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
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. firstname.lastname@example.org
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.