Rocky Mountains Formed From Squishy Layers Like a Peanut Butter Sandwich

The Rocky Mountains are beautiful, sure, but to geologists they have also been a bit frustrating—because they aren't located where mountains are supposed to be. Now, one team of geologists think they have the explanation, and it relies on the fact that different layers of the crust have different levels of squishiness.

Consider a sandwich's basic ingredients: bread, peanut butter and jelly all behave slightly differently when they are pushed around because of their inherent physical properties. The same basic idea holds true for Earth, too. "What we've done is mapped out where it's a peanut butter sandwich and where it's a jelly sandwich," Derek Schutt, a geologist at Colorado State University, told the Coloradoan. Schutt and colleagues presented their argument in a study published recently in the journal Geology.

Most mountains are found between about 200 and 400 miles from the edge of a tectonic plate, one of the slabs that makes up Earth's crust and generally carry one landmass apiece. Mountains usually form near the edges, as different plates bang against each other and crumple up.

Even the Himalayas, which seem like inland mountains, come at the border of two plates since the Indian subcontinent is a separate hunk of crust. But that's where the 70-million-year-old Rockies defy expectations—they're more like 600 miles away from the coast, and there are no plate edges in sight.

That probably has something to do with the Pacific plate being sucked under North America at a more shallow angle than usual, but the details are still difficult to sort out.

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Scientists have previously suggested that perhaps something caused the edge of the dense ocean plate to start rising again far inland, pushing up against the crust and raising the mountains above it. A more recent theory holds that a dense slab of rock got stranded beneath Wyoming and blocked the traffic of the smoothly moving plates, eventually forcing rock upward.

The new paper doesn't try to solve the dilemma, but by measuring the rock characteristics across the Western U.S.—and assessing whether layers are more like the bread, the peanut butter or the jelly—starts geologists on the path to doing so. Those characteristics included how quickly seismic waves travel through the rock and how viscous it is, or how prone it is to flowing. "It really gets at some fundamental questions about why plates deform and what processes are important," Schutt told the Coloradoan.