Interior Of The Earth

The interior of the Earth is not directly visible to us, but its structure can be understood through rock samples, volcanic materials, mining, seismic waves, meteorites and changes in temperature, pressure and density.

The Earth is made up of different layers. These layers are different in composition, density, temperature and physical state.

Classification Of Earth’s Interior

The Earth can be divided in two main ways:

• Mechanical division
• Chemical or compositional division

Mechanical Division

Mechanically, the Earth is divided into:

• Lithosphere
• Asthenosphere
• Mesospheric mantle
• Outer core
• Inner core

Chemical Or Compositional Division

On the basis of composition, the Earth is divided into:

• Crust
• Mantle
• Core

The mantle is further divided into upper mantle and lower mantle.

The core is divided into outer core and inner core.

Lithosphere

The lithosphere is the rigid outer part of the Earth.

It includes:

• Crust
• Uppermost part of the mantle

Its thickness varies between 10–200 Km.

The lithosphere is broken into tectonic plates. The movement of these plates causes large-scale geological changes such as folding and faulting.

Crust

The crust is the outermost solid part of the Earth.

Its thickness is different under oceans and continents.

Oceanic Crust

The oceanic crust is thinner than continental crust.

Its average thickness is about 5 Km.

It is mainly made up of basaltic rocks.

The lower part of the crust contains mainly:

• Silica
• Iron
• Magnesium

This part is also known as SIMA.

Continental Crust

The continental crust is thicker than oceanic crust.

Its average thickness is around 30 Km.

In major mountain systems, it may become thicker than 70 Km.

In the Himalayan region, it may be 70–100 Km thick.

The upper part of the crust consists mainly of granitic rocks.

It contains mainly:

• Silica
• Aluminium

This part is also known as SIAL.

Temperature And Density In Crust

Density increases with depth.

The average density of Earth is 5.51 g/cm³.

The temperature of the crust also increases with depth. Near the boundary with the mantle, it may reach about 200°C to 400°C.

Mantle

The mantle lies below the crust.

It extends up to a depth of about 2900 Km.

The mantle is made up of very dense rocks and is rich in minerals like olivine.

It is divided into:

• Upper mantle
• Lower mantle

The upper part of the mantle, along with the crust, forms the lithosphere.

Asthenosphere

The asthenosphere lies below the lithosphere.

It is a weak and partially molten zone.

The weakest zone of the asthenosphere lies roughly at a depth of about 200 Km from the Earth’s surface.

This layer is important because lithospheric plates move over it.

Core

The core is the innermost part of the Earth.

It is mainly made up of iron and nickel, so it is also called NiFe.

The core is divided into:

• Outer core
• Inner core

Outer Core

The outer core is in a liquid state.

Its density ranges from 9.9 g/cm³ to 12.2 g/cm³.

The temperature of the outer core ranges from about 4400°C to 6000°C.

The movement of electrically conducting material in the liquid outer core is considered the main reason for Earth’s magnetism.

Inner Core

The inner core is solid.

Its density ranges from 12.6 g/cm³ to 13 g/cm³.

Discontinuities Inside The Earth

Different layers inside Earth are separated by transition zones called discontinuities.

These discontinuities show clear changes in density, seismic wave velocity, temperature and pressure.

Major Discontinuities

Discontinuity	Location
Conrad Discontinuity	Between outer crust and inner crust
Mohorovicic Discontinuity	Between crust and mantle
Repetti Discontinuity	Between upper mantle and lower mantle
Gutenberg Discontinuity	Between mantle and core
Lehmann Discontinuity	Between outer core and inner core

The core-mantle boundary is located at about 2900 Km depth. This boundary is called the Gutenberg Discontinuity.

Sources Of Information About Earth’s Interior

Information about Earth’s interior is obtained through direct and indirect sources.

Direct Sources

Direct sources include:

• Rocks from mining areas
• Volcanic eruptions

These give actual material from inside the Earth, but only from limited depths.

Indirect Sources

Indirect sources include:

• Seismic waves
• Temperature, pressure and density changes
• Meteors
• Magnetic sources

Seismic waves are especially important because they help scientists understand the internal structure of the Earth.

Seismic Waves

Seismic waves are waves generated during earthquakes.

They are recorded by an instrument called a seismograph.

