Location of the triangle (the shaded area in the center of the map) and the local fault lines. It is located at 11 30 N, 43 00 E

The Afar Triple Junction is a junction of three tectonic rifts (or ridges) centered in the Afar Depression, informally known as the Afar Triangle, of northeastern Africa. Here, the Red Sea Rift meets the Aden Ridge and the East African Rift. It also represents the junction of the Arabian Plate and the two protoplates which are beginning to form as the African Plate splits apart along the East African Rift. These protoplates, or subplates, are referred to as the Nubian Plate and the Somali Plate, with the Somali Plate to the southeast and the Nubian Plate to the northwest. The Red Sea Rift runs northwest to southeast along the Red Sea and separates the Nubian (northwestern part of African plate) and the Arabian plates. The Aden Ridge runs east to west separating the Somali Plate (southeastern part of African plate) and Arabian Plate.

Plate tectonics

The Aden Ridge, Red Sea Rift, and the East African Rift all meet at the Afar Triple Junction. Here, the Arabian plate and Nubian and Somali proto-plates are being pulled apart, which forms three divergent plate boundaries. Divergent plate boundaries are fundamental features of plate tectonics and are associated with rifts and mid-ocean ridges. Plate separation causes deeper mantle asthenosphere to rise into the widening region (shown in figure). The Aden Ridge and the Red Sea Rift began to widen before the East African Rift with the separation of the African and Arabian plates about 25 million years ago.^[1] Prior to the opening of these rifts, Africa and Arabia were connected. The East African Rift is the third limb, or branch, in the triple junction occurring when the African Plate began to split into the Somali Plate to the southeast and the Nubian Plate to the northwest.^[2] The East African Rift began extending about 15 million years after both the Aden and Red Sea rifts. The double sided arrows represent the spreading rates of the rifts. The single sided arrows represent general plate movement.

Rifts

A rift is the result of pulling apart or extension of both the lithosphere and crust (note that the crust is a part of the lithosphere). This is a product of what is referred to as mantle upwelling where hotter asthenosphere rises up into colder lithosphere. This rise is associated with thinning and stretching of the lithosphere. left

East African Rift

The East African Rift is an active rift between the Nubian and Somali protoplates. This rift is caused by elevated heat flow from the mantle under Kenya and the Afar region. Trending NNE to SSW, the East African Rift is composed of a western and an eastern branch. The eastern branch (sometimes called the Gregory Rift) is characterized by high volcanic activity and the western branch (sometimes called the Albertine Rift) is characterized by deeper basins, which contain lakes and sediments. The extension rate for this rift starts at about 6 millimeters per year in the north, and declines to the south.^[3]

Red Sea Rift

The Red Sea Rift is between the African (or Nubian) and Arabian Plates. The rift runs along the length of the Red Sea, starting from the Dead Sea to the Afar triple junction. Within the rift, in the Red Sea, there are many volcanoes, including the Jabal al-Tair. The extension rate for this rift varies from about 7 to 17 millimeters per year.^[4]

Aden Ridge

The Aden Ridge is a divergent plate boundary that divides the African (or Somali) and Arabian Plate. It extends from the triple junction eastward to the Owen Fracture Zone. The Aden Ridge is also a part of another triple junction in the Indian Ocean to the east, called the Aden-Owen-Carlsberg triple junction, which include the African, Arabian, and Indo-Australian plates. The spreading rate for Aden Ridge is about 17 millimeters per year near the Afar Triple Junction.^[5]

Geology

The dotted lines represent political country borders. The red lines represent the plate boundaries. The point at which the red lines meet is the Afar Triple Junction.

The geology of the Afar Triple Junction incorporates multiple rock types. Most of this area is composed of basalt, a volcanic rock formed by rapid cooling of magma at the Earth s surface. There are also sedimentary and metamorphic rocks, but not as much as the igneous (basalt). The vast majority of the basalt was created between 1 and 24 million years ago, with production peaking towards the end of this period.^[6] Volcanic activity continues today, although on a smaller scale, as active volcanoes continue to erupt; Erta Ale is perhaps the most famous active volcano in the Afar. Volcanism today is mainly in northern Ethiopia and farther southward along the East African Rift. Thicknesses of basalt can reach upwards of 4 kilometers within the Afar Triple Junction.^[7]

Before the creation of the rifts, which now define the Afar Triple Junction, the area was dominated by igneous and metamorphic rocks. These were formed between 500 and 600 million years ago during a mountain building event caused by the collision of East and West Gondwana. These rocks are referred to as basement rocks, which are buried by future successions of rocks and represent the earliest portion of geologic history.^[6]

Clues as to the history of the Afar Triple Junction lies in the presence of limestones, evaporites, and ancient coral reefs found in and around the Afar Depression, centralized in western Djibouti. The presence of these types of rocks is strong evidence that the area known as the Afar Depression was once covered by shallow seawater, possibly an extension of the Red Sea. The rifts of the Afar Triple Junction and the valleys they form are mostly filled with sedimentary rocks (see yellow bands on geologic map) which are representative of the rocks from which they were formed. These include sediments eroded from both basement rocks and volcanic rocks, good candidates for erosion based on their topography, as they sit at higher elevations on the borders of the rifts.^[6]

References

fr:Jonction triple de l'Afar pl:W ze potr jny Afar uk: