STRUCTURE OF THE EARTH

STRUCTURE OF THE EARTH
The nature of earth's interior is understood by scien­tists through studying the lavas emitted by volcanoes and studying the behaviour of earthquake waves. These obser­vations have led to the conclusion that earth's structure has three parts--the core, the mantle and the crust. Until recently it was thought that the earth's outer crustal layer was composed of rafts of sial which floated on a sea of sima. But recent studies have revealed that large areas of the outer crustal layer are made of basaltic rocks similar to the sima. The earth's crust is now regarded as a series of plates which are gradually being pushed apart. The sial rocks carried on some of the plates from the continents.

CORE The centre Qf the earth is occupied by a spherical zone called core, about 3475 km in radius. The innermost part of the core may be solid or crystalline, with a radius of about 1255 km, while the outer core has properties of a liquid. The liquid core is considered to comprise iron and a small. proportion of nickel. The temperatures in the earth's core lie between 22000C and 2750°C. Pressures are as high as three to four million times the pressure of annosphere at sea level.

MANTLE The mantle lies outside the core, a layer about 2895 km thick, composed of mineral matter in a solid state. It is probably composed largely of magnesium iron silicate, which comprises an ultramafic rock called dunite. This rock exhibits great rigidity and high density in response to earthquakes that pass through it. It can also adjust to unequal forces acting over great periods of time.

THE CRUST It is the outermost and thinnest layer of the earth's surface, about 8 to 40 km thick.I The base of the crust is sharply defined where it contacts the mantle. This surface of separation between the mantle and the crust is called Moho (Mohorovicic Discontinuity). The crust varies greatly in thickness and composition-as small as 5 km thick in some places beneath the oceans, while under some moun­tain ranges it extends upto 70 km in depth.

The rocks of this layer can be sub-divided into (i) basaltic rocks, under­lying the ocean basins, containing much iron and magne­sium, and (ii) the rocks that make up the continents which are rich in silicon and aluminium and are lighter in colour and density.

Ccmposition of Earth's Crust The earth's crust is the most significant zone of the solid earth. With an average thickness of 17 km, this mineral skin contains the continents and ocean basins and is the source of soil and other sediments vital to life, of salts of the sea, of gases of the atmosphere and of all free water of the oceans, atmosphere and lands. Oxygen is the predominant element accounting for 46,6 per cent of the weight. It occurs in combination with silicon (27.7 per cent), aluminium (8.1 per cent), iron (5 per cent), calcium (3.6 per cent) and other elements.