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Geomorphology

Study Material > Geography

Origin of the Earth

  • The origin of the Earth is surrounded in mystery and no complete and comprehensive theory about the Earth's origin has been propounded till now. Most of the theories believe that since the Earth is an important member of the solar system, its origin cannot be different from that of the other members of the solar system. Majority of the theories believe the Earth got separated from the sun as a hot gaseous mass, which on cooling with the passage of time, turned into a liquid and then into a solid sphere. A large number of theories have been put forward to explain the origin of the earth. Some of the outstanding hypothesis are briefly described here.
    1. Gaseous hypothesis: Professor Immanuel Kant was a German scholar who put forward this hypothesis in 1755. According to him, pre mordial Matter was evenly distributed in the shape of small and cold particles from which our earth and other planets of solar system were formed. Since this hypothesis is based on gas, it is popularly known as gaseous hypothesis.
    2. Nebular hypothesis: Laplace was French mathematician who put forward his hypothesis of origin of the Earth in 1796. According to Laplace, the pre mordial matter existed in the form of intensely hot and rotating gaseous mass in the beginning. This hot and slowly rotating gaseous mass was called Nebula after which it is called the nebular hypothesis. As the tme passed, the gaseous mass cooled, its volume decreased and its speed of rotation increased. The increase in speed of rotation resulted in increase in centrifugal force. When the centrifugal force exceeded the gravitational force, a ring moved away from the Nebula and broke into many smaller rings. This rings, on cooling became planets and satellites. The remaining Central part of Nebula is the present Sun. Planesimal Hypothesis : In the year 1900, Chamberlin and Moulton  put forward their planesimal hypothesis according to which a wandering star approached the sun and exerted its gravitational pull on the sun. As a result, a Cigar shaped extension of material was separated from the Solar surface. As the wandering star moved away from the sun, the material separated from the Solar surface started revolving around the sun and condensed into planets at later stage.
    3. Tidal Hypothesis: A British scientists Sir James Jeans propounded his “tidal hypothesis” in 1919 and another British scientist, Harold Jeffreys suggested some modifications in his hypothesis in 1929. According to this hypothesis the sun was a gaseous mass in the beginning. Another star, several times larger than the sun, accidentally came close to it and pulled the gaseous material away from the sun due to its gravitational pull. Giant tongues of matter come out of the sun and the planets were formed.
    4. Big Bang Theory: Also known as expanding Universe hypothesis. It can be credited to Edwin Hubble who provide evidence of expanding Universe in 1920. This theory was postulated in 1950s and 1960s and validated in 1972. According to this theory everything in the Universe emerged from a point known as singularity, about 13.7 billion years ago. The Galaxy is moved apart as the space between them expanded. Obviously, the universe was much smaller in the beginning. As the Universe expanded the hot radiation in the originals Firewall also expanded and cooled down.  Misty clouds of matter already existed. As those clouds collapsed upon themselves they were pulled together by their own gravity and formed clusters of galaxies with the galaxies themselves breaking up into stars like those of the Milky Way. The stars might have broken up to form their planets like those of our solar system.

   Interior of the Earth

  • The factual evidence concerning the interior of the Earth is not readily available. Therefore, the knowledge about the interior of the Earth is built on indirect evidence is such as:
    1. Density Studies: Using the spherical shape, the mean (or average) radius and mass of the earth, it is possible to determine the average density of the earth as 5.5 gram/cm3 (core: 11.0 gram/cm3, Earth surface 2.6 to 3.3 gram/cm3 ). By the density studies, it is proved that the density of the core of the Earth is highest of all parts of the Earth.
    2. Pressure: Though, the density increases with increasing pressure. Yet, every rock has a critical limit beyond which its density cannot be increased inspite of the increasing pressure. It shows that the high density in the core is the result of its constituting heavy metallic materials of high density.
    3. Temperature: Normally, the temperature increases by 10C for every 32 meters of depth. But, this rise in temperature while going down the surface of the Earth is not uniform. Relatively higher temperature is found in the tectonically active regions. Heat flows outwards from the interior of the earth and takes place in the form of thermal convective currents.
    4. Vulcanicity: The molten lava coming out of the volcanoes during volcanic eruption indicates that there is at least a layer below the Earth's crust which is in liquid or semi liquid state.
    5. Meteorites: The remains of unknown planets, allow us to directly analyse the density, chemistry and mineralogy of the nickel-iron cores of bodies having a similar composition to that of the earth.
    6. Seismic Studies: The study of seismic waves i.e., the vibrations, usually created by the disturbances within the earth's outer crust, reveals some useful information about the earth's internal structure.

