Geologic History of the Western U.S.

Cenozoic (66 million years ago to present)

The geography of the Western States became progressively more recognizable during the Cenozoic.  Around 66 million years ago, the entire west coast of North America was a convergent boundary, where subduction created volcanic mountain ranges. Accretion of terranes also continued. 

All of the earliest volcanic mountain ranges have ceased to erupt, often being replaced by newer ranges farther westward. As more land accreted to the edge of the continent, subduction became increasingly more difficult, until it finally stopped and a new subduction zone was formed farther seaward. When this happened, the source of magma for the old volcanic arcs was cut off, and a new arc formed closer to the new boundary. This accretion caused the western edge of North America to expand further westward towards its present configuration.

Southern and Central California and Basin and Range

San Andreas Fault

A major change in the plate boundary along California also contributed to the extinction of the older Cenozoic volcanic arcs. Beginning about 30 million years ago, a mid-ocean ridge called the East Pacific Rise collided with the North American continent. As this ridge subducted, the plate boundary adjacent to California gradually changed, becoming a transform or sideways-moving boundary. As a result, subduction ceased, as did the last of the volcanic activity in what we now know as the Sierra Nevada in eastern California and western Nevada. 

Basin and Range

By the Neogene, the Farallon plate lay shallowly under the North American plate for hundreds of kilometers eastward of the West Coast. Now situated more fully beneath what are now parts of the south-central, southwestern, western, and northwest-central states, this extra layer of crust caused uplift and extension of the region, as the added thickness of buoyant rock (relative to the mantle) caused the entire area to rise isostatically. The Farallon plate was subjected to increasing temperatures as it subducted, causing it to expand. As heat dissipated to the overlying North American plate, that rock expanded as well. Finally, the high temperatures in the upper mantle caused the Farallon plate to melt, and the resulting magma was injected into the North American plate, destabilizing it.

These processes, along with the complex crustal movements taking place along the San Andreas fault, caused the surface of the North American plate to pull apart and fault into the north-south trending mountainous blocks and valleys of the huge Basin and Range province. This province covers part of eastern California, most of Nevada, and southern Oregon, as well as parts of other states stretching from Idaho and Utah into Arizona, New Mexico, and Texas.

Sierra Nevada mountains

Much of the present-day Sierra Nevada is a large exposure of granite called a batholith. A batholith is the remains of the magma chambers that previously fueled volcanic mountain ranges that are exposed once the ancestral ranges have been eroded away. In the case of the Sierra Nevada, the stretching of the Basin and Range uplifted the solidified magma chambers of the ancestral range as rivers, landslides, and glaciers eroded away the overlying rock. The continued erosion of this rock is the source of much of the gold-laden gravel found in the rivers of California’s Central Valley.

Northern California, Oregon, and Washington

Cascadia subduction zone

Like the mountain ranges further south in California, the mountain ranges found from southern British Columbia, Canada, to northern California reflect the standard features of a subduction zone. The Coast Ranges and the Olympic Mountains are part of an accretionary wedge; the Willamette Valley and Puget Sound are parts of a forearc basin; and the Cascades are inland volcanoes.

In the early Cenozoic, the Farallon plate was being subducted beneath the North American plate in this region. As this plate continued to subduct, in broke into smaller plates, which were each given their own names. Today, the plate being subducted under the North American plate in the region spanning southern British Columbia to northern California is called the Juan de Fuca plate (sometimes itself considered three small plates: the Explorer, Juan de Fuca, and Gorda plates).

In the Paleogene, a terrane called Siletzia docked on the edge of the North American continent, creating southern Vancouver Island as well as the western parts of Oregon and Washington. The present-day Cascade Range is made up of a series of volcanoes that have built up a large platform of volcanic debris. These volcanoes, the Cascade Volcanic Arc, began to arise 36 million years ago (in the late Eocene). However, the major volcanic peaks that make up the High Cascades formed more recently, within the Pleistocene.

Active subduction still occurs off the west coast of northern California, Oregon, Washington, and southern British Columbia. As long as subduction continues, the Cascades will remain volcanically active; there is evidence that the rate of subduction is slowing, and, as a result, volcanism in the Cascades will eventually cease. 

