geography- Our Changing Earth-I, NCERT-VII

 

3. Our Changing Earth-I

 

Earth is dynamic, i.e., it is constantly changing due to the action of forces operating inside and on its surface. Most of the changes taking place on the Earth are very slow and therefore, not easy to detect. For example, the Himalayas are still increasing in height. But this change is so small, it is not noticed easily. On the contrary, some natural processes such as earthquakes and volcanic eruptions can bring about sudden and drastic changes on the Earth, which are hard to miss. In this chapter, we will study about the dynamic nature of the Earth and the movements taking place both inside the Earth and on its surface, which are constantly destroying or giving rise to landforms.

 

CONTINENTAL DRIFT AND PLATE TECTONICS THEORIES

Earlier, people believed that the position of the continents was fixed. But we now know the distribution of various landforms and water bodies on the Earth has not always been the same. In the 20th century, a German meteorologist called Alfred Wegener proposed that the continents were not fixed. He came up with the theory that the continents were once joined together into a single supercontinent called Pangaea (a Greek word, meaning 'all land' or 'all Earth'). The Pangaea later split up into pieces, which began drifting away from each other. Water took up the space between the land masses to form oceans. This theory is known as the Continental Drift Theory. If we bring together all the continents that exist today, we will find they fit into each other, much like the pieces of a big jigsaw puzzle.

 

Wegener's theory was not widely accepted for a long time. Later scientists added newer ideas to his hypothesis and came up with the Theory of Plate Tectonics.

 

The crust and the upper mantle make up the lithosphere. The lithosphere comprises plates of different shapes, sizes, and thickness, which float on the molten magma of the upper mantle. These plates are called lithospheric or tectonic plates. There are seven major tectonic plates. It is these plates which carry continents and ocean floors. There are seven major plates and a number of smaller ones. The lithospheric plates are constantly moving due to the circular movement of molten magma inside the Earth. The high temperatures in the mantle cause rocks inside this layer to melt and flow up towards the surface of the Earth. As they come closer to the surface, the molten rocks (or magma) begin to cool. They start flowing sideways under the crust and then move down towards the core, only to heat up and rise once again. The sideways movement of molten rocks beneath the crust causes tectonic plates to move.

 

The rate at which the plates move is extremely slow. The movement of tectonic plates is known as tectonic movement. Tectonic movements lead to many changes on the surface of the Earth, for example, formation of landforms, such as mountains, valleys, etc.

 

Info Bubble The tectonic plates move almost as slowly as the rate at which our fingernails or hair grow. The seven major tectonic plates are the Pacific Plate, the North American Plate, the South American Plate, the Eurasian Plate, the African Plate, the Indo-Australian Plate, and the Antarctic Plate.

 

ENDOGENIC AND EXOGENIC FORCES

We know that tectonic movements are the result of forces acting inside the Earth. Such forces are called endogenic forces (endo means 'inside'). Endogenic forces include diastrophic forces and sudden forces. Diastrophic forces cause horizontal and vertical movements on the Earth. These forces operate very slowly and their effects become visible after many years (for example, formation of mountains). Sudden forces cause earthquakes, volcanic eruptions, and landslides. They work quickly and their effects become visible within minutes.

 

Besides plate tectonics, the various landforms on the surface of the Earth are also constantly changing due to the action of natural agents, like wind, water, ice, etc. These agents are constantly breaking down or giving rise to new landforms such as lakes, sea cliffs, floodplains, etc. Such forces that act on the surface of the Earth are called exogenic forces (exo means 'outside'). Exogenic forces are exerted by agents, such as rivers, winds, sea waves, and glaciers. These lead to breaking up of rocks or formation of landforms, like river deltas and plains.

WORK OF DIASTROPHIC FORCES

Diastrophic forces act deep inside the Earth and lead to both vertical and horizontal movements of the crust.

 

Vertical movements: Sometimes diastrophic forces cause parts of the Earth's crust to rise up or slump suddenly. When a part of the Earth's crust rises in relation to the surrounding area, it is called an uplift. When a part of the Earth's crust sinks in comparison to the surrounding area, it is called subsidence. Such vertical movements lead to the formation of continents and plateaus.

