Important Questions: Distribution of Oceans & Continents | Geography Class 11 - Humanities/Arts PDF Download

Very Short Answer Type Questions

Q1: Who first propounded the theory of continental drift?
Ans: Alfred Wegner propounded the theory of continental drift in 1912.

Q2: How did the Himalayas rise?
Ans: The collision between the Indian Plate and the Eurasian plate took place north of the Indian plate giving rise to the Himalayas.

Q3: Name the continents into which the supercontinent, Pangaea, got split.
Ans: They are Laurasia and Gondwanaland.

Q4: What are hot spots?
Ans: The centers of volcanic activity are called hot spots.

Q5: The collision between which two plates gave rise to the Himalayas?
Ans: The collision between Indian and Eurasian plates gave rise to the Himalayas.

Q6: What is Palaeornagnetism?
Ans: It is the fossil magnetism evident today in igneous rocks such as magnetite, hematite, pyrrhotite, etc.

Q7: In which parts Pangaea was split?
Ans: Pangaea was split into northern continent Laurasia and Southern continent, Gondwanaland.

Q8: What is meant by sea-floor spreading?
Ans: The crust spreads away from the ridge and the ocean basin widens. This phenomenon is known as sea-floor spreading.

Short Answer Type Question

Q9: Why do geographers focus their attention on plate boundaries?
Ans: Geographers focus their attention on plate boundaries because each plate is a tectonic plate. It is rigid and moves as a single unit. Nearly all major tectonic activity occurs along the plate boundaries.

Q10: Discuss the causes of plate movement.
Ans: Sub-crustal convection currents invoke the mechanism of thermal convection that acts as a driving force for the movement of plates. Hot currents rise, then cool as they reach the surface. At the same time, cool currents sink down. This convectional movement moves the crustal plates. .. Owing to current movements, the rigid plates of the lithosphere, which ‘float’ on more mobile asthenosphere, are in constant motion.

Q11: What is subduction?
Ans: The process in which the overridden plate slips down into the mantle and is submerged is known as subduction. Besides volcanism and shallow to deep-focus earthquakes, boundaries also formed deep trenches and folded mountains.

Q12: Account for the formation of the Himalayas according to the theory of plate tectonics in three points.
Ans:

• South-east Indian ocean ridge collided between the Indian plate and European plate. The Indian plate gave rise to the Himalayas.
• The suture between the Indian and Eurasian plates in the Himalayan region has been along the Indus and Brahmaputra.
• The mid-Indian ocean ridge has been spreading faster.

Q13: Describe the driving force for the movement of plates.
Ans: The sub-crustal convection currents invoke the mechanism of thermal convection that acts as a driving force for the movement of plates. As these currents reach the surface, the hot current rises, then cools. Sometimes, cooler currents sink down. This type of convectional movement causes the movement of crustal plates.

Long Answer Type Questions

Q14: Discuss the Continental Drift Theory.
Ans: Continental Drift Theory was proposed by Alfred Wegner in 1912. He postulated that all landmasses of the world have been formed from one super-continent called Pangaea. The supercontinent, Pangaea, had evolved some 280 million years ago, at the end of the Carboniferous period and by the mid-Jurrasic age, 150 million years ago. Pangaea had split into a northern continent, Laurasia, and a southern continent Gondwanaland. About 65 million years ago, i.e., at the end of the Cretaceous, Gondwanaland further broke up to give rise to several other continents, such as South America, Africa, Australia, and Antarctica. Thus, the present distribution of the continental masses and the oceans are the n .uit of fragmentation of one or more pre-existing masses. These landmasses have drifted apart the intervening hollows having been occupied by the oceans.

There are several pieces of evidence that suggest the existence of Pangaea. This evidence is available in the form of geological watching, palaeoclimatic units, and paleomagnetism.

Q15: Explain the plate tectonic theory and its mechanism.
Ans: Plate tectonic theory proves that the earth’s crust consists of several plates that carry continents and the ocean floors and float on the asthenosphere, moving very slowly, the movement probably resulting from currents in the asthenosphere. As shown in fig., the lithosphere is broken into seven major plates. They include :

• Pacific plate
• Eurasian plate
• Indo-Australian plate
• African plate
• North American plate
• South American plate
• Antarctic plate

The latest among these is the Pacific plate which is composed of oceanic crust almost entirely. The other plates have both continental and ocean crust. No plate consists of only continental crust. Plates range in thickness from about 70 km. beneath oceans to 150 km. under continents. Each tectonic plate is rigid and moves on a single unit. Nearly all major tectonic activities occur along the plate boundaries.
Based on the relative motion of plates, three kinds of plate boundaries have been recognized. They are :

• Zones of divergence: These are boundaries along which plates separate and in the process of separation molten materials upwell. This is commonly observed along linear ocean edge where the new lithosphere is created in the form of new ocean floors. Active volcanism and shallow focus earthquakes make such boundaries,
• Zones of convergence: There are the boundaries along which, the edge of one plate overrides the other. The overridden plate slips down into the mantle and is absorbed. This process is known as subduction.
• Fracture zones or Transform faults: The plates slide past each other effect creating or destructing the boundaries. However, shallow focus earthquakes may result due to this friction.

