Chapter 3 Interior of the Earth | class 11th | ncert quick revision notes geography

Notes of Chapter 3 Interior of the Earth Class 11th Geography

Sources of Information about the Interior

• The earth’s radius is 6,370 km.

• As no one can reach the centre of the earth, most of our knowledge about the interior of the earth is largely based on estimates and inferences.

• There are two types of source of information available:

→ Direct Sources

→ Indirect Sources (Analysis of materials)

Direct Sources

• Surface rock or the rocks we get from mining areas.

→ Example: Gold mines in South Africa which are as deep as 3-4 km.

• Scientists have taken up a number of projects to penetrate deeper depths to explore the conditions in the crustal portions.

→ Example: “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.

→ 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

• We know through the mining activity that temperature and pressure increase with the increasing distance from the surface towards the interior in deeper depths.

→ Moreover, it is also known that the density of the material also increases with depth.

→ Knowing the total thickness of the earth, scientists have estimated the values of temperature, pressure and the density of materials at different depths.

• Another source of information are the meteors that at times reach the earth.

→ However, it may be noted that the material that becomes available for analysis from meteors, is not from the interior of the earth but the material and the structure observed in the meteors are similar too that of the earth.

→ They are solid bodies developed out of materials same as, or similar to, our planet.

• The other indirect sources include gravitation, magnetic field, and seismic activity.

→ 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.

→ 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 is one of the most important sources of information about the interior of the earth.

Earthquake

• An earthquake in simple words is shaking of the earth.

• It is caused due to release of energy, which generates waves that travel in all directions.

Why does the earth shake?

• 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.

• The point on the surface, nearest to the focus, is called epicentre. It is the first one to experience the waves.

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 

→ 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.

• Body Waves are of two types – P-Waves and S-Waves

P-Waves

• These waves move faster and are the first to arrive at the surface and are also called ‘primary waves’.

• These are similar to sound waves and travel through gaseous, liquid and solid materials as sound.

• P-waves vibrate parallel to the direction of the wave.

S-Waves

• These waves arrive at the surface with some time lag and are called secondary waves.

• These waves can travel only through solid materials which has helped scientists to understand the structure of the interior of the earth.

• Reflection causes waves to rebound whereas refraction makes waves move in different directions.

• These waves are more destructive as 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

• Earthquake waves get recorded in seismo-graphs 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

• Tectonic Earthquakes: generated due to sliding of rocks along a fault plane.

• Volcanic Earthquakes:  A special class of tectonic earthquake. These are confined to areas of active volcanoes.

• Collapse earthquakes: In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors.

• Explosion earthquakes: Ground shaking may also occur due to the explosion of chemical or nuclear
devices.

• Reservoir induced earthquakes: The earthquakes that occur in the areas of large reservoirs.

Measuring Earthquakes

• The earthquake events are scaled either according to the magnitude or intensity of the shock.

Richter Scale

• The magnitude scale is known as the Richter scale. The magnitude is expressed in absolute numbers, 0-10.

Mercalli Scale

• The intensity scale is named after Mercalli, an Italian seismologist. The range of intensity scale is from 1-12.

Effects of Earthquakes

(i) Ground Shaking
(ii) Differential ground settlement
(iii) Land and mud slides
(iv) Soil liquefaction
(v) Ground lurching
(vi) Avalanches
(vii) Ground displacement
(viii) Floods from dam and levee failures
(ix) Fires
(x) Structural collapse
(xi) Falling objects
(xii) Tsunami

• The first six listed above have some bearings upon landforms, while others may be considered the effects causing immediate concern to the life and properties of people in the region.

• The effect of tsunami would occur only if the epicentre of the tremor is below oceanic waters and the magnitude is sufficiently high.

Structure of the Earth

The Crust

• It is the outermost solid part of the earth.

• It 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.

• The continental crust is thicker in the areas of major mountain systems.

• The type of rock found in the oceanic crust is basalt. The mean density of material in oceanic crust is 2.7 g/cm³.

The Mantle

• The portion of the interior beyond the crust is called the mantle.

• It extends from Moho’s discontinuity to a depth of 2,900 km.

• The upper portion of the mantle is called asthenosphere.
→It is the main source of magma that finds its way to the surface during volcanic eruptions.

• 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 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/cm³3

• 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.

• Volcanoes are classified on the basis of:
→ Nature of eruption
→ Form developed at the surface.

Types of Volcanoes

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.

• They become explosive if somehow water gets into the vent; otherwise, they are characterised by low-explosivity.

Composite Volcanoes

• These volcanoes are characterised by eruptions of cooler and more viscous lavas than basalt.

• These volcanoes often result in explosive eruptions.

• 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 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 and these forms are called intrusive forms.

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.

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.

Batholiths

• Batholiths are the cooled portion of magma chambers.

• 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.

Lacoliths

• 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.

Lapolith, Phacolith and Sills

• 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 lapolith.

• A wavy mass of intrusive rocks, at times, is found at the base of synclines or at the top of anticline 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 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.

• These are the most commonly found intrusive forms in the western Maharashtra area.