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Columbia River Flood Basalts
Sami Butler — GEO 208, Green River Community College
Origins
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
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 Backarc spreading theory.
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
Fig. 5. Saddle Mountain basalt (Elephant
Mountain member) east of Ice Harbor.
The Back-arc Spreading Theory
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. 2. the extent of the Columbia
River Basalt Group.
Fig. 3. The Weiser, Clearwater,
and St. Maries Embayments.
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.
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.
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.
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/cas
cade%20Episode.html
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. [email protected]
Or take Bob Filson’s Geo 208 class.
Fig. 6. An illustration of back-arc
spreading.