TheEast African Rift (EAR) orEast African Rift System (EARS) is an active continentalrift zone inEast Africa. The EAR began developing around the onset of theMiocene, 22–25 million years ago.[1] It is considered to be part of a larger system, formerly known as theGreat Rift Valley, that extends north toAsia Minor, also known as Anatolia.
A narrow zone, the rift is a developingdivergenttectonic plateboundary where theAfrican plate is in the process of splitting into two tectonic plates, called theSomali plate and theNubian plate, at a rate of 8–9 mm (0.31–0.35 in) per year.[2] The rift system consists of threemicroplates, theVictoria microplate to the north, and theRovuma andLwandle microplates to the south. The Victoria microplate is rotating anti-clockwise with respect to the African plate. Its rotation is caused by the configuration of mechanically weaker and stronger lithospheric regions in the EARS.[3][4]
Many of theAfrican Great Lakes lie within the Rift Valley.
A series of distinct rift basins, the East African Rift System extends over thousands of kilometers.[5] North of theAfar triple junction, the rift follows two paths: west to theRed Sea Rift and east to theAden Ridge in theGulf of Aden.
Southward from the Afar triple junction, the EAR consists of two main branches. The Eastern Rift Valley (also known asGregory Rift) includes theMain Ethiopian Rift, runs southward from the Afar triple junction, and continues south as the Kenyan Rift Valley,[6] into northern Tanzania. The Western Rift Valley includes theAlbertine Rift, which transectsDemocratic Republic of the Congo,Uganda,Rwanda, andBurundi through theRuzizi Plain, and farther southTanzania,Zambia, the valley ofLake Malawi andMozambique.[7]
The rift also continues offshore from the coast of Mozambique along the Kerimba and Lacerdagrabens, 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.[6] 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.[6][8] Its movement is concurrent with the EAR.[9]
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.[10] Others proposed an Africansuperplume causing mantle deformation.[11][12][13] Although the effects of deep-rootedmantle plumes are an important hypothesis, their location and dynamics are poorly understood, and a matter of active research.[14] The question is still debated.
The most recent and accepted view is the theory put forth in 2009: thatmagmatism andplate tectonics have a feedback with one another, controlled by oblique rifting conditions. According to this theory, lithospheric thinning generates volcanic activity, further increasing magmatic processes such asintrusions and numerous small plumes. These processes further thin thelithosphere in saturated areas, making the thinning lithosphere behave like amid-ocean ridge.[12] According to marine geologistKathleen Crane, the rift could eventually cause eastern Africa to separate from the mainland, although this potential event could take tens of millions of years.[16]
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.
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.[17] In accordance, a 2014 study compares the geochemical signature ofrare earthisotopes fromxenoliths 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.[18]
The geophysical method ofseismic 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 ofP wave andS wave velocities suggest that a superplume upwelling from the lower mantle at the northeastern EAR feeds plumes of smaller scale into theupper mantle.[19][20]
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 geodynamic model of the plume-crust coupling was capable of reproducing the lateral asymmetry of the EAR around theTanzania craton.[21] 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.[22]
Prior to the rift's formation, enormouscontinental flood basalts erupted, uplifting theEthiopian, Somali, and East African plateaus. The first stage of rifting of the EAR was characterized by rift localization and magmatism along the entire rift zone. Periods of extension alternated with relative inactivity. There was also the reactivation of a pre-Cambrian weakness in the crust, asuture zone of multiplecratons, displacement along large boundary faults, and the development of deep asymmetric basins.[5] The second stage of rifting was 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 normalfaults 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.[14]
.jpg)
The Africancontinental crust is generally cool and strong. Manycratons are found throughout the EAR, such as theTanzania andKaapvaal cratons. The cratons are thick, and have survived for billions of years with little tectonic activity. They are characterized bygreenstone belts,tonalites, and other high-grade metamorphic lithologies. The cratons are of significant importance in terms ofmineral resources, with major deposits ofgold,antimony,iron,chromium andnickel.[23]
A large volume of continental flood basalts erupted during theOligocene, with the majority of the volcanism coinciding with the opening of the Red Sea and the Gulf of Aden approximately 30 Ma.[11][14] 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.[24]
The East African Rift Zone includes a number of active and dormant volcanoes, among them:Mount Kilimanjaro,Mount Kenya,Mount Longonot,Menengai Crater,Mount Karisimbi,Mount Nyiragongo,Mount Meru andMount Elgon, as well as theCrater Highlands in Tanzania. Although most of these mountains lie outside of the rift valley, the EAR created them.[24]
Notable active examples of EAR volcanism includeErta Ale,Dalaffilla (also called Gabuli, Alu-Dalafilla),Hayli Gubbi (which erupted in 2025), andOl Doinyo Lengai. Erta Ale is a basaltic shield volcano in the Afar Region of northeastern Ethiopia, active continuously since at least 1967,[25] with a summit lava lake documented since at least 1906.[26] The 2008 eruption of Dalafilla, its only documented activity since the start of theHolocene,[27] is the largest recorded eruption in Ethiopian history.[citation needed] Ol Doinyo Lengai is currently the only activenatrocarbonatite volcano on Earth.[28] Its magma contains almost no silica; typical lava flows haveviscosities of less than 100 Pa⋅s,[29] comparable to olive oil at 26 °C (79 °F). EAR-related volcanic structures with dated activity since the onset of the Holocene include approximately 50 in Ethiopia,[5]17 in Kenya, and9 in Tanzania.
The EAR is the largest seismically active rift system on Earth today. The majority ofearthquakes occur near the Afar Depression, with the largest typically occurring along or near major border faults.[14] Seismic events in the past century are estimated to have reached a maximum moment magnitude of 7.0.[citation needed] 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 be below 30 km (19 mi).[14][30]Focal mechanism solutions strike NE and frequently demonstrate normal dip-slip faulting, although left-lateral motion is also observed.[5]
The East African Rift system affects regional, continental and even global climate. Regions of higher elevation, including theEthiopian Highlands and the Kenya Highlands are hotspots of higher rainfall amid the semi-arid to arid lowlands of East Africa.[31] Lakes which form within the rift, includingLake Victoria, have a large effect on regional climate.[32] They are a source of water vapour, and also lead to the formation oflake breeze systems, which affect weather across large areas of East Africa. The east to west river valleys within the rift system, including the Turkana Channel in northernKenya and theZambezi river valley, concentrate low-level easterly winds and accelerate them towardsCentral Africa.[33] This leaves East Africa drier than it otherwise would be, and also supports the high rainfall in theCongo Basinrainforest.[34] The formation of the east–west valleys could in turn be important for thearidification of East Africa over millions of years.[34]
The barrier presented by EARS concentrates monsoonal winds (known as the Somali Jet) in the westernIndian Ocean.[35] The Somali Jet supplies water vapour for the high rainfall during theIndian Monsoon[36] and is responsible for roughly half the global cross-equatorial atmospheric mass flux in the lower-branch ofHadley Circulation.[37]
The Rift Valley in East Africa has been a rich source ofhominid fossils that allow the study of human evolution.[5][38] 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 partialaustralopithecine skeleton discovered by anthropologistDonald Johanson dating back over 3 million years.Richard andMary Leakey have also done significant work in this region.[39]In 2008, two other hominid ancestors were discovered here: a 10-million-year-old ape calledChororapithecus abyssinicus, found in the Afar rift in eastern Ethiopia, andNakalipithecus nakayamai, which is also 10 million years old.[40]
| International | |
|---|---|
| Other | |
