rift valley

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

A rift valley near Quilotoa, Ecuador.

The Ottawa-Bonnechere Graben

A rift valley is a linear-shaped lowland between highlands or mountain ranges created by the action of a geologic rift or fault. This action is manifest as crustal extension, a 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 it will do so such that a center block will drop down relative to its flanking blocks. This creates the nearly parallel steeply dipping walls. This feature is the beginning of the rift valley. As this process continues the valley gets wider and wider until it becomes a large basin that fills with sediment from the rift walls and the surrounding area. One of the better long term examples of this process is the Basin and Range province in Nevada and Utah. Rifts can occur at all elevations, from the sea floor to plateaus and mountain ranges. They can occur in continental crust or in oceanic crust. Rift valleys are often associated with a number of adjoining subsidiary or co-extensive valleys which are typically considered geologically part of the principal rift valley.

The most extensive rift valley is located along the crest of the mid-ocean ridge system and is the result of sea floor spreading. Examples of this type of rift include the Mid-Atlantic Ridge and the East Pacific Rise.

Many existing continental rift valleys are the result of a failed arm (aulacogen) of a triple junction, although there are two, the East African Rift and the Baikal Rift Zone, which are currently active, as well as a third which may be, the West Antarctic Rift. In these instances, not only the crust, but also entire tectonic plates are in the process of breaking apart to create 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, 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 or landslides.

Rift valley lakes

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. The largest subglacial lake, Lake Vostok, may also lie in an ancient rift valley.^[4] Lake Nipissing and Lake Timiskaming in Ontario and Quebec, Canada lie inside a rift valley called the Ottawa-Bonnechere Graben.^[5] Þingvallavatn, Icelands largest lake, is also an example of a rift lakes.

References

1. ^ "The World's Greatest Lakes". http://www.schoolofflyfishing.com/resources/worldslakes.htm. Retrieved 2007-01-07. 
2. ^ "Lake Baikal - World Heritage Site". World Heritage. http://whc.unesco.org/en/list/754. Retrieved 2007-01-13. 
3. ^ "The Oddities of Lake Baikal". Alaska Science Forum. http://www.gi.alaska.edu/ScienceForum/ASF9/986.html. Retrieved 2007-01-07. 
4. ^ Siegert, Martin J. (1999). "Antarctica's Lake Vostok". American Scientist 87 (6): 510. doi:10.1511/1999.6.510. http://www.americanscientist.org/template/AssetDetail/assetid/15820/page/3;jsessionid=baa9...#26308. Retrieved 2007-12-12. "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.". 
5. ^ John Grotzinger .... (2006). Understanding Earth.. New York: W. H. Freeman. ISBN 0716776960. 

Further reading

• 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.