Rifts

Squeeze the crust together and blocks move upwards either in folds or, through brittle fracture, faults. Stretch the crust and the result is rifting. In its simplest form a single block moves down wards leaving steep ridge on either side. More often the process happens many times, so in effect a flight of steps is produced on either side. This is often accompanied by uplift since it is not pulling from the sides but the pushing of upwelling mantle rock underneath that does the stretching. So the process is often accompanied by volcanoes. The same process operates at mid ocean ridges: beneath continents it is as if a new ocean is trying to open. Recent examples of rifting processes at work include the valley of the River Rhine and Africa’s Great Rift Valley.

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.