CONSTITUTION OF THE EARTH’S INTERIOR
Ideas about the interior of the earth are based upon indirect evidences. Modern view of the earth's internal structure takes into account evidences from sources such as:
- Density Studies: As the relative density of the earth has been calculated to be 5.5 and that of the upper rocks to be 2.7, the relative density of the in-depth rocks must be more than 5.5.
- Temperature and Pressure: Complex interplay of temperature and pressure largely determines the state of the matter in the interior of the earth. At depths of 2,900 km, the temperature is expected to be around 25,000°C at which most pan of the earth's interior would have melted. However, this is not so. Enormous underlying pressure raises the melting point of the rocks.
- Seismic Waves: The most authentic source of information about the earth's interior are the earthquake waves coming from within the earth. These are of three types, as follows:
(a) Primary Waves (P-waves): (longitudinal waves or compressional waves)
(b) Secondary Waves (S-waves): (transverse or distortional waves)
(c) Surface Waves (L-waves): (long-period waves)
Seismic waves would move in straight lines if the earth were a homogenous solid sphere. But the path of seismic waves has been found to be curved indicating non-homogenous structure of the earth. After the study of seismic waves and `Seismic Tomography', the sphere of the earth has been found to be constituting of three concentric layers, as follows:
- Thickness of this outermost layer varies from 30-40 km. beneath continents, to about 10 km. beneath the oceanic floor.
- The crust is divided into two shells : upper, discontinuous, lighter layer of `SiAl’ (Silica + Aluminium) and the lower, continuous, denser layer of the `SiMa' (Silica + Magnesium). ).
- The SiAlic shell is thicker under the continents and nearly disappears under the oceanic surface (composed of SiMa).
- The surface of the earth is covered with sedimentary rocks, below which lies a layer of crystalline rocks comprising granite and gneisses in its upper section, and basaltic rocks in the lower section.
- The mantle is separated from the crust by a discontinuity called the Mohorovicic or Moho Discontinuity, where the speed of 'P' wave increases suddenly from 6.9 km/s. to 7.9-8.1 km/s.
- The mantle extends from this discontinuity (having an average depth of 30-35 km) to a depth of 2,900 krn.
- The mantle accounts for 83 per cent of volume and 68 per cent of the mass of the earth.
- The mantle is composed of dense and rigid rocks which have a predominance of magnesium and iron.
- Mean density of the mantle is 4.6
- This can be divided into two parts: (i) The Upper Mantle (density of 3.3-4.0) extends down to 700 km, and (ii) The Lower Mantle or Mesosphere (density range of 4.0-5.5) which extends between 700-2,900 km.
- This is the innermost layer of the earth.
- It starts from the Weichart—Gutenberg discontinuity at a depth of 2,900 km, where there is an abrupt reduction in P wave velocity and the disappearance of S waves (which cannot pass through liquids).
- This part of the core, categorized as the Outer Core (2,900-5,150 km) is in liquid state since the pressure at such great depth is also very high.
- The core, also called the barysphere, is composed of heavy metallic elements of Nickel and Iron (NiFe).
- The core accounts for 16 per cent of volume and 32 per cent of the mass of the earth, with relative density ranging from 9.9 to 13.6 or even higher (average relative density being 11.0).
Lithosphere and Asthenosphere
- Lithosphere comprises of 80 to 100 km of the uppermost mantle on which the crust rests. It is cool and rigid like the crust, and along with the crust behaves as a unit. Lithosphere is a combination of this rigid part of the crust and the uppermost mantle.
- Beneath this rigid layer of Lithosphere is the Asthensophere, the upper part of which is hot and plastic, as well as relatively soft.
Continental Drift Theory
- This theory was proposed by Alfred Wegener in 1915. According to him, in the carboniferous period (about 250 million years ago), all the continents were united as a super continent known as Pangaea which was surrounded by a large ocean, called Panthalsa.
- Pangaea started breaking up during the Carboniferous period.
- Continents made up of the lighter SiAl were moving over the ocean basins, which are composed of the denser SiMa.
- The continents drifted in two directions — towards the equator due to gravitational attraction of equatorial bulge (resulting in the formation of Himalayas, Alps, Atlas etc) and towards the west owing to tidal forces of the moon and the sun (forming the mountains of Rockies and Andes).
- The theory of Plate Tectonics postulates that the outer rigid lithosphere comprises a mosaic of rigid segments, called Plates„ that move on the plastic upper mantle (asthenosphere) carrying the continents and oceans along with them. Their thickness varies from 80-100 km along the oceans, to over 100 km in the continents. Six major and many minor plates have been identified.