INTERIOR OF THE EARTH

Start music   Dec 30 2016 || 1:07 PM

The interior of the earth can be understood by indirect evidences as neither any one has nor anyone can reach the interior of the earth.

Exogenic as well as endogenic processes are constantly shaping the landscape.

A proper understanding of the physiographic character of a region remains incomplete if the effects of endogenic processes are ignored.

 

Why to study Interior of the Earth?

  • Human

    life is largely influenced by the physiography of the region. Therefore, it is necessary that one gets acquainted with the forces that influence landscape development.

  • To understand why the earth shakes or how a tsunami wave is generated, it is necessary that we know certain details of the interior of the earth.

 

SOURCES OF INFORMATION ABOUT THE INTERIOR

The earth’s radius is 6,370 km

 

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.
  • Scientists world over are working on two major projects such as “Deep Ocean Drilling Project” and “Integrated Ocean Drilling Project”. The deepest drill at Kola, in Arctic Ocean, has so far reached a depth of 12 km.
  • Volcanic eruption forms another source of obtaining direct information. As and when the molten material (magma) is thrown onto the surface of the earth, during volcanic eruption it becomes available for laboratory analysis.

 

Indirect Sources

  1. It is known that the density of the material also increases with depth.
  2. Knowing the total thickness of the earth, scientists have estimated the values of temperature, pressure and the density of materials at different depths.
  3. The meteors that at times reach the earth can be an indirect source. The material and the structure observed in the meteors are similar to that of the earth. They are solid bodies developed out of materials same as, or similar to, our planet.
  4. The other indirect sources include

Gravitation:

  • The gravitation force (g) is not the same at different latitudes on the surface.
  • It is greater near the poles and less at the equator.
  • This is because of the distance from the centre at the equator being greater than that at the poles. The gravity values also differ according to the mass of material.
  • The uneven distribution of mass of material within the earth influences this value.
  • The reading of the gravity at different places is influenced by many other factors.
  • These readings differ from the expected values. Such a difference is called gravity anomaly.
  • Gravity anomalies give us information about the distribution of mass of the material in the crust of the earth.

Magnetic field:

  • Magnetic surveys also provide information about the distribution of magnetic materials in the crustal portion, and thus, provide information about the distribution of materials in this part.

Seismic activity:

  • It is one of the most important sources of information about the interior of the earth.
  • Hence, we shall discuss it in some detail.

 

Earthquake

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

It is a natural event and is caused due to release of energy, which generates waves that travel in all directions.

Why does the earth shake?

  • The release of energy occurs along a fault. A fault is a sharp break in the crustal rocks.
  • Rocks along a fault tend to move in opposite directions. As the overlying rock strata press them, the friction locks them together.
  • However, their tendency to move apart at some point of time overcomes the friction. As a result, the blocks get deformed and eventually, they slide past one another abruptly.
  • This causes a release of energy, and the energy wave travels in all directions.
  • The point where the energy is released is called the focus of an earthquake, alternatively, it is called the hypocentre.
  • The energy waves travelling in different directions reach the surface.
  • The point on the surface, nearest to the focus, is called epicentre. 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.
  • An instrument called ‘seismograph’ records the waves reaching the surface. A curve of earthquake waves recorded on the seismograph.
  • Earthquake waves are basically of two types — body waves and surface waves.
  • Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth
  • The body waves interact with the surface rocks and generate new set of waves called surface waves. These waves move along the surface.
  • The velocity of waves changes as they travel through materials with different densities.
  1. The denser the material, the higher is the velocity.
  2. Their direction also changes as they reflect or refract when coming across materials with different densities.
  3. As a result, it creates density differences in the material leading to stretching and squeezing of the material.

 

  1. Body waves:

In Body waves the speed decreases with increasing density of rock and increases with increasing rock elasticity.

Rock elasticity increases faster than density with depth.

There are two kinds of body waves viz. P-waves and S-waves.

  1. Primary Waves or P-waves:

The Primary waves or Push waves are longitudinal / compression waves that vibrate parallel to the direction of wave movement.

They have shortest wavelength, fastest speed {5-7 km/s} and can travel through solid, liquid and gas. They travel fast in denser, solid materials.

  1. Secondary waves or S-waves:

Secondary waves or Sheer waves or shock waves are transverse waves which create vibrations perpendicular to the direction of wave movement.

The S waves only travel through solids because liquids and gases have no sheer strength.

They have a medium wavelength and cause vibrations at right angles to the direction of propagation of waves.

