GEOMORPHOLOGY

INTERIOR OF THE EARTH

Man with his technological up gradation had send the rovers like “Curiosity” to the other planets and even the instruments like “Voyeger-1 and voyager-2” to the areas beyond Neptune (area recently named as Magnetic Highway).  Exploring the earth beneath our feet is a more difficult task than mapping the moon.  The deepest drill on this earth till date is near the kola peninsular of just 12 kms deep.  Direct observation of the earth’s interior is not possible because of the tremendous heat energy inside the earth but there are certain indirect sources through which we can able to know what is there inside the earth.  The sources which provide the knowledge about the inside of the earth can be grouped into two.

1. Natural sources

2. Artificial sources

Natural sources like Volcanism, Seismology and evidence from meteorites, where as the artificial sources like Temperature, Densityof its different shells & the pressure inside the earth.  It is these sources which provide enough knowledge about the interior of the earth. By using seismograph a graphic recording of the earthquake waves or vibrations is made.  An earthquake occurs when ever a rock breaks inside the earth, this will release energy in the form of waves called seismic waves, Seismic waves are vibrations in Earth caused by the rupture and sudden movement of rock, they will travel all over the globe just like ripples in the well.  The seismic waves are of two types

1.       The Body waves

2.       The Surface waves.

  The body waves are those waves which travel through the earth’s interior and are of two types, Primary waves denoted by the letter P and the Secondary wave denoted by the letter S.  The surface waves are those waves which travel along the surface of the earth for long period waves and are denoted with the letter L, these are of two types, one Rayleigh waves denoted by R and the Love waves denoted by Q.

Primary waves: also called Longitudinal waves or compression waves which are analogous to the sound waves wherein particles move both to and fro in the line of the propagation of the wave.  These waves travel faster through solids, though, these also travels through liquids and gaseous but their speed is slowed down.

Secondary waves:   also called as transverse waves which are analogous to the light waves.  These can travel only through solid media couldn’t travel through liquid or gaseous media.

Surface waves:  also called Long period waves or simply L waves. These affect the surface of the earth and die out at smaller depths.  Though their speed is less than P and S waves but these are most violent and destructive.

There is a change in the course and velocity of the waves on crossing the boundaries of different zones inside the earth.  If the ground through which the wave travels is solid, they behave in one way.  If it is liquid, the waves behave in a different way.  Their velocity in both the cases differs for example the velocity of the P and S waves increases with depth but up to 2900kms.  Afterwards, S wave passing across the direction of their movement do not pass and the P waves travelling in the direction of their movement generally pass at a reduced velocity.  The long L waves do not pass and do not go deeper in the earth.  The S waves cannot pass through a liquid and are transmitted only through a rigid solid body.  The velocity of the 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 solid iron core and the outer core has probably the properties of a liquid.  It may be concluded that as a whole behaves like a solid even when the parts of its interior are reported to be in a plastic or semi- liquid state.  The rock samples from various depths have also provided the evidence of the density of the materials. 

    The Meteorites belonging to our solar system are another source of our information for a better understanding of the earth’s structure.  The meteors outer layered burnt during their fall to the earth, as the stony material of the meteorites are similar to those found on the earth’s surface, these are scarcely recognized.  But the composition of the heavier materials confirms the similar composition of inner core of the earth. 

Temperature and Pressure:  The evidence of the volcanic eruptions and the hot springs indicate that high temperatures prevail in the interior of the earth.  A progressive rise in the temperature with the increasing depth is recorded in mines and deep wells all over the world, the average rate of the increase being 1 degree for every 32 meters of descent.  Normally, at this rate of increase in temperature, the rocks at great depths should be in a liquid state.  At one time, on the basis of this view, it was accepted that thin solid crust of the earth was perhaps resting on a molten core.  But the behavior of the earthquake waves as hinted above has led us to revise our views.  The rate of increase in temperature is now considered to be 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 100kms, the increase is estimated at 12 degrees per km.  it is 2 degree centigrade per km in the next 300km and 1 degree per km below it.  By this calculation the temperature is actually 2000 degree centigrade at the core of the earth.  The heat or rise in temperature is the result of internal forces, automatic disintegration of radioactive substances, chemical reaction and other sources keeping the earth’s interior hot.  It indicates the liquid or perhaps the gaseous conditions prevailing at the 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 a 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 temperatures prevailing there.  While solid material melts inside the earth, the liquid also takes up the properties of a solid as alluded above.

DENSITY: The velocity and the path followed by earthquake waves, temperature and pressure conditions inside the earth tell us of varying physical properties, density and composition prevailing there.  The structure of earth’s interior is therefore layered.  The arrangement of layers is comparable to onion with its shells, one inside another.  The inside of the earth has a shallow crust as thin as an eggshell which can be compared with the size of a ball.  Below the surface capped by the sedimentary material, upper layer of the crust is mainly composed of crystalline igneous and metamorphic rocks, acid in nature.  The lower layer of the crust has basaltic and ultra basic rocks.  The layer of heavier or inner silicate is not found beneath oceans.  The oceans are mostly under laid by a thick greenish and tremendously hot layer.  The continents are composed of silicates termed as SIMA or silica and Aluminium.  The oceans have heavier silicates named SIMA or silica and 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 and Ferrous.  A zone of mixed heavy metals and silicates separates the core from the other layers.

