Travel Reference
In-Depth Information
lustrates the theory that the
Yellowstone hot spot
is caused by a mantle plume. Mantle plumes
are thought to be formed very deep, perhaps 1,800 miles (2,900 km), under Earth's surface, in
locations where there is hot rock. Because hot rock is more buoyant than cold rock, it rises
like a column to the surface. A very distorted column containing a small percentage of melted
rock extends down 310 miles (500 km), for which there is scientific evidence. This would be
a very shallow plume. The dashed lines at the right in part
B
are drawn to 600 miles (1,000
km) deep, where there may be some evidence of melted rock, and the dotted lines are shown
to 1,800 miles (2,900 km), where there is no evidence of a plume so far. Whether there is a
shallow or a deep plume, it would have to punch through the descending subduction plate.
The distorted column may be evidence of this collision.
C
illustrates another way that the Yel-
lowstone hot spot could form. The shallowly dipping plate has broken off from a now steeply
descending plate to the southwest—and is foundering in a not quite horizontal position to the
northeast. Magma is being generated at the older plate's leading eastern edge, which is just
under Yellowstone, while the Cascade Range is forming to the west. There is evidence for the
positions of the subducting plates. Research is continuing.
—J.S.
The Rocky Mountains
Starting around 110 million years ago and lasting until about 40 million years ago, today's
Rocky Mountains were formed. First, thick layers of sedimentary rock were pushed up and
over one another, moving from west to east. Next, great blocks of granite and gneiss with sed-
imentary rock on top were also squeezed and pushed up. Most of this movement and moun-
tain building took place outside the park. Almost entirely north and east of the park, the
Beartooth Mountains arose about 60 million years ago, and the Absaroka volcanics (locations
shown on the map on
page 159
) poured forth massive amounts of
lava
about 45 million years
ago.
he Absarokas
During plate collisions, one plate sinks below another deeply into the earth (called subduc-
tion). This drags seafloor sediments and other near-surface rocks down to great depths, where
they reach high temperatures and pressures. There they melt, forming magma. Since magma
is less dense than the solid rock, it rises toward the surface. When magma reaches the sur-
face, it forms a chain of volcanoes, such as the Absarokas. The Absaroka Range, which extends
from Montana north of the park for approximately 100 miles (160 km) to the southeast into
Wyoming, was formed about 45 million years ago when an oceanic plate from the west dove
under the North American Plate. Most of the rocks in these mountains are either lava flows
or the debris that resulted when the rocks were rapidly eroded to form mudslides. The out-
standing example of these processes in the park is Mount Washburn
[GEO.4]
. he Northeast
