Arift valley is a linear shaped lowland between severalhighlands or mountain ranges produced by the action of ageologic rift. Rifts are formed as a result of the pulling apart of thelithosphere due toextensional tectonics. The linear depression may subsequently be further deepened by the forces of erosion. More generally the valley is likely to be filled with sedimentary deposits derived from the rift flanks and the surrounding areas. In many casesrift lakes are formed. One of the best known examples of this process is theEast African Rift.[1] On Earth, rifts can occur at all elevations, from the sea floor to plateaus and mountain ranges incontinental crust or inoceanic crust. They are often associated with a number of adjoining subsidiary or co-extensive valleys, which are typically considered part of the principal rift valley geologically.
The most extensive rift valley is located along the crest of themid-ocean ridge system and is the result ofsea floor spreading. Examples of this type of rift include theMid-Atlantic Ridge and theEast Pacific Rise.
Many existing continental rift valleys are the result of a failed arm (aulacogen) of atriple junction, although there are three, theEast African Rift,Rio Grande rift and theBaikal Rift Zone, which are currently active, as well as a fourth which may be, theWest Antarctic Rift System. In these instances, not only the crust but entiretectonic plates are in the process of breaking apart forming new plates. If they continue, continental rifts will eventually become oceanic rifts.
Other rift valleys are the result of bends or discontinuities in horizontally-moving (strike-slip) faults. When these bends or discontinuities are in the same direction as the relative motions along the fault, extension occurs. For example, for a right lateral-moving fault, a bend to the right will result in stretching and consequent subsidence in the area of the irregularity. In the view of many geologists today, theDead Sea lies in a rift which results from a leftward discontinuity in the left lateral-movingDead Sea Transform fault. Where a fault breaks into two strands, or two faults run close to each other, crustal extension may also occur between them, as a result of differences in their motions. Both types of fault-caused extension commonly occur on a small scale, producing such features assag ponds orlandslides.
Many of the world's largest lakes are located in rift valleys.[2]Lake Baikal inSiberia, aWorld Heritage Site,[3] 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.[4]Lake Tanganyika, second by both measures, is in theAlbertine Rift, the westernmost arm of the activeEast African Rift.Lake Superior inNorth America, the largestfreshwaterlake by area, lies in the ancient and dormantMidcontinent Rift. The largest subglacial lake,Lake Vostok, may also lie in an ancient rift valley.[5]Lake Nipissing andLake Timiskaming inOntario andQuebec,Canada lie inside a rift valley called theOttawa-Bonnechere Graben.[6]Þingvallavatn,Iceland's largest natural lake, is also an example of a rift lake.
Rift valleys are also known to occur on other terrestrial planets and natural satellites. The 4,000 km longValles Marineris on Mars is believed by planetary geologists to be a large rift system.[7][8] Some features of Venus, most notably, the 4,000 kmDevana Chasma[9] and a part of the western Eistla, and possibly also Alta and Bell Regio have been interpreted by some planetary geologists as rift valleys.[10][11] Some natural satellites also have prominent rift valleys. The 2,000 km longIthaca Chasma on Tethys in the Saturn system is a prominent example. Charon'sNostromo Chasma is the first confirmed in the Pluto system, however large chasms up to 950 km wide observed on Charon have also been tentatively interpreted by some as giant rifts, and similar formations have also been noted on Pluto.[12] A recent study suggests a complex system of ancient lunar rift valleys, includingVallis Rheita andVallis Alpes.[13] The Uranus system also has prominent examples, with large 'chasma' believed to be giant rift valley systems, most notably the 1492 km longMessina Chasma on Titania, 622 kmKachina Chasmata on Ariel,Verona Rupes on Miranda,[14] andMommur Chasma on Oberon.[15]
The best explanation is that Lake Vostok may lie in a rift valley, as does Lake Tanganyika in East Africa and Lake Baikal in Russia. The geography of Lake Vostok is indeed consistent with this notion, in that the lake has a crescent shape, just like Tanganyika and Baikal, and the side walls of the lake are relatively steep, at least on one side.
