Great Rift Valley

This is the Rift Valley as it is visible near Eldoret, Kenya

The Great Rift Valley is a vast geographical and geological feature that runs north to south for around four thousand miles (six thousand kilometers), from northern Syria to central Mozambique in East Africa. Astronauts say it is the most significant physical detail on the planet that is visible from space. In Africa, the terrain ranges from salt flats more than five hundred feet below sea level to towering, snow-capped mountains. The Rift is known for its spectacular scenery and abundant, sometimes unique, wildlife. A series of some thirty lakes along its length, known as the Great Lakes of Africa, includes Lake Tanganyika, the deepest lake in the world, and Lake Victoria, the second largest freshwater lake in the world.

Geography

The valley varies in width from thirty to one hundred kilometers (twenty to sixty miles), and in depth from a few hundred to several thousand meters. The northernmost part of the Rift forms the Beqaa Valley in Lebanon. To the south, in Israel, it is known as the Hula Valley separating the Galilee mountains and the Golan Heights. Farther south, the valley becomes the Jordan River, which flows southward through Lake Hula into Lake Tiberias in Israel and then continues south through the Jordan Valley into the Dead Sea on the Israeli-Jordanian border. From the Dead Sea southward, the Rift is occupied by the Wadi Arabah and then the Gulf of Aqaba and the Red Sea.

The Jordan Rift Valley from space

The southern end of the Red Sea marks a fork in the rift. The Afar Triangle or Danakil Depression of Ethiopia and Eritrea is the probable location of a triple junction, which is possibly underlain by a mantle plume. The Gulf of Aden is an eastward continuation of the rift - before the rift opened, the Arabian Peninsula was attached to the Horn of Africa - and from this point the rift continues as part of the mid-oceanic ridge of the Indian Ocean. In a southwest direction the fault continues as the Great Rift Valley, which split the older Ethiopian highlands into two halves.

In eastern Africa the valley divides into the Eastern Rift and the Western Rift. The Western Rift, also called the Albertine Rift, is edged by some of the highest mountains in Africa, including the Virunga Mountains, Mitumba Mountains, and Ruwenzori Range, and contains the Rift Valley lakes, which include some of the deepest lakes in the world (up to 1,470 meters deep at Lake Tanganyika). Lake Victoria, the second largest freshwater lake in the world, is considered part of the Rift Valley system although it actually lies between the two branches. The other Great Lakes are also formed by the rift.

Many sites in the Albertine Rift are protected as national parks, wildlife reserves, or forest reserves. But there are several other sites important for conservation that require legal protection in the future, mainly in the Democratic Republic of Congo.

Northern section of the Great Rift Valley. The Sinai Peninsula is in the center and the Dead Sea and Jordan River valley are above.

In Kenya the valley is deepest to the north of Nairobi. As the lakes in the Eastern Rift have no outlet to the sea, these lakes tend to be shallow and have a high mineral content as the evaporation of water leaves the salts behind. For example, Lake Magadi is almost solid soda (sodium carbonate), and Lake Elmenteita, Lake Baringo, Lake Bogoria, and Lake Nakuru are all strongly alkaline, while Lake Naivasha needs to be supplied by freshwater springs to support its biological variety. Subterranean movement is common today as the Rift Valley is home to thirty active and semi-active volcanoes and countless hot springs along its length. This string of alkaline lakes and boiling springs northwest of Nairobi includes Lake Baringo, Lake Bogoria, Lake Nakuru, Lake Elementaita, Lake Naivasha, and Lake Magadi in the south. These lakes are unique because their water is highly concentrated sodium carbonate. This situation is caused by the high alkalinity from the surrounding volcanic rocks coupled with poor drainage outlets due to the steep sides of the valley. The high evaporation of the surface lake water results in sodium carbonate which, in turn, creates an ideal breeding ground for algae. Several species of fish, tilapia in particular, thrive in this environment. As a result, millions of birds flock to these soda lakes to feast on the abundant food supply of algae and fish. Each of the lakes in the Rift Valley string have a slightly different water composition ranging from freshwater to extremely alkaline, highly saline to brackish.

