- Geologic Forces: Earth has undergone great changes over million of years. Generally processes of gradation, tectonism and volcanism.
(a) Gradation
- Degradation: Erosion results from wearing of rocks by water, air and ice.
- Aggradation: Deposition results in accumulation of sediment and ultimate building up of rock strata.
(b) Tectonism
- Plate tectonics - a dynamic process of the lithospheric plate which moves over a weak plastic layer in the upper mantle known as asthenosphere.
- These plates interact with one another along their boundaries. Indicative of crustal instability, produce faulting (fracture and displacement), folding, subsidence and uplift of rock formation. Responsible for formation of mountain ranges.
- Earth’s lithosphere is composed of seven large plates
with thickness ranging from 75 to 125 km.
| Pacific Plate | Eurasian Plate |
| Antartic Plate | |
| Indian Plate | South American Plate |
| Africian Plate | 20 other small plates in between |
(c) Volcanism
- Volcano - a vent in the earth's crust through which molten rock materials within the earth, lavas, ashes, steam and gas are ejected and responsible for the formation of plutonic rocks, once solidified at great depth.
- Majority of volcanoes are located along the margins of tectonic plates.
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