Based on the chemical composition of the earth’s interior, different scientists have given different views about the interior of the earth.

1.       Edward Suess has identified three zones of different matter below the outer thin sedimentary layer.

SIAL : It is located just below the outer sedimentary cover.  It composed of Granite.  This layer is dominated by Silica and Aluminium – SIAL.  The average density is 2.9 whereas thickness ranges between 50 – 300 kms.  Continents have been formed by sialic layer.

SIMA : It is located just below the sialic layer.  This layer composed of basalt and is the source of Magma and Lava during the volcanic eruptions.  Silica and Magnesium are the dominating materials in this layer.  Average density of this layer is 2.9 to 4.7 whereas the thickness varies from 1000km to 2000km.

NIFE :  It is located just below the SIMA layer.  This layer is composed of Nickel and Ferrium.  It is thus apparent that this layer made of heavy metals which are responsible for very high density.  The diameter of this zone is 6880 km.  The presence of the Iron (ferrium) indicates the magnetic property of the earth’s interior.  This property also indicates the rigidity if the earth.

2.       Van Der Gracht has identified 4 layer system of the composition of the earth.

Outer sialic crust : This layer varies in thickness, under the continents it is 60 km and under the oceans it is 20km esp Atlantic ocean, completely absent under the Pacific ocean. Density of this layer is between 2.75 to 2.9

Inner silicate mantle : it extends between 16km to 1140 km and has the density of 3.1 to 4.75.

Zone of mixed metals and silicates : the thickness of this layer from 1140km to 2900km where as the density of this layer varies between 4.75 to 5.0

Metallic nucleus : This layer thickness is from 2900km to 6371km and density is 11.0.

It appears that there are different opinions about the number, thickness and density of the layers.  In order to avoid the confusion a commonly accepted view has been brought by majority of the scientists.

     The scientific study and the analysis of various aspects of seismic waves – mainly its velocity and path of the waves, also the earthquakes have enabled the scientists to unravel the mystery of the earth’s interior.  Three zones of varying properties have been identified in the earth e.g. crust, mantle and the core.

CRUST:  The very first layer of the earth is the Crust, it extends up to a depth of 50 km on an average.  It is divided into two the Oceanic crust and the Continental crust, the oceanic crust is denser as it is made up of basalt rock which is denser than the granitic rock, with which the continental crust is made of.  The thickness of the crust is more under the continents the thickness is less under the oceans.  This crust based upon the depth is again divided into two, outer and lower crust.  The average density of the outer and lower crust is 2.8 and 3.0 respectively.  This difference in the density is because of the Pressure of superincumbent load.  The formation of the minerals of the upper crust was accomplished at relatively lower pressure than the minerals of the lower crust.

MANTLE: It is the second layer just below the crust, it extends up to a depth of 2900 km inside the earth.  This mantle is divided initially into two zones based on the changes in the velocity of seismic waves and density, the upper mantle and the lower mantle.  The upper mantle starts from 50 km and extends up to 1000km and the lower mantle from 1000km to 2900km but now the mantle is divided in to three based on the information received from the discovery of International Union of Geodesy and Geophysics.  (i) first zone extending from the crust to 200km depth. (ii) Second zone extends from 200km to 700km and  (iii) third zone extends from 700km to 2900km depth.  The velocity of the seismic waves relatively slows down in the upper most zone of the mantle for a depth of 100 to 200 km.  This zone is called the zone of low velocity, i.e the rocks here are in the semi molten or Gel like form, this layer is also called as Asthenosphere.   There is a sudden increase in the velocity of the seismic waves at the base of the lower crust and the upper mantle this trend of seismic waves denotes discontinuity.  This discontinuity was discovered by Mohorovicic in 1909 and thus it is called as Moho discontinuity. 

CORE:  The core, the deepest and the most inaccessible zone of the earth, extends from the lower boundary of the mantle at the depth of 2900km to the centre of the earth.  The mantle  - core boundary is determined by the “Weichert – Gutenberg Discontinuity” at the depth of 2900km there is a pronounced change in the density of the core from 5.5 to 10.0, this sudden change in the density is indicated by sudden increase in the velocity of P waves along the mantle core boundary i.e the Gutenberg discontinuity.  The density further increases from 10.0 to 13.0 with the increasing depth of the core.  Thus it appears that the density of the core is twice the density of the mantle.  The core is further divided into two zones the outer and the inner core, the dividing line being at the depth of 5150km.  S, waves disappears in this outer core indicating it as molten state.  The next layer is the inner core which extends from 5150km to 6371km i.e the centre of the earth.  The density of this layer is 13.0 where the P waves travel at greater velocity than that of the outer core indicating this layer as  a solid zone.  It is believed that the core is composed of Iron and Nickel and so it the most density part in the interior of the earth.