Model Answers- Answer Writing Practice- Day 2

Day 2: Model Answers for Mains Questions posted on 18th June

1. What are mantle plumes? Indicate the formation and the role played by these plumes to shape up the landforms on the surface. 

Approach:

1. Discuss in detail about the mantle plume phenomenon
2. Mention about the formation of landforms because of its spread
3. Discuss in detail about various landforms created by it

Answer:

Across the core-mantle boundary of the earth's interior, heat is carried upward in a narrow, rising columns, driven by heat exchange. This upwelling of abnormally hot rock within the earth’s mantle is known as “mantle plume”. As an effect of mantle plume, temperature in the lower lithosphere is raised to above melting point and forms magma chambers that feed volcanoes at the surface. 

With the shape like a mushroom with a long tail connecting the bulbous head to its base, the plume plays a crucial role in formation of various landforms on the surface.

Mantle plumes are directly linked with rifting and the formation of plates thus results in growth and breakdown of supercontinents, the formation of passive volcanic-type continental margins and the origin of time-progressive volcanic chains on oceanic and continental plates. Moreover, the plume in some scenarios rises through the centre and diverges in all directions just below the lithospheric plates and this exerts extensional stress on the lithospheric plate above and causes the plate to stretch and rupture and then diverge to form a rift in between.

Role of mantle plume in shaping the landforms on earth’s surface:

• In the middle of the Pacific Plate, Hawaiian Island and Emperor Seamount chain are formed because of the mantle plume.
• Formation of the flood basalts namely the Deccan traps in India, the Siberian traps of Asia etc are linked to the mantle plume eruptions.
• Volcanic hotspots such as Hawaii or Iceland are formed because the plume head partially melts on reaching shallow depths.
• Large igneous provinces like Iceland, Siberian Traps, Deccan Traps, and Ontong Java Plateau, are extensive regions of basalts on a continental scale resulting from flood basalt eruptions.
• Afar hotspot in Africa got ruptured due to the mantle plume below which caused extensional stress on the lithospheric plate above and resulted in the plate to stretch.
• Highlands structures like Ethiopian Highlands are formed as the plume reaches the lithosphere and spreaded out laterally doming zones of the Earth.  
• Supervolcanoes are formed when a large volume of magma accumulates under the lithospheric plate but is unable to break through it and the pressure keeps building up over thousands of years until the plate can no longer contain the pressure, resulting in an eruption.

Thus mantle plumes are important geological activities in the supercontinent cycle and shape the Continental landforms (eg: Islands) of a region depending on the intensity of mantle plumes. 

2. What are the different landforms created as a result of convergence of different types of plates? Elaborate the process using examples. 

Approach:

1. Define briefly plates and their associated movement.
2. Mention the three kinds of movements of plates
3. Discuss in detail about the three types of convergent plate interactions with landforms associated

Answer:

The earth’s lithosphere which extends from the surface of the Earth to a depth of about 100 km is made up of the crust and upper mantle. In geological terms, the plates refer to the broken pieces of the Lithosphere. Plates may be of continental or oceanic and they tend to move over the asthenosphere, just like boats over the water. The plates while moving interact with one another plates and the activity that takes place at the boundaries is of great interest for the geologists.

Based on the types of plates, there are three kinds of boundaries.

1. Convergent Plate Boundaries - The plates move towards each other (or one plate moves towards the other plate) and destruction of material takes place.

2. Divergent Plate Boundaries - The plates move away from each other. Magma erupts to fill in the void and new rocks are formed.

3. Transform Boundaries - The plates simply slide against each other, without much construction or destruction.

CONVERGENT PLATE BOUNDARY MOVEMENTS

These can be further classified into 3 cases.

Convergence between the oceanic and oceanic plates:

In this, the cooler and denser oceanic lithosphere sinks beneath the warmer and less dense oceanic lithosphere.

The denser or the fast-moving plate (in case the densities are equal) subducts beneath the less dense plate. For example, the Pacific plate converges with the Japanese plate in which the denser Pacific-plate subducts beneath the Japanese plate.

The frictional force between the 2 plates creates heat, which causes the rocky material of the denser oceanic crust to melt and form magma. The magma thus plumes up and forms volcanic mountains.If the peaks of these mountains become visible above the water level then islands are formed. Almost all volcanic islands are formed due to such ocean-ocean plate convergence. If there is more volcanism, then more islands are formed, creating a chain of islands known as island archipelagos. Example of this is Ryukyu islands.

Convergence between the oceanic and continental plates:

In this, the denser oceanic plate plunges beneath the continental plate and causes subduction at the oceanic trenches. 

The example for this is the convergence of North America and South America on one side with the Pacific plate, on the other side.Density is inversely proportional to the volume.The oceanic plate being denser subducts beneath the continental plate.This deforms the continent to form fold mountains.

Also, due to frictional force, the crust melts (both oceanic and continental crust melts) and forms magma, which plumes up to form volcanic mountains.For example, fold mountains like the Rockies in North America and Andes in South America. Volcanic mountains there are Mount Cotopaxi of Ecuador and Mount Aconcagua of Chile.

Convergence between the continental and continental plates:

In this, continental lithosphere is low in density and very thick thus cannot subduct. So when two continental plates collide, they just smash together.

One of the landform created under this category of convergence is Himalays. Asia on one side and peninsular India on the other side and their convergence leads to the folding of continents to form the fold mountains like HImalayas.

Since there are no rocks to subduct, there is no melting of rocks and hence no volcanism. 

Similarly, because of the collision of the African Plate with the Eurasian Plate, the series of ranges extending from the Alps in Europe to the Zagros Mountains in Iran is formed.

Thus convergence of plates due to plate-tectonics movement develops different landforms based on the convergence of different types of plates (Continental or Oceanic plates).