 

Horizontal movements: Diastrophic forces can also lead to folding or fracturing in the Earth's crust. When two tectonic plates move towards each other, they create a compressional force. This force pushes up the crust at the point the plates meet, causing multiple folds to form. This is known as folding. Folding leads to creation of mountains, called fold mountains. Such mountains are usually arc-shaped and quite tall. The Himalayas, the Andes, and the Alps are all fold mountains. When tectonic plates move away from each other or get pulled apart, they create a tensional force which causes fractures or cracks in the crust. This is known as faulting. When faults develop, a portion of the crust gets displaced vertically to form a block mountain or a rift valley. When land between two faults is pushed up, block mountains, such as the Black Forest Mountains are formed. When the land between two faults subsides, rift valleys, such as the Rhine Valley are formed.

 

WORK OF SUDDEN FORCES

Some forces build up deep inside the Earth over a period of time. When they become very strong, they give rise to sudden movements on the Earth's surface, which cause a lot of destruction. Such forces are called sudden forces. These forces lead to movements such as earthquakes or volcanic eruptions.

 

EARTHQUAKES

Earthquakes are caused by the movement of lithospheric plates inside the Earth. When lithospheric plates move, they cause vibrations on Earth's surface. These vibrations are called earthquakes. The point on the Earth's crust from where the plates begin to move is called the seismic focus. The point on the Earth's surface where these vibrations seem to emerge from is called the epicentre. In other words, the epicentre is the point on the Earth's surface that is directly above the focus. The vibrations produced by the movement of the lithospheric plates travel away from the epicentre in waves and are called seismic waves. The vibrations are the strongest at the epicentre and become weaker as they move away from it. Therefore, they cause maximum damage at the epicentre.

 

We know the lithospheric plates move constantly. As a result, the Earth experiences more than 10,00,000 earthquakes every year.

 

However, we do not feel most earthquakes as they are not strong enough. The intensity of an earthquake is measured by a device called a seismograph. The Richter Scale measures the magnitude of an earthquake. The Richter Scale has a range of zero to nine. Earthquakes measuring 2.0 2 or less on the scale are very slight. Earthquakes measuring 6.0 are considered strong, while those measuring 7.0 are considered major earthquakes. An increase of just one point on the scale represents a ten-fold increase in the magnitude of a quake. Earthquakes are hard to predict. However, many people believe animals can sense earthquakes before they strike and studying their behaviour can help predict quakes. For example, before an earthquake strikes, snakes come out of their holes while animals like cats and dogs make strange sounds.

 

No part of the Earth is safe from earthquakes. But some areas experience more frequent quakes than the others. The Circum-Pacific belt accounts for the most earthquakes on the Earth. It includes the west coasts of North and South America, Central America, West Indies, east coast of Russia, China, Japan, Philippines, and Indonesia. The Mid-Continental belt includes the Alpine mountain system, Mediterranean Sea, northern and eastern Africa, and the Himalayas. It accounts for less than a quarter of the earthquakes taking place on the Earth. Areas falling under the Mid-Atlantic Ridge belt (Mid-Atlantic ridge and adjoining islands) experience moderate earthquakes. In India, the Himalayan region and the Ganga-Brahmaputra Valley are most prone to earthquakes.

 

Info Bubble There are three types of seismic waves - primary, secondary and surface. Primary or push waves (P waves) are the first to originate. They travel through the Earth at a fast rate and cause small displacements. Secondary or shake waves (S waves) also pass through the Earth. These are slower than P waves and cannot pass through liquids. They cause the crust to shake. Surface waves (L waves) travel on the Earth's surface. They travel the greatest distance and cause the greatest damage.

 

How earthquakes change the Earth?

Strong earthquakes can cause serious damage. They can cause landslides in hilly areas, which can even change the course of rivers. They can make buildings and other human-made structures fall or damage them. Therefore, buildings in quake-prone areas are usually made of lighter materials so they do not cause much harm to life and property. Strong earthquakes can even lead to the formation of hills and mountains.

