Geography Class 13

 Geography Class 13

Ocean-Ocean convergence

• When 2 oceanic plate margins converge oceanic plates of higher density descend into the asthenosphere, this process is called a subduction zone.
• The process of subduction leads to the formation of trenches which are the deepest regions on the surface of the earth.
• For example subduction of the pacific plate below the Eurasian plate has led to the formation of the Mariana Trench, and Japan Trench.
• The subducting plate undergoes deformation, intense compression, metamorphism, and melting as it reaches the deeper parts.
• Some of the molten material from the subducting plate rises upwards and accumulate continually on the other oceanic plates, this results in the formation of Volcanic mountains on the seafloor.
• When these Volcanic mountains rise above the sea level it results in the formation of volcanic islands.
• The continued volcanic activity gradually increases the size and elevation of volcanic islands.
• These islands are arranged parallel to the trenches in an arc shape and are called island arcs, for example, japan, Aleutian, and the Caribbean islands.
•  An Archipelago is a group of scattered islands in the ocean formed due to ocean-ocean convergence plate boundary with intense volcanic activity, for example, Indonesia, Philippines, etc.
• A subduction zone is the site of widespread volcanic activity and earthquakes. 
• In ocean-ocean convergent, all 3 kinds of earthquakes shallow, intermediate, and deep-seated are found.
• The majority of the earthquake appears to be confined to a dipping zone along the subduction plate called Benioff Zone.

Ocean-Continent convergent plate boundary:

• When a plate with an oceanic margin collides with a plate of the continental margin oceanic crust is denser and subducts beneath the continental plate which is more buoyant, for example, the Nasca plate subducting below the South American plate.
• During the subduction, the thick sequence of rocks along the continental margin is compressed and deformed to form a chain of fold mountains, for example, Andes and rocky mountains.
• As the oceanic plate subducts it creates trenches and earthquakes are generated at Benioff zones.
• As the oceanic plate subducts deeper it gets melted and the magma starts to rise.
• The rising magma accumulates within the adjacent continental crust which eventually rises to the surface resulting in the formation of volcanoes.
• For example, the subduction of the Nasca plate below the  South American plate has resulted in a series of volcanos in the Andes, such as Ozes del Salado and Cotopaxi.
• St. Helens volcano in the Rockies is also similar.
• Ocean-continent convergence results in all the 3 types of earthquakes.
• The Pacific Ocean is surrounded by trenches on all sides, which are zones of intense volcanic activities and earthquakes, therefore it is called as Pacific ring of fire.

Continental-continental convergence:

• Before the collision of the continental margin, the land masses are usually separated by the oceanic crust.
• As the two plates converge, the oceanic crust located in between starts to subduct beneath one of the plates.
• The oceanic plate completely breaks from the continental block and assimilated into the mantle, at this point, Volcanic activity stops.
• The continued convergence causes the two continual masses to get stitched together along a zone called a suture zone.
•  When continental margins continue to collide the low density of continental material doesn't permit its subduction.
• The continued convergence forces the continental crust partially under the other one creating an unusually thick layer of intense folding.
• The sediments deposited in the basis between the continental margins undergo continued compression resulting in the formation of very high-fold mountains, for example, the Himalayas.
• Shallow and intermediate earthquakes are absorbed along this margin.

Parallel plate/Conservative plate margin:

• At the conservative plate margins, the plate slightly passes each other without any formation of new crust.
• The transform fault roughly moves parallel to the direction of plate movement, for example, the San Andreas fault of Califonia.
• A conservative plate margin involves no volcanic activity.
• The earthquakes observed are shallow in depth but they are more frequent and intense.
• Such a series of transform faults are also found along the oceanic plate margins where they offset the oceanic ridges throughout their length.

Table summarizing plate boundaries:

• 	Divergent(O-O) plates	Divergent(C-C) plates	Convergent(O-O) plates	Convergent(O-C) plates	Convergent(C-C) plates	Conservative/parallel plates
  Features.	Mid-oceanic ridges, transform faults.	Rift valley, shallow sea, MOR.	Trenches, subduction zones, archipelago,  island arcs, Benioff zone.	Trenches, subduction zones, fold mountains, volcanoes Benioff zone.	suture zone, partial subduction, geo-syncline, fold mountains, nappe.	Series of transform faults
  Earthquakes.	shallow.	shallow.	shallow, intermediate, and deep.	shallow, intermediate, and deep.	shallow and intermediate	shallow.
  volcanism	Yes	Yes	Yes	Yes	No	No
  Examples	Mid Atlantic Ridge, Carlsberg Ridge.	East African rift valley, Red sea, Mid Atlantic ridges	Japan, Aleutian, Indonesia, etc.	Andes, Rockies mountains.	The Himalayas.	San Andreas.

