Interior of the Earth [UPSC Geography Notes]

Geography is an important part of the UPSC Syllabus. It features prominently in both the UPSC Prelims and the Mains exams. In this article, we give you detailed notes for the chapter ‘Interior of the Earth’. You will also learn about earthquakes, parts of an earthquake, volcanoes, volcanic landforms, etc. for the IAS exam.

Earth’s Interior

Under this section, you can read more about the interior structure of the earth.

Structure of the Earth

The earth’s surface is being continuously reshaped by both the endogenic forces (internal) and exogenic forces (external). The changes brought about by these processes are known as ‘geomorphic processes’.

• Diastrophism is the process by which the earth’s surface is reshaped through rock movements and displacements. This includes both orogenic (mountain building) and epeirogenic processes (continental forming).

Earth’s interior can be divided into the crust, upper mantle, lower mantle, outer core and inner core. The temperature increases as we move from the crust to the core, generally the temperature increases by 1℃ for every 32 m towards the interior of the earth.

Crust

• It is the outermost solid part of the earth.
• The crust is further divided into upper crust (continental crust) composed of silica and aluminium (sial) and lower crust (oceanic crust) made up of silica and magnesium (sima). The boundary between the upper crust and the lower crust is called the “Conorod boundary”.
• The thickness of the crust varies under the oceanic and continental areas. Continental crust is thicker as compared to the oceanic crust. The mean thickness of the continental crust is about 32 km whereas that of oceanic crust is 5 km. The continental crust is thicker in the areas of major mountain systems. It is about 70 km in the Himalayan region.
• The density of the crust is less than 2.7g/ cm³.

Mantle

• The portion of the earth beyond the crust is called the mantle. It is made up of magnesium, silica and iron. It extends to a depth of about 2900 km.
• The mantle is divided into upper mantle and lower mantle. The upper portion of the mantle is called the asthenosphere. The word “astheno” means weak. Asthenosphere extends up to 400 km and is the main source of magma which comes over to the surface during volcanic eruptions.
• The boundary which divides the lower crust and the upper mantle is called the “Mohorovicic”.
• Its density is 3.9g/ cm³.
• The crust and the uppermost part of the mantle is called lithosphere. Its thickness ranges from 10 – 200 km.

Core

• The core is composed of heavy material mainly iron and nickel which is called NiFe (Barysphere).
• It forms the centre of the earth and its density is 13g/ cm³.
• The outer core is in liquid state and the inner core is in solid state.
• The temperature of the core ranges between 5500℃ – 6000℃.
• Guttenberg margin is the boundary between the lower mantle and the outer core. The Lehmann boundary separates the outer core and the inner core.
• The core extends from 2900 km to 6378 km from the surface of the earth.

Read more UPSC Geography notes in the link

Earthquake

An earthquake is the sudden shaking of the earth’s surface. Earthquakes occur due to the release of energy which generates seismic waves that travel in all directions. The study of seismic waves provides information about the interior of the earth.

• Focus is the point where the energy is released during an earthquake, also called hypocentre. The seismic waves travel in all directions and reach the surface. Epicentre is the point on the surface nearest to the focus. It is directly above the focus.
• All natural earthquakes occur in the lithosphere. It extends up to the depth of 200 km from the surface of the earth.
• An instrument called a “seismograph” records the waves reaching the earth’s surface. The earthquake waves/seismic waves are broadly of two types – body waves and surface waves.
  • Body waves – Body waves are generated due to release of energy at the focus and these waves travel in all directions through the interior of the earth. There are two types of body waves:
    • P-waves or Primary waves or Compressional waves – P-waves travel faster, about 6 km per sec in the upper crust and are first to arrive at the surface. These waves are similar to sound waves as they travel through gas, liquid as well as solid materials. P-waves vibrate parallel to the direction of the wave. These waves exert pressure on the material in the direction of propagation thereby, creating a density difference in the material leading to stretching and compression of the material.
    • S-waves or Secondary waves or Shear waves – S-waves arrive at the surface with some time lag, and are slower (about 3.5 km per sec in the upper crust). S-waves only travel through solid materials. This characteristic feature of S-waves helps in understanding the structure of the interior of the earth. S-waves vibrate perpendicular to the wave direction in the vertical plane. They create crests and troughs in the material through which they travel.
  • Surface waves – The body waves interact with the surface rocks and generate new sets of waves called surface waves. These waves move along the surface. These waves are more destructive, causing the displacement of rocks thereby collapsing the structure.

