Account for the formation of the East African rift valley. 

Candidates are expected to define a Rift Valley                                                            .

As an elongated trough or depression bordered or surrounded by in- facing fault scarps along more or less parallel faults.

Areas covered by the East African Rift Valley should be identified by either description or with the help of a sketch map. i.e. Eastern and Western arms/ section in Kenya, Uganda and Tanzania

The origin of the Rift valley formation is still more of debate i.e. it involves many theories related to radio-active and gee-chemical reactions within the interior of the earth crust hence convective currents producing tensional, compressional, horizontal and vertical differential forces causing faulting i.e. fracturing and displacement of the earth crust along fault lines:

The major theories include

Tensional theory by J.W Gregory

• That radio-active and convective currents produced tensional forces within the earth crust.
• Tensional forces acted / pulled apart/ in opposite directions.
• Normal faults were produced, displacing rock strata.
• Side blocks were separated from the middle block, which was later lowered/ sunk under its own weight, forming a rift valley with gentle slopes.
• Erosion and mass wasting modified the slopes.
• The theory is more applicable to the Eastern Kenya Rift valley (Gregory Rift).

Compression force theory by E.J Wayland

• That strain developed in the East Africa crust as compressional forces pushed/moved in the same directions (convergent).
• Reversed faults were produced.
• The side blocks were forced to over-ride (up thrust), hanging above the central block. The central block thus formed rift valley with steep/ sharp edges.
• The sharp edges were later modified by erosion and mass wasting.
• The theory is more relevant to Western arm of the East African Rift Valley, especially the Albertine Rift valley.

Differential uplift theory

• Dealt with the Nairobi part of the East African Rift valley of Kenya that is step faulted.
• That there was a period of general uplift of part of the East African Crust.
• It led to the formation of several parallel fault-lines.
• Blocks on either side of the central block rose faster as the middle lagged behind in stages.
• At each stage, a mass or of block formed a terrace·
• Examples include Kendang scar near Nairobi appearing as several terraces rising from the Rift Valley floor.
• The gap in the middle of terraces formed the rift valley.
• Diagrammatic illustration of differential uplift

Relative Sinking (subsidence theory)

• Just like in differential uplift theory, there was extensive faulting in East Africa which created multiple parallel faults.
• Within the mantle / interior of the earth, there is intense heat originating from radioactivity and geochemical reactions.
• The heat melts down the interior rocks and they begin to move in form of convective currents upwards towards the crustal plates.
• When they become colder and therefore heavy, they sink or flow back down in the mantle and as they do so they exert a drag force which pull the crustal blocks downwards thus sagging of crustal blocks along each fault.
• The central blocks sagged more than those or, the extreme ends to form a step or terraced rift valley