Geology

The Great Rift System of Africa runs over 7000 km and includes the East and West branch of the Rift system and its extension through the Red Sea into the Dead Sea. The eastern branch of the Rift is arid with little rainfall, while the west is humid with lots of rivers and lakes. The land plates separate to form deep valleys in which water fills creating lakes and rivers. Volcanic sites in the rift give the lakes a peculiar chemistry, containing unique suits of evaporitic minerals.

The Rift system has many of the geophysical characteristics of a mid-ocean ridge and is laterally continuous with the Red Sea and Gulf of Aden spreading centers. Although all three were initiated simultaneously on continental crust, in pre-Miocene times, that in East Africa did not evolve into oceanic crust, although the crust is thin (unlike continental crust), and the underlying upper mantle has similar features to mid ocean ridges. The rift is a mid-plate phenomenon involving magmatic, seismic or deformational activity which is not directly attributable to the interations between plate margins.

The main rift phase was during the Cretracious when substantial and rapid stretching of the lithosphere caused thinning of the crust and passive up welling of the hot asthenosphere. It developed as a result of large extensional stresses, continental disruption and high sea-floor spreading rates. The rift consists of strike-slip/oblique-slip movement between two laterally moving blocks. Not far away from the rift are mountainous zones of compression and uplift which, when eroded, send the sediments down to the rift basin.

The main controversy with regard to extensional rift valleys is whether magmatic activity in rifts reflects an underlying heat source (such as a convection plume in the mantle or a hot spot), the rifting then being a consequence of expansion and uplift brought about by thermal activity, or whether the magmatic activity is merely the result of regional or local extensional stresses which have passively allowed magma to come to the surface.

A mantle plume could have initiated the Afar Triple Junction breakup between the East African rift (continental crust), the Red Sea and the Gulf of Aden (both oceanic crusts), but since the Miocene, the continent has moved 10 degrees in latitude and probably has moved longitudinally as well, so it is not understood how the supposedly "fixed" mantle plume is still under the rift today (a slowly migrating plume-like mantle flow could account for the heat under the rift today).

Basaltic volcanism began in the early Tertiary along the rift. Volcanism has been particularly important in preserving faunal remains in East Africa, volcanic activity was a source of sediment for burial, and in East Africa it supplied carbonatite ash, which chemically aided fossilization. Moreover, subaerial ash falls have preserved faunal samples of a type not generally represented in riverine or lacustrine sediment.

The Hadar Formation, in the Afar depression of Ethiopia, is renowned for its superb mammalian fossils, hominid fossils and ardeulian or oldowan artifacts. Hominid-bearing sands and clays which were laid down at Hadar towards 3.1-2.8Ma may be mainly fluviatile, reflecting both local and allochthorious sediment influx.

Early Environments and Hominid Evidence

Looking at the fauna and species indicates well what environments might have been taking place at that time. A change in diversity of fauna depict change in environment, and that the species were adapting. During the first half of the Cenozoic era (after 65 Ma), warm, moist conditions are thought to have prevailed over much of Africa, with the result that lowland-type rain forests were probably more widespread than they have been since. By around 18 Ma, however, relatively dry, open woodlands had interrupted the more continuous African forest of earlier times. During this time we see the appearance of the first ape-like hominids.

African climates, especially after mid-Miocene time (around 12 Ma), were becoming dryer and more seasonal in character. Changes in African climate and hominoid evolution paralleled changes in the landscape itself, as rifting and uplift produced the highlands of East Africa.

By the early Pliocene (around 5 Ma), the trend to cooler, drier environments was apparently well established, and open grasslands were perhaps becoming a more prevalent feature of the vegetation. The famous "Lucy" was from this time; a bipedal but still well-adapted to climbing trees.

Early man adapted to bipedalism (walking on two feet) because of the environment. It was advantageous because they could now carry things, including dependent offspring or food, it changed their feeding posture, and bipedalism increased locomotor efficiency. Because there was a pronounced drying trend in East Africa at this time, most scenarios invoke environmental change as the underlying cause of the bipedalism.

During the late Pliocene and early Pleistocene (especially around 2.4 and 1.8 Ma) further cooling and drying of the climate resulted in a major expansion of grass land and desert environments, enhancing the opportunity for our ancestors to evolve feeding strategies adapted to open country. This was a time of maximum hominid diversity.

Exactly what role carnivory played in the diets of various hominid species is still a matter of contention among paleoanthropologists, but evidence for cut-marks on bones of large mammals, apparently the result of butchery with stone tools, dates back to 1.8 Ma.

Homo erectus with the use of a simple tool-kit of choppers and hand axes, and possibly the use of fire, made its mark on the world. It may also have created a wave, or waves of megafaunal extinctions in the Middle Pleistocene. The evolution of intelligence and tool use over the last few million years represented the beginning of what humans have been doing ever since: modifying the environment to suit the needs of the organism, rather than passively adapting to the environment.

Gregory Rift Valley

550 hominid fossils have been discovered with in the last eighteen years (as of 1978) in the Gregory Rift Valley. The valley is only 40-80 km in width, but over 2000 km in length. It is located in Kenya and was the last part of the Rift System to become known. The first penetrations into the rift area were in 1883. Then in 1892-3, J.W. Gregory journeyed to the rift valley. He published a paper about it which marked the beginning of the scientific understanding of the rift valley. He recognized the rift to be a true graben (fault trough) and observed much of the faulting to be recent because closely spaced block-and-trough strips were relatively shallow features, due to crustal extension. Only during the last 2 million years has the largest vertical movements in the Gregory Rift occurred.

Evidence of Hominid Activity with in the Gregory Rift Valley

The large expanse of sedimentary exposures to the east of Lake Turkana in North Kenya contains a unique fossil and artifact record that documents the existence of early hominids during the Pliocene/early Pleistocene and in addition provides evidence of their activities. For example, rock flakes and flake fragments have been found associated with bone debris. One interpretation for the association of flakes and small stone fragments with bones, is that they functioned as cutting and scraping implements in the butchering and processing of meat.

The majority of the artifacts were made of lava (mostly trachybasalt); also used were feldspathic ignimbrite and rhyolite. Early hominids were hunters and gatherers who used a wide range of resources and habitats. This is consistent with the idea that early hominids were mobile. The distribution of known sites indicates that the center of hominid activities was at or near to a water source (streams or river channels). This was advantageous for food and resources.

References

Edited by L.E. Frostick, R.W. Renaut, I. Reid and J-J. Tiercelin 1986, Sedimentation in the African Rifts, Blackwell Scientific Publications, England

Edited by T.R.McClanahan and T.P. Young 1996, East African Ecosystems and their Conservation Oxford University Press, England

Edited by Walter W. Bishop 1978, Geological Background to Fossil Man; Recent research in the Gregory Rift Valley, East Africa, Scottish Academic Press LTD, England