WBBSE Chapter 2 Unstable Earth Synopsis
The movement of lithospheric plates is responsible for natural events like earthquakes, volcanic activities, mountain-building processes, landslides, avalanches etc.
Alfred Wegener propounded the Continental Drift Theory. He assumed that all landmasses of the world had formed from one supercontinent, called Pangea, about 300 million years ago.
The Plate Tectonic Theory had evolved in the 1960s on the basis of the Continental Drift Theory and Seafloor Spreading Theory.
Scientists such as Le Pichon, Wilson, McKenzie, Parker and Morgan elaborated the various aspects of Plate Tectonics.
Read and Learn More WBBSE Solutions For Class 8 Geography
All lithospheric plates are floating on the asthenosphere. These plates are on an average 70km-150km thick.
6 major and 20 minor plates have been identified in the world.The major plates are Eurasian Plate, Indo-Australian Plate, American Plate, Pacific Plate, African Plate and Antarctic Plate.
WBBSE Class 8 Plate Tectonics Overview
When two tectonic plates move apart from each other, a Divergent or Constructive plate boundary is formed.
Earthquakes: and volcanic eruptions can occur here.
When two plates move towards each other and collide, a Convergent or Destructive plate boundary is formed. Earthquakes, volcanic eruptions and folds can occur here. ons and folds can occur here.
When two tectonic plates slide or glide past each other, a Conservative or Neutral plate boundary is formed. Earthquakes and faults can occur here. The San Andreas Fault in California is an example of a neutral plate boundary.
The fold mountains formed about 10-25 million years ago are known as young fold mountains. The Rockies, the Andes and the Himalayas are young-fold mountains.
The fold mountains formed more than 200 million years ago are known as the old fold mountains. The Urals, the Appalachians and the Aravallis are classic examples of this category.
According to Plate Tectonic Theory, plates move horizontally at different speeds. The Pacific Plate is moving about 10cm towards the west annually, while the American Plate is moving towards the same direction at a speed of just 2-3cm/year.
Class 8 Geography Chapter 2 Plate Tectonics Notes
Volcanism is the process of upwelling of viscous magma, gas, and water vapour from the interior of the earth to the surface through cracks, joints or weak points of the lithosphere and the solidification of lava on the earth’s surface.
Viscous, molten rock materials in the earth’s interior is known as magma.
When magma comes out onto the earth surface through cracks, joints or weak points of the lithosp of the lithosphere, it is known as lava.
Active volcanoes are the volcanoes which have been erupting continuously or frequently since their formation. Example-Etna in Sicily, Barren in India.
The volcanoes which have remained inactive for a long period after one. the eruption, but suddenly erupt violently are called dormant volcanoes. Example- Fujiyama in Japan, Narcondam in India.
Volcanoes are considered to be extinct or dead when there are no indications of future eruptions. Example-Mt. Popa in Myanmar.
The dense and viscous lava ejected from volcanoes is known as A’a lava and the comparatively less viscous or highly fluid lava, which spreads like thin sheets, is known as Pahoehoe lava in the Hawaiian language.
The place in the interior of the earth where an earthquake originates is the focus of the earthquake. These are located at a depth of about 50km-100km under the surface.
The point on the earth’s surface vertically above the focus of an earthquake is known as its epicentre.
Class 8 Geography Solution WBBSE
The different types of waves generated due to the earthquake are called seismic waves.
Seismic waves are of three types
- P-wave
- S-wave and
- L-wave.
The seismograph is an instrument that records seismic waves, focus and the epicentre of an earthquake.
Richter Scale measures the magnitude or intensity of energy released during an earthquake. The number indicating an earthquake’s intensity on the Richter scale ranges from 0-10.
Seismogram is a graph output that we get using a seismograph. It is a record of the ground motion during an earthquake. The science that deals with seismic waves, is called Seismology.
Most of the active volcanoes in the world flank the Pacific Ocean and form a ring around it. This is called the Pacific Ring of Fire. Krakatau in Indonesia, Cotopaxi in Ecuador, etc., belong to this belt.
Class 8 Geography Solution WBBSE
Predicting an earthquake is very difficult. However, different measures can be adopted to mitigate the hazardous effects of earthquakes, such as constructing earthquake-resistant buildings, monitoring radon gas emissions, etc.
WBBSE Chapter 2 Topic A Plate Tectonics Analytical Type Questions
Question 1. What are plates? Discuss the Plate Tectonic Theory.
Answer:
Plates:
The rigid lithospheric slabs including SIAL and SIMA, or the solid crustal layer including oceanic and continental crust are called plates.