Causes of Plates movements:

• Diagramatic representation of various causes:
• 
• The convection currents:
• They are generated due to the intense heat released from the interior of the earth.
• The theory of convention current was proposed by Arthur homes.
• As the currents ascend from below they diverge and spread laterally.
• Mantle plumes:
• It is a type of mantle convention that involves jet-like plumes of low-density magma material from the core-mantle boundary.
• Mantle plumes are also the cause of the formation of volcanic hot spots, for example, reunion, Hawai, etc.
• Ridge push:
• The magma rushes along the oceanic ridges from wedges of the new lithosphere on either side trail edge of the plate and causes the plates to be pushed apart.
• Gravity sliding:
• The spreading centers along the mid-ocean ridges stand high on the oceanic floor this results in the gravitation side of the lithospheric slab being away from the oceanic ridges.
• Slab pull:
• It is considered to be along the subduction zone where the subducting plate pulls the rest of the slab along.

The topic for the next class: Dictation for causes of plate movement and volcanism.

Here are 40 short answer type questions with answers based on the provided notes on Geography Class 13:

 

1. What happens when two oceanic plate margins converge?

   - The oceanic plate with higher density descends into the asthenosphere, creating a subduction zone.

 

2. What is formed as a result of the subduction process?

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

 

3. Give examples of trenches formed due to ocean-ocean convergence.

   - The Mariana Trench and the Japan Trench.

 

4. What happens to the subducting plate as it reaches deeper parts?

   - It undergoes deformation, intense compression, metamorphism, and melting.

 

5. What results from the molten material rising from the subducting plate?

   - The formation of volcanic mountains on the seafloor.

 

6. What forms when volcanic mountains rise above sea level?

   - Volcanic islands.

 

7. What is an island arc?

   - A series of volcanic islands arranged parallel to the trenches in an arc shape, e.g., Japan, Aleutian, and the Caribbean islands.

 

8. What is an archipelago?

   - A group of scattered islands in the ocean formed due to ocean-ocean convergence with intense volcanic activity, e.g., Indonesia, Philippines.

 

9. What kind of seismic activity is common in subduction zones?

   - Widespread volcanic activity and earthquakes.

 

10. What types of earthquakes are found in ocean-ocean convergent boundaries?

    - Shallow, intermediate, and deep-seated earthquakes.

 

11. What is the Benioff Zone?

    - A dipping zone along the subducting plate where the majority of earthquakes are confined.

 

12. What happens at an ocean-continent convergent plate boundary?

    - The denser oceanic crust subducts beneath the more buoyant continental plate.

 

13. Give an example of an ocean-continent convergence.

    - The Nazca plate subducting below the South American plate.

 

14. What forms as a result of the compression and deformation along the continental margin?

    - A chain of fold mountains, e.g., the Andes and Rocky Mountains.

 

15. What geological features are created by the subduction of an oceanic plate beneath a continental plate?

    - Trenches and earthquakes.

 

16. What happens to the magma from a subducting oceanic plate?

    - It rises and accumulates within the adjacent continental crust, forming volcanoes.

 

17. Give examples of volcanoes formed by ocean-continent convergence.

    - Ojos del Salado and Cotopaxi in the Andes, St. Helens in the Rockies.

 

18. What surrounds the Pacific Ocean that indicates intense volcanic activity and earthquakes?

    - Trenches, forming the Pacific Ring of Fire.

 

19. What happens during continental-continental convergence?

    - The land masses are stitched together along a suture zone, forming high-fold mountains.

 

20. What is a suture zone?

    - The zone where two continental masses are stitched together during convergence.

 

21. Why doesn't subduction occur in continental-continental convergence?

    - The low density of continental material prevents subduction.

 

22. Give an example of high-fold mountains formed by continental-continental convergence.

    - The Himalayas.

 

23. What types of earthquakes are observed in continental-continental convergence?

    - Shallow and intermediate earthquakes.

 

24. What happens at a parallel or conservative plate margin?

    - Plates slide past each other without forming new crust.

 

25. Give an example of a transform fault at a conservative plate margin.

    - The San Andreas Fault in California.

 

26. Is volcanic activity observed at conservative plate margins?

    - No, volcanic activity is not observed.

 

27. What types of earthquakes are frequent at conservative plate margins?

    - Shallow but frequent and intense earthquakes.

 

28. What causes offset oceanic ridges along their length?

    - Transform faults along the oceanic plate margins.

 

29. What features are observed at divergent (ocean-ocean) plate boundaries?

    - Mid-oceanic ridges and transform faults.