There are two major types of seismic waves:

• Body waves
• Surface waves

Body Waves

Body waves travel through the interior of the Earth.

They are of two types:

• P-waves
• S-waves

P-Waves

P-waves are also called primary waves.

They are compressional and longitudinal in nature.

They can travel through:

• Solids
• Liquids
• Gases

P-waves are the fastest seismic waves.

S-Waves

S-waves are also called secondary waves.

They are transverse waves.

They can travel only through solids.

They cannot pass through liquids.

This is important evidence that the outer core of Earth is liquid.

Surface Waves

Surface waves travel along the Earth’s surface.

They decay more rapidly but are more destructive.

Shadow Zones

Shadow zones help scientists understand the structure of Earth’s interior.

P-Wave Shadow Zone

P-waves can pass through solids and liquids, but when they enter the liquid outer core, they bend due to refraction.

This creates a P-wave shadow zone between 105° and 145° from the epicentre.

S-Wave Shadow Zone

S-waves cannot pass through liquids.

When S-waves reach the liquid outer core, they are blocked.

The entire region beyond 105° from the epicentre becomes an S-wave shadow zone.

The pattern of shadow zones proves that the outer core is liquid and the inner core is solid.

Earthquakes

An earthquake is the shaking of the Earth’s surface caused by the sudden release of energy inside the Earth.

This energy travels in the form of seismic waves.

Focus And Epicentre

The point inside the Earth where energy is released is called the focus or hypocentre.

The point on the Earth’s surface directly above the focus is called the epicentre.

The epicentre is the first surface point to experience earthquake waves.

Isoseismic And Homoseismal Lines

An isoseismic line connects all places where earthquake intensity is the same.

A homoseismal line connects places affected by earthquake waves at the same time.

Causes Of Earthquakes

Natural Causes

• Movement of tectonic plates
• Volcanic activity
• Collapse of underground rocks

Human-Induced Causes

Human activities may also cause earthquakes.

Important causes include:

• Reservoir-induced seismicity
• Underground mining
• Blasting
• Hydraulic fracturing or fracking

The 1967 Koyna Dam Event is an example of reservoir-induced seismicity.

Types Of Earthquakes

Tectonic Earthquake

This is the most common type of earthquake.

It is caused by the movement of tectonic plates.

Volcanic Earthquake

This type of earthquake occurs before or after a volcanic eruption.

It is caused by the movement of magma.

Collapse Earthquake

This occurs in underground mines due to pressure generated within rocks.

Explosion Earthquake

This is artificial and is caused by high-density explosions such as nuclear explosions.

Reservoir-Induced Earthquake

This occurs due to the filling or operation of large reservoirs behind dams.

The weight of water and seepage into rocks may trigger seismic activity.

Effects Of Earthquakes

Earthquakes can produce primary, secondary and tertiary effects.

Primary Effects

Primary effects occur directly due to ground shaking.

They include:

• Ground shaking
• Surface rupture
• Damage to buildings and roads

Secondary Effects

Secondary effects are indirect results of earthquakes.

They include:

• Landslides
• Liquefaction
• Floods
• Fire hazards
• Gas pipeline rupture
• Electrical short circuits

The 1906 San Francisco earthquake is an important example linked with fire hazards.

Tertiary Effects

Tertiary effects include long-term social, economic and environmental impacts.

They may include:

• Destruction of property
• Homelessness
• Migration
• Psychological trauma
• Environmental damage
• Pressure on administration and relief work

Measurement Of Earthquakes

Earthquakes are measured mainly by:

• Magnitude scale
• Intensity scale

Richter Scale

The Richter Scale measures the magnitude or energy released by an earthquake.

It is also called the Local Magnitude Scale.

It was developed in 1935 by Charles F. Richter and Beno Gutenberg.

It is a logarithmic scale based on the maximum amplitude of seismic waves recorded by a seismograph.

Each increase of one unit represents a 10-fold increase in wave amplitude.

Mercalli Scale

The Mercalli Scale measures the intensity of an earthquake at a specific location.

It is based on:

• Shaking felt by people
• Damage to buildings
• Effects on the environment

It is expressed in Roman numerals from I to XII.

It was originally developed by Giuseppe Mercalli in 1902 and later revised as the Modified Mercalli Intensity Scale in 1931.

