Extra Questions For Class 11 Geography Interior of the Earth -

Q 1 – What are Body waves?

Ans. The earthquake waves which travel through the solid body of the earth are called Body waves

Q 2 – Name two types of Body waves.

Ans. ‘P’ (primary) and ‘S’ (secondary) waves.

Q 3 – Where does the asthenosphere lie?

Ans. Asthenosphere lies in the upper mantle, or below the earth’s crust.

Q 4 – What is Gutenberg discontinuity?

Ans. A plane of the discontinuous surface between the core and the mantle is known as Gutenberg discontinuity.

Q 5 – What is the effect on ‘P’ waves at the mantle-core boundary?

Ans. The ‘P’ waves make an abrupt drop in velocity at the mantle- core boundary.

Q 6 – What does ‘M’ discontinuity stand for?

Ans. It stands for Mohorovicic discontinuity.

Q 7 – Which region in the earth’s interior is referred to as the low-velocity zone?

Ans. Asthenosphere in the upper part of the mantle is referred to as the low-velocity zone.

Q 8 – Which layer in the interior of the earth consists of ‘shadow zone’?

Ans. Shadow zone lies within the earth’s core.

Q 9 – What is Love wave?

Ans. It is an earthquake wave that travels along the surface of the ‘ earth with a motion entirely horizontal.

Q 10 – Name the three layers of the earth.

Ans. They are core (barysphere), mantle (mesosphere) and crust (lithosphere).

Q 11 – What is a seismograph?

Ans. It is a sensitive instrument that records the intensity of vibrations of earthquake waves and helps in earthquake prediction.

Q 12 – Name two types of surface waves.

Ans. Rayleigh waves
Love waves

Q 13 – What is the temperature of the core?

Ans. The temperature of the core is about 2000°C.

Q 14 – What are the various sources of heat in the earth?

Ans. Radioactivity
The heat of aggradation
The temperature of materials of which the earth is made

Q 15 – What is the depth of the earth’s crust?

Ans. 0-100 km. from the surface of the earth.

Q 16 – What is the radius of the earth’s core?

Ans. 3470 km. approximately.

Q 17 – What do you mean by Sial and Sima?

Ans. It is the uppermost layer of the earth. It consists of silica and magnesium. Its average depth is 2800 km. It has a density of 4.75.

Q 18 – Define the term Nife.

Ans. The core of the earth is called Nife. It consists of Nickel and ferrous. It has an average depth of 3500 km. and a density of 13 g/cm3.

Q 19 – What is a caldera?

Ans. Caldera complexes are the most explosive of earth’s volcanoes.

Q 20– Name the basic parts of a volcano.

Ans. Crater, caldera, vent, fumaroles are the basic parts of a volcano.

Q 21 – Name the various intrusive forms.

Ans. Name the various intrusive forms.

Q 22 – Define ‘Pacific ring’.

Ans. It is the region around the Pacific ocean. There are active volcanoes all around the Pacific OceAnswer:

Q 23 – What is lithosphere?

Ans. The crust and the uppermost part of the mantle together are called the lithosphere

Q 24 – Name the different types of earthquakes.

Ans. Tectonic/Volcanic, Collapse, Explosion and Reservoir induced earthquake.]

Q 25 – How explosion earthquakes occurs?

Ans. Explosion earthquakes occur due to the explosion of chemical or nuclear devices

Q 26 – Name the direct sources which provide information about the earth’s interior.

Ans. Volcanic eruptions, Deep Ocean Drilling Project and mining.

Q 27 – How do the rocks of the earth’s mantle behave when subjected to the earthquake waves?

Ans. The earth’s mantle behaves as an elastic solid when subjected to the earthquake waves. It changes the shape of the landscape when stresses are applied, but returns exactly to its original shape when these stresses are removed.

Q 28 – What is asthenosphere?

Ans. The asthenosphere is the zone of hot rocks, believed to be in a plastic condition, underlying the solid lithosphere or the earth’s crust. It is sometimes termed as the soft layer of mantle or the low-velocity zone because the earthquake waves travel in it at reduced velocities.

Q 29 – Distinguish between Body waves and surface waves.

Ans. The earthquake waves which travel longitudinally through the solid body of the earth are called Body waves. They move-faster. Those waves which move transversally along the surface are known as Surface waves. They move slowly and cause more disaster.

Q 30 – Define surface waves along with their sub-types.

