The Afar Triangle (also called the Afar Depression) is a geological depression caused by the Afar triple junction, which is part of the Great Rift Valley in

East Africa East Africa, also known as Eastern Africa or the East of Africa, is a region at the eastern edge of the Africa, African continent, distinguished by its unique geographical, historical, and cultural landscape. Defined in varying scopes, the regi ...

. The region has disclosed fossil specimens of the very earliest hominins; that is, the earliest of the human clade, and it is thought by some paleontologists to be the cradle of the evolution of humans. The Depression overlaps the borders of

Eritrea Eritrea, officially the State of Eritrea, is a country in the Horn of Africa region of East Africa, with its capital and largest city being Asmara. It is bordered by Ethiopia in the Eritrea–Ethiopia border, south, Sudan in the west, and Dj ...

Djibouti Djibouti, officially the Republic of Djibouti, is a country in the Horn of Africa, bordered by Somalia to the south, Ethiopia to the southwest, Eritrea in the north, and the Red Sea and the Gulf of Aden to the east. The country has an area ...

and the entire

Afar Region The Afar Region (; ; ), formerly known as Region 2, is a Regions of Ethiopia, regional state in northeastern Ethiopia and the homeland of the Afar people. Its capital is the planned city of Semera, which lies on the paved Awash, Ethiopia, Awash� ...

Ethiopia Ethiopia, officially the Federal Democratic Republic of Ethiopia, is a landlocked country located in the Horn of Africa region of East Africa. It shares borders with Eritrea to the north, Djibouti to the northeast, Somalia to the east, Ken ...

; and it contains the lowest point in

Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...

, Lake Assal,

, at below sea level. The

Awash River } The Awash River (sometimes spelled Awaash; Oromo language, Oromo: ''Awaash OR Hawaas'', Amharic: ዐዋሽ, Afar language, Afar: ''Hawaash We'ayot'', Somali language, Somali: ''Webiga Dir'', Italian language, Italian: ''Auasc'') is a major river ...

is the main waterflow into the region, but it runs dry during the annual dry season, and ends as a chain of saline lakes. The northern part of the Afar Depression is also known as the

Danakil Depression The Danakil Depression is a large valley of approximately , across the north of the Afar Region of Ethiopia and Eritrea. It is about below sea level and is bordered to the west by the Ethiopian Plateau and to the east by the Danakil Alps, bey ...

. The lowlands are affected by heat,

drought A drought is a period of drier-than-normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D. Jiang, A. Khan, W. Pokam Mba, D. Rosenfeld, J. Tierney, ...

, 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 the Ethiopian Plateau and

escarpment An escarpment is a steep slope or long cliff that forms as a result of faulting or erosion and separates two relatively level areas having different elevations. Due to the similarity, the term '' scarp'' may mistakenly be incorrectly used inte ...

; to the north-east (between it and the Red Sea) by the Danakil block; to the south by the Somali Plateau and escarpment; and to the south-east by the Ali-Sabieh block (adjoining the Somali Plateau). Many important fossil localities exist in the Afar region, including the Middle Awash region and the sites of Hadar, 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.

Environment

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 , with even less rain falling closer to the coast. Daily mean temperatures at Dallol ranged from in January to in July in six years of observations from 1960 to 1966. AfarDrape

The

, 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 the Afars, the nomadic people living in the Danakil Desert. About from the

Red Sea The Red Sea is a sea inlet of the Indian Ocean, lying between Africa and Asia. Its connection to the ocean is in the south, through the Bab-el-Mandeb Strait and the Gulf of Aden. To its north lie the Sinai Peninsula, the Gulf of Aqaba, and th ...

the Awash ends in a chain of salt lakes, where its waterflow evaporates as quickly as it is supplied. Some of the Afar Depression is covered by

salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...

deposits, and mining salt is a major source of income for many Afar groups. The Afar Depression

biome A biome () is a distinct geographical region with specific climate, vegetation, and animal life. It consists of a biological community that has formed in response to its physical environment and regional climate. In 1935, Tansley added the ...

is characterized as desert scrubland. Vegetation is mostly confined to drought-resistant plants such as small trees (e.g. species of the dragon tree), shrubs, and grasses. Wildlife includes many

herbivore A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat ...

