The real interior of the Earth is nowhere exposed to our direct observation. With our present scientific skills we can hardly penetrate up to a few kilometers below the surface of the Earth whereas the average radius of the earth is 6,371 km. Therefore, the entire discussion given below about the internal structure of the Earth is based on the evidence yielded by indirect geophysical methods. The study of seismic waves (released during earthquakes and nuclear shocks) forms the single most important source of information for the interior of the Earth.
Seismology is a branch of geophysics that deals with the study of elastic or seismic waves generated within the earth during an earthquake. Shock waves developed during big explosions are similar in all details to seismic waves.
The most important facts about seismic waves as are relevant to the internal structure of the earth are summarized as follows:
(i) In every earthquake elastic waves of three main types are generated at the focus (point of origin below the surface) of the earthquake. These are named in short as P-waves, S-waves and L-waves. The P and S waves travel through the body of the earth and hence are distinguished as body waves. The L-waves are confined mostly to near the surface of the earth and are hence called surface waves. The seismic waves travel in all directions from the focus.
(ii) The three types of seismic waves are recorded during each earthquake in a definite sequence at various seismographic stations; their records are known as seismograms. The P and S waves are recorded on the surface after having passed through materials deep within the earth and hence are considered important in the study of internal structure of the earth.
These waves travel with characteristic velocities through different media, so that from their arrival times many important conclusions can be made regarding the nature of the materials lying in their paths.
(i) From the study of travel time curves of these waves as obtained from various earthquake records, it is possible to calculate the velocity of any one type of these waves at any depth within the earth. This gives the very important and revealing velocity depth curves. (Fig. 2.5)
(ii) A thorough analysis of the velocity depth curves, especially of sharp and prominent changes in them observed repeatedly from different records is taken as indicative of major variations in the nature of the medium at those respective depths below the surface. The analysis when extended from surface to the center of the earth enables us to obtain a generalized picture of the internal structure of the Earth.
The interpretation of the internal structure of the Earth from the study of seismic waves is based on detection of abrupt changes in the velocity of P and S waves during their travel from the focus to various seismographic stations on the earth.
These waves reach the surface after being reflected and refracted at various depths below (Fig. 2.6). If the earth were of a uniform nature from the surface to the center, seismic waves traveling through it would be recorded on the opposite end without undergoing any change in their velocity.
Conversely, a major change in the velocity of seismic waves at some specific depths below the surface in numerous records can be taken to mean there is a change in the nature of medium (material) at that particular depth. Such a major change in the velocity of a seismic wave is called a seismic discontinuity. It is of fundamental importance in the interpretation of the internal structure of the Earth.
In actual practice, a number of such seismic discontinuities have been repeatedly observed from the records of many earthquakes. Further, the depths calculated from these discontinuities show remarkable agreement and hence may be taken as demarcating zones of different material composition within the earth.
The two most significant seismic discontinuities are:
(i) The Mohorovicic discontinuity
(ii) The Mantle-Core discontinuity.
(i) The Mohorovicic Discontinuity:
This is the first major discontinuity in the seismic record for the earthquakes and is named after its discoverer A.Mohorovicic (Pronounced as Moharoveechick). It occurs in the seismic records at depths of 30-40 km below the continents, 5-6 km below the oceans and 60-70 km below the mountains.
It is observed that both P and S waves on reaching these depths undergo sharp increase in their velocity. The P-waves attain a velocity of 7.75 km/sec from an original velocity of 5.4 km/sec in the immediately overlying layer. Similarly, the S-waves traveling at 3.35km/sec attain velocity of 4.35 km/sec. at this junction.
Thus the Mohorovicic discontinuity marks the lower limits of the skin of the earth commonly known as the Crust. This layer (the crust) is merely 35 km thick (on an average) below the continental surfaces and about 5-6 km thick under the oceans. Comparison of the earth with a large old apple will help in clarifying the concept of crust. The crust is to the earth what skin is to an old apple – a very thin, hard and wrinkled sheet of rock covering.