Ans. Hie earthquake waves which move along the free upper crust of the earth are called surface waves. Surface waves are of two types, viz, Rayleigh waves and Love waves. Rayleigh waves can be visualised as water waves travelling across the surface of a still pond after a pebble has been tossed into the water. But the motion in the Love waves is entirely horizontal, at right, angles to the direction of the wave motion. The Rayleigh and Love waves travel more or less the same length, but with different speeds

Q 31 – Describe Mohorovicic Discontinuity.

Ans. The surface of a sudden increase in wave velocity, which separates the crust above from the mantle below is Mohorovic or ‘Moho’ or ‘M’ discontinuity. It is named after the Yugoslavian seismologist Mohorovicic, who first detected it in 1909.

Q 32 – How is the crust distinguished from the mantle?

Ans. The crust is distinguished from the mantle by the presence of an abrupt change in the velocity of seismic waves. This corresponds to the abrupt change in the rigidity of the rock from crust to mantle. This change in rigidity is due to the change in the mineral composition or in the physical state of rocks.

Q 33 – Write a short note on the earth’s core.

Ans. ‘Core’ means the innermost part. Earth’s core is the innermost or central part of the earth’s interior. It has a radius of approximately 3470 km (2160 miles). On the basis of the behaviour of the seismic waves, it has been proved that the earth’s outer core is in a liquid state. Earth’s magnetic field is generated here. Between the outer core and the mantle that envelops it is the Gutenberg discontinuity.

Q 34 – Describe the earth’s crust.

Ans. Earth’s crust is the topmost layer of the earth’s interior. It is also called the lithosphere. It has an average density of 3.0 g/cm. Its thickness varies from about 7 km. beneath the oceans to 70 km. under some parts of the continents. It is separated from the mantle below by Mohorovicic Discontinuity.

Q 35 – Discuss the properties of the upper mantle.

Ans. Based on the behaviour of seismic waves, the mantle is subdivided into two major parts – the upper mantle and the lower mantle. The upper mantle extends from the crust to a depth of about 650 km. including the asthenosphere, which occupies the upper 300 to 400 km. The upper boundary of the upper mantle is forward by Mohorovicic Discontinuity, which separates it from the overlying crust.

Q 36 – How do the rocks of the earth’s mantle behave when subjected to the earthquake waves?

Ans The P’ waves move faster and make an abrupt drop in velocity of the mantle-core boundary, whereas ‘S’ waves move slowly and terminate at the mantle-core boundary. Through earth’s mantle (nearly ’ 2900 km) the speed of the earthquake waves is so high that only a very rigid and dense rock will satisfy the observed condition; the rocks behave as an elastic solid so that the mantle changes its shape when shear stresses are applied and returns exactly to its former shape when stresses are removed.

Q 37 – What is the main evidence in favour of the layered structure of the earth?

Ans. The scientists accept that the earth has a layered structure. The earth has three layers or shells :

crust
mantle
core.

These layers are distinguished on the basis of their physical and chemical properties, i.e.,

thickness,
density,
temperature,
metallic contents and
rocks.

Q 38 – What do you mean by the term ‘Barysphere’?

Ans. The Barysphere is the central core of the earth. It has a depth of 4980 km to 6400 km. It has a density of 17 g/cm3. It consists of heavy minerals and it is also called ‘core’. The core of the earth is metallic in > nature.

Q 39 – Describe the three types of earthquake waves.

Ans. These waves are :

‘P’waves or longitudinal waves-These are also known as primary waves. These travel in the direction of their movement. They can travel through solids as well as liquid and gaseous matter.
‘S’ waves or transverse waves – These are also known
as secondary waves. These travel at a right angle to the direction of their oscillation. They can travel in solid ‘? medium only.
‘L’ waves – These are known as surface waves. These waves do hot go deep into the earth.

Q 40 – What do you understand by the structure of the earth?

Ans. The earth is made up of many layers of different materials. The density and temperature of each layer vary according to the depth. The study of the layered composition of the earth is known as the structure of the earth.

