East African Rift

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A map of East Africa showing some of the historically active volcanoes (as red triangles) and the Afar Triangle (shaded at the center), which is a so-called triple junction (or triple point) where three plates are pulling away from one another: the Arabian Plate and two parts of the African Plate--the Nubian and Somali--splitting along the East African Rift Zone. EAfrica.png
A map of East Africa showing some of the historically active volcanoes (as red triangles) and the Afar Triangle (shaded at the center), which is a so-called triple junction (or triple point) where three plates are pulling away from one another: the Arabian Plate and two parts of the African Plate—the Nubian and Somali—splitting along the East African Rift Zone.

The East African Rift (EAR) is an active continental rift zone in East Africa. The EAR began developing around the onset of the Miocene, 22–25 million years ago. [1] In the past it was considered to be part of a larger Great Rift Valley that extended north to Asia Minor.

Rift A linear zone where the Earths crust is being pulled apart, and is an example of extensional tectonics

In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics.

Africa The second largest and second most-populous continent, mostly in the Northern and Eastern Hemispheres

Africa is the world's second largest and second most-populous continent, being behind Asia in both categories. At about 30.3 million km2 including adjacent islands, it covers 6% of Earth's total surface area and 20% of its land area. With 1.2 billion people as of 2016, it accounts for about 16% of the world's human population. The continent is surrounded by the Mediterranean Sea to the north, the Isthmus of Suez and the Red Sea to the northeast, the Indian Ocean to the southeast and the Atlantic Ocean to the west. The continent includes Madagascar and various archipelagos. It contains 54 fully recognised sovereign states (countries), nine territories and two de facto independent states with limited or no recognition. The majority of the continent and its countries are in the Northern Hemisphere, with a substantial portion and number of countries in the Southern Hemisphere.

The Miocene is the first geological epoch of the Neogene Period and extends from about 23.03 to 5.333 million years ago (Ma). The Miocene was named by Charles Lyell; its name comes from the Greek words μείων and καινός and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene is preceded by the Oligocene and is followed by the Pliocene.

Contents

The rift, a narrow zone, is a developing divergent tectonic plate boundary where the African Plate is in the process of splitting into two tectonic plates, called the Somali Plate and the Nubian Plate, at a rate of 6–7 mm (0.24–0.28 in) annually. [2] As extension continues, lithospheric rupture will occur within 10 million years; the Somali Plate will break off and a new ocean basin will form.

African Plate Tectonic plate underlying Africa west of the East African Rift

The African Plate is a major tectonic plate straddling the equator as well as the prime meridian. It includes much of the continent of Africa, as well as oceanic crust which lies between the continent and various surrounding ocean ridges. Between 60 million years ago and 10 million years ago, the Somali Plate began rifting from the African Plate along the East African Rift. Since the continent of Africa consists of crust from both the African and the Somali plates, some literature refers to the African Plate as the Nubian Plate to distinguish it from the continent as a whole.

Somali Plate Minor tectonic plate including the east coast of Africa and the ajoining seabed

The Somali Plate is a minor tectonic plate, which straddles the equator in the eastern hemisphere. It is approximately centered on the island of Madagascar and includes about half of the East coast of Africa, from the Gulf of Aden in the North through the East African Rift Valley. The southern boundary with the Nubian-African plate is a diffuse plate boundary consisting of the Lwandle plate.

Extent

A series of distinct rift basins, the East African Rift System extends over thousands of kilometers. [3] The EAR consists of two main branches. The Eastern Rift Valley (also known as Gregory Rift) includes the Main Ethiopian Rift, running eastward from the Afar Triple Junction, which continues south as the Kenyan Rift Valley. [4] The Western Rift Valley includes the Albertine Rift, and farther south, the valley of Lake Malawi. To the north of the Afar Triple Junction, the rift follows one of two paths: west to the Red Sea Rift or east to the Aden Ridge in the Gulf of Aden.

Gregory Rift

The Gregory Rift is the eastern branch of the East African Rift fracture system. The rift is being caused by the separation of the Somali plate from the Nubian plate, driven by a thermal plume. Although the term is sometimes used in the narrow sense of the Kenyan Rift, the larger definition of the Gregory Rift is the set of faults and grabens extending southward from the Gulf of Aden through Ethiopia and Kenya into Northern Tanzania, passing over the local uplifts of the Ethiopian and Kenyan domes. Ancient fossils of early hominins, the ancestors of humans, have been found in the southern part of the Gregory Rift.

