The orientation of the rifts, highlands and associated flood basalts in the Afar region indicate that the EAR most likely formed as the result of elevated heat flow from the asthenosphere beneath Kenya and Ethiopia. This increased heat caused the regions to uplift resulting in stretching and fracturing of the brittle continental crust.
The source of this elevated heat flow is under debate but most geologists agree that it is related to mantle plume activity heating the overlying crust and causing it to expand and fracture. Today the EAR remains above sea level however in the future, as extension continues along the rift, the rift valley will sink lower and lower eventually allowing ocean waters to flood into the basin.
If rifting continues, new basaltic oceanic crust may form along the centre of the rift producing a new narrow ocean basin with its own mid ocean ridge between the Nubian and Somalian plates. Whilst both the Vale of Eden and the East African Rift Valley are formed by continental rifting they are formed in response to different crustal processes.
The Eastern rift branch is a magmatic rift dominated by volcanism and formed by an ascending mantle plume bringing hot asthenosphere the upper layer of the earth's mantle, below the lithosphere to shallow crustal levels.
The Vale of Eden is a sedimentary rift with extension related to plate stretching and more comparable to the Western rift branch. Morley ed. This website uses cookies This website uses cookies to give you the best user experience.
The purpose of this paper is to discuss the general geology of these rifts are and highlight the geologic processes involved in their formation. Smaller segments are sometimes given their own names, and the names given to the main rift segments change depending on the source. The oldest and best defined rift occurs in the Afar region of Ethiopia and this rift is usually referred to as the Ethiopian Rift.
Further to the South a series of rifts occur which include a Western branch, the "Lake Albert Rift" or "Albertine Rift" which contains the East African Great Lakes, and an Eastern branch that roughly bisects Kenya north-to-south on a line slightly west of Nairobi Figure 2.
These two branches together have been termed the East African Rift EAR , while parts of the Eastern branch have been variously termed the Kenya Rift or the Gregory Rift after the geologist who first mapped it in the early 's. The complete rift system therefore extends 's of kilometers in Africa alone and several more if we include the Red Sea and Gulf of Aden as extensions.
In addition there are several well-defined but definitely smaller structures, called grabens, that have rift-like character and are clearly associated geologically with the major rifts. Thus, what people might assume to be a single rift somewhere in East Africa is really a series of distinct rift basins which are all related and produce the distinctive geology and topography of East Africa.
Figure 3: "Textbook" horst and graben formation left compared with actual rift terrain upper right and topography lower right. Notice how the width taken up by the trapezoidal areas undergoing normal faulting and horst and graben formation increases from top to bottom in the left panel.
Rifts are considered extensional features continental plates are pulling apart and so often display this type of structure. The exact mechanism of rift formation is an on-going debate among geologists and geophysicists.
One popular model for the EARS assumes that elevated heat flow from the mantle strictly the asthenosphere is causing a pair of thermal "bulges" in central Kenya and the Afar region of north-central Ethiopia. These bulges can be easily seen as elevated highlands on any topographic map of the area Figure 1. As these bulges form, they stretch and fracture the outer brittle crust into a series of normal faults forming the classic horst and graben structure of rift valleys Figure 3.
Most current geological thinking holds that bulges are initiated by mantle plumes under the continent heating the overlying crust and causing it to expand and fracture. Ideally the dominant fractures created occur in a pattern consisting of three fractures or fracture zones radiating from a point with an angular separation of degrees.
The point from which the three branches radiate is called a "triple junction" and is well illustrated in the Afar region of Ethiopia Figure 4 , where two branches are occupied by the Red Sea and Gulf of Aden, and the third rift branch runs to the south through Ethiopia. These eruptions are considered by some geologists to be "flood basalts" - the lava is erupted along fractures rather than at individual volcanoes and runs over the land in sheets like water during a flood.
Such eruptions can cover massive areas of land and develop enormous thicknesses the Deccan Traps of India and the Siberian Traps are examples.
If the stretching of the crust continues, it forms a "stretched zone" of thinned crust consisting of a mix of basaltic and continental rocks which eventually drops below sea level , as has happened in the Red Sea and Gulf of Aden. Further stretching leads to the formation of oceanic crust and the birth of a new ocean basin. Twenty-five million years ago, as plate tectonics began to pull apart eastern Africa, the landscape that would eventually be home to the first humans began to take shape.
Now, new research says that landscape — and its lakes, rivers and climate — may have looked quite different than scientists thought. The eastern branch of the rift passes through Ethiopia and Kenya, and the western branch forms a giant arc from Uganda to Malawi. Millions of years ago, the Arabian Peninsula was connected to Africa.
Seafloor spreading caused the Arabian and African plates to rift apart. The Indian Ocean flooded the rift valley between the continents, creating the Red Sea. Today, Africa and Asia are connected by the triangle of the Sinai Peninsula. East African Rift. Throughout the East African Rift, the continent of Africa is splitting in two.
The African plate, sometimes called the Nubian plate, carries most of the continent, while the smaller Somali plate carries Horn of Africa. Two arms of the Afar Triple Junction continue to widen in the process of seafloor spreading—the arm extending into the Red Sea and the arm extending into the Gulf of Aden. As these rifts continue, the narrow valley created by the Gregory Rift the arm of the Afar Triple Junction located above sea level may sink low enough that the Arabian Sea will flood it.
Separated from Africa by this new strait , Horn of Africa sitting on the Somali plate would become a continental island , like Madagascar or New Zealand. The Western Rift is one of the most biodiverse regions in Africa, featuring a narrow corridor of highland forest s, snow-capped mountains, savanna s, and chains of lake s and wetland s. Rift lake s, formed as freshwater floods rift valleys, often mark rift valley systems. More than a billion years ago, for instance, the North American plate began a rifting process.
A triple junction formed in the middle of the young continent, and deep rift valley developed. Freshwater drained and collected in this rift valley, creating a lake. After millions of years, however, the rift failed. Today, the remains of that ancient rift lake, Lake Superior, rest atop one of the oldest and deepest rift valleys in the world. Lake Baikal, the rift lake over the Baikal Rift Valley in Siberia, is the deepest and oldest freshwater lake in the world. The deepest parts of Lake Baikal are 1, meters 5, feet , and are getting deeper every year.
In addition, over the past 25 million years, layers of soft sediment have accumulate d on the lakebed. The actual floor of the rift valley is more than 5 kilometers 3 miles deep. Lake Baikal also has the largest volume of liquid freshwater in the world—a staggering 23, cubic kilometers 5, cubic miles.
Although the Dead Sea is not the world's deepest lake, the deep Jordan Rift makes it the lowest land elevation on Earth. Unlike Lake Baikal, however, the Dead Sea is not a true rift lake as it was not formed entirely by the rift beneath it.
The so-called Dead Sea Transform is a geologically complex area, where tectonic plates interact in many ways. The most famous rift lakes in the world may be the series of narrow, deep rift valleys in the East African Rift known simply as the Rift Valley lakes. The Rift Valley lakes, stretching from Ethiopia to Malawi, are sites of amazing biodiversity.
Only Lake Baikal is deeper and holds more water. Like many freshwater Rift Valley lakes, Lake Tanganyika is home to hundreds of endemic species of cichlid fish.
Rift valleys are typically deep and narrow. Photograph by Emory Kristof, National Geographic.
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