continent

The North American continent is growing in the west as a result of collision with the Pacific plate. On the east of the wide area of the Ozark Plateau shield lie the Appalachian Mountains, showing where the continent once collided with another continent. The eastern coastal rifting formed when the continents broke apart. On the western edge, new impact mountains have formed.

Any one of the seven large land masses of the Earth, as distinct from the oceans. They are Asia, Africa, North America, South America, Europe, Australia, and Antarctica. Continents are constantly moving and evolving (see plate tectonics). A continent does not end at the coastline; its boundary is the edge of the shallow continental shelf, which may extend several hundred kilometres out to sea.

Continental crust, as opposed to the crust that underlies the deep oceans, is composed of a wide variety of igneous, sedimentary, and metamorphic rocks. The rocks vary in age from recent (currently forming) to almost 4,000 million years old. Unlike the ocean crust, the continents are not only high standing, but extend to depths as great at 70 km/45 mi under high mountain ranges. Continents, as high, dry masses of rock, are present on Earth because of the density contrast between them and the rock that underlies the oceans. Continental crust is both thick and light, whereas ocean crust is thin and dense. If the crust were the same thickness and density everywhere, the entire Earth would be covered in water.

At the centre of each continental mass lies a shield or craton, a deformed mass of old metamorphic rocks dating from Precambrian times. The shield is thick, compact, and solid (the Canadian Shield is an example), and is usually worn flat. Around the shield is a concentric pattern of fold mountains, with older ranges, such as the Rockies, closest to the shield, and younger ranges, such as the coastal ranges of North America, farther away. This general concentric pattern is modified when two continental masses have drifted together and they become welded with a great mountain range along the join, the way Europe and northern Asia are joined along the Urals. If a continent is torn apart, the new continental edges have no mountains; for instance, South America has mountains (the Andes) along its western flank, but none along the east where it tore away from Africa 200 million years ago.