The East African Rift System: a belt of natural wonders

The East African Rift System is home to Africa's grandest geological landscapes. Majestic volcanoes loom over endless savannahs through which the Big Seven roam, plateau lands abruptly give way to deep rift valleys forming great scarps; and lakes are so large they feel like inland seas. Remains of our ancestors and their relatives were found in ancient river and lake deposits. In the future these landscapes will undergo a huge transformation: the African continent will break into two pieces and a new ocean will invade the space between both.

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. As a giant zipper, the Great Rift first formed in the north and then expanded southwards. 

In this article we will go on a north to south journey through the Great Rift to marvel at the remarkable geology. But let first see how the Great Rift came into being, and what is its fate.

Africa breaks in two, a new ocean forms

The earth's outer shell, known as the lithosphere, is fractured into a puzzle of rigid tectonic plates, which are driven by movements deep within the earth's mantle. 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. 

Africa splits in two along the Great Rift: the Nubian plate in the west will break away from the Somali plate in the east (see illustrated map). As the deep chasm between both broadens, 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. The journey starts in the Middle East.

The Afar Triangle

The large and deadly faults of Turkey are connected to fault systems that run southward parallel to Mount Lebanon through 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, flat area, also known as the Danakil Depression, with volcanoes, faults, salt and sulfur lakes. The Danakil Depression is an inferno 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, bulging lava lake. 

The Afar Triangle is a so-called triple junction, because it lies where not two, but three tectonic plates (Arabian, Nubian and Somali plates) are getting dislodged from each other. The Gulf of Aden and the Red Sea are two branches of the junction already occupied by nascent oceans. The sea will one day submerge the Afar Triangle and eventually advance southwards, into the Ethiopian rift, the third branch. 

From here the great zipper stretches into Africa.

The Afar Triangle is also an important palaeo-archaeological site. In the 1970s 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. She's been called an ancestor of humans, but the australopitecus may well be more related to the other great apes than to humans. The evolutionary tree of humans is far from bing resolved, which is exciting. 

Blue Nile and Egypt

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 (known as the Afar Plume) that preceded rifting 30 million years ago. The Blue Nile has its sources in these volcanic highlands. As the basalt bedrock weathers into soil, volcanic minerals are loosened and carried away by the rivers. Thousands of kilometers further downstream, the volcanic sediment settles on riverbanks of the Nile, resulting in incredibly fertile soils. Good soils, the base of all terrestrial life, must have been beneficial for farming and cultural prosperity in Ancient Egypt.

As we move south, the Great Rift splits into two large branches: a western and an eastern branch (shown in the illustration of the Afar Triangle, above).

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

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 up 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.

The lakes are framed by mountains and volcanoes, for example the 5,000-metre-high Rwenzori Mountains in Uganda. These rugged, snow-capped peaks are probably what 19th century explorers referred to as the Mountains of the Moon, where the source of the Nile was believed to be located. 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.

Gorillas in the mist

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 ongoing 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. Upstream, Dr. David Livingstone encountered an imposing 1700-meter-wide curtain of waterfalls, which are known locally as the 'Smoke that Thunders', or Mosi-oa-Tunya. Livingstone named the falls after his queen, and as such, the Victoria Falls became known to the western world. In a more recent article, I wrote about the geological origin of Victoria Falls.

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 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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You may want to read more on the origins of humans, in or out of Africa, and how we lived near the water. My fascination with Africa started in Tanzania, which I visited the first time in 2003. In the article on my meeting with Jane Goodall I wrote on some of these early beginnings. Get a better grasp of plate tectonics and how it became accepted (an arduous battle with much opposition), and of Africa's geological history in general. Go west and read on Africa's strangest river, the Niger. It makes a huge bend near Timbuktu; and then head back south, to learn about the Zambezi River and the Smoke that Thunders and how its evolution was influenced by great volcanic episodes. 

Sources:

Chambers, R. 2021, Mountains of the Moon in Africa, https://roseannechambers.com/mountains-of-the-moon-in-africa/

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.

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Kathelijne: I am intrigued by how earth, air, ocean, life and societies interact on geological and human timescales.

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