Paleogeography

(geology) The geography of the geologic past; concerns all physical aspects of an area that can be determined from the study of the rocks. Paleogeography is used to describe the changing positions of the continents and the ancient extent of land, mountains, shallow sea, and deep ocean basins.

The geography of the ancient past. Paleogeographers study the changing positions of the continents and the ancient extent of land, mountains, and shallow-sea and deep-ocean basins. The Earth's geography changes because its surface is in constant motion due to plate tectonics. The continents move at rates of 2–10 cm/yr (0.75–4 in./yr). Though this may seem slow, over millions of years continents can travel across the globe. As the continents move, new ocean basins form, mountains rise and erode, and sea level rises and falls. Paleogeographic maps are necessary in order to understand global climatic change, migration routes, oceanic circulation, mountain building, and the formation of many of the Earth's natural resources, including oil and gas. See also Basin; Continents, evolution of; Geography; Mid-Oceanic Ridge; Paleoclimatology; Plate tectonics; Subduction zones.

In the late Precambrian the continents were colliding to form supercontinents, and the Earth was locked in a major ice age. About 1100 million years ago (Ma), the supercontinent of Rodinia was assembled. Rodinia split into halves approximately 750 Ma, opening the Panthalassic Ocean. By the end of the Precambrian three continents came together to form the supercontinent of Gondwana(land). This major continent-continent collision is known as the Pan-African orogeny. See also Orogeny; Precambrian; Proterozoic; Supercontinent.

The supercontinent that formed at the end of the Precambrian Era, approximately 600 Ma, had already begun to break apart by the beginning of the Paleozoic Era. Gondwana, which was considerably larger than any of the other continents, stretched from the Equator to the south. See also Ordovician; Paleozoic.

By the end of the Paleozoic Era, the continents had collided to form the supercontinent of Pangea. Centered on the Equator, Pangea stretched from the South Pole to the North Pole. Though the supercontinent that formed at the end of the Paleozoic Era is called Pangea (literally, “all land,”), this supercontinent probably did not include all the landmasses that existed at that time.

The supercontinent of Pangea did not rift apart all at once, but in three main episodes. The first episode of rifting began in the Middle Jurassic, about 180 Ma when North America rifted away from northwest Africa, opening the Central Atlantic. See also Jurassic.

The second phase in the breakup of Pangea began in the Early Cretaceous, about 140 Ma. Gondwana continued to fragment as South America separated from Africa, opening the South Atlantic, and India together with Madagascar rifted away from Antarctica and the western margin of Australia, opening the Eastern Indian Ocean. See also Cretaceous.

The third and final phase in the breakup of Pangea took place during the early Cenozoic. North America and Greenland split away from Europe, and Antarctica released Australia. Australia, like India some 50 million years earlier, moved rapidly northward on a collision course with Southeast Asia. See also Cenozoic.

About 18,000 years ago, all of Antarctica and much of North America, northern Europe, and the mountainous regions of the world were covered by glaciers and great sheets of ice. These ice sheets melted approximately 10,000 years ago, giving rise to familiar geographic features such as Hudson's Bay, the Great Lakes, the English Channel, and the fiords of Norway.