Essay on the Earth: Top 8 Essays on Earth |Solar System | Geography

Here is a compilation of essays on ‘Earth’ for class 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Earth’ especially written for school and college students.

Essay on the Earth

Essay Contents:

1. Essay on the Origin of Earth
2. Essay on the Geological History of Earth
3. Essay on the Evolution of Earth
4. Essay on the Chemical Composition of Earth
5. Essay on the Role of Ice in Earth’s Surface
6. Essay on the Interior of the Earth
7. Essay on the Different Zones of Earth
8. Essay on the Depth of Different Layers of Earth

Essay # 1. Origin of the Earth:

Unlike monistic concept (e.g. gaseous hypoth­esis of Immanuel Kant and nebular hypothesis of Laplace) the planetesimal hypothesis envisaged the origin of the solar system (and the earth) with the help of two heavenly bodies. According to Chamberlin initially there were two heavenly bodies (stars) in the universe-proto-sun and its companion star.

The behav­iour and properties of the proto-sun were not like other stars. It was formed of very small particles which were cold and solid. Thus, the proto-sun, unlike Laplace’s nebula, was not hot and gaseous rather it was formed of solid particles and was cold and circular in shape.

There was another star, termed as ‘intruding star’ or ‘companion star’ which was destined to pass very close to the proto-sun. When the intruding star came very close to the proto-sun infinite number of small parti­cles were detached from the outer surface of the proto- sun due to massive gravitational pull exerted by the giant intruding star. Chamberlin termed these detached small particles as planetesimals.

Initially, the detached particles or planetesimals were just like dust particles. The planetesimals were not of uniform size rather a few planetesimals around the proto-sun were of fairly big size. These larger planetesimals became nuclei for the formation of fu­ture possible planets. Gradually, large planetesimals started attracting small planetesimals.

Thus, numerous small planetesimals were accreted (added) to the nu­clei of large planetesimals and ultimately these large planetesimals grew in the form of planets due to continuous accretion of infinite number of planetsimals.

With the passage of time, the remaining proto-sun changed into the present-day sun. The satellites of the planets were created due to the repetition of the same processes and mechanisms.

According to the planetesimal theory the main force of the ejection of small jets or planetesimals from the proto-sun was the tidal force exerted by the ap­proaching or intruding star on the outer surface of the proto-sun. “It was not necessary to assume that the earth as a whole was ever in molten condition. It grew from small beginnings by the addition of planetesimal matter, rapidly at first, but with decreasing speed”.

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Essay # 2. Geological History of the Earth:

The geological history of the earth or the ‘geologi­cal clock’ refers to the reconstruction of evolutionary sequence of the geological events involving the infor­mation of various zones (crust, mantle and core) of the earth, formation and evolution of geomaterials (rocks), formation and development of mountains and faults, evolution of different lives etc.

The whole geological history right from the origin of the earth to its present form has been divided into major and minor periods on the basis of forms of life (organic remains), character­istic rock deposits, places of rock formation, major tectonic events etc.

The whole geological history of the earth has been divided into five eras (the largest time division of the earth’s history has been termed Era) based on five major groups of deposits as follows:

Each era is numbered in sequence as first (pri­mary), second (secondary), third (tertiary) and fourth (quaternary) and is named period. Further, each pe­riod is divided into several epochs.

The names of epochs have been assigned on various grounds e.g. names of the places of characteristic systems of deposits, the names of tribes, the characteristics of deposits, dominance of certain elements and minerals etc. as follows:

Some scientists have put together all the geologi­cal events of the past history of the earth in the form of a clock. Thus, the spiral system representing the whole geological and geomorphic history together is called as ‘geological clock’ wherein one billion years represent each revolution of the clock’s arm.

Each revolution is further subdivided into ‘hours’ where each division (hour) corresponds to 100 million years and ‘minutes’ represent the time period of 10 million years. F.g. 3.1 represents the geological clock suggested by Frank Press and Raymond Siever (1974).

