The East African Rift System: a belt of natural wonders

16/12/2020

In the East African Rift System, we find Africa's emblematic landscapes, such as Kilimanjaro, the Ngorongoro Crater, the vast savannahs that are home to the Big Five, and the Great Lakes. Furthermore, remains of our own ancestors and their relatives were found here. But in the future the East African Rift will undergo a major transformation: the African continent will break apart and the space between both parts will be invaded by the ocean.

Author: 2020 Kathelijne Bonne

The Great Rift of East Africa is an elongated region of depressions, valleys,  faults, fractures and volcanoes that stretches from north to south across the entire length of the African continent, from Turkey, through Ethiopia, to Mozambique and Botswana. The Great Rift first formed in the north and then expanded southwards, as a giant zipper. 

In this article we will go on a north to south journey through the Great Rift and we will marvel at some of the most remarkable geological landscapes. But let us first quickly explain how the Great Rift came into being, and what is its fate.

Africa splits into two

The earth's outer shell, known as the lithosphere, is fractured into a puzzle of rigid plates, which are driven by movements deep within the earth's mantle, as described in our article on Plate Tectonics. When two tectonic plates move towards each other, as is the case for convergent plates, mountains are thrust up. Divergent plates on the other hand move away from each other, creating depressions or rifts (elongated valleys that become increasingly deeper), as in Africa. 

East African Rift.
East African Rift.

As the African tectonic plate splits into two along the Great Rift, the Nubian plate in the west will get detached from the Somali plate in the east (see illustrated map). In the future, a new ocean will occupy the space between both plates. The separation of plates proceeds slowly, a few centimeters per year. In Africa, the first signs of rifting began more than 40 million years ago, during the Eocene Epoch, but most activity took place during the last fifteen to ten million years.

Equipped with this background knowledge, we embark on our epic journey. We will encounter some fellow Hominids and cross large rivers and volcanoes. Our journey starts in the Middle East.

The Afar Triangle

Faults of the Great Rift system start in Turkey, running southward parallel to Mount Lebanon through the depression of the Dead Sea, one of the lowest and saltiest lakes on earth. Through faults along the Gulf of Aqaba, this area is linked to the famous Afar Triangle, located in northern Ethiopia, Djibouti and Eritrea. The Afar Triangle is a low lying area, also known as the Danakil Depression, with volcanoes, faults, salt and sulfur lakes. Here, not two, but three tectonic plates,  the Arabian, Nubian and Somali plates, move in opposite directions from each other and a 'young' ocean, the Gulf of Aden and the Red Sea, already invaded what was, previously, land. From this region the great zipper stretches into Africa, via the Ethiopian rift. 

The Afar Triangle is also an important palaeo-archaeological site. In the seventies of last century, a skeleton of a young female Hominid, Lucy, was unearthed. She belonged to the genus Australopithecus afarensis and lived about 3.2 million years ago.

Afar Triangle.
Afar Triangle.
Erta Ale lava lake. Note the people, top left (Enrique Pidal / CC BY-SA 4.0)
Erta Ale lava lake. Note the people, top left (Enrique Pidal / CC BY-SA 4.0)

From hell to fertile riverbanks

Both the plateaus of the Ethiopian highlands and the deepest depths of the Danakil Depression were formed by rifting. The Danakil Depression is hell on earth in terms of heat, drought and volcanism. Only the sturdiest of species thrive in this type of environment. The volcano Erta Ale is one of only six volcanoes worldwide with an active lava lake.

At the edges of the Danakil Depression, the land rises steeply to the Ethiopian Highlands. The rocks of these highlands consist of basalt, testimony of massive volcanic eruptions that preceded rifting 30 million years ago. The Blue Nile has its sources in these volcanic highlands. As the basalt bedrock weathers to sediment, volcanic minerals are loosened and carried away by the rivers. Thousands of kilometers further downstream, the volcanic sediment settles out of the Nile on the Egyptian riverbanks, resulting in incredibly fertile soils. This fortunate geological quirk certainly must have benefited cultivation and cultural prosperity in Ancient Egypt.

As we move south, the Great Rift splits into two large branches: a western and an eastern branch.

Turkana Boy

The eastern branch leads us via Lake Turkana to Kenya. At the shores of this huge lake, a very complete human-like skeleton was excavated in 1984. It belonged to a nine-year-old boy who lived about 1.5 million years ago, now known as Turkana Boy, a Homo ergaster.

On the border with Tanzania the eastern branch of the Great Rift stops, and here some majestic, solitary volcanoes rise above the great plains: Kilimanjaro, Mount Kenya, Mount Meru and Mount Elgon. A very special and photogenic volcano is the Oldoinyo Lengai, sacred mountain of the Maasai. It produces carbonate lava, the only volcano in the world to do so. The Ngorongoro crater on the other hand, is an old, eroded volcano.

