The Rockies are fifteen hundred kilometers, or one thousand miles, to the east. The cause must be the tectonic plate that built these mountains. Its name is Farallon. Farallon started off normally enough. It plunged beneath the North American Plate at a forty-five degree angle. This process sprouted volcanoes to form the Sierra Nevada in what is now California. Next, mantle motions pulled North America westward over Farallon, and the plate scraped along the bottom of the continent - for fifteen hundred kilometers. As North America continued its westward trek, Farallon settled to the bottom of the mantle. The image is output from a model run using the TERRA mantle software. To read more about Hans-Peter Bunge's work of that time (2000), visit the following story: http://ct.gsfc.nasa.gov/insights/vol14/story1.htm. To learn more about Bunge's more recent work, visit his web site at: http://www.geophysik.uni-muenchen.de/Members/bunge.