Interior Of The Earth
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Interior Of The Earth

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GEOGRAPHY

Interior Of The Earth 

The configuration of the surface of the earth is largely a product of the processes operating in the interior of the earth. This article will take you further of the article Origin and evolution of earth.

Interior Of The Earth

Introduction

  • The configuration of the surface of the earth is largely a product of the processes operating in the interior of the earth.
  • Exogenic as well as endogenic processes are constantly shaping the landscape.

Why know about the earth’s interior

  • Understanding the earth’s interior is essential to understand the nature of changes that take place over and below the earth’s surface.
  • To understand geophysical phenomena like volcanoes, earthquakes, etc.
  • To understand the internal structure of various solar system objects

Sources of information about the interior

Direct Sources

  • The most easily available solid earth material is surface rock or the rocks we get from mining areas. 
  • Gold mines in South Africa are as deep as 3 - 4 km.
  • Besides mining, scientists have taken up a number of projects to penetrate deeper depths to explore the conditions in the crustal portions.
  • Mponeng gold mine and TauTona gold mine in South Africa are the deepest mines reaching a depth of 3.9 km
  • The deepest drill at Kola, in the Arctic Ocean, has so far reached a depth of 12 km. 
  • This and many deep drilling projects have provided a large volume of information through the analysis of materials collected at different depths.
  • As and when the molten material (magma) is thrown onto the surface of the earth, during volcanic eruption it becomes available for laboratory analysis.
  • However, it is difficult to ascertain the depth of the source of such magma.

 Indirect sources

Depth: With depth, pressure and density increase and hence temperature. This is mainly due to gravitation.

Meteors: Meteors and Earth are solar system objects that are born from the same nebular cloud. Thus they are likely to have a similar internal structure.

Gravitation:

  • Changes with latitude
  • It is greater near the poles and less at the equator. This is because of the distance from the center at the equator being greater than that at the poles.
  • The gravity values also differ according to the mass of the material. The uneven distribution of mass of material within the earth influences this value. Such a difference is called a gravity anomaly. 
  • Gravity anomalies give information about the distribution of mass of the material in the crust of the earth.
  • Magnetic field: The geodynamo effect helps scientists understand what’s happening inside the Earth’s core. 

Magnetic field of the earth

  • Our planet’s magnetic field is believed to be generated deep down in the Earth’s core.
  • Right at the heart of the Earth is a solid inner core, two-thirds of the size of the Moon and composed primarily of iron.
  • At 5,700°C, this iron is as hot as the Sun’s surface, but the crushing pressure caused by gravity prevents it from becoming liquid.
  • Surrounding: This is the outer core, a 2,000 km thick layer of iron, nickel, and small quantities of other metals. Lower pressure than the inner core means the metal here is fluid.
  • Differences in temperature, pressure, and composition within the outer core cause convection currents in the molten metal as cool, dense matter sinks whilst warm, less dense matter rises.
  • This flow of liquid iron generates electric currents, which in turn produce magnetic fields. 
  • Charged metals passing through these fields go on to create electric currents of their own, and so the cycle continues. This self-sustaining loop is known as the geodynamo.
  • The spiraling caused by the Coriolis force means that separate magnetic fields created are roughly aligned in the same direction, their combined effect adding up to produce one vast magnetic field engulfing the planet.

High Levels of Temperature and Pressure Downwards

  • Volcanic eruptions and the existence of hot springs, geysers, etc. point to an interior that is very hot.
  • The high temperatures are attributed to the automatic disintegration of the radioactive substances.
  • Gravitation and the diameter of the earth help in estimating pressures deep inside.

Evidence From The Meteorites

  • When they fall to earth, their outer layer is burnt during their fall due to extreme friction, and the inner core is exposed.
  • The heavy material composition of their cores confirms the similar composition of the inner core of the earth, as both evolved from the same star system in the remote past.
  • The most important indirect source is seismic activity. 
  • The major understanding of the earth’s internal structure is mainly from the study of seismic waves.

Seismic waves

  • The study of seismic waves provides a complete picture of the layered interior.

