The East African Rift Valley System

The East African Rift Valley System

One of the world’s geologic wonders of the world is the East African Rift Valley System, which is the largest rift or fracture in the surface of the earth that has widened over time. People also call it the Afro-Arabian Rift Valley. It is a place where the tectonic forces of this planet have created new African plates by splitting the old continental crust apart in the same way it does mid-ocean ridges. So, the East African Rift Valley is a divergent boundary in a region of continental crust. Debates are still going on as to how exactly rift comes into being. But the process is well displayed in East Africa.

The two branches

The rifting of East African Rift Valley System has paved the way to two branches – the Eastern Rift Valley and the Western Rift Valley. On the west are the African Great Lakes and another parallel rift to the east which divides Kenya north-to-south and enters Tanzania where it dies out. Lake Victoria is between these two branches. These fractures follow the old sutures between continental masses and the Tanzania craton.

The continental masses are ancient that collided billions of years ago to form the African craton. The Tanzania craton is a small core of metamorphic rock that was too hard for it to go straight through. Instead, it diverged around it forming the two branches – east and west. The plateaus adjacent to the East African Rift System slope upward toward the valley. They provide an average of 2,500 feet to the valley floor. In the Gikuyu and Mau escarpments, it drops about 8,500 feet.

Eastern Rift Valley

The eastern branch system is the main branch of the East African Rift Valley System and extends along the length of the rift system. The branch is in the north by the Gulf of Aqaba, the Dead Sea, and the Jordan River. It comprises the Main Ethiopia Rift from the Afar Triple Junction and continues along the Red Sea and the Ethiopian Denakil Plain towards the south to Lakes Naivasha, Rudolf, and Magadi in Kenya along the Kenyan Rift Valley. It is less obvious in Tanzania, due to the erosion in the eastern rim, but the rift continues southward through the Mozambique Plain and the Shire River valley to the Indian Ocean coast near Beira, Mozambique.

The east branch is arid, which one can see in the salt-loving algae of the salty and shallow Lake Magadi. A nearby lake to the north appears dark due to the depth. The white salt deposits of the lake floor host a few commercial salt pans. The lakes are where the secondary and main fault trends intersect.

Western Rift Valley

The west branch of the East African Rift Valley System forms an arc all the way from Uganda to Malawi. It extends northward from the northern part of Lake Nyasa in an arc that includes Lakes Rukwa, Kivu, Tanganyika, Edward, and Albert. The lakes within the Western Rift Valley are in-depth and fjord-like. The floors of some of these lakes are well below sea level.

The rifts have different characteristics

The western and eastern branches of the East African Rift Valley System have very different characteristics even though the same tectonic processes developed them. The west branch is characterized by deeper basins with large lakes and lots of sediment while volcanic activity typifies the eastern branch. Active crevice formation and basalt eruptions feature the east rift. Most such phenomenon in the world is now either under water or sediments and hard to study directly. The East African Rift System, on the other hand, is an excellent laboratory to study an actively developing system. Geologists and scholars explore the formation of an ocean on land by directly observing the Ethiopian Rift.

Volcanic hazards along the rift

The stretching process results in tension causing the crust of the East African rift valley to weaken. Hence, the system that has been forming for 30 million years is often associated and marked by individual and substantial volcanic activity. There is a variety of volcano types along the East African Rift System. They are usually on the fissures and edges along the parts of its length ranging from effusive basaltic fissure systems to large explosive silicic centers. Some geologists consider these eruptions as “flood basalts” because the lava erupts along fractures and runs over like water during a flood.

The styles, frequency, and magnitude of these volcanoes vary. The eruptions can even cover vast areas of land and develop large thicknesses. Some of those volcanoes are large, producing such massifs as Mount Kenya and Mount Kilimanjaro. Deeply eroded slopes indicate that the volcanoes have been inactive for a long time, whereas the volcanoes that are on the faults are much younger than that. Some curious geologists have been investigating the creation and movement of past and current rift magmatism and active volcanism from the surface all the way to deep within the earth in the Afar Depression and the Main Ethiopian Rift.

Human evolution in the valley

The unique geological setting called the East African Rift System is always high on the list of marvels to visit not only for volcanic studies but also to understand how continents break apart. Equally significant is its contribution towards understanding the beginnings of human evolution. Many hominid fossils remain within the rift. Therefore, the development of the East African Rift Valley has played a key role in shaping human development. Its structure may have made East Africa sensitive to climate changes which led to alternations between dry and wet periods. The pressure caused by these environmental changes may have altered the local climate, just the motivation needed for humans to become brainier, bipedal, develop culture and wonder how such a rift came into being to adapt to the shifting climates.

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