STRUCTURE OF THE EARTH
Initially earth was considered to be a uniform solid but it consists of different concentric shells, like a boiled egg. These shells or layers are divided into three parts, crust, mantle and core. The core is further divided into two layers the outer core and the inner core.
(a) Crust: It is the outer layer of earth on which our continents and ocean basins rest. It is thickest in the continental regions where it has an average thickness of 40 kilometres and thinnest in the oceans where it may have maximum thickness of 10 to 12 km.
(b) Mantle: The layer beneath the earth's crust is called the mantle. Mantle, is important because it accounts for nearly half of the radius of the earth (2900 km). The dynamic processes which determine the movements of the crust plates are powered by the mantle. The mantle is a shell of red hot rock and separates the earth's metallic and partly melted rock (both the inner and the outer rocks) form the cooler rocks of the earth's crust. It consists of silicate minerals rich in magnesium and iron. The density of mantle increases with depth from about 3.5 grams per cubic centimetre to around 5.5 grams per cubic centimetre, near the outer core.
(c) Outer Core: The outer core is around 2300 km thick. The outer core appears to be in molten state. It contains iron and nickel in molten state. It also contains sulphur.
(d) Inner Core: The inner core is about 1300 km thick and is surrounded by outer core . It is a solid ball and is composed of iron.
An earthquake is a sudden movement or a fracture in the crust and the upper layer of the mantle (together called the lithosphere). This causes a series of shocks (movements). An earthquake may range from a mild tremor, which will result in the vibrations of the ground to a large-scale movement of earth causing a widespread damage over a large area.
The branch of science which deals with the earthquakes and related phenomena is called seismology. All the phenomena related to the emergence and manifestations of earthquakes are called seismic.
The surface of the earth is broken into many large pieces called plates. Tectonics means the study of rock structures involved in earth movements. The world is divided into seven large plates and several smaller plates. Due to slow movement inside the earth, these plates move with respect to each other by about a few inches in a year. The energy developed by this 'small movement is very large. The boundaries of the plates are prone to rigorous geological activity and the result of these movements with respect to each other are earthquakes, volcanoes, mountain formation, etc.
The theory that the surface of earth is made of lithospheric plates which move, is called plate tectonics. This theory explain the locations of earthquakes, volcanic eruptions and newly forming mountains.
NOTE: The mountain system of the Himalayas is believed to have been formed by the collision of two continental plates, the Eurasian Plate and the Indian Plate.
OCCURRENCE OF EARTHQUAKE
Earthquakes generally occur at plate boundaries as follows:
(i) As the plates move, they rub against each other. At the edges of the plates where the earth'