Faulting refers to the processes by which the landscape is affected in any of the following ways:

a) Vertical displacement of rocks. (Also known as uplifting)-

b) Down warping or depression of the land.

c) Horizontal displacement of rocks.

Causes of Faulting

Faulting and associated landscapes are caused by forces whose origins are related to thermal convections within the earth's mantle. Where these convections cause lateral forces or movements away from the point of rupture of a continental block, they are equated to tensional forces that generally bring about the occurrence of faulting. On the other hand, thermal convections may cause the upper mantle and oceanic rocks to converge towards each other. This convergence is equated to compressional forces and are also one of the forces that bring about faulting. In East Africa faulting began in the Cainozoic era about 40 million years ago continued to the Quaternary Era (Pleistocene Period). The diagrams below depict the formation of rift valleys and associated landscapes.

Tensional Theory

In the above diagram tensional forces include two parallel faults or lines of weakness where relative displacement of rocks has occurred. Those are normal faults. As the tensional forces continue to pull apart, the two masses on either side of the central block sinks downwards as indicated by the arrows. In this type of faulting, the main bounding faults are normal faults.

Tensional Forces and Formation of Normal Faults

An elongated trough or depression (Rift valley) is formed and is surrounded tby in-facing steep cliffs known as escarpments. The two blocks which stand above the rift valley are block mountains or horsts.

In the above diagram, as congressional forces push the two masses on either side of the central block towards each other, these two blocks are thrust up and over­ride the central section holding it down as a trough.

(HI) Tear Faulting: Sometimes as plates move past each other instead of towards or away from each other, fracturing of the land occurs horizontally along a fault line.

DIRECT EFFECTS OF FAULTING ON THE LANDSCAPE

As indicated earlier, faults are of various types. Normal faults are by far the most common. They result from tensional stresses. By contrast, reverse faults tesult from congressional stresses often connected with converging of plates. Faults are reflected in the landscape mainly in the form of escarpments or scarps, two types of which can be distinguished,

a) Fault escarpments: This type may also be called a fault-line scarp. This is a cliff which is sub-parallel to a fault trace. The original feature has been modified by erosion so that it does not reflect the original fault plane in any way. The East African Rift Valley system provides the best examples of fault line scarps in Africa and even in the whole world, e.g. Manyara escarpment in Tanzania and Butiaba in Uganda.

b) Fault Scarps: By contrast in fault scarps, the evidence of faulting is still fresh. Fault scarps are therefore only seen in areas of recent earth movements, particularly in arid or semi-arid regions. In East Africa there are numerous examples of fault scarps in the bounding walls of the troughs such as the Elgayo scarp in Kenya and the Chunya scarp in Southern Tanzania.

c) Step Faulting: This is where new faults are caused on parts of the rift floor and induce parts of the rift valley floor to drop further down e.g. The 1964 earthquake of Toro caused part of the rift valley floor to drop by upto 2.5 metres, fe Kenya an example of step faulting ca»%e found on the edge of the Gregory (Kenya) rift valley near Kijabe.

Step Faulting and rift valley formation

2. Block Mountains or Horsts: This by definition is a fault block that is, in areas of normal faulting. It is generally elongated and has been raised relative to the blocks on either side of it and is usually not associated with major tilting or folding.

In order to produce a horst, all the blocks may have moved but ifee middle block rises more than the outer two blocks as indicated in the diagram below.

Ruwenzori Horst

Examples of block mountains in East Africa include the Ruwenzori Range lying at the junction of two branches of the Western Rift valley between lake George and Edward. Ruwenzori rises over 5000 metres and is the highest non volcanic mountain in Africa.

The uplift of this giant structure is about 100 km long and up to 45 km wide. Ruwenzori is composed largely of ancient schists and gneisses surrounding a core of granites, diorites and other igneous rocks. It is the igneous core that forms the six chief mountains of Ngaliema (Stanley), Baker, Speke, Gessi and Luigi di Savoia. They are separated from each other by deep glaciated valleys and high passes. Mount Ngalioma (Stanley) is the highest with Magherita as the highest point (5111 metres).

Smaller horsts exist in Tanzania such as the Usambara mountains, 1500 metres above sea level, the Ulugurs, the Mbeya Range and the Iramba Plateau.

The diagram below illustrates a cross section of the Ruwenzori mountains.

Rift Vallevs or Grabens

These are the opposite of horsts in that they are depressions or down thrust areas with faults on either side. The East African Rift Valleys are one «f the most comprehensively studied. They are compos^ of fault escarpment of up to 6(X) metres high in some places. The rift is also marked by a system of lakes most of which are narrow and elongated. Some of these lakes are the deepest in world e.g. L. Tanganyika while others such as Lake Albert is quite shallow (only about 56 metres at its deepest point) and others such as George, Elementeita, Nakuru and Naivasha are no more than 3-16 metres deep. A number of volcanic peaks not only adjoin the rift valley system but also mark the floor of the rift valley as indicated in the map below.