 

Earthquakes on the ocean floor give rise to huge waves which can submerge entire islands and coastal regions. Such waves are called tsunami waves. A severe tsunami, caused due to an earthquake measuring over 9 on the Richter Scale, struck the coasts of Indonesia, Sri Lanka, India, Thailand, and Maldives in 2004. It caused more than 200,000 casualties.

 

Protective measures

We cannot predict earthquakes with much accuracy. However, there are some protective measures that we can take when an earthquake strikes:

(i) Keeping our movements to a minimum.

(ii) Finding a safe spot such as a kitchen counter, or a table and taking shelter under it, or standing against a corner wall.

(iii) Keeping away from anything that can fall or break, like mirrors, cupboards, etc.

(iv) Keeping away from lifts or elevators.

(v) Keeping a disaster supply kit ready at all times. The kit must have all important documents, a first-aid box, and some non-perishable food items.

 

What is a landslide?

A landslide is the sliding down of a mass of the Earth or rocks from a mountain or cliff after an earthquake or heavy rainfall.

 

VOLCANIC ERUPTIONS

Molten magma lies under great pressure inside the Earth. When this magma reaches a weak point or a crack on the Earth's surface, it comes out with great force. A volcano is an opening in the Earth's crust from where molten magma, ashes, gases, and steam are emitted with a sudden force. Molten magma which erupts from a volcano and falls on Earth's surface is called lava. The tunnel-like part of the volcano through which lava and other molten material are emitted is called a vent or shaft. The vent has a large cup-shaped mouth called crater.

 

Type of volcanoes

Volcanoes are differentiated on the basis of how frequently they erupt. Volcanoes that erupt frequently are called active volcanoes, for example, the Barren Island in Andaman and Nicobar Islands. Volcanoes that have not erupted for the past many years but may become active at some stage are called dormant volcanoes, for example, Mount Fuji Yama in Japan. Volcanoes that have not been known to erupt for hundreds of years and are not likely to do so in future are called extinct volcanoes, for example, Mount Kilimanjaro in Africa and Lonar Lake in Maharashtra.

 

Most of the volcanoes are found in a circular zone around the Pacific Ocean called the Pacific Ring of Fire. The Barren Island is the only volcano in India.

 

How volcanoes change the Earth?

Often the lava coming out of a volcano accumulates around it. This lava cools and solidifies to form a volcanic mountain such as Mount Kilimanjaro in Africa. When molten magma erupts from a long crack or fissure in the Earth's surface, it spreads to form a volcanic plateau, such as the Deccan Plateau in India. Lava emitted from volcanoes solidifies on the Earth's surface to form volcanic rocks, which in tum break down and form a very fertile soil called black soil. Volcanic eruptions also bring valuable minerals to the Earth's surface.

 

 

Rewind

1. The Earth is constantly changing due to the action of forces operating inside (endogenic forces) and on its surface (exogenic forces).

2. Exogenic forces are exerted by agents such as rivers, winds, sea waves, and glaciers.

3. Endogenic forces include sudden forces and diastrophic forces.

4. Sudden forces lead to movements such as earthquakes or volcanic eruptions. Disastrophic forces lead to the formation of continents, plateaus, mountains, etc.

5. Earthquakes are caused by the movement of lithospheric plates inside the Earth. They can lead to landslides, change the course of rivers, and even give rise to tsunami waves.

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6. A volcano is an opening in the Earth's crust from where molten magma, ashes, gases, and steam are emitted with a sudden force.

7. Volcanic eruptions often lead to the formation of volcanic mountains, volcanic plateau, and volcanic rocks.

 

New Terms

Exogenic forces: Forces that act on the surface of the Earth.

Endogenic forces: Forces acting inside the Earth.

Diastrophic forces: Act deep inside the Earth and cause horizontal and vertical movements of the crust.

Sudden forces: Cause earthquakes, volcanic eruption, and landslides.

Uplift: When a part of the Earth's crust rises in relation to the surrounding area.

Subsidence: When a part of the Earth's crust sinks in comparison to the surrounding area.

Folding: Takes place when two tectonic plates move towards each other, creating a compressional force which pushes up the crust at the point the plates meet.

