The earth consists of very huge plates, which show constant motion in the form of folding, deforming and displacement. Such compression and uplift has been possible only because of the operation of internal forces on a large scale. Such internal forces, called plate tectonics are responsible for volcanic eruptions, earthquakes etc.
Two major types of movement occur inside the earth’s crust: Vertical and Horizontal. Vertical movements either uplift or subside the earth’s crust along lines of weakness called faults. Displacement of the earth’s crust takes place along the fault line. When such displacement takes place along two adjoining faults, the portion between them may get uplifted to form a block mountain or plateau, or subside to form a basin. The Rift Valleys in East Africa are typical examples of such fault-based structures. Large-scale vertical movements of the crust are called continent-building movements. Horizontal movements of the earth’s crust are responsible for folding and displacement of the rock layers. Simple folding consists of upfolds called anticlines and downfolds called synclines. Often, these folds get compressed such an extent that the rock layers get displaced over long distances resulting in complex structures. Large –scale horizontal movements are called orogenic or mountain-building movements because they are responsible for the fold-mountains like the Himalayas.
A volcano is a hole in the earth’s crust through which magma, ash, gases and rock material from the earth’s interior gets erupted. Magma consists of molten rock with gases and steam. The molten mass called lava cools down gradually and gets solidified. The opening through which 5t erupts takes place is called the vent. The erupted material accumulates around the vent to form a cone-shaped hill, which grows with the deposits 6f rock particles, boulders and other materials produced by explosive eruptions. Explosive eruption may cause a depression around the vent called the crater. Most major volcanoes are of the cone and crater type in origin and appearance.
Activitywise, volcanoes may be active; those which have erupted in the recent past. Around 500 of them exist world-wide, mostly in and around the Pacific Ocean. Dormant volcanoes are those, which have erupted in early history but are now quiet. Extinct volcanoes have not erupted in the historical periods.
Whenever a part of the crust displaces due to tectonic forces, it causes tremors, which travel in all directions from the centre of disturbance. These sudden tremors are called earthquakes. The centre from which the earthquake originates is called the Seismic Focus. The Epicentre of an earthquake is a point vertically above the seismic focus. Most earthquakes have a seismic focus at depths of less than 60 km. The intensity of the tremors is the maximum near the epicentre and decreases with increasing distance from the epicentre.
Earth movements along the lines of weakness and volcanic eruptions cause tremors on the earth’s surface. Hundreds of mild quakes occur daily, while severe trem6rs are relatively rare. Earthquakes occur more frequently in the recently-formed fold mountain regions, which are relatively unstable. About 66 % of all earthquakes originate in the Circum-Pacific belt along the west coast of North and South America and the east coast of Asia.