Afar Triangle - Geology

Geology

See also: Afar Triple Junction and Plate Tectonics

The Afar Depression is the product of a tectonic triple junction (the Afar Triple Junction), where the spreading ridges that form 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 1–2 centimetres (0.4–0.8 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 (as of late 2005) a continuous sequence of earthquakes, fissures hundreds of metres long and deep appearing in the ground, and the valley floor sinking as much as 100 metres. Between September and October 2005, 163 earthquakes of magnitudes greater than 3.9 and a volcanic eruption occurred within the Afar rift at the Dabbahu and Erta Ale volcanoes. 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, known as the Dabbahu fissure. Related eruptions have taken place in Teru and Aura woredas. The rift has recently been recorded by means of three-dimensional laser mapping.

The salt deposits which local people mine for income were created, over the course of history, as water from the Red Sea periodically flooded the Depression and evaporated; the most recent such flood was roughly 30,000 years ago. 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, and separating the Horn of Africa from the rest of the continent.

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, a great upwelling of mantle that melts to yield basalt as it approaches the surface.