East African Rift Valley (EAR) – Discoveries, Geologic Evolution and More

East African Rift Valley (EAR) – Discoveries, Geologic Evolution and More

East African Rift System (EARS) or East African Rift Valley (EAR) is a dynamic zone of continental found in East Africa. It emerged roughly 22 to 25 million years ago, around the start of the Miocene epoch. It was once thought to be a part of the wider Great Rift Valley that stretched north all the way to end in Asia Minor.

East African displaying various historical active volcanoes
East African displaying various historical active volcanoes

The East African Rift Valley, a narrow area, is a divergent tectonic plate border at which the African Plate gradually continues to split into 2 tectonic plates, namely the Nubian and the Somali Plates, at a pace of 6 to 7 millimeters (0.24 to 0.28 inches) each year. This rift system is made up of 3 microplates:  Northwards is the Victoria Microplate, and southwards is the Lwandle and Rovuma microplates. In comparison to the African plate, the Victoria Microplate rotates counter-clockwise. The arrangement of mechanically stronger and weaker lithospheric zones in East African Rift Valley causes it to rotate.

Lithospheric rupture is projected to take place in about 10 million years if extension persists; the Somali Plate is predicted to be broken off and another ocean basin shall arise.

Extent of the East African Rift Valley

The East African Rift Valley is made up of a number of different rift basins that span for several thousand kilometers. The East African Rift Valley is divided into two sections. The Eastern Rift Valley (Sometimes referred to as Gregory Rift), inclusive of the Major Ethiopian Rift, runs east from the junction at Afar Triple and continues southwards as the Rift Valley in Kenya. The Albertine Rift as well as the Lake Malawi Valley are both parts of the Western Rift Valley. The rift takes one of the two pathways north of the junction of Afar Triple: west towards the Red Sea Rift or eastwards to the Gulf of Aden at the Aden Ridge.

The major plates, rift faults, GPS plate velocities, plate boundaries between overlooking blocks and minimum horizontal stress directions
The major plates, rift faults, GPS plate velocities, plate boundaries between overlooking blocks and minimum horizontal stress directions

The East African Rift Valley stretches from the junction at the Afar Triple in Ethiopia’s Afar Triangle to eastern Africa to Mozambique. Ethiopia, Uganda, Burundi, Tanzania, Rwanda, Kenya, Zambia, Malawi Uganda, Rwanda, and Mozambique are all crossed by the East African Rift Valley. It also extends offshore of Mozambique’s coast along the Lacerda and Kerimba grabens, that are connected by the Davie Ridge, which is a 2,200-kilometer (1,400-mile) relic fracture zone which cuts through the basin of West Somali and straddles the Tanzania-Mozambique border. The Davie Ridge is 30 to 120 kilometers (19 to 75 miles) broad, with an escarpment facing west (an arch plunging east) that climbs to about 2,300 meters (7,500 feet) above sea level in the southern part of its length.

Challenging Theories on the Geologic Evolution of the East African Rift Valley

Many ideas have attempted to explain the East African Rift Valley’s evolution over time. In 1972, the hypothesis was that East African Rift Valley was generated by crustal difference in density rather than tectonic activity. Evidence of mantle plumes under the East African Rift Valley has been discovered since the 1990s. Others suggested that a superplume in Africa was responsible for mantle deformation. Although the consequences of deep-seated mantle plumes are a compelling concept, where they are located and behavior are not well understood and still under investigation. The issue is still being argued.

The most current and widely accepted idea is that plate tectonics and magmatism are in feedback with each other, regulated by oblique rifting conditions, as proposed in 2009. Lithospheric thinning, as per this idea, causes volcanic activity, which leads to more magmatic processes like intrusions and many tiny plumes. The processes make the lithosphere thin even more in saturated places, causing it to resemble a mid-sea ridge in behavior.

Isotope geochemistry, geodynamical modeling, and seismic tomography are among the approaches that have contributed to a better understanding of rift evolution.

Geochemistry of Isotopes

The geochemical characteristics of a group of lavas in Ethiopia point to numerous plume origins, including a minimum of one from the deep mantle and another from the subcontinental lithosphere. In line with this, research published in 2014 examines rare-earth isotopes’ geochemical signature in lava and xenoliths samples obtained within the East African Rift Valley. These findings support the existence of a superplume which is “common in the whole rift” in conjunction with a subcontinental or mid-sea ridge-type mantle material source.

The northeast corner of Jacob's Ford
The northeast corner of Jacob’s Ford

Seismic Tomography

A seismic tomography acts as a geophysical approach for investigating Earth’s subsurface features deep to the earth’s crust. It’s an inverse approach to the problem that simulates the interior Earth’s velocities and reproduces seismographic data from all over the planet. Recent advancements in tomographic Earth simulations of S-wave and P-wave velocities show that a superplume upsurge originating from the lower part of the mantle feeds smaller-scale plumes into the upper part of the mantle near the northeastern East African Rift Valley.

