Geography Class 12

 Geography Class 12

Seafloor spreading 

• Mapping of the oceanic floor revealed the following information:
• Presence of mid-oceanic ridges along the seafloor.
• The ridges are volcanically active resulting in a continuous eruption of magma.
• Rocks on either side of the ridges are of the same age with similar composition and magnetic properties.
• The age of rocks along the oceanic floors increases away from the ridge.
• The rocks of the oceanic crust are younger than the continental crust.
• The oceanic crust is thinner than the continental crust.
• Theory:
• Based on the above observation, Herry Hess proposed the theory of seafloor spreading in 1961.
• According to it, the continued magma eruption at the oceanic ridges causes a rupture of the oceanic crust.
• The new lava wedges on to oceanic crust, which pushes the oceanic crust onto either side therefore the ocean floor spreads.
• The spreading crust sinks at the oceanic trenches and gets consumed.

Plate tectonics theory 

• Introduction:
• The term plate was coined by JT Wilson in 1965.
• The theory of plate tectonics was proposed by Morgan, Mckenzie, and Parker in 1967.
• Plate tectonics:
• Plates are broad and rigid segments of the lithosphere which includes the ridge upper part of the upper mantle and crust.
• The plates are in motion on underlined asthenosphere, which is in a semi-solid and semi-liquid state.
• Plate tectonics is the study of deformation within plates and of the interaction of plates around their margins.
• Plates are nearly 100 km thick and have high rigidity and are unable to deform except in response to very strong and prolonged force.
• There are 7 major plates on the earth's surface, pacific, north American, south American, Eurasian, Indo-Australian, and Antarctican.
• There are many minor plates such as Nazca Plates, cocos plates, Arabain plates, etc
• The difference concerning continental drift theory:

• CDT	PTT
  Sial, Sima, Nefi	Lithosphere(SiAl and Sima) and asthenosphere
  Freely floating SiAl	Offer High resistance

• Plate movements:
• The plates are constantly in motion but with different speeds and directions, this causes 3 types of plate boundaries, Divergent, convergent and transform.
• A plate boundary or margin is a zone of motion between two plates.
• Diagrammatic representation of different plates:
•  

Divergent/constructive plate boundary

• The ocean-ocean divergent plate boundary:
• It is a type of plate boundary where two oceanic plate margins are moving apart in opposite directions
• It is a zone of tension where the lithosphere split and hot magma comes up through the cracks and solidifies leading to the formation of a new oceanic crust.
• The continues built of solid magma results in the formation of mid-oceanic ridges along the plate margins.
• In this plate boundary, shallow earthquakes with a focus of up to 70 km are observed.
• The continental-continental divergent plate boundary:
• The formation of divergent plate boundaries along continents involves 3 stages:
• Intra-continental rifting:
• The upward movement of magma below the continental crust causes the fragmentation of the continent through the creation of numerous cracks and faults.
• Such a series of faults is called a rift valley.
• Rising magma starts to come out through this rift.
• For example, the East African rift valley.
• Inter-plate thinning:
• It involves partial melting of the lithosphere and gradual thinning of continental crust.
• Rift valley starts to widen and may gradually get filled with ocean water resulting in the formation of a shallow sea.
• For example, the Red sea.
• Formation of mid-oceanic ridge:
• The continues spreading of continental plates away from each other and the creation of new oceanic crust along the rift valley by the rising magma pulls the continental mass sufficiently apart.
• At this stage, there will be a new oceanic basin along both the sides of mid-oceanic ridge.
• For example mid-Atlantic ridge.

Convergent/destructive plate boundaries 

• Oceanic-oceanic convergence:
• When two oceanic plate margins converge the ocean plate of higher density decent into the asthenosphere.
• This process is called subduction and the zones are called subduction zones.
• Subduction leads to the formation of trenches, that is the deepest regions on the surface of the earth.
• These subduction zones are the sites of the most widespread and intense earthquakes.

The topic for the next class: Convergent/destructive plate boundaries to continue.

Here are 40 short answer type questions with answers based on the provided notes on Geography Class 12:

 

1. What did mapping of the oceanic floor reveal?

   - The presence of mid-oceanic ridges along the seafloor.

