A rift valley
is a linear shaped lowland between several highlands
or mountain ranges created by the action of a geologic rift
. A rift valley is formed on a divergent plate boundary, a crustal extension or spreading apart of the surface, which is subsequently further deepened by the forces of erosion. When the tensional forces are strong enough to cause the plate to split apart, a center block drops between the two blocks at its flanks, forming a graben
. The drop of the center creates the nearly parallel steeply dipping walls of a rift valley when it is new. That feature is the beginning of the rift valley, but as the process continues, the valley widens, until it becomes a large basin that fills with sediment from the rift walls and the surrounding area. One of the best known examples of this process is the East African Rift
On Earth, rifts can occur at all elevations, from the sea floor to plateaus and mountain ranges in continental crust
or in oceanic 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.
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, the Dead Sea
lies in a rift which results from a leftward discontinuity in the left lateral-moving Dead 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 as sag ponds
Rift valley lakes
Many of the world's largest lakes are located in rift valleys. Lake Baikal
, a World Heritage Site
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. Lake Tanganyika
, second by both measures, is in the Albertine Rift
, the westernmost arm of the active East African Rift
. Lake Superior
in North America
, the largest freshwater lake
by area, lies in the ancient and dormant Midcontinent Rift
. The largest subglacial lake, Lake Vostok
, may also lie in an ancient rift valley. Lake Nipissing
and Lake Timiskaming
lie inside a rift valley called the Ottawa-Bonnechere Graben
largest natural lake, is also an example of a rift lake.
Extraterrestrial rift valleys
Extraterrestrial rift valleys are also known to occur on other terrestrial planets and natural satellites. The 4,000 km long Valles Marineris
on Mars is believed by planetary geologists to be a large rift system.
Some features of Venus, most notably, the 4,000 km Devana Chasma
and a part of the western Eistla, and possibly also Alta and Bell Regio have been interpreted by some planetary geologists as rift valleys.
Some natural satellites also have prominent rift valleys. The 2,000 km long Ithaca Chasma
on Tethys in the Saturn system is a prominent example. Charon's Nostromo 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.
A recent study suggests a complex system of ancient lunar rift valleys, including Vallis Rheita
and Vallis Alpes
The Uranus system also has prominent examples, with large 'chasma' believed to be giant rift valley systems, most notably the 1492 km long Messina Chasma
on Titania, 622 km Kachina Chasmata
on Ariel, Verona Rupes
and Mommur Chasma
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Last edited on 31 December 2020, at 12:44
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