Geography

Module 5: Earth’s Interior

INTERIOR OF THE EARTH

The earth consists of three major concentric layers – the Crust, the Mantle and the Core.

The Crust

  • The outermost layer of the earth is the crust. The crust is in solid form and accounts 0.5% of the earth.
  • It extends between 0 to 40 km below the earth and average density is 2.9 g/cm3.
  • The crust is made up of two layers: Upper crust with density 2.7 gm/cm3 and lower crust with density 3 gm/cm3.
  • Conrad Discontinuity separates the outer and inner crusts.
  • The upper crust is composed of crystalline igneous and metamorphic rocks, acidic in nature. Lower crust is composed of denser basaltic rocks.
  • Upper crust forms continents and its main mineral constituents are Silica (Si) and Aluminium (Al); so it is collectively referred to as Sial.
  • Lower crust forms ocean floor and it is mainly composed of Silica and Magnesium. So it is collectively called Sima.
  • Sial and Sima together make up the earth’s crust.
  • The ocean floors that we see today are of more recent origin. No portion of the seafloor is more than 200 million years old. These are made by denser rocks than those of the continents.
  • The boundary between the crust and the mantle is called ‘Moho’ or Mohorovic Discontinuity.

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The Mantle

  • Below the earth’s crust is a thick layer called mantle.
  • It extends between 40 to 2900 km below the earth.
  • It has an average density of 4.5 g/cm3
  • Mantle is the thickest layer of the earth and accounts 82% of its volume and 65% of its mass
  • The upper portion of the mantle is in a solid state. The upper portion of the mantle and the crust together is called the lithosphere. The maximum thickness of the Lithosphere is about 100 kilometres.
  • Just below the upper mantle, the rocks are found in molten state because of high temperature. This part of mantle is known as aesthenosphere.
  • Mantle consists predominantly of solid olivine rocks made of silicates of magnesium and iron.
  • The inner and outer mantles are separated by Repetti Discontinuity.
  • Gutenberg Discontinuity separates the mantle from outer core.

The Core

  • The core is the innermost layer of the earth and has highest density.
  • It is also known as Nife, because this layer contain large concentration of iron nickel (Ni) and (Fe).
  • The core consists of outer core and inner core. Lehman discontinuity separates inner core and outer core.
  • The outer core extends between 2900 km to 5150 km with an average density of 10.7 g/cm3. It is in molten state.
  • The inner core extends from 5150 km to 6371 km.
  • The inner core has an average density of 15 g/cm3
  • The temperature in the inner core is 110000C and the pressure is very high. Hence the inner core is in solid form.

DISCONTINUITIES

  • Conrad Discontinuity : Between inner crust and outer crust.
  • Mohorovic Discontinuity : Between crust and mantle
  • Repetti Discontinuity : Between upper mantle and lower mantle
  • Guttenberg Discontinuity : Between core and mantle
  • Lehman Discontinuity : Between inner core and outer core

MATERIALS OF THE EARTH’S CRUST

The outermost part of lithosphere constitutes earth’s crust. The material of the crust is made up of rocks. The rocks are of different types. They have a great variety of colour, weight and hardness.

Rocks

  • A rock is an aggregate of one or more minerals.
  • Rocks may be hard or soft, large or small and in varied colors.
  • Based on the mode of formation, rocks can be grouped into three types:
    • Igneous Rocks
    • Sedimentary Rocks
    • Metamorphic Rocks

Igneous Rocks

  • The word igneous is derived from the Latin word ‘ignis’ meaning fire.
  • Igneous rocks are formed by the solidification of molten magma ejected from the interior of the earth. (When molten magma reaches the surface of earth, it is called lava)
  • More than 95% of earth’s crust is made up of igneous rocks.
  • Igneous rocks are called Primary rocks or Parent Rocks because all the other rocks are formed directly or indirectly from igneous rocks.
  • Granite, Basalt, Anthracite etc. are examples of igneous rocks.
  • The valuable minerals like iron ore, gold, silver, zinc, lead, aluminum, manganese, mica etc. are generally found in the igneous rocks.
  • All igneous rocks are of magmatic origin. Igneous rocks are formed from solidified molten magma below or on the earth’s surface.
  • They are massive, having no layers, hard, compact and free of fossils.
  • On the basis of mode of occurrence, igneous rocks are subdivided into:
  • Extrusive Igneous rocks
  • Intrusive Igneous rocks

Extrusive Igneous Rocks

  • Extrusive Igneous rocks are rocks formed by solidification of magma above the earth’s surface. These types of rocks are found in volcanic areas and are also known as volcanic rocks.
  • Since cooling of magma is rapid, structure of extrusive rocks are very small.
  • Basalt is a typical example of extrusive type covering 500,000 km of Peninsular India in its north-western part.
  • The basaltic material is used for building roads
  • The regur soil found in Deccan plateau in India is derived from lava.

