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									                                                                                                            Columbia River Flood Basalts
                                                                                           Sami Butler — GEO 208, Green River Community College

   Introduction                                                                     Origins                                                                                                                                      Other Ideas
                                                                                    The obvious origins of the basalts are, well, obvious. They
   The Columbia River Basalt Group is a group of vast flood                                                                                                                                                                      Though the mantle plume theory and the back-arc spreading
                                                                                    erupted from massive North/Northwest trending fissures who’s
   basalts that make up the North American Columbia Plateau.                                                                                                                                                                     theory are the most popular ideas of how the Columbia River
                                                                                    feeder dikes litter the East and Southeast part of the Plateau.
   They erupted in the Miocene epoch. The series includes the                                                                                                                                                                    Basalt group came into existence, there are other suggestions
                                                                                    The dike swarms include the Monument swarm (which fed
   Imnaha, Grande Ronde, Picture Gorge, Wanapum, and Saddle                                                                                                                                                                      as well.
                                                                                    only the isolated Picture Gorge basalt) the Grande Ronde
   Mountain basalts. They erupted to create the Clearwater,                                                                                                                                                                      Some evidence might suggest that a narrow, wedge-shaped
                                                                                    swarm, the Cornucopia swarm, and the largest, the Chief
   Weiser, and St. Maries embayments.                                                                                                                                                                                            area North of the Blue Mountains was underlain by oceanic
                                                                                    Joseph swarm.
                                                                                                                                                                                                                                 crust. The main focus of the eruptions lie at the apex of this
                                                                                    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                                                                                        Another idea is that the eruptions had to do with the plate
                                                                                    Yellowstone Hotspot, or mantle plume theory, and the Back-                                                                                   overriding the East Pacific Rise. This mid-ocean ridge
                                                                                    arc spreading theory.                                                                                                                        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.

                                                                                                                                                                                                                                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
                                                                                                                                                                      Fig. 5. Saddle Mountain basalt (Elephant
                                                                                                                                                                      Mountain member) east of Ice Harbor.                      The mantle plume could very well have been the source. If
                                                                                                                                                                                                                                so, will it ever uplift and send basalt flooding across the land
                                                                                                                                                           The Back-arc Spreading Theory                                        again?
Fig. 1. Stratigraphy of the Columbia River Basalt Group. (modified after                                                                                                                                                        Back-arc spreading is very real, but is it the answer to this
                                                                                                                                                           Another very popular theory is back-arc spreading. (fig 6.) The
Swanson and others, 1979c) Asterisks denote basalt units that have not                                                                                                                                                          puzzle? Could the supposed back-arc basin of the Columbia
been found in Idaho. R=reversed, N=Normal, T=Transitional polarity.                                                                                        tectonic setting for the Columbia River Basalt Group eruptions
                                                                                                                                                                                                                                Plateau ever become active again, as long as the arc of the
                                                                                                                                                           is North-South compression and East-West extension as
                                                                                                                                                                                                                                Cascades is still active?
                                                                                                                                                           terranes accreted Northward. At the time, the Cascade
The basalts are spectacular, yet still a mystery. It is clear that                                                                                                                                                              In another few million years, we’ll let you know.
                                                                                                                                                           mountains were being created as the Juan De Fuca plate was
they must have erupted from deep within the Earth as they are                         Fig. 4 Quarry East of Waitsburg. Wanapum basalt banded lava          subducting underneath the North American plate. Stretching
                                                                                      flow. (Frenchman Springs member.)
relatively uncontaminated by the Earth’s crust. Yet they existed                                                                                           occurred east of the Cascades inducing faulting behind the arc
within a short time period of only 13-17 million years ago,                                                                                                which the subduction was creating. This was apparently
never before and never again. Where did these extensive floods                         The Mantle Plume Theory                                             sufficient for shallow magma to erupt. Not much is known             Literature cited
of basalt come from, and why did they erupt and cease so                                The mantle plume theory, or Yellowstone Hotspot theory,            about back-arc spreading. However, at places like the Lau            Columbia River Basalt in Idaho: Physical and Chemical
suddenly?                                                                           suggests that the source of the eruptions are from a mantle            Basin (between the islands of Fiji and Tonga) and the Mariana            Characteristics, Flow Distribution, and Tectonic
                                                                                    plume, possibly the hotspot that now rests underneath                  Trough, back-arc spreading is occurring today; proof that it             Implications. Camp, Hooper, Swanson, Wright
                                                                                    Yellowstone National Park. This hotspot did indeed move                does happen.                                                         The Columbia River Basalts Peter R. Hooper
                                                                                    through the general area across the Snake River plain around           The roadblock with back-arc spreading is the confined time           Flood Basalts and Glacier Floods: Roadside Geology Carson
                                                                                    the time the basalts were erupting. It explains very well the fact     period of the basalt eruptions, and the fact that it is not known        and Pogue
                                                                                    that a lot of basalt erupted in a very specific time, in a very        why back-arc spreading really occurs in the first place. It          The Origin of the Columbia River Flood Basalt Province:
                                                                                    specific area. The biggest problem with this theory is the             seems that the back-arc basin should still be as active as the arc       Plume versus Nonplume Models. Hooper, Camp, Reidel,
                                               Fig. 2. the extent of the Columbia   discrepancy in the supposed movement of the hotspot and the            that it lies behind.
                                               River Basalt Group.
                                                                                    area of eruption. The Cascade arc seems to have rotated                                                                                     Geology of the Pacific Northwest W. Orr, E. Orr
                                                                                    clockwise due to plate tectonics, possibly explaining the                                                                                   http://www.washington.edu/burkemuseum/geo_history_wa/cas
                                                                                    awkward movement of the hotspot.                                                                                                                cade%20Episode.html
                                                                                    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                                                                               Acknowledgments
                                                                                    models.                                                                                                                                     Thanks to Bob for providing sources of information, and
                                                                                     Yet another suggestion is that the hotspot was deflected at the                                                                            everyone in GEO208, and to the nice ladies at the Natural
                                                                                    Chief Joseph dike swarm by the subducting Farallon plate, and                                                                               Resource building for their help.
                                                                                    deflected further by the thick margin of North America.
                                                                                    A further difficulty lies in the fact that it is hard to explain why
                                                Fig. 3. The Weiser, Clearwater,
                                                and St. Maries Embayments.
                                                                                    the same mantle plume that is causing rhyolitic activity at
                                                                                    today’s Yellowstone National Park, spit out a tremendous
                                                                                                                                                                                                                                For further information
                                                                                    amount of basalt across the Columbia Plateau. Apparently,                                                                                   Please contact Sami. riyuki_chan@yahoo.com
                                                                                    some typical plume models go through uplift, which could                                                                                    Or take Bob Filson’s Geo 208 class.
                                                                                    have occurred with the Yellowstone hotspot at that time,                               Fig. 6. An illustration of back-arc
                                                                                    allowing the magma to rise more quickly and have less time                             spreading.
                                                                                    for partial melting.

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