Q 41 – Write short notes on :
1. Shadow zone
2. The earth’s crust

Ans. 1. Shadow zone – It lies beneath the surface of the earth, i.e., in its interior. The seismic waves bend as they travel through the core and, therefore, ‘P’ waves are not directly received in a zone known as the shadow zone. Also, ‘S’ waves are not received there because they do not travel through the liquid outer core. Only surface waves are received in the shadow zone

2. The earth’s crust – This is also known as the lithosphere. The crust is the outermost shell 1 of the earth. It consists of the surface granite SIAL and the intermediate basic SIMA layers. It is separated from the under layer MANTLE by the Mohorovicic Discontinuity. There are two kinds of crust – continental and oceanic. Continental crust has an average density of 3 g/cnt3, the average thickness of 35 to 40 km. (22 to 25 miles) with large areas older than 1500 million years. Continental crust is a complicated structure and has a variable composition. Oceanic crust is thinner than continental crust. Its average density is 2.7 g/cm3 and average thickness of only 6 km. (3.7 miles), with the simple layered structure of the uniform composition.

Q 42 – Discuss how do seismic waves suggest layering of the earth’s interior.

Ans. Seismic waves are the earthquake waves. The movement of seismic waves tells us a great deal about the earth’s internal structure. The shock waves arising from earthquakes pass through the interior of the earth in different ways and provide evidence about the inaccessible interior region of the earth. Seismic waves are of two types -: body waves and surface waves. Body waves travel through the solid body of the earth, whereas surface waves move along the free upper crust of the earth. Surface waves, in turn, are of two types, viz, Rayleigh waves and Love waves.

Body waves also have two sub-types, viz, ‘P’ waves and ‘S’ waves. ‘P’ (primary waves) pass through all the medium solid, liquid and gaseous, whereas ‘S’ or secondary waves do not pass through liquids.

Based on the above observation related to seismic waves, the interior has been divided into three layers – core, mantle and crust.

The core is the innermost or central layer, the crust is the outermost layer, whereas mantle lies in the middle. Since ‘S’ waves cannot pass through the central part of the earth or core, it suggests that this part is made up of a medium which is not solid. This proves that the earth’s outer core is in a liquid state in contrast to the surrounding mantle which is solid.

The ‘P’ waves make an abrupt drop in velocity at the mantle-core _ boundary, whereas ‘S’ waves terminate here. Based on the behaviour of seismic waves, the mantle is sub-divided into two major parts – the upper mantle and the lower mantle.

The crust is distinguished from the mantle by the presence of an abrupt change in velocity of seismic waves. The change in rigidity, in turn, is due to change in the universal composition or in the physical state of the rocks

Q 43 – Describe the earth’s mantle.

Ans. The mantle is that part of the earth’s interior which lies between the core and the crust. It consists of solid ultrabasic rocks. The mantle is about 2900 km thick with a density of about (3.0 to 3.4) g/cm3. The lower surface of the mantle forces Gutenberg Discontinuity, the uppermost layer forces the Asthenosphere.

The ‘P’ or primary earthquake waves make an abrupt drop in velocity at the mantle-core boundary, whereas ‘S’ or secondary waves terminate at this boundary. . It is because of the presence of a plane of the discontinuous surface between the core and the mantle known as Gutenberg Discontinuity.

Based on the behaviour of seismic waves, the mantle is sub-divided into two major parts – the upper mantle and the lower mantle. The upper mantle extends from the crust to a depth of about 650 km. and includes 300 to 400 km wide Asthenosphere, the uppermost part of the mantle. Rocks in Asthenosphere behave as an elastic solid. This region is referred to as the low-velocity zone.

Q 44 – How do the waves of different types tell us about the changes in the nature of different layers of the earth’s interior?

Body waves also have two sub-types, viz, ‘P’ waves and ‘S’ waves. ‘P’ (primary waves) pass through all the medium – solid, liquid and gaseous, whereas ‘S’ or secondary waves do not pass through liquids.

Based on the above observation related to seismic waves, the interior has been divided into three layers – core, mantle and crust.

The ‘P’ waves make the abrupt drop in velocity at the mantle-core boundary, whereas ‘S’ waves terminate here. Based on the behaviour of seismic waves, the mantle is sub-divided into two major parts – the upper mantle and the lower mantle.

The mantle is that part of the earth’s interior which lies between the core and the crust. It consists of solid ultrabasic rocks. The mantle is about 2900 km thick with a density of about (3.0 to 3.4) g/cm3. The lower surface of the mantle forces Gutenberg Discontinuity, the uppermost layer forces the Asthenosphere.

Based on the behaviour of seismic waves, the mantle is sub-divided into two major parts – the upper mantle and the lower mantle. The upper mantle extends from the crust to a depth of about 650 km. and includes 300 to 400 km wide Asthenosphere, the uppermost part of the mantle. Rocks in Asthenosphere behave as elastic solid. Th,is region is referred to as the low-velocity zone.

