The Afar Depression (also called the Afar Triangle, the Danakil Depression, or the Denakil Plain) is a geological depression in the Horn of Africa, where it overlaps Eritrea and the Afar Region of Ethiopia, and slightly touches Djibouti and Somalia. Live volcanoes (the “Denakil Alps”) separate it from the Red Sea. Nomadic pastoralists, related to the Afar people of Djibouti, are virtually the plain’s only inhabitants.
The Afar Depression is a formidable landscape which includes the Danakil Desert and the lowest point in Africa, Lake Asal, less than 155 meters (510 ft) below sea level. Dallol, Ethiopia is also part of the Depression, one of the hottest places year round anywhere on Earth. The only river that flows into the Depression is the Awash River, which ends in a chain of salt lakes, where its water evaporates as quickly as it is supplied. About 1,200 square kilometers (460 sq mi) of the Afar Depression is covered by salt, and salt mining remains a major source of income for many Afar tribes, who cut the salt into bars and carry it by mule and/or camel to other parts of Ethiopia and Eritrea.
The Afar Depression is well known as one of the cradles of hominids with the oldest bones discovered in the world having come from this area. The Middle Awash is the site of many fossil hominid discoveries; Gona, site of the world’s oldest stone tools; and Hadar, site of “Lucy,” the famous fossilized specimen of Australopithecus afarensis.
The Afar Depression results from the presence of a tectonic triple junction (the Afar Triple Junction) where the spreading ridges that are forming the Red Sea and the Gulf of Aden emerge on land and meet the East African Rift. The central meeting place for these three pieces of Earth’s crust is around Lake Abbe. The Afar Depression is one of two places on Earth where a mid-ocean ridge can be studied on land, the other being Iceland.
In the Depression, the earth’s crust is slowly rifting apart at a rate of 1 to 2 centimeters (0.3–0.8 in) per year along each of the three rifts which form the “legs” of the triple junction. The immediate consequence of this is that there are a continuous sequence of earthquakes, fissures hundreds of meters long and deep appearing in the ground, and the valley floor sinking as much as 100 meters. Between September and October 2005, 163 earthquakes of magnitudes greater than 3.9 and a volcanic eruption occurred within the Afar rift. 2.5 cubic kilometers of molten rock was injected into the plate along a dyke between depths of 2 and 9 km, forcing open an 8 meter wide gap on the surface.
Over millions of years, geologists expect the Red Sea to erode through the highlands surrounding the Afar Depression and flood the valley. In about 10 million years, geologists predict that the whole 6,000 km length of the East African Rift will be submerged, forming a new sea as large as the Red Sea is now. At that point, Africa will have lost its horn.
The floor of the Afar Depression is composed of lava, mostly basalt. One of Earth’s great active volcanoes, Erta Ale, is found here. The Afar Depression is, in the views of some geologists, underlain by a mantle plume, a great up-welling of mantle that melts to yield basalt as it approaches the surface.