Map of East Africa showing some of the historically active volcanoes (red triangles) and the Afar Triangle (shaded, center) — a triple junction where three plates are pulling away from one another: the Arabian Plate, and the two parts of the African Plate (the Nubian and the Somalian) splitting along the East African Rift Zone (USGS).

Pre-colonial history

When the Bantu peoples left West Africa about five thousand years ago, one branch moved east and reached the Great Lakes region two thousand years later. The descendants of the groups that remained established the Buganda, Bunyoro, and Karagwe Kingdoms, while others continued on to the Indian Ocean coast or moved south to Lake Malawi.

History of exploration

The first Europeans to penetrate Central and East Africa reported snow at the equator but were not initially believed. The Royal Geographical Society sent an expedition to the region in search of the Nile's source. The detailed reports of that trip helped Austrian geologist Eduard Suess formulate his theory that the system of valleys and lakes that crossed Arabia and Africa was part of a continuous trough. In 1891 he published a paper attributing the geology and structural geography between Syria and Malawi to interconnected movements of the earth. The following year, John Gregory set out for East Africa. His comparisons of geological strata led him to conclude that, indeed, faulting, not erosion, had formed the valley to which he gave the name "Great Rift Valley," the name still used for the entire African Rift.

Geothermal activity

The formation of the Rift Valley continues, probably driven by mantle plumes and ultimately a result of the African superswell. The associated geothermal activity and spreading at the rift has caused the lithosphere to thin from a typical 100 km thickness for continents to a mere 20 km. Though it is common for one arm of a triple junction to fail, if spreading continues the lithosphere may rupture several million years hence, splitting eastern Africa off to form a new landmass. In short, this will lead to the formation of a new mid-ocean ridge.

The volcanic activity at this site and unusual concentration of hotspots has produced the volcanic mountains Mount Kilimanjaro, Mount Kenya, Mount Karisimbi, Mount Nyiragongo, Mount Meru and Mount Elgon as well as the Crater Highlands in Tanzania. The Ol Doinyo Lengai volcano remains active, and is currently the only natrocarbonatite volcano in the world.

Discoveries in human evolution

The Rift Valley has been a rich source of fossils that allow study of human evolution, especially in an area known as Piedmont. Richard Leakey has speculated that the diversity of environments resulting from the creation of the Rift Valley provided "an ideal setting for evolutionary change." The presence of multiple lakes in the region would have drawn animals, including hominids. Furthermore, the rise and fall of lake levels and seasonal flooding of rivers aided in burying and preserving bones. The numerous active volcanoes in the region provided volcanic ash that would later allow dating of the fossils that wre found.

There is no reason to conclude that early hominids were confined to East Africa, but because the rapidly eroding highlands filled the valley with sediments, a favorable environment for the preservation of remains was created. The bones of several hominid ancestors of modern humans have been found there, including those of "Lucy", a nearly complete australopithecine skeleton, which was discovered by anthropologist Donald Johanson. Richard as well as Mary Leakey have done significant work in this region.

It has been suggested by scientists that the Rift's trough may have been an important route for human dispersal.

Flora and fauna

A wide variety of fauna inhabit the Rift Valley. Those parts that are grasslands contain a greater concentration of wild animals than any other area of comparable size. Species unique to the Rift Valley include the mountain gorilla that is found on the forested slopes of the Virunga volcanoes and fish that dwell in the alkaline springs.

The wide variety of terrains—from desert to mountain—results in adaptation of flora to altitude, temperature, and rainfall totals.

References

ISBN links support NWE through referral fees

• Donald Johanson and Blake Edgar, From Lucy to Language, 1996. Simon & Schuster, New York, NY. ISBN 0684810239
• Nigel Pavitt, Africa's Great Rift Valley, 2001. Harry N. Abrams, New York, NY. ISBN 0810906023
• Richard E. Leakey, The Making of Mankind, 1981. E.P. Dutton, New York, NY. ISBN 0525150552

External links

• Small simple-coloured map
• Maps from the Albertine Rift Programme

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