Home General Knowledge GK SPECIAL TOPIC : EARTH'S INTERIOR
GK SPECIAL TOPIC : EARTH'S INTERIOR
Friday, 18 July 2014 08:43




EARTH'S INTERIOR

The scientific study and analysis of various seismic waves of natural and man induced earthquakes unrevealed the mystery of earth’s interior and divided the earth in three different layers explained below:

a) Earth's Crust:   There are two different types of crust: thin oceanic crust that underlies the ocean basins and thicker continental crust that underlies the continents. These two different types of crust are made up of different types of rock. The thin oceanic crust is composed of primarily of basalt and the thicker continental crust is composed primarily of granite. The low density of the thick continental crust allows it to "float" in high relief on the much higher density mantle below. The boundary between the crust and the mantle is MOHOROVICIC DISCONTINUITY

b) Earth's Mantle: Earth's mantle is thought to be composed mainly of olivine-rich rock. It has different temperatures at different depths. The temperature is lowest immediately beneath the crust and increases with depth. The highest temperatures occur where the mantle material is in contact with the heat-producing core. This steady increase of temperature with depth is known as the geothermal gradient.

The geothermal gradient is responsible for different rock behaviors which divide the mantle into two different zones. Rocks in the upper mantle are cool and brittle, while rocks in the lower mantle are hot and soft (but not molten). Rocks in the upper mantle are brittle enough to break under stress and produces earthquakes. However, rocks in the lower mantle are soft and flow when subjected to forces instead of breaking. The lower limit of brittle behavior is the boundary between the upper and lower mantle.

c) Earth's Core: Earth's Core is thought to be composed mainly of an iron and nickel alloy. This composition is based on calculations of its density. The core is earth's source of internal heat because it contains radioactive materials which release heat as they break down into more stable substances.

The core is divided into two different zones. The outer core is a liquid because the temperatures there are adequate to melt the iron-nickel alloy. However, the inner core is a solid even though its temperature is higher than the outer core. Here, tremendous pressure, produced by the weight of the overlying rocks is strong enough to crowd the atoms tightly together and prevents changing to the liquid state.

• TYPES OF SEISMIC WAVES

There are two types of seismic waves, body wave and surface waves.

Body waves travel through the interior of the Earth. They follow ray paths refracted by the varying density and stiffness of the Earth's interior which in turn, vary according to temperature, composition, and phase.

Body waves are divided as

* P waves (primary waves) are compression waves that are longitudinal in nature. These waves can travel through any type of material, and can travel at nearly twice the speed of S waves. In air, they take the form of sound waves; hence they travel at the speed of sound. Typical speeds are 330 m/s in air, 1450 m/s in water and about 5000 m/s in granite.

 

* S-waves (secondary waves) are shear waves that are transverse in nature. These waves typically follow P waves during an earthquake and displace the ground perpendicular to the direction of propagation. S waves can travel only through solids, as fluids (liquids and gases) do not support shear stresses. S waves are slower than P waves, and speeds are typically around 60% of that of P waves in any given material.

Surface waves are analogous to water waves and travel along the Earth's surface. They travel slower than body waves. Because of their low frequency, long duration, and large amplitude, they can be the most destructive type of seismic wave.

There are two types of surface waves.

Rayleigh waves, also called ground roll, are surface waves that travel as ripples with motions that are similar to those of waves on the surface of water. The existence of these waves was predicted by John William Strutt, Lord Rayleigh, in 1885.

They are slower than body waves.

Love waves are surface waves that cause circular shearing of the ground. They are named after A.E.H. Love, a British mathematician who created a mathematical model of the waves in 1911. They usually travel slightly faster than Rayleigh waves, about 90% of the S wave velocity, and have the largest amplitude.