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Laurentia
Laurentia (North American Craton) is a large continental craton that forms the ancient geological core of the North American continent. Many times in its past, Laurentia has been a separate continent as it is now in the form of North America, although originally it also included the cratonic areas of Greenland and also the northwestern part of Scotland, known as the Hebridean Terrane. During other times in its past, Laurentia has been part of larger continents and supercontinents and itself consists of many smaller terranes assembled on a network of Early Proterozoic orogenic belts. Small microcontinents and oceanic islands collided with and sutured onto the ever-growing Laurentia, and together formed the stable Precambrian craton seen today.
The craton is named after the Laurentian Shield, which in turn is named after the Laurentian Mountains.
==Interior platform==
In eastern and central Canada, much of the stable craton is exposed at the surface as the Canadian Shield; when subsurface extensions are considered, the wider term Laurentian Shield is more common, not least because large parts of the structure extend outside Canada. In the United States, the craton bedrock is covered with sedimentary rocks of the interior platform in the Midwest and Great Plains regions. Only in northern Minnesota, Wisconsin, the Adirondacks of New York and the Upper Peninsula of Michigan is the craton bedrock exposed. The sequence of rocks varies from about 1,000 to in excess of 6,100 m (3,500–20,000 ft) in thickness. The cratonic rocks are metamorphic and igneous, while the overlying sedimentary rocks are composed mostly of limestones, sandstones, and shales. These sedimentary rocks were deposited from 650 to 290 million years ago.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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