Precambrian Period:

The Precambrian period started 700 million year B.P. (before present). The earth changed from gaseous state to liquid state. The solid outer crust was formed due to cooling and solidification of liquid materials. This phase was followed by the formation of dense atmosphere surrounding the earth.

Due to gradual but continued cooling and contraction of the earth and resultant condensation of water vapour there began the precipitation process which ultimately resulted into the formation of rivers and seas.

The sequence of warm climate was broken by many glacial periods. The rocks were subjected to maximum metamorphism due to heat and pressure. Among the plant kingdom only marine grasses were evolved. Soft bodied invertebrate animals were evolved in warm seas but the land areas were devoid of animals.

Cambrian Period:

The Cambrian Period lasted from 600 million years B .P. to 500 million years B.P. The earth’s surface was characterized by shallow seas and the land areas were frequented by transgression and regression of seas. Cambrian rocks of Wales, north-west Scottland, western England, Canada and USA were formed dur­ing this period. Europe was characterized by vulcanicity but no trace of any mountain building could be found.

The earth’s surface was characterized by warm and uniform climate.

Evolution of plants was still confined to the seas only. Most of the vertebrate animals includ­ing 1000 species were evolved in the seas but they are not found at the present time. These animals depended on marine grasses for their food. No land animals could evolve during this period.

Ordovician Period:

The Ordovician Period continued from 500 mil­lion years B.P. to 440 million years B.P. The transgressional and regressional phases of seas contin­ued. Many shallow seas became dry because of depo­sition of sand and mud. Ordovician rocks were formed in north-west Europe and North America. This period was characterized by the initiation of mountain build­ing. Marine areas were affected by active vulcanicity.

The climate on the entire earth’s surface was warm and hence no zonal pattern of climate was evolved. Vegeta­tion and animals were still confined to the seas only. Animals species included only vertebrates.

Silurian Period:

The Silurian Period spread from 440 million to 400 million years B.P. Sea level was characterized by periodic rise and fall which introduced changes on earth’s crust. The mountain building continued but the vulcanicity was less active than during ordovician period. On an average the climate was warm but some areas were also characterized by relatively dry climate.

Leafless plants were evolved on the land areas. The remains of such vegetation have been found in Aus­tralia. There was increase in the species of vertebrate animals of marine environment. The plant community was diversified because of evolution of new species. Corals were evolved at large scale. Plants were evolved for the first time on land areas.

Devonian Period:

The Devonian Period continued from 400 to 350 million years B.P. and experienced increase in land and decrease in marine areas. Mountain building and vulcanicity were more active. The newly formed moun­tains were subjected to denudation and eroded materi­als were deposited as pebbles, sands and red sandstones. Most of the areas of North-West Europe and North America were characterized by warm and semi-arid climate.

The remaining areas were dominated by uniform climate. The earth’s surface was covered with green vegetation as the plants developed their leaves, branches, stems and roots. The vegetation comprised of small shrubs to tall trees measuring 14-15 m in height. Fern vegetation was evolved by the end of this period. Marine vertebrate animals were again evolved.

This period was characterized by the evolution of a large number of species of fish. Amphibians were evolved towards the end of this period. There was dispersal of vertebrate animals (mites, spiders etc.) from seas to the land areas due to evolution of such flora on land areas which could provide them food.

Carboniferous Period:

The carboniferous period spread from 350 million to 270 million years B.P. and was characterized by numerous small and shallow seas on the earth’s sur­face. Most of the areas of Europe including Russia were submerged under water. Some land areas in North America and Europe were depressed and thus were covered with water giving birth to swamps. The coal formation of northern hemisphere was accom­plished during this period.

Dry climate continued for most period but some areas were characterized by warm and wet climate which became responsible for dense vegetation cover over such areas. Land areas were covered with green tall trees measuring more than 30m in height. The number and species of am­phibious animals continued to increase in water areas. Reptiles were evolved on land areas. Smaller animals were evolved in swamps and marshes but their length increased up to 4-5m by the end of this period.

