West's landscape may be flowing like molasses
May 17, 1996
The changing topography of the western United States over the eons may be caused by Earth's crust and underlying mantle deforming under its
own weight like a mound of molasses rather than tectonic plates yanking the
landscape around, according to a new study.
University of Colorado at Boulder geologist Craig Jones said
sound-wave measurements of the 100-mile thick crust and upper mantle
indicate the topography of the West has behaved like a "viscous fluid" over
geologic time. The surprising gravitational force inside the continent
appears to be causing the landscape to "spread" in places, causing
significant deformation in areas like the Great Basin of Nevada and western
Utah and the Sierra Nevada in eastern California.
The western United States may be the single best example of an
area that has deformed much differently than one would expect from
traditional plate tectonics," said Jones. "We think we have finally
succeeded in measuring the forces that are active on the continental crust
in this region."
A cover article on the subject by Jones, Jeffrey Unruh of William
Lettis and Associates of Walnut Creek, Calif., and Leslie Sonder of
Dartmouth College appeared in the May 2 issue of Nature. Jones is a
research associate at the Cooperative Institute for Research in
Environmental Sciences, a joint program of CU-Boulder and the National
Oceanic and Atmospheric Administration.
The researchers used data obtained by measuring the velocity of
sound waves bounced into Earth's crust -- the top layer of surface ranging
from 15 to 30 miles deep -- which allowed them to estimate the crust's
density and thickness. They then were able to infer the weight of the
crust and upper mantle -- known collectively as the lithosphere -- to a
depth of about 100 miles.
The study indicates parts of the Western landscape are being
subjected to high amounts of a force known as "gravitational potential
energy" that deforms the landscape under its own weight, Jones said. The relatively large number of earthquakes in the Great Basin are probably the result of a relatively
thinner and weaker crust being "leveled out" by the gravitational potential
energy, he said.
Surprisingly, some of the highest amounts of gravitational
potential energy were measured in Colorado's Rio Grande Valley from
Leadville south to the San Luis Valley, he said. A large "fault scarp"
visible at Colorado's Great Sand Dunes National Monument in the Rio Grande
Valley indicates a large earthquake with a magnitude greater than 7 may
have struck the area from 10,000 to 14,000 years ago.
Although the valley has been seismically quiet in recent centuries,
"the forces available to drive deformation appear to be quite large there,"
said Jones. "It looks like gravitational potential energy is trying to
drive southern Colorado apart. But we need to better understand the
strength of the lithosphere in that region."
The lithosphere's strength appears to be dependent on both its
thickness and temperature, said Jones. The crust, which is heated by
reactions in Earth's deep interior, appears to be warmer in the Basin and
Range region of Nevada than in Colorado's Rio Grande Valley and therefore
more susceptible to earthquakes.
"The situation is analogous to heating up a frozen chunk of
molasses," he said. "If the crust is colder and stronger, it is better
able to resist deformation."
Although the study provides evidence that deformation in the West and perhaps other continental regions are driven by topography, most
scientists still believe deformation beneath the oceans is caused primarily
by lateral movements of the huge, rigid plates beneath the continents and
sea floor, he said.
Researchers believe the continental crust floats on the mantle like
an iceberg floating on water, said Jones. The crust and upper mantle
appear to be more "buoyant" in some areas than others, "essentially holding
up or pulling down the landscape," he said.
The mantle looks to be a much more complex player in all of this than we previously thought," Jones said. "But I think this study helps us
to better understand the topography we have in the West, and to understand
why we see deformation where we do."
Many of the sound-wave measurements of Earth's crust used for the Nature study had previously been made by other research groups conducting
related research, said Jones. "We are the beneficiaries of a lot of hard
work by other people."
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