Molasses Mountains

Sunday, September 01, 1996
It’s no surprise that California, home to the boundary between the Pacific and the North American tectonic plates, should experience earthquakes with disturbing frequency as the two plates lurch past each other. But quakes also occur well east of the boundary, as far away as Colorado. Trying to explain those quakes in terms of plate tectonics is a little bit disingenuous, says Craig Jones, a geophysicist at the University of Colorado in Boulder. We are a thousand miles away from the plate boundary, and deformation over such a broad region is directly in contrast with the fundamental premise of plate tectonics--namely, that plates are rigid.

Jones has come up with an explanation for these puzzling quakes, one that doesn’t involve plate tectonics. Earthquakes far from plate boundaries, like those in Colorado, are caused not by the jostling of distant tectonic plates, he says, but by thick, massive portions of the western crust spreading out under its own weight. If you had a mountain of molasses, it would slowly flow away to try to equalize the topography, Jones explains. Since the crust, of course, is not as fluid as molasses, as it spreads, says Jones, parts of it at times stretch to the breaking point and snap, producing an earthquake.

To find evidence for this theory, Jones and his colleagues made use of known data on the travel time of seismic waves through the North American crust. Because seismic waves travel more rapidly through denser crust, geologists can use them to gauge the crust’s thickness and density. Jones’s group then compared that information with satellite measurements of crustal motion in North America. Since satellites can be placed in highly precise orbits, they can be used as a reference to carefully measure even minute changes in the longitude and latitude of any point on Earth’s surface.

As Jones had expected, denser parts of the crust typically coincided with areas undergoing the most crustal movement and seismic activity, although such motions usually amounted to no more than half an inch or so a year. Surprisingly, though, some massive regions that the researchers had expected to be most actively deforming, such as the Rocky Mountains, seemed to be relatively stable.

The Rockies’ stability may result from their position over a relatively cool part of Earth’s mantle. If you had very cold molasses, it would not move very much, says Jones. But if you heated it up, it would flow much more easily. The crust of the Rockies is rather cold, so it is deforming slowly.
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