Internal Land Forming Processes | High School Geography

Displacement of the earth’s crustal rocks

They are brought about by tectonic forces which originate and operate in the interior of the earth e.g. tensional forces (which operate along horizontal plane moving away from each other), compressional forces (which operate along horizontal plane moving towards each other), shear forces (which move past each other with unequal strength) and gravitational forces (which attracts things to the earths centre).
There are two types of earth movements

1. Horizontal/lateral/orogenic movements
2. Vertical/epeirogenic movements

Horizontal Earth Movements

Movements which act along a horizontal plane within crustal rocks.
They are caused by tensional and compressional and shear forces.
Effects of these movements include

• Strain and stretching of crustal rocks due to stretching caused by tensional forces which cause formation of cracks or faults.
• Squeezing and shortening of crustal by compressional forces rocks which cause them which also cause formation of faults.
• Crustal rocks to shear by slipping past each other or by dividing into layers which is caused by shear forces.

Horizontal Earth Movements result in the formation of the following features:

1. Faults
2. Rift valleys
3. Fold mountains
4. Escarpments
5. Basins
6. Tilt blocks
7. Block mountains

Vertical Earth Movements

Movements which occur along the earth’s radius or towards the earth’s surface or towards its centre.
Effects of these movements include

• Subsiding/sinking/downwarping or pulling of crustal rocks downwards.
• Uplifting/upwarping or pushing of crustal rocks upwards
• Tilting of crustal rocks or shearing in vertical direction due to greater uplift on one side.

Vertical Earth Movements result in the formation of the following features:

1. Raised cliffs
2. Tilt blocks
3. Rift valleys
4. Fault scarps/escarpments
5. Plateaus
6. Basins

The following are the main causes of earth movements
a) Magma movement within the earths crust.
b) Gravitational force
c) Convectional currents in the mantle
d) Isostatic adjustment

(a) Magma Movement within the Earths Crust

When magma moves with force pushing crustal rocks horizontally or vertically.
When magma moves from reservoir and leaves empty spaces onto which crustal rocks are pulled inwards.

(b) Gravitational Force

When the attractive force of the earth pulls crustal rocks into empty spaces left after magma escaping from the reservoir.

(c) Convectional Currents within Mantle

When convectional currents in magma in mantle drug crustal rocks by friction.
Horizontal movement of currents cause horizontal movements while vertical cause vertical movements.

(d) Isostatic Adjustment

Rising of continental masses to restore the upset state of balance between sial and sima layers.
Isostacy is the state of balance between sial and sima layers. It can be disturbed by erosion on continents and melting of continental ice sheets.
The reduced weight causes continental masses to rise.

A theory is reasoned ideas intended to explain facts or ideas.
There are 2 theories which explain the earth’s movements namely the Continental Drift Theory and the Plate tectonics theory.

- Its proponent was A. Wegener.
- It explains the origin of 6 continents.
- It explains as follows:

• The earth was a single sialic land mass called Pangaea surrounded by a huge ocean called Panthalasa whose floor was a mass of sima.
• Pangaea broke into two parts called Laurasia (N. Hemisphere) which lay around equator and Gondwanaland (S. Hemisphere) which lay around south pole which were separated by a narrow ocean called Tethys (the present Mediterranean Sea).
• Laurasia broke into Laurentian Shield and Fennoscandia (Europe, Asia and N. America) and moved northwards to their present positions.
• Gondwanaland broke into Africa, Australia, S. America and Antarctica and India subcontinent.
• Africa and India drifted northwards.

Evidences Supporting the Theory.
1. Fitting of western coast of Africa and S. America into a jigsaw.
2. Discovery of coal 40◦N and 55◦N which was formed by burying of tropical vegetation.
3. Considerable displacement of rocks along some faults e.g. along the Great Glen Fault of Scotland.
4. Cape and Buenos Aires folds resemble one another by having east west trend.
5. Red sea shores show evidence of having undergone lateral displacement an indication that it was formed by movement of the earth’s crust.
6. Evidence of ancient Glaciation to the south of equator in Africa in Madagascar and India where there is presence of ancient glacial deposits suggesting these areas were once around south pole.

The theory explains that the earth's crust is made of blocks called plates. There are seven large ones as follows;

1. Eurasian plate
2. Australian plate
3. Africa plate
4. Antarctic plate
5. N. American plate
6. S. American plate
7. Pacific plate

Then there are some smaller ones as follows;

• Indian
• Arabian
• Caribbean
• Cocos
• Somali plates
• Juan de Fuca
• Nazca
• Philippine
• Scotia

These plates are two types

1. Oceanic plates which form major areas of the ocean floor including coastal lowland.
2. Continental plates which form the bulk of the continental land mass.

The plates float on molten mantle layer called Asthenosphere.
The plates move relative to each other due to convectional currents in the mantle.
They move away from each other forming extension or constructive boundary called so because magma fills the space between

They move towards each other forming compressional or destructive boundary called so because materials between are crushed. The movements of those two types of plates have the following effects:

When two oceanic plates meet;

There is subduction and the ocean floor is pulled inwards forming a trench e.g. Java Trench .Subduction is the passing of edge of one plate beneath the edge of another.
- Sediments on the sea floor in the region of subduction are compressed to form Fold Mountains.
1. When an oceanic plate meets a continental plate the edge of the oceanic plate slides beneath the continental plate in a movement called subduction.
- Sediments on the sea floor in the region of subduction are compressed to form Fold Mountains.
- Fold Mountains are also formed at the edge of the continent when the sial layer is compressed.
- The edge of the oceanic plate bends into the mantle forming a trench.
2. When two continental plates collide the sial layer is folded into mountains.

They move past each other forming transform or conservative boundary called so because there is neither construction nor destruction which occurs where the plates are separated by a major fault.

Significance of Plate Movements
1. Are sources of earthquakes and Volcanicity.
2. Causes formation of land forms such as Fold Mountains and ocean trenches.
3. Spectacular landscapes formed are a tourist attraction.
4. Eruption of magma can result in formation of valuable minerals.