Interior of the Earth - Physical Geography, UPSC, IAS. | Geography (Prelims) by Valor Academy PDF Download

Interior of the Earth

CHAPTER 6

INTERIOR OF THE EARTH

Only upper part of the earth’s crust just below its surface could be known more or less by direct observations. Lower part is beyond the reach of direct observations and our knowledge about it is based upon indirect scientific evidences. These are:

1. Behaviour of earthquake waves
2. Evidence from meteorites,
3. Temperature,
4. Pressure inside the earth,
5. Density of its different shells.

BEHAVIOUR OF EARTHQUAKE WAVES

There is a change in the course and velocity of the waves on crossing the boundaries of different zones inside the earth. There are three (3) types of waves (recorded by seismograph) –

1)  P or Longitudinal or Primary Waves.

1. S or Transverse or Secondary Waves.
2. L or Long or Surface Waves.

The velocity of P and S waves increases with depth but only up to 2900 Kilometers. Afterwards S waves do not pass and P waves travels at a reduced velocity.

The L waves do not pass and do not go deeper in the earth.

The S waves can not pass through a liquid and are transmitted only through a rigid body. The velocity of P waves passing through inner core again increases as compared to their passage through the outer core.

This data briefly point out that the inner core of the earth is of solid iron and the outer core has probably the properties of a liquid. It may be concluded that as a whole the earth behaves as a solid even when parts of its interior are reported to be in a plastic pr semi-liquid state.

EVIDENCE FROM METEORITES

The meteorites belonging to our solar family are another source of our information for a better understanding of the earth’s structure. Their outer layer is burnt during their face to the earth. As the strong material of meteorites is similar to those found on the earth’s surface, they are scarcely recognized. But the composition of the meteorites consisting of heaviest materials confirms that similar composition of inner core of the earth.

TEMPRATURE AND PRESSURE INSIDE THE EARTH

The evidence of volcanic eruptions and hot springs indicates the high temperature prevail in the interior of the earth. A progressive rise in temperature with increasing depth is recorded in mines and deep wells all over the world, the average rate of increase being 1°C for every 32 metres of descent. But there is no uniform increase from the surface to the center of the earth.

In upper 100 km, the increase is estimated at 12°C per km. it is 2°C per km in the next 300 km and 1°C per km below it. By this calculation the temperature is actually 2000°C at the core of the earth.

The heat or rise in temperature is the result of internal forces, automatic disintegration or radio-active substances, chemical reaction and other sources keeping the interior hot. It indicates the liquid or perhaps gaseous conditions prevailing at greater depths. But at the same time there is a tremendous increase in the pressure of overlying layers on earth’s interior. Thus, even under extremely high temperature towards the central part pf the earth, the liquid nature of its core has acquired the properties of a solid and is probably in a plastic state.

DENSITY AND COMPOSITION OF EARTH’S INTERIOR

The structure of the earth’s interior is layered. The arrangement of layer is comparable to onion with its shells, one inside another. Below the surface capped by sedimentary material, upper layer of the crust is mainly composed of crystalline igneous and metamorphic rocks, acidic in nature. The lower layer of the crust has basaltic and ultra-basic rocks. The layer of heavier or inner silicates is not found beneath oceans. The oceans are mostly underlaid by dark coloured basalt followed down by a thick greenish and tremendously hot layer.

The continents are composed of lighter silicates turned as ‘Sial’ or Silica+Aluminium. The oceans have heavier silicates named as ‘Sima’ or Silica+ Magnesium. The continents of lighter material are floating in a Sea of heavier and denser material. The central core has the heaviest mineral materials of highest density. It is composed of ‘Nife’ or Nickel + Iron. A zone of mixed heavy metal and silicates separates the core from the other layers.

There are three layers- the crust, mantle and the core.

Crust           0.5% of Earth’s volume

Mantle         16 % of Earth’s volume

Core             83 % of Earth’s volume