Class 11 Geography Chapter 4 Distribution of Ocean and Continents Competency Based Question Answer

Distribution of Oceans and Continents

Question: Explain the theory regarding the distribution of oceans and continents given by a German Meteorologist.

Answer: The German meteorologist Alfred Wegener proposed the concept of the distribution of oceans and continents, which is known as 'The continental drift theory'. 

Continental drift theory:

1. Pangaea and Panthalassa:

According to Wegener, all the continental masses were together in the form of a supercontinent, known as Pangaea, and this Pangaea was surrounded by Panthalassa. According to Wegener, this supercontinent began to split around 200 million years ago.

2. Laurasia, Gondwanaland, and others:

Around 200 million years ago, the single supercontinent began to split and was divided into two parts - the northern part was known as Laurasia, and the southern part was known as Gondwanaland. After that, these two broad landmasses began to break into several parts, forming several continents.

3. Forces for Drifting:

According to Wegener, there are mainly two forces responsible for continental drift -

• Pole-fleeing force means the tendency of the continental masses to move towards the pole due to the rotation and revolution of the Earth.
• Tidal force - due to the gravitational attraction of the Sun and the Moon.

Though the other scholars criticised and shared that these forces are not sufficient for the drifting of large continental masses.

Question: "A variety of evidences were proposed in support of continental drift." - Support the statement. 

Answer: The statement is correct. The German Meteorologist Alfred Wegener proposed this continental drift theory, and in support of his theory, many evidences were proposed, these are as follows:

1. Jig-Saw-Fit:
   The matching of either side of the Atlantic Ocean is like a jigsaw puzzle. The shoreline of South America and the shoreline of Africa have remarkable similarities, which proves that a long time ago, all these continental parts were joined together.
2. Matching of rock age across the ocean:
   It is also observed through the radiometric dating that the rock age of both sides of the Atlantic Ocean are same. The rock age of the eastern Brazilian coast and the western African coast match with each other.
3. Tillite:
   Tillite is a sedimentary rock formed by glacial deposits. This tillite deposition of the Indian Gondwana system has remarkable similarities with Africa, the Falkland Islands, Antarctica, Madagascar, and Australia. Therefore, it proves a prolonged glaciation in these places and the continental drift.
4. Placer Deposit:
   The placer deposits of golds in both sides of the Atlantic Ocean, areas like Brazil and Ghana, have remarkable similarities.
5. Distribution of Fossils:
   The skeletons of a special type of species were found in South Africa and South America, which species of animals were living in the same type of area.

Question: Explain the forces responsible for the continental drift according to Wegener.

Answer: Wegener proposed the theory of Continental Drift in 1912. In his theory, he proposed that a long time ago, all the continental masses were together in a form of supercontinent, called 'Pangaea', which was surrounded by the mega ocean, called 'Panthalassa'. Thereafter, about 200 million years ago pangaea began to split and gave rise to the rise of continents.

Forces for drifting the continents:

He has suggested two types of forces responsible for the drifting of continents, which are as follows: 

1. Pole-fleeing force:
   This force is related to the rotation of the Earth. Due to this force, all the continental masses are trying to move towards the pole, which is directly related to the continental drift.
2. Tidal force:
   Wegener believes that the tidal force is another important factor for the drifting of the continents. Tidal force is the force created by the sun and the moon's attraction. According to Wegener, if a huge amount of tidal bulge may occur due to this attraction, it may causes for continetal drfit also, as these forces are acting over millions of years.

However, many scholars do not agree to accept these forces for continental drift. According to them, these forces are not sufficient for continental drift.

Question: 'The new information after World War II has strengthened the concept of continental drift theory.' - Evaluate the sentence.

Answer: Yes, the statement is correct. Because, in the concept of continental drift theory, maximum evidences were given from the continental parts only. But, after the Second World War, new information was added to the concept of continental drift theory, and this information was regarding the ocean floor mapping.

Ocean floor mapping: After World War II, the information about ocean floor mapping gave some amazing information regarding the distribution of the oceans and the continents. All of this information proves the continental drift, which includes the following:

1. Age of the oceanic crust:
   This ocean floor mapping proved that the age of the ocean floor is younger than the continental part. 
2. Same rock age around the mid-oceanic ridge:
   Rocks found on both sides of the mid-oceanic ridge have remarkable similarities in terms of age and composition.
3. Thin sediment cover on the ocean floor:
   It was observed that the sediment deposition on the ocean floor is very thin, and these are not older than about 200 million years. It also proves the continental drift theory.

Question: Explain the concept of the ocean floor spreading.

Answer: Among the post-drift studies, the 'ocean floor spreading' concept became most popular as this type of information was missing in the continental drift theory. Because this concept revealed the following facts:

1. The volcanic eruption near the mid-oceanic ridge is more common.
2. The rocks on equidistant points from both sides of the mid-oceanic ridge are the same age and the same type of composition.
3. The oceanic crusts are younger than the continental crust. It was observed that the oceanic crusts are about 200 million years old.
4. The sediment layers on the oceanic floor are thin, and nowhere have sediments been found younger than 200 million years old.
5. The earthquake foci below the deep oceanic trench are much and below the mid-oceanic ridge have shallow depth.

All this information about the ocean floor led Harry Hess to propose his hypothesis, 'Ocean Floor Spreading'.

Question: Explain how the different types of movement of plates have influenced the present-day distribution of continents and oceans.

Answer: The present-day continental and oceanic plate movements are related to the concept of 'Plate tectonic theory'. This concept was proposed in the year of 1967 by McKenzie, Parker, and Morgan.

• Plates:
  According to this theory, the whole Earth's crust is created by seven major and some minor plates. Here plate means the lithospheric plate, which is made up of irregular-shaped, rigid slabs of rock masses. The thickness of these plates varies from 5 to 100 km in the oceanic part and from around 200 km in the continental part.
• Plate movement:
  According to the plate tectonic theory, all the plates are continuously moving in different directions. This movement has three types, which are as follows:

1. Divergent movement:
   In this type of plate movement, two plates move far from each other, and in between the two plates ocean forms. The formation of the mid-oceanic ridge in the Atlantic Ocean is the best example of this type of plate movement. Here American plate is separated from the African and Eurasian plates.
2. Convergent movement:
   Here, two plates come closer to each other, and as a result, folded mountains may form. The Himalayas are the best example of such a type of mountain formed in the convergent boundary of the Indo-Australian plate and Eurasian plate.
3. Transform movement:
   This type of plate movement occurs when either two plates don't come close to each other or move far from each other but move parallel to each other. As a result, faults on the crust may form.

• Rates of plate movement:
  The plate movement rates vary from place to place. As per the existing record, it may be 2.5 cm per year to 15 cm per year. It was recorded that, in the Arctic ridge area, this plate movement rate is very slow (2.5 cm/year), and in the East Pacific Rise in the South Pacific Ocean area, it is faster (15 cm/year).