It is important to note that the formation of the East African rift valleys can be explained in terms of Plate Tectonics and Sea floor Spreading (continental Drift) i.e. The East African Rift Valley was caused by the tearing apart of the East African Plateau a process which is still in progress and is'likely to cause the areas to the east of the rift valleys to break and form islands millions of years to come. The map below illustrates the geographical extent of the East African Rift Valley system.

There is evidence to support this theory. Step faulting can be seen on the edge of the rift valley north of Nairobi. Geological drillings to a depth of 1300 metres in Lake Albert area show that normal faults are being stretched. This causes the rift valley floors to sink into the molten rock below and hence forces some of the molten rock up to the surface along the fault lines. This explains why active volcanic activity is constantly taking place in the Gregory rift (eastern rift valley).

Near Mbeya Range are two good examples of down- thrust features of faulted landscape. To the South West, there is lake Rukwa rift, a typical graben, and in the south east a similar down faulted landscape around the area of Usangu.

INDIRECT EFFECTS OF FAULTING

1. Volcanic Activity: This is nearly always associated with major fault zones though not all fault zones give rise to volcanic land forms. On the rift valley floor are found a number of volcanoes, e.g. Menengai, Longqnot, Suswa, Ol doinyo Lengai etc. This has already been explained under formation of rift valleys.

Formation of Waterfalls: When this takes place across a river valley it causes a water-fall, since the long profile of rivers is usually concave as shown in the diagram below, uplifting may disrupt the concave profile so that the river's waters have to fall through its local base level or the mouth, the diagrams below illustrate changes in a rivers profile.

(i) Before vertical faulting across a river

(ii) After vertical Uplifting across a river valley

Examples of waterfalls formed through uplifting across a river valley include: Karuma and Murchison falls along the Nile river in Uganda and Nyahururu falls in Kenya, where the Ngare Naru river drops for almost 75 metres into a gorge.

In the case of Murchision falls, the formation of the rift valley with escarpments forming along fault lines resulted in the Nile river flowing over the central plateau of Uganda and drop down into the rift valley bottom. Gradually the falls have retreated upstream away from the main lines of faulting. In the Western rift valley area the Nkusi and Wambabeya rivers kept their westward flow when faulting occurred. They both have water falls in their valleys as they drop down the eastern fault escarpment of lake Albert rift valley.

3. Fault Guided Valleys: Rivers may follow natural fault valleys where differential movements have raised some parts while lowering others e.g. the Aswa river in northern Uganda which follows a north-west trending tear fault.

Tear faulting refers to horizontal displacement of rocks and if this takes place across a river valley, it causes the river to be offset at the point where the river crosses the fault. The diagrams below illustrate tear faulting across a river valley.

Other examples of fault guided valleys are found in Kenya such as the Kerio river which flows between Elgayo Fault and Tugen Fault in north eastern Kenya, the Ewaso-Ngiro river along the Nkruman faults in south central Kenya and draining into Lake Natron and the Melawa river that drains into Lake Naivasha in Central Kenya.

In Tanzania fault guided valleys include Mzimu river which How from north east , to Southwest, the Nglumi river which flows from Southwest to north east. These two rivers have clearly aligned themselves to the line of faults and have emphasized this fault weakness to form a deep gorge. V

4. Rift Valley Lakes: Where differential movements that are associated with faulting occur on the rift valley floor, then down faulted basins are created into which rivers may drain e.g. lakes Tanganyika, Turkana, Albert, Rukwa etc. are some examples of rift valley lakes.

5. Reversal of Drainage: As with every other feature on the landscape around us, the drainage patterns we see today are gradually changing all the time. This is well illustrated in East Africa where considerable changes have taken place in drainage patterns as the rift valley systems were being formed during the plio-Pleistocene period.

Before the formation of the rift valleys, the rivers flowed westwards across present Uganda into Zaire on the one hand, and on the other, they flowed eastwards to the Indian Ocean from a highland area then known as Mozambique ridge in Kenya. None of the lakes we find in East Africa today existed that time. The map below illustrates the drainage pattern at that time.

Before Reversal

The gradual formation of the western and eastern Rift valleys with highland areas oh either side brought about changes in the drainage patterns. The rise of the highlands on the eastern side of the western rift valley blocked the paths of Kafu, Katonga and Kagera rivers so that their flow was reversed. Instead of flowing to the west they were turned round and began flowing eastwards. The map below shows reversal of drainage in East Africa.

At about the same time when the East African rift valleys were being formed, two large shallow basins were formed between the western and eastern rift valleys. The reversed rivers flowed from the west and the rivers flowing from Kenya gradually began to flood these basins to form lakes Victoria and Kyoga.