Shadow Zone

• Seismographs are located at far off places to record the seismic waves. However, there are certain areas where the waves are not reported. Such a zone is called a “shadow zone”. For each earthquake, there exists an altogether different shadow zone.
• When a seismograph is located at any distance within 105° from the epicentre, it records both P-waves and S-waves.
• When a seismograph is located beyond 145° from the epicentre, it records only P-waves. A zone between 105° and 145° from the epicentre was identified as the shadow zone for both types of seismic waves.
• The shadow zone of S-waves is much larger than that of P-waves and is over 40% of the earth’s surface. The shadow zone of P-waves appears as a band around the earth between 105°-145° away from the epicentre.

Measuring Earthquakes

• Earthquakes are measured either in terms of magnitude or intensity of the shock. Earthquake magnitude is measured on the Richter scale (named after the seismologist who devised it). The magnitude implies the energy released during the earthquake and is expressed in numbers 0 to 10.
• Earthquake intensity is measured on the modified Mercalli scale, which ranges from 0 to 12 depending upon the intensity. The intensity scale takes into account the visible damage caused by the earthquake.

Causes of Earthquakes

Some of the major factors which cause earthquakes are –

• Plate tectonic movements – These are generated due to the sliding of rocks along a fault plane.
• Volcanic eruptions – Earthquakes due to volcanic activity are confined to areas of active volcanoes.
• Construction of large dams results in an earthquake e.g., Koyna dam (Maharashtra).
• Nuclear explosions release enormous energy which causes tremors in the earth’s crust.
• In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors.

Distribution of Earthquakes

Earthquakes have a definite distribution pattern. There are three belts in the world that are frequented by earthquakes of varying intensity. These are:

1. Circum-Pacific region (Ring of fire) – This belt is located around the coast of the Pacific Ocean. It extends through the coasts of Alaska, Aleutian Islands, Japan, the Philippines, New Zealand, west coast of north and south America. This belt has about 68% of the total earthquakes that are recorded in the world. These belts being the zones of convergent plate boundaries (the subduction zones) are isostatically very unstable. Japan experiences about 1500 earthquakes every year.
2. Mediterranean-Himalayan region – This extends from the Alps mountain to the Himalayan mountains and Tibet to China. About 31% of the world’s earthquakes occur in this region.
3. Other areas – These include Northern Africa and rift valley areas of the Red Sea and the Dead Sea.

Volcano

A volcano is an opening in the earth’s crust through which gases, ashes and molten rock material are released to the earth’s surface.

• The upper position of the mantle of the earth is called the asthenosphere which is a weaker zone. It is from this weaker zone the molten rock materials find their way to the surface.
• The molten rock material found in the interior of the earth is called magma. Once the magma reaches the earth’s surface it is called lava.
• The materials that reach the earth’s surface include lava flows, pyroclastic debris, volcanic bombs, ash, dust and gases such as sulphur compounds, nitrogen compounds and some amounts of chlorine, hydrogen and argon.
• Fumaroles are the gushing fumes through the gap in the volcano. Crater is a saucer shaped depression in the mouth of a volcano. When the crater is widened it is called Caldera. Volcano generally erupts either through the vent (Mt. Fujiyama, Japan) or fissure (The Deccan Plateau, India).

Causes of Volcanic Eruptions

The following are the causes of volcanic eruptions –

1. Magma, in the interior of the earth, is often saturated with gases like carbon dioxide and hydrogen sulphide. These gases with water vapours make the magma highly explosive. Magma is forced out as lava on the surface of the earth due to pressure exerted by these gases.
2. When two tectonic plates collide against each other or drift apart the region becomes weak. Volcanoes may erupt in such zones, e.g., Africa and Eurasian plates.