Types of seismic waves

  • P waves or  Primary waves or Compressional waves: These waves are identical to sound waves – the particle motion is parallel to the direction of  propagation of waves.
  • S waves or secondary waves or transverse or shear waves: These waves are characterized by particle motion that is perpendicular to the direction of propagation.
  • L waves or Long waves or Rayleigh waves or Surface waves: These waves are confined to the surface of the earth.

Types of seismic waves

  • P waves or  Primary waves or Compressional waves: These waves are identical to sound waves – the particle motion is parallel to the direction of  propagation of waves.
  • S waves or secondary waves or transverse or shear waves: These waves are characterized by particle motion that is perpendicular to the direction of propagation.
  • L waves or Long waves or Rayleigh waves or Surface waves: These waves are confined to the surface of the earth.

 Chemical composition of Earth

  • Based on the chemical composition the Earth as a whole has been divided into three broad zones - Crust, Mantle and core.
  • The outermost layer of the Earth is the crust. This is divided into Continental and Oceanic crust.
  • The Continental crust varies in thickness between 35 to 70 Km.
  • The upper continental crust is composed mainly of alumino-silicates.
  • The Oceanic crust is made up of igneous rocks rich in Iron and magnesium, such as basalt and peridotite.
  • The thickness of the Oceanic crust varies from 10 to 12 km.
  • The next layer Mantle is about 29000 km thick and separated into the upper and lower Mantle. The volume of the mantle is about 83% of the total volume of the earth and its mass is about 68% of the total mass of the Earth. Silica and magnesium are the major constituting elements of this layer, hence it is also called SIMA.
  • Between 100 km to 200 km depth of the upper Mantle, the velocity of the seismic waves is slowed down to 7.8 km per second. Therfore, this zone is known as the zone of low velocity.
  • Core is the last layer which is separated into the liquid outer core and the solid inner core. Earth's magnetic field is believed to be controlled by the liquid outer core. The outer Core is 2300 km thick, twice as thick as the inner core. The volume of the core is merely 16% of the Earth but its mass is 32 % of the total mass of the Earth. Though some silica is found in the inner core, yet its main constituents are nickel and iron, so it is also known as NIFE.

Mechanical composition of Earth

  • The layers can be separated based on mechanical properties or strength (resistance to flowing or deformation) in addition to composition.
  • Lithosphere: The uppermost layer is the Lithosphere (sphere of rock), which comprises the crust and a solid portion of the upper mantle. The Lithosphere is divided into many plates that move in relation to each other due to tectonic forces. The solid Lithosphere floats atop a semiliquid layer known as the Asthenosphere(weak sphere), which enables the Lithosphere to move around. Compared to the Lithosphere above, this region is more elastic or less viscous, that is, it is softer, more pliable and capable of bending or deforming without breaking.
  • Mesosphere: It includes the Earth’s zone that lies below the Asthenosphere and includes the whole of mantle.

Discontinuities

  • Discontinuities are the boundaries between the different layers of the Earth. It represents the difference in the physical and chemical properties of the Earth’s interior. The discontinuity in density between upper crust and the lower crust is known as Conard Discontinuity. The discontinuity between the upper mantle and the lower crust is known as Mohorovicic or Moho discontinuity. The discontinuity in density between upper mantle and lower mantle is known as Repetti discontinuity. The discontinuity between the lower mantle and the outer core is known as Weichert-Gutenberg discontinuity.

