Great Rift Valley

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Great Rift Valley,

geological faultfault,
in geology, fracture in the earth's crust in which the rock on one side of the fracture has measurable movement in relation to the rock on the other side. Faults on other planets and satellites of the solar system also have been recognized.
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 system of SW Asia and E Africa. It extends c.3,000 mi (4,830 km) from N Syria to central Mozambique. The northernmost extension runs S through Syria and Lebanon, the Jordan valley, the Dead Sea, and the Gulf of Aqaba. It continues into the trough of the Red Sea and at the southern end branches into the Gulf of Aden, where it continues as part of the Mid-Oceanic Ridge of the Indian Ocean. The main section of the valley in Africa continues from the Red Sea SW across Ethiopia and S across Kenya, Tanzania, and Malawi to the lower Zambezi River valley in Mozambique. Many small lakes in Ethiopia and several long narrow lakes, notably lakes Turkana and Nyasa, lie on its course. Just N of Lake Nyasa there is a western branch, which runs north, chiefly along the eastern border of Congo (Kinshasa); this branch is marked by a chain of lakes, including lakes Tanganyika, Kivu, Edward, and Albert (Mobutu). Lake Victoria does not lie in the Great Rift Valley but between its main and western branches. The Great Rift Valley ranges in elevation from c.1,300 ft (395 m) below sea level (the Dead Sea) to c.6,000 ft (1,830 m) above sea level in S Kenya. Erosion has concealed some sections, but in places, notably in Kenya, there are sheer cliffs several thousand feet high. The present configuration of the rift, which dates from the mid-Pleistocene epoch, is probably a result of a rifting process associated with thermal currents in the earth's mantle (see rift valleyrift valley,
elongated depression, trough, or graben in the earth's crust, bounded on both sides by normal faults and occurring on the continents or under the oceans. The central flat block forming the trough slips downward relative to the crustal blocks on either side.
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); there is evidence of earlier rift structures.

Great Rift Valley

 

a system of major faults and grabens (rifts) that have developed against a background of recent uplifts in east Africa. The valley stretches meridionally from the northern edge of the Red Sea to the lower course of the Zambezi River. It also includes the graben of the Red Sea and its spurs (the Gulfs of Suez and Aqaba), as well as the graben of the Gulf of Aden. To the north of the Gulf of Aqaba, the Rift Valley continues across the Dead Sea and the Jordan River valley to the foothills of the Taurus Mountains, paralleling the Mediterranean coast of the Arabian peninsula.

Within Africa, the valley consists of two branches. The Afar graben in Ethiopia and, further, the graben belt running to the east of Lake Victoria, past the Elgon, Kenya, and Kilimanjaro volcanoes, and from the basin of Lake Rudolf to Lake Nyasa, are a continuation of the graben of the Red Sea. Roughly parallel to this eastern rift is the western rift, which skirts the eastern shore of Lake Victoria across Lakes Albert, Edward, Kivu, Tanganyika, and Rukwa to the northern end of Lake Nyasa. To the south of Lake Nyasa, the Rift Valley follows the graben of the Shire River valley to the Zambezi River.

The Great Rift Valley in its present form began to develop in the Oligocene period simultaneously with the formation of the major uplifts and orogeny in the eastern part of Africa and Arabia. Here the recent faults have in part used the strikes of the more ancient faults, which date back as far as the Precambrian period. The movements along the faults led to a great outburst of volcanic activity, which reached a peak in the Neocene period and has continued until the modern era. All the active volcanoes in Africa (with the exception of Mount Cameroun) are located in this zone. The seismicity of the valley is evidence of the recent movements along the faults. The Great Rift Valley is part of the world rift system and shows a great similarity with oceanic rifts.

In the opinion of some geologists, the general cause of the development of the rifts, volcanic activity, and seismicity in the Rift Valley is the breakup of the subcrusted matter of the upper mantle as a consequence of the heat flow from deeper parts of the earth’s interior. Others view these phenomena as related to the extension of the earth’s crust.

REFERENCES

Dixie, F. Velikie afrikanskie razlomy. Moscow, 1959. (Translated from English.)
Milanovskii, E. E. “Osnovnye cherty stroeniia i formirovaniia riftovoi sistemy Vostochnoi Afriki i Aravii.” Vestnik MGU: Geologiia, 1969, no. 1.
V. E. KHAIN

Great Rift Valley

the most extensive rift in the earth's surface, extending from the Jordan valley in Syria to Mozambique; marked by a chain of steep-sided lakes, volcanoes, and escarpments
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