Permian Period:

The Permian period continued from 270 million to 225 million years B.P. Inland lakes were formed due to faulting. The evaporation of these lakes resulted into the formation of major potash reserves of the world. High mountains were formed due to great tectonic movements in Europe, Asia and eastern North America (Applachians).

Different climatic conditions prevailed over the earth’s surface. British Isles were character­ized by semi-arid climate. The most parts of the north­ern hemisphere were dominated by dry climate but were periodically frequented by warm and wet cli­mate. Most parts of the southern hemisphere were under the influence of glacial period.

With increasing seasonal variations in the climatic conditions the ratio of evergreen trees continued to decrease. Consequently, deciduous trees, which could resist dry weather and frost, were evolved. The number and species of land animals further increased and numerous species of insects were evolved on land areas.

Triassic Period:

The Triassic Period continued from 225 million to 180 million years B.P. Most of the mountains were covered with deserts and bushes. The entire Britain was covered with saline lakes which were surrounded by desert areas. Marl and sandstones were formed in warm seas. Warm and dry climate was dominant over entire areas but the climate became wet by the end of this period.

Consequently, coniferous trees and ferns were developed in the northern hemisphere. Carnivore fishes like reptiles and lobsters were evolved in seas. The land areas were still dominated by reptiles. For the first time, mammals evolved from reptiles on land areas. Flies and termites also appeared on the land.

Jurassic Period:

The Jurassic Period spread from 180 million to 135 million years B.P. and was characterized by re-extension in marine areas. Land areas were domi­nated by forests and swampy plains having lakes and meandering rivers. The mountains were transformed into low hills due to continued denudation. Major areas of Asia and Europe and surrounding areas of Great Britain were submerged under sea water.

This period is characterized by widespread deposition of lime mostly in France, southern Germany, Switzerland etc. The climate became subtropical towards the end of this period. The rainfall was such that dense vegetation could be evolved and developed in many areas. For the first time, flowering plants were evolved during this period.

Essay # 3. Evolution of the Earth:

T.C. Chamberlin has attempted to describe and explain the evolution of different components of the earth e.g., continents and ocean basins, folds and faults, volcanoes and earthquakes, mountains and plains, heat of the interior of the earth and its structure.

The origin and evolution of its atmosphere through specific periods or stages:

(1) First stage-‘the period of planetesimal accession’ or ‘the period of acquisition of the present shape and size by the earth’.

(2) Second stage—”the period of dominant vulcanism’ or ‘the period of the evolution of the earth’s interior and the evolution of continents and ocean basins’.

(3) Third stage—’the actual geological period’ or ‘the period of the formation of the folds and faults, mountains and plateaux etc.’

It may be pointed out that these stages of the evolution of the earth are separated from each other only for the sake of convenience; other-wise these are so interlinked with each other that it is quite difficult to differentiate one stage from the other.

(i) Period of Planetesimal Accession:

As the solar tides caused by the gravitational force of the intruding star became greater and greater, huge quantity of planetesimals (jets) were thrown out of the surface of the proto-sun and these planetesimals began to whirl. Some planetesimals followed the intruding star which ultimately vanished in the space while others were attracted by proto-sun and thus started moving around it.

The great solar tide subsided when the intruding star moved away from the proto-sun and ultimately van­ished in the space. Gradual accretion of smaller planetesimals around larger planetesimals ultimately gave birth to the planets and the earth. The mechanism of the accretion of planetesimals has already been described above.

Evolution of the earth’s atmosphere:

Chamberlin maintained that in the initial stage of the origin of the earth there was no atmosphere on it but as the earth grew in size it captured ‘atmospheric material and elements’ by gravitational force which was continu­ously increasing due to ever increasing size of the earth.

The earth’s atmosphere was formed from two basic sources:

(1) External Source:

When the earth grew in size it became successful in capturing free atmospheric molecules. The supply of atmospheric molecules was more but it decreased with the passage of time as most of the molecules were already captured by the earth.