Faulting: Takes place when tectonic plates move away from each other or get pulled apart, creating a tensional force which causes fractures or cracks in the crust.

Lithospheric plates: Plates of different shapes and sizes on the crust which float on top of the molten magma.

Volcano: Opening in the Earth's crust from where molten magma, ashes, gases, and steam are emitted with a sudden force.

Seismic focus: Point on the Earth's crust from where lithospheric plates begin to move.

Epicentre: Point on the Earth's surface directly above the focus; it is the point from where vibrations seem to emerge from.

Seismograph: Measures the intensity of an earthquake.

Richter Scale: Measures the magnitude of an earthquake.

Circum-Pacific belt: Includes the west coasts of North and South America, Central America, West Indies, east coast of Russia, China, Japan, Philippines, and Indonesia; accounts for the most Earthquakes on the Earth.

Mid-Continental belt: Includes the Alpine mountain system, Mediterranean Sea, northern and eastern Africa, and the Himalayas; accounts for less than a quarter of earthquakes on the Earth.

Mid-Atlantic Ridge belt: Includes the Mid-Atlantic ridge and adjoining islands; areas in this belt experience moderate earthquakes.

Tsunami waves: Giant waves caused by earthquakes on the ocean floor; they are powerful waves which can submerge entire islands and coastal regions.

Lava: Molten magma which erupts from a volcano.

Vent of a volcano: Also known as shaft; it is a tunnel-like part through which lava and other molten material are emitted.

Crater: Cup-shaped mouth of a volcano.

Active volcano: A volcano which erupts frequently.

Dormant volcano: A volcano that has not erupted for the past many years but can become active at some stage.

Extinct volcano: A volcano that has not been known to erupt for hundreds of years and is not likely to do so in future.

Volcanic mountain: Formed when the lava coming out of a volcano accumulates around it, cools and solidifies.

Volcanic plateau: Formed when molten magma erupts from a long crack or fissure in the Earth's surface and spreads around it.

Volcanic rocks: Formed by the solidification of lava on the Earth's surface.

 

 

 

Practice MCQs

 

Q1. The Himalayas are still increasing in height due to:

a) Erosion by rivers

b) Movement of tectonic plates

c) Volcanic eruptions

d) Earthquakes

Answer: b) Movement of tectonic plates

Explanation: The Himalayas are still rising slowly because of the collision between the Indian Plate and the Eurasian Plate.

 

Q2. Who proposed the Continental Drift Theory?

a) Charles Lyell

b) Alfred Wegener

c) James Hutton

d) Harry Hess

Answer: b) Alfred Wegener

Explanation: Alfred Wegener, a German meteorologist, proposed the Continental Drift Theory in the 20th century.

 

Q3. The supercontinent described in the Continental Drift Theory was called:

a) Gondwana

b) Laurasia

c) Pangaea

d) Panthalassa

Answer: c) Pangaea

Explanation: Wegener proposed that all continents were once joined in a single supercontinent called Pangaea, meaning “all land.”

 

Q4. The crust and upper mantle together form the:

a) Lithosphere

b) Hydrosphere

c) Atmosphere

d) Asthenosphere

Answer: a) Lithosphere

Explanation: The lithosphere is the rigid outer layer of the Earth comprising the crust and upper mantle.

 

Q5. How many major tectonic plates exist on Earth?

a) Five

b) Seven

c) Ten

d) Twelve

Answer: b) Seven

Explanation: There are seven major tectonic plates along with many minor plates that move constantly.

 

Q6. Which tectonic plate is the largest?

a) Eurasian Plate

b) Pacific Plate

c) African Plate

d) Indo-Australian Plate

Answer: b) Pacific Plate

Explanation: The Pacific Plate is the largest tectonic plate, covering most of the Pacific Ocean floor.

 

Q7. The movement of tectonic plates is caused by:

a) Solar radiation

b) Ocean currents

c) Convection currents in magma

d) Earth’s rotation

Answer: c) Convection currents in magma

Explanation: The circular movement of molten magma in the mantle creates convection currents that move tectonic plates.

