What Is Internal Constitution Of The Earth?

The internal constitution of the earth primarily relates to the structural and compositional aspects of the layered earth.

The internal structure of the earth deals primarily with concentric layering of the earth based on their physical seismologic characteristics which distinctively vary in their densities and seismic (earthquake) wave characteristics. On the other hand, the compositional layerings of the earth are characterized by their totality of the nature of the bulk chemistry, analogy with the composition of meteorites, smelter differentiated products and three distinctive established compositional layers.

Earth’s internal structure:

Man can hardly peep into the earth down to a depth of about 11 km through very deep underground mining and deep drilling. This depth, however, represent a minute fraction (less than 0.15%) of the earth radius (6371 km). Some direct geological observations could hardly read through few tens of km into the earth’s interior.

Hence our knowledge about the interior is obtained through indirect means, which include a number of geophysical sources of which the seismology singularly affords the most valuable and efficient tool to probe into the realm of the earth’s deep interior. Seismology, the science of earthquake studies is able to scan through the earth’s interior like X-rays that probe into the materials including human / animal bodies.

The seismic body waves such as P (primary – compressional) and S (secondary – shear) waves are recorded in seismograms (seismic records) from which the velocities of P- and S-waves could be numerically and graphically plotted against the distance along the radius of the earth. From these, the density, rigidity and bulk modulus factors against depth could be calculated to throw light on the nature of variation of these factors vis-a-vis depth (radius) of the earth. This is known as velocity – depth graph, which shows a number of breaks / discontinuities at certain depth (radius). These are physically expressed in terms of sharp rise or fall indicating 1st order discontinuities at definite depths. These discontinuities / breaks rationally corroborate to interpret the following aspects of the earth’s internal structure.

(i) The earth is a spheroid of revolution.

(ii) Its interior is concentrically divisible into a number of onion like layers or shells of varying density and elastic properties in which the interlayer / zone boundaries are qualified by either 1st

(iii) The internal structural layers are concentrically more or less homogeneous but radially somewhat heterogeneous.

(iv) Its interior is denoted by spherically symmetrical distribution of elastic and density properties.

(v) The nature and values of P- and S-wave velocities indicate the state of materials i.e. whether rigid, solid, soft solid, liquid or viscously flowing.

(vi) The density increases from about 2.6 near to surface to about 13 in the centre of the earth. The average density of the earth is 5.52 gmJ cm³ as determined by Cavendish in 1798.

Based on the above geophysical premise, various types of zonal mode have been advocated by a large number of pioneer workers. The earliest was a simple three-fold zonal model of crust-mantle-core by Oldha (1906). A four-fold zonal model with further division of the core (inner and outer) was interpreted by Lehman (1936).

The present knowledge of the earth’s interior is based on the discovery of a number of 1st and 2nd order discontinuities there by dividing the interior into a multi-zoned concentric layers characterized by distinctive seismi-physical attributes, After 1950, the more improved concentric models have been proposed by Gutenberg, Wadati, Wiechert, Geiger, Jeffrey, Bullen and Bullen- Hadon and others. It is just not possible here to deal with such models; however, a comprehensive but simplistic structural model is presented in

Compositional models of the Earth are interior:

These models present various layering / shells characterized by their (a) similarity with meteorite composition (b) the concept of primary geochemical differentiation of elements in primordial molten stage of earth’s evolution (c) mineral and rock assemblages of the interior and (d) laboratory experiments based on theoretical and hypothetical deductions. Of the various, two compositional models are briefly presented below.

(i) Goldschmidt’s (1922)

Compositional model based on the analogy with copper smelter products comprising three density differentiated products of top slag, intermediate sulphide-oxide matte and the bottom most metal alloy of Fe and Ni. The model is presented in characterised by:

(a) Variable thickness that ranges from 33 km (average) on the continents down to about 5 km in the oceanic sector.

(b) Its maximum thickness on land is about 60 km in the areas of fold mountain belts.

(c) It is broadly divisible into two sub-layers such as upper sialic crust (Sial) and lower simatic crust (Sima) by a somewhat inclined separating surface termed as Conrad discontinuity.

(d) ‘Sial’ stands for upper sub-layer which is rich in silicon (Si) and aluminium (Al).

(e) ‘Sima’ stands for silicon (Si), magnesium (Mg) and iron (Fe).

(0 Sial is composed of igneous and metamorphic rocks such as granite and granitic gneisses with or without a thin veneer of sediments. The average density of sial is about 2.6 gm/cm³.

(g) Sima forms the bulk of oceanic part comprising mostly basaltic rocks having density between 3 – 3.2 gm/cm³.

(h) Simatic crust is much thinner in comparison to sialic one.

(i) The base of the crust is characterized by a pronounced first-order discontinuity termed as Mohorovicic discontinuity below which rock layer shows an abrupt increase of seismic velocities of P- and S- waves and also density.

(J) The internal temperature at the base of the crust is about 1000°C.

(k) In general, the crust forms uppermost concentric layer of the earth’s interior, which is solid, strong and rigid.

Mantle:

It is the second major concentric layer from the top that underlies the crust. Principal characteristics of the mantle are:

(a) This layer is about 2867 km thick.

(b) It accounts for about 83% of the volume and 64% of the mass of the earth’s interior.

(c) This concentric zone is bounded in between the Mohorovicic discontinuity at ± 33 km depth at the top and the Gutenberg- Wiechert discontinuity at about 2900 km radial depth at the bottom,

(d) It is divisible into two major sub-zones called the upper mantle and the lower mantle respectively above and below the pronounced second order discontinuity termed as Repetti discontinuity at 960 km depth.

(e) Lower mantle is about two times thicker than the upper mantle.

(f) There is an about 150 km thick viscous low velocity zone in the lowermost part of the mantle.

(g) There are about six second order discontinuities within the mantle.

(h) The densities in the mantle increases with depth and varies from 3.5 to 4.4 gm/cm³ in the upper mantle and in the base of the lower mantle, the density goes up to 5.6 gm/cm³.

(i) Mantle is the major storehouse for the earth’s internal energy and forces, which support ocean-floor spreading and continental drifting.

(j) Upper mantle is composed of a mixture of ultrabasic-basic rocks called pyrolite.

(k) Lower mantle is composed of heavy silicates mixed with Fe and Ni.

Core:

It is a two-part core that forms the lowermost concentric zone of the earth’s interior. Thus, it forms the lowermost composite zone lying below the mantle from which it is demarcated by a first-order discontinuity called Gutenberg – Wiechert discontinuity at a depth of 2900 km. It is also known as ‘Centrosphere’. Its radial thickness is about 3471 km. It represents 11% of the volume of the earth. It is principally composed of Fe and Ni and hence, in general, referred to as ‘NiFe’. Density of the core varies from 9.8 gm/cm³ at the top to about 13 gm/cm³ at the very centre of the earth.

Later findings reveal a major two-fold division of the core into (i) outer core and (ii) inner core, which are separated from one another by inner- outer core boundary, which is a pronounced first-order discontinuity placed at a depth of 5000 km. between the two cores, lays a 300 km wide transition zone.

The outer core (2100 km thick) is thicker than the inner core (1370 km). Densities in the outer core vary from 9.8 gm/cm³ to 10.7 gm/cm³. Outer core does not transmit S-waves and hence said to be in liquid/semi-solid state. It controls the earth’s magnetic field. Inner core is said to be in soft-solid condition as it distinctly transmits P-wave and very feebly weak S-wave. The outer core is said to be composed of Fe, Ni and a little sulphur. Inner core is composed of solid Fe and Ni alloy and has a high density varying from about 1’1-13 gm/cm³ at the very centre.