What causes earthquakes?

  • Abrupt release of energy along a fault causes earthquake waves.
  • A fault is a sharp break in the crustal rock layer.
  • Rocks along a fault tend to move in opposite directions. But the friction exerted by the overlying rock strata prevents the movement of the rock layer.

Earthquake

  • Under intense pressure, the rock layer, at a certain point, overcomes the friction offered by the overlying layer and undergoes an abrupt movement generating shockwaves.
  • This causes a release of energy, and the energy waves travel in all directions.
  • The point where the energy is released is called the focus of an earthquake, it is also called the hypocentre.
  • The energy waves traveling in different directions reach the surface. The point on the surface, nearest to the focus, is called the epicenter. It is the first one to experience the waves. It is a point directly above the focus.

 Earthquake Waves

  • All-natural earthquakes take place in the lithosphere (depth up to 200 km from the surface of the earth).
  • An instrument called ‘seismograph’ records the waves reaching the surface.
  • Earthquake waves are basically of two types—body waves and surface waves.

Body waves:

  • Generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. Hence, the name body waves.
  • There are two types of body waves. They are called P and S-waves.

P-waves:

  • Move faster and are the first to arrive at the surface. These are also called ‘primary waves’. 
  • Also called longitudinal or compressional waves.
  • Similar to sound waves.

P-waves

  • They travel through gaseous, liquid, and solid materials.
  • This gives clues about the Solid inner core
  • Particles of the medium vibrate along the direction of propagation of the wave.
  • These waves are of high frequency.
  • They can travel in all mediums.
  • The velocity of P waves in Solids > Liquids > Gases
  • The shadow zone for ‘P’ waves is an area that corresponds to an angle between 103 and 142

S-waves

  • Arrive at the surface with some time lag.
  • These are called secondary waves. 
  • Also called as transverse or distortional waves.
  • Can travel only through solid materials. 
  • Reflection causes waves to rebound whereas refraction makes waves move in different directions. 
  • The variations in the direction of waves are inferred with the help of their record on the seismograph. 
  • The surface waves are the last to report on seismographs.
  • Analogous to water ripples or light waves.
  • A secondary wave cannot pass through liquids or gases.
  • These waves are high-frequency waves.
  • Travel at varying velocities (proportional to shear strength) through the solid part of the Earth’s crust, mantle.
  • The shadow zone of ‘S’ waves extends almost halfway around the globe from the earthquake’s focus.
  • The shadow zone for ‘S’ waves is an area that corresponds to an angle between 103 and 180
  • This observation led to the discovery of a liquid outer core. 
  • Since S waves cannot travel through liquid, they do not pass through the liquid outer core.

Why S-waves can’t pass through a liquid?

  • S-waves are shear waves, which move particles perpendicularly to their direction of propagation.
  • They can propagate through solid rocks because these rocks have enough shear strength.
  • The shear strength is one of the forces that hold the rock together, and prevent it from falling into pieces.
  • In fact, it is just a matter of rigidity: S-waves need a medium rigid enough to propagate. Hence, S-waves do not propagate through liquids

Surface waves (L waves)

Surface Waves (L waves)

  • Also called long-period waves.
  • They are low frequency, long wavelength, and transverse vibration.
  • Affect the surface of the Earth only and die out at smaller depth.
  • Develop in the immediate neighborhood of the epicenter.
  • They cause the displacement of rocks, and hence, the collapse of structures occurs.
  • These waves are responsible for most of the destructive force of earthquakes.
  • Recorded last on the seismograph.

