Geography of North America - Paleogeography

Paleogeography

Seventy percent of North America is underlain by the Laurentia craton, which is exposed as the Canadian Shield in much of central and eastern Canada around the Hudson Bay, and as far south as the U.S. states of Michigan, Wisconsin, and Minnesota. The continental crust started to form 4 billion years ago (Ga), and six of the microcontinents collided to form the craton about 2 Ga. This core has been enlarged by plate tectonics, most notably between 1.8 and 1.65 Ga when a piece currently stretching from Arizona to Missouri fused with the south and west portion of the craton. The craton started to rift about 1.1 Ga, and the fissure (now the Midcontinent Rift System) ran between Kansas and Lake Superior before stopping, perhaps due to the Grenville collision in the east. Otherwise the craton has remained relatively stable, with some rocks dating from 2.5 to 4 Ga, including what may be the world's oldest known rock: Specimens from the Nuvvuagittuq greenstone belt on the coast of the Hudson Bay have been dated to 4.38 Ga, though the dating methods are disputed. Periodic flooding by inland seas, most recently the Western Interior Seaway during the Cretaceous, caused the layer of sedimentary rock over the remainder of the craton. The Laurentia craton is the center of the Proterozoic supercontinent Rodinia in most models, and was also part of the later Laurussia, Pangea, and Laurasia supercontinents.

Roughly 3 million years ago (Ma), the volcanic Isthmus of Panama formed between the North and South American continents creating a bridge over what was the Central American Seaway and allowing the migration of flora and fauna between the two landmasses in the Great American Interchange. Starting 2.58 Ma, the Quaternary glaciation covered much of the continent with ice, centered west of Hudson Bay, the floor of which is slowly rebounding after being depressed by the great weight of the ice. Glaciers descended the slopes of the Rocky Mountains and those of the Pacific Margin. Extensive glacial lakes, such as Glacial Lake Missoula, Bonneville, Lahontan, Agassiz, and Algonquin, formed by glacial melt water. "Remnants of them are still visible in the Great Basin and along the edge of the Canadian Shield in the form of the Great Salt Lake, the Great Lakes, and the large lakes of west central Canada." The last glacial period of the current ice age caused a lowering of the sea level, exposing the Bering Land Bridge between Alaska and Siberia, which resulted in human migration from Asia to the Americas between 40,000 and 15,000 years ago.

North America can also be divided into four great regions:

• Great Plains: stretching from the Gulf of Mexico to the Canadian Arctic;
• the geologically young, mountainous west: including the Rocky Mountains, the Great Basin, California and Alaska; the raised but relatively flat plateau of the Canadian Shield in the northeast;
• the varied eastern region: including the Appalachian Mountains, the coastal plain of the Atlantic Seaboard, and the Florida peninsula.
• Mexico and its long plateaus and cordilleras fall largely in the western region, although the eastern coastal plain does extend south along the Gulf.