Geology | White Paper

Geology of the Great Rift Valley

Written by  John C. Cannon 2/23/2016
The creation of the cradle of humanity, as well as the vast herds of iconic wildlife we hold synonymous with the African continent, begins 30 million years ago, long before we humans stood up on two legs and before most of our more ape-like ancestors even existed in East Africa (3). The daily struggle for life in the Great Rift Valley that now plays out mimics the tumult under the surface, at least if you’re looking at geologic time.

In fact, the concept of the Great Rift Valley has become more of a cultural notion, enveloping the people, the animals and the ecosystems that characterize much of the East African countries of Tanzania and Kenya. Geologically, however, what we think of as the Great Rift Valley is the result of several rifts created as tectonic plates jostle, shift and shrug for space below the earth’s surface.

As continental plates pull apart – in the Great Rift’s case, the African (or Nubian) plate and the Somali plate – the lithosphere, which comprises the upper layer of the mantle and Earth’s crust, stretches. Normally, it’s about 100 kilometers thick, but in East Africa, it dwindles down to as little as 20 kilometers. In some places this thinning of the Earth’s surface creates oceans of great depths (1,5). Much rarer, at least in the time of humans, are the continental rifts, like what we think of when we think of the Great Rift Valley. The rift is the first stage of continents tearing apart: Geologists expect a portion of East Africa to split off from the rest of the continent within a few million years (4,7).

These vast geological movements have created the great diversity of habitats and ecosystems that support so much unusual and diverse life in East Africa, where dry deserts, life-rich savannas, mountaintop rainforests and snow-capped peaks all share space.

In some places, as the crack formed, it ran into “cratons” of metamorphic rock that wouldn’t budge or pull apart, causing the cracks to go in different directions around the obstacle (1). The tectonic movements have led to verdant valleys and bulging highlands that break up the plains where heat from the mantle weakens the crust (1,2).

The meeting and separation of these great plates has produced a system that includes around 30 active and semi-active volcanoes such as Mount Kilimanjaro (as well as extinct ones such as Mount Kenya) along its length, stretching 6,400 kilometers (about 4,000 miles) from Jordan in the Middle East to Mozambique (3).

To the east the rifts, the Ruwenzori – the “mountains of the moon” – and the Virungas jut from equatorial Africa to some of the greatest heights on the continent and house the last 800 remaining mountain gorillas. To the south of the eastern spine, one of the world’s deepest lakes, Lake Tanganyika, plunges nearly one and a half kilometers down and highlights the Great Lakes of Africa (4). Recent research pegs the Virungas as having the greatest number of endemic vertebrates and more vertebrate species than anywhere else in Africa (11).

Perhaps the most identifiable part of the Great Rift Valley is the East African – or Gregory, named after the person who identified it to Western science – Rift, running about 600 kilometers east of the Great Lakes, through Kenya and Tanzania and encompassing the Serengeti Plain.

It’s because of the rapidly changing environment and the sediment that that process produces that human remains have been so well preserved in this area (9). And the volcanic ash, spread over the region, has added a new dimension of fertility to the region, activated by the seasonal rains (6). Much of the tuff – that is, light volcanic ash – on the plains owes its existence to the eruption of Kerimasi 150,000 years ago. Consistent streams of ash from other volcanoes have helped keep the Serengeti fertile, thanks to volcanos like Ol Doinyo Lengai, which has erupted 15 times in the past 140 years (10,12).

The upper layer of soil, a fertile mix of sand and clay called loam deposited across much of the plains, supports the hardy grasses that in turn support so much life on the plains. But less than a meter down in most places, a layer of limestone keeps larger root systems from reaching the water table. Hence, grasses, which have evolved to survive the rainy and dry seasons, predominate (10).

The way the rift evolved may have created a tumultuous climate with its constant shifting and transitions between wet and dry periods could have driven our immense adaptability as a species, allowing us the refine our adaptability to different environments. And it may even have helped nudge us into full upright, bipedal motion (1).

Today, the geothermal activity bubbling through the belly of the region could mean huge changes for humans living in the area. The United Nations Environmental Program is working to harvest this energy with steam wells in Kenya. One well could power 5,700 homes. What’s more, this energy would be a sustainable source of power for many people who have limited or no access to electricity. (5)

Citations
1. “ East Africa's Great Rift Valley: A Complex Rift System,” Geology.com
2. “ Rift Valley,” National Geographic Education.
3. “ The Great Rift,” Gorongosa National Park.
4.  “
Great Rift Valley, Tanzania,” Odyssei Travel Community.
5. “ Rift Valley,” National Geographic Education.
6. “ Road will ruin Serengeti,” Nature, 2010.
7. “ Rift Valley: definition and geologic significance,” Ethiopian Rift.
8. “ East African Rift System,” Brittanica.com.
9. “ Great Rift Valley,” New World Encyclopedia.
10. “ The Serengeti Lion,” University of Chicago Press, 1976.
11. “ Alpine grassland palaeoecology of the Virunga Volcanoes, East Africa: A new phytolith record from Mt. Muhavura,” Quaternary International, 2016.
12. “
Oldoinyo Lengai,” 2009.