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. In the past, it was considered to be
part of a larger Great
Rift Valley that extended north to
Asia Minor .
The rift is a narrow zone that is a developing divergent tectonic
plate boundary , in which 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 annually. As extension
continues, lithospheric rupture will occur within 10 million years,
the Somalian plate will break off, and a new ocean basin will form.
* 1 Extent
* 2 Competing theories on geologic evolution
* 3 Geologic evolution
* 4 Petrology
* 5 Volcanism and seismicity
* 6 Discoveries in human evolution
* 7 See also
* 8 References
A series of distinct rift basins, the East African
extends over thousands of kilometers. The EAR consists of two main
branches. The Eastern
Rift Valley (also known as Gregory
includes the Main Ethiopian
Rift , running eastward from the Afar
Triple Junction , which continues south as the Kenyan
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
Gulf of Aden .
The EAR runs from the
Afar Triple Junction in the
Afar Triangle of
Ethiopia through eastern Africa, terminating in Mozambique. The EAR
Mozambique . It also runs offshore of
the coast of
Mozambique along the Kerimba and Lacerda grabens , which
are joined by the Davie Ridge, a 2200 km-long relic fracture zone that
cuts across the West Somali basin, straddling the boundary between
Tanzania and Mozambique. The Davie Ridge ranges between 30–120 km
wide, with a west facing scarp (east-plunging arch) along the southern
half of its length that rises up to 2300 m above the sea floor. Its
movement is concurrent with the EAR.
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.
Others proposed an African superplume causing mantle deformation.
However, 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.
Additionally, the subject of deep-rooted mantle plumes is still a
matter of controversy, and therefore cannot be confirmed.
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 .
Prior to rifting, enormous continental flood basalts erupted on the
surface and uplift of the Ethiopian , Somalian, 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.
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
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
The African continental crust is generally cool and strong. Many
cratons are found throughout the EAR, such as the
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.
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
Gulf of Aden approximately 30 Ma. 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.
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
Mount Karisimbi ,
Mount Nyiragongo , Mount
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.
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
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
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. 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
beneath the rift axis. Further away from the rift axis, focal depths
can reach depths of over 30 km.
Focal mechanism solutions strike NE
and frequently demonstrate normal dip-slip faults, although
left-lateral motion is also observed.
DISCOVERIES IN HUMAN EVOLUTION
Human evolution and
Timeline of human evolution
Timeline of human evolution
Rift Valley in
East Africa has been a rich source of hominid
fossils that allow the study of human evolution. 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. 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.
Northern Cordilleran Volcanic Province
* West Antarctic
* West and Central African
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Major African geological formations
CRATONS AND SHIELDS
* West African
* Broodkop Shear Zone
Central African Shear Zone
Central African Shear Zone
* Chuan Shear Zones
Foumban Shear Zone
Kandi Fault Zone
Mwembeshi Shear Zone
* Todi Shear Zone
* Western Meseta Shear Zone
Cape Fold Belt
* East African
* Pan-African orogens
* Terra Australis
Bahr el Arab rift
Blue Nile rift
* East African Rift
* Gulf of Suez
* Lamu Embayment
* Red Sea
* Sangha Aulacogen
* Atbara rift
White Nile rift
Blue Nile Basin
* Douala Basin
* Foreland Karoo Basin
* Gabon Basin
* Kufra Basin
Niger Delta Basin
* Orange River basin
Ouled Abdoun Basin
* Reggane Basin
* Rio del Rey Basin
* Somali Coastal Basin
Tanzania Coastal Basin
* Blue Mountains
Central Pangean Mountains
East African mountains
* Great Escarpment
Great Karas Mountains
Serra da Leba
Serra da Chela
* WorldCat Identities
* VIAF : 246083336
* GND : 4043997-5