These are relatively mobile in nature. The whole mechanism of the evolution, nature and motion of plates and the resultant reaction is known as plate tectonics.
Plate Tectonic Theory:
The Plate Tectonic Theory came into light mainly in the 1960s. It was able to explain clearly and scientifically the causes and effects of most of the geo-tectonic phenomena.
divergent movement (the plates move away from each other), convergent movement (the plates move towards each other) and neutral movement (the plates slide past each other).
Chief propounder:
In 1965, J.T. Wilson, a Canadian geophysicist, first used the term ‘Plate. McKenzie and Parker, too discussed about the movement of the plates in 1967. Later on, French geophysicists Le Pichon and Jason Morgan established a dependable theory from this base.
Basic Concept:
The Plate Tectonic Theory is based on two concepts-
- Continental Drift and
- Seafloor Spreading. According to this theory, the lithosphere consists of some plates which continuously glide over the weak asthenosphere due to the action of internal convection currents. It drives the plates to move away from or towards each other.
Major plates of the world:
There are 6 major and 20 minor plates in the world. The major plates are
- Eurasian Plate
- Indo- Australian Plate
- American Plate
- Pacific Plate
- African Plate and
- Antarctic Plate.
Movement of the plates:
The plates are mobile in nature. Plate movement can be of three types
- Divergent movement (the plates move away from each other),
- Convergent movement (the plates move towards each other) and
- Neutral movement (the plates slide past each other)
Causes of plate movement:
Plates move due to
- upwelling of magma along the plate boundary
- gravitational force
- buoyancy
- convection current
- increasing temperature due to radioactivity within the earth.
Impact of plate tectonics:
Understanding Plate Tectonics for Class 8
The impact of plate tectonics includes the following
- Formation of new landmasses along the divergent plate boundaries.
- Volcanic eruptions and earthquakes along the subduction zone.
- Formation of young fold mountains along the convergent plate boundaries example, the Alps, Himalayas, Rockies and Andes.
- Formation of faults and rift valleys along the boundary where the plates slide past each other.
Question 2. Explain the formation of different types of landforms due to plate movement.
Answer:
Formation Of Different Types Of Landforms Due To Plate Movement:-
Due to plate movement, different types of landforms are formed. The three kinds of plate movement and their related landforms are discussed below-
1. Divergent plate movement:
The upwelling of the internal convection current in reverse direction forces the plates to move away from each other and is known as divergent plate movement.
Landforms Due To Divergent Plate Movement :
- The emergence of the magma forms mid-oceanic ridges and rift valleys along this plate boundary.
- Formation of faults on the earth’s crust occurs and lava erupts along this zone.
2. Convergent plate movement:
The movement of the plates towards each other is called convergent plate movement.
WBBSE Class 8 Geography Plate Boundaries
Landforms Due To Divergent Plate Movement :
- Geosynclines, oceanic trenches and young fold mountains may form along the ocean-continent convergent plate boundary
- Island arcs may form along the ocean-ocean convergent plate boundary.
- Fold mountains form along the continent-continent convergent plate boundary.
3. Neutral plate movement:
The boundary along which the plates slide or glide past each other is called a conservative plate boundary and the movement is called neutral plate movement.
Class 8 Geography WBBSE
Landforms Due To Divergent Plate Movement :
- There is no formation of new landforms.
- Along this plate boundary, long cracks and faults may be found and this zone is earthquake-prone.
Question 3. Discuss the origin of fold mountains in light of the Plate Tectonic Theory.
Answer:
Origin Of Fold Mountains In Light Of The Plate Tectonic Theory:-
Recently, the Plate Tectonic Theory has been accepted unanimously to discuss the origin of fold mountains
1. Continent-continent plate collision:
In this case, the two continental plates move towards each other and there exists a narrow and shallow sea in between. Due to this movement, the plates create pressure on the deposited sediments of the intra- plate ocean and give rise to, fold mountains.
Example-the Himalayas were formed in this way. The Eurasian Plate and the Indian Plate collided with each other and the Indian Plate got subducted under the Eurasian Plate.
Consequently, the sediments of the intra-plate Tethys sea got folded to form the Himalayan mountain range.
Plate Tectonics Concepts for Class 8 Students
2. Ocean-continent plate collision:
The heavier oceanic plate subducts under the lighter continental plate along this plate boundary. This creates pressure, due to which a long crack forms in front of the continental plate and that crack turns into synclines.
Increasing movement of the plates leads to the folding of sediments and thus fold mountains are formed. Example-formation of the Andes and the Alps mountains.
WBBSE Chapter 2 Topic A Plate Tectonics Short Analytical Type Questions
Question 1. Write a short note on Pangea.
