Continents Essay ⋆ Environment Essay Examples ⋆ EssayEmpire

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Continents are the large dry landmasses of the earth. They are areas of continental crust atop subterranean areas of the lithosphere. The continental crust is thicker, but less dense than the oceanic crust, and is floating on the molten mantle. While the continents hold deposits of heavy materials such as iron or gold, they are mostly felsic materials that are lighter rocks composed mostly of silicate minerals, sodium, potassium, and aluminum. Each continent is connected to the oceanic crust or the sea floor that adjoins it. Continental margins are the sides of the continents. They are composed of the continental shelf, the continental shelf break, the continental slope, and the continental rise. The bottoms of the continental margins exist in the ocean basins as zones that are directly adjacent to a continent. They include the belt of continental crust and lithosphere that is in contact with the oceanic crust and its associated lithosphere. This is an area that often has an active plate boundary. Continental shelves are areas covered by relatively shallow seawater. Barrier islands are areas of sand built up on the continental shelf such as the barrier islands off of the coasts of Georgia or Texas.

The continental shelves surrounding the continents usually reach a maximum depth of 600 feet (200 meters). They extend from a few miles to several hundred miles from shore. At the outer limits of the continental shelf, there is a drop off called the continental shelf break. The transition from the gently sloping continental shelf to the deep ocean basin is called the continental slope. The continental shelf break marks the beginning of the continental slope, and occurs at an average depth of 430 feet (130 meters). The dropoff moves rapidly down thousands of feet to the boundary between the continental crust and the oceanic crust. Here, the ocean depth increases rapidly, reaching several thousand meters within a few kilometers. The continental rise is the gently sloping seafloor lying at the foot of the continental slope and leading gradually into the abyssal plain of the deep ocean floor. At the base of the continental slope is the boundary between the continental crust and the oceanic crust. Sediments washing over the edge of the slope or scraping off of a subducting plate form the slope. Generally, continental slopes located at active margins, for example the Chile Trench, are steeper than those located at passive margins like those in the Drake Passage.

Plate Tectonics

Continental shelf wedges are thick bodies of sediment formed by deposition on a subsiding passive continental margin in shallow waters of the continental shelf. Continental accretion is the increase in volume of the area of the continental crust due to the formation of granitic and andesitic rocks. These formations occur within mobile belts. They are also formed by collisions between continents that causes plate subduction and orogeny. The continents are all oceanic islands and include Africa, Asia and Europe, North and South America, Australia, and Antarctica. Asia and Europe are part of the same landmass, in which Europe is a branchy peninsula extending from Asia. Some islands such as Ireland, Britain, Greenland, Sicily, Sumatra, Java, New Guinea, and Tasmania are continental islands, areas of land that rise up from the continental shelf. Continental drift is an older hypothesis introduced by Alfred Wegener and others in the early 1900s. The hypothesis asserted that several hundred million years ago there was a single continent that Wegner called Pangaea (“universal mother earth”). It began to break up in the Mesozoic Era, and the pieces drifted apart to the present positions of the continents.

The current theory of plate tectonics is similar to continental drift, but not exactly the same. The continents have been compared to ships of rock that are “drifting” on plates of rock riding over the earth’s molten core. The splitting of continents happens along rift fault lines. Continental ruptures occur when there is a doming up of the continental lithosphere and its crust to create a continental rift or rift valley. One of the most dramatic examples is the Great Rift Valley that begins in eastern Africa and runs through the Dead Sea Valley to Turkey. Continental rifts can also create continental rift islandssuch as the micro-continental islands of Madagascar, some of the Seychelles, and the Kerguelen Islands. Continents have varied features, which are mostly variations of plains or mountains. In addition, there are inland seas such as the Caspian, Aral, or the Great Lakes that are virtually freshwater seas. The vast areas of the continents allow for the formation of continental air masses, which can be polar, dry, or wet. These continental air masses may be dry if coming from the interior of a continent. Others may be moist equatorial air masses.

Continental divides are mountain areas on continents from which water flows to one ocean rather than to another. In the eastern United States, the continental divide separates waters that flow eastward to the Atlantic Ocean from water that flows west and then south to the Gulf of Mexico through the Mississippi River Valley. There are similar divides on other continents for waters flowing into the Pacific, Indian, or Arctic Oceans. Continental glaciers occur in high mountain areas where the temperature remains cold even in the summer. During ice ages, continental glaciers spread from a base location to cover enormous areas of continents. Continental shields are areas of continental crust, such as the Canadian Shield (Laurentian Plateau). It is a vast area of rock of igneous and metamorphic rock, most of it Precambrian or Archaean-age. In the case of the Canadian Shield, the rock was elevated above the sea and has never been covered with sedimentary deposits. Rock in the shield was exposed by the actions of continental glaciers, enormous ice sheets that covered much of the northern hemisphere in the most recent Ice Ages.

Continent collisions happen when plate tectonics brings continents into headlong contact. The ocean between them shrinks, causing a continential suture, a continental suture is a long, narrow zone of crust deformation. The Himalayan Mountains and the European Alps are being pushed ever higher by under-thrusting and intense folding as India drives into the Asian continent or as the African continent moves toward Europe. Continents are sometimes subdivided by geographers into subcontinents, such as the Indian subcontinent. Because the continents are isolated from one another, different fauna and flora have developed on each continent.

In recent years, there has been a general rethinking of the categorical status of continents. Geographer Martin Lewis and historian Karen Wigen have argued, for example, that for the most part, our distinction of continents (Asia from Europe as a most egregious example) has been historically conditioned by social and cultural preconceptions, rather than tectonic or biogeographical considerations. Insofar as Europe, Asia, and Africa are part of one land mass, and South and North America are contiguous, the insistence of historians, geographers and others to distinguish them must be seen as part of larger historical habits including Orientalism (imposing essential definitions of East and West) and colonialism. Continents in most respects better reflect historical worldview than distinct metageographic realities.

Bibliography:

Peter Farb, Face of North America: The Natural History of a Continent (Harper & Row, 1963);
Hallam, A Revolution in the Earth Sciences: From Continental Drift to Plate Tectonics (Oxford University Press, 1973);
W. Lewis and K. E. Wigen, The Myth of Continents: A Critique of Metageography. (Berkeley, University of California Press, 1997);
Russell Miller, Continents in Collision (Time-Life Books, 1983);
National Geographic Society, Our Continent: A Natural History of North America (National Geographic Book Service, 1976);
John Reader, Africa: Biography of a Continent (National Geographic Society, 2001);
Ron Redfern, The Making of a Continent (Times Books, 1983).