Structure and Characteristics of the Earth's crust and interior

Structure and Characteristics of the Earth's crust and interior

The structure and characteristics of the earth's crust and interior has always remained a matter of great controversy among the geologist and geophysics. At present a lot of reliable knowledge is available about the crust and it's interrior.

Since their is no way of finding the chemical composition of the unseened material, ideas on the interrior regions must be based on the indirect evidences and somewhat speculatives.

The earth solidified heavier elements towards the centre and lighter elements slowly welled upward to the surface concentrating the crust. In the absence of direct evidence the structural characteristics of the unseen region of the earth can be measured for estimated more readily.

The change in temperature, pressure and density with depth can be estimated from the velocity of earthquake waves through the earth.

• Temperature : It is an established fact that as we go down from the surface inside the earth the temperature goes increasing at the rate of 1° centigrade for every 32 metres of descent. At this rate the temperature at the depth of 48 km would be between 1200°c - 2000°c. The rocks and minerals cannot remain in a solid state at such high temperature.

It is therefore concluded the source of lava eruption of the volcanoes is at a depth of about 48 km.

If we assume that the temperature goes on increasing at this rate the temperature at the centre of the earth should be more than 4000°c.

• Pressure : The pressure exerted by the weight of the atmosphere inside the earth has also been used to determine the composition and structure of the interior of the earth.

On atmospheric unit equals to a pressure of about 14.7 lb per square Inch. At this rate, at a depth of 2500 km the pressure is about 1 million atmosphere. Such a high temperature in the interior of the earth exercises a powerful influence on the temperature and physical state of the earth.

• Density : The average density of the earth is 5.5. The surface layer has a density of about 2.7.

From this facts it may be conclude that the density of materials in the core of the earth is much more than the density of rocks found in the uppermost layer.

The geologists have consensus on the point that the earth is composed of several shells or layers, the density of which goes on increasing with depth.

• Seismic Waves : The behaviour of seismic waves in the different layers of the earth provides the most authentic evidence of the earth. During the occurrence of an earthquake, it generated different types of waves and are categorised as -

(i) Primary Waves (P-Waves)

(ii) Secondary Waves (S-Waves)

(iii) Surface Waves (L-Waves)

The velocities of these waves traveling through a specific part of earth thus give an indication of the type of rock there. Abrupt changes in the seismic waves velocities indicate significant changes in the earth interior.

The earthquake waves behave differently in different media. When they travel through a homogeneous solid body they move in straight line with a constant velocity, but when they travel through a body having heterogeneous composition and varying densities, they adopt curved paths and travel with varying velocities, the velocity increases with increase in the density.

In other words, the seismic waves suffer refraction and reflection. While traveling through heterogeneous medium of the earth interior and follow curved path. This is a clear indication that the density inside the earth increases regularly and sometimes rather abruptly.

Due to increased density inside the earth p-waves follow a curve path. They are also refracted abruptly at the boundaries between the layers particularly as the boundary between mantle and core.

Similarly, S-Waves follow curve path due to increased in density inside the earth these waves cannot pass through liquids and gases and are unable to enter the core of the earth.

All the three waves (P, S and L-Waves) are present near the focus of the earthquake. After traveling some distance L-Waves disappear and only P and S-Waves are detected.

It has been found that none of the earthquake waves reach the region between 103° and 143°.

From the result or information supplied by earthquake waves, it can be conclude that the earth is layered by structure. The uppermost layer is named as crust which is solid and then followed by mantle layer and the centre is the core.

The characteristics of each layers are also different. Their density, temperature of rocks, pressure and the chemical composition, physical characteristics of the rocks are also different from one part to another party.