Great Rift Valley
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The Great Rift Valley is a geographical and geological feature, approximately 6,000 kilometres (3,700 mi) in length, that runs from northern Syria in Southwest Asia to central Mozambique in East Africa. Caused by the geological process of rifting, it is a complex feature where several tectonic plates of the earth's crust join. The rift valley varies from 30 to 100 kilometers in width and from a few hundred to several thousand meters in depth. It was named by the explorer John Walter Gregory.
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[edit] Geography
The great rift system extends from Lebanon in the north to Mozambique in the south, and constitutes one of two distinct physiographic provinces of the East African Highlands physiographic division. The northernmost part of the Rift forms the Beqaa Valley in Lebanon separating the Lebanon Mountains and Anti-Lebanon Mountains. Further south it is known as the Hula Valley separating the Galilee mountains and the Golan Heights. The Jordan River begins here and flows southward through Lake Hula into the Sea of Galilee in Israel, then continues south through the Jordan Valley into the Dead Sea on the Israeli-Jordanian border. From the Dead Sea southwards, the Rift is occupied by the Wadi Arabah, then the Gulf of Aqaba, and then the Red Sea.
The southern end of the Red Sea marks a fork in the rift. The Afar Triangle or Danakil Depression of Ethiopia and Eritrea is the probable location of a triple junction which is possibly underlain by a mantle plume. The Gulf of Aden is an eastward continuation of the rift - before the rift opened, the Arabian Peninsula was attached to the Horn of Africa - and from this point the rift continues as part of the Mid-oceanic ridge of the Indian Ocean. In a southwest direction the fault continues as the Great Rift Valley, which split the older Ethiopian highlands into two halves.
In eastern Africa the valley divides into two, the Western Rift and the Eastern Rift Valley.
The Western Rift, also called the Albertine Rift, is edged by some of the highest mountains in Africa, including the Virunga Mountains, Mitumba Mountains, and Ruwenzori Range. It contains the Rift Valley lakes, which include some of the deepest lakes in the world (up to 1,470 meters deep at Lake Tanganyika). Lake Victoria, the second largest area freshwater lake in the world, is considered part of the Rift Valley system although it actually lies between the two branches. All of the African Great Lakes were formed as the result of the rift, and most lie within its rift valley.
In Kenya the valley is deepest to the north of Nairobi. As the lakes in the Eastern Rift have no outlet to the sea and tend to be shallow they have a high mineral content as the evaporation of water leaves the salts behind. For example, Lake Magadi has high concentrations of soda (sodium carbonate) and Lake Elmenteita, Lake Baringo, Lake Bogoria, and Lake Nakuru are all strongly alkaline, while the freshwater springs supplying Lake Naivasha are essential to support its current biological variety.
[edit] Geothermal activity
The formation of the Rift Valley continues, probably driven by mantle plumes and ultimately a result of the African superswell.[1] The associated geothermal activity and spreading at the rift has caused the lithosphere to thin from a typical 100 km thickness for continents to a mere 20 km. Though it is common for one arm of a triple junction to fail, if spreading continues the lithosphere may rupture several million years hence, splitting eastern Africa off to form a new landmass. In short, this will lead to the formation of a new mid-ocean ridge.
The volcanic activity at this site and unusual concentration of hotspots has produced the volcanic mountains Mount Kilimanjaro, Mount Kenya, Mount Karisimbi, Mount Nyiragongo, Mount Meru and Mount Elgon as well as the Crater Highlands in Tanzania. The Ol Doinyo Lengai volcano remains active, and is currently the only natrocarbonatite[2] volcano in the world.
The island Jabal al-Tair including the basaltic stratovolcano of the same name, is located northwest of the Bab al-Mandab passage at the mouth of the Red Sea, about half way between Yemen and Eritrea. This volcano erupted on 30 September 2007, after 124 years of dormancy.
[edit] Discoveries in human evolution
The Rift Valley has been a rich source of fossils[3] that allow study of human evolution, especially in an area known as Piedmont.[4] Because the rapidly eroding highlands have filled the valley with sediments, a favorable environment for the preservation of remains has been created. The bones of several hominid ancestors of modern humans have been found there, including those of "Lucy",[5] a nearly complete australopithecine skeleton, which was discovered by anthropologist Donald Johanson. Richard and Mary Leakey have also done significant work in this region.
More recently, 2 other hominid ancestors have been discovered there: a 10-million-years-old big ape called Chororapithecus abyssinicus, found in the Afar rift, in eastern Ethiopia,[6] and the Nakalipithecus nakayamai, who is also 10 million years old[7]
[edit] Further reading
- Africa's Great Rift Valley, 2001, ISBN 0810906023
- Tribes of the Great Rift Valley, 2007, ISBN 9780810994119
- East African Rift Valley lakes, 2006, OCLC 76876862
- Photographic atlas of the Mid-Atlantic Ridge Rift Valley, 1977, ISBN 0387902473
- Rift Valley fever : an emerging human and animal problem, 1982, ISBN 9241700637
[edit] See also
[edit] Notes
- ^ http://72.14.253.104/search?q=cache:SrozakeRnuIJ:www.geo.lsa.umich.edu/~crlb/RESEARCH/PAPERS/CLBSilver98.pdf+%22great+rift+valley%22+-wiki+superswell&hl=en&ct=clnk&cd=12&gl=us accessdate 2007-08-16
- ^ http://lycos.com/info/great-rift-valley.html accessdate 2007-08-16
- ^ http://whc.unesco.org/en/tentativelists/1580/ accessdate 2007-08-16
- ^ http://www.spiritus-temporis.com/great-rift-valley/ accessdate 2007-08-16
- ^ http://www.ethiopianriftvalleysafaris.com/ accessdate 2007-08-16
- ^ http://news.bbc.co.uk/2/hi/science/nature/6958313.stm accessdate 2007-08-23
- ^ http://news.bbc.co.uk/2/hi/science/nature/7092424.stm accessdate 2007-11-14.
[edit] External links
General sub-fields of Physical Geography | |
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