Colorado Plateau

From Academic Kids


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The Colorado Plateau, also called the Colorado Plateaus Province, is a physiographic region of the Intermontane Plateaus, roughly centered on the Four Corners region of the southwestern United States. The province covers an area of 130,000 square miles (337,000 km²) within western Colorado, northwestern New Mexico, southeastern Utah, and northern Arizona. The area is drained by the Colorado River and its tributaries, the Green, San Juan and Little Colorado rivers.

The province is bounded by the Rocky Mountains, Uinta Mountains, Rio Grande Rift, Mogollon Rim and the Basin and Range. In Utah, the province includes several higher fault-separated plateaus:

Four corners region and Colorado plateau
Enlarge
Four corners region and Colorado plateau

The rock units that make up these plateaus are mostly flat-lying sedimentary rock that are between 5000 feet (1500 m) to 11,000 feet (3350 m) above sea level. The crust of the region is 35 to 50 kilometers (22 to 30 miles) thick. A supersequence of these rocks are exposed in the various cliffs and canyons (including the Grand Canyon) that make up the Grand Staircase.

Geology

One of the most geologically intriguing features of the Colorado Plateau is its remarkable stability. Relatively little rock deformation (ex. faulting and folding) has affected this high, thick crustal block within the last 600 million years or so. In contrast, provinces that have suffered severe deformation surround the plateau. Mountain building thrust up the Rocky Mountains to the north and east and tremendous, earth-stretching tension created the Basin and Range province to the west and south.

The Precambrian history of the Colorado Plateaus is only known at its southern end where the Grand Canyon has exposed the 2000 million year old Vishnu Schist to the 600 million year old Kaibab Limestone. Most of the formations were deposited in warm shallow seas and near-shore environments (such as beaches and swamps) as the seashore repeatedly advanced and retreated over the edge of a proto-North America (for detail, see geology of the Grand Canyon area). The province was probably on a continental margin throughout the late Precambrian and most of the Paleozoic era. Igneous rocks injected millions of years later form a marbled network through parts of the Colorado Plateau's darker metamorphic basement.

In late Paleozoic and much of the Mesozoic era the region was affected by a series of orogenies (mountain-building events) that deformed western North America and caused a great deal of uplift. By 600 million years ago North America had been beveled off to a remarkably smooth surface.

Throughout the Paleozoic Era, tropical seas periodically inundated the Colorado Plateau region. Thick layers of limestone, sandstone, siltstone, and shale were laid down in the shallow marine waters. During times when the seas retreated, stream deposits and dune sands were deposited or older layers were removed by erosion. Over 300 million years passed as layer upon layer of sediment accumulated.

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Erosion-resistant sandstones of Mesozoic age result in bands of continuous cliffs, central Colorado Plateau

It was not until the upheavals that coincided with the formation of the supercontinent Pangea began about 250 million years ago that deposits of marine sediment waned and terrestrial deposits dominate. Eruptions from volcanic mountain ranges to the west buried vast regions beneath ashy debris. Short-lived rivers, lakes, and inland seas left sedimentary records of their passage. Streams, ponds and lakes created formations such as the Chinle, Moenave, and Kayenta in the Mesozoic era. Later a vast desert formed the Navajo and Temple Cap formations and dry near-shore environment formed the Carmel (see geology of the Zion and Kolob canyons area for details).

The area was again covered by a warm shallow sea when the Cretaceous Seaway opened in late Mesozoic time. The Dakota Sandstone and the Tropic Shale were deposited in the warm shallow waters of this advancing and retreating seaway. Several other formations were also created but were mostly eroded following two major periods of uplift.

The Laramide orogeny created uplift that closed the seaway and uplifted a large belt of crust from Montana to Mexico, with the Colorado Plateau region being the largest block. Thrust faults in Colorado are thought to have formed from a slight clockwise movement of the region, which acted as a rigid crustal block. Minor uplift events continued through the start of the Cenozoic era and was accompanied by some basaltic lava eruptions and mild deformation. The colorful Claron Formation that forms the delicate hoodoos of the Bryce Canyon was then laid down as sediments in cool streams and lakes (see geology of the Bryce Canyon area for details).

Tectonic activity resumed in Mid Cenozoic time and started to unevenly uplift and slightly tilt the Colorado Plateaus region and the region to the west starting 20 million years ago (as much as 3 kilometers of uplift occurred). Streams had their gradient increased and they responded by downcutting faster. Headward erosion and mass wasting helped to erode cliffs back into their fault-bounded plateaus, widening the basins in-between. Some plateaus have been so severely reduced in size this way that they become mesas or even buttes. Monoclines form as a result of uplift bending the rock units. Eroded monoclines leave steeply tilted resistant rock called a hogback and the less steep version is a cuesta.

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Cliffs of Navajo Sandstone in Zion National Park

Great tension developed in the crust, probably related to changing plate motions far to the west. As the crust stretched, the Basin and Range province broke up into a multitude of down-dropped valleys and elongate mountains. Major faults, such as the Hurricane Fault, developed that separate the two regions. The dry climate was in large part a rainshadow effect resulting from the rise of the Sierra Nevada further west. Yet for some reason not fully understood, the neighboring Colorado Plateau was able to preserve its structural integrity and remained a single tectonic block. Eventually, the great block of Colorado Plateau crust rose a kilometer higher than the Basin and Range. As the land rose, the streams responded by cutting ever deeper stream channels. The most well-known of these streams, the Colorado River, began to carve the Grand Canyon less than 6 million years ago.

The Pleistocene epoch brought periodic ice ages and a cooler, wetter climate. This increased erosion at higher elevations with the introduction of alpine glaciers while mid-elevations were attacked by frost wedging and lower areas by more vigorous stream scouring. Pluvial lakes also formed during this time. Glaciers and pluvial lakes disappeared and the climate warmed and became drier with the start of Holocene epoch.

Protected lands

Covering a large part of the Colorado River catchment, this relatively high semi-arid province produces many distinctive erosional features such as arches, arroyos, canyons, cliffs, fins, natural bridges, pinnacles, hoodoos, and monoliths that, in various places and extents, have been protected. There are eight national parks, more than a dozen U.S. National Monuments and dozens of wilderness areas in the province along with millions of acres in national forests and other protected lands.

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Erosional features within Glen Canyon National Recreation Area. Lake Powell, in foreground, is not a natural lake but a reservoir impounded by Glen Canyon Dam.

National parks (from south to north clockwise):

National Monuments:

Wilderness areas:

Other notable protected areas include: Glen Canyon National Recreation Area, Dead Horse Point State Park, Kodachrome State Park, Goblin Valley State Park and Barringer Crater.

References and further reading

ja:コロラド高原 pl:Wyżyna Kolorado

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