Interior of the Earth Revision Notes - UPSC PDF Download

The earth’s radius is 6,370 km.
Two type of source of information:
(i) Indirect
(ii) Direct

Direct Sources

• Surface rocks and  the rocks we get from mining areas.
  Example- Gold mines in South Africa are as deep as 3 to 4 km.
• Scientific drilling projects to explore the conditions inside the Earth’s crust.
  Example- 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.

Kola Borehole

• Volcanic eruption forms another source of obtaining direct information. The molten material (magma) thrown onto the surface of the earth, during volcanic eruptions are used for laboratory analysis.

These have provided large volumes of information through the analysis of materials collected at different depths.

Indirect Sources

• Mining: It is known that temperature, density and pressure increase with increasing depths as we mine deeper.
• Meteors: The material and the structure observed in the meteors are similar to that of the Earth and hence provides a clue to Earth’s interiors.
• Other indirect sources include gravitation, magnetic field, and seismic activity.
• Gravity: The gravitational force (g) is not the same at different latitudes on the surface. It is greater near the poles and less at the equator as the distance from the centre at the equator is greater than that at the poles. Such differences in gravity are called gravity anomalies. They provide information about the distribution of mass of the material in the crust of the earth.
• Geomagnetism: The existence of a magnetic field around Earth indicates the presence of moving charged particles in the Earth’s interior. Magnetic surveys also show the distribution of magnetic materials in the crust portion.
• Seismic activity: It is one of the most important sources of information about the interior of the earth. The seismic waves play an important part in this.

Earthquakes

• The study of seismic waves provides a complete picture of the layered interior.
• An earthquake in simple words is the shaking of the Earth.
• It is a natural event.
• It is caused due to the release of energy along a fault, which generates waves that travel in all directions.
• 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 waves travel in all directions.
• The point where the energy is released is called the focus of an earthquake, alternatively, it is called the hypocentre. (somewhere inside the earth)
• 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 (depth up to 200 km from the surface of the earth.)
• An instrument called ‘seismograph’ records the waves reaching the surface.
• Two types of waves — 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. Hence, the name body waves.
• The body waves interact with the surface rocks and generate a 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.
• The denser the material, the higher is the velocity.
• Their direction also changes as they reflect or refract when coming across materials with different densities.

Seismic waves

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’.

• The P-waves are similar to sound waves.
• They travel through gaseous, liquid and solid materials.(as sound)
• 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

S-waves: arrive at the surface with some time lag. These are called secondary waves.

• An important fact about S-waves is that they can travel only through solid materials.
• Reflection causes waves to rebound whereas refraction makes waves move in different directions.
• These waves are more destructive. They cause displacement of rocks, and hence, the collapse of structures occurs.
• 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.

Shadow Zone

Shadow Zones

• Earthquake waves get recorded on 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’.
• 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.

Types of Earthquakes

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

Measuring Earthquakes

Seismograph: Used for measurement 
of Earthquake intensity

• 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. It 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 and ranges from 1-12.

Effects of Earthquakes
Earthquakes are a natural hazard. The following are the immediate hazardous effects of earthquake:

• Ground Shaking
• Differential ground settlement
• Land and mudslides
• Soil liquefaction
• Ground lurching
• Avalanches
• Ground displacement
• Floods from dam and levee failures
• Fires
• Structural collapse
• Falling objects
• Tsunami
  (i) The effect of a tsunami would occur only if the epicentre of the tremor is below oceanic waters and the magnitude is sufficiently high.
  (ii) Tsunamis are waves generated by tremors and not an earthquake in itself.

Structure of The Earth

The Crust

• It is the outermost solid part of the earth and is brittle in nature.
• The thickness of the crust varies under the oceanic and continental areas. Oceanic crust is thinner as compared to the continental crust.
• Continental crust is made up of heavier rocks having density of 3 g/cm³.
• This type of rock found in the oceanic crust is basalt.
• The mean density of material in the oceanic crust is 2.7 g/cm³.

The Mantle

• The portion of the interior beyond the crust is called the mantle.
• The mantle extends up to a depth of 2,900 km.
• The upper portion of the mantle is called the asthenosphere, extending upto 400 km. It is the main source of magma that finds its way to the surface during volcanic eruptions. It has a density higher than the crust (3.4 g/cm³).
• The crust and the uppermost part of the mantle together forms the lithosphere. Its thickness ranges from 10-200 km.
• The lower mantle extends beyond the asthenosphere and is in solid state.

The Core

• 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 mantle contains a weaker zone called the asthenosphere.
• The material in the upper mantle(asthenosphere ) 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 chlorine, hydrogen and argon.
• 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

Shield Volcano

• 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.

Composite Volcanoes

Composite Volcano (Mt Cotopaxi, Ecuador)

• 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.

Mid-Ocean Ridge Volcanoes

Mid Oceanic Ridge Volcano

• These volcanoes occur in the oceanic areas formed as a result of divergent plate boundaries.
• 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 is released during volcanic eruptions on cooling develops into igneous rocks.
• The cooling may take place either on reaching the surface or also while the lava is still in the crustal portion.
• Depending on the location of the cooling of the lava, igneous rocks are classified as volcanic rocks (cooling at the surface) and plutonic rocks (cooling in the crust).
• The lava that cools within the crustal portions assumes different forms. These forms are called intrusive forms.

Caldera

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

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

Batholiths

Batholith

• 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

Laccolith

• 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 volcanoes, only these are located at deeper depths.
• It can be regarded as the localised source of lava that finds its way to the surface.

Lopolith, Phacolith and Sills

Phacoliths, lopoliths, sills, dykes 
and other intrusive volcanic landforms

• As and when the lava moves upwards, a portion of the same may tend to move in a horizontal direction wherever it finds a weak plane.
• It may get rested in different forms. In case it develops into a saucer shape, concave to the sky body, it is called lopolith.
• A wavy mass of intrusive rocks, at times, is found at the base of synclines or at the top of anticlines in folded igneous country. Such wavy materials have a definite conduit to source beneath in the form of magma chambers. These are called the phacoliths.
• The near horizontal bodies of  the intrusive igneous rocks are called sill or sheet, depending on the thickness of the material. The thinner ones are called sheets while the thick horizontal deposits are called sills.

Dykes

• When the lava makes its way through cracks and the fissures developed in the land, it solidifies almost perpendicular to the ground.
• It gets cooled in the same position to develop a wall-like structure. Such structures are called dykes.