East Africa's Great Rift Valley: A Complex Rift System Structural Geology Steffanie Mahusay Rosfel Bernal Ivan Fritz Esguerra

The East African Rift System Is one of the geologic wonders of the world. The Nubian Plate makes up most of Africa, while the smaller plate that is pulling away has been named the Somalian Plate. These two plates are moving away form each other and also away from the Arabian plate to the north. The point where these three plates meet in the Afar region of Ethiopia forms what is called a triple-junction.

The East African Rift System

What is the East African Rift System? 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.

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.   The two EAR branches are often grouped with the Ethiopian Rift to form the East Africa Rift System (EARS). The complete rift system therefore extends 1000's of kilometers in Africa alone and several 1000 more if we include the Red Sea and Gulf of Aden as extensions.

How did these rifts form? One popular model for the EARS assumes that elevated heat flow from the mantle is causing a pair of thermal "bulges" in central Kenya and the Afar region of north-central Ethiopia. Bulges are initiated by mantle plumes under the continent heating the overlying crust and causing it to expand and fracture.

The stretching process associated with rift formation is often preceded by huge volcanic eruptions which flow over large areas and are usually preserved/exposed on the flanks of the rift.

The East African Rift   The rifting of East Africa is complicated by the fact that two branches have developed, one to the west which hosts the African Great Lakes and another nearly parallel rift about 600 km to the east which nearly bisects Kenya north-to-south. The East African Rift is referred to as “continental rifting” – a rifting process that occurs at continental setting. The Eastern and Western branches were developed by the same processes but they have very different characters. 

These two branches are generally following old sutures between ancient continental masses that collided billion years ago to form the African craton. The split around the Lake Victoria region occurred due to the presence of a small core of ancient metamorphic rock, the Tanzania craton, that was too hard for the rift to tear through. It instead diverged around it leading to the two branches that can be seen today.

The Eastern Branch is characterized by greater volcanic activity while the Western Branch is characterized by much deeper basins that contain large lakes and lots of sediment.  Recently, basalt eruptions and active crevice formation have been observed in the Ethiopian Rift which permits us to directly observe the initial formation of ocean basins on land. Most rifts in other parts of the world have progressed to the point that they are now either under water or have been filled in with sediments and are thus hard to study directly. 

Triple Junction in the Afar region of Ethiopia Triple Junction in the Afar region of Ethiopia. Image shows areas of stretched and oceanic crust as well as areas of exposed flood basalts that preceded rifting. Areas unshaded or covered by flood basalts represent normal continental crust.

Afar East African Rift System

This image shows several fault scarps that are progressively farther away. Edges of several horst blocks and a graben that contains Lake Baringo.

The gorge was carved by water, and an igneous dike cutting through the wall of the canyon.

Rift Valley

Jordan Rift Valley

Jordan Rift Valley Is an elongated depression located in modern-day Israel and Jordan. Includes the Jordan River, Jordan Valley, Hula Valley, Lake Tiberias, and the Dead Sea.

Jordan rift Valley

Origins and Physical Features The Jordan Rift Valley was formed many million years ago in the Miocene epoch (23.8 - 5.3 Myr ago) when the Arabian tectonic plate moved northward and then eastward away from Africa. The lowest point in the Jordan Rift Valley is at the shores of the Dead Sea, which is also the lowest point (on land) on the surface of the earth at 400 meters below sea level.

A 2003 satellite image of the region showing the Jordan Rift Valley.

Rising sharply to almost 1,000 meters in the west, and similarly in the east, the rift is a significant topographic feature over which few narrow paved roads and difficult mountain tracks lead.

Northern section of the Great Rift Valley Northern section of the Great Rift Valley. The Sinai Peninsula is in the center and the Dead Sea and Jordan River valley above.

Jordan river is a river in Southwest Asia flowing to the Dead Sea

Panorama of Jordan Valley

The Dead Sea Transform The plate boundary which extends through the valley is variously called the Dead Sea Transform or Dead Sea Rift. The boundary separates the Arabian plate from the African plate, connecting the divergent plate boundary in the Red Sea (the Red Sea Rift) to the East Anatolian fault in Turkey. The interpretation of the tectonic regime that led to the development of the Dead Sea Transform is highly contested.

Anatolian Plate

Some consider it as a transform fault that accommodates a 105 km northwards displacement of the Arabian plate. Others presume that the Rift is an incipient oceanic spreading center, the northern extension of the Red Sea Rift, and the displacement along it is oblique, with approximately 10–15 km of extension in addition to the more substantial left lateral (sinistral) strike-slip. The evolution of the rift, according to this latter model, started in the late Miocene with the linear series of basins that propagated gradually along their axes to form the present rift valley.

Dead Sea