rift valley

A valley that has developed along a rift, especially one bounded by normal faults in an area of lithospheric thinning.

One of the geomorphological expressions between two tectonic plates that are opening relative to each other or sliding past each other. The term originally was used to describe the central graben structures of such classic continental rift zones as the Rhinegraben and the East African Rift, but the definition now encompasses mid-oceanic ridge systems with central valleys such as the Mid-Atlantic Ridge. See also Mid-Oceanic Ridge; Plate tectonics.

Continental and oceanic rift valleys are end members in what many consider to be an evolutionary continuum. In the case of continental rift valleys, plate separation is incomplete, and the orientation of the stress field relative to the rift valley can range from nearly orthogonal to subparallel. Strongly oblique relationships are probably the norm. In contrast, oceanic rift valleys mark the place where the trailing edges of two distinctly different plates are separating. The separation is complete, and the spreading is organized and focused, resulting in rift valleys that tend to be oriented orthogonal or suborthogonal to the spreading directions.

The basic cross-sectional form of rift valleys consists of a central graben surrounded by elevated flanks. It is almost universally accepted that the central grabens of continental rift valleys are subsidence features. The crystalline basement floors of some parts of the Tanganyika and Malawi (Nyasa) rift valleys in East Africa lie more than 5 mi (9 km) below elevated flanks. See also Graben.

In continental rift valleys the true cross-sectional form is typically asymmetric, with the rift floors tilted toward the most elevated flank. Most of the subsidence is controlled by one border fault system, and most of the internal faults parallel the dip of the border faults. See also Fault and fault structures.

Oceanic rift valleys are also distinctly separated into segments by structures known as transform faults. The cross-sectional form of oceanic rift valleys can be markedly asymmetric. It is unlikely that the cross-sectional form of oceanic rift valleys is related genetically to that of continental rift valleys, except in the broadest possible terms. See also Transform fault.

Also known as a graben, this is a long strip of country let down between normal faults, or between a parallel series of step faults. The Valle de Cibão Graben, Hispaniola, has a length of 250 km and a maximum width of 40 km—roughly the same as the Rhine rift valley.

Rift valleys only form in brittle, resistant rocks, since more plastic rocks will thin and deform under pressure. They are among the largest, structurally controlled landforms, and the biggest terrestrial rift valley system is the East African system, at 3000 km long.

Plate tectonic theory suggests that rift valleys are the result of large-scale doming above a mantle plume, followed by fracturing along the crest of the dome as plates diverge. Many rifts, like the Rhine rift valley, have the Y-shaped pattern characteristic of a triple junction indicating that they arise from plate separation, but some have been attributed to the presence of hot spots.

For more information on rift valley, visit Britannica.com.

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

A rift valley near Quilotoa, Ecuador.

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 deep 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.

Notes

1. ^ The World's Greatest Lakes. Retrieved on 2007-01-07.
2. ^ Lake Baikal - World Heritage Site. World Heritage. Retrieved on 2007-01-13.
3. ^ The Oddities of Lake Baikal. Alaska Science Forum. Retrieved on 2007-01-07.

• 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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