Afar Triple Junction Place where three tectonic rifts meet in East Africa

The Afar Triple Junction is located along a divergent plate boundary dividing the Nubian, Somali, and Arabian plates. This area is considered a present-day example of continental rifting leading to seafloor spreading and producing an oceanic basin. Here, the Red Sea Rift meets the Aden Ridge and the East African Rift. It extends a total of 6,500 kilometers (4,000 mi) in three arms from the Afar Triangle to Mozambique.

Albertine Rift

The Albertine Rift is the western branch of the East African Rift, covering parts of Uganda, the Democratic Republic of the Congo (DRC), Rwanda, Burundi and Tanzania. It extends from the northern end of Lake Albert to the southern end of Lake Tanganyika. The geographical term includes the valley and the surrounding mountains.

The EAR runs from the Afar Triple Junction in the Afar Triangle of Ethiopia through eastern Africa, terminating in Mozambique. [5] The EAR transects through Ethiopia, Kenya, Uganda, Rwanda, Burundi, Zambia, Tanzania, Malawi and Mozambique. It also runs offshore of the coast of Mozambique along the Kerimba and Lacerda grabens, which are joined by the Davie Ridge, a 2,200 km-long (1,400 mi) relic fracture zone that cuts across the West Somali basin, straddling the boundary between Tanzania and Mozambique. [4] The Davie Ridge ranges between 30–120 km (19–75 mi) wide, with a west-facing scarp (east-plunging arch) along the southern half of its length that rises to 2,300 m (7,500 ft) above the sea floor. [4] [6] Its movement is concurrent with the EAR. [7]

Afar Triangle A geological depression caused by the Afar Triple Junction

The Afar Triangle 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 Asal, Djibouti, at 155 m below sea level.

Ethiopia country in East Africa

Ethiopia, officially the Federal Democratic Republic of Ethiopia, is a country in the northeastern part of Africa, popularly known as the Horn of Africa. It shares borders with Eritrea to the north, Djibouti to the northeast, and Somalia to the east, Sudan and South Sudan to the west, and Kenya to the south. With over 102 million inhabitants, Ethiopia is the most populous landlocked country in the world and the second-most populous nation on the African continent that covers a total area of 1,100,000 square kilometres (420,000 sq mi). Its capital and largest city is Addis Ababa, which lies a few miles west of the East African Rift that splits the country into the Nubian Plate and the Somali Plate.

Kenya republic in East Africa

Kenya, officially the Republic of Kenya, is a country in Africa with 47 semiautonomous counties governed by elected governors. At 580,367 square kilometres (224,081 sq mi), Kenya is the world's 48th largest country by total area. With a population of more than 52.2 million people, Kenya is the 27th most populous country. Kenya's capital and largest city is Nairobi while its oldest city and first capital is the coastal city of Mombasa. Kisumu City is the third largest city and a critical inland port at Lake Victoria. Other important urban centres include Nakuru and Eldoret.

Competing theories on geologic evolution

Over time, many theories have tried to clarify the evolution of the East African Rift. In 1972 it was proposed that the EAR was not caused by tectonic activity, but rather by differences in crustal density. Since the 1990s, evidence has been found in favor of mantle plumes beneath the EAR. [8] Others proposed an African superplume causing mantle deformation. [9] [10] [11] The question is still debated.

The conceptual extensional difference between plume models and the superplume model placed beneath the East African Rift. Modified from Hansen et al. 2012. Diff plumes superplumes mod Hansen 2012.pdf
The conceptual extensional difference between plume models and the superplume model placed beneath the East African Rift. Modified from Hansen et al. 2012.
Maps of four different depth slices of the Shear-velocity (Vs) model developed by Emry et al. 2018. The forms of the zones with lower Vs (colors toward red) suggest the hotter structures in the Mantle. The distinguishing fourth map depicts a depth below the 410 km discontinuity where Vs steeps up (getting overall bluer), but it still displays the signature of a plume at the substrate of the East African Rift. In the white box, the Vs vertical profile at 10degN, 40degE illustrates the increase of velocity with depth and the effect of the 410 km discontinuity. Emryetal2018 Africa Vs Depth Slices.pdf
Maps of four different depth slices of the Shear-velocity (Vs) model developed by Emry et al. 2018. The forms of the zones with lower Vs (colors toward red) suggest the hotter structures in the Mantle. The distinguishing fourth map depicts a depth below the 410 km discontinuity where Vs steeps up (getting overall bluer), but it still displays the signature of a plume at the substrate of the East African Rift. In the white box, the Vs vertical profile at 10°N, 40°E illustrates the increase of velocity with depth and the effect of the 410 km discontinuity.