Propagation of Earthquake Waves

  • Different types of earthquake waves travel in different manners. As they move or propagate, they cause vibration in the body of the rocks through which they pass.
  • P-waves vibrate parallel to the direction of the wave. This exerts pressure on the material in the direction of the propagation.
  • As a result, it creates density differences in the material leading to stretching and squeezing of the material.
  • The other two waves vibrate perpendicular to the direction of propagation.
  • The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane. Hence, they create troughs and crests in the material through which they pass.
  • Earthquake waves get recorded in seismographs located at far-off locations.
  • However, there exist some specific areas where the waves are not reported. Such a zone is called the ‘shadow zone’.
  • The study of different events reveals that for each earthquake, there exists an altogether different shadow zone. 
  • Seismographs located at any distance within 105 ° from the epicenter, records the arrival of both P and S-waves.
  • Seismographs located beyond 145 ° from epicenter, record the arrival of P-waves, but not that of S-waves.
  • Thus, a zone between 105 ° and 145 ° from epicenter was identified as the shadow zone for both the types of waves.
  • The entire zone beyond 105 ° does not receive S-waves.
  • The shadow zone of S-wave is much larger than that of the P-waves. 
  • The shadow zone of P-waves appears as a band around the earth between 105 ° and 145 ° away from the epicenter.
  • The shadow zone of S-waves is not only larger in extent but it is also a little over 40 per cent of the earth surface

How these properties of ‘P’ and ‘S’ waves help in determining the earth’s interior

  • Reflection causes waves to rebound whereas refraction makes waves move in different directions.
  • The variations in the direction of waves are inferred with the help of their record on seismograph.
  • Change in densities greatly varies the wave velocity.
  • By the observing the changes in direction of the waves (emergence of shadow zones), different layers can be identified.

Earth’s layers

Earth’s Layers

  • Earth’s layers are identified by studying various direct and indirect sources 
  • The structure of the earth’s interior is made up of several concentric layers.
  • Broadly three layers can be identified—crust, mantle and the core

Earth’s Layers based on chemical properties

  1. crust
  2. mantle
  3. Core

Earth’s Layers – The Crust

  • Crust is the outer thin layer
  • Thickness: 30-50 km.
  • The thickness of the crust varies under the oceanic and continental areas.
  • Oceanic crust is thinner (5-30 km thick) as compared to the continental crust (50-70 km thick).
  • The continental crust is thicker in the areas of major mountain systems. It is as much as 70 -100 km thick in the Himalayan region.
  • The outer covering of the crust is of sedimentary material (granitic rocks) and below that lie crystalline, igneous and metamorphic rocks which are acidic in nature.
  • The lower layer of the crust consists of basaltic and ultrabasic rocks.
  • The continents are composed of lighter silicates—silica + aluminium (also called ‘sial’) while the oceans have the heavier silicates—silica + magnesium (also called ‘sima’).

Earth’s Layers – Mantle

  • The mantle extends from Moho’s discontinuity (35 km) to a depth of 2,900 km (Moho-Discontinuity to the outer core).
  • The crust and the uppermost part of the mantle are called lithosphere. Its thickness ranges from 10-200 km.
  • The lower mantle extends beyond the asthenosphere. It is in solid state.
  • The density of mantle varies between 2.9 and 3.3.
  • The density ranges from 3.3 to 5.7 in the lower part.
  • Composed of solid rock and magma.
  • It forms 83 per cent of the earth’s volume.
  • The outer layer of the mantle is partly simatic while the inner layer is composed of wholly simatic ultra-basic rocks.

Earth’s Layers – Asthenosphere

  • The upper portion of the mantle
  • The word astheno means weak.
  • Extending up to 400 km.
  • Main source of magma that finds its way to the surface during volcanic eruptions. It has a density higher than the crust’s.

Earth’s Layers – Core

  • Lies between 2900 km and 6400 km below the earth’s surface.
  • Accounts for 16 per cent of the earth’s volume.
  • Core has the heaviest mineral materials of highest density.
  • It is composed of nickel and iron [nife].
  • The outer core is liquid while the inner core is solid.
  • A zone of mixed heavy metals + silicates separates the core from outer layers.

Earth’s Layers – Seismic Discontinuities

  1. Mohorovicic Discontinuity (Moho) – separates the crust from the mantle, its average depth being about 35 km.
  2. A soft asthenosphere (highly viscous, mechanically weak and ductile). It’s a part of mantle.

Gutenberg Discontinuity – lies between the mantle and the outer core. Below 2900 km from earth’s surface of older alluvial and is spread towards the west.

 

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