Their velocity is 3 to 4 km per second.

  1. Surface Waves:

Surface waves are of two types viz. Rayleigh Waves and Love waves

  1. Rayleigh Waves or L-waves:

L Waves or Surface Waves travel near the earth’s surface and within a depth of 30-32 kilometers from the surface.

These are also called Rayleigh waves after Lord Rayleigh who first described these waves.

Behave like water waves with elliptical motion of material in wave.

Generally slower than Love waves.

  1. Love waves:

Love waves make the ground vibrate at right angles to the direction of waves.

They are a variety of S-waves where the particles of an elastic medium vibrate transversely to the direction of wave propagation, with no vertical components.

Involve shear motion in a horizontal plane. Most destructive kind of seismic wave.

How Seismic waves help in defining Earth’s interior?

  • The speed of the seismic waves varies with the composition of the medium.
  • In earth crust their speed is around 2-8 kilometers per second, while in mantle the speed is up to 13 kilometer per second, because mantle is denser.
  • In his observations, Mohorovicic found that when the focus of the Earthquake is not too deep, some waves are propagated along the surface and remains in the crust, while other set enters the mantle, speeds up and reaches the seismometer first.
  • This means that for a seismograph stations located at about 150 Kilometers from a shallow focus earthquake epicentre received those waves first which came from beneath the ground via mantle.
  • This was enough to conclude that there is something below earth crust which has a greater density and varied composition. It was later called Mohorovicic discontinuity or simply Moho.
  • The above finding led to determine that mantle is denser than crust and is viscous, semi-molten material. P-wave velocities are much slower in the outer core than in the deep mantle while S-waves do not travel at all in the liquid portion of the outer core.

 

Emergence of Shadow Zone:

  • Earthquake waves get recorded in seismographs located at far off locations.
  • 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.
  • It was observed that seismographs located at any distance within 105° from the epicentre, recorded the arrival of both P and S-waves.
  • However, the seismographs located beyond 145° from epicentre, record the arrival of P-waves, but not that of S-waves.
  • Thus, a zone between 105° and 145° from epicentre 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 epicentre.
  • 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.
  • You can draw the shadow zone for any earthquake provided you know the location of the epicentre.

 

Types of Earthquakes:

  1. The most common ones are the tectonic earthquakes. These are generated due to sliding of rocks along a fault plane.
  2. A special class of tectonic earthquake is sometimes recognised as volcanic earthquake. However, these are confined to areas of active volcanoes.
  3. The earthquakes that occur in the areas of large reservoirs are referred to as reservoir induced earthquakes.
  4. In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors. These are called collapse earthquakes.
  5. Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes.

 

Measuring Earthquakes:

  • The earthquake events are scaled either according to the magnitude or intensity of the shock.
  • The magnitude scale is known as the Richter scale. The magnitude relates to the energy released during the quake. The magnitude is expressed in absolute numbers, 0-10.
  • The intensity scale is named after Mercalli, an Italian seismologist. The intensity scale takes into account the visible damage caused by the event. The range of intensity scale is from 1-12.

 

EFFECTS OF EARTHQUAKE

Earthquake is a natural hazard. The following are the immediate hazardous effects of earthquake:

Bearings upon landforms:

  1. Ground Shaking
  2. Differential ground settlement
  3. Land and mud slides
  4. Soil liquefaction
  5. Ground lurching
  6. Avalanches

Effects causing immediate concern to the life and properties of people:

  1. Ground displacement
  2. Floods from dam and levee failures
  3. Fires
  4. Structural collapse
  5. Falling objects
  6. Tsunami

 

 

STRUCTURE OF THE EARTH

The Crust

  • It is the outermost solid part of the earth. It is brittle in nature.
  • Oceanic crust is thinner as compared to the continental crust. The mean thickness of oceanic crust is 5 km whereas that of the continental is around 30 km.
  • The continental crust is thicker in the areas of major mountain systems. It is as much as 70 km thick in the Himalayan region. It is made up of heavier rocks having density of 3 g/cm3.
  • Rock found in the oceanic crust is basalt. The mean density of material in oceanic crust is 2.7 g/cm3.
  • Crust contains SiAl ie., Silica and Aluminum.

The Mantle

  • The portion of the interior beyond the crust is called the mantle.
  • The mantle extends from Moho’s discontinuity to a depth of 2,900 km.
  • The upper portion of the mantle is called asthenosphere. The word astheno means weak. It is considered to be extending upto 400 km.
  • It is the main source of magma.
  • It has a density higher than the crust’s (3.4 g/cm3).
  • 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.
  • Mantle contains SiMa ie., Silica and Magnesium.

