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, Jones 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, Jones said.
Researchers believe the continental crust floats on the mantle like an iceberg floating on water. The crust and upper mantle appear to be more “buoyant” in some areas than others, “essentially holding up or pulling down the landscape.”
“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.”