Great Rift Valley
The
Great Rift Valley is a vast geographical and geological feature that runs north to south for some 5,000 km, from northern
Syria in
Southwest Asia to central
Mozambique in
East Africa. The
valley varies in width from thirty to one hundred kilometers, and in depth from a few hundred to several thousand metres. It was named by the explorer
John Walter Gregory.
The Great Rift Valley is the result of the
rifting and separation of the
African and
Arabian tectonic plates that began around 35 million years ago in the north, and by the ongoing separation of East Africa from the rest of Africa along the East African Rift, which began about 15 million years ago.
The northernmost part of the Rift forms the
Beqaa Valley in
Lebanon separating the
Lebanon Mountains and
Anti-Lebanon Mountains. To the south in
Israel, it is known as the
Hula Valley separating between the
Galilee mountains and the
Golan Heights. Further south, the valley becomes the
Jordan River, which flows southward through from
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 southwards, 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 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 two, 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 area 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.
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.
 |
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). |
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. Within a few million years, the lithosphere may rupture and eastern Africa will split off to form a new landmass. If spreading continues, 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.
The Rift Valley has been a rich source of fossils that allow study of
human evolution, especially in an area known as Piedmont. Because the rapidly eroding highlands have filled the valley with sediments, a favourable environment for the preservation of remains has been 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 and
Meave Leakey have also done significant work in this region.
*
Graben*
Horst (geology)*
Small simple-coloured map*
Maps from the Albertine Rift Programme