Q 45 – How will you locate epicentre of an earthquake?

Ans. Check the scale on your map. It should look something like a piece of a ruler. All maps are different. On your map, one centimetre could be equal to 100 kilometres or something like that. Figure out how long the distance to the epicentre (in centimetres) is on your map. For example, say your map has a scale where one centimetre is equal to 100 kilometers. If the epicentre of the earthquake is 215 kilometers away, that equals 2.15 centimetres on the map. Using your compass, draw a circle with a radius equal to the number you came up within the centre of the circle will be the location of your seismograph. The epicentre of the earthquake is somewhere on the edge of that circle

Q 46 – Write a note on composite volcanoes.

Ans. 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.

Q 47 – Give a brief description of caldera.

Ans. These are the most explosive of the earth’s volcanoes. Their features are as follows:

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.
The hills produced by them are similar to the one made by composite volcanoes.

Q 48 – Write a short note on flood basalt provinces.

Ans. 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.
It is believed that initially the trap formations covered a much larger area than the present.

Q 49 – Explain the uppermost layer of the earth- crust.

Ans. It is the outermost solid part of the earth. It is brittle in nature.

Thickness: The thickness of the crust varies under the oceanic and continental areas. 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.
Composition: It is made up of heavier rocks.
Density: It is having density of 3 g/cm3.
In Oceans: This type of rock found in the oceanic crust is basalt. The mean density of material in oceanic crust is 2.7 g/cm3.

Q 50 – Explain the innermost layer of the earth- core.

Ans. The Core is the innermost layer of the earth.

Information: The earthquake wave velocities helped in understanding the existence of the core of the earth.
Depth: The core- mantle boundary is located at the depth of 2,900 km.
Form: The outer core is in liquid state while the inner core is in solid state.
Density: 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.
Composition: The core is made up of very heavy material mostly constituted by nickel and iron.
Other name: It is sometimes referred to as the nife layer.

Q 51 – Explain the middle layer of the earth- mantle.

Ans. The Mantle: The portion of the interior beyond the crust is called the mantle.

Depth: The mantle extends from Moho’s discontinuity to a depth of 2,900 km.
Asthenosphere: 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 that finds its way to the surface during volcanic eruptions.
Density: It has a density higher than the crust’s (3.4 g/cm3).
Upper Mantle: The crust and the uppermost part of the mantle are called lithosphere. Its thickness ranges from 10-200 km.
Lower Mantle: The lower mantle extends beyond the asthenosphere. It is in solid state.

Q 52 – Write a short note on shield volcanoes.

Ans. Shield Volcanoes:

Composition: 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.
Largest of all: Barring the basalt flows, the shield volcanoes are the largest of all the volcanoes on the earth.
Examples: The Hawaiian volcanoes are the most famous examples.
Movement of Lava: 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.

Q 53 – What are the effects of earthquake?

Ans. The following are the immediate hazardous effects of earthquake

Loss of life and property: Ground shaking takes place in earthquake. It leads to loss of life and property. Many buildings fall off and take life of people who were in and around the building.
Change in land: Due to earthquake we can see many changes in the land. Many areas get converted into pits. There are cracks in mountains.
Causes landslides and tsunami: It leads to landslides in mountainous regions and tsunami in oceans. It may further aggravate the calamity and loss of human and property.
Destruction of means of communication and transportation: It leads to destruction of means of communication and transportation. It creates problem in sending help and relief to victims.
Other effects: Earthquake may also lead to breaking or damage of dams which mav lead to floods.

Q 54 – What is earthquake? Discuss focus/ hypocentre and epicentre. How do we measure its magnitude and intensity?

Ans. An earthquake is shaking of the earth. It’s a nature event. It is caused due to release of energy, which generates waves that travel in all directions.

Focus/Hypocentre: The point where the energy is released is called the focus/ Hypocentre of an earthquake.
Epicentre: The point on the surface which is nearest to the focus of energy is called epicentre. It is the first one to experience the waves. It is a point directly above the focus. Measurement of earthquake:
- Magnitude: It is measured by Richter Scale
- Intensity: It is measured by Mercalli Scale.

Q 55 – Differentiate between Body waves and Surface waves?

Ans. Main differences between body waves and surface waves are given below:

Body waves:
1. These are generated due to the release of energy at the focus.
They move in all directions travelling through the body of the earth.
These are less destructive than the surface waves.
Surface waves:
The body waves interact with the surface rocks and generate new set of waves called surface waves.
These waves move along the surface.
These waves are more destruction.