Answer:
Pangea:-
Alfred Wegener propounded in his Continental Drift Theory that in the pre-Carboniferous period, around 300 million years ago, all the present-day landmasses were part of a single landmass or supercontinent, which he named Pangea, a Greek word meaning ‘all lands.
In the Triassic period, there was a geosyncline named Tethys in the middle of the Pangea. The northern and southern parts of the Pangea were known as Laurasia and Gondwanaland respectively.
Wegener envisaged that Pangea began to break apart in the Mesozoic era and the present landmasses and waterbodies originated due to drifting apart of the fragments of Pangea by differential gravitational force, buoyancy and tidal force.
Question 2. What is plate movement?
Answer:
Plate Movement:-
In geology, plate refers to a rigid and solid lithospheric slab. Even the SIAL and SIMA are plates. There are 6 major and 20 minor plates in the world. The plates are moving very slowly in the slippery asthenosphere due to various causes.
They can move towards each other or away from each other. They can even slide past each other. This continuous movement of the
plates is known as plate movement.
WBBSE Class 8 Geography Chapter 2 Questions
Question 3. Why do the tectonic plates move?
Answer: Tectonic plates move due to the following reasons
1. Upwelling of magma:
Plates move away from each other at divergent plate boundaries due to upwelling of magma. Again, the formation of dykes at the divergent plate boundaries causes the drifting of plates.
2. Gravitational force:
Volcanic activities lead to the formation of oceanic ridges at the divergent plate boundaries and the plates begin to move away from each other due to gravitational force.
3. Convection current:
According to Holmes, the viscous and lubricating materials of asthenosphere act as the medium of heat transfer, which gives rise to convection current. This causes the rigid plates to drift apart on the fluid mantle.
Question 4. What do you mean by convection current and convection cell?
Answer:
Convection Current And Convection Cell:-
Convection currents play an important role in the movement of the plates. According to Holmes, viscous and lubricating materials of the asthenosphere act as the medium of heat transfer, which generate convection currents.
As the materials move upwards with the current, they move away from each other just beneath the surface of the earth and this creates convection cells.
Two consecutive cells cause the plates to drift away on the fluid mantle. Similarly, downward currents cause two plates to come towards each other.
Question 5. What is a divergent plate boundary?
Answer:
Divergent Plate Boundary:-
When two tectonic plates move apart from each other, a divergent or constructive plate boundary is formed. The upwelling magma coming out through the intra-plate crack develops new landforms along this boundary.
Along this plate boundary, the sea floor spreads due to the prevalence of upwelling magma and forms MOR (Mid-Oceanic Ridges), e.g., Mid-Atlantic Ridge. Thus, a divergent plate boundary is also called constructive plate boundary.
Question 6. What is a convergent plate boundary?
Answer:
Convergent Plate Boundary:-
When two plates move towards each other and collide, a convergent plate boundary is formed. The collisions form fold mountains and subducted parts begin to melt due to the high temperature of the interior of the earth.
WBBSE Class 8 Geography Chapter 8 Question Answer
Due to the loss of parts of the earth’s crust, it is also called destructive plate boundary.
Question 7. How is the altitude of the Himalayas increasing due to the movement of the plates?
Answer:
Altitude Of The Himalayas Increasing Due To The Movement Of The Plates:-
The Himalayan mountain is located at the convergent plate boundary between the Indian and Eurasian Plates. The Indian Plate is subducting under the Eurasian Plate at a high rate every year.
As a result, folding and upheaving of the sediments of the Tethys Geosyncline due to high pressure is increasing the altitude of the Himalayan range further. As long as the plates continue to move, the height of the Himalayas will go on increasing.
Question 8. Most fold mountains are formed due to collision of the continental plates- Why?
Answer:
Most Fold Mountains Are Formed Due To Collision Of The Continental Plates:-
A geosyncline usually occurs between the continental plates which meet along a boundary. As these plates get closer to each other, the geosyncline becomes narrower and the sediments of the geosyncline crumble.
This way, the upheaval of the sediments leads to the birth and development of fold mountains through plate collisions. Hence, most of the fold mountains form as a result of the collision of continental plates.
Question 9. What do you mean by Mid-World exogenic and endogenic forces. Processes that Mountain Belt?
Answer:
Mid-World Exogenic And Endogenic Forces:-
Mid-World Mountain Belt is the young fold mountain belt extending over Eurasian Plate, Indo-Australian Plate and African Plate. It has two knots- Pamir and Armenian.
The mountains like Himalayas, Karakoram, Kunlun, Altyn Tagh and Arakan Yoma are located on the east and Hindu Kush, Suleiman, Elburz, Zagros, Pontic, Taurus and Alps are located on the west of this belt.
