Afar Triangle

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. 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, see Middle Awash, Hadar. The Depression overlaps the borders of Eritrea, Djibouti and the entire Afar Region of Ethiopia; and it contains the lowest point in Africa, Lake Assal, Djibouti, at 155 m (or 509 ft) below sea level.

Location map of the Afar Triangle (the shaded area in the center of the map) and the East African Rift zones; red triangles show historically active volcanoes.
Topographic map showing the Afar Triangle, which correlates to the shaded area in the location map shown above

The Awash 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 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 the Ethiopian Plateau and escarpment; 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).[1]

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
MODIS satellite image of the Afar Depression and surrounding regions of the Red Sea, Gulf of Aden, Arabia, and the Horn of Africa

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 millimetres (4 to 7 in), with even less rain falling closer to the coast. The climate varies from around 25 °C (77 °F) during the very short rainy season (November–February) to 48 °C (118 °F) during the dry season (February–November); see climate of the lowlands of the Danakil Depression.

Perspective view of the Afar Depression and environs, generated by draping a Landsat image over a Digital elevation model.

The Awash 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 the Afars, the nomadic people living in the Danakil Desert. About 128 kilometres (80 mi) from the Red 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 by salt deposits, and mining salt is a major source of income for many Afar groups.

The Afar Depression biome 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 herbivores such as Grevy's zebra, Soemmering'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 (Heteromirafra archeri), the secretary bird, Arabian and Kori bustards, Abyssinian roller, and crested francolin. In the southern part of the plain lies the Mille-Sardo Wildlife Reserve in Ethiopia.

The Afar Triangle is a cradle source of the earliest hominins. It contains a paleo-archaeological district that includes the Middle Awash region and numerous prehistoric sites of fossil hominin discoveries, including: the hominids 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, the fossilized specimen of Australopithecus afarensis; and Dikika, the site of the fossilized child Selam, an australopithecine hominin.[2]

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.[3]

Geology
See also: Afar Triple Junction
A simplified geologic map of the Afar Depression.

The Afar Depression is the product of a tectonic triple-rifts junction (the Afar Triple Junction), where the spreading ridges forming the Red Sea and the Gulf of Aden emerge on land and meet the East African Rift. The conjunction of these three plates of Earth's crust is near 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.[4]

Within the Triangle, the Earth's crust is slowly rifting apart at a rate of 1–2 centimetres (0.4–0.8 in) per year along each of the three rift zones forming the "legs" of the triple junction. The immediate consequences are recurring sequences of earthquakes with deep fissures in the terrain hundreds of metres long, and the valley floor sinking broadly across the Depression. During September and October 2005 some 163 earthquakes of magnitudes greater than 3.9 and a volcanic eruption occurred within the Afar rift at the Dabbahu and Erta Ale volcanoes. Some 2.5 cubic kilometers of molten rock was injected from below into the plate along a dyke between depths of 2 and 9 km, forcing open an 8 meter wide gap on the surface, known as the Dabbahu fissure.[5][6]

Satellite image of a graben in the Afar Depression.

Related eruptions have taken place in Teru and Aura woredas. The rift has recently been recorded by means of three-dimensional laser mapping.[7]

The region's salt deposits were created over time as water from the Red Sea periodically flooded the Depression and evaporated; the most recent such flood was roughly 30,000 years ago.[8] Over the next millions of years, geologists expect erosion and the Red Sea to breach the highlands surrounding the Afar Depression and flood the valley. Geologists predict that in about 10 million years the whole 6,000 km length of the East African Rift will be submerged, forming a new ocean basin as large as today's Red Sea, and separating the Somali plate and the Horn of Africa from the rest of the continent.[9]

The floor of the Afar Depression is composed of lava, mostly basalt. One of Earth's five lava lakes, Erta Ale is found here, as well as Dabbahu Volcano. It has been proposed that the Afar Depression is underlain by a mantle plume,[10] a great upwelling of mantle that melts to yield basalt as it approaches the surface.

See also

References

Citations
  1. "Geology of the Afar Depression". Afar Rift Consortium. Retrieved 27 October 2013.
  2. Shreeve, Jamie (July 2010). "The Evolutionary Road". National Geographic. Washington, D.C.: National Geographic Society. ISSN 0027-9358. Retrieved 2015-05-28.
  3. White, Tim D.; Asfaw, Berhane; Beyene, Yonas; Haile-Selassie, Yohannes; Lovejoy, C. Owen; Suwa, Gen; WoldeGabrie, Giday (2009). "Ardipithecus ramidus and the Paleobiology of Early Hominids" (PDF). Science. 326 (5949): 75–86. Bibcode:2009Sci...326...75W. doi:10.1126/science.1175802. PMID 19810190. S2CID 20189444.
  4. Beyene, Alebachew & Abdelsalam, Mohamed G. (2005). "Tectonics of the Afar Depression: A review and synthesis". Journal of African Earth Sciences. 41 (1–2): 41–59. Bibcode:2005JAfES..41...41B. doi:10.1016/j.jafrearsci.2005.03.003.
  5. Wright, TJ; Ebinger, C; Biggs, J; Ayele, A; Yirgu, G; Keir, D; Stork, A (July 2006). "Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode" (PDF). Nature. 442 (7100): 291–294. Bibcode:2006Natur.442..291W. doi:10.1038/nature04978. hdl:2158/1078052. PMID 16855588. S2CID 4319443.
  6. "Inside the Hottest Place on Earth". BBC News. 2009-03-19. Retrieved August 30, 2009.
  7. Hottest Place On Earth, Episode 1 at bbc.co.uk Archived November 14, 2012, at the Wayback Machine
  8. Morell, Virginia (January 2012). "Hyperactive Zone". National Geographic. 221 (1): 116–127.
  9. Bojanowski, Axel (2006-03-15). "Africa's New Ocean: A Continent Splits Apart". Spiegel Online. Retrieved 2006-03-16. Includes a photo essay of the region and its geologic changes. }}
  10. Hammond, J. O. S., J.- M. Kendall, G. W. Stuart, C. J. Ebinger, I. D. Bastow, D. Keir, A. Ayele, M. Belachew, B. Goitom, G. Ogubazghi, and T. J. Wright. "Mantle Upwelling and Initiation of Rift Segmentation beneath the Afar Depression." Geology 41.6 (2013): 635–38. DOI:10.1130/G33925.1

Sources

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