Intrusive Igneous rocks

  • Intrusive Igneous rocks are formed by solidification of magma at moderate depths beneath the earth’s surface.
  • Here cooling is very slow because of high heat at these depths and therefore result in the formation of large crystals in the rocks.
  • Granite and dolerite are the common examples of intrusive igneous rocks.
  • The huge blocks of granitic rocks are found both in the Himalaya and the Deccan Plateau.
  • Intrusive igneous rocks that are formed at great depths are called plutonic rock.
  • Plutonic rocks are much larger in size because of slow rate of cooling. Granite is an example of plutonic rock.
  • Intrusive igneous rocks that are formed at shallow depth are known as hypabyssal rocks.
  • On the basis of chemical properties, igneous rocks are classified into acidic and basic rocks. These are formed as a result of solidification of acidic or basic lava.
  • Acidic igneous rocks are composed of 65% or more of silica. These rocks are light coloured, hard and very strong. Granite is an example of an acidic rock.
  • Basic igneous rocks contain less than 55% of silica and have more of iron and magnesium. These rocks are dark coloured and weak enough for weathering.
  • Gabbro, basalt and dolerite are examples of basic rocks.

Sedimentary Rocks

  • Sedimentary rocks are formed due to the aggregation and compaction of sediments. These sediments may be the debris eroded from any previously existing rock which may be igneous rock, metamorphic or old sedimentary rock.
  • Sedimentary rocks have layered or stratified structure. The thickness of strata varies from few millimeters to several metres. So these rocks are also called stratified rocks or fragmental rocks.
  • Sedimentary rocks constitute only 5 percent of the volume of earth’s crust. They are widely spread on the earth surface but to a shallow depth.
  • Most of the sedimentary rocks contain fossils. Fossil is the solid part or an impression of a prehistoric animal or plant embedded in strata of sedimentary rocks.
  • The sedimentary rocks are the sources of major fossil fuels like coal, petroleum, natural gas etc.
  • The accumulated remains of plants and animals that were trapped and entombed in sediments millions of years ago are subsequently transformed into coal. On the basis of the decreasing content of carbon, coal is classified as anthracite, bituminous, lignite and peat.
  • The cementation, compaction and hardening of sediments into sedimentary rocks are known as lithification.
  • Depending upon the mode of formation, sedimentary rocks are classified into three major groups:
  • Chemically formed sedimentary rocks – eg: Rock salt, Gypsum, Dolomite.
  • Organically formed sedimentary rocks – eg: coal, chalk, limestone.
  • Mechanically formed sedimentary rocks – eg: sandstone, shale
  • All sediments are carried by running water, wind or ice.
  • As sedimentation is favoured by water, most of sedimentary rocks are formed under water.
  • All sedimentary rocks are layered, lying in horizontal beds.
  • Most of sedimentary rocks are permeable and porous.
  • Loess is an example of fine sand carried by wind and deposited as sedimentary rock in north-western China and Indian subcontinent.

Metamorphic Rocks

  • Igneous Rocks and Sedimentary Rocks are transformed into Metamorphic Rock due to pressure and temperature.
  • Tremendous pressure and high temperature change the colour, hardness, structure and composition of all types of pre-existing rocks.
  • The process through which metamorphic rocks are formed is called Metamorphism.
  • Metamorphic rocks are hard and tough in comparison to the parent rocks from which they are formed
  • The arrangement of minerals or grains in layers or lines in metamorphic rock is called lineation or foliation.