Great Lakes and Great Apes

The western branch is no less majestic in terms of landscapes. We are in the area of Africa's Great Lakes; the Albert, Edward, Kivu, Tanganyika, Rukwa and Malawi Lakes lie in a great S from north to south. These lakes are deep rift depressions, filled with water from rain and rivers. The Tanganyika and Malawi lakes are 1400 and 600 metres deep respectively, and they have existed for 12 and 8 million years, ranking them amongst the oldest lakes in the world.

Dr. Jane Goodall.
Dr. Jane Goodall.

The lakes are framed by mountains and volcanoes, for example the 5,000-metre-high Rwenzori Mountains in Uganda, also known as Mountains of the Moon. In Congo, the lava lake of the notorious Nyirangongo volcano near Goma shrouds the often-overcast night sky in an ominous reddish glow. In Tanzania, on the steep, pristine slopes of Lake Tanganyika, Jane Goodall set foot in 1960 to study our closest living cousins, the chimpanzees. Her discoveries prompted anthropologists change the definition of man.

Crater lake in western Uganda (photo: K. Bonne).
Crater lake in western Uganda (photo: K. Bonne).

The western branch is the water tower of Africa: Africa's large rivers, Congo, Nile and Zambezi all start here. The climate is superhumid and therefore the vegetation is exuberant. Uganda, an emerald green country dotted by crater lakes, wetlands and rivers, is rightfully called the pearl of Africa. Due to the extreme climatic and geographic variety, biodiversity is very high in the western branch. The Virungas are the last refuge of the endangered mountain gorilla, defended, literally, to death by Dian Fossey, who was murdered in 1985. Today, the gorillas are protected by the rangers of Virunga National Park, trained especially for this purpose and risking their lives on a daily basis because of the armed conflict in eastern DRC.

Between the eastern and the western branch lies Lake Victoria, a vast, shallow lake. It is supplied with water by the many rivers from the surrounding highlands. Unfortunately it is also severely polluted. Near Jinja in Uganda, Lake Victoria waters spill over the edge, and here, the White Nile starts its long journey to Egypt.

Young parts of the Great Rift

We head further south. Beyond Lake Malawi, the Great Rift splits into two branches again. One branch stretches eastwards to the plains of southern Mozambique. Rifting activity started here 'recently', perhaps less than a million years ago. This is manifested in an overall flat relief with some minor hills, much less spectacular than elsewhere in the Great Rift. Nevertheless, these hills are significant enough to divert the course of the great Limpopo River, which is forced to flow to the south.

Then we head into the south-western branch of the Great Rift. It stretches through Zambia to Botswana, along some very deep rift basins, Mana Pools and the Gwembe Trough, prime safari locations. The Zambezi River runs eastwards through these rifts. Right in the heart of southern Africa, Dr. David Livingstone encountered an imposing 1700-meter-wide curtain of waterfalls. The 'Smoke that Thunders', or Mosi-oa-Tunya, are formed by the difference in height between the low-lying rift depressions and the surrounding high plateau. Livingstone named the falls after his queen, and as such, the Victoria Falls became known to the western world.

Victoria Falls, on the frontier between Zambia and Zimbabwe. (photo: K. Bonne)
Victoria Falls, on the frontier between Zambia and Zimbabwe. (photo: K. Bonne)

Okavango Delta

We have reached Botswana, at the end of our journey. This is at first glance a fairly flat country. However, as in southern Mozambique, radar images reveal some gentle hills and slopes, a sign of the earliest stages of rifting. And just as in Mozambique, this is sufficient to influence the course of the rivers. The Okavango Delta formed on a slightly subsided rift, creating a unique ecosystem of wetlands and watercourses.

Further east in Botswana we find a few shallow, bowl-shaped depressions, the Makgadikgadi Salt Pans, whose water levels vary enormously depending on rainfall. Recent research shows that this is the actual cradle of our own species, Homo sapiens, which lived and thrived here, some 200,000 years ago, and embarked on its own great journey from here. 

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Sources:

Chan, E.K.F., Timmermann, A., Baldi, B.F. et al. Human origins in a southern African palaeo-wetland and first migrations. Nature 575, 185-189 (2019). https://doi.org/10.1038/s41586-019-1714-1

Dawkins, R & Wong Y, 2019, The Ancestor's Tale: A Pilgrimage to the Dawn of Life. Weidenfeld & Nicolson, 800 p.

Modisi, M. P. et al, 2000, Rift kinematics during the incipient stages of continental extension: Evidence from the nascent Okavango rift basin, northwest Botswana. Geology, v. 28; no. 10; p. 939-942.

MacGregor, D. 2014, African Rift System: A series of interpreted maps through time. Journal of African Earth Sciences, Vol. 101, 232-252.

Yirgu et al, 2006, The Afar volcanic province within the East African Rift System: introduction. Geological Society, London, Special Publications, 259, 1-6, https://doi.org/10.1144/GSL.SP.2006.259.01.01

Pictures:

Title picture: Wildebeest migration: Daniel Rosengren / CC BY 4.0.

(*) Turkana Boy: Neanderthal Museum / CC BY-SA 4.0.

No annotation: public domain.

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