GEOGRAPHY- CLASS-11: FUNDAMENTALS OF PHYSICAL GEOGRAPHY
CHAPTER-4
Distribution of Oceans and Continents
1. Multiple Choice Questions
(i) Who amongst the following was the first to consider the possibility of Europe, Africa and America having been located side by side?
Answer: (c) Abraham Ortelius
Explanation: Abraham Ortelius first suggested that the continents were once joined together before they drifted apart. Later, Alfred Wegener developed the Continental Drift Theory.
(ii) Polar fleeing force relates to:
Answer: (c) Rotation of the Earth
Explanation: Polar fleeing force is caused by the Earth's rotation. Wegener believed it was one of the forces responsible for the movement of continents.
(iii) Which one of the following is not a minor plate?
Answer: (d) Antarctica
Explanation: The Antarctic Plate is one of the seven major tectonic plates, whereas Nazca, Arabian and Philippine Plates are minor plates.
(iv) Which one of the following facts was not considered by those while discussing the concept of sea floor spreading?
Answer: (c) Distribution of fossils in different continents
Explanation: Fossil distribution was an important evidence for Continental Drift Theory, not for Sea Floor Spreading. Sea floor spreading was based on volcanic activity, magnetic stripes and the age of ocean-floor rocks.
(v) Which one of the following is the type of plate boundary of the Indian plate along the Himalayan mountains?
Answer: (d) Continent-continent convergence
Explanation: The Himalayas were formed when the Indian Plate collided with the Eurasian Plate. This is called a continent-continent convergent boundary.
2. Answer the following questions in about 30 words.
(i) What were the forces suggested by Wegener for the movement of the continents?
Answer:
Wegener suggested that the movement of continents was caused by two forces: the polar fleeing force produced by the Earth's rotation and the tidal force produced by the gravitational pull of the Moon and the Sun.
(ii) How are the convectional currents in the mantle initiated and maintained?
Answer:
Convection currents are generated by the heat produced inside the Earth due to radioactive decay. Hot molten material rises while cooler material sinks, creating continuous circulation in the mantle.
(iii) What is the major difference between the transform boundary and the convergent or divergent boundaries of plates?
Answer:
At transform boundaries, plates slide horizontally past each other without creating or destroying crust. At convergent boundaries, crust is destroyed, while at divergent boundaries, new crust is formed.
(iv) What was the location of the Indian landmass during the formation of the Deccan Traps?
Answer:
During the formation of the Deccan Traps about