FOLDING AND FAULTING
Tectonic forces result in earth movements
These movements involve vertical or horizontal movements of the earth’s crust
These movements result in the land being raised or uplifted, depressed or lowered, folded/bent or
fractured or erupting
FOLDING
Folding is the bending of rock layers in the earth’s crust
Folds form when compressed rock fails to break and only bends.
Folding results from compressional forces acting on the earth’s crust
These forces act horizontally on a land mass and this results in the different types of folds
The nature of folds depend on the amount of force exerted on the rock
1. Anticline Fold
This is an arch-like shaped fold with oldest bedrocks at its core.
It is convex up in which the crest is the location where curvature is greatest
One slope is steeper than the other.
2. Symmetrical fold/ Simple fold
In a symmetrical fold, the limbs/slopes are of the same steepness
The limbs bend upwards.
3. Syncline Fold
This is the opposite of an anticline
This is a downfold because the limbs are curving downwards
The oldest bed rock layers are found near the core of the fold
Horizontal layers (strata) are bent upwards
4. Overfold
This forms when compressional forces continue to be exerted on the limbs.
This too much compression forces one limb over the other leading to an overfold/ overthrust or
recumbent structure.
NB: if faulting then occurs over a plane, a nappe is formed as shown below.
LANDFORMS RESULTING FROM FOLDING
1. Fold mountains
The formation of Fold Mountains is called orogenesis which means the beginning of mountain
formation. (oros in Greek means mountain, genesis means formation or beginning)
Fold mountains result due to the convergence of plates at plate margins
In Africa most of the well-known mountain chains such as Cape folded ranges of South Africa, the
Eastern Highlands (Chimanimani uplands) in Zimbabwe and the Atlas mountains are classical
examples of fold mountains
Outside Africa examples of fold mountains include the Himalayas, the Rockies of North America,
the Andes of South America and the Appalachians of North America
GROUPS OF FOLD MOUNTAINS
Fold mountains are classified into two major groups determined by their age as seen in their
altitude or elevation
A. OLD FOLD MOUNTAINS
These are mountains that have been downwarped by denudational processes such as erosion,
weathering and mass fall
These mountains are plateau-like shaped and as a result they are not very high
Fold mountains that were built 400 million years ago (in the Caledonian Orogeny) and 300 million
years ago (in the Hercynian Orogeny) belong to this group
Their altitude range between 2000 and 3000meters on average
Example include the Harz mountains of Germany
B. YOUNG FOLD MOUNTAINS
o These fold mountains were built during the last Orogeny called the Alpine Orogeny (35 million years
ago)
o They have great heights of over 6000 meters high
o Examples include Alps, Pyrennes, Rockies, Andes and Himalayas
o Young fold mountains are rugged and jagged ranges or ridges form
OTHER FEATURES RESULTING FROM FOLDING
2. Anticlinal mountains
This is the initial landform produced by folding
This mountain does not last for a long geological period since it is quickly downwarped by
denudation (weathering and erosion)
3. Synclinal valleys
This is the initial landform produced by folding
A Zimbabwean example is the famous Mberengwa Greenstone Belt.
4. Anticlinal valleys
These form when an anticline’s top is attacked by erosion. This quick erosional attack is because
the compressional forces are operating away from the fold axis.
When the anticline is eroded, an anticlinal valley forms.
5. Synclinal mountains
Due to the fact that compressional forces are operating towards the centre of the fold axis, the
syncline is more resistant to erosion
Therefore it remains standing as the surrounding anticlines are eroded.
FAULTING
This involves an upward or downward movement of a rock along a fault line
A joint differs from a fault in that a joint is simply a crack in a rock
There is no displacement of the rock along the crack/ fracture but fault results in displacement of
rock
Faulting results from tensional and compressional forces that causes stress in the rock
MOVEMENTS ASSOCIATED WITH FAULTING
Vertical displacement of a block (a very large rock ) is known as the throw
Lateral displacement is called the heave
The angle between the fault plane and the horizontal is called the hade
Tensional forces result in normal faults while compressional forces result in reverse faults or
thrust faults
Types of Faults
1. Tear Fault
This is formed where there is horizontal movements along a fault, resulting in a sideward or
lateral displacement
2. Step Faults
This is when faults are arranged in the form of steps resulting from tensional forces
Examples include parts of the Great African Rift Valley and Rhone Rift valley in Europe.
3. Oblique Fault
This fault forms when displacement is both vertical and horizontal
4. Reverse Faults/ Normal faults
These form as a result of tensional forces which results in downward displacement of the block
LANDFORMS RESULTING FROM FAULTING
A. FAULT SCARPS
A fault scarp is the steep slope created on the face of a block when displacement occurs along a
fault line
On diagrams it is indicated by a an ‘f’
A fault scarp is simply referred to as a scarp and sometimes it develops into an escarpment.
Example is the Zambezi escarpment in northern Zimbabwe which includes the Matusadonha and
Mamvuradonha scarps
B. A HORST OR BLOCK MOUNTAIN
Chizarira Mountain Range (Horsts)
Normal faulting may occur, raising a central block and lowering the two adjacent blocks
The raised block is called a horst or a block mountain
Examples include Ruwenzoni range of mountains in Uganda, Kenya highlands, the great Kharas
mountains of Namibia and the Chizarira range to the south of Lake Kariba in Zimbabwe
Horsts are characterized by flat tops and steep sides
C. A GRABEN
A graben is the downthrow segment usually between the horst features
It results in the formation of rift valleys such as the Great East African Rift Valley
Grabens can be formed by tensional forces or by compressional forces
FORMATION OF RIFT VALLEYS
Formation of rift valleys can be explained using the compression theory or tension theory
1. The compression theory
This theory explains that the middle block is downthrown resulting in a fault-bounded rift valley
2. The tension theory
Here the tensional forces are responsible for pulling apart blocks of rock and the middle block is
downthrown to result in a rift valley
This theory explains the divergence of plates. Therefore the rift valleys are associated with mid-
oceanic points which mid-oceanic ridges.
Down warping also lead to natural depressions such as Lake Victoria and Lake Cahorra Bassa.
VOLCANOES AND EARTHQUAKES