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Parent Rock and its Metamorphic Changed Form

  • The formation of metamorphic rocks under the stress of pressure is known as dynamic metamorphism.
  • The formation of metamorphic rocks from igneous rocks and sedimentary rocks under the influence of high temperature within earth’s crust is known as thermal or contact metamorphism.
  • Metamorphic rocks are recognized by their great hardness, closely banded structures, and interlocking of crystals.
  • Different types of metamorphic rocks are found all over the world. In India, marble is found in Rajasthan, Bihar and Madhya Pradesh, whereas slates are available in plenty in Orissa, Andhra Pradesh and Haryana. In Kangra and Kumaun regions of Himalaya, slates of different colours are found.

Rock Cycle

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PLATE MOVEMENTS

Continental Drift Theory

  • The movements of continents relative to each other across the earth’s surface are called continental drift.
  • The continental drift theory was proposed by Alfred Wegner in 1912. According to this theory, about 200 million years ago the present seven continents was a single super continent called ‘Pangea’ (pan mean all, gea means earth)
  • Pangea was encircled by a super single ocean called ‘Panthalasa’.
  • Pangea was split into two megacontinents called ‘Laurasia’ in the north and ‘Gondwanaland’ in the south and created ‘Tethys’ sea between them.
  • ‘Laurasia’ comprised North America, Asia and Europe. Gondwana land comprised Peninsular India, Africa, South America, Arabian Peninsula, Australia and Antarctica.
  • The splitting of ‘Laurasia’ and ‘Gondwanaland’ created all the present continents.
  • Some of the important evidence given in support of the theory are:
  • The Geographical similarity in the opposing lands of the Atlantic ocean.
  • The similarity of certain fossils found on the continents on both sides of the Atlantic ocean
  • Anomalous deposits of ancient deposits

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Plate Tectonics Theory

  • The term ‘plate tectonics’ was coined by J. Tuzo Wilson in 1965.
  • According to this theory lithosphere is divided into a number of parts called ‘plates’ and are moving over aesthenosphere.
  • A tectonic plate (lithospheric plate) is a massive irregularly-shaped slab of solid rock generally composed of both continental and oceanic crust.
  • The theory of plate tectonics proposes that the earth’s lithosphere is divided into seven major and some minor plates. All these plates are clustered together to exist.
  • Eurasian plate, North American Plate, South American Plate, African Plate, Indo-Australian Plate, Antarctic an Plate and Pacific Plates are the seven major plates.
  • Pacific Plate is the largest plate. Eurasian Plate is the largest continental plate.
  • Most of the earth’s seismic, activity, mountain building and volcanism occur along with these plate boundaries
  • Most of the plates include both of the continental crust and oceanic crust. There are also plates which include either oceanic crust or continental crust alone

PLATE BOUNDARIES

  • Based on the relative motion of plates with respect to adjacent plates, there are three types of plate boundaries: Divergent Plate Boundary, Convergent Plate Boundary and Shear Plate Boundary.
  • These are responsible for various tectonic activities like volcanoes, earthquakes and the formation of fault zones.

Divergent Plate Boundary

  • When two adjacent plates moves in the opposite direction, very long fissure will be formed between them and molten magma from mantle comes out to the surface and a new crust will be formed. Such plate boundaries are called Divergent Plate Boundary.
  • Rift valleys are formed due to such movements in continental plates. In these areas, earthquakes and volcanoes are frequent, even though of moderate to low intensity.

Convergent Plate Boundary

  • When two plates collide with each other, one plate descends under the adjacent plate. This type of plate boundary is called Convergent Plate Boundary.
  • This is also known as Destructive Plate Margin or Consuming Plate Margin.
  • When the two continental plates collide with each other, mountains may be formed. The collision of the Indian plate with the Eurasian plate has resulted in the formation of the Himalayas.
  • The Indian plate, which was a part of Gondwana land, collided with Eurasian Plate about 40-50 million years ago and gave rise to Himalayas.
  • Indian plate is moving from South to North at a speed of 5 cm/year towards the static Eurasian Plate.
  • It is because of this collision, frequent earthquakes occur in the Himalayan region of India.
  • This collision also results in the increase of height of Himalayas at the speed of 1cm per year.

Shear Plate Boundary

  • The two lithospheric plates that slide past each other along a common boundary neither create nor destroy the crust, but may cause fissures on the earth’s crust. These are called zones of faulting. San Andreas Fault Zone of California is an example for this.
  • Boundaries which locate in this region are called Shear Plate Boundaries. They are zones of frequent earthquakes of varied intensity.