 

Q8. The rate of tectonic plate movement is compared to the growth of:

a) A plant

b) Human bones

c) Fingernails or hair

d) Coral reefs

Answer: c) Fingernails or hair

Explanation: Tectonic plates move as slowly as fingernails or hair grow.

 

Q9. Forces operating inside the Earth are called:

a) Exogenic forces

b) Endogenic forces

c) Sudden forces

d) Hydro forces

Answer: b) Endogenic forces

Explanation: Endogenic forces originate inside the Earth and cause movements like folding, faulting, earthquakes, and volcanism.

 

Q10. The forces acting on the surface of the Earth are called:

a) Endogenic forces

b) Exogenic forces

c) Centrifugal forces

d) Elastic forces

Answer: b) Exogenic forces

Explanation: Exogenic forces are external forces like rivers, wind, glaciers, and sea waves shaping the Earth’s surface.

 

Q11. When a part of the Earth’s crust rises compared to surrounding areas, it is called:

a) Faulting

b) Folding

c) Uplift

d) Subsidence

Answer: c) Uplift

Explanation: Uplift occurs when the crust rises in relation to surrounding areas due to diastrophic forces.

 

Q12. The sinking of a part of Earth’s crust compared to surrounding areas is called:

a) Uplift

b) Folding

c) Subsidence

d) Faulting

Answer: c) Subsidence

Explanation: When the crust sinks in comparison to surrounding areas, it is termed subsidence.

 

Q13. Which mountains are formed due to folding of the Earth’s crust?

a) Black Forest Mountains

b) Himalayas

c) Rhine Valley

d) Rockies

Answer: b) Himalayas

Explanation: The Himalayas are fold mountains formed by compressional forces pushing the crust upwards.

 

Q14. When land between two faults is uplifted, which landform is created?

a) Fold mountains

b) Rift valley

c) Block mountain

d) Plateau

Answer: c) Block mountain

Explanation: Block mountains form when land between two faults is pushed up, such as the Black Forest Mountains.

 

Q15. The Rhine Valley is an example of:

a) Fold mountain

b) Rift valley

c) Plateau

d) Volcanic mountain

Answer: b) Rift valley

Explanation: The Rhine Valley was formed when land between two faults subsided, creating a rift valley.

 

Q16. The point inside the Earth where an earthquake originates is called:

a) Epicentre

b) Crustal point

c) Seismic focus

d) Hypocentre

Answer: c) Seismic focus

Explanation: The seismic focus is the underground point of origin of an earthquake.

 

Q17. The point directly above the focus on Earth’s surface is called:

a) Epicentre

b) Fault line

c) Seismograph

d) Shock point

Answer: a) Epicentre

Explanation: The epicentre lies directly above the seismic focus and experiences the strongest tremors.

 

Q18. The instrument used to measure earthquake vibrations is:

a) Seismograph

b) Richter meter

c) Barometer

d) Anemometer

Answer: a) Seismograph

Explanation: A seismograph records vibrations caused by earthquakes.

 

Q19. The Richter Scale measures earthquake magnitude from:

a) 0 to 5

b) 0 to 7

c) 0 to 9

d) 0 to 12

Answer: c) 0 to 9

Explanation: The Richter Scale ranges from 0 to 9, with each step representing a ten-fold increase in energy released.

 

Q20. The region most prone to earthquakes is:

a) Mid-Atlantic Ridge

b) Circum-Pacific Belt

c) Mid-Continental Belt

d) African Rift Valley

Answer: b) Circum-Pacific Belt

Explanation: The Circum-Pacific Belt, also called the “Ring of Fire,” experiences the most frequent and intense earthquakes.

 

Q21. Which seismic waves are the fastest and the first to reach the Earth’s surface?

a) Surface waves

b) Primary waves (P waves)

c) Secondary waves (S waves)

d) Tsunami waves

Answer: b) Primary waves (P waves)

Explanation: P waves are the fastest seismic waves; they cause small displacements and travel through solids and liquids.

