The Rocky Mountains, which stretch along the western part of North America, are one of the longest mountain chains on Earth. Although geologists pretty well understand how the southern and northern portions formed, the central Rockies, from Wyoming to New Mexico, have always been an enigma. Now, scientists have a possible answer to the long-standing riddle—an answer that may also shed light on Colorado's rich silver, gold, and other mineral deposits.
Most of the Rockies began forming between 66 million and 75 million years ago. The northern and southern portions of the mountain range arose within 200 kilometers of the continent's western coast through the same largely volcanic processes taking place along today's Cascade Range, which runs from southern British Columbia to northern California.
But the central Rockies formed almost 1000 kilometers inland, says geologist Craig Jones of the University of Colorado, Boulder. There's no modern analog for such mountain building, he notes. Although today's Himalayas are forming far inland, those mountains result from the Indian subcontinent crashing into southern Asia. There was no such landmass crashing into North America's western shores when the Rockies began forming, Jones says. Instead, a dense slab of crust moving eastward beneath the Pacific was subducting, or being forced beneath, the lighter North American tectonic plate.
Previously, various teams of researchers proposed that the slab subducting beneath North America floated back from the mantle toward Earth's surface. As it jostled along the lower surface of the crust on its journey eastward, it would have scraped away large amounts of material that had solidified on the crust. But that explanation doesn't fit the geological evidence unearthed in recent years, says Jones. Magma brought to the surface by subsequent volcanic activity in California and Arizona contains bits of the material that, according to this hypothesis, should have been scraped away by the tectonic slab. Because that material remained in place after the slab had supposedly passed, this hypothesis is wrong, Jones says.
Now, in the February issue of Geosphere, Jones and his colleagues set forth a new notion: As the subducting slab slipped beneath a chunk of North American crust called the Wyoming craton, friction carried the flow of the underlying mantle  past a downward bulge in the craton, causing that bit of crust to be sucked downward. Stresses induced in the crust by that flow, added to the compressive stresses caused there by distant collisions of tectonic plates, were large enough to fracture the crust, the team's analyses suggest. That weak spot then served as the focal point for subsequent mountain building, says Jones: "Once the process was started, it was easy for tectonic forces to keep it going," he notes.
The new hypothesis "isn't straightforward, but it's reasonably plausible and it's consistent with what we know about the geological record," says geologist Basil Tikoff of the University of Wisconsin, Madison. "It's the best explanation we have."
Besides elucidating how and why the central Rockies formed where they did, the new model may help explain the presence of a broad swath of gold, silver, and other mineral deposits stretching across Colorado. The large volumes of molten rock streaming to Earth's surface across a narrow band of territory focused the minerals and created these lucrative deposits, Jones and his colleagues contend.