s such as Grévy's zebra, Soemmerring's gazelle, beisa and, notably, the last viable population of African wild ass ('' Equus africanus somalicus''). Birds include the ostrich, the endemic Archer's lark, the secretary bird,

Arabian The Arabian Peninsula (, , or , , ) or Arabia, is a peninsula in West Asia, situated north-east of Africa on the Arabian plate. At , comparable in size to India, the Arabian Peninsula is the largest peninsula in the world. Geographically, the ...

and Kori bustards, Abyssinian roller, and crested francolin. In the southern part of the plain lies the Mille-Serdo Wildlife Reserve. The Afar Triangle is a cradle source of the earliest

hominin The Hominini (hominins) form a taxonomic tribe of the subfamily Homininae (hominines). They comprise two extant genera: ''Homo'' (humans) and '' Pan'' (chimpanzees and bonobos), and in standard usage exclude the genus '' Gorilla'' ( gorillas) ...

s. It contains a paleo-archaeological district that includes the Middle Awash region and numerous prehistoric sites of fossil hominin discoveries, including: the

hominids The Hominidae (), whose members are known as the great apes or hominids (), are a taxonomic family of primates that includes eight extant species in four genera: '' Pongo'' (the Bornean, Sumatran and Tapanuli orangutan); '' Gorilla'' (the ...

and possible hominins, Ardi, or '' Ardipithecus ramidus'', and '' Ardipithecus kadabba'', see below; the Gawis cranium hominin from Gona; several sites of the world's oldest stone tools; Hadar, the site of

Lucy Lucy is an English language, English feminine given name derived from the Latin masculine given name Lucius with the meaning ''as of light'' (''born at dawn or daylight'', maybe also ''shiny'', or ''of light complexion''). Alternative spellings ar ...

, the fossilized specimen of ''

Australopithecus afarensis ''Australopithecus afarensis'' is an extinct species of australopithecine which lived from about 3.9–2.9 million years ago (mya) in the Pliocene of East Africa. The first fossils were discovered in the 1930s, but major fossil finds would not ta ...

''; and Dikika, the site of the fossilized child Selam, an australopithecine hominin. In 1994, near the Awash River in Ethiopia, Tim D. White found the then-oldest known human ancestor: 4.4 million-year-old ''Ar. 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.

Geology

The Afar Depression is a

tectonic Tectonics ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...

triple junction (the Afar triple junction), where the spreading ridges of the

and the

Gulf of Aden The Gulf of Aden (; ) is a deepwater gulf of the Indian Ocean between Yemen to the north, the Arabian Sea to the east, Djibouti to the west, and the Guardafui Channel, the Socotra Archipelago, Puntland in Somalia and Somaliland to the south. ...

meet the

East African Rift The East African Rift (EAR) or East African Rift System (EARS) is an active continental rift zone in East Africa. The EAR began developing around the onset of the Miocene, 22–25 million years ago. It was formerly considered to be part of a l ...

. These rifts are caused by the northeastward movement of the Arabian plate (approximately 20 mm/yr) and the much slower eastward movement of the Somalian plate (approximately 5 mm/yr) relative to the Nubian (African) plate. At smaller scale, the

tectonics Tectonics ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...

of the Afar Depression is more complex. An independent microplate, the Danakil (or Arrata) microplate, is carrying a piece of continental material (the Danakil block) between the Afar and the Red Sea and is rotating counterclockwise, causing the slow propagation of the Afar Rift to the north and the propagation of the Red Sea rift to the south. The recent geological history of the Afar Depression started around 33 million years ago, before any

rifting In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear Fault (geology), downfaulted depression, called a graben, or more commonly ...

, with the eruption of the Ethiopian Flood Basalts that covered large parts of

and

Yemen Yemen, officially the Republic of Yemen, is a country in West Asia. Located in South Arabia, southern Arabia, it borders Saudi Arabia to Saudi Arabia–Yemen border, the north, Oman to Oman–Yemen border, the northeast, the south-eastern part ...

with hundreds to thousands of meters of volcanic rocks. These eruptions were cause by a hot rising

mantle plume A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic ho ...

that impacted the

continental crust Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as '' continental shelves''. This layer is sometimes called '' si ...