The most recent and accepted view is the theory put forth in 2009: that magmatism and plate tectonics have a feedback with one another, controlled by oblique rifting conditions. At that time it was suggested that lithospheric thinning generated volcanic activity, further increasing the magmatic processes at play such as intrusions and numerous small plumes. These processes further thin the lithosphere in saturated areas, forcing the thinning lithosphere to behave like a mid-ocean ridge. [10]

Magmatism geological process

Magmatism is the emplacement of magma within and at the surface of the outer layers of a terrestrial planet, which solidifies as igneous rocks. It does so through magmatic activity or igneous activity, the production, intrusion and extrusion of magma or lava. Volcanism is the surface expression of magmatism.

Plate tectonics The scientific theory that describes the large-scale motions of Earths lithosphere

Plate tectonics is a scientific theory describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of the Earth's lithosphere, since tectonic processes began on Earth between 3 and 3.5 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s.

Mid-ocean ridge An underwater mountain system formed by plate tectonic spreading

A mid-ocean ridge (MOR) is an underwater mountain system formed by plate tectonics. It consists of various mountains linked in chains, typically having a valley known as a rift running along its spine. This type of oceanic mountain ridge is characteristic of what is known as an 'oceanic spreading center', which is responsible for seafloor spreading. The production of new seafloor results from mantle upwelling in response to plate spreading; this isentropic upwelling solid mantle material eventually exceeds the solidus and melts. The buoyant melt rises as magma at a linear weakness in the oceanic crust, and emerges as lava, creating new crust upon cooling. A mid-ocean ridge demarcates the boundary between two tectonic plates, and consequently is termed a divergent plate boundary.

Although reasonably considered, the exact conformation of deep-rooted mantle plumes is still a matter of active research. [13] Studies that contribute to the broader understanding on the evolution of rifts can be grouped into the techniques of isotope geochemistry, seismic tomography and geodynamical modeling.

Isotope Geochemistry

The varying geochemical signatures of a suite of Ethiopian lavas suggest multiple plume sources: at least one of deep mantle origin, and one from within the subcontinental lithosphere. [14] In accordance, a study of Halldórsson et al. in 2014 compare the geochemical signature of rare earth’s isotopes from Xenolith and lava samples collected in the EAR. The results corroborate the coexistence of a superplume “common to the entire rift” with another mantle material source being either of subcontinental type or of mid-ocean ridge type. [15]

Seismic Tomography

The geophysical method of Seismic_tomography is a suitable tool to investigate Earth’s subsurface structures deeper than the crust. It is an inverse problem technique that models which are the velocities of the inner Earth that reproduce the seismographic data recorded all around the world. Recent improvements of tomographic Earth models of P-wave and S-wave velocities suggest that a superplume upwelling from the lower mantle at the northeastern EAR feeds plumes of smaller scale into the upper mantle. [16] [17]

Geodynamical Modeling

Parallel to geological and geophysical measures (e.g. isotope ratios and seismic velocities) it is constructive to test hypotheses on computer based geodynamical models. A 3D numerical geodynamical model of the plume-crust coupling was capable of reproducing the lateral asymmetry of the EAR around the Tanzania craton. [18] Numerical modeling of plume-induced continental break-up shows two distinct stages, crustal rifting followed by lithospheric breakup, and the upwelling between stages of an upper mantle plume. [19]

Geologic evolution

Prior to rifting, enormous continental flood basalts erupted on the surface and uplift of the Ethiopian, Somali, and East African plateaus occurred. The first stage of rifting of the EAR is characterized by rift localization and magmatism along the entire rift zone. Periods of extension alternated with times of relative inactivity. There was also the reactivation of a pre-Cambrian weakness in the crust, a suture zone of multiple cratons, displacement along large boundary faults, and the development of deep asymmetric basins. [3] The second stage of rifting is characterized by the deactivation of large boundary faults, the development of internal fault segments, and the concentration of magmatic activity towards the rifts.

