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African Rift Valley. From left to right: Lake Upembe, Lake Mwaru, Lake Tanganyika (largest), and Lake Rukwa.

A rift valley in geology is a valley created by the formation of a rift. The Great Rift Valley, located in the Middle East and Africa, is the most famous of the world's rift valleys. Rift valleys are produced by tensional tectonic forces which occur at divergent plate boundaries. Rift valleys typically appear as a downdropped graben between a pair of faults, or vertical Earth movements. Rift valleys are often associated with and flanked by volcanoes. The margins of rifts are commonly uplifted, so that the downfaulting of the rift floor is associated with the uplift of both margins. Which of these two processes initiates the composite rifting penomenon is still debated, but their co-occurrence is common. Where the extension that generates rifting is not normal to the rift axis but is somewhat oblique, a series of basins may develop along the axial zone, then, as extension continues, the basins merge to form the rift. Such basins are commonplace in the central and northern Red Sea, as well as other rifts.

The most extensive rift valley is located along the crest of the mid-ocean ridge system and is the result of seafloor spreading. Existing continental rift valleys are usually the result of a failed arm (aulacogen) of a triple junction. Examples besides the Great Rift Valley include the Mississippi embayment and the Rio Grande Rift in North America.

The largest freshwater lakes in the world are all located in rift valleys.^[1] Lake Baikal in Siberia, a World Heritage Site,^[2], lies in an active rift valley. Baikal is both the deepest lake in the world and, with 20% of all of the liquid freshwater on earth, has the greatest volume.^[3] Lake Tanganyika, second by both measures, is in the Albertine Rift, the westernmost arm of the active Great Rift Valley of East Africa and Southwest Asia. Lake Superior in North America, the largest freshwater lake by area, lies in the ancient and dormant Midcontinent Rift.

1. ^ The World's Greatest Lakes. Retrieved on January 7, 2007.
2. ^ Lake Baikal - World Heritage Site. World Heritage. Retrieved on January 13, 2007.
3. ^ The Oddities of Lake Baikal. Alaska Science Forum. Retrieved on January 7, 2007.

• Bonatti, E., 1985. Punctiform initiation of seafloor spreading in the Red Sea during transition from a continental to an oceanic rift. Nature, 316: 33-37.
• Mart, Y., Dauteuil, O., 2000. Analogue experiments of propagation of oblique rifts. Tectonophysics, 316: 121-132.

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