Question 10. What do you mean by endogenic force?
Answer:
Endogenic Force:-
The earth is always in an unstable condition and changes occur on it due to both are caused by forces from within the earth are called endogenic forces. The endogenic processes are generally of two types
Slow and continuous processes, which include orogenic and epeirogenic movement.
Rapid processes, like earthquakes, volcanism etc.
Types of Plate Boundaries for Class 8
Question 11. Compare among divergent, convergent and neutral plate boundaries.
Answer: A comparison between divergent, convergent and neutral plate boundaries includes the following which are mentioned in the table below
Question 12. Write the differences between a continental plate and a oceanic plate.
Answer: The differences between a continental plate and a oceanic plate are
WBBSE Chapter 2 Topic A Plate Tectonics Short Answer Type Questions
Question 1. What is Continental Drifting?
Answer:
Continental Drifting:-
According to Alfred Wegener, drifting of the fragments of Pangea towards different directions due to tidal force, buoyancy and differential gravitational force is known as Continental Drifting.
Question 2. What do you know about convection current and plate movement?
Answer:
Convection Current And Plate Movement:-
According to Arthur Holmes, hot, viscous and molten lava produces convectional current through heat transfer under the surface of the earth.
Upwelling molten lava move in opposite directions just beneath the earth’s crust. Due to that convection current, the plates begin to drift apart.
Short Answer Questions on Plate Tectonics for Class 8
Question 3. Why is the convergent plate boundary are called destructive plate boundary?
Answer: After the collision between the two plates along a convergent plate boundary, the heavier plate gets subducted under the lighter one. Consequently, the subducted parts begin to melt due to the high temperature of the interior of the earth.
Due to this loss of the earth’s crust, a convergent plate boundary is also called a destructive plate boundary.
Question 4. Name the major plates of the earth.
Answer:
Major Plates Of The Earth:-
According to Le Pichon, there are 6 major plates on the Earth.
These are
- American Plate
- Antarctic Plate
- Indo- Australian Plate
- Eurasian Plate
- Pacific Plate and
- African Plate.
Question 5. Write the names of the continental plates and an oceanic plate.
Answer:
The continental plates are
- American Plate
- Antarctic Plate
- Indo- Australian Plate
- Eurasian Plate and
- African Plate.
The Pacific Plate is an oceanic plate.
Question 6. How is the area of the Pacific plate decreasing due to the plate tectonic movement?
Answer:
Area Of The Pacific Plate Decreasing Due To The Plate Tectonic Movement As Follows:-
The tectonic plates are constantly drifting. The American plate is moving eastwards while the Eurasian plate is moving north-eastwards. As a result, the area of the Pacific plate is decreasing gradually.
The trenches bordering the Pacific Ocean are regions where the Pacific plate is sinking.
Question 7. Which plates are responsible for the formation of the Himalayas?
Answer: The movement of the Indian plate and the Eurasian plate towards each other caused a collision between them. Consequently, the Eurasian plate overrode the other one.
Thus, the formation of the Himalayan mountains started. So, the Indian Plate and the Eurasian Plate are responsible for the formation of the Himalayas.
WBBSE Class 8 Geography Study Guide on Plate Tectonics
Question 8. Why are young fold mountains called so?
Answer:
young fold mountains called so:-
The age of the earth is 4600 million years. The old fold mountains had been formed 200 million years ago while the young fold mountains have developed only 10-25 million years ago.
Formation and development of the young fold mountains is still ongoing. With respect to the geological time scale from the origin of the earth to the present, 10-25 million years is a very short span. Thus these mountains are called ‘young’ fold mountains.
Question 9. What do you mean by geological time?
Answer:
Geological Time:-
The entire span of time, starting from the origin of the earth to the present, is known as geological time, i.e., Pre-Cambrian, Holocene, etc.
Question 10. Define Suture Line.
Answer:
Suture Line:-
Suture Line is a transitional line along which two continental plates collide with each other. One of the prominent examples of suture line is found in the Himalayan mountainous region.
Question 11. Why can we not feel the plate movement? Or, Can we feel the plates moving?
Answer: The plates of the earth’s crust are in continuous motion on the asthenosphere. However, their movement is too slow to be felt by us. As per information collected from GPS (Global Positioning System), the plates move at a rate of 2-10 cm/year.
To cite an example, the American Plate is moving westward at a rate of 2-3cm/year, whereas, the Pacific Plate is moving westward at a rate of 10cm/year.
Therefore, the movements of the plates are very slow in terms of human pace and hence, we cannot feel their movement.
