Laurentia

Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. 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, through the Laurentian Mountains, which received their name from the Saint Lawrence River, named after Lawrence of Rome. In eastern and central Canada, much of the stable craton is exposed at the surface as the Canadian Shield, an area of Precambrian rock covering over a million square miles. This includes some of the oldest rock on Earth, such as the Archean rock of the Acasta Gneiss of Canada, which is 4.04 billion years (Ga) old, and the Istaq Gneiss Complex of Greenland, which is 3.8 billion years old. 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 on the broad interior platform in the Midwest and Great Plains regions and is exposed only in northern Minnesota, Wisconsin, the New York Adirondacks, and the Upper Peninsula of Michigan. The sequence of sedimentary rocks varies from about 1,000 m to in excess of 6,100 m (3,500–20,000 ft) in thickness. The cratonic rocks are metamorphic or igneous with the overlying sedimentary layers composed mostly of limestones, sandstones, and shales. These sedimentary rocks were largely deposited from 650 to 290 million years ago.