The geology of North America is a subject of regional geology and covers the North American continent, the third-largest in the world. Geologic units and processes are investigated on a large scale to reach a synthesized picture of the geological development of the continent. The divisions of regional geology are drawn in different ways, but are usually outlined by a common geologic history, geographic vicinity or political boundaries. The regional geology of North America usually encompasses the geographic regions of Alaska, Canada, Greenland, the continental United States, Mexico, Central America, and the Caribbean. The parts of the North American Plate that are not occupied by North American countries are usually not discussed as part of the regional geology. The regions that are not geographically North American but reside on the North American Plate include parts of Siberia (see the Geology of Russia), and Iceland, and Bermuda. A discussion of North American geology can also include other continental plates including the Cocos Plats and Juan de Fuca Plate being subducted beneath western North America. A portion of the Pacific Plate underlies Baja California and part of California west of the San Andreas Fault.
North American Craton
The stable core of the continent is the North American Craton. Much of it was also the core of an earlier continent, Laurentia. The part of the craton where the basement rock is exposed is called the Canadian Shield. Surrounding this is a stable platform where the basement is covered by sediment; and surrounding that are a series of orogenic zones.
Canadian Shield
The Canadian Shield is a large area of Archean through Proterozoic igneous and metamorphic rocks in eastern Canada and north central and northeastern United States. The earliest part of the shield is metamorphosed Archean rocks, originally volcanic in origin. Numerous terranes were accreted onto this Archean core during the Proterozoic to form the Canadian Shield. The southern Archean province is the Superior Craton, it is formed by the combination of a greenstone-granite and a gneiss terrane. The margins of the Canadian Shield have been covered by sedimentary rocks, such as in Michigan where a series of sediments has filled in the Michigan Basin. The exposed sections are often where glaciers have removed this overlying regolith to reveal the underlying glacially scarred crystalline rock.
Stable platform
The stable platform is an area in which the North American Craton forms a basement and is covered by sediment. This area now forms much of the Interior Plains and the slope of the Appalachians below the mountains proper. This area has been covered by a shallow inland sea, which became the site of deposition for most of the overlying sedimentary rock. The sea receded as the continent rose becoming covered by stream, lake, and wind deposits. Orogenies in the surrounding provinces have had little effect on the craton, making it an epeirogenic region, and, as such, the stable platform is mostly a crystalline basement, covered by sedimentary rocks, interrupted only by occasional domes, such as the Cincinnati Arch, Wisconsin Dome, and Ozark Dome.
Midcontinent rift system One billion years ago, the Midcontinent Rift System began to extend along a 2,000 kilometres (1,200 mi) path, across both the Canadian Shield and the Stable Platform. The rift failed, then crustal movement reversed. A range formed then eroded, forming basins on either side of a horst. These rocks have been buried beneath sediment in many areas, but are exposed in some areas, especially around Lake Superior.
Grenville Orogen The Grenville Orogen developed during the Proterozoic along eastern and southern margin of the North American Craton. The largest outcrop of Grenville age rocks is an approximately 400 kilometres (250 mi) wide band southeast of the Grenville Front which stretches from the central Labrador coast southwest across southern Quebec and southeastern Ontario to Georgian Bay on Lake Huron. The southeastern boundary of this area is approximately the St. Lawrence River. Rocks of the Grenville outcrop in the Adirondack Mountains of northern New York and throughout the Appalachians. The Llano Uplift of central Texas and the Franklin and Hueco Mountains of west Texas have been correlated with the Grenville as have occurrences in Mexico.
Appalachian Orogen
The fold and thrust belt of the Appalachians is continuously exposed for 2,000 kilometres (1,200 mi) from Pennsylvania to Alabama. In the south, it extends under the coastal plain, but is covered by Mesozoic sediments. North of this fold and thrust belt, the Acadian Orogen of the middle Devonian is an area where deformation has exposed granite plutons. The center of the range is a pair of provinces running north and south parallel to each other, the eastern Blue Ridge Province and the western Valley and Ridge provinces. These are surrounded by the Appalachian Plateau on the west, and the Piedmont Province to the east. Faulting extends throughout the region and is caused by numerous spatially and temporally varied sources. Inliers of Late Mesoproterozoic age are present on the west of the core of the Appalachians, and these inliers are associated with the Grenville orogeny. During the Proterozoic terranes were accreted onto the province. During the Taconic orogeny 445 to 435 million years ago, accretion continued, an island arc collided with the North American continent, and mountains were raised. These mountains slowly eroded and deposited sediment into the Catskill delta, stretching from New York to Pennsylvania.
Piedmont
The eastern portion of the orogen is made up of the Piedmont plateau, a 150 to 300 metres (490 to 980 ft) elevation area composed of Paleozoic marine and volcanic sediments deformed into crystalline metamorphic rocks and intruded by granite domes. During the Proterozoic a series of terranes were accreted onto the North American craton, forming the Piedmont of the central Appalachians. Following the Grenville orogeny, mountains eroded, and the sediments from this erosion were deposited below the mountains. The bedrock of the plateau formed about 470 million years ago during the Taconic orogeny, when a volcanic island arc collided with the ancestral North American Continent.
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