Columbia River Basalts & Yellowstone Hot
A Mantle Plume?
Geology, Washington State University, Pullman, WA
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According to what Marcia McNutt labels “the
standard model”, the Yellowstone hot spot first
manifested itself beneath what is now south-eastern
Oregon at 16.6 Ma. The hot spot was then over-ridden
by the North American plate travelling in a westerly
direction, leaving its trace in sequential volcanic
outbursts across southern Idaho, along the eastern
Snake River Plain (ESRP) to reach its present position
beneath the Yellowstone caldera. The track of the
hot spot, or rather the track of the North American
plate across the stationary hot spot in the mantle,
is based on independently derived plate motions (Engebretson
et al., 1985; Pierce & Morgan, 1992).
There are four immediately apparent problems with
this model as many workers have pointed out. First,
the eastern Snake River Plain reflects a much more
ancient tectonic boundary and it seems coincidental
that the hot spot track should follow this structural
discontinuity so exactly. Second,
that while magmatism can be traced sequentially along
this track from southern Oregon to Yellowstone Park,
that magmatism is primarily silicic, not the basaltic
magmatism expected from a mantle plume. Third,
the huge outpourings of tholeiitic magma of the lower
Steens Mountain and the Columbia River Basalt
Group (CRBG) that are regarded as representing
the original plume head, initially move north rather
than east with time, with the great majority of the
CRBG erupting in northeast Oregon and southeast Washington,
rather than in southeast Oregon; that is, the main
flood basalt eruptions occur well off the main track
of the supposed plume. Fourth,
there is the problem of apparently mirror-image, primarily
silicic volcanism along the Brothers Fault Zone to
To a degree some of these problems may be explained
by the exact position of eruption and the type of
magmatism being controlled by the tectonic regime
in the overlying lithosphere (Thompson & Gibson,
1991; Geist & Richards, 1993; Camp,
1995; Hooper et al., 2002).
So what is the evidence in support
of a mantle plume as the standard model for the origin
of the CRBG flood basalts?
First, there is the exceptionally
large volume of tholeiitic magma of the Steens-CRBG
province erupted in an extremely short time. If
we accept the recent evidence that the lower Steens
basalt is the first part of the whole CRBG flood
basalt event (Hooper et al., 2002; Camp
et al., 2003) then this series of eruptions
produced 234,000 km3 of tholeiitic basalt,
almost all erupted between 16.6 and 15.0 Ma (Camp
et al., 2003). This exceptional volume and
eruption rate is reflected in other continental
flood basalt provinces. The Deccan eruptions created
approximately ten times the volume seen on the Columbia
Plateau in less than one million years (Hofmann
et al., 2000). The Parana, Karoo and Siberian
Traps erupted equivalent volumes of tholeiitic magma
to the Deccan in similarly short periods of time.
How much less than 1 m.y. will remain unclear until
isotopic age-dating techniques get still more precise.
Such huge volumes and high eruption rates are unique
to continental flood basalt provinces. As such they
appear to require a unique tectonic/magmatic event.
A mantle plume appears to fit that requirement.
From the prospective of flood basalt provinces,
then, no other model appears to provide for the
unique volumes and eruption rates of these large
Second, these huge eruptions
frequently can be shown to occur at the beginning
of a long trail of lesser eruptions which end at
a currently active volcanic center. The Deccan is
by far the best example of this correlation with
the traditional hot spot/plume model, but the CRBG
fits adequately with the Yellowstone hot spot model,
albeit with the problems and plausible solutions
noted above. The Parana flood basalt province also
seems to conform. On the other hand evidence of
such hot spot tracks is much less clear for the
Karoo and the Siberian Traps.
The third line of evidence in
support of the mantle plume - hot spot model for
the origin of continental flood basalt provinces
lies in the composition of the magmas. Here, again,
the evidence is not unambiguous and certainly does
not prove the existence of mantle plumes. But it
does fit the mantle plume model and can be used
to provide appropriate explanations for the lack
of primitive upper mantle magmas. As with the Deccan,
some of the earlier CRBG eruptions contain those
elevated helium isotope ratios that are equated
with an origin deep in the mantle (Dodson et
al., 1997). Assertions that such helium isotope
ratios can be formed elsewhere requires substantiation
beyond rhetoric or the circular argument that, because
Yellowstone is not a product of a hot spot then
elevated helium isotope ratios must be produced
in the upper mantle. Are there, for example, clear
examples of tholeiitic volcanic rocks not plausibly
connected with hot spots which carry such elevated
helium isotope ratios?
The CRBG lacks any primary magma that could have
been in equilibrium with a normal peridoditic upper
mantle and is dominated by relatively low Mg tholeiites
and basaltic andesites, typified by the Grande Ronde
Basalt Formation with 52-58% SiO2 and an
Mg number (Mg/Mg+Fe+2) less than 55. Attempts
to explain this by gabbro fractionation fail because
the incompatible elements are no more evolved in the
Grande Ronde basalts than those of the more mafic
Imnaha Basalt flows and there is no evidence of large
volumes of gabbro cumulate at depth below the main
feeder dikes. Virtually all CRBG magmas have undergone
gabbro fractionation within the crust, probably in
large reservoirs at depths close to the mantle/crust
boundary, but fractionation appears to have started
from magma in equilibrium with a source composition
more evolved and more iron-rich than typical upper
mantle. A more iron-rich source has long been advocated
by Tom Wright and his co-workers and supported by
experimental evidence (Wright et al., 1988;
Takahahashi et al., 1998; Yaxley,
2000). An eclogite-bearing mantle plume source derived
from subducted ocean basalts recycled through the
deep mantle appears to satisfy this requirement (Cordrey
et al., 1997) and to satisfy the trace element
These appear to be the principal reasons why virtually
all workers on flood basalt provinces subscribe to
the mantle plume model. Other, such as plate margin,
models do not appear capable of accounting for either
the huge volumes of magma, the high magma eruption
rates, nor the restricted eruptive centers required
of flood basalt provinces. Nor do these models account
for their compositional peculiarities. In addition
many CFBs can be shown to lie at the starting end
of typical hot spot tracks which would surely be coincidental
in any plate margin model.