(2) Internal Sources:

Internal sources provided carbon dioxide, water vapour and nitrogen gases. Another source of the ‘atmospheric material’ was occluded gases carried by the planetesimals captured by the ‘nucleus’ of the earth. These occluded gas particles came out of the interior of the earth through volcanic and fissure eruptions and became part and parcel of the present day atmosphere. Oxygen was, thus, provided by the volcanic eruption.

(ii) Period of Dominant Vulcanism:

Gradual accu­mulation of heat inside the earth during its early stage of evolution resulted into selective melting of rocks in the outer parts of the earth and thus began widespread volcanic activity. It may be pointed out that in the initial stage of the evolution of the earth; its surface was very much rough and fragmented due to ‘infalling’ of planetesimals.

There were huge crevices between the planetesimals. The fragments of the earth’s surface were also not well cemented. The-escape of volatile substances of the interior of the earth resulted into violent volcanic explosions which created ‘crater-like hollows’ on the surface of the earth.

Evolution of continents and ocean basins:

Chamberlin opined that the primitive oceans were first formed under the fragmented and crevice-ridden outer permeable zone of the earth’s surface. Later on the crevices were cemented and thus water derived through the condensation of water vapour accumulated in these crevices and volcanic craters and the earth’s surface, thus, looked as if filled with numerous lakes.

Gradually and gradually these lakes were connected due to their expanding areal extents and thus different oceans were formed. Basic materials were weathered and eroded and were ultimately carried away by running water from the upstanding land masses (continents) and were deposited in the submerged areas of the earth (oceans).

Thus, there was gradual increase in the acidic material of the land- masses because most of the basic material was removed in solution form from the landmasses. This caused reduction of the specific gravity of the continental material. In other words, the weight of continental material started decreasing whereas there was increase in the weight of oceanic material.

This caused further submergence of the low-lying parts of the continents. Continuous deposition of weathered and eroded debris and the weight of the water itself further depressed the submerged parts of the earth (oceans). This process caused further extension of the oceans. According to J.A. Steers as long as the earth as a whole continued appreciably to grow by the accession of the planetesimals, the oceanic regions expanded and deepened.’

(iii) Actual geological period:

The final stage of the evolution of the earth was characterized by domi­nant tectonic events including dominant vulcanicity, folding and faulting and submergence and emergence and thus the ancient surface features of the earth’s surface were formed.

Evaluation:

No doubt, Chamberlin made a sincere effort through his ‘planetesimal hypothesis’ to solve the riddle of the origin of the earth, the structure of its interior, the evolution of continents and ocean basins and the origin of mountains and faults but he commit­ted certain mistakes while doing so because of the fact that he attempted to paint a very large canvas (several problems of the earth) with a single stroke of a brush.

Had he concentrated on the single problem of the origin of the earth he might not have left a long fissure of loop-holes unplugged.

Essay # 4. Chemical Composition of the Earth:

E. Suess has thrown light on the chemical com­position of the earth’s interior. The crust is covered by a thin layer of sedimentary rocks of very low density. This layer is composed of crystalline rocks, mostly silicate matter. The dominant minerals are felspar and mica. The upper part of this layer is composed of light silicate matter while heavy silicate matter dominates in the lower part.

Suess has identified three zones of different matter below the outer thin sedimentary cover:

(i) SIAL located just below the outer sedimen­tary cover is composed of granites. This layer is domi­nated by silica and aluminium (SIAL = SI + AL). The average density of this layer is 2.9 whereas its thick­ness ranges between 50 to 300 km. This layer is dominated by acid materials and silicates of potas­sium, sodium and aluminium are abundantly found. Continents have been formed by sialic layer.

(ii) SIMA is located just below the sialic layer. This layer is composed of basalt and is the source of magma and lava during volcanic eruptions. Silica (sisilica+ma-magnesium) and magnesium are the domi­nant constituents. Average density ranges between 2.9. to 4.7 whereas the thickness varies from 1,000 km to 2,000 km. There is abundance of basic matter. The silicates of magnesium, calcium and iron are most abundantly found.

