Forty million years ago upwelling in the mantle was splitting Africa apart. It lifted the Atlas Mountains and split open the Red sea. The crack continued down east Africa forming the Great Rift Valley. The stretching was at its greatest 3.5 million years ago when volcanoes erupted. In Kenya the volcanic material filled up the valley as fast as it was created. On the western branch of the rift, that did not happen and deep lakes fill the valley.