Tsunami

A tsunami is a series of long-wavelength sea waves caused by sudden displacement of a large volume of water.

The word tsunami comes from Japanese words:

• Tsu – harbour
• Nami – wave

Causes Of Tsunami

Tsunamis may be caused by:

• Undersea earthquakes
• Submarine volcanic eruptions
• Landslides
• Asteroid impacts

Tsunamis can travel across ocean basins at speeds of about 700–800 Km/h in deep oceans.

In the open ocean, tsunami waves may be less than one metre high and may pass unnoticed by ships.

Volcanoes

A volcano is an opening in the Earth’s crust through which molten material, gases, ash and rock fragments come out.

Volcanic eruptions are often accompanied by earthquakes.

The molten material below the Earth’s surface is called magma.

When magma comes out on the surface, it is called lava.

Types Of Volcanoes Based On Activity

Active Volcanoes

Active volcanoes erupt frequently or have erupted in recent times.

Dormant Volcanoes

Dormant volcanoes have not erupted in recent times but may erupt again.

They still retain volcanic features.

Extinct Volcanoes

Extinct volcanoes have not erupted in the distant geological past and are not expected to erupt again.

They show no signs of magma supply, seismic activity or volcanic gas emission.

Examples include:

• Mount Buninyong
• Dhosi Hill
• Ben Nevis
• Parts of the Deccan Plateau

Types Of Volcanoes Based On Nature Of Eruption

Shield Volcanoes

Shield volcanoes are broad and gently sloping.

They are formed by low-viscosity basaltic lava.

Examples:

• Mauna Loa
• Kilauea

Composite Volcanoes

Composite volcanoes are also called stratovolcanoes.

They erupt thick, sticky lava rich in silica.

They are often explosive and are commonly found at convergent plate boundaries.

Examples:

• Mayon Volcano
• Mount Fuji
• Mount Rainier

Calderas

Calderas form when a large volume of magma is emptied and the overlying land collapses inward.

Flood Basalt Provinces

These are formed by highly fluid basaltic lava that spreads over large areas.

Mid-Ocean Ridge Volcanoes

These volcanoes occur underwater along mid-ocean ridges.

They are found at divergent plate boundaries, where magma rises and forms new oceanic crust.

Intrusive Volcanic Forms

When magma does not reach the surface and solidifies inside the crust, it forms intrusive volcanic bodies.

Batholiths

Batholiths are large intrusive igneous bodies formed deep inside the Earth.

Laccoliths

Laccoliths are dome-shaped intrusive bodies.

They form when magma pushes overlying rock layers upward.

Lopoliths

Lopoliths are saucer-shaped intrusive bodies.

They are concave upward in shape.

Phacoliths

Phacoliths are lens-shaped intrusive bodies found in folded rock structures.

They occur at the crest of anticlines or troughs of synclines.

Sills

Sills are horizontal or near-horizontal intrusive sheets.

They are parallel to the bedding planes of surrounding rocks.

Dykes

Dykes are vertical or steeply inclined intrusive bodies.

They cut across existing rock layers.

Dykes are widely found in western Maharashtra and are considered feeder channels for the Deccan Traps.

Geyser

A geyser is a natural fountain of hot water and steam that erupts intermittently from the Earth’s surface.

It is a type of hot spring, but unlike ordinary hot springs that flow continuously, geysers erupt periodically because of underground heating and pressure build-up.

Formation Of A Geyser

• Rainwater seeps deep into the Earth through cracks and fissures.
• This groundwater is heated by hot rocks near magma.
• At greater depth, water becomes superheated but remains liquid due to high pressure.
• When pressure crosses a critical limit, superheated water changes into steam and escapes forcefully through surface vents.
• After eruption, the system refills with groundwater, reheats and repeats the cycle.

Fumaroles

Fumaroles are steam vents found in volcanic and geothermal areas.

Instead of lava, they release hot gases and water vapour, often containing carbon dioxide and sulphur compounds.

Formation Of Fumaroles

• After a volcanic eruption, magma beneath the surface cools and releases dissolved gases.
• Volcanic gases escape through cracks and fractures in the crust.
• These gases reach the surface through vents called fumaroles.
• They often produce steam clouds and hissing sounds.
• The Valley of Ten Thousand Smokes in Alaska is a well-known example of extensive fumarolic activity.

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