WBBSE Chapter 2 Topic A Plate Tectonics Multiple Choice Questions
Choose the correct option
Question 1. Continental Drift Theory was propounded by-
- Le Pichon
- Kober
- McKenzie
- Alfred Wegener
Answer: 4. Alfred Wegener
Question 2. The term ‘Plate’ was first used by-
- Le Pichon
- McKenzie
- J.T. Wilson
- Alfred Wegener
Answer: 3. J.T. Wilson
Question 3. Plate Tectonic Theory was successfully applied by-
- Le Pichon
- Kober
- Alfred Wegener
- McKenzie
Answer: 1. Le Pichon
WBBSE Class 8 Geography Plate Movement Explanation
Question 4. Two plates that move away from each other are called-
- Convergent plates
- Neutral plates
- Divergent plates
- None of these
Answer: 3. Divergent plates
Question 5. Two plates that move towards each other are called-
- Convergent plates
- Neutral plates
- Divergent plates
- None of these
Answer: 1. Convergent plates
Question 6. The boundary between two convergent plates is known as-
- Destructive plate boundary
- Constructive plate boundary
- Neutral plate boundary
- Indefinite plate boundary
Answer: 1. Destructive plate boundary
Question 7. The highest fold mountain of the world is the-
- Appalachian
- Andes
- Alps
- Himalayas
Answer: 4. Himalayas
Question 8. The longest fold mountain of the world is the-
- Appalachian
- Andes
- Alps
- Himalayas
Answer: 2. Andes
Question 9. The San Andreas Fault is an example of-
- Convergent plates
- Divergent plates
- None of these
- Neutral plate
Answer: 4. Neutral plate
WBBSE Chapter 2 Topic A Plate Tectonics Fill In The Blanks
Question 1. All the continents of the world existed together as a united landmass named ________, about 300 million years ago.
Answer: Pangea
Question 2. The tectonic plates move due to the ________ currents of the asthenosphere.
Answer: Convection
Question 3. The drifting away of the American and African Plates from each other along the ________ Ridge has led to the formation of the Atlantic Ocean.
Answer: Mid-Atlantic
Question 4. ________ of the Pacific Ocean is the deepest oceanic trench in the world.
Answer: Mariana Trench
Question 5. The Himalayas were formed due to the collision of the ________ Plate and the Eurasian Plate.
Answer: Indian
Question 6. The Pacific Ocean plate is moving at a rate of ________ /year.
Answer: 2-3cm
Question 7. The Greek word ‘Tekton’ means ________.
Answer: To from
WBBSE Chapter 2 Topic A Plate Tectonics Write True or False
Question 1. The Continental Drift Theory was propounded by Le Pichon.
Answer: False
Question 2. The continental plate is thicker than the oceanic plate.
Answer: False
Question 3. There are 6 minor plates in the world.
Answer: False
Question 4. Many volcanoes and volcanic islands are formed along a submerged plate boundary.
Answer: True
Question 5. The plates drifting on an upward divergent current between two consecutive convection cells move away from each other.
Answer: True
Question 6. The depth of the plates is less than their area.
Answer: True
Question 7. The boundary where the plates move parallel to each other is called the transform plate boundary.
Answer: True
Question 8. The Himalayan mountain was formed due to the collision of the African and the Eurasian Plates.
Answer: False
Question 9. A convergent plate boundary is earthquake earthquake-prone area.
Answer: True
Question 10. The mechanism of convection of heat is also active on other planets apart from the earth.
Answer: True
Question 11. Aravalli is an old fold mountain.
Answer: True
Question 12. Plates move apart along the mid-oceanic ridges.
Answer: True
WBBSE Chapter 2 Topic A Plate Tectonics Match The Columns
1.
Answer: 1. C, 2. D, 3. E, 4. A, 5. B
WBBSE Chapter 2 Topic A Plate Tectonics Answer In One Or Two Words
Question 1. Who propounded the concept of Plate Tectonics?
Answer: J.T. Wilson.
Question 2. What do the tectonic plates float on?
Answer: Asthenosphere.
Question 3. How many major plates are there in the world?
Answer: 6.
Question 4. Along which type of plate boundary are volcanoes formed?
Answer: Convergent plate boundary.
Question 5. Along which type of plate boundary, does volcanism occur without any explosion or scattering?
Answer: Divergent and neutral plate boundaries.
Question 6. Which plates are responsible for the genesis of the Himalayan mountain range?
Answer: Eurasian and Indian Plates.
Question 7. What do you call the boundary where there is neither any destruction nor any construction of landforms?
Answer: Neutral or conservative plate boundary.