The Afar Triple Junction is a junction

Triple junction

A triple junction is the point where the boundaries of three tectonic plates meet. At the triple junction a boundary will be one of 3 types - a ridge, trench or transform fault - and triple junctions can be described according to the types of plate margin that meet at them...

 of three tectonic

Plate tectonics

Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...

 rift

Rift

In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....

s (or ridges) centered in the Afar Depression

Afar Depression

The Afar Triangle is a geological depression that is caused by the Afar Triple Junction which is part of the Great Rift Valley. It overlaps Eritrea, Djibouti and the entire Afar Region of Ethiopia. The Afar Triangle includes the Danakil Depression and the lowest point in Africa, Lake Asal...

, informally known as the Afar Triangle, of northeastern Africa

Africa

Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...

. Here, the Red Sea Rift

Red Sea Rift

The Red Sea Rift is a spreading center between two tectonic plates, the African Plate and the Arabian Plate. It extends down the length of the Red Sea, stretching from the southern end of the Dead Sea Transform to a triple junction with the Aden Ridge and the East African Rift in the Afar...

 meets the Aden Ridge

Aden Ridge

The Aden Ridge is a divergent tectonic plate boundary which runs offshore in the Gulf of Aden and Indian Ocean along the southeastern coastline of the Arabian Peninsula. It extends eastward from the Afar Triple Junction to a junction with the Owen Fracture Zone, and separates the Arabian Plate...

 and the East African Rift

East African Rift

The East African Rift is an active continental rift zone in eastern Africa that appears to be a developing divergent tectonic plate boundary. It is part of the larger Great Rift Valley. The rift is a narrow zone in which the African Plate is in the process of splitting into two new tectonic plates...

. It also represents the junction of the Arabian Plate

Arabian Plate

The Arabian Plate is one of three tectonic plates which have been moving northward over millions of years and colliding with the Eurasian Plate...

 and the two protoplates which are beginning to form as the African Plate

African Plate

The African Plate is a tectonic plate which includes the continent of Africa, as well as oceanic crust which lies between the continent and various surrounding ocean ridges.-Boundaries:...

 splits apart along the East African Rift. These protoplates, or subplates, are referred to as the Nubian Plate and the Somali Plate

Somali Plate

The Somali Plate or Somalian Plate is a tectonic plate that is being formed as the African Plate is splitting along the East African Rift. The part of the African Plate which lies on the other side of the rift is sometimes referred to as the Nubian 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

Red Sea

The Red Sea is a seawater inlet of the Indian Ocean, lying between Africa and Asia. The connection to the ocean is in the south through the Bab el Mandeb strait and the Gulf of Aden. In the north, there is the Sinai Peninsula, the Gulf of Aqaba, and the Gulf of Suez...

 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 boundary

In plate tectonics, a divergent boundary or divergent plate boundary is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent boundaries within continents initially produce rifts which produce rift valleys...

. Divergent plate boundaries are fundamental features of plate tectonics and are associated with rifts and mid-ocean ridge

Mid-ocean ridge

A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges , typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading...

s. Plate separation causes deeper mantle asthenosphere

Asthenosphere

The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...

 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. 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. The East African Rift began extending about 15 million years after both the Aden and Red Sea rifts (seemingly a long time, but geologically speaking, just a brief amount of time).

Rifts

A rift

Rift

In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....

 is the result of pulling apart or extension of both the lithosphere

Lithosphere

The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

 and crust

Continental crust

The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...

 (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.

East African Rift

The East African Rift

East African Rift

The East African Rift is an active continental rift zone in eastern Africa that appears to be a developing divergent tectonic plate boundary. It is part of the larger Great Rift Valley. The rift is a narrow zone in which the African Plate is in the process of splitting into two new tectonic plates...

 is an active rift between the Nubian and Somali protoplates. This rift is caused by elevated heat flow from the mantle under Kenya

Kenya

Kenya , officially known as the Republic of Kenya, is a country in East Africa that lies on the equator, with the Indian Ocean to its south-east...

 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

Volcano

2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

 activity and the western branch (sometimes called the Albert Rift) is characterized by deeper basins

Sedimentary basin

The term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification...

, 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.