 

30. What features are observed at divergent (continental-continental) plate boundaries?

    - Rift valleys, shallow seas, and mid-ocean ridges.

 

31. What features are observed at convergent (ocean-ocean) plate boundaries?

    - Trenches, subduction zones, archipelagos, island arcs, and the Benioff Zone.

 

32. What features are observed at convergent (ocean-continent) plate boundaries?

    - Trenches, subduction zones, fold mountains, volcanoes, and the Benioff Zone.

 

33. What features are observed at convergent (continental-continental) plate boundaries?

    - Suture zones, partial subduction, geo-synclines, fold mountains, and nappes.

 

34. What features are observed at conservative or parallel plate margins?

    - Series of transform faults.

 

35. What causes the movement of tectonic plates?

    - Convection currents, mantle plumes, ridge push, gravity sliding, and slab pull.

 

36. What generates convection currents in the earth's mantle?

    - The intense heat released from the interior of the earth.

 

37. Who proposed the theory of convection currents?

    - Arthur Holmes.

 

38. What are mantle plumes?

    - Jet-like plumes of low-density magma material from the core-mantle boundary.

 

39. What geological features are formed by mantle plumes?

    - Volcanic hot spots, e.g., Reunion, Hawaii.

 

40. What is slab pull?

    - The process where the subducting plate pulls the rest of the slab along.

QUESTIONS FOR PRACTICE

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1)

Which plate is the principal cause of the Andes Mountains?

(a)The Nazca Plate

(b)The Arabian Plate

(c)The North American Plate

(d)  The Caribbean Plate

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2)

Consider the following statements regarding the Conservative plate boundary:

1. There is no generation of magma and volcanic activity.
2. A series of transform faults are generated along the conservative plate margin.
3. The San Andreas Fault is an example of a conservative plate boundary.

Which of the above statements is/are correct?

(a)Only 1 and 2

(b)Only 2 and 3

(c)1,2, and 3

(d)None of the above.

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3)

Which of the following can cause the movement of tectonic plates?

1. Rising convectional current below the lithosphere.
2. Tidal forces
3. Ridge push
4. Slab pull

Select the correct answer using the code given below the list.

(a)1, 3, and 4 only

(b)2, 3, and 4 only

(c)1 and 2 only

(d)1, 2, 3, and 4

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4)

Which of the following are formed as a result of Convergent plates?

1. The Aleutian Islands
2. The Himalayas
3. The Philippines
4. Red sea

Select the correct answer using the codes given below:

(a)1 and 4 only

(b)1, 2, and 3 only

(c)2 and 3 only

(d)1, 2, 3 and 4

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5)

Which of the following are earthquake-prone regions?

1. Alpine-Himalayan belt.
2. East African rift valley system.
3. The rim of the Pacific Ocean.

Select the correct answer using the code given below:

(a)1 only

(b)1 and 3

(c)2 and 3.

(d)1, 2 and 3

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6)

Consider the following statements:

1. At a conservative plate margin, the plates slide past each other without creating a new crust or destroying them.
2. Continental–Oceanic Convergence gives rise to extensive mountain systems.
3. Volcanism is observed in Continental–Oceanic convergence and is almost absent in Continental – Continental convergence.

Which of the statements given above is/are correct?

(a)1 and 2 only

(b)2 and 3 only

(c)1 and 3 only

(d)1, 2 and 3

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7)

Consider the following statement regarding Mantle Convection Current Theory:

1. Convection Current Theory was proposed by Arthur Holmes a marine geologist in the 1930s.
2. These currents are generated due to radioactive elements causing thermal differences in the asthenosphere.
3. The movement of the lithospheric plates is caused by the movement of the magma in the asthenosphere.

Which of the statements given above is/are not correct?

(a)1 and 2 only

(b)2 and 3 only

(c)3 only

(d)None of the above

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8)

which of the following is/are correctly matched?

         Type of plate boundary:                           Example of landform:

1. Divergent(Oceanic-Oceanic) plate            Mid Atlantic Ridge.
2. Convergent(Oceanic-Oceanic)) plates      Rockies mountains.
3. Conservative plates.                                 The Himalayas.

Select the correct answer using the code given below:

(a)1 only

(b)1 and 2 only 

(c)2 and 3 only 

(d)all of the above.

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9)

Define mantle plume and explain its role in plate tectonics. (10 Marks/150 Words)

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10)

Explain in detail the various features of conservative plate bounadry with example?(150 words/10 marks)

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

Q1.

What are the different types of plate boundaries? Explain the convergent plate boundary movements with relevant examples. (10 Marks/150 words)