 

2. What is a significant feature of the mid-oceanic ridges?

   - They are volcanically active and result in continuous magma eruptions.

 

3. What is the age relationship of rocks on either side of the ridges?

   - Rocks on either side of the ridges are of the same age with similar composition and magnetic properties.

 

4. How does the age of oceanic crust rocks compare to that of continental crust rocks?

   - The oceanic crust rocks are younger than the continental crust rocks.

 

5. What is the difference in thickness between oceanic crust and continental crust?

   - The oceanic crust is thinner than the continental crust.

 

6. Who proposed the theory of seafloor spreading and when?

   - Herry Hess proposed the theory of seafloor spreading in 1961.

 

7. What causes the ocean floor to spread according to the theory of seafloor spreading?

   - The continued magma eruption at the oceanic ridges causes a rupture of the oceanic crust, and the new lava pushes the oceanic crust to either side.

 

8. What happens to the spreading crust in seafloor spreading?

   - The spreading crust sinks at the oceanic trenches and gets consumed.

 

9. Who coined the term "plate" in the context of plate tectonics?

   - JT Wilson coined the term "plate" in 1965.

 

10. Who proposed the theory of plate tectonics and when?

    - Morgan, McKenzie, and Parker proposed the theory of plate tectonics in 1967.

 

11. What are plates in the context of plate tectonics?

    - Plates are broad and rigid segments of the lithosphere which include the ridge upper part of the upper mantle and crust.

 

12. What is the state of the asthenosphere beneath the plates?

    - The asthenosphere is in a semi-solid and semi-liquid state.

 

13. What does plate tectonics study?

    - Plate tectonics is the study of deformation within plates and the interaction of plates around their margins.

 

14. How thick are the plates in the context of plate tectonics?

    - Plates are nearly 100 km thick.

 

15. How many major plates are there on the earth's surface?

    - There are 7 major plates: Pacific, North American, South American, Eurasian, Indo-Australian, Antarctic, and African.

 

16. Name some minor plates.

    - Nazca Plates, Cocos Plates, Arabian Plates, etc.

 

17. What is the primary difference between the continental drift theory (CDT) and plate tectonics theory (PTT)?

    - CDT describes SiAl, Sima, Nefi layers and freely floating SiAl, while PTT involves the lithosphere (SiAl and Sima) and asthenosphere, which offer high resistance.

 

18. What are the three types of plate boundaries?

    - Divergent, convergent, and transform boundaries.

 

19. What is a divergent plate boundary?

    - It is a type of plate boundary where two plate margins are moving apart in opposite directions.

 

20. What happens at an ocean-ocean divergent plate boundary?

    - The lithosphere splits, and hot magma comes up through the cracks and solidifies, leading to the formation of new oceanic crust.

 

21. What geological feature is formed due to the continuous buildup of solid magma at divergent plate boundaries?

    - Mid-oceanic ridges.

 

22. What kind of earthquakes are observed at ocean-ocean divergent plate boundaries?

    - Shallow earthquakes with a focus of up to 70 km.

 

23. What is an intra-continental rifting?

    - The upward movement of magma below the continental crust causes the fragmentation of the continent through the creation of numerous cracks and faults, forming a rift valley.

 

24. Give an example of an intra-continental rift valley.

    - The East African Rift Valley.

 

25. What happens during inter-plate thinning?

    - It involves partial melting of the lithosphere and gradual thinning of the continental crust, leading to the formation of a shallow sea.

 

26. Give an example of inter-plate thinning.

    - The Red Sea.

 

27. What is formed due to the continued spreading of continental plates away from each other?

    - A new oceanic basin along both sides of a mid-oceanic ridge.

 

28. Give an example of a mid-oceanic ridge formed by continental spreading.

    - The Mid-Atlantic Ridge.

 

29. What is a convergent plate boundary?

    - A type of plate boundary where two plate margins are moving towards each other, often resulting in one plate being forced below the other.

 

30. What happens during oceanic-oceanic convergence?

    - One oceanic plate descends into the asthenosphere, a process called subduction, forming trenches and causing intense earthquakes.

 

31. What is a subduction zone?

    - An area where one tectonic plate is being forced below another.

 

32. What geological feature is formed at subduction zones?

    - Trenches, which are the deepest regions on the earth's surface.