 Rocks

  • Rock or stone is a natural substance, a solid  aggregate of one or more minerals.
  • However, there are a few substances of organic origin, which the geologists also accept as rocks; the chief of these are coal, peat and guano.
  • The minerals and metals found in rocks have been essential to human civilization.
  • Three major groups of rocks are: igneous, sedimentary, and metamorphic.
  • The scientific study of rocks is called petrology, which is an essential component of geology.
  • The 98 % of the Earth’s crust is composed of 8 major rock-forming elements – Oxygen(47%), Aluminium(8%), Iron(5%), Calcium, Sodium, Potassium, and Magnesium.

Igneous rock

  • Igneous rocks are formed when magma (molten rock deep within the earth) cools and hardens. Sometimes the magma cools inside the earth, and other times it erupts onto the surface from volcanoes (in this case, it is called lava). When lava cools very quickly, no crystals form and the rock looks shiny and glasslike. Sometimes gas bubbles are trapped in the rock during the cooling process, leaving tiny holes and spaces in the rock.
  • Depth of cooling determines the texture in deep-seated condition; this is coarsely crystalline; fast cooling on the surface leads to fine-grained texture; and in between at moderate depth, phenocrystalline. Ex: Granite, basalt

Sedimentary rocks

  • Rocks formed on the surface of the Earth due to the erosion and deposition of igneous and metamorphic rocks are known as sedimentary rocks. Sedimentary rocks are found over about 75% area of the crust, but they contribute only 5% in the formation of the crust. This rocks contain fossils. On the basis of the nature of the sediments, the sedimentary rocks are classified into:
    1. Mechanically formed sedimentary rocks, such as sandstones, conglomerates, clay rocks, shale and loess .
    2. Organically formed sedimentary rocks, such as limestones, coal and peat.
    3. Chemically formed sedimentary rocks, such as Chalk rocks, Gypsum and Salt Rock.
    4. Shale, limestone and sandstone make up over 99% of all sedimentary rocks. Shale is most abundant because of the abundance of the feldspars.

Metamorphic rocks

  • Metamorphic rocks are formed due to complete alternation in the appearance and constitution of Pre-existing rocks due to change in mineral composition and texture through temperature and pressure.
  • These are the hardest rocks and do not contain fossils. Metamorphism usually takes place at depth in the roots of Mountain chains or adjacent to large intrusive igneous bodies. The presence of inter-granular fluid greatly speeds up metamorphic reaction. Metamorphic rocks formed through the sedimentary rocks:
    1. Slate -  from Shale
    2. Marble - from Limestone
    3. Quartzite - from Sandstone and conglomerate
    4. Marble - from Chalk and Dolomite
  • Metamorphic rocks formed through the igneous rocks
    1. Gneisses - from Granites
    2. Amphibolite - from Basalt
    3. Schist - from basalt
  • Metamorphic rocks formed by the further metamorphosis of metamorphic rocks
    1. Phyllite - from Slate
    2. Schist - from phyllite
    3. Serpentine - from Gabbro

Composition of Earth's crust

  • oxygen 46.60 %,  Silicon 27.72 %, Aluminium – 8.13 % , Iron 5 %, Calcium 3.63 %, Sodium 2.83%, Potassium 2.59 %, Magnesium 2.09%.
  • The composition of whole Earth : Iron 35 %, Oxygen 30 %, Silicon 15 %, Magnesium 13 %, nickel 2.4 % , Sulphur 1.9%, Calcium 1.1 %, Aluminium 1.1 %.
  • Cratons: A craton is an old and stable part of the continental lithosphere, where the lithosphere consists of the Earth's two topmost layers, the crust and the uppermost mantle. These regions have not been affected by any Earth movements for over half billion years except for broad gentle warping. Cratons include the shields and platforms.
  • Shields: A shield is generally a large area of exposed Precambrian crystalline igneous and high-grade metamorphic rocks that form  tectonically stable   Shields are the regional surface of low relief and are broadly convex and relatively immobile regions. E.g., Indian shield, African shield, Angaran shield, Canadian shield, Australian shield and Antarctican shield.

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