Great Rift Valley
From Wikipedia, the free encyclopedia
The Great Rift Valley is a vast geographical and geological feature, approximately 6,000 km in length, which 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 plates of the earth's crust join. The rift valley thereby formed varies in width from thirty to one hundred kilometers, and in depth from a few hundred to several thousand meters. 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. 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 Lake Tiberias in Israel and 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 Eastern Rift and the Western Rift.
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, these lakes tend to be shallow and 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 Lake Naivasha needs to be supplied by freshwater springs to support its 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. 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 volcano in the world.
[edit] Discoveries in human evolution
The Rift Valley has been a rich source of fossils that allow study of human evolution, especially in an area known as Piedmont. Because the rapidly eroding highlands have filled the valley with sediments, a favourable 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", a nearly complete australopithecine skeleton, which was discovered by anthropologist Donald Johanson. Richard and Mary Leakey have also done significant work in this region.
