Why has East Africa been such a productive place to look for evidence of early hominids? The answer to that question lies in the geological history of Africa. There are two general conditions necessary for the discovery of very old fossils of land animals. The first is that the animals have to inhabit an area where rapid deposition, preferably by very fine grained materials, occurs. Moreover, that deposition has to occur over a long period of time. The second is that eventually erosion must expose the lowest strata so that the fossils can be detected by human observers. The tectonic and geological history of Africa has provided both of these conditions in the Rift Valleys of East Africa, where many of the earliest fossil remains of humans and their tools have been discovered.

Map of Africa and Adjacent Eurasia Showing Important Geophysical Features. Features are described below.

CAMEROON RIFT

The Cameroon Rift is an extremely long and straight rift valley dating to the Cenozoic Era (65-0 million years ago). Its formation resulted in substantial volcanic activity in west and central Africa. The rift was caused by severe warping and uplifting of the craton (as much as 2,000 meters). The uplifting caused expansion of the crust and the resultant collapse of the arch crests along normal faults. The collapse formed the rift valley, and the breaks in the crust along the fault lines allowed magma to escape near the surface, causing increased volcanism. The rift is continued off of the Afro-Arabian Craton by a 2500 km long sub-oceanic ridge, the Guinea Ridge, that terminates at St. Helena Island. It is possible that continued continental shifting may extend the rift as far north as the Mediterranean Sea, effectively splitting West Africa from Central Africa.

WEST AFRICAN MOBILE BELT

The West African Mobile Belt is a Cenozoic relict area where materials eroded from the Afro-Arabian Craton were deposited at the western continental margin. Much of this area was later uplifted by continental displacement. The Mobile Belt may have at one time been coterminous with the mobile belt along the eastern margin of North America that eventually gave rise to the Appalachian Mountains. The Belt dates to the Late Paleozoic (500-230 million years ago).

CAPE MOBILE BELT

The Cape Mobile Belt is a Cenozoic relict area where materials eroded from the Afro-Arabian Craton were deposited at the southern continental margin. Much of this area was later uplifted by continental displacement. This Mobile Belt was probably never coterminous with any of the other land masses of the period. The Belt dates to the Late Paleozoic (500-230 million years ago).

TETHYAN MOBILE BELT

The Tethyan Mobile Belt actually extends from this area, across northern India, through the Himalaya Mountain Range, and finally terminates in the vicinity of Indonesia. This Mobile Belt was formed as the result of the deposition of erosional materials from the Afro-Arabian craton, the European craton, the Ural-Mongolian Mobile Belt, the Chinese craton, and the Indian craton. The belt dates to the Late Mesozoic-Early Cenozoic eras (100-25 million years ago). As you can see, most of the current landmass of southern Europe is built from this material.

RED SEA RIFT

The Red Sea Rift began during the Miocene Epoch (about 25 million years ago) and continues today. Its formation is related to the formation of the Aden Rift. The two rifts have now effectively separated Africa from Arabia, although the two were once part of the same landmass, the Afro-Arabian craton. It has been suggested that the rifting was caused by Africa being displaced 200 kilometers south and west of its original position. Both Rifts attain oceanic depths (i.e. 2,000 meters below sea level) over most of their extent. It is possible that Africa is actually pivoting away from Arabia.

ADEN RIFT

The Aden Rift began during the Miocene Epoch (about 25 million years ago) and continues today. Its formation is related to the formation of the Red Sea Rift. The two rifts have now effectively separated Africa from Arabia, although the two were once part of the same landmass, the Afro-Arabian craton. It has been suggested that the rifting was caused by Africa being displaced 200 kilometers south and west of its original position. Both Rifts attain oceanic depths (i.e. 2,000 meters below sea level) over most of their extent. It is possible that Africa is actually pivoting away from Arabia.

EAST AND WEST GREAT RIFT VALLEYS

The East and West Great Rift Valleys of East Africa offer some of the richest beds of fossils dating from the Miocene and younger to be found in the world. With the exception of the South African cave deposits, all of the most important type fossils for Australopithecus and early Homo have been found within the Rift zone. The rifts were caused by severe warping and uplifting of the craton (as much as 2,000 meters). The uplifting caused expansion of the crust and the resultant collapse of the arch crests along normal faults. The collapse formed the rift valleys, and the breaks in the crust along the fault lines allowed magma to escape near the surface, causing increased volcanism. The rifts began forming during the Miocene Epoch (65 million years ago).

	1. Contents 2. Introduction 3. Tectonic Hist. of Af. 4. Hominid Sites 5. Map Old. Gorge 6. Olduvai Animation 7. Chronology 8. Conclusion Courseware Page
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