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Evolution of the Earth

Study Material > Geography
  • In earlier chapter we have studied about the origin of earth in this chapter we will study about the evolution of the earth.
  • Initially earth’s surface and atmosphere was totally different from present. Earth was a rocky, barren and hot planet with no traces of life.
  • Initially earth’s atmosphere contains hydrogen and helium. Therefore there must happened some episode on the earth which has changed the earth and made it favourable for the life.
  • Earth is having a different layer having different composition from outermost atmosphere to the core of the earth. We will discuss all the layer of earth one by one.
  • In this chapter we will discuss evolution of lithosphere and its layer.
  • Origin of earth takes place from gaseous state to rocky and metallic planets with high density. This journey of earth from gaseous state to rocky and metallic state has much repercussion.
  • With the increase in density of the earth pressure inside the earth also increases which in turn increases the inside temperature.
  • With the increase in temperature, interior of the earth changes to molten state and materials are getting separated from each other density wise.
  • Heavier materials like iron sink towards earth’s centre and become part of earth’s core whereas lighter material moves upward forming different layers according to their densities.
  • Scientists also said that the giant impact of earth during moon formation has also produced heat which further continued the process of differentiation.
  • Hence the earth is having a different layer from surface to core and density of these layer increases from surface towards the core.
  • Studying about interior of the earth is very difficult and hence knowledge about the interior of the earth is limited and is built mainly on indirect evidences though few direct sources are also available.
  • Direct Sources for studying interior of the earth:
    1. Study from surface rocks.
    2. Rocks obtained from mining. In this case one cannot go beyond certain depth because temperature increases with increasing depth and after certain point it is not possible to resist such a high temperature.
    3. Drilling project to explore interior of the earth by the scientists are another source of obtaining evidences about interior of the earth.
    4. Two major projects “Deep Ocean Drilling Project” and “integrated Ocean Drilling Project” have provided valuable information through the studying of materials extracted from different depths.
    5. Deepest drill of 12 km is made at Kola, in Arctic Ocean.
    6. Volcanic eruption also provides a direct source of analysis by studying the molten material which comes out during volcanic eruption. However in this case analysis of depth is difficult.
  • Indirect sources for studying about interior of the earth are:
    1. Through mining activities it has been found that temperature and pressure increases with increasing depth. Moreover density of the material also increases with increase in depth. So by calculating rate of change of these properties a rough estimation can be made about temperature, pressure and density of material at different interior layers of the earth.
    2. Study of meteorites is yet another source of information as meteorites and earth both formed with the same process and at the same time.
    3. Gravitational force, magnetic field and seismic activity are another indirect sources about study of interior of the earth.
      1. Gravity anamoly: It has been found that gravity at pole is different than the gravity at equator. This difference is called as “gravity anomaly”. Gravity value differs due to different factor and difference in mass of the material is one of those factors. Hence through gravity anomaly distribution of the mass of material in the crust of the earth can be analysed.
      2. Magnetic Field: Through magnetic survey we can analyse the distribution of magnetic material in the earth’s crust.
      3. Seismic Activity: Seismic activity is one of the most important source as it provides a complete picture of the interior of the earth. We will discuss it in details in later chapter.
  • Majority of findings by different scientists have an opinion that the earth is having a concentric layers lying one above another.
  • According to the chemical composition interior structure of the Earth is divided in to three broad layers:
    1. Crust
    2. Mantle
      1. Upper mantle
      2. Lower Mantle
    3. Core
      1. Outer Core
      2. Inner Core
  • The Crust: It is the outermost and thinnest layer among all the three layers of the earth. Its average thickness varies between 8 to 40 km. Crust is further divided into:
    1. Continental Crust.
    2. Oceanic Crust.

Continental Crust

Oceanic Crust

Mean thickness of continental crust is about 30 km however its thickness increases in the major Mountain system of the world. It is about 70 km thicker in Himalayan region.

Mean thickness of oceanic crust is about 5 km

It is less dense than oceanic crust

It is more dense than continental crust

It is a layer of igneous, sedimentary and metamorphic rocks

It is mainly made up of igneous rocks rich in iron and magnesium. Ex basalt.

Composition of continental crust is richer in silicon and aluminium hence it is also called SIAL.

Composition of oceanic crust is richer in silicon and magnesium hence it is also called SIMA

Mean density of this layer is about 2.7 g/cm3

Mean density of this layer is about 3 g/cm3
  • Mantle:
    1. Beneath crust next portion is called mantle which extends up to a depth of 2900 km.
    2. Composition of this layer is richer in Silica and Magnesium; hence it is also called
    3. Crust and uppermost part of the mantle together form Lithosphere. Thickness of lithosphere is about 10-200 km.
    4. In the upper mantle, between 100 km to 200 km depth scientists found that the velocity of seismic waves is dramatically slowed down. This zone is named as Zone of Low Velocity.
    5. Mantle occupies about 83 % of total volume of the earth however it occupies only 68 % of the total mass of the earth.
    6. It is further divided into upper mantle and lower mantle.
      1. Upper Mantle: Upper mantle is known as asthenosphere which is in semi solid state. It extends up to a depth of about 400 km and is the main source of magma which comes out to the surface during volcanic eruption. It is denser than crust and is having a density of 3.4 g/cm3.
      2. Lower mantle: Beneath the upper mantle lies the lower mantle. Lower mantle is in solid state and having a density of about 4.4 g/cm3.
  • Core:
    1. Core of the earth lies between depths of 2900 km to 6370 km.
    2. Because this layer is rich in iron (Fe) and nickel (Ni), it is also known as Nife.
    3. This is the densest among all the layer of the earth. Its density value is about 13 g/cm3.
    4. It occupies 16 % of the total volume of the earth. However it occupies 32 % of the total mass of the earth.
    5. Core is further divided into outer core and inner core.
      1. Outer Core: It is in liquid state. It is believed that "earth’s magnetic field" is controlled by this liquid outer core.
      2. Inner core: This is in solid state.
  • Discontinuities:
    1. Inside the earth there is a drastic change in the physical and chemical properties between three concentric layers of crust, mantle and core.
    2. Basically discontinuities are the transition zone between different layers of the earth. It represents the density difference between them.
      1. Conard Discontinuity: Between upper crust and lower crust.
      2. Mohorovicic or Moho Discontinuity: Between lower crust and upper mantle.
      3. Repetti Discontinuity: Separates upper mantle and lower mantle.
      4. Gutenberg Discontinuity: Between lower mantle and outer core.
      5. Lehman Discontinuity: Between outer core and inner core.
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