Red Sea Rift

The Red Sea Rift

Red Sea Rift

The Red Sea Rift is a spreading center between two tectonic plates, the African Plate and the Arabian Plate. It extends down the length of the Red Sea, stretching from the southern end of the Dead Sea Transform to a triple junction with the Aden Ridge and the East African Rift in the Afar...

 is between the African (or Nubian) and Arabian Plates. The rift runs along the length of the Red Sea, starting from the Dead Sea

Dead Sea

The Dead Sea , also called the Salt Sea, is a salt lake bordering Jordan to the east and Israel and the West Bank to the west. Its surface and shores are below sea level, the lowest elevation on the Earth's surface. The Dead Sea is deep, the deepest hypersaline lake in the world...

 to the Afar triple junction. Within the rift, in the Red Sea, there are many volcanoes, including the Jabal al-Tair

Jabal al-Tair Island

Jabal al-Tair Island is a roughly oval volcanic island northwest of the constricted Bab al-Mandab passage at the mouth of the Red Sea, about half way between Yemen and Eritrea...

. The extension rate for this rift varies from about 7 to 17 millimeters per year.

Aden Ridge

The Aden Ridge

Aden Ridge

The Aden Ridge is a divergent tectonic plate boundary which runs offshore in the Gulf of Aden and Indian Ocean along the southeastern coastline of the Arabian Peninsula. It extends eastward from the Afar Triple Junction to a junction with the Owen Fracture Zone, and separates the Arabian Plate...

 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

Owen Fracture Zone

The Owen Fracture Zone is a transform fault which runs along the eastern boundary of the Arabian Plate, separating it from the Indo-Australian Plate for most of its length, and from the African Plate for a much shorter distance...

. The Aden Ridge is also a part of another triple junction in the Indian Ocean

Indian Ocean

The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...

 to the east, called the Aden-Owen-Carlsberg triple junction, which include the African, Arabian, and Indo-Australian

Indo-Australian Plate

The Indo-Australian Plate is a major tectonic plate that includes the continent of Australia and surrounding ocean, and extends northwest to include the Indian subcontinent and adjacent waters...

 plates. The spreading rate for Aden Ridge is about 17 millimeters per year near the Afar Triple Junction.

Geology

The geology of the Afar Triple Junction incorporates multiple rock types. Most of this area is composed of basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

, a volcanic rock

Volcanic rock

Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it is an igneous rock of volcanic origin...

 formed by rapid cooling of magma

Magma

Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

 at the Earth’s surface. There are also sedimentary

Sedimentary rock

Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

 and metamorphic

Metamorphic rock

Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...

 rocks, but not as much as the igneous

Igneous rock

Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...

 (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. Volcanic activity continues today, although on a smaller scale, as active volcanoes continue to erupt; Erta Ale

Erta Ale

Erta Ale is a continuously active basaltic shield volcano in the Afar Region of northeastern Ethiopia, the most active volcano in Ethiopia. It is in the Afar Depression, a badlands desert area spanning the border with Eritrea, and the volcano itself is surrounded completely by an area below sea...

 is perhaps the most famous active volcano in the Afar. Volcanism today is mainly in northern Ethiopia

Ethiopia

Ethiopia , officially known as the Federal Democratic Republic of Ethiopia, is a country located in the Horn of Africa. It is the second-most populous nation in Africa, with over 82 million inhabitants, and the tenth-largest by area, occupying 1,100,000 km2...

 and farther southward along the East African Rift. Thicknesses of basalt can reach upwards of 4 kilometers within the Afar Triple Junction.

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

Pan-African orogeny

The Pan-African orogeny was a series of major Neoproterozoic orogenic events which related to the formation of the supercontinents Gondwana and Pannotia about 600 million years ago....

 caused by the collision of East and West Gondwana

Gondwana

In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...

. These rocks are referred to as basement rocks

Basement (geology)

In geology, the terms basement and crystalline basement are used to define the rocks below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin...

, which are buried by future successions of rocks and represent the earliest portion of geologic history.

Clues as to the history of the Afar Triple Junction lies in the presence of limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

s, evaporite

Evaporite

Evaporite is a name for a water-soluble mineral sediment that result from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporate deposits, marine which can also be described as ocean deposits, and non-marine which are found in standing bodies of...

s, and ancient coral reef

Coral reef

Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...

s found in and around the Afar Depression, centralized in western Djibouti

Djibouti

Djibouti , officially the Republic of Djibouti , is a country in the Horn of Africa. It is bordered by Eritrea in the north, Ethiopia in the west and south, and Somalia in the southeast. The remainder of the border is formed by the Red Sea and the Gulf of Aden at the east...

. 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

Erosion

Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 based on their topography

Topography

Topography is the study of Earth's surface shape and features or those ofplanets, moons, and asteroids...

, as they sit at higher elevations on the borders of the rifts.