(iii) NIFE is located just below the ‘sima’ layer. This layer is composed of nickel (NI) and ferrium (Fe). It is, thus, apparent that this layer is made of heavy metals which are responsible for very high density (11) of this layer. The diameter of this zone is 6880 km. The presence of iron (ferrium) indicates the magnetic property of the earth’s interior. This property also indicates the rigidity of the earth (fig. 4.3).

Essay # 5. Role of Ice in Earth’s Surface:

Why the earth’s surface was not covered with ice in its childhood in spite of the fact that sun’s rays were much fainter in its fragile beginning than today?

This ‘faint early sun paradox’ was formulated by astronomer Carl Sugan and his colleague George Mullen in 1972.

This paradox consisted of the following:

i. The earth’s age ranging between 4 to 4.5 billion years,

ii. The earth’s climate has been fairly consistent during its 4 billion out of 4.5 billion years age inspite of the fact that solar radiation increased by 25-30 per cent.

Then paradoxical question arises:

Why the earth’s surface in its childhood was not covered with ice?

Explanation:

1. JAIM KASTING’S Explanation:

American atmospheric scientist Jim Kasting pre­sented his explanation in 1993. According to him 4billion years ago 30 percent of the earth’s atmosphere consisted of greenhouse gas carbon dioxide (CO₂,). Thus CO₂ formed a protective greenhouse gas thick layer around the earth. This resulted in the warming of ocean surface which in turn prevented the earth’s surface from freezing.

2. MINIK ROSTING’S Explanation:

Minik Rosting of Natural History Museum of Denmark propounded that it was not high concentration of CO₂ in the atmosphere in the earth’s early history which prevented the earth’s early surface from being covered which ice layers but it was thin cloud cover which pre­vented early ice age on the basis of the following facts:

1. Cloud layer in the earth’s childhood was much thinner than today, and

2. Most of the earth’s surface was covered with water with the result oceans were warmed unin­terruptedly and hence water surface of the earth could not be frozen.

3. Paradox of faint sun and ice-free oceans solved:

The team of Carl Sugan, George Mullen and Minik Rosting analysed the samples of 3.8 billion year- old mountain rocks from world’s oldest bedrocks of Isua in western Greenland to solve the said paradox. The an­alysis revealed the maximum concentration of one part per thousand of CO₂, in the childhood of the earth. Thus, this CO₂, concentration was only 3 to 4 times more than the present CO₂, concentration in the atmosphere.

Thus they refuted the earlier finding of 30 percent share of CO₂, in the composition of the atmosphere. They finally concluded that the concentration of CO₂, continent in the atmosphere has not changed substantially through billions of geological history of the earth.

It was thin cloud cover and dominance of water surface on the early earth’s surface which prevented the earth’s surface from being frozen in the beginning of the earth’s geological history.

Essay # 6. Interior of the Earth:

It has been possible to have an idea about the interior of the earth through the study of earthquake or seismic wave records. Many sources of evidence also indicate that the interior of the earth has a varied structure, consisting of some what concentric shells differing in chemical composition, density, elasticity and also in state (solid, liquid or gaseous).

Study of seismic waves has indicated that the earth consists of some zones. As the seismic waves travel from one zone to the other, they are subjected to velocity changes depending on the material of the zone through which they pass. These seismic waves get reflected and refracted as they cross the boundaries between them. Such boundaries where the waves are so reflected or refracted are called discontinuities.

The specific gravity of the material of the earth also differs from a value of 2.5 or 3 for the surface rocks to a high value at great depths. The shape of the earth shows an equatorial bulge caused due to its rotation and this is indicative of the presence of very heavy central core.

Observations of changes in the precession, i.e. the angle through which the tilt of the earth’s axis changes also indicates the presence of a heavy central core. Presence of tides caused by gravitational pull of the sun and the moon also indicates the existence of a central heavy core.

Essay # 7. Different Zones of the Earth:

The different zones of the earth are as follows:

(i) Barysphere:

Represents the innermost zone of the interior of the earth and extends from 2800 km. depth to the nucleus of the core. The average density ranges between 8 and 11.

