transitional polarity. The basalts are spectacular, yet still a mystery. It is clear that they must have erupted from deep within the Earth as they are relatively uncontaminated by the Earth’s crust. Yet they existed within a short time period of only 13-17 million years ago, never before and never again. Where did these extensive floods of basalt come from, and why did they erupt and cease so suddenly? Fig. 4 Quarry East of Waitsburg. Wanapum basalt banded lava flow. (Frenchman Springs member.) The Mantle Plume Theory The mantle plume theory, or Yellowstone Hotspot theory, suggests that the source of the eruptions are from a mantle plume, possibly the hotspot that now rests underneath Yellowstone National Park. This hotspot did indeed move through the general area across the Snake River plain around the time the basalts were erupting. It explains very well the fact that a lot of basalt erupted in a very specific time, in a very specific area. The biggest problem with this theory is the discrepancy in the supposed movement of the hotspot and the area of eruption. The Cascade arc seems to have rotated clockwise due to plate tectonics, possibly explaining the awkward movement of the hotspot. Another explanation is that while the plume tail of the hotspot is tracked to the Oregon-Nevada border at this time, the plume head could be underlain at the source of the basalts. This relationship of head and tail is typical in traditional plume models. Yet another suggestion is that the hotspot was deflected at the Chief Joseph dike swarm by the subducting Farallon plate, and deflected further by the thick margin of North America. A further difficulty lies in the fact that it is hard to explain why the same mantle plume that is causing rhyolitic activity at today’s Yellowstone National Park, spit out a tremendous amount of basalt across the Columbia Plateau. Apparently, some typical plume models go through uplift, which could have occurred with the Yellowstone hotspot at that time, allowing the magma to rise more quickly and have less time for partial melting. Literature cited Columbia River Basalt in Idaho: Physical and Chemical Characteristics, Flow Distribution, and Tectonic Implications. Camp, Hooper, Swanson, Wright The Columbia River Basalts Peter R. Hooper Flood Basalts and Glacier Floods: Roadside Geology Carson and Pogue The Origin of the Columbia River Flood Basalt Province: Plume versus Nonplume Models. Hooper, Camp, Reidel, Ross Geology of the Pacific Northwest W. Orr, E. Orr http://www.washington.edu/burkemuseum/geo_history_wa/cascade%20Episode.html Fig. 2. the extent of the Columbia River Basalt Group. Acknowledgments Thanks to Bob for providing sources of information, and everyone in GEO208, and to the nice ladies at the Natural Resource building for their help. For further information Please contact Sami. riyuki_chan@yahoo.com Or take Bob Filson’s Geo 208 class. Fig. 3. The Weiser, Clearwater, and St. Maries Embayments. Fig. 6. An illustration of back-arc spreading.