 

Q22. Which seismic waves cannot pass through liquids?

a) Primary waves

b) Secondary waves

c) Surface waves

d) Tsunami waves

Answer: b) Secondary waves

Explanation: S waves cannot travel through liquids and cause the crust to shake.

 

Q23. Which seismic waves travel the greatest distance and cause the maximum damage?

a) P waves

b) S waves

c) Surface waves (L waves)

d) Tsunami waves

Answer: c) Surface waves (L waves)

Explanation: L waves travel along the Earth’s surface, covering long distances and causing maximum destruction.

 

Q24. What natural disaster can earthquakes on the ocean floor trigger?

a) Cyclone

b) Tsunami

c) Volcanic eruption

d) Tornado

Answer: b) Tsunami

Explanation: Earthquakes under the ocean cause giant waves called tsunamis that can submerge coastal regions.

 

Q25. The 2004 Indian Ocean tsunami was caused by an earthquake measuring over:

a) 7 on the Richter Scale

b) 8 on the Richter Scale

c) 9 on the Richter Scale

d) 10 on the Richter Scale

Answer: c) 9 on the Richter Scale

Explanation: The 2004 tsunami was triggered by a massive undersea earthquake of magnitude over 9.0.

 

Q26. Landslides are caused by:

a) Heavy rainfall only

b) Earthquakes or heavy rainfall

c) Wind erosion

d) Glacier movement

Answer: b) Earthquakes or heavy rainfall

Explanation: Landslides occur when rocks or earth masses slide down slopes due to earthquakes or heavy rain.

 

Q27. Which of the following is NOT a protective measure during earthquakes?

a) Standing under a doorway or table

b) Staying near mirrors and cupboards

c) Avoiding lifts and elevators

d) Keeping a disaster kit ready

Answer: b) Staying near mirrors and cupboards

Explanation: One should avoid mirrors, cupboards, or anything that can fall or break during an earthquake.

 

Q28. What is the purpose of a disaster supply kit?

a) Entertainment during disasters

b) To provide safety, food, and important documents

c) To help predict earthquakes

d) To measure seismic intensity

Answer: b) To provide safety, food, and important documents

Explanation: A disaster kit must have documents, first aid, and non-perishable food for emergencies.

 

Q29. What is a volcano?

a) A mountain formed by wind

b) An opening in Earth’s crust where magma erupts

c) A crack in tectonic plates without magma

d) A hill formed by erosion

Answer: b) An opening in Earth’s crust where magma erupts

Explanation: A volcano is an opening from where magma, gases, and ashes are released with great force.

 

Q30. What is magma called when it comes out on Earth’s surface?

a) Crust

b) Lava

c) Ash

d) Vent

Answer: b) Lava

Explanation: Magma that reaches Earth’s surface is known as lava.

 

Q31. The tunnel-like passage of a volcano is called:

a) Crater

b) Vent or shaft

c) Magma chamber

d) Rift

Answer: b) Vent or shaft

Explanation: Lava and gases escape through the vent or shaft of a volcano.

 

Q32. The cup-shaped opening of a volcano is called:

a) Vent

b) Crater

c) Rift valley

d) Fault

Answer: b) Crater

Explanation: The crater is the large cup-shaped mouth of a volcano.

 

Q33. Which is the only active volcano in India?

a) Mount Abu

b) Lonar Lake

c) Barren Island

d) Nilgiri Hills

Answer: c) Barren Island

Explanation: Barren Island in the Andaman & Nicobar Islands is India’s only active volcano.

 

Q34. Which volcano is an example of a dormant volcano?

a) Mount Kilimanjaro

b) Mount Fuji Yama

c) Barren Island

d) Lonar Lake

Answer: b) Mount Fuji Yama

Explanation: Dormant volcanoes have not erupted for many years but may erupt again, such as Mount Fuji Yama in Japan.

 

Q35. Mount Kilimanjaro is an example of:

a) Active volcano

b) Dormant volcano

c) Extinct volcano

d) Volcanic plateau

Answer: c) Extinct volcano

Explanation: Mount Kilimanjaro is an extinct volcano that has not erupted for hundreds of years.