(ii) Core:

Is the deepest and most inaccessible zone of the interior of the earth and extends from the lower boundary of the mantle at the depth of 2900 km. to the center of the earth i.e., up to 6371 km. The density increases from 10 at the mantle-core boundary down word to 11.6 at the center of the core. The core has two subzones i.e. outer core (from 2900 km. to 5150 km. depth) and inner core (from 5150 to 6371 km. depth).

(ii) Crust:

Is the outermost layer (zone) of the earth with average density of 2.8 to 3.0 and average thickness of 30 km.

Density:

Refers to the amount of mass per unit volume of substance, usually measured in gram per cubic centimeter (g/cm³).

(iii) Lithosphere:

Literally means rocksphere (lithos means rock) which represents the solid portion of the continents having a thickness of about 100 km. and average density of 3.5. It is composed of mostly silicate min­erals.

(iv) Mantle:

Represents the second zone of the interior of the earth and extends from 30 km. to 2900 km. depth. The lower crust and upper mantle is separated by Moho-discontinuity.

Mantle is divided into 2 subzones i.e.:

i. upper mantle (200 km. to 700 km. depth), and

ii. lower mantle (700 km. to 2900km. depth).

Mantle is formed of silicate minerals.

Essay # 8. Depth of Different Layers of the Earth:

According to Daly:

Daly has recognized three layers of different density in the earth:

(i) Outer zone is composed of silicates. Average density is 3.0 and the thickness is 1,600 km.

(ii) Intermediate layer is composed of the mixture of iron and silicates. Average density is from 4.5 to 9 and the thickness is 1,280 km.

(iii) Central zone is made of iron and is in solid state. Average density and diameter are 11.6 and 7,040 km respectively.

According to Harold Jeffreys:

Jeffreys has identified, on the basis of the study of seismic waves, four layers in the earth e.g.:

(i) Outer layer of sedimentary rocks,

(ii) Second layer of gran­ites,

(iii) Third layer of thachylyte or diorite and

(iv) Fourth layer of dunite, peridotite or eclogite.

According to Holmes:

Arthur Holmes has recognized two major layers in the earth. The upper layer is termed as crust which is composed of whole of Suess’ sialic layer and upper portion of ‘sima’. The lower layer has been named by Holmes as substratum which represents lower portion of Suess’ sima.

Holmes has determined the thickness of sial below the continental surface on the basis of different sources and evidences as given below:

(i) On the basis of thermal conditions – 20 km or less.

(ii) On the basis of surface seismic waves (L waves) – 15 km or more.

(iii) On the basis of longitudinal waves – 20- 30 km.

(iv) On the basis of subsidence of the deepest geosynclines- 20 km or more.

According to Van Der Gracht:

Van der Gracht has identified 4 – layer system of the composition of the interior of the earth.

He has summarized the various properties of the earth’s inte­rior in the following manner:

It appears from the foregoing discussion that there is difference of opinions about the number, thickness and various properties of the layers of the earth.

In order to avoid confusion the following gener­alized pattern of the layering system of the earth’s interior is commonly accepted by majority of the scientists:

(i) Lithosphere with a thickness of about 100 km is mostly composed of granites. Silica and aluminium are dominant constituents. Average density is 3.5.

(ii) Pyrosphere stretches for a thickness of 2780 km having an average density of 5.6. The dominant rock is basalt.

(iii) Barysphere is composed of iron and nickel. Average density ranges between 8 and 11 and this layer stretches from 2800 km to the nucleus of the core.

Recent Views:

The aforesaid views about the composition and structure of the earth’s interior have now become obsolete. The scientific study and analysis of various aspects of seismic waves (mainly velocity and travel paths) of natural and man-induced earthquakes have enabled the scientists to unravel the mystery of the earth’s interior based on authentic information.

Three zones of varying properties have been identified in the earth on the basis of changes in the velocity of seismic waves while passing through the earth (fig. 4.4) e.g. crust, mantle and core. It may be pointed out that there is still difference of opinions about the thickness of these zones, mainly about the thickness of the crust. Various sources put the thickness of the crust between 30 km and 100 km.

