Introduction

Geomorphology which is a branch of physical geography, is a science that emphasizes study of landforms that have resulted from various landscapes. Landforms, in reality, originate from various geomorphological processes which are operating both internally as well as externally over periods of time in the earth's crust. These processes in short are known as geomorphic processes. It is these processes which bring about reshaping or moulding and sculpturing of the earth's crust. Their result is the production of landforms we see both on the ground surface and underground.

One interesting thing about geomorphic processes is that they have the following characteristics:

• They are governed by climatic condition. When a geomorphic process is influenced by an intense climatic parameter it will tend to produce a pattern of unique landforms which are very much related to that process. As a matter of fact such landforms become dominant in those areas. This is why we today have landscapes like desert, glaciated coastal, river-valley and karst.

• They are affected by the parent rock. The rock structure is another factor that plays a big role as climate does. It tends to resist them. But due to the fact that rock structure is never uniform all through, it is often overcome in the portions where it is weakest. The removal of the weakest sections results into the formation of the definite landforms which are recognisable. These are the landforms we refer to as physical, physiographic or relief features. They are synonymous terms. The activity leading to these features is referred to as sculpturing, moulding or reshaping.

The two branches of physical geography are absolutely very important for the determination of "human landscape which gives way to yet another important branch of geography referred to as human geography.

They are also determined by time. The extent of a geomorphic operation is very important in that the longer it operates on a rock.

(i) the weaker the rock and

(ii) the more distinct the landform becomes.

Geomorphic processes are both internal and external leading to formation of different landforms in different parts of East Africa.

Internal processes:

These processes are sometimes known by either names: endogenetic or hypogene. As the name suggests, they are processes which operate from within the earth's crust. They are initiated by high convective currents which are brought by high heat and pressure otherwise called, heat plumes or 'hot spots that is subjected to core rocks. These high pressure levels and temperature are the products of the "ever moving plates that make up the earth's crust. This is because they are in the first instance mobile i.e change over space and time; secondly, they occasionally collide or sometimes by pass each other.

These activities stress the crust and as result produce effects which take the form of compression and tension. These effects are so huge that it is better to refer to them as forces, hence tensional and compressional forces. The net result of these forces is the fracturing of the earth' s crust so that either Magmatic materials are introduced to the ground surface or remain underground or the ground surface is uplifted while others are subsided. Through the two ways a number of geomorphic processes and their associated landforms are found. They include:

Faulting

It is the breaking and fracturing of the rocks in the earth's crust into large cracks called joints and faults. In fact it occurs when the rock is subjected to violent stress. In East Africa this process is clearly shown by the major faulted areas like the Great Rift Valley.

Folding

This is the bending and breaking repeatedly in which some parts of the earth's crust are raised over geological times into anticlines (up slopes) and synclines (down slopes). When a rock is subjected to folding it sustains the stress. The process of folding takes place imperceptibly in that it is very difficult to discern it practically.

To understand its effect, needs close study on the resulting landforms. Yet to the surprise its huge compressive forces from within the crust have raised deposited (Sedimentary) rocks to high elevations; above the sea level and today they stand as new lands. For instance in East Africa, the coastal plain which largely consist of layers of coral rocks today is lying between 15 and 445 metres above the sea level.

In Tanzania, the Bukoban sandstone hills of the west lake province also show quite good examples of this process.

There are also many of the world's greatest mountain ranges that have been formed through up-folding of piles of buried sedimentary rocks. They include: The Cape Ranges of South Africa, the Atlas of N. Africa, the Andes of South America, the Rockies of N. America, Alpo-Himalayan mountain chains of Eurasia.

Warping

It involves both up and down-warping of the earth's crust resulting to the formation of plateaux, highlands and depressional surfaces. Warping is one special form of folding. Like folding, warping also produces two forms of surfaces: the down warped and the up warped. The former in particular has resulted into the formation of;

(a) Lake basins which tend to contain accumulations of sediments eroded from neighbouring plateaus and as such are often filled with deposits of sand stones, limestones, clays and shales.

(b) The river systems and plains which have also been filled with sediments. In East Africa, land drainage system like L. Victoria and Kyoga, Kano Tune plains of Kenya, and Wembere plains of Tanzania are today filled with sediments. L. Amboseli basin and the upper Pangani river valley were also affected by down-warping and sediments infilling which have flat plains drained by tributaries of Wami and the Kilombero rivers. The latter, on the other hand has resulted primarily to the formation of:

Plateaux surfaces like the rimmed plateau that is surrounding the lake Victoria Basin.

The arms of the Great Rift Valley of East Africa.

Generalized cross-section of East, Africa, illustrating the effect of compression and Tension.

Vulcanism

This is a process in which the magmatic materials are injected either into the earth' crust or onto the surface. The Resulting landforms are called intrusives and extrusives with their order of formation intrusion and extrusion. In East Africa there are several areas which exhibit both extrusive and intrusive features. The former appears to be the most pronounced.

Earth-Quake

It is a sudden intense movement of the earth crust as a result of the passage of seismic waves that have been released paroxysmal inside the earth.

Earthquakes primarily result from the movements of the plates underneath ground. The resulting seismic waves or tremors are mainly of two types; Namely: Foreshocks which are extremely large and are the first to-be released. Aftershocks. These are the ending waves as the earthquake fades away. These are smaller and one may find them interesting to experience as they pass by.