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Lithospheric plates

Sea Floor Spreading Theory

  • Sea Floor Spreading Theory was first proposed by Hary Hess in 1961.
  • This theory states that mid oceanic ridges are situated on rising thermal convective current coming from the mantle.
  • When ocean crust moves in opposite directions from mid oceanic ridges, the molten magma rises from the fractures and solidifies to form new sea floor.

EARTH QUAKES

  • Earthquakes are vibrations of earth caused by sudden movements in the earth’s crust.
  • It is a form of energy of wave motion transmitted through the surface layer of the earth.
  • The point of origin of an earthquake in the interior of the earth is called the seismic focus. It is also known as hypocenter.
  • The place on the ground surface directly above the focus is called epicenter. The maximum damage is caused at the epicenter.
  • Seismic waves are the vibrations from earthquake that travel through the earth. They are recorded on instrument called seismograph.
  • The recorded information in the seismograph is known as seismogram.
  • Seismology is the branch of geology that deals with the study of earthquakes.
  • Isoseismal lines: Lines joining the regions of same seismic intensity are called isoseismal lines.
  • Homoseismal lines: Lines joining the places which experience the earthquake tremors at the same time are called Homoseismal lines.

SEISMIC WAVES

  • The energy being released from the focus propagates as waves. These cause tremors on the surface of the earth and cause destructions.
  • Seismic waves are classified into three:
    • Primary waves (P waves)
    • Secondary waves (S waves)
    • Surface waves or Long waves (L waves)

Primary Waves (P-Waves)

  • Primary waves are longitudinal waves and can travel through solid, liquid or gas.
  • They travel faster through liquid and slowly through liquids and reach the epicenter from the focus first.
  • Velocity of P waves increase with depth but only up to 2900 km. Its velocity decrease when it passes through liquid outer core and again increase when it reaches the solid inner core.

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Secondary Wave (S-Waves)

  • Secondary waves are transverse wave that can travel only through solid.
  • S-waves cannot pass through liquid outer core.
  • These reach the epicenter following the primary waves. The P-waves and S-waves are together known as body waves.

Surface waves or Long waves (L-Waves)

  • L-waves travels with long wave length
  • Surface waves reach the epicenter last. They travel only through the surface of the earth and cause destruction on the earth’s surface.
  • They do not go deeper in the earth.

Measuring Earthquake

  • The magnitude of earth quake is measured by using Richter Scale. It was developed by Charles F. Richter in 1935
  • The magnitude of an earthquake refers to the energy released during the earthquake. The magnitude is expressed in absolute number, from 0 to 10.

Characteristics of earthquakes of different magnitude

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  • The Mercalli Intensity Scale is a Seismic Scale used for measuring the intensity of earthquake.
  • The intensity scale is named after Mercalli, an Italian seismologist. The range of intensity scale is from 1 to 12.

EARTH QUAKE ZONES IN INDIA

  • Earthquake – prone areas of the country have been identified on the basis of scientific inputs relating to seismicity, earthquakes occurred in the past and tectonic setup of the region.
  • Based on these inputs, Bureau of Indian Standards has grouped the country into four seismic zones, viz. Zone II, III, IV and V. Of these, Zone V is seismically the most active region, while zone II is the least.
Zones Regions included
Zone – V Entire northeastern India, parts of Jammu and Kashmir, Himachal Pradesh, Uttaranchal, Rann of Kutch in Gujarat, part of North Bihar and Andaman & Nicobar Islands.
Zone – IV The remaining parts of Jammu and Kashmir and Himachal Pradesh, National Capital Territory (NCT) of Delhi, Sikkim, Northern Parts of Uttar Pradesh, Bihar and West Bengal, parts of Gujarat and small portions of Maharashtra near the west coast and Rajasthan.
Zone – III Kerala, Goa, Lakshadweep islands, remaining parts of Uttar Pradesh, Gujarat and West Bengal, Parts of Punjab, Rajasthan, Madhya Pradesh, Bihar, Jharkhand, Chhattisgarh, Maharashtra, Orissa, Andhra Pradesh, Tamil Nadu and Karnataka.
Zone – II The remaining parts of country.

Earth quake zones in India

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