On the basis of the change in the velocity of seismic waves crust is further divided into:

(i) upper crust and

(ii) lower crust because the velocity of P waves suddenly increases in the lower crust.

For example, the average velocity of P waves in the upper crust is 6.1 km per second while it becomes 6.9 km per second in the lower crust, Fig. 4.4 depicts the different velocities of P and S waves in different parts of the earth and the relationship between velocities of seismic waves and different zones of the earth.

Crust:

The average density of the outer and lower crust is 2.8 and 3.0 respectively. It may be pointed out that in the beginning vast difference be­tween the structure and composition of upper and lower crust was reported by the scientists but now the evidences of seismology have revealed almost identi­cal structure and composition of these two sub-zones of the crust.

The difference of density between the upper (2.8) and lower crust (3.0) 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:

There is sudden increase in the velocity of seismic waves at the base of lower crust as the velocity of seismic waves is about 6.9 km per second at the base of lower crust but it suddenly becomes 7.9 to 8.1 km per second. This trend of seismic waves denotes dis­continuity between the boundaries of lower crust and upper mantle.

This discontinuity was discovered by A. Mohorovicic in the year 1909 and thus it is called as ‘Mohorovicic Discontinuity’, or simply ‘Moho Dis­continuity.’ The mantle having mean density of 4.6 g cm^-3 extends for a depth of 2900 km inside the earth. 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 per cent of the total mass of the earth.

Previously the mantle was divided into two zones on the basis of changes in the velocities of seismic waves and density e.g.:

(i) upper mantle from Moho Discontinuity to the depth of 1000 km and

(ii) lower mantle from 1000 km to 2900 km depth but now the mantle is divided on the basis of the information received from the discovery of the International Union of Geodesy and Geophysics into 3 sub-zones e.g.:

(a) first zone extending from Moho Discontinuity to 200 km depth,

(b) second zone extending from 200 km to 700 km depth and

(c) third zone extending from 700 km to 2900 km depth.

The velocity of seismic waves relatively slows down in the uppermost zone of the upper mantle for a depth of 100 to 200 km (7.8 km per second). This zone is called the zone of low velocity. Mantle is believed to have been formed largely of silicate minerals rich in iron and magnesium.

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 (upto 6371 km). The mantle-core boundary is determined by the ‘Weichert-Gutenberg Discontinuity’ at the depth of 2900 km. It is significant to note that there is pronounced change of density form 5.5 g cm^-3 to 10.0g cm^-3 along the Gutenberg Discontinuity.

This sudden change in density is indicated by sudden increase in the velocity of P waves (13.6 km per second) along the mantle-core boundary or Gutenberg Discontinuity. The density further increases from 12.3 to 13.3 and 13.6 with increasing depth of the core. It, thus, appears that the density of the core is more than twice the density of the mantle but the volume and mass of the core are 16 per cent and 32 per cent of the total volume and mass of the earth respectively.

The core is further divided into two sub-zones e.g. outer core and inner core, the dividing line being at the depth of 5150 km. S waves disappear in this outer core. This means that the outer core should be in molten state. The inner core extends from the depth of 5150 km to the centre of the earth (6371 km).

This lowermost zone of the interior of the earth is in solid state, the density of which is 13.3 to 13.6. P waves travel through this zone with the speed of 11.23 km per second. It is generally believed that the core is com­posed of iron and nickel but according to the second view point the core may be formed of silicates.

It is also believed that after disintegration on high pressure the electronic structures have changed into heavy metallic materials, thus the density of the core has increased. According to the third view point initially the core was composed of hydrogen but later on hydrogen was transformed into metallic materials due to excessive pressure (over 3 million atmospheres).

This possibility is questioned on the ground that though the transfor­mation of silicate or hydrogen due to very high pres­sure in the core may be believed tentatively but this process cannot increase the density of the core as high as it is at present.

For example, the planet Mercury is smallest of all the planets of our solar system. It may be argued that least compression and pressure cannot generate highest density in the core of Mercury. Most of present-day geophysicists and geochemists believe that the core is made of metallic materials mainly iron and nickel.