(ii) The Mantle-Core Discontinuity:
The seismic waves that cross the mohorovicic discontinuity continue to travel downwards with almost a uniform increase in their velocities. Such a gradual and uniform increase with increasing depth is on expected lines since their velocity is related to density of the medium which is normally expected to increase with depth.
At a depth of 2,900 km below the surface, however, another major discontinuity is suddenly observed in the records of the seismic waves. At this depth, the P waves become very sluggish and suffer a decrease in velocity from 13.64 km/sec to as low as 8.1 km/sec. Not only that, the S-waves are practically stopped from going deeper into the earth at this depth (of 2,900 km).
The zone of the earth lying between these two discontinuities, the M- Discontinuity and the Mantle-Core Discontinuity, is called MANTLE.
The second discontinuity, recorded at depth of 2,900 km, while demarcating the end of Mantle also marks the beginning of the third major zone of the Earth that is named as CORE. This discontinuity is, therefore, aptly known as mantle-core discontinuity. It was first discovered from the seismic records by B.Gutenberg in 1918 and subsequently confirmed by Jefferys in 1939.
The behavior of P and S waves below the depth of 2,900 km throws sufficient light on the existence of the third major shell, the Core. In every major earthquake, P and S waves are recorded at all the stations lying between the epicenter and 142° arc distance (11,000 km); further between 105° and 142° arc distances (11,000-16,000 km), only P waves reappear.
There is thus a shadow zone free from P and S waves in the record of each deep-seated earthquake. The shadow zone indicates existence of a zone made up of material of completely different nature compared with that of the upper two zones of the earth. This zone starts at a depth of 2,900 km (2,898 km to be precise) and continues right up to the center of the earth (6,371 km) which defines the boundaries of the core within the earth.
R.D. Oldham established the existence of core in the body of the earth in 1906.
The final picture of the internal structure of the Earth as developed from the study of the seismic wave records of the earthquakes divides it into three well defined shells or zones or spheres- the Crust, the Mantle and the Core. In each of these shells, there are records of significant variations on the basis of which each shell can be further subdivided into different layers with definite characteristics.
Following is only an outline of major zones and shells of which the earth is believed to be made up:
1. The Crust:
It is the uppermost shell of the Earth that extends to variable depths below the mountains (75 km), continents (35 km) and oceans (5 km). The Mohorovicic discontinuity marks the lower boundary of the crust.
Study of seismic waves reveals following details about thickness of the crust:
i. Mountainous Areas:
Under the Himalayas, the crust is believed to be 70-75 km thick; under the Hindukush Mountains it is 60 km thick and under the Andes 75 km thick.
ii. Continental Areas:
Thickness of the crust in continents varies from 30 to 40 km along the continental slopes, thickness of the crust shows considerable variation.
iii. Oceanic Areas:
The crustal cover below the oceanic water varies in thickness from a maximum of 19 km to low value of 5 km in deep oceans.
The Continental Crust is further distinguished into three layers- A, B and C.
The A or the Upper Layer is between 2-10 km thick and is of low density (2.2 g/cc). It is mostly made up of sedimentary rocks. In this layer, the P wave velocities range from 1.8 to 5.0 km/sec.
The B or the Middle layer of the continental crust is relatively dense (2.4 to 2.6 g/cc) Seismic waves attain velocities of 5 to 6.2 km/sec. This layer is also sometimes called the Granite Layer and is made up mostly of granites, gneisses and other related igneous and metamorphic rocks. At places, it acquires thickness of 20 km or more.
In fact at many places in the world, it is the B layer of the crust which is exposed on the surface because the overlying A layer has already been removed due to prolonged erosion by weathering agents. Since granite layer is mostly made up of silicates of aluminium and potassium, it is also sometimes referred as SIAL (Si = silica, AL = alumina) layer while discussing internal structure of the earth.
The C layer is the lowermost layer of the continental crust and has a density of 2.8 to 3.3 g/cc in which P waves attain as high velocity as 6 to 7.6 km/sec. This layer is also referred as Basaltic Layer of the crust and acquires a thickness of 25 to 40 km under the continents. It is made predominantly of basic minerals (rich in magnesium silicates) and hence is sometimes named as SIMA (Si for silica and Ma for magnesium).
The Oceanic Crust:
It is generally the extension of C layer of the continental crust that makes the top layer of the oceans in most cases; A and B layers being practically absent from there. The oceanic crust is estimated to have a volume of 2.54 × 109 cc with an average density of 3.00g/cc.
2. The Mantle:
It is the second concentric shell of the Earth that lies beneath the crust everywhere. This zone starting from the lower boundary of the crust continues up to a depth of 2,900 km. The exact nature of the mantle is as yet incompletely understood. It has been sub-divided into an upper and lower mantle, the boundary between the two layers being placed at 900-1,000 km below the earth. The upper mantle is further divided into two layers of 400 and 600 km thickness respectively.
Enough seismic data is available to suggest that density in the mantle rises from 3.3 g/cc from just below the crust to about 5.7g/cc at the base of the mantle. Recent studies indicate that a part of the upper mantle, from 100 km to 500 km depth, is in a plastic rather than solid state.
This zone has been named as asthenosphere (Greek “asthenes” – without strength). It is believed to be the source of much volcanic activity of the Earth and many other processes. The asthenosphere is believed to be located entirely in the upper mantle and supports the slowly moving tectonic plates.
3. The Core:
It is the innermost concentric shell of the Earth as concluded from the record of seismic waves. Its existence was suggested by R.D. Oldham in 1906 and subsequently confirmed by other seismologists. The core boundary begins at depth of 2,900 km from the surface and it extends to the center of the earth at 6,371 km.
Further studies of seismic waves with special reference to core indicate that the core itself can be distinguished into two distinct zones- the outer core and the inner core.
The outer core comprises the region from a depth of 2,900 km to 4,580 km below the earth surface and behaves more like a liquid because the S-waves from the earthquake shocks reaching this zone are not transmitted through this zone at all. (It is characteristic of S waves, also called shear waves that these are unable to travel through liquids).
The inner core, with a thickness of around 1,790 km is believed to be a solid metallic body. Much variation in composition is also suggested tor the material lying between the outer core and the inner core but nothing can be said conclusively.
Very significant variations in the density of material immediately outside and inside of the core are suggested by seismic observations. At the base of the mantle, density is inferred as 5.7 g/cc that jumps to 9.9 g/cc at the top of the core. This value reaches a figure of 12.7 g/cc at the boundary of the inner core and becomes 13.0 g/cc at the center of the earth.
As regards, the chemical composition of the inner core, the hypothesis that it is made up chiefly of iron and nickel has found support from many accounts. Seismologically, velocities of P waves recorded in the core bear close resemblance to those recorded for nickel iron alloys.
Other non-seismological grounds suggesting to a nickel iron core of the Earth are:
(i) The High Density of the Earth as a Planet:
It is fairly established that the mean density of the Earth is 5.517 g/cc. The density of the rocks of the crust is put at 2.7 g/cc. The density for the materials of mantle is also calculated to range from 3.3 to 5.7 g/cc. Hence all calculations suggest a density of 12 g/cc for the material of the core. This density is comparable to alloys of nickel and iron.
(ii) The Composition of Meteorites:
Meteorites are considered to be the wandering fragments of planetary matter. Most of these are made up of ferruginous composition, with iron being an important metal in them.
In fact three types of iron bearing meteorites have been studied:
a. Hexahedrites- These have a nickel content of 5-6.5%
b. Octahedrites- In these nickel content varies from 6.5 to 16%
c. Ataxites- Fine grained in nature and contains nickel in still higher proportions.
It is argued by many that the iron meteorites are actually fragments from the core of a planet or planet like body of our solar system that has suffered disintegration during the process of evolution of solar system. As such, the core of our earth can also be assumed to have a similar composition.