The Core

  • As indicated earlier, the earthquake wave velocities helped in understanding the existence of the core of the earth.
  • The core-mantle boundary is located at the depth of 2,900 km.
  • The outer core is in liquid state while the inner core is in solid state.
  • The density of material at the mantle core boundary is around 5 g/cm3 and at the centre of the earth at 6,300 km, the density value is around 13g/cm3.
  • The core is made up of very heavy material mostly constituted by nickel and iron. It is sometimes referred to as the nife layer.

 

VOLCANOES AND VOLCANIC LANDFORMS

  • A volcano is a place where gases, ashes and/or molten rock material – lava – escape to the ground. A volcano is called an active volcano if the materials mentioned are being released or have been released out in the recent past. The layer below the solid crust is mantle. It has higher density than that of the crust. The mantle contains a weaker zone called asthenosphere.
  • It is from this that the molten rock materials find their way to the surface. The material in the upper mantle portion is called magma.
  • Once it starts moving towards the crust or it reaches the surface, it is referred to as lava.
  • The material that reaches the ground includes lava flows, pyroclastic debris, volcanic bombs, ash and dust and gases such as nitrogen compounds, sulphur compounds and minor amounts of chlorene, hydrogen and argon.

Volcanoes

Volcanoes are classified on the basis of nature of eruption and the form developed at the surface. Major types of volcanoes are as follows:

Shield Volcanoes:

Barring the basalt flows, the shield volcanoes are the largest of all the volcanoes on the earth.

The Hawaiian volcanoes are the most famous examples.

These volcanoes are mostly made up of basalt, a type of lava that is very fluid when erupted. For this reason, these volcanoes are not steep. They become explosive if somehow water gets into the vent; otherwise, they are characterised by low-explosivity.

The upcoming lava moves in the form of a fountain and throws out the cone at the top of the vent and develops into cinder cone.

 

Composite Volcanoes:

 

  • These volcanoes are characterised by eruptions of cooler and more viscous lavas than basalt. These volcanoes often result in explosive eruptions.
  • Along with lava, large quantities of pyroclastic material and ashes find their way to the ground.
  • This material accumulates in the vicinity of the vent openings leading to formation of layers, and this makes the mounts appear as composite volcanoes.

Caldera:

  • These are the most explosive of the earth’s volcanoes. They are usually so explosive that when they erupt they tend to collapse on themselves rather than building any tall structure.
  • The collapsed depressions are called calderas. Their explosiveness indicates that the magma chamber supplying the lava is not only huge but is also in close vicinity.

Flood Basalt Provinces:

  • These volcanoes outpour highly fluid lava that flows for long distances. Some parts of the world are covered by thousands of sq. km of thick basalt lava flows.
  • There can be a series of flows with some flows attaining thickness of more than 50 m. Individual flows may extend for hundreds of km.
  • The Deccan Traps from India, presently covering most of the Maharashtra plateau, are a much larger flood basalt province.

Mid-Ocean Ridge Volcanoes:

  • These volcanoes occur in the oceanic areas.
  • There is a system of mid-ocean ridges more than 70,000 km long that stretches through all the ocean basins.
  • The central portion of this ridge experiences frequent eruptions.

 

VOLCANIC LANDFORMS

Intrusive Forms

The lava that cools within the crustal portions assumes different forms. These forms are called intrusive forms.

Batholiths:

  • A large body of magmatic material that cools in the deeper depth of the crust develops in the form of large domes.
  • They appear on the surface only after the denudational processes remove the overlying materials.
  • They cover large areas, and at times, assume depth that may be several km. These are granitic bodies. Batholiths are the cooled portion of magma chambers.

Laccoliths:

  • These are large dome-shaped intrusive bodies with a level base and connected by a pipe-like conduit from below.
  • It resembles the surface volcanic domes of composite volcano, only these are located at deeper depths.
  • It can be regarded as the localised source of lava that finds its way to the surface.
  • The Karnataka plateau is spotted with domal hills of granite rocks.

Sill:

  • A sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock.

Dykes:

  • A dyke is called a -usually more or less vertical- flat, sheet-like magma body that cuts unconformingly through older rocks or sediments.
  • Most dykes can be described as fractures into which magma intrudes or from which they might erupt.