These lakes grew in size until the level of water in them reached the lowest part of the surrounding water sheds. Lake Victoria over flowed as the Nile into Lake Kyoga through Ripon falls. In turn, lake Kyoga over flowed through the Nile river. The Nile found its way around the northern end of the highlands bordering the western rift valley, thus forming the Murchision Falls into the rift valley. The map below shows reversal of drainage in Uganda alone.

After Reversal

6. Rejuvenated River Valleys: The term rejuvenation refers to the revival or renewal of erosive activity of a river so that it superimposes new features on a landscape already modified by previous river action. These land forms will be better understood when a fuller discussion of river action and landscapes is undertaken in the Chapter ahead. However for the purpose of illustrations let us assume that a river has attained a graded profile, that is, it has reached the old stage where its long profile has a uniform gradient where erosion equals deposition. (As propounded by W. M. Davis).

If uplifting occurs along any section of this river valley, then a new cycle of erosion

will be initiated. The result will be the formation of rejuvenated landscapes. These include:

i) Incised meanders found north of Lushoto in Tanzania where numerous streams have been deeply incised e.g. on the Sine river. Furthermore the Songwe river is another example of incised meanders in Tanzania, others in Kenya include the Mwachi river.

ii) The production of river terraces such as on the Nyando and Ngaila rivers in Kenya. The same applies to Kagera river in Uganda and Rwanda.

iii) The development of parallel consequent drainage Pattern (or the trellis Pattern) depicted in River Mlovo south- west of the Mbeya Range.

7. Antecedent drainage System: The antecedent gorge of the Great Ruaha river in Tanzania is a, result of slow uplifting as the river was able to cut fast along its bed thus producing a. gorge. Furthermore the Malagarasi river in Tanzania kept eroding down as the highlands bordering the western rift rose. The rate of erosion was sufficient to allow the river to continue its westward course and it is therefore an example of another antecedent river. The level of the water in lake Tanganyika also rose for it to continue to flow through river Lukuga into Zaire basin. The Lukuga is therefore an antecedent river.

Economic Importance of Faulting to Man ;

Faulting has created one of the most impressive sceneries in the world e.g. horsts, escarpments, rift valleys and other related landscapes. These32 landscapes are a potential attraction to tourists. These help to generate foreign exchange to the nation and at the same time provide employment to those involved in the tourist industry.

Faulting has created rapids and waterfalls which are a potential source of hydro electric power e.g. this could easily be harnessed at Murchision falls. In the light of the fact that to-date there is energy crisis, this could easily be developed to alleviate the country's energy requirements and at the same time surpluses could be exported to neighbouring countries to generate foreign exchange.

Ruwenzori Ranges were formed through vertical displacement of rocks. In the process of its formation, copper bearing rocks were also brought near the surface e.g. at Kilembe. This in the past has been a major source of revenue to the government and source of employment when fully rehabilitated it is hoped that it will continue to be beneficial to the nation.

Faulting has been responsible for the reversal of the drainage in Uganda. This led to the formation of lakes such as Kyoga. Furthermore faulting has also created rift valley lakes in East Africa. These lakes are beneficial in that ' they serve in the following ways.

The high relief features formed due to faulting such as the Ruwenzori mountains and Usambara Ranges have improved rainfall reliability and totals. Furthermore, the higher percentage of relative humidity in these areas means higher cloud cover, low evapo-transpiration rates and above all a general coolness of these areas. Such areas are usually very productive agriculturally.

Faulting poses problems of transport and communications both in terms of construction of communication lines and in terms of fuel consumption. Roads have to meander along the slopes e.g. in Bundibugyo.

Faulting has also hampered the navigability of rivers at certain sections e.g. the Nile at Murchision falls cannot be crossed by boats etc.

Faulting can cause distraction to life and property.

Faulting is responsible for the creation of rain shadow areas especially along the Kasese-Mbarara corridor. This is the reason why at Mubuku, irrigation is practised. Also rift valley areas of Narok in Kenya experience very dry climatic conditions,

The permanent snow fields on Ruwenzori mountains provide water to rivers such as Nyamuwamba and Mubuku which may be used for irrigation, for domestic use, industrial use etc.

Generally similium fly (Mbwa) which causes river blindness breeds in fast flowing water and this is associated with where rapids and waterfalls exist. S«r such areas are potential sites where these vectors could easily live and breed.

The slope of high mountains such as the Rwenzori, Usambara mountains, the Uluguru mountains etc. generally encourage growth of alpine forests that can be exploited for timber, fuel, modifiers of climate etc.

EXERCISE

1. With the aid well illustrated examples explain the influence of faulted landscape on the people of East Africa.
2. What are some of the evidences that points to the tectonic movements of the Pleistocene period have influenced drainage patterns in East Africa.
3. Explain the relationships that exist between faulting, earthquarkes and volcanic eruptions in any given area these phenomena are frequent..