TheAwash River is the main waterflow into the region, but it runs dry during the annual dry season, and ends as a chain ofsaline lakes. The northern part of the Afar Depression is also known as theDanakil Depression. The lowlands are affected by heat,drought, and minimal air circulation, and contain the hottest places (year-round average temperatures) of anywhere on Earth.
The Afar Triangle is bordered as follows (see the topographic map): on the west by theEthiopian Plateau andescarpment; to the north-east (between it and the Red Sea) by theDanakil block; to the south by the Somali Plateau and escarpment; and to the south-east by the Ali-Sabieh block (adjoining the Somali Plateau).[1]
Many important fossil localities exist in the Afar region, including theMiddle Awash region and the sites ofHadar,Dikika, and Woranso-Mille. These sites have produced specimens of the earliest (fossil) hominins and of human tool culture, as well as many fossils of various flora and fauna.
Dallol in the Danakil Depression is one of the hottest places year-round anywhere on Earth. There is no rain for most of the year; the yearly rainfall averages range from 100 to 200 mm (4 to 8 in), with even less rain falling closer to the coast.Daily mean temperatures at Dallol ranged from 30 °C (86 °F) in January to 39 °C (102 °F) in July in six years of observations from 1960 to 1966.
Perspective view of the Afar Depression and environs, generated by draping aLandsat image over adigital elevation model.
TheAwash River, flowing north-eastward through the southern part of the Afar Region, provides a narrow green belt which enables life for the flora and fauna in the area and for theAfars, the nomadic people living in theDanakil Desert. About 128 kilometres (80 mi) from theRed Sea the Awash ends in a chain of salt lakes, where its waterflow evaporates as quickly as it is supplied. Some 1,200 km2 (460 sq mi) of the Afar Depression is covered bysalt deposits, andmining salt is a major source of income for many Afar groups.
In 1994, near the Awash River in Ethiopia,Tim D. White found the then-oldest known human ancestor: 4.4 million-year-oldAr. ramidus. A fossilized almost complete skeleton of a female hominin which he named "Ardi", it took nearly 15 years to safely excavate, preserve, and describe the specimen and to prepare publication of the event.[3]
At smaller scale, thetectonics of the Afar Depression is more complex. An independentmicroplate, theDanakil (or Arrata) microplate, is carrying a piece of continental material (the Danakil block) between the Afar and the Red Sea and is rotating counterclockwise,[6][7][8] causing the slow propagation of the Afar Rift to the north and the propagation of the Red Sea rift to the south.[8]
Thisvolcanic activity weakened the crust and allowed the beginning of the separation between the Arabian plate and the Nubian plate.[20][21] The Gulf of Aden rift propagated westwards and rifting started in the Afar region approximately 28 million years ago, at the same time as in the southern Red Sea.[8] Between 13 and 8 Ma, a major reorganization of the region took place.[8] TheDanakil microplate started rotating, causing the secession of tectonic activity in thesouthernmost Red Sea, and propagation of the Afar rift in theDanakil Depression (i.e. the northern part of the Afar Triangle).[8] At the same time, theMain Ethiopian Rift (the northernmost part of theEast African Rift System) started to form and the Afar Depression became atriple junction.[22] This movement in three different directions by three majorplates caused extension and thinning of the crust, explaining the general morphology of the Afar Depression.
During theextension,volcanism remained very important in the Depression, with kilometers ofvolcanic rocks dominated bybasalts emplaced in central Afar.[6][23][24] So much magmatic rocks were added to crust, at the surface as lava flows, but also in the crust asintrusions and below the crust asunderplated material, that it did not thin as much as expected.[8][25][26] This phenomenon is calledmagma-compensated thinning[27] and it can explain why the central Afar is the only part of the Gulf of Aden - Red Sea system that do not feature normaloceanic crust.[8][25] Because of this high volcanic activity, some researchers propose that this region might never form a normal ocean, but instead form anoceanic plateau, similar toIceland.[25]
Satellite image of agraben in the Afar Depression.
Volcanic andtectonic activity is still very strong in the Depression. In different regions of the Afar, the extension is accommodated byfaulting ormagmatic intrusions.[28][29] The faults form a complex system ofhorst andgraben[30][29][26][31] easily observable onorthophotos thanks to thedesertic environment.Magmatic intrusions intrude the crust asdikes that can also erupt at the surface.[28][32][33] Both processes cause importantearthquakes reaching magnitude 6[34] and having devastating consequences for the local population.[35] In 2005, an important magmatic and tectonic crisis inDabbahu caused up to 8 meters of extension along a 60 km rift segment and theintrusion of 2.5 km3 of lava in only two weeks.[36][28][32] Accounting for an extension rate of approx. 20 mm/yr in the area, 8 m of extension corresponds to the release of 400 years ofstress accumulation in thecrust.
In the northern part of the Afar Depression, called theDanakil Depression, the volcanic activity was less intense until approx. 0.6 My ago.[8] This allowed the crust to thin more than the central part of the Afar and the topography to reach elevations below sea level.[8] This allowed the Red Sea to invade the Danakil Depression during at least fourperiods of high sea-level in thePleistocene.[37][38] The last flooding happened approx. 130'000 years ago.[37][38] These flooding are testified by fossilcoral reefs[37][38] and by thick (>500 m)evaporites deposits (mainlyhalite, i.e.salt) found in the central part of the basin.[39][40]
Geologists predict that in about 10 million years the whole 6,000 km (3,700 mi) length of the East African Rift will be submerged, forming a newocean basin as large as today's Red Sea, and separating theSomali plate and theHorn of Africa from the rest of the continent.[41]
Barberi, F.; Borsi, S.; Ferrara, G.; Marinelli, G.; Santacroce, R.; Tazieff, H.; Varet, J. (1972). "Evolution of the Danakil Depression (Afar, Ethiopia) in Light of Radiometric Age Determinations".Journal of Geology.80 (6):720–729.Bibcode:1972JG.....80..720B.doi:10.1086/627797.S2CID128757919.
Jon Kalb:Adventures in the Bone Trade. The Race to Discover Human Ancestors in Ethiopia's Afar Depression. Copernicus Books, New York 2001,ISBN0-387-98742-8
