How was the great valley formed?

It is formed by an upwelling of hot mantle material under the valley (which has elevated the land) and then split it as the hot mantle moves sideways away form the center of upwelling. This results in volcanoes and a Graben fault system (a rift valley) above the upwelling. Essentially the Rift Valley is the start of a new ocean - the two sides of Africa are splitting apart.