Today, the narrow rift segments of the East African Rift system form zones of localized strain. These rifts are the result of the actions of numerous normal faults which are typical of all tectonic rift zones. As aforementioned, voluminous magmatism and continental flood basalts characterize some of the rift segments, while other segments, such as the Western branch, have only very small volumes of volcanic rock. [13]

Petrology

An artificial rendering of the Albertine Rift, which forms the western branch of the East African Rift. Visible features include (from background to foreground): Lake Albert, the Rwenzori Mountains, Lake Edward, the volcanic Virunga Mountains, Lake Kivu, and the northern part of Lake Tanganyika Albertine Rift, East African Rift (artificial rendering).jpg
An artificial rendering of the Albertine Rift, which forms the western branch of the East African Rift. Visible features include (from background to foreground): Lake Albert, the Rwenzori Mountains, Lake Edward, the volcanic Virunga Mountains, Lake Kivu, and the northern part of Lake Tanganyika

The African continental crust is generally cool and strong. Many cratons are found throughout the EAR, such as the Tanzania and Kaapvaal cratons. The cratons are thick, and have survived for billions of years with little tectonic activity. They are characterized by greenstone belts, tonalites, and other high-grade metamorphic lithologies. The cratons are of significant importance in terms of mineral resources, with major deposits of gold, antimony, iron, chromium and nickel. [20]

A large volume of continental flood basalts erupted during the Oligocene, with the majority of the volcanism coinciding with the opening of the Red Sea and the Gulf of Aden approximately 30 Ma. [9] [13] The composition of the volcanics are a continuum of ultra-alkaline to tholeiitic and felsic rocks. It has been suggested that the diversity of the compositions could be partially explained by different mantle source regions. The EAR also cuts through old sedimentary rocks deposited in ancient basins. [21]

Volcanism and seismicity

The East African Rift Zone includes a number of active as well as dormant volcanoes, among them: Mount Kilimanjaro, Mount Kenya, Mount Longonot, Menengai Crater, Mount Karisimbi, Mount Nyiragongo, Mount Meru and Mount Elgon, as well as the Crater Highlands in Tanzania. Although most of these mountains lie outside of the rift valley, the EAR created them. [21]

Active volcanos include Erta Ale, DallaFilla, and Ol Doinyo Lengai, the former of which is a continuously active basaltic shield volcano in the Afar Region of northeastern Ethiopia. When DallaFilla erupted in 2008 it was the largest volcanic eruption in Ethiopia in recorded history. The Ol Doinyo Lengai volcano is currently the only active natrocarbonatite volcano in the world. The magma contains almost no silica, making the flow viscosity extremely low. “Its lava fountains crystallize in midair then shatter like glass” according to the National Geographic. Approximately 50 volcanic structures in Ethiopia alone have documented activity since the onset of the Holocene. [3]

The EAR is the largest seismically active rift system on Earth today. The majority of earthquakes occur near the Afar Depression, with the largest earthquakes typically occurring along or near major border faults. [13] Seismic events in the past century are estimated to have reached a maximum moment magnitude of 7.0. The seismicity trends parallel to the rift system, with a shallow focal depth of 12–15 km (7.5–9.3 mi) beneath the rift axis. Further away from the rift axis, focal depths can reach depths of over 30 km (19 mi). [13] [22] Focal mechanism solutions strike NE and frequently demonstrate normal dip-slip faults, although left-lateral motion is also observed. [3]

Discoveries in human evolution

The Rift Valley in East Africa has been a rich source of hominid fossils that allow the study of human evolution. [3] [23] The rapidly eroding highlands quickly filled the valley with sediments, creating a favorable environment for the preservation of remains. The bones of several hominid ancestors of modern humans have been found here, including those of "Lucy", a partial australopithecine skeleton discovered by anthropologist Donald Johanson dating back over 3 million years. Richard and Mary Leakey have done significant work in this region also. [24] More recently, two other hominid ancestors have been discovered here: a 10-million-year-old ape called Chororapithecus abyssinicus , found in the Afar rift in eastern Ethiopia, and Nakalipithecus nakayamai , which is also 10 million years old. [25]

See also

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Coordinates: 3°00′00″S35°30′00″E / 3.0000°S 35.5000°E / -3.0000; 35.5000