Geography Class 14

Causes of Plates movements 

• The convection currents:
• Arthur homes proposed the theory of convention current.
• There are two types of convection current, one involves the whole of the mantle and the second involves only the asthenosphere.
• They are generated due to the intense heat released from the interior of the earth due to radioactivity.
• As the currents rise from below, diverge and spread laterally.
• The convection causes the lithosphere to crack and rift forming a mid-oceanic ridge.
• As the plates move laterally the currents carry the slab of the lithosphere with them.
• When these currents encounter a similar current from the opposite direction, they descend into the deeper part of the mantle and drag the lithosphere along with them causing subduction.
• Mantle plumes:
• It is a type of mantle convention and involves jet-like plumes of low-density magma material from the core-mantle boundary.
• As the plume reaches the lithosphere it spreads out latterly doming the surface zones of the earth and moving them along the direction of the mantle plume.
• Mantle plumes are also the cause of the formation of volcanic hot spots, for example, the reunion hotspot, Hawai hotspot, etc.
• Mantle plumes through a supply of magma transfer heat to the lithospheric plates and keeps them in active motion.
• Ridge push:
• The magma rising along the oceanic ridges creates new lithosphere along either side of the plates.
• This causes both plates to be pushed apart.
• Gravity sliding:
• The spreading centres along the mid-ocean ridges stand high on the oceanic floor.
• This results in the gravitation sliding of the lithospheric slab away from the oceanic ridges.
• Slab pull:
• It is experienced along the subduction zone.
• As the subducting plate descends it pulls the rest of the slab along.

The criticism of plate tectonic theory 

• The theory although widely expected is not able to explain the following:
• 1. Both Africa and Antarctica are surrounded by ridges but have no subduction zones.
• The future of these plates is not clear yet.
• 2. Plate tectonics in the past like the formation of earlier mountains, movements of different plates, and how they were different from the present ones.

Volcanism

• Volcanism includes all the phenomena associated with the movement of molten material from the interior of the earth to the surface.
• It involves three stages:
• 1. Generation of magma in the earth's interior that is asthenosphere and the mantle due to an increase in heat, decrease in pressure or increase in water content, which lowers the melting point of rock causing it to generate magma.
• 2. The intrusion of magma from lower layers into the lithospheric layers
• 3. Extrusion of magma when sufficient pressure builds up in lithospheric chambers the magma erupts onto the surface of the earth.
• Magma and Lava:
• Magma is hot, mobile and moltem silcate material.
• It is made up of a combination of solids, liquids, and gases.
• When magma reaches the surface, it erupts as lava.
• Lava that erupts is hotter and more volatile
• It cools either on the surface or underwater.
• Types of Magma:
• There are mainly two types of magma:

• Granitic magma:	Basaltic magma:
  It has a high silica content( more the 66%).	It has less than 52% silica content.
  It has a lower density	It has a higher density
  The Continental crust is made up of granite rocks	The oceanic crust is made up of basaltic rocks
  The temperature is lower.	The temperature is higher.
  Viscous.	Fluid.
  It is generated at ocean-continent convergence and it is generated rarely.	It is generated at O-O divergence C-C divergence, hotspots, etc.

Magma generation and plate tectonics 

• 	O-O Divergence	C-C Divergence	O-O Convergence	O-C Convergence	Hot spots
  Location:	Mid oceanic Ridge	Rift Valley	Along the subduction Zone through volcanic mountains or islands.	Along the subduction Zone through volcanoes along the fold mountains	Intra-plate Hotspots
  Cause:	Convection Currents	Convection current	Melting of Subducting plate.	Melting of Subducting plate.	Mantle Plume
  Type of Magma:	Basaltic	Basaltic	Andesitic	Andesitic	Basaltic
  Nature of eruption:	Fissure. Slow and continues.	Along volcanoes of the rift valley. Slightly explosive.	Violent and explosive	Violent and explosive	Smooth

Distribution of volcanoes

• Ridge volcanism:
• Along the oceanic floor through mid-oceanic ridges
• For example mid-oceanic ridges in the Atlantic Ocean, East Pacific Rise, and Carlsberg ridge.
• Arc Volcanism:
• Found along ocean-ocean convergence boundaries forming island arcs.
• For example Japan, the Aleutian Islands, the Caribbean islands, etc.
• Volcanic chains:
• Found along ocean-continent convergent plate boundaries with a straight chain of volcanoes on the continental margin.
• For example Andes and Rockies.
• Volcanic clusters:
• Found along the continent-continent divergent boundary.
• For Example east African rift valley 
• Volcanic lines:
• Line of volcanoes arranged along with the hotspots.
• For Example Hawaii islands and the Reunion islands.