Q 56 – Explain different types of earthquakes.

Ans. The various types of earthquakes are:-

Tectonic earthquake: The most common ones are the tectonic earthquakes. These are generated due to sliding of rocks along a fault plane.
Volcanic earthquake: A special class of tectonic earthquake is sometimes recognised as volcanic earthquake. However, these are confined to areas of active volcanoes.
Collapse earthquake: In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors. These are called collapse earthquakes.
Explosion earthquake: Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes.
Reservoir Induced earthquake: The earthquakes that occur in the areas of large reservoirs are referred to as reservoir induced earthquakes. Sometimes earthquakes also occur in mines due to mining processes. Sometimes earthquakes also occur below the oceans on surface of the ocean causing tsunamis.

Q 57 – What are different sources of information | about the interior of the earth?

Ans. Some of the direct sources are:-

Mining: It is a process by which commercially variable valuable mineral resources are extracted from Earth’s surface which includes precious stones, rocks and solid fuels.
Drilling: Scientists world over are working on two major projects such as “Deep Ocean Drilling projects” and “integrated ocean drilling project”. The deepest drill at kola, in Arctic Ocean, has so far reached a depth of 12 km.
Volcanic Eruptions: When molten material is thrown onto the surface of the earth during volcanic eruption it becomes available for analysis.
Some of the indirect sources of information: Analysis of properties of matter indirectly provides information about the interior. Knowing the total thickness of the earth, scientists have estimated the values of temperature, pressure and the density of materials at different depths.
- 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. 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. Hence, this becomes yet another source of information about the interior of the earth.
- 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.
- Magnetic surveys: 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.

Q 58 – Explain different types of earthquake waves.

Ans. Earthquake waves are basically of two types body waves and surface waves.

Body waves: These are generated due to the release of energy at the focus. They move in all directions in all directions travelling through the body of the earth. These are less destructive than the surface waves.
Surface waves: The body waves interact with the surface rocks and generate new set of waves called surface waves. These waves move along the surface. These waves are more destruction. The surface waves are the last to report on seismograph. These waves are more destructive. They cause displacement of rocks, and hence, the collapse of structures occurs.
There are two types of body waves. They are called P- and S-waves,
P-waves: They 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.
S-Waves: 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. This characteristic of the S-waves is quite important. It has helped scientists to understand the structure of t he interior of the earth. Reflection causes was es to rebound whereas refraction makes waves move in different directions. The variations in the direction of waves are inferred with the help of their record on seismograph.

Q 59 – Explain how does shadow zone emerge. Use a diagram.

Ans. Earthquake waves get recorded in 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’. The study of different events reveals that for each earthquake, there exists an altogether different shadow zone.

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. Figures given below show the shadow zones of P- and S-waves.

Q 60 – Explain the interior structure of the earth.

Ans. The interior structure f the earth can be classified into three parts:
1. Crust; (ii) Mantle; (iii) Core (i) 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 mean thickness of oceanic crust is 5 km w’hereas 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. It is having density of 3 g/cm3. This type of rock found in the oceanic crust is basalt. The mean density of material in oceanic crust is 2.7 g/cm3.

2. 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 that finds its way to the surface during volcanic eruptions. 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.

3. Core: 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 also called the nife layer.

Q 61 – In how many groups are volcanoes classified on the basis of nature of eruption and the form developed at the surface? Explain.

Ans. Volcanoes are classified on the basis of nature of eruption and the form developed at the surface into following types:

Shield Volcanoes: 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. 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. 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.

2. 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.

3. Calderas: 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. The hills produced by them are similar to the one made by composite volcanoes.

4. Basalt Flood Provinces: These volcanoes outpour highly fluid lava that flows for long distances. Some parts of the world are cohered 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. It is believed that initially the trap formations covered a much larger area than the present.

5. 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.

Q 62 – How does shadow zone emerge?

Ans. The P-wave slows down (the rigidity G is 0.0) and the wave is deflected towards the centre of the Earth. From 105° to 140° from the epicentre no P-waves are recorded. This creates a belt from 105° to 140° around the Earth called the P-Wave Shadow Zone.

Although it is not illustrated think what would happen to an S-wave that just entered the outer liquid core. S-waves cannot travel through a liquid so from 105° to 105° degrees on either side of the epicentre no S-waves are recorded. This is the S-Wave Shadow Zone.

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