Q16: What do you mean by ‘jig-saw-fit’? Describe the similarities found on the east and west coasts of the Atlantic ocean. What do they suggest
Ans: The ‘jig-saw-fit’ means the landmasses on the east and west coast of the Atlantic ocean can be fitted together. There are many similarities found on the two coasts :

• The Gulf of Guinea can be fitted into Cape San-Rogue of Brazil, the shoulder of Africa can be fitted into Gulf of Mexico, Western Europe and Eastern coast of North America along with Greenland can be fitted together.
• Gold deposits of Ghana lands are found on the east and in Brazil on the west.
• The glacial deposits are found in all landmasses of Gondwana land.

These similarities suggest that these continents were together in the geological past.

Q17: Describe the main features of the Indian plate.
Ans: Indian ocean floor presents striking topography consisting of a number of elevated ridges and plateaus. Two of the ocean ridges, namely, the Mascarene Plateau, Chagos-Maldweep-Lakshadweep island ridge are said to be the volcanic tracts of two hot spots. The Northward extension of Ninety-east Ridge indeed in the trench which consumed the seafloor to the north of the Indian continental mass. The Chagos- Lakshadweep ridge linked the ancient Carlsberg ridge with the Southeast Indian ridge during the Eocene period (50 million years ago).
Consequent to the Carlsberg-Southeast Indian ocean ridge, the collision between the Indian plate and the Eurasian plate took place north of the Indian plate giving rise to the Himalayas. The suture between Indian and Eurasian plates in the Himalayan region has been along the Indus and Brahmputra rivers.

Q18: Describe sea-floor spreading in brief.
Ans: The present distribution of continents has taken place in the last 65 million years. The drift of continents still continues. The ridges down the middle of ocean floors have been emitting lava continuously. Those mid-oceanic ridges are cracks on the floor of the ocean where molten rocks push up to form a new crust. The crust spreads among the ridge and the ocean basin widens. This phenomenon is known as sea-floor spreading. The Atlantic ocean is getting smaller and the Red Sea is the part of a crack in the crust that will widen to produce new ocean millions of years ahead in the future. In the past, the widening of the South Atlantic ocean has separated Africa and South America.

Q19: Name the major plates of the earth. Which plate is composed of the oceanic plate?
Ans: The Earth's lithosphere is divided into several major tectonic plates. These plates are composed of both continental and oceanic crust. Some of the major plates include:

• Pacific Plate: This is mostly an oceanic plate and is the largest tectonic plate on Earth.
• North American Plate: This plate includes North America, Greenland, and parts of the Atlantic Ocean.
• Eurasian Plate: This plate includes Europe and Asia.
• African Plate: This plate includes Africa as well as oceanic crust in the Atlantic Ocean.
• South American Plate: This plate includes South America and parts of the Atlantic Ocean floor.
• Antarctic Plate: This plate includes Antarctica and some parts of the surrounding Southern Ocean.
• Indo-Australian Plate: This is a combination of the Indian Plate (which includes the Indian subcontinent and part of the Indian Ocean) and the Australian Plate (which includes Australia and surrounding oceanic crust).
• Nazca Plate: This plate is an oceanic plate located in the eastern Pacific Ocean.
• Cocos Plate: Another smaller oceanic plate in the Pacific Ocean, located off the western coast of Central America.

The composition of these plates can vary. For example, the Pacific Plate is largely oceanic, while others like the North American Plate and Eurasian Plate contain both continental and oceanic crust. Tectonic plate boundaries, where these plates interact, are often sites of geological activity, including earthquakes and volcanic eruptions.

Q20: What is polar wandering?
Ans: Polar wandering refers to the movement of the Earth's rotation axis (the North and South Poles) over geological time scales. It is a phenomenon observed in paleomagnetism, the study of the Earth's magnetic field as preserved in rocks and sediments. The Earth's magnetic field has not always been constant; it has undergone changes and reversals over millions of years. When molten rocks solidify to form igneous rocks, such as basalt, the magnetic minerals in these rocks align themselves with the prevailing magnetic field at that time. By studying the orientation of these magnetic minerals in ancient rocks, scientists can determine the past positions of the Earth's magnetic poles.
Polar wandering is significant because it provides information about the movement of tectonic plates and the dynamics of the Earth's interior over long periods of time. By comparing the paleomagnetic data from rocks of different ages and locations, scientists can reconstruct the movement of continents and the drift of tectonic plates, which contributes to our understanding of the processes of plate tectonics and the geological history of the Earth.