The top layer of the Earth’s crust is covered by giant pieces called tectonic plates. There are seven large plates and several smaller ones, all moving very slowly at different speeds (2cm to 10cm a year), and in different directions. Some, those under the ocean, are oceanic plates. Those under continents are continental plates.
The place where two plates meet is called a boundary, and it is at these boundaries where new crust forms and old crust is returned to the core to be melted again. The cycle of crust forming and later melting down in the core takes about 100 million years.
At some boundaries, called Divergent Boundaries, the plates move away from each other, and the very top layer of the crust breaks, and the space that forms sinks down into the second layer of the crust and forms a valley called a rift. Liquid rock called magma oozes up from deep inside the Earth to fill cracks. The magma hardens and becomes a new part of the Earth’s crust.
Africa's Great Rift Valley
When the African and Arabian plates separated about 35 million years ago, a huge rift was created in Africa, and is today called the Great Rift Valley.
It is over 5000 km long, stretching from Syria to Mozambique. Its width varies from 30 to 100 kilometres, and its depth varies from several hundred to several thousand metres. In eastern Africa the Great Rift Valley separates into two branches, the eastern rift and the western rift.
The western rift is very deep, and some of the deepest lakes in the world have developed along it. The African great lakes, including Lake Tanganyika and Lake Victoria, the second largest lake in the world, were formed by the rift . The highest mountains in Africa are along the edge of the western rift.
The eastern rift runs through Kenya and Tanzania, and is deepest north of Kenya. The lakes of the eastern rift are shallow and contain lots of minerals because when the sun evaporates the water in the shallow lakes, salt is left behind and absorbed back into the lakes.
In Tanzania a place in the rift valley called Olduvai Gorge was excavated by Louis and Mary Leakey in the mid-1950s. The gorge area is 14 kms long and 106 metres deep and includes evidence of earliest human existence.
Olduvai Gorge, Tanzania
The excavation sites are mostly by streams and lakes, and show that early humans worked with stone, making shelters and places where meat was butchered and food was stored. The almost complete skeleton of a very early type of human species was found, nicknamed 'Lucy'.
The eastern and western rifts join up again at Lake Nyasa and continue south to Mozambique.
The original plate movement that created the Great Rift Valley weakened the Earth’s crust, and there is volcanic activity and earthquakes in the area. Volcanic mountains found along the rift include Mount Kilimanjaro, Mount Kenya and Mount Elgon, among others.
In Tanzania, an active volcano, Mount Oldoinyo Lenkai (or Oldonyo Lenkai), which means Mountain of God, is the only known volcano to erupt lava that contains very little silicon. The mountain erupts about every seven years. Nearby is Lake Natron, whose water is intensely salty, and sometimes red due to algae blooms. Tens of thousands of flamingos flock to this lake annually to nest.
In a few million years, the eastern part of Africa will probably split off from the continent and form a new landmass.
Click to see a map showing the Rift Valley: http://www.nhnz.tv/FactsAF
Read about Mount Oloinyo Lenkai here:
Read about Lake Natron here:
If you use any of this information in your own work acknowledge this source in your bibliography like this:
Sydenham, S. & Thomas, R. The Great Rift Valley [Online] www.kidcyber.com.au 
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updated May 2014