Volcanic landforms

• There are two types of volcanic landforms, Intrusive and extrusive landforms.
• Extrusive landforms:
• Volcanic cones.
• Flood basalt province.
• Hot springs.
• Geysers.
• Fumarole.
• Mud volcano.
• Intrusive landforms include:
• Batholiths.
• Laccoliths.
• Lopoliths.
• Sill.
• Dykes.
• Type of Volcanic cones:
• There are 3 major types of volcanic cones:
• Shield volcano.
• Ash-cinder volcano.
• Composite volcano.

The topic of the next class: Volcanism (to continue)

1. Who proposed the theory of convection currents?
• Arthur Holmes.
2. What are the two types of convection currents?
• One involves the whole mantle, and the other involves only the asthenosphere.
3. What generates convection currents?
• Intense heat released from the interior of the earth due to radioactivity.
4. What happens when convection currents rise from below?
• They diverge and spread laterally.
5. What geological feature is formed due to convection causing the lithosphere to crack and rift?
• A mid-oceanic ridge.
6. What happens to the lithospheric plates as convection currents move laterally?
• The currents carry the slab of the lithosphere with them.
7. What occurs when convection currents encounter a similar current from the opposite direction?
• They descend into the deeper part of the mantle and drag the lithosphere along, causing subduction.
8. What is a mantle plume?
• A jet-like plume of low-density magma material from the core-mantle boundary.
9. What effect does a mantle plume have on the lithosphere when it reaches it?
• It spreads out laterally, doming the surface zones of the earth and moving them along the direction of the mantle plume.
10. What are examples of volcanic hotspots caused by mantle plumes?
• Reunion hotspot and Hawaii hotspot.
11. How do mantle plumes keep lithospheric plates in motion?
• Through the supply of magma transferring heat to the lithospheric plates.
12. What happens during ridge push?
• Magma rising along the oceanic ridges creates new lithosphere on either side, pushing the plates apart.
13. What causes gravity sliding in the context of plate movements?
• The spreading centers along mid-ocean ridges stand high on the oceanic floor, causing gravitational sliding of the lithospheric slab away from the oceanic ridges.
14. What is slab pull?
• The process where the subducting plate descends and pulls the rest of the slab along.
15. What criticism is associated with plate tectonic theory regarding Africa and Antarctica?
• Both are surrounded by ridges but have no subduction zones, making their future unclear.
16. What aspects of plate tectonics in the past are not explained by the theory?
• The formation of earlier mountains, movements of different plates, and how they were different from present ones.
17. What does volcanism include?
• All phenomena associated with the movement of molten material from the earth's interior to the surface.
18. What is the first stage of volcanism?
• Generation of magma in the earth's interior due to increased heat, decreased pressure, or increased water content.
19. What is the second stage of volcanism?
• The intrusion of magma from lower layers into the lithospheric layers.
20. What is the third stage of volcanism?
• The extrusion of magma when sufficient pressure builds up in lithospheric chambers, causing it to erupt onto the surface.
21. What is magma?
• Hot, mobile, and molten silicate material made up of solids, liquids, and gases.
22. What is lava?
• Magma that reaches the surface and erupts.
23. How does basaltic magma differ from granitic magma in silica content?
• Basaltic magma has less than 52% silica content, while granitic magma has more than 66%.
24. How does the density of basaltic magma compare to granitic magma?
• Basaltic magma has a higher density, while granitic magma has a lower density.
25. What type of magma is found at ocean-ocean divergence?
• Basaltic magma.
26. What type of magma is found at ocean-continent convergence?
• Andesitic magma.
27. What type of magma is found at hotspots?
• Basaltic magma.
28. What nature of eruption is associated with mid-oceanic ridges?
• Fissure, slow, and continuous.
29. What nature of eruption is associated with ocean-ocean convergence?
• Slightly explosive.
30. What nature of eruption is associated with ocean-continent convergence?
• Violent and explosive.
31. What type of volcanic activity is associated with intra-plate hotspots?
• Smooth eruptions.
32. Where is ridge volcanism found?
• Along mid-oceanic ridges on the oceanic floor.
33. Where is arc volcanism found?
• Along ocean-ocean convergence boundaries forming island arcs.
34. Where are volcanic chains found?
• Along ocean-continent convergent plate boundaries with a straight chain of volcanoes on the continental margin.
35. Where are volcanic clusters found?
• Along continent-continent divergent boundaries, e.g., East African Rift Valley.
36. What are volcanic lines?
• Lines of volcanoes arranged along hotspots, e.g., Hawaii and Reunion islands.
37. What are the two types of volcanic landforms?
• Intrusive and extrusive landforms.
38. Give examples of extrusive volcanic landforms.
• Volcanic cones, flood basalt provinces, hot springs, geysers, fumaroles, and mud volcanoes.
39. Give examples of intrusive volcanic landforms.
• Batholiths, laccoliths, lopoliths, sills, and dykes.
40. What are the three major types of volcanic cones?
• Shield volcano, ash-cinder volcano, and composite volcano.

 

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