Geology Reference
In-Depth Information
The enormous batholiths in Idaho, British Columbia, Canada,
and the Sierra Nevada of California were emplaced during
the Mesozoic (see Chapter 22), but intrusive activity con-
tinued into the Paleogene Period. Numerous small plutons
formed, including copper- and molybdenum-bearing stocks
in Utah, Nevada, Arizona, and New Mexico.
Volcanism was more or less continuous in the Cordil-
lera, but it varied in location, intensity, and eruptive style,
and it ceased temporarily in the area of the Laramide orogen
(Figure 23.5b). In the Pacifi c Northwest, the Columbia Pla-
teau (Figure 23.6) is underlain by 200,000 km
3
of Miocene
lava flows called the Columbia River basalts that have an
aggregate thickness of about 2500 m. These vast lava fl ows
are well exposed in the walls of the canyons eroded by the
Columbia and Grand Ronde rivers (
These rocks are youngest in the southwest part of the area and
become older toward the northeast, leading some to propose that
North America has migrated over a mantle plume that now lies
beneath Yellowstone National Park in Wyoming. Other geologists
disagree, thinking that these volcanic rocks erupted along an in-
tracontinental rift zone.
Bordering the Snake River Plain on the northeast is the
Yellowstone Plateau (Figure 23.6), an area of Pliocene and
Pleistocene volcanism. Perhaps a mantle plume lies beneath
the area, as just noted, that accounts for the ongoing hydro-
thermal activity there, but the heat may come from an in-
truded body of magma that has not yet completely cooled.
Some of the most majestic and highest mountains in the
Cordillera are in the
Cascade Range
of northern California,
Oregon, Washington, and southern British Columbia, Can-
ada (see Geo-inSight on pages 650 and 651). Thousands of
volcanic vents are present, the most impressive of which are
the dozen or so large composite volcanoes and Lassen Peak
in California, the world's largest lava dome. Volcanism in this
region is related to subduction of the Juan de Fuca plate
beneath North America. Volcanism in the Cascade Range
goes back at least to the Oligocene, but the most recent epi-
sode began during the Late Miocene or Early Pliocene.
Figure 23.6b). The rela-
tionship of this huge outpouring of lava to plate tectonics
remains unclear, but some geologists think that it resulted
from a mantle plume beneath western North America.
The Snake River Plain (Figure 23.6), which is mostly in Idaho,
is actually a depression in the crust that was filled by Miocene
and younger rhyolite, volcanic ash, and basalt (Figure 23.6a, c).
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◗
Figure 23.6
Cenozoic Volcanism
Columbia
Plateau
About 20 lava fl ows of Columbia River basalts are
exposed in the canyon of the Grand Ronde River in
Washington.
b
Basalt lava fl ows of the Snake River Plain at Malad
Gorge State Park, Idaho.
c
Distribution of Cenozoic volcanic rocks in the western United States.
a
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