and produced large quantities of

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

. This impact of the mantle plume also caused the high topography of the region, an effect still visible today. This

volcanic activity Volcanism, vulcanism, volcanicity, or volcanic activity is the phenomenon where solids, liquids, gases, and their mixtures erupt to the surface of a solid-surface astronomical body such as a planet or a moon. It is caused by the presence of a he ...

weakened the crust and allowed the beginning of the separation between the Arabian plate and the Nubian plate. 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. Between 13 and 8 Ma, a major reorganization of the region took place. The Danakil microplate started rotating, causing the secession of tectonic activity in the southernmost Red Sea, and propagation of the Afar rift in the

(i.e. the northern part of the Afar Triangle). At the same time, the Main Ethiopian Rift (the northernmost part of the East African Rift System) started to form and the Afar Depression became a triple junction. This movement in three different directions by three major plates caused extension and thinning of the crust, explaining the general morphology of the Afar Depression. During the extension,

volcanism Volcanism, vulcanism, volcanicity, or volcanic activity is the phenomenon where solids, liquids, gases, and their mixtures erupt to the surface of a solid-surface astronomical body such as a planet or a moon. It is caused by the presence of a he ...

remained very important in the Depression, with kilometers of volcanic rocks dominated by basalts emplaced in central Afar. So much magmatic rocks were added to crust, at the surface as lava flows, but also in the crust as

intrusions In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of Intrusive rock, intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety o ...

and below the crust as underplated material, that it did not thin as much as expected. This phenomenon is called ''magma-compensated thinning'' and it can explain why the central Afar is the only part of the Gulf of Aden - Red Sea system that do not feature normal

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramaf ...

. Because of this high volcanic activity, some researchers propose that this region might never form a normal ocean, but instead form an oceanic plateau, similar to

Iceland Iceland is a Nordic countries, Nordic island country between the Atlantic Ocean, North Atlantic and Arctic Oceans, on the Mid-Atlantic Ridge between North America and Europe. It is culturally and politically linked with Europe and is the regi ...

Volcanic A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often fo ...

and

activity is still very strong in the Depression. In different regions of the Afar, the extension is accommodated by

faulting In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

or magmatic intrusions. The faults form a complex system of horst and

graben In geology, a graben () is a depression (geology), depressed block of the Crust (geology), crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German language, German, meaning 'ditch' or 't ...

easily observable on orthophotos thanks to the desertic environment. Magmatic intrusions intrude the crust as dikes that can also erupt at the surface. Both processes cause important earthquakes reaching magnitude 6 and having devastating consequences for the local population . In 2005, an important magmatic and tectonic crisis in Dabbahu caused up to 8 meters of extension along a 60 km rift segment and the

intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

of 2.5 km³ of lava in only two weeks. Accounting for an extension rate of approx. 20 mm/yr in the area, 8 m of extension corresponds to the release of 400 years of stress accumulation in the crust. In the northern part of the Afar Depression, called the

, the volcanic activity was less intense until approx. 0.6 My ago. This allowed the crust to thin more than the central part of the Afar and the topography to reach elevations below sea level. This allowed the Red Sea to invade the Danakil Depression during at least four periods of high sea-level in the

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

. The last flooding happened approx. 130'000 years ago. These flooding are testified by fossil

coral reefs A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. ...

and by thick (>500 m) evaporites deposits (mainly

halite Halite ( ), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride ( Na Cl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, pi ...

, i.e.

) found in the central part of the basin. Geologists predict that in about 10 million years the whole length of the East African Rift will be submerged, forming a new

ocean basin In hydrology, an oceanic basin (or ocean basin) is anywhere on Earth that is covered by seawater. Geologically, most of the ocean basins are large  geologic basins that are below sea level. Most commonly the ocean is divided int ...

as large as today's Red Sea, and separating the Somali plate and the

Horn of Africa The Horn of Africa (HoA), also known as the Somali Peninsula, is a large peninsula and geopolitical region in East Africa.Robert Stock, ''Africa South of the Sahara, Second Edition: A Geographical Interpretation'', (The Guilford Press; 2004), ...

from the rest of the continent. Includes a photo essay of the region and its geologic changes.

References

Citations

Sources

* * Includes a photo essay of the region and its geologic changes. * * * Jon Kalb: ''Adventures in the Bone Trade. The Race to Discover Human Ancestors in Ethiopia's Afar Depression.'' Copernicus Books, New York 2001, *

External links