Specific objectives
By the end of the topic, the learner should be able to:
- Define earth movements.
- Explain the causes of horizontal and vertical earth movements.
- Explain the theories of continental drift and plate tectonies.
- Define folding, faulting, vulcanicity and earthquakes.
- Explain the processes of folding, faulting and vulcanicity.
- Describe types of folds, faults and forms of vulcanicity.
- Explain the resultant features due to folding, faulting and vulcanicity.
- Explain the causes of earthquakes the causes of earthquakes.
- Describe how earthquakes are measured.
- Account for the world’s distribution of earthquake zones, fold mountain systems and features due to faulting and vulcanicity.
- Explain the significance of the resultant features of folding, faulting, vulcanicity and the effects of earthquakes.
Land forming processes
Physical features include valleys, hills, plateaus, basins and escarpments. These features were formed by natural internal and external land forming processes. The internal land forming processes occur within the earth. The external land forming processes occur on the surface of the earth.
The internal land forming process is also known as endogenetic or endogenic process. The external land forming process is also known as exogenic process. Both internal and external land forming processes are caused by tectonic forces. These are the forces that originate and operate in the interior of the earth. Tectonic forces are responsible for earth movement and formation of land forms.
Earth movements
The earth spins on its own axis (rotation) and it also moves around the sun on its orbit (revolution). These are referred to as movements of the earth. There are also movements that take place within the earth, these are the earth movements. The earth movements are changes in location of crustal rocks caused by tectonic forces. These tectonic forces continually affect the crustal rocks leading to slow earth movements. However, sometimes these movements can be fast and very sudden.
Tensional forces move away from each other. When the crustal rocks are subjected to tensional forces, they may pull apart or stretch leading to the formation of a fracture in the crustal rock.
Horizontal Earth Movements
When horizontal forces move parallel to each other in opposite directions or when two forces of unequal strength move in the same direction, they cause the crustal rocks to shear.
Vertical Earth Movements
The vertical Earth movements result from forces that act upwards or downwards along the earth’s radius. These forces either from the earth’s interior to the surface or from the surface towards the earth’s centre.
The upward movement of the crustal rocks results in the upwarping of the rocks. The downward movement results in the subsidence or downwarping of the crustal rocks. These forces can also result in tilting of the crustal rocks if forces are of unequal magnitude. (See Fig. 1.3 (a), (b), (c) and (d).
The horizontal and vertical earth movements may act on the crustal rock at the same time. The crustal rock move resulting to processes such as;
- Faulting
- Folding
- Upwarping or upthrusting
- Down warping or down thrusting.
These processes lead to the formation of many features on the earth’s surface. Some of the features formed are; fault blocks, rift valleys, basins, fault scarps and tilt blocks. These features are discussed in detail under folding and faulting.
The formation of features as a result of earth movements is determined by:
- The type of movement involved, either vertical or horizontal.
- The strength of forces involved, either weak or strong.
- Nature and age of crustal rocks, either young and elastic or old and rigid.
Causes of Earth Movements
The crustal rock moves due to the following reasons:
- Magma movement within the earth’s crust.
- The gravitative pressure of the earth.
- The convectional currents in the mantle.
- Isostatic adjustment.
Magma movement within the earth’s crust
The rocks in the mantle are under intense pressure and experience high temperatures. Therefore, these rocks occur in molten state known as magma. The molten rocks are full of gases and therefore highly explosive.
Magma may forcefully move from the mantle through cracks or lines of weakness in the crustal rocks. The crustal rocks are pushed away by magma in either horizontal or vertical direction. This movement creates space which is filled in by the intruding magma. This process causes horizontal or vertical earth movements.
Gravitative pressure of the earth.
The gravitational force pulls all objects towards the centre of the earth. Therefore, the gravitative pressure of the earth pulls the rocks towards the centre of the earth. After volcanic eruptions voids are left in the upper mantle and the lower sima. The crustal rocks above the voids collapse into the voids due to gravitational pull. This causes earth movement.
Convectional currents in the mantle.
Convection is the process of heat transfer in fluids. Convectional currents occur in fluids. Magma is fluid state of the mantle, thus exhibit convectional currents. Convectional currents develop in the magma contained in the mantle and move in a circular motion towards the earth’s crustal rocks. These convectional currents cause a frictional drag with the overlying sima rock. The frictional drag causes the crustal rocks to move horizontally. The eroded sediments are transported by water and deposited on the ocean floor. This adds extra weights of the oceanic floor causing isostatic sinking of ocean floor. The eroded continent experience isostatic uplift due to the lose of sediments by erosion. Isostatic adjustment may lead to earth movements and formation of landforms.
Isostatic adjustment
Isostacy is the state of equilibrium balance that exists between the earth’s continental crust and the oceanic crust.
Isostatic adjustment is the constant adjustment between continental crust and oceanic crust. This balance must be maintained at all times. It occurs when disturbances are caused by other processes such as erosion on the continents.
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