GEOTHERM supports East Africa - regional boundary conditions for geothermal energy use
Federal Institute for Geosciences and Natural Ressources (BGR), Stilleweg 2, 30655 Hannover, Germany (GEOTHERM working group)
The 4000 km long and in average 60 km wide East African Rift System is visible from the moon. It is the largest crack in the continental crust on Earth and consists of two branches (Figure 1). Eastern branch of East African Rift System
30°E 35° R 45° e Eritrea d S Tanzania Kenya Ethiopia e Djibouti a 15°N Afar triangle f A f o fa ul n tr r G de ian A gl 10° e S N Lengai Erta Ale al crust continent 5° Lake Turkana
Figure 3 Schematic sketch of eastern rift with central volcanoes and changing thickness of the continental crust 0° Indian The continental crust of the eastern rift area (Figure 3) thickens Rifting of the eastern branch with a rate of millimetre per year is Ocean from North (Afar triangle) to South (Tanzania) and the amount of caused by uprising mantle plumes. These plumes are transporting Lake dilatation and age of rifting decreases in the same direction melts and heat to the earth surface and bulge the continental crust. Victoria (Figure 4). If the tensional forces exceed the breaking strength of the up- warped crustal rocks cracks occur (i.e. the tension is released by 5° earthquakes) leading to a Y-shaped triple junction at the spherical Lake Tanganiyka earth surface (Figure 4). Rifting started 30 million years ago in the Afar region (dilatation of continental crust up to 60 km), xx Ma ago in northern Kenya (dilatation 35-40 km), 12 Ma ago in central Kenya (dilatation 5-10 East km) and less than 5 Ma ago in northern Tanzania (dilatation less 10° than 5 km). The continental crust is composed of units different in 15° 30°E 35° 45° Lake African age, composition, thickness, rigidity etc. and the rift mainly follows A pre-existing zones of weakness along contacts between these fa Malawi t r Rift units. The rift is not a symmetrical structure because it consists ria mainly of linked half-grabens (Figure 5). ng 10° le 15° System
35° R 45°E 20°S 45°E S e 5° e d a Lake Turkana Figure 1 Simplified map of the East African Rift System 15°N The eastern branch is extending from the southern Red Sea (Afar triangle) to Tanzania and the western branch is extending from of Uganda to Mozambique. A much bigger volume of volcanic rocks Gulf 0° were erupted in the eastern branch of the rift compared to the 30 my en western branch (Figure 2). Most of the eastern rift belongs to Ad Indian Ethiopia and Kenya and therefore the highest numbers of active Ocean volcanoes and geothermal resources are found in these two 10° Lake countries. Victoria 5° 60 km Lake Tanganiyka Vol- Half-Graben Model xx my Rifting canic 5° Pro- 40 km 10° 12 my Lake Malawi ducts
10 km 15°S 45°E 0° Indian Figure 2 Difference in volcanic activity in the two branches of the Ocean East African Rift System 5 my 5 km
5°S 45°E Two arms of the triple junction will open to an ocean by forming Figure 4 Maximum extension of continental crust, age of initial new oceanic crust and the third arm will be a “failed rift”. The Red rifting in million years before present and triple junction formed by Sea and the Gulf of Aden are the two new oceanic arms spreading Red Sea, Gulf of Aden and East African Rift System with a rate of centimetre per year and the rift valley is the remaining third arm which will never open to an ocean. Therefore it is most unlikely that East Africa will be split off from the rest of the African continent like it happened to Madagascar 165 million years ago.. Figure 5 Schematic sketch of linked half-grabens
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