Valley of the great rift
The valley of the great rift (or valley of the African rift , or great East-African rift ) is a geological element major, which extends from the south of the Red Sea (in north) with the Zambezi (in the south) on more: 9500 km length, 40 to 60 km of width and a few hundreds to a few thousands of meters of depth.
GeologyThis zone of Extension intracontinentale joined in north two extensive structures (old oceanized rifts ) which limit the Arabic plate: the Red Sea and the Gulf of Aden. The Point triples Afar which connects these three structures is a major volcanic zone cut out by many normal faults. The Rifting begins with the Miocène, and collapse causes an important lake sedimentation (until: 8000 m). Many lakes currently occupy the rift (Lac Kivu, Lac Tanganyika, Lac Malawi).
The speed of opening is about 10 mm/an and decreases towards the south. The two branches of the rift are connected by a zone of important fracturing, the feature of Assoua. The Kilimandjaro and the Mont Kenya are located at the intersection between the Eastern branch and this feature. The continuation of this extension intracontinentale can succeed, in the next million years to a Océanisation and with the individualization of a Somali plate .
The valley of the great rift knows a very great volcanic activity: there nowhere exists a very great complexity of visible volcanicity elsewhere. One can quote here:
- the Erta Ale, Volcano very active shield effusive projecting fluid lava;
- the Dallol, a volcanic site of the Afar known by its single geology. This volcanic crater is composed of extended of salt and sulfur. Acid lakes overhang terraces limestones and the hornitos and other small geysers pullulate there;
- the Ol Doinyo Lengaï, mixed volcano rejecting a single fluid lava in the world, the Carbonatite. While cooling, this lava becomes white. On the minis Caldeira of the crater, cones of this white Basalte emit small carbonatite castings and gray smoke.
- the Kilimandjaro and the Mount Kenya, effusive volcanos very high equipped with a Glacier and respectively natural symbols with the Tanzania and Kenya.
A theory concerning the appearance of the human line makes play a leading role with the formation of the rift. Known under the name of East Side Story , she was proposed by A. Kortlandt then popularized by Yves Coppens , . The formation of the rift would have led to a climatic and environmental differentation major between the area located at the west, wet and timbered, and the area located at the east, much drier and occupied by savanna. Starting from a common stock, two evolutionary lines would have diverged leading to the west with the large arboricolous monkeys, and the east with the Australopithèque S, group of biped Hominides including the ancestors of Homo sapiens . The appearance of the bipédie would be an adaptation to savanna.
This tempting model was called into question by the locomotion still largely arboricolous of certain Australopithecus, then by the discovery of Australopithecus bahrelghazali in Chad: 2500 km in the west of the rift.
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