Yellowstone hotspot and Columbia River Basalts

During the Miocene (about 17 to 14 million years ago), the Columbia River Basalts erupted. These massive basalt flows may have been caused by the formation of the Yellowstone hotspot beneath the North American Plate. They blanketed parts of southeastern Washington, northern and eastern Oregon, western Idaho, and a bit of northern Nevada in thick layers of basalt. Volcanics more clearly formed by the Yellowstone hotspot are found in the Miocene McDermitt and Owyhee-Humboldt volcanic fields of the southern Oregon-northern Nevada border. (Due to drift of the North American plate over the Yellowstone hotspot, the hotspot currently sits beneath Yellowstone National Park in Wyoming).

Alaska

Subduction has occurred along the southern coastline of Alaska throughout the Cenozoic. The Aleutian Islands are a chain of islands that occur where an oceanic plate, the Pacific Plate, is being subducted beneath the North American plate. The islands themselves are volcanoes that have formed on the overriding North American plate, similar to the Cascade volcanoes. The Alaska Peninsula and the southern coastline of Alaska also experience volcanism related to subduction. In Alaska, accretion and volcanism continue to add more land to the coastline. Currently, the Yakutat terrane is accreting to the southeastern coast of Alaska near the Alaska-Yukon (Canada) border.

Denali (also known as Mt. McKinley) is the highest mountain in North America at 6190 meters (20,310 feet). This mountain is in the Alaska Range and was uplifted in the Paleogene. Denali is thought to be so high because it is located on a bend in the Denali Fault, a transform fault that curves across southern Alaska. As in the Sierra Nevada mountains, the rock that makes up Denali is a batholith (in other words, magma that cooled underground and that is now exposed at the surface). In the case of Denali, however, the batholith dates to the Paleogene.

Fossils found in Alaska tell us that during parts of the Cenozoic, its climate was temperate, perhaps even subtropical. Although plate motion can account for some of this change—North America has been slowly moving north throughout the Cenozoic—other important factors also played a role in Alaska’s changing climate history. The majority of the Cenozoic was characterized by greenhouse conditions, during which sea levels are generally higher and glaciers diminish. During periods of changing global climate, polar areas tend to see a greater shift in climate than do areas close to the equator. During a shift to greenhouse conditions, tropical climate zones could have moved into areas that are now temperate or even subpolar.

Quaternary

At the start of the Quaternary period, approximately 2.5 million years ago, continental ice sheets began to form in northernmost Canada. During the Pleistocene, ice sheets advanced south and retreated north several dozen times, reaching their last maximum extent 25,000–18,000 years ago. Throughout this period, the northern half of North America has been periodically covered by continental glaciers that originated in northern Canada. During each ice age, sea level dropped as more and more seawater became locked up in glacial ice. As the oceans fell, coastlines moved farther out to sea. Later, as the climate warmed and sea level rose, the former coastal lands were flooded, drowning river valleys, glacial valleys, and coastal plains.

Last glacial advance

During the most recent glacial advance, approximately 20,000 years ago, portions of the Cordilleran Ice Sheet buried southern Alaska and northern Washington under a mile of ice, carving deep fjords and glacial valleys. This ice sheet deposited huge quantities of glacial sediment in low-lying areas such as Puget Sound and also carved rugged mountain landscapes.

All of Washington east of the Cascades was inundated and scoured by numerous enormous, violent floods, forming the Channeled Scablands. These floods occurred when an ice sheet alternately blocked and retreated from what is now the Clark Fork River in northwestern Montana and northern Idaho. When the river was blocked, an enormous lake called Glacial Lake Missoula built up behind the ice dam. When the ice dam later failed, the water was released catastrophically. These floods cut through the dust deposits and basalt that covered much of the region, leaving islands, escarpments, and channels so large that geologists did not at first recognize their origins. 

Farther to the south, what are now modest mountain glaciers grew to become ice caps covering entire mountain ranges such as the Sierra Nevada and the Blue Mountains of Oregon.

Holocene

The Holocene epoch is the most recent (and current) period of retreat. The beginning of the Holocene is considered to be 11,700 years ago, or about 9700 BCE. While the “ice age” continues today, the Earth is in an interglacial stage, since the ice sheets have retreated for now. The glacial-interglacial cycling of ice ages indicates that the world will return to a glacial stage in the future, unless the impacts of human-induced climate change radically shift these natural cycles.