Interior Plains

The Interior Plains is a vast international region that spreads across the Laurentian craton of central North America. It extends along the east flank of the Rocky Mountains from the Gulf Coast region to the Arctic Beaufort Sea. In Canada, it encompasses the Canadian Prairies separating the Canadian Rockies from the Canadian Shield, as well as the Boreal Plains and Taiga Plains east of the Mackenzie and Richardson Mountains. In the United States, it includes the Great Plains of the West/Midwest and the tallgrass prairie region to the south of the Great Lakes extending east to the Appalachian Plateau region.

A series of tectonic plate collisions in the crust that formed the center of the North American continent laid the groundwork for the modern-day interior plains. Mountain building and erosion around the plains as well as flooding from inland seas provided sediments that make up the rock strata of the interior plains.

Between 2.0 and 1.8 billion years ago the Hearne-Rae, Superior, and Wyoming cratons were sutured together to form the North American/Laurentia craton in an event called the Trans-Hudson Orogeny (THO). The tectonic activity that followed along the edges of the four main cratons sparked mountain building in those regions. The formation of the Himalayas is following the same process after the Indian Plate began colliding with the Eurasian Plate about 10 millions years ago.

The interior of Laurentia remained relatively flat and became a basin for eroded sediment from mountains at the beginning of the current time period, the Phanerozoic Eon. The only remaining outcrops from this orogeny in the interior plains are in the Black Hills of South Dakota. The sediments that formed the Black Hills were granite and different types of igneous rocks, which make up the basement of bedrock in central North America. However, much of the Black Hills sediment has been metamorphosed and deformed, so it is uncertain what the conditions were like at the time of their formation.

This period has a large importance in Earth's history as it saw the Cambrian explosion and Permian extinction. When global sea level rose and continents became partially submerged, the oceans had an explosion of complex life, which was the first time an event like this occurred on Earth. However, the center of Laurentia remained above sea level and as the continent moved east towards other supercontinents like Gondwana, the Appalachian Mountains began to form around 400 MYA. This coincided with the formation of Pangea around 300 MYA, when the Appalachians were at their peak height. The central plains of Laurentia were subjected to deposition of eroded sediment from these mountains. The oldest sediments from this period are felsic igneous rocks and granite that have since been metamorphosed, while the younger sediments are made up of sandstone, shale, limestone, and coal. Sediments deposited in the interior plains from this era are currently buried deep beneath the surface where they are difficult to study.

Around 220 MYA, the supercontinent Pangea broke apart, and the North American continent began to move west and isolate itself. For much of this period the interior plains were covered by inland seas. During the Jurassic period, the Sundance Sea formed along the western coast of the North American continent and extended from northern Canada to the interior plains, covering parts of Wyoming, Montana, North Dakota, and South Dakota. Coquina and sandstone layers from marine deposition were deposited on top of rock layers from the Paleozoic Era. During the Cretaceous period, another inland sea called the Western Interior Seaway was formed. This body of water extended from present-day Alaska to the Gulf of Mexico and covered almost all the interior plains west of the current boundary of the Mississippi River. Limestone-shale couplets, as well as carbonate layers, are commonly found in sedimentary deposits from this inland sea. Towards the end of this period, the inland seas began to drain due to uplift from the formation of the Rocky Mountains.

The Laramide Orogeny event was when the western Cordillera was formed due to the flat-slab subduction of the Farallon Plate under the North American Plate. This created the frontal range of the Rockies from Montana through New Mexico. The outcrops seen at the surface of the Rockies are made up of sandstone, granite, and limestone; as well as metamorphic rocks uplifted from the Proterozoic Period. The interior plains have remained relatively flat during this period and recent sedimentation is from erosion of the newly formed Rocky Mountains as well as continued erosion from Appalachia. In general, Rocky Mountain sediment is deposited on the plains west of the Mississippi River, and Appalachian sediment is deposited to the east of the Mississippi River.

2.6 million years ago at the start of the Pleistocene Epoch, the Laurentide Ice Sheet began to spread southwards to cover North America down to the northern Great Plains on the western side of the Interior Plains and down into most of Minnesota and Wisconsin. The Laurentide Ice Sheet had a large influence on the morphology of the Interior Plains during the end of the Pleistocene. During the retreat, the Laurentide scoured numerous pockets of sediment. Upon the plate's melting, those pockets were filled, resulting in the kettle lakes. The Great Lakes and both Great Slave Lake and Great Bear Lake of Canada were formed by the Laurentide. During the retreat, the Laurentide gouged and filled the glacial paleolake McConnell in northern Canada. As the region uplifted and rebounded isostatically from the mass of the ice sheet, paleolake McConnell was split into Great Slave Lake and Great Bear Lake. Great Slave Lake's basin formed under the 4-kilometer thick Keewatin Dome that today is the deepest lake in North America. A vast amount of smaller lakes were formed as well and serve an integral part of ethos in the surrounding regions. For example, Minnesota is often referred to as “the Land of 10,000 Lakes” due to the number and widespread recreational use of the state's lakes.