 

33. What type of earthquakes are associated with subduction zones?

    - The most widespread and intense earthquakes.

 

34. Who proposed the theory of seafloor spreading?

    - Herry Hess.

 

35. What is the primary cause of seafloor spreading according to Hess?

    - The continued eruption of magma at oceanic ridges.

 

36. What term did JT Wilson coin in 1965?

    - The term "plate" in the context of plate tectonics.

 

37. What are the components of the lithosphere according to plate tectonics theory?

    - The upper mantle and crust.

 

38. How do plates move according to plate tectonics theory?

    - Plates move on the semi-solid and semi-liquid asthenosphere.

 

39. What causes divergent plate boundaries?

    - The movement of two plate margins apart from each other.

 

40. What process occurs at oceanic-oceanic convergent boundaries?

    - Subduction, where one oceanic plate descends into the asthenosphere.

QUESTIONS FOR PRACTICE

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1)

What evidence did Wegener give in support of his observations?

1. Jig-saw fit
2. Rock types of the same ages.
3. Glacial evidence
4. Placer deposits
5. Fossil remains

Select the correct answer using the codes given below:

(a)1, 2,3, 4 only

(b)2, 3, 4 only

(c)All of the above

(d)1,4,5 only

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2)

Who among the following propounded Continental Drift Theory?

(a)Fredwine and Mathews

(b)Arthur Holmes

(c)Harry Hess

(d)Alfred Wegner

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3)

With reference to Continental drift theory, consider the following statements:

1. The big landmass was called Panthalassa which was covered by one big ocean called Pangaea.
2. Tethys divided the Panthalassa into two huge landmasses.
3. Around 20 million years ago, Arabia got separated from Africa and merged into Asia.

Which of the statements given above is/are correct?

(a)1 and 3 only

(b)3 only

(c)1 and 2 only

(d)1, 2 and 3.

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4)

With reference to the forces responsible for the continental drift by Wegner consider the following statements:

1. The tidal force was responsible for the movement of continents.
2. Equatorward or North-South movement is caused by the Pole-fleeing force.
3. He proposed the primordial heat as one of the forces.

Which of the statements given above is/are correct?

(a)1 and 2 only

(b)2 only

(c)3 only

(d)1, 2 and 3.

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5)

Which of the following was the criticism/s of continental drift theory?

1. Rocks of continental crust and oceanic crust are very rigid and would not permit a smooth drifting of continents over the oceanic floor.
2. The geophysical mechanic envisaged for the movement of continents i.e. forces suggested are inadequate.

Select the correct answer using the codes given below:

(a)1 only

(b)2 only

(c)Both 1 and 2

(d)Neither 1 nor 2

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6)

With reference to Isostatic movements consider the following statements:

1. Isostatic movements involve vertical movements under the action of floatation displacement between the rock layers.
2. Isostasy is the state of equilibrium or balance in the earth"s crust.
3. There is no role of density in Isostatic movements.

Which of the statements given above is/are correct?

(a)1 only

(b)2 and 3 only

(c)3 only

(d)1 and 2 only

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7)

Eustatic movements are related to?

(a)Worldwide movement of sea level resulting from changes in the total volume of liquid seawater or capacity of ocean basins.

(b)Movement of galaciers in northern Hemisphere.

(c)Movement of earth"s outer core.

(d)None of the above.

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8)

Consider the following Assumptions made by Alfred Wegner in Continental drift theory:

1. The three layers of the earth with outer SiAl, intermediate SiMa, and inner NiFe.
2. The Crust of the earth is divided into many tectonic plates floating upon the mantle.
3. The continental masses were assumed to be floating on oceanic crust without any resistance.

How many of the above assumptions were made by Alfred Wegner in Continental drift theory?

(a)Only One

(b)Only two

(c)All three

(d)None

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9)

Explain Isostatic and Eustatic movement with examples.(150 words/10 marks)

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10)

Point out the assumption and major criticism of continental drift theory(150 words/10 marks).

Answers
1) c
2) d
3) b
4) a
5) c
6) d
7) a
8) a

Q1.

Enumerate the major aspects of the Continental Drift Theory. Explain major pieces of evidence that support this theory. (150 words/10 marks)