Types of Volcanoes

There are three types of volcanoes based on the frequency of eruption. These are –

1. Active Volcanoes – A volcano is called an active volcano if the eruptions are frequent. Their vent usually remains open. For example, Mount Etna ( in Italy), Cotopax (in Ecuador is the world’s highest active volcano).
2. Dormant Volcano – These volcanoes may not have erupted in the recent past but there is a possibility of eruption at any time. For example, Mt. Vesuvius (Italy) and Mt. Fujiyama (Japan).
3. Extinct Volcano – These volcanoes have not erupted during the known geological period. Their vent remains closed with solidified lava. The craters may be filled with water giving rise to crater lakes. The slopes of these landforms may be covered with vegetation. For example, Popa (Myanmar) and Mt. Kenya (Eastern Africa).

On the basis of the nature of eruption and form developed at the surface, volcanoes are classified into the following types –

1. Shield Volcanoes – These volcanoes are mostly made up of basalt, a type of lava that is very fluid when erupted. They become explosive if water gets into the vent, otherwise they are less explosive. The upcoming lava moves in the form of a fountain and throws out the cone at the top of the vent and develops into a cinder cone. For example, Hawaiian volcanoes.
2. Composite volcanoes/Stratovolcanoes – Along with the lava, large quantities of pyroclastic material and ashes form part of the eruptions. This material accumulates in the vicinity of the vent openings resulting in the formation of layers which makes the mount appear as composite volcanoes. These volcanoes often result in explosive eruptions. For example, Mount Vesuvius (Italy), Mount Fuji in Japan.
3. Caldera – These volcanoes are most explosive. When these volcanoes erupt they collapse on themselves rather than building any tall structure. The resulting depressions are called calderas.
4. Flood Basalt Provinces – These volcanoes erupt highly fluid lava that flows for long distances. There are some regions in the world which are covered by thousands of sq. km of thick basalt lava flows. Individual flows may extend for hundreds of kilometers. For example, the Deccan Traps in Maharashtra, India.
5. Mid Ocean Ridge Volcanoes – These volcanoes are found 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

During volcanic eruptions, the lava that is released on cooling develops into igneous rocks (primary rocks). The cooling may take place either on reaching the surface or within the crust. Depending on the location of cooling, igneous rocks are of two types – volcanic rocks (when cooling takes place at the surface) and plutonic rocks (when cooling occurs within the crust). The lava that cools within the crust assumes different forms called intrusive forms. These are –

1. Batholiths – These are large rock masses that are formed due to cooling and solidification of hot magma inside the earth. These are granitic bodies.
2. Laccoliths – Laccoliths are large dome-shaped intrusive rock connected by a pipe-like conduit from below. It resembles the surface volcanic domes of composite volcanoes, only these are located at deeper depths. The Karnataka plateau is spotted with domal hills of granite rocks. Most of these, now exfoliated, are examples of Laccoliths or Batholiths.
3. Lapoliths – When the magma moves upwards, a saucer-shaped, concave-shaped body called lapolith is formed.
4. Phacoliths – 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 (subsequently developed as batholiths). These are called phacoliths.
5. Sill or Sheet – The near-horizontal solidified lava layer (intrusive igneous rocks) are called sill or sheet, depending on the thickness of the lava. The thick deposits are called sills while the thinner ones are called sheets.
6. Dykes – When magma 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. Mostly found in the western Maharashtra region. These are believed to be the feeders for the eruptions that led to the formation of the Deccan Trap.

Distribution of Volcanoes across the World

The major regions of volcanic distribution are as follows:

1. Pacific Ring of Fire – Circum-Pacific region, also called the Pacific Ring of Fire, has the highest number of active volcanoes. The earthquake belt and volcanic belt closely overlap along the “Pacific Ring of Fire”. It almost includes two-thirds of the world’s volcanoes.
2. Mid-Atlantic region – This region has comparatively less active volcanoes but many extinct or dormant volcanoes. For example, Cape Verde Islands, St. Helena and the Canary Islands. The volcanoes of Iceland and Azores are active.
3. The Great Rift Valley of Africa – Some volcanoes are present along the East African Rift valley. Kilimanjaro and Mt. Kenya are extinct volcanoes. Mt. Cameroon is the only active volcano in west Africa.
4. Mediterranean Region – For example, Mt. Vesuvius, Mt. Stromboli, known as the “lighthouse of the Mediterranean sea”. Volcanoes of the Mediterranean are mainly associated with the Alpine folds.
5. Other regions – In the interiors of continents of Asia, North America and Europe active volcanoes are rare. There are no volcanoes in Australia.

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