Modeling of Geodynamics

It is beneficial to test theories on geodynamical models based on computers in addition to geophysical and geological measurements (e.g. seismic velocities and isotope ratios). The East African Rift Valley’s lateral asymmetry over the Tanzanian craton was reproduced using a 3D computational geodynamical simulation of the plume-crust interaction. Numerical simulation of plume-caused continental break-off reveals two different stages: the crustal rifting which was preceded by lithospheric breakdown, and upper part of the mantle plume upwelling between stages.

Evolution of the Earth’s Crust

Massive continental flood basalts blew up on the earth’s surface prior to rifting, causing elevation of the East African, Ethiopian and Somali plateaus. The East African Rift Valley’s initial rifting stage is marked by rift magmatism and localization throughout the rift zone. Expansion periods were interspersed with periods of relative dormancy. A suture region of several cratons, Reactivation of pre-Cambrian faults, displacement along major border faults, and the formation of deep basins that are asymmetric was also among the events. The formation of interior fault segments, as well as deactivation of massive boundary faults, define the second stage of rifting.

The East African Rift Valley’ tiny rift segments now produce zones of concentrated tension. The operations of multiple normal faults, that usually occur in all tectonic rift regions, have resulted in these rifts. Some of the rift sections have a lot of magmatism as well as continental flood basalts, while others, like the western branch, only have relatively little volcanic rock.

Petrology

Maps of four different depth slices of the Shear-velocity (Vs) model developed by Emry et al. 2018
Maps of four different depth slices of the Shear-velocity (Vs) model developed by Emry et al. 2018

In general, the continental crust of Africa is cool and robust. The Kaapval and Tanzania cratons, for example, maybe found all over the East African Rift Valley. The cratons are typically thick and have withstood tectonic action for billions of years. Tonalities, Greenstone belts as well as various metamorphic lithologies which are high-grade define them. When it comes to mineral resources, these cratons are quite important as they have significant deposits of antimony, gold, chromium, iron, and nickel.

In the Oligocene period, a vast amount of continental flood basalts exploded, with most of the volcanism occurring around 30 Ma, when the Gulf of Aden and the Red Sea opened. The volcanic range in composition from ultra-alkaline to felsic and tholeiitic rocks. Various mantle source areas have been postulated as a possible explanation for the variation in compositions. The East African Rift Valley may also be used to cut through ancient sedimentary rocks that have been deposited in old basins.

Seismology and Volcanism

Mount Kilimanjaro, Mount Longonot, Mount Kenya, Menengai Crater, Mount Elgon, Mount Karisimbi, Mount Meru, Mount Nyiragongo, and the Crater Highlands found in Tanzania, are among the dormant and active volcanoes in the East African Rift Valley Zone. Despite the fact that many of the mountains are not located inside the rift valley, they were produced by the East African Rift Valley.

Dalaffilla (also known as Alu-Dalafilla, Gabuli), Erta Ale, and Ol Doinyo Lengai are three active East African Rift Valley volcanoes. Erta Ale mountain is a basaltic shield volcano in northern Ethiopia’s Afar Region that has been active since 1967 and has had a laval summit lake since 1906. Dalafilla’s solitary reported activity since the Holocene began in 2008 was the greatest eruption ever eruption in the history of Ethiopia. Ol Doinyo Lengai is the sole natrocarbonatite volcano on the planet that is active. Its magma is virtually completely devoid of silica; normal lava flows contain viscosities of under 100 Pa s, which are equal to olive oil at about 26 degrees Celcius. Around 50 volcanic structures in Ethiopia, seventeen in Kenya, nine in Tanzania related to the East African Rift Valley have been dated with activity since the Holocene began.

The East African Rift Valley ranks as the world’s greatest seismically rift system that is seismically active. The Afar Depression is home to numerous earthquakes, with the greatest often occurring near or along main border faults. Seismic occurrences with a momentary maximum magnitude of 7.0 are thought to have occurred in the last century. With a modest focal depth of 12km–15 (7.5mi–9.3 miles) under the rift axis, seismicity runs parallel to this rift system. Focal depths may be less than 30 kilometers (19 mi) distant from the axis of the rift. Solutions of focal mechanism solutions typically strike NE, often exhibiting standard dip-slip faulting, however, there is also left-lateral motion.

Human Evolution Discoveries

The East African Rift Valley is a huge hominid fossils source that may be used to research human evolution. The valley was swiftly filled with sediments as the mountains eroded, offering a good habitat for fossil preservation. Several hominid progenitors of contemporary human beings have been unearthed here, like “Lucy,” a3-million-year-old fragmentary australopithecine skeleton that was discovered by researcher Donald Johanson. Mary and Richard Leakey also worked extensively in this area. Two more hominid predecessors were discovered here in 2008: Chororapithecus abyssinicus, a ten-million-year-old ape discovered in eastern Ethiopia’s Afar Rift, as well as Nakalipithecus nakayamai, a ten-million-year-old ape discovered in the same region.

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