4 Folding and Faulting Activity CASE STUDY: EAST AFRICAN RIFT VALLEY Background
The East African Rift Valley has formed as a result of faulting on the landscape. Created over the last 20 million years, it stretches approximately 4,800 km from Syria to the Zambezi River (Fig. 4.8). The movement of convection currents in a magma plume under the earth’s crust caused the African continent to bulge, stretch and split. Two normal faults were created by the forces of tension and the centre portion of land slipped into the earth’s crust forming a flat-floored rift valley. The land subsided to such an extent that some places in the rift valley lie more than 153 m below current sea level. The rifting process continues with the valley widening at a rate of 4 mm per year. Unless this stops, it is likely that the continent of Africa will eventually split apart. A number of features have formed in the valley as a result of the rifting process:
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volcanoes such as Mount Kilimanjaro formed as magma forced its way through fissures on the earth’s surface;
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numerous lakes such as Lake Tanganyika have also been created on the floor of the rift valley.
Fig. 4.8 East African Rift Valley. Faults resulting from compression
Reverse fault A reverse fault is formed at a convergent plate boundary when compression in the earth’s crust causes the crust to fracture and one block of land moves upwards (Fig. 4.9).
Chapter 1:
Plate Tectonics Chapter 2:
Volcanic Activity Tanganyika Lake
EURASIAN PLATE
Nile River
ARABIAN PLATE
NUBIAN PLATE
Mt Kenya Mt Kilimanjaro Victoria Lake
Danakil Depression
SOMALIAN PLATE
Hanging wall
Footwall Fault plane
Reverse fault: compression force – one section of crust is uplifted
Thrust fault
A thrust fault is a type of reverse fault commonly found in areas where one plate is being subducted under another (e.g. Japan). The fault plane is inclined at an angle equal to or less than 45° (Fig. 4.10).
Hanging wall
Footwall Fault plane
Thrust fault: compression force – angle of fault is equal or less than 45°
