How Rift Valleys are Formed

For the last 30 million or so years immense tectonic forces in action around north-east Africa have been slowly pulling old continental plates apart and in the process creating new ones.

Earth’s rocky outer shell, or lithosphere, floats atop a hotter, denser and more fluid layer called the asthenosphere. The continental lithosphere is about 150 kilometres (93 miles) thick, while the oceanic lithosphere beneath our seas is thinner, 100 kilometres (60 miles) at its thickest.

This brittle surface layer consists of 12 major tectonic plates and numerous smaller ones, all of which are in constant motion due to convection currents within the Earth’s mantle. The plates are moving very slowly but surely at roughly five to ten centimeters (two to four inches) per year.

This movement stretches and squeezes the lithosphere in all directions. At their boundaries, or faults, these plates interact – either converging (colliding), diverging (moving apart) or slipping past each other.

In the so-called East African Rift System (EARS) region there are three large diverging plates: the Arabian Plate and Africa’s Somalian and Nubian protoplates. The intersection of these plates – known as the Afar Triangle or Afar Depression – is located at the mouth of the Red Sea.

The Eastern Rift Valley (which is more commonly referred to as the Great Rift Valley) is a zone of tectonic subsidence that extends the full length of the EARS.

A rift valley is the long, narrow, flat-bottomed trough of land created when a block of Earth’s lithospheric crust drops into the space that’s left when two divergent tectonic plates drift apart. They can be anything from ten kilometres (six miles) to several hundred kilometres wide, with a roughly symmetrical cross-section. The entire East African Rift System, meanwhile, extends some 6,400 kilometres (4,000 miles) north to south from Syria in south-west Asia, down through the Red Sea and along the African coast all the way to Mozambique in the south-east of the continent.

If this rift continues the valley could sink low enough for the Gulf of Aden to flood the region, turning the Horn of Africa into an island.

The cradle of humanity?

North-east Africa’s Afar region is a site of scientific interest as some of the biggest palaeoanthropological breakthroughs have arisen there. This includes the discovery of the 3.2-million-year-old fossilized remains of a female nicknamed Lucy. The rift’s history holds the answers to many questions about our ancestors’ evolution and how early humans developed the intelligence to walk on two feet and adapt to major climate changes.

Tanzania’s Olduvai Gorge in particular has unearthed over 60 hominids (early humans). The fossil deposits in this steep-sided, 40-kilometre (25-mile)-long gorge saw it become a UNESCO World Heritage Site back in 1979. No other location has revealed ancient remnants so closely connected to their environment.