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Geologic Provinces of the United States: Rocky Mountains

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Rocky Mountain System

The Rockies form a majestic mountain barrier that stretches from Canada through central New Mexico. Although formidable, a look at the topography reveals a discontinuous series of mountain ranges with distinct geological origins.

List of National Parks exhibiting the Rocky Mountain Province

The Rocky Mountains took shape during a period of intense plate tectonic activity that formed much of the rugged landscape of the western United States. Three major mountain-building episodes reshaped the west from about 170 to 40 million years ago (Jurassic to Cenozoic Periods). The last mountain building event, the Laramide orogeny, (about 70-40 million years ago) the last of the three episodes, is responsible for raising the Rocky Mountains.

Northern Teton Range and Jackson Lake
Northern Teton Range and Jackson Lake. Mt. Moran on left. Photo by Marli Miller.

Setting the stage

During the last half of the Mesozoic Era, the Age of the Dinosaurs, much of today's California, Oregon, and Washington were added to North America. western North America suffered the effects of repeated collision as slabs of ocean crust sank beneath the continental edge. Slivers of continental crust, carried along by subducting ocean plates, were swept into the subduction zone and scraped onto North America's edge.

About 200-300 miles inland, magma generated above the subducting slab rose into the North American continental crust. Great arc-shaped volcanic mountain ranges grew as lava and ash spewed out of dozens of individual volcanoes. Beneath the surface, great masses of molten rock were injected and hardened in place.

For 100 million years the effects of plate collisions were focused very near the edge of the North American plate boundary, far to the west of the Rocky Mountain region. It was not until 70 million years ago that these effects began to reach the Rockies.

Raising the Rockies

The growth of the Rocky Mountains has been one of the most perplexing of geologic puzzles. Normally, mountain building is focused between 200 to 400 miles inland from a subduction zone boundary, yet the Rockies are hundreds of miles farther inland. What geologic processes raise mountains at this scale? Although geologists continue to gather evidence to explain the rise of the Rockies, the answer most likely lies with an unusual subducting slab.

oceanic vs. continental plate subduction zone
Sketch of an oceanic plate subducting beneath a continental plate at a collisional plate boundary. The oceanic plate typically sinks at a fairly high angle (somewhat exaggerated here). A volcanic arc grows above the subducting plate.
oceanic vs. continental plate subduction zone
This sketch shows the plate tectonic setting during the growth of the Rocky Mountains (Laramide orogeny). The angle of the subducting plate is significantly flatter, moving the focus of melting and mountain building much farther inland than is normally expected.

At a 'typical' subduction zone, an oceanic plate typically sinks at a fairly high angle (see above). A volcanic arc grows above the subducting plate. During the growth of the Rocky Mountains, the angle of the subducting plate may have been significantly flattened, moving the focus of melting and mountain building much farther inland than is normally expected.

It is postulated that the shallow angle of the subducting plate greatly increased the friction and other interactions with the thick continental mass above it. Tremendous thrusts piled sheets of crust on top of each other, building the extraordinarily broad, high Rocky Mountain range.

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