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Rocky Mountain System

clickable province index map Atlantic Coastal Plain Pacific Mountains Colorado Plateau Ozark/Ouachita Interior Highlands Appalachian Highlands Laurentian Upland Columbia Plateau Interior Plains Basin and Range Rocky Mountains

spacer image 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.

Northern Rocky Mountains

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

Setting the stage

spacer image 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.
spacer image 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.
spacer image 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

spacer image 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.
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. oceanic vs. continental plate subduction zone
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Middle Rocky Mountains

spacer image "Complex mountains with many intermontane basins and plains"
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Southern Rocky Mountains

spacer image "Complex mountains rising to over 14,000 feet"
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Wyoming Basin

spacer image "Elevated plains and plateaus on sedimentary strata"
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-Northern Rocky Mountains
-Middle Rocky Mountains
-Wyoming Basin
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This page was last updated on 10/10/00