Definition of Afar Depression, Ethiopia

One of the world’s largest,

deepest regions below sea level that is subaerially exposed on

the continents, home to some of the earliest known hominid

fossils. It is a hot, arid region, where the Awash River drains

northward out of the East African rift system, and is evaporated

in Lake Abhe before it reaches the sea. It is located in eastern

Africa in Ethiopia and Eritrea, between Sudan and Somalia, and

across the Red Sea and Gulf of Aden from Yemen. The reason

the region is so topographically low is that it is located at a tectonic

triple junction, where three main plates are spreading

apart, causing regional subsidence. The Arabian plate is moving

northeast away from the African plate, and the Somali plate is

moving, at a much slower rate, to the southeast away from

Africa. The southern Red Sea and north-central Afar Depression

form two parallel north-northwest-trending rift basins,

separated by the Danakil Horst, related to the separation of

Arabia from Africa. Of the two rifts, the Afar depression is

exposed at the surface, whereas the Red Sea rift floor is submerged

below the sea. The north-central Afar rift is complex,

consisting of many grabens and horsts. The Afar Depression

merges southward with the northeast-striking Main Ethiopian

Rift, and eastward with the east-northeast-striking Gulf of

Aden. The Ethiopian Plateau bounds it on the west. Pliocene

volcanic rocks of the Afar stratoid series and the Pleistocene to

Recent volcanics of the Axial Ranges occupy the floor of the

Afar Depression. Miocene to recent detrital and chemical sediments

are intercalated with the volcanics in the basins.

The Main Ethiopian and North-Central Afar rifts are

part of the continental East African Rift System. These two

kinematically distinct rift systems, typical of intracontinental

rifting, are at different stages of evolution. In the north and

east, the continental rifts meet the oceanic rifts of the Red Sea

and the Gulf of Aden, respectively, both of which have propagated

into the continent. Seismic refraction and gravity studies

indicate that the thickness of the crust in the Main Ethiopian

Rift is less than or equal to 18.5 miles (30 km). In Afar the

thickness varies from 14 to 16 miles (23–26 km) in the south

to 8.5 miles (14 km) in the north. The plateau on both sides

of the rift has a crustal thickness of 21.5–27 miles (35–44

km). Rates of separation obtained from geologic and geodetic

studies indicate 0.1–0.2 inches (3–6 mm) per year across the

northern sector of the Main Ethiopian Rift between the

African and Somali plates. The rate of spreading between

Africa and Arabia across the North-Central Afar rift is relatively

faster, about 0.8 inches (20 mm) per year. Paleomagnetic

directions from Cenozoic basalts on the Arabian side of the

Gulf of Aden indicate 7 degrees of counterclockwise rotation

of the Arabian plate relative to Africa, and clockwise rotations

of up to 11 degrees for blocks in eastern Afar. The initiation

of extension on both sides of the southernmost Red Sea

Rift, Ethiopia, and Yemen appear coeval, with extension starting

between 22 million and 29 million years ago.

See also DIVERGENT OR EXTENSIONAL BOUNDARIES; RIFT.

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