East African Rift

The East African Rift is an active area where the Earth's surface is slowly splitting apart. It is located in East Africa and began forming about 22–25 million years ago during a time called the Miocene. Scientists consider it part of a much larger system that was once called the Great Rift Valley, stretching all the way north to Asia Minor, also known as Anatolia.

This narrow zone marks where two huge pieces of the Earth's crust, called tectonic plates, are moving apart. The African plate is dividing into two smaller plates, the Somali plate and the Nubian plate, at a slow rate of about 8–9 mm (less than half an inch) each year. The East African Rift includes several smaller moving pieces of crust, known as microplates, such as the Victoria microplate, which turns in a counter-clockwise direction.

Many of Africa's large lakes, including the famous African Great Lakes, sit inside this rift valley, making the area very important for both nature and people living there.

Extent

The East African Rift System stretches over many kilometers. North of the Afar triple junction, it splits into two paths: one goes west to the Red Sea Rift and the other goes east to the Aden Ridge in the Gulf of Aden.

From the Afar triple junction, the rift has two main branches. The Eastern Rift Valley, also called the Gregory Rift, runs through Main Ethiopian Rift and continues down into Tanzania. The Western Rift Valley includes the Albertine Rift, passing through Democratic Republic of the Congo, Uganda, Rwanda, Burundi, and further into Tanzania, Zambia, and along Lake Malawi to Mozambique.

The rift also extends offshore from Mozambique along certain underwater features, connected by the Davie Ridge, a long underwater ridge that lies between Tanzania and Mozambique.

Competing theories on geologic evolution

Scientists have many ideas about how the East African Rift formed. In 1972, some thought it was caused by differences in the thickness of Earth's outer layer, not by movements in the Earth's crust. Since the 1990s, evidence suggests that hot spots deep inside the Earth, called mantle plumes, may play a role. However, exactly where these plumes are and how they work is still being studied.

A popular idea from 2009 says that volcanic activity and the movement of Earth's plates work together. As the Earth's outer layer thins, it causes more volcanoes, which thin the layer even more. This process might eventually make eastern Africa split from the rest of the continent, but this would take many millions of years.

Researchers use different methods to study these ideas, such as studying chemical signs in rocks, using seismic waves to look inside the Earth, and creating computer models to test theories.

Geologic evolution

Before the East African Rift formed, huge continental flood basalts erupted, lifting up the Ethiopian, Somali, and East African plateaus. The rift began with magma forming along the whole area and the Earth's crust stretching and relaxing in turns. Old weaknesses in the ground also became active again, leading to big cracks and deep basins.

Today, the narrow parts of the rift show where the Earth's crust is stretching. These areas have many cracks typical of places where continents are pulling apart. Some parts have lots of magma and old lava flows, while others, like the Western branch, have only a little volcanic rock.

Petrology

The African continental crust is cool and strong. There are large, ancient landmasses called cratons in the East African Rift, such as the Tanzania and Kaapvaal cratons. These cratons are very thick and have stayed mostly unchanged for billions of years. They contain important minerals like gold, antimony, iron, chromium, and nickel.

During a time called the Oligocene, large amounts of volcanic rock, known as flood basalts, erupted. Most of this happened around the same time as the Red Sea and the Gulf of Aden began to form, about 30 million years ago. These volcanic rocks have many different types of compositions.

Volcanism and seismicity

The East African Rift Zone has many active and quiet volcanoes. Some well-known ones are Mount Kilimanjaro, Mount Kenya, Mount Longonot, Menengai Crater, Mount Karisimbi, Mount Nyiragongo, Mount Meru, and Mount Elgon, as well as the Crater Highlands in Tanzania. Even though many of these mountains are not inside the rift valley itself, they were created by it.

Some notable active volcanoes linked to the East African Rift include Erta Ale, Dalaffilla, Hayli Gubbi, and Ol Doinyo Lengai. Erta Ale, found in Ethiopia, has been active since at least 1967 and has a lava lake at its top. Ol Doinyo Lengai is special because it is the only active volcano on Earth that produces a unique kind of lava. The East African Rift is also very active with earthquakes, especially near the Afar Depression. These earthquakes are usually not very deep and can sometimes be strong.

Effect on climate

The East African Rift system changes the weather in East Africa and even around the world. Higher places, like the Ethiopian Highlands and the Kenya Highlands, get more rain than the dry lands around them. Lakes inside the rift, such as Lake Victoria, help create weather patterns called lake breeze systems that affect large areas. River valleys, like the Turkana Channel in northern Kenya and the Zambezi valley, push winds that make East Africa drier but help bring rain to the Congo Basin rainforest.

The rift also influences winds over the western Indian Ocean. These winds, known as the Somali Jet, carry water vapor that helps create heavy rains during the Indian Monsoon. They are important for moving air around the world as part of the Hadley Circulation.

Discoveries in human evolution

Main articles: Human evolution and Timeline of human evolution

The Rift Valley in East Africa has been very important for learning about how humans evolved. The land there erodes quickly, which helped preserve many old bones. Scientists have found the bones of early humans, including "Lucy," an ancient skeleton that is more than 3 million years old. Other important discoveries include two older types of apes found in Ethiopia, one of which lived 10 million years ago.