I am not persuaded
by the arguments outlined by Christiansen et al.
(2002) that their five geologic constraints are
inconsistent with the mantle plume model or that their
own or any other model explains these geologic factors
any better. I do concede that the supposed track of
the hot spot coincides with an older major tectonic
structure: this is a coincidence that has long troubled
me. Yet that it is a coincidence cannot be denied: the
entirely independently derived reconstruction of the
plate motions determined by Engebretson et al.
(1985) places the hot spot beneath southeast Oregon
below the massive lower Steens Basalt eruption at 16.5
Ma. We now know that the Steens Basalt was the first
episode in the CRBG sequence of flood basalt eruptions.
Given the older structure along the eastern Snake River
Plain (ESRP), this is a coincidence whatever model is
adopted to explain the magmatism. The Brothers Fault
Zone, which is a mirror image of the ESRP, is also an
older structure, almost certainly involving originally
right-lateral motion separating an area of greater east-west
extension to the south from an area of less extension
to the north (Lawrence, 1976; Hooper &
Conrey, 1989; Hooper et al., in press).
Given that the other parallel WNW-ESE trending fault
zones recognised by Lawrence are not followed by silicic
or bimodal magmatism, it seems plausible that the magmatism
along this older structure was due to the expanding
mantle plume beneath SE Oregon, as suggested by others.
The argument that CRBG volcanism is caused by the
Basin & Range extension makes no sense from the
view of the Columbia River basalt eruption. First,
it is surely now widely accepted that east-west extension
has been going on from the Eocene to the present (of
many papers one could quote, there is the work of
Gains and others to the south and Hawkesworth
et al., 1995, and many to the north including,
for example, Janecke, 1992; Hooper et
al., 1995; Morris & Hooper, 1997,
Morris et al., 2000 and, the most recent,
Breitsprecher et al., 2003). Certainly from
north of the Canadian border to Nevada Eocene extension
is well established. The Pasco Basin in central Washington
State, for example, is a NS rift which has been actively
extending from before the Eocene, throughout the Miocene
CRBG eruptions, and apparently continues developing
The magmatism associated with Eocene and subsequent
east-west extension from British Columbia to Nevada
is of small volume and of calc-alkaline to alkaline
affinity. This volcanic activity is physically and
chemically distinct from the huge burst of tholeiitic
activity that was superimposed on the extension magmatism
for a brief period in the Miocene (16.6 to 15.3 Ma
in east central Oregon). The details of this relationship
have been recently documented and need not be repeated
here (Hooper et al., 2002, Camp et al.,
2003, and references therein). It is logical that
the superimposition of the hot spot, plume or otherwise,
on the stretching lithosphere in the eastern Oregon
area, would have softened the lithosphere and accelerated
extension immediately following the CRBG-Steens eruption.
Thus, although significant extension and associated
volcanism had preceded the Steens eruptions, the large
and conspicuous Oregon-Idaho graben lying between
the Oregon-Idaho border and Steens Mountain began
to form immediately after the hot spot magmatism ceased.
An equivalent example of rapid extension following
the main CFB eruptions is also well documented on
the western side of the Deccan (Hooper, 1990;
Sethna, 2003). Thus, the most recent field
and geochemical studies in eastern Oregon do not support
the argument that the Yellowstone magmatism coincidentally
began as the extension started. Again, to view the
concept of lithospheric structures controlling the
funnelling of hot spot generated magmas to the surface
as a “difficult to explain coincidence”
seems unrealistic. Rather it might be viewed as inevitable
that such structural control would play a part in
the final eruptions on to the surface. In brief I
find neither the geologic constraints nor the 3He/4He
arguments against a mantle plume model at all convincing.
The bottom line to this contributor
is the huge volume, exceptional rate of eruption, and
restricted area of continental flood basalt eruptions
in general and the CRBG in particular. None of the non-plume
models appear to recognize, far less explain, these
very basic facts. Until they do flood basalt workers
are likely to retain their belief in a mantle plume
model as the best explanation available for the origin
of continental flood basalts, despite the model’s
Having said all that, this worker would
be the first to concede that the mantle plume model
is poorly constrained and in need of more rigorous investigations.
Current programs with this in mind are to be greatly
welcomed. The evidence against the mantle plume model
appears to be almost entirely geophysical, not geological.
As a basalt petrologist I can have no adequate answer
to the geophysical problems, but cannot but note that
such techniques as seismic tomography are relatively
new and, as we have so often found of exciting new techniques
in the past, may not yet be fully understood. To that
extent and to a non geophysicist its results appear
susceptible to misinterpretation.
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confirmation of the Kula-Farallon slab window beneath
the Pacific Northwest in Eocene time. Geology,
Camp,V.E., 1995, Mid-Miocene
propagation of the Yellowstone mantle plume head
beneath the Columbia River basalt source region.
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Camp, V.E., Ross, M.E., Hanson,
W.E., 2003, Genesis of flood basalts and Basin and
Range volcanic rocks from Steens Mountain to the
Malheur River Gorge, Oregon. GSA Bull.,
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DePaolo, D.J., 1997, Helium and neon isotopes in
the Imnaha Basalt, Columbia River Basalt group:
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last updated 24th April, 2003