The Birth of the Rockies

A new theory might solve the mystery of how these mountains formed.

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If you want to see a geologist squirm, ask him how the central Rocky Mountains were created. The truth is that, until now, “it’s been an enigma,” says Craig Jones, a research fellow of the Cooperative Institute for Research in Environmental Sciences (CIRES) and a professor of geological sciences at the University of Colorado at Boulder. Mountains generally arise where continental plates collide into each other, but the central Rockies (from Wyoming to New Mexico) are about 600 miles inland from the nearest plate boundary, which runs along the West Coast. Although the Himalayas are sited similarly far inland, they have the Indian plate crashing into the Eurasian plate to push them up. So geologists have long been puzzled by which forces raised the Rockies.

The conventional theory suggests that about 200 million years ago, an oceanic plate to the west (the Farrallon plate) started sliding under the North American tectonic plate at a shallow angle, “grating along underneath,” Jones says. “The forces added up as it went, concentrating at the farthest point”—and pushing up the Rockies by about 70 million years ago. This is kind of like sliding your hand under a tablecloth and seeing rumples form at the far end. By about 70 million years ago, this force pushed up the Rockies. However, several problems exist with this model. First, if the oceanic plate were scraping underneath the continental plate, you would expect the former to have scoured the latter plate’s underbelly along the way, shearing away certain ancient rocks. But you can still find those rocks in places like California and Arizona.

This hypothesis also doesn’t explain where Colorado’s broad belt of gold, silver, and other minerals originated from, or how a marine basin developed in Colorado and Wyoming before the Rockies formed. Overtime, sediments deposited in the basin, creating a thick layer of dark-gray shale—called the Pierre Shale—along the Front Range.“This is the stuff that causes people’s houses to bow up because the shale swells when wet,” Jones says.

Now, Jones and his team have proposed a new model, published in the February issue of Geosphere, that may unravel the mystery. Their theory centers on the fact that under Wyoming, the North American plate is thicker than elsewhere, protruding like the keel of a boat into the more fluid part of the mantle underneath. As the oceanic plate subducted (slid under it), the fluid layer flowed into the space between the two plates. As it tried to flow back out, it got cut off under the keel, creating a strong suction force. The suction flexed the crust downward and formed a basin in Colorado and southern Wyoming.

In a counterintuitive fashion, this depression then gave rise to the Rockies: The suction weakened the crust, so when the plate pressed in from the side—adding compressive forces to the mix—the force was great enough to fracture the crust and push up mountains. Jones uses an analogy of a sheet of molasses to explain it: “If you spooned out part [of the molasses], making a basin, the surrounding molasses would flow in, thickening the molasses in the hole,” he says. “Such thickening in the Earth causes thrust faults [where ground on one side of the fault moves up and over adjacent ground] like those that pushed up the Front Range or Wind Rivers.”

The circulation of the more-fluid mantle around the keel also drew up magma from below—in a line parallel to the northeasterly movement of the oceanic plate—creating the belt of gold, silver, and other precious metals we see today.

The next step, says Jones, is to test the predications made by the model. Meanwhile, the next time you’re hiking in the central Rocky Mountains, consider this: The lofty spires towering above you might have emerged from a giant hole.