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At the inception of the plate-tectonic scheme of events, the continental crust splits into a series of rifts. First, a piece of land, called the domal rises due to mineral changes in the Earth's mantle. This structure may be about 100 km wide 250 km across and 1 km in thickness1, and ideally, three rift valleys develop symmetrically around the domal. Such domes and rift valleys are clearly seen today in Africa. A good example would be where the Ethiopian Rift meets the Red Sea and the Gulf of Aden, in north-eastern region of the continent of Africa. Both these rift valleys have opened into narrow seas at a triple junction. The three rifts may all widen and become narrow oceans that will eventually become an ocean as wide as the Atlantic. However, it is possible that a rift may fail to become a ocean and therefore become a failed rift arm or aulacogen. This will eventually become an extension to it's neighbouring continent. The Benue Trough is such an example, where one plate failed whereas it's neighbouring two plates have opened to form the South Atlantic Ocean. Other examples of aulacogens that failed to become oceans or seas are the East African Rift System, the Midland Valley of Scotland, and Scandinavia's Oslo Graben. When an ocean has reached a mature stage, it may begin to contract by subduction. A subduction zone or oceanic rift which is bordered by a continent, may collide with that continent. This collision may create a fold, which will eventually develop to make a mountain range.
These rifts and aulacogens are marked by heat flow and magma injection. Continental rifting is the basic phenomena that gives rise to volcanoes and dikes. After a rift has evolved into an ocean, some of the lava and dikes that may have ejected may be preserved on the continental coasts. For example, the volcanoes and sediments ejected at the time of the Pacific ocean are still found along the eastern coast of the United States. The early-Jurassic dike ejections that occurred parallel to the coast in south-western Greenland and Liberia still exist. These examples date from the early stage of the development of the Atlantic Ocean.
Oceans are constantly being formed as continets move apart. New additions are made to the land as continental rifts fail and land is submerged as plates move apart and rifts become oceans.
1 The Grolier Multimedia Encyclopedia
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