 

Q36. Which type of volcano erupts frequently?

a) Active volcano

b) Dormant volcano

c) Extinct volcano

d) Rift volcano

Answer: a) Active volcano

Explanation: Active volcanoes erupt frequently, e.g., Barren Island.

 

Q37. Most volcanoes are found around which region?

a) Himalayas

b) Andes

c) Pacific Ring of Fire

d) Mid-Atlantic Ridge

Answer: c) Pacific Ring of Fire

Explanation: The Pacific Ring of Fire is a circular zone with the highest volcanic activity.

 

Q38. Which volcanic landform is formed when lava accumulates around a volcano and solidifies?

a) Rift valley

b) Volcanic mountain

c) Plateau

d) Fold mountain

Answer: b) Volcanic mountain

Explanation: Lava accumulates around a volcano, cools, and forms volcanic mountains like Kilimanjaro.

 

Q39. The Deccan Plateau was formed by:

a) Folding

b) Volcanic fissure eruptions

c) Faulting

d) Landslides

Answer: b) Volcanic fissure eruptions

Explanation: The Deccan Plateau was formed by lava spreading out from fissures on Earth’s surface.

 

Q40. Fertile black soil is formed from:

a) Sandstone

b) Volcanic rocks

c) Granite

d) Limestone

Answer: b) Volcanic rocks

Explanation: Lava solidifies into volcanic rocks, which break down into fertile black soil.

 

Q41. Which of these minerals are brought to the surface by volcanic eruptions?

a) Gold and Silver

b) Petroleum

c) Valuable minerals

d) Salt

Answer: c) Valuable minerals

Explanation: Volcanic eruptions bring valuable minerals like copper and sulfur to the surface.

 

Q42. Which of the following is an exogenic force?

a) Folding

b) Faulting

c) River erosion

d) Earthquake

Answer: c) River erosion

Explanation: Exogenic forces include rivers, winds, glaciers, and sea waves acting on the Earth’s surface.

 

Q43. Sudden forces include:

a) Folding and faulting

b) Earthquakes and volcanic eruptions

c) Mountain building

d) Plateau formation

Answer: b) Earthquakes and volcanic eruptions

Explanation: Sudden forces act quickly, causing earthquakes, volcanic eruptions, and landslides.

 

Q44. Diastrophic forces result in:

a) Earthquakes

b) Plateaus and mountains

c) Volcanic eruptions

d) Tsunami

Answer: b) Plateaus and mountains

Explanation: Diastrophic forces are slow and lead to vertical/horizontal movements forming continents and plateaus.

 

Q45. Which of the following is NOT a sudden force?

a) Landslides

b) Earthquakes

c) Volcanic eruptions

d) Folding

Answer: d) Folding

Explanation: Folding is a diastrophic force that works slowly over long periods.

 

Q46. What is the seismic focus of an earthquake?

a) Point on Earth’s surface above epicentre

b) Point inside Earth where vibrations originate

c) Zone of maximum damage

d) Area where waves end

Answer: b) Point inside Earth where vibrations originate

Explanation: The seismic focus is the underground origin point of an earthquake.

 

Q47. What is the epicentre of an earthquake?

a) Deepest point of magma chamber

b) Point inside Earth where quake starts

c) Point directly above the focus on surface

d) Area least affected

Answer: c) Point directly above the focus on surface

Explanation: The epicentre is the surface point above the seismic focus.

 

Q48. Which instrument records earthquake vibrations?

a) Barometer

b) Seismograph

c) Anemometer

d) Richter meter

Answer: b) Seismograph

Explanation: A seismograph records the intensity and vibrations of earthquakes.

 

Q49. The Richter Scale has a maximum reading of:

a) 7

b) 8

c) 9

d) 10

Answer: c) 9

Explanation: The Richter Scale measures earthquake magnitude from 0–9.

 

Q50. Which area in India is most prone to earthquakes?

a) Thar Desert

b) Himalayan region and Ganga-Brahmaputra Valley

c) Deccan Plateau

d) Coastal Andhra

Answer: b) Himalayan region and Ganga-Brahmaputra Valley

Explanation: These regions lie in earthquake-prone zones and experience frequent seismic activity.