Intermountain Region

Regional Geology Provinces

Anyone who has experienced the Basin and Range province is familiar with the rollercoaster feel of the area. Alternating mountain ranges and valley floors make this province very distinctive. This province has a long history of being pulled apart by plate tectonic forces, more specifically, extension, that created a series of north-south trending mountain ranges and valleys. Extensional forces uplift mountains and down-drop valley floors along fractures. Movement along these fractures creates faults.

The weathering and erosion of materials from these mountains is carried by wind and water and deposited in the valleys. Some areas of bedrock are covered by thousands of feet of sediments and rocky debris.

If you would like to experience the Basin and Range Province up close and personal, visit the Humboldt-Toiyabe, Payette, Wasatch-Cache, Fishlake, and Dixie National Forests.

Visit the Columbia Plateau and you'll find an intriguing story of recent and extensive lava flows pouring out from the earth's surface over the past 17 million years. Scientists studying the area have noted that the youngest, igneous rocks are present near the Yellowstone Plateau while the oldest igneous rocks lay further away towards the west coast. This evidence suggests the presence of a hot spot or area beneath the earth's crust where the mantle has been stretched thin and plumes of hot magma rise to the surface. In the past, as the North American Plate slid west across the hot spot, the magma rose and erupted creating extensive lava flows. The Yellowstone Plateau is still geologically active with geysers, like the famous Old Faithful in Yellowstone National Park, as well as hot springs, and hot pots.

If you would like to see some of the wonders of the Columbia Plateau Province, visit the Payette National Forest.

Enter the Rocky Mountain System and you'll find yourself entering a tumultuous past of massive plate tectonic forces at work. During one of the last major mountain building events, the Laramide orogeny, that took place 40-70 million years ago, the Rocky Mountains were raised to form a long, discontinuous chain of mountains stretching from Canada to central New Mexico. As the Pacific Plate was subducted beneath the North American Plate, hot magma rose to form a volcanic chain of individually erupting volcanoes as well as massive magma chambers that cooled and hardened below the surface 100 million years ago. Nevertheless, most of this activity was focused 200-400 miles in and the Rockies occur much further inland. Not until 70 million years ago, did intense thrusting and crumpling of sheets of rock really begin to form the Rocky Mountain System.

The most perplexing thing about the formation of the Rocky Mountains is the fact that they formed so far inland from the subduction zone. Scientists still continue to gather evidence and speculate that the angle at which the Pacific Plate was subducted beneath the North American Plate was significantly shallower than most subduction zones which allowed the focus of the melting--and thus the mountain formation--to occur further inland.

If you would like to explore the Rocky Mountain System Province, visit the Ashley, Wasatch-Cache, Bridger-Teton, Caribou-Targhee, Salmon-Challis, Boise, and Sawtooth National Forests.

Explore the Colorado Plateau and you'll see quite a contrast from the Basin and Range Province and yet, at one time, their stories were very similar. During the Miocene epoch, about 20 million years ago, something happened to cause three kilometers of uplift in both regions. The Basin and Range province reacted to the stretching by fracturing and forming valleys that down-dropped and mountains that rose. Alternatively, the Colorado Plateau remained one large tectonic block that continued to rise another kilometer above the Basin and Range.

Eons of time are encompassed in the beautiful Colorado Plateau rocks telling stories of warm, shallow seas and drifting sand dunes. Delicately carved layers of limestone, siltstone, sandstone, and shale color this country in reds, purples, and grays. Deep canyons carved by river systems expose millions of years of rich, geologic history.

If you would like to see the Colorado Plateau Province first-hand, visit the Ashley, Manti-LaSal, Fishlake, and Dixie National Forests.

These pages were developed by Debbie Morgan, a 2006 GeoCorps America participant,  in partnership with the U.S. Forest Service and the Geological Society of America's GeoCorps America Program. The GeoCorps America Program strives to place geoscientists in working relationships with the U.S. Forest Service, National Park Service, and Bureau of Land Management.

Comments and suggestions regarding these pages may be directed to J. Gurrieri, regional geologist.