In this question a student should try to: explain what is meant by the theory of plate tectonics and what causes movement of plates. The mechanism of movement should also be described. show how plate-tectonics has led to the formation of landforms in East Africa. Show that plate-tectonics alone can not entirely explain all the landforms in East Africa and therefore bring out other processes responsible for landform evolution in East Africa

 

The idea of plate tectonics is a very recent one which caused a revolution in geological sciences in the 1960s. Plate tectonics is a process through which movements of the Earth crust are effected so that the surface is divided along lines of weakness pre-determined by the internal structure of the Earth. This division has produced a number of separate cmstal blocks known as tectonic plates. These plates are relatively light and therefore "float" on the denser rocks of the mantle and asthenosphere. The plates are also mobile and move extremely slowly on rocks of the upper mantle, driven by convective currents. A given plate is so rigid that if one part moves, then the rest also moves. A plate therefore moves as a unit

Tectonic plates move because of energy generated by radioactivity and geo- chemical reactions in the interior of the Earth. The resultant intense heat causes partial melting of rocks which then rise up in form of convection current and drive the plates,

There are three major plate boundaries, namely, divergent, convergent and transform. A divergent plate boundary is said to exist when plates are moving slowly away from each other. New rocks come to the surface and push old ones away. This phenomenon is therefore closely linked with sea floor-spreading On ocean floors, this produces an under-water mountain known as the mid-ocean ridge. If plates move apart beneath a continent, continental land masses undergo faulting and this causes formation of several landforms such as grabens and rift valleys. Divergence tends to lead to expansion of the crust.

A convergent boundary is said to exist when plates move and meet, that is. converge. When this happens one of the plates is forced to descend into the Earth's interior where it becomes slowly heated and eventually assimilated into the surrounding material. It is the dense oceanic lithosphere plate that descends beneath the continental plate. Continental rocks are rather light and so do not sink. Instead, they are compressed into fold mountains when the plates meet Convergence tends to cause contraction of the crust.

The transform boundary exists where two tectonic plates are sliding past each other. The plates by pass each other without creating new or destroying the old lithosphere but sometimes causing earthquakes as the two surfaces rub against each other. The San Andreas fault in California is one such boundary.

Plate tectonics causes movements in the crust and thus formation of landforms It causes the formation of rift valleys, block mountains, down warped basins, volcanoes and several other related landforms. In other words, all these features can be explained in terms of plate tectonics, Where plates diverge or move. Away from each other beneath a continent, they tend to pull the Earth's surface apart resulting in tensional forces. If tension is great, the crust fractures and this can lead to the formation of a rift valley. This is how the East African rifl: valley especially the Gregory branch was formed.

Plate tectonics also causes formation of down warped basins. A down warped basin can be an indirect effect of faulting. The down warped basins occupied by lakes Kyoga and Victoria are believed to be connected to the formation of the East African rift valley. In the process of formation of the rift valley in Western Uganda, the sides (shoulders) of the valley were uplifted. At the same time, a rift valley also formed in central Kenya and its shoulders similarly rose. The plateau between these two rift valleys in central Uganda was therefore gently deformed downwards. The land sagged on a great scale and developed into big depressions that are currently occupied by lakes Victoria and Kyoga.

Another alternative explanation but which is also related to plate tectonics is the effect of converging convective currents below the Earth's crust. It has been suggested that when convective currents sink back into the interior below the crust, they pull the Earth's surface towards the Earth's centre leading to down

Currents sink back into the interior and pull Earth's crust downwards Plate tectonics also causes vertical movement of the cmst. As convective currents rise towards the surface, they exert an upward push on the crust leading to uplift This has led to the formation of block mountains in East Africa. Plate tectonics

definitely agrees with formation of block mountains according to the theory of differential uplift. The block mountains in East Africa include Rwenzori, Kamasiya, Ndoto, Pare, Usambara, Uluguru and Mahenge.

Plate tectonics is also responsible for the formation of volcanic landforms in East Africa. Radioactivity and geo-chemical reactions produce enormous heat which melts mantle and asthenosphere rocks. The molten rocks rise towards the surface of the Earth where there is less pressure. Vulcanicity normally takes advantage of lines of weakness and therefore magma rises easily if there are weak points created by faulting. This is why most volcanic landforms are within or near the rift valley. It has been suggested that even mountains Elgon and Moroto which are away from the main faulted regions are located on ancient fault lines.

Plate tectonics is therefore greatly responsible for the formation of many landforms in East Africa. It would be wrong, however, to assert that the present landforms in East Africa are entirely a product of plate tectonics- It would certainly he wronp; to suggest that plate tectonics alone and exclusively wholly explains all the present landforms in East Africa. For example, while plate tectonics originally produced the Rwenzori block mountain during the Pleistocene period, it is not responsible for the Bujuku and Mubuku valleys that evolved later. If the sub-aerial processes of glaciation had not taken place the mountain would not be grooved with deep valleys. It would certainly not be having the characteristic glacial landforms of aretes, corries and pyramidal peaks.

Similarly, while plate tectonics originally created intrusive igneous features below the crust, it is not responsible for their subsequent exposure at the surface of the Earth. If the sub-aerial exogenic processes of weathering and differential erosion had not taken place, these igneous intrusions would have remained buried underground. In that case the impressive inselbergs in the Northern and North Eastern parts of Uganda would not have formed. Therefore in an attempt to explain the present landforms in East Africa, one needs to examine plate tectonics and other processes.

External or exogenic processes of weathering, erosion and deposition have all played a big role in the evolution of today's landforms in East Africa. Mechanical weathering in semi-arid areas has led to the formation of exfoliation domes Evidence of exfoliation can be found on Ngetta, Akia and Mubende hills in Uganda.

The process of erosion has also created many landforms in East Africa. The major agents of erosion are running water, waves, glaciers and wind of which the first three are very important in East Africa. Glacial erosion in this region has produced aretes, pyramidal peaks, glacial troughs and hanging valleys. River erosion has produced such landforms as river cliffs, gorges and rejuvenation terraces.

Lacustrine and marine (wave) erosion has produced wave-cut platforms, cliffs, caves, arches and stacks. These are evident at the coast of East Africa and, to a lesser extent, the northern shores of lake Victoria.

Deposition is another process that explains the present landforms in East Africa. Glacial deposition on the gentle slopes or foothills of high mountains has led to the development of terminal moraines. This is evident on the Kilimanjaro. Kenya and Rwenzori mountains. River deposition has led to the development of deltas, levees and flood plains in the lower courses of river Tana and Semuliki.

Finally wave deposition has led to the formation of such depositional landforms as spits, mud flats, tombolos, sand bars and beaches. Big beaches abound on the East African coast while small ones also exist on the northern shores of lake Victoria at Lido and Kasenyi.

Thus in conclusion, plate tectonics is indeed a fundamental process responsible for the formation of landforms in East Africa- It is responsible for the formation of a "background structure". Other external processes, however, have operated on this initial structure and modified it to give rise to the present landforms. This is why it is right and fitting to maintain that plate tectonics is not responsible for the formation of the present landforms to a "full extent". These landforms are a product of interaction of processes of which plate-tectonics is only a part.