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A large rift in the East African Rift is evidence of the division of the continent into two parts



Suddenly a big, several kilometers wide rift appeared in the southwest of Kenya. The still-growing tear caused part of the Nairobi Narok highway to collapse and was accompanied by seismic activity in the area.

Earth is an ever-changing planet, though in some ways the changes are barely noticeable to us. Plate tectonics is a good example of this. But again and again it comes to something dramatic and leads to renewed questions about the division of the African continent.

The lithosphere of the earth (consisting of the crust and the upper part of the mantle) is divided into several tectonic plates. These plates are not static but move at different speeds relative to each other and "glide" across a viscous asthenosphere. Which mechanism or mechanisms are behind their movement is still under discussion, but there are likely to be convective currents within the asthenosphere and the forces generated at the boundaries between the plates.

These forces do not simply move the plates around, they can also cause plates to break, form a crack, and possibly lead to the formation of new plate boundaries. The East African Rift system is an example of where this is happening.

The East African Rift Valley stretches over 3,000 kilometers from the Gulf of Aden in the north to Zimbabwe in the south, dividing the African plate into two unequal parts: the Somali and Nubian plates. Activity along the eastern arm of the Rift Valley along Ethiopia, Kenya and Tanzania became apparent as the large rift suddenly appeared in southwestern Kenya.

See the BBC video about the crack on Narok ̵

1; Nairobi Highway

Why is Rifting happening?

When exposed to horizontal expansive force, the lithosphere expands and becomes thinner. Eventually it will break, which will lead to the formation of a rift valley.

Great Rift Valley, Tanzania Shutterstock

This process is accompanied by surface phenomena along the rift valley in the form of volcanic activity and seismic activity. Cracks are the initial stage of continental fragmentation and, if successful, can lead to the formation of a new sea basin. An example of a place on earth where this happened is the South Atlantic, which arose from the dissolution of South America and Africa about 138 million years ago – ever noticed how their shorelines fit together like parts of the same puzzle?

Snider-Pellegrini's 1858 maps showing his idea of ​​how the American and African continents might once go together. https://en.wikipedia.org/wiki/Antonio_Snider-Pellegrini#/media/File:Antonio_Snider- Pellegrini_Opening_of_the_Atlantic.jpg

Continental Rifting requires the existence of expansion forces large enough to break the lithosphere. The East African Rift is described as an active type of rift in which the source of these stresses lies in the circulation of the underlying mantle. Below this trench, the rise of a large mantle cloud overhangs the lithosphere, causing it to weaken as a result of the increase in temperature, expanding and breaking through fault.

Magma Plume Doming DBoyd13, CC BY-SA

Evidence for the existence of this hotter than normal mantle cloud has been found in geophysical data and is often referred to as "African superswell". Not only is this supernumerary a widely accepted source of extractive forces leading to the formation of the Rift Valley, but it has also been used to explain the anomalous high topography of the South and East African plateaus.

Breaking Up Is Not Easy

Rifts have a very characteristic topography characterized by a series of flawed depressions surrounded by higher ground. In the East African system, a series of aligned trench valleys, separated by large bounding errors, can be clearly seen from outer space.

Topography of the Rift Valley James Wood and Alex Guth, Michigan Technological University. Basemap: Space Shuttle Radar Topography Image from NASA

Not all of these fractures occurred at the same time, but followed a sequence that began in the Afar region of northern Ethiopia about 30 million years ago and occurred at a mean velocity of between 2 , 5-5 cm per year.

Although rifting is mostly unnoticeable to us, the formation of new faults, cracks and tears, or re-movement along old faults, as the Nubian and Somali plates move apart, can lead to earthquakes.

In East Africa, however, most of this seismicity is spread over a wide area above the rift valley and is relatively small. The parallel volcanism is another superficial manifestation of the progressive continental decay and the proximity of the hot molten asthenosphere to the surface.

A timeline in action

The East African Rift is unique in that it allows it to observe various stages of rifting along its length. In the south, where the gap is young, the expansion rates are low and there is a large-scale upheaval. Volcanism and seismicity are limited.

Towards the Afar region, the entire trench bottom is covered with volcanic rock. This indicates that the lithosphere has almost completely dissolved in this area. When this happens, the solidification of magma in the space created by the broken plates will create a new ocean. Eventually, the spread of the seabed will continue over a period of tens of millions of years along the entire length of the ditch. The ocean will flood and the African continent will become smaller and there will be a large island in the Indian Ocean consisting of parts of Ethiopia and Somalia, including the Horn of Africa.

A Twitter user with the handle & # 39; @ Nel-kimz & # 39; shared this picture on March 19th.

 The Conversation Dramatic events such as sudden motorway fissions or major earthquake disasters could cause continental rifts a sense of urgency, but most of the time it's about splitting Africa without anyone noticing.

By Lucia Perez Diaz, postdoctoral fellow, Fault Dynamics Research Group, Royal Holloway

This article was originally published on The Conversation published. Read the original article.


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