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 indirect sources are temperature and pressure inside the earth, density of its different shells, behavior of earthquake waves and evidence from meteorites.
Although earthquakes can have catastrophic effects, they can also reveal a great deal about the earth’s internal structure. By using seismograph, a graphic recording of the earthquake waves or vibrations is made and scientists are able to get some idea of the kind of rocks which are found below the earth’s surface.
STRUCTURE OF THE EARTH
The waves travel at different rates from a common source. Therefore time interval between their arrivals at the recording station will also vary. Besides, the density of rocks and nature of the medium, whether solid or liquid, through which the P and S waves pass also affect the propagation of waves. Based on these observations the earth’s interior has been divided into three layers – crust, mantle and core.
- THE CRUST
It is the outermost solid part of the earth and it is brittle in nature. It comprises of two distinct parts. The upper part consists of granitic rocks and forms the continents. Its main mineral constituents are Silica and Alumina, so it is collectively referred to as Sial. It has an average density of 2.7g/cm3. The lower part is a continuous zone of denser basaltic rocks forming the ocean floors, comprising mainly silica, iron and magnesium.
It is therefore called Sima and has an average density of 3.0gm/cm3. The Sial and Sima together form the earth’s crust. Since the Sial is lighter than the sima, the continents can be said to be ‘floating’ on a sea of denser sima. Oceanic crust is thinner as compared to the continental crust. The mean thickness of oceanic crust is 5 Km whereas that of the continental crust is around 30Km. The continental crust is thicker in the areas of major mountain systems. It is as much as 70 km, thick in the Himalayan region.
The crust is distinguished from the mantle by the presence of abrupt change in the velocity of seismic waves. This corresponds to the abrupt change in rigidity of the rock from crust to mantle.
The change in rigidity in turn is due to change in mineral composition or in physical state of the rocks. The ‘P’ waves near the surface travel at about 6 km per second and this velocity increases gradually or abruptly to the base of the crust, where it is 7 Km per second.
About 98 per cent of the total crust of the earth is composed of eight elements like Oxygen, Silicon, Aluminium, Iron, Calcium, Sodium, Potassium and Magnesium, and the rest is constituted by Titanium, Hydrogen, Phosphorous, Manganese, Sulphur, Carbon, Nickel and other elements.
The elements in the earth’s crust are rarely found exclusively but are usually combined with other elements to make various substances. These substances are recognised as minerals.
- THE MANTLE
The portion of the interior beyond the crust is called mantle. The velocity of seismic waves is about 7Km/sec at the base of lower crust but it suddenly becomes 7.9 to 8.1 Km per second i.e., there is sudden increase in the velocity of seismic waves at the base of lower crust. This trend of seismic waves denotes discontinuity between the boundaries of lower crust and upper mantle.
This discontinuity is called as Mohorovicic Discontinuity or Moho Discontinuity or M Discontinuity. Through the earth’s mantle, upto nearly 2,900 Km, the speed of earthquake waves is so high that only a very rigid and dense rock will satisfy the observed conditions. Solid or rigid in this case means either crystalline or glassy.
It also means that, when subjected to the sudden twists and bends of earthquake waves, the rock behaves as an elastic solid, that is, it changes shape when shear stresses are applied, but returns exactly to its former shape when those stresses are removed. Thus the mantle consists of solid rock and it extends from Moho discontinuity to a depth of 2900 Km.
The upper portion of the mantle is called Asthenosphere. The word astheno means weak. It is considered to be extending from 300Km up to 400 Km. It is the main source of magma that finds its way to the surface during volcanic eruptions. It has a density higher than the crust (3.4 g/cm3). Rocks in the Asthenosphere behave as both a plastic solid and an elastic solid.
The matter possessing these remarkable properties is an elastic-viscous substance – it can be elastic and plastic at the same time, depending on whether the forces that tend to deform it are applied and released suddenly or steadily. The velocity of seismic waves relatively slows down in the uppermost zone of the upper mantle below 150 km after first increasing rapidly from the surface to that depth. This region is referred to as the low velocity zone.
The crust and the upper most part of the mantle together are called Lithosphere. Its thickness ranges from 10-200Km. The lower mantle extends beyond the asthenosphere and it is in solid state.
It may be mentioned that the thickness of the mantle is less than half of the radius of the earth (6371 Km) but it contains 83 per cent of the total volume and 68 percent of the total mass of the earth.
- THE CORE
The core, the deepest and most inaccessible zone of the earth extends from the lower boundary of the mantle at the depth of 2900 Km to the centre of the earth (6378 km). The ‘P’ waves make abrupt drop in velocity at the mantle-core boundary, whereas S waves terminates at the mantle-core boundary. Thus making a plane of discontinuous surface between the core and the mantle known as Gutenburg discontinuity.
Study of Seismogram (a seismograph record) has conformed of a spherical core at the earth’s centre and has added insights into its physical nature. In case the earths were entirely solid, both P and S waves would travel in all directions. The body waves of any large earthquake could be recorded directly opposite its focus.
It was, however, found that there is a region on the globe opposite the earthquake focus where S waves are not received. That means the S waves cannot pass through the central part of the earth because this part is made of a medium which is not solid.
Physicists have proved through experiments that S waves cannot be send through a liquid medium. This proves that earth’s core is in liquid state in contrast to the surrounding mantle which is solid.
The core-mantle boundary is located at the depth of 2900 Km. The outer core is in liquid state, while the inner core is in solid state. The density of material at the mantle core boundary is around 5g/cm3 and at the centre of the earth at 6300 Km, the density value is around 13g/cm3. The core is made up of very heavy material mostly constituted by nickel and iron. It is sometimes referred to as the Nife layer.
Scientists have discovered why the crystallised iron core of the Earth remains solid, despite being hotter than the surface of the Sun. An energy distribution cycle keeps the core solid despite it being hotter than the surface of the Sun. Spinning within Earth’s molten core is a crystal ball — actually a mass formation of almost pure crystallised iron — nearly the size of the moon.
|Name of the layer
|Structure and composition
|Density of rocks
|Outer and thinnest layer of the earth. It is composed mainly of Silica and Aluminium.
(Si + Al = SIAL)
|5 – 40 km
Asthenosphere Upper and lower mantle are composed mainly of Silica and Magnesium.
|Composed mainly of Nickel and ferrous (Ni + Fe = Nife)
Liquid or in Plastic State
TEMPERATURE AND PRESSURE INSIDE THE EARTH
The evidence of volcanic eruption and hot springs indicate that high temperatures 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 rate of increase in temperature is considered to the variable and there is no uniform increase from the surface to the centre of the earth. The rate of increase in the overlying pressure makes the melting point higher but only to a certain degree. In upper 100 km, the increase is estimated at 120 C/km. It is 20 C/km in the next 300 km and 10 C/km below it.
The heat or rise in temperature is the result of internal forces, automatic disintegration of radioactive 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 of the earth, the liquid nature of its core has acquired the properties of a solid and is probably in a plastic state. The earth is rigid and behaves mostly as a solid down to the depth of 2900 km because of such pressure conditions.
Wherever even a slight release of pressure occurs, the underlying matter escapes to the surface and becomes molten because of high temperature prevailing there. While solid materials melt inside the earth, the liquid also takes up the properties of a solid. The interior is inaccessible to human technology due to the progressive increase in temperature and pressure due to the over lying rocks.
PREVIOUS YEAR QUESTION (PRELIMS)
- Which one of the following is present in the largest amount in terms of percent by mass in the earth’s crust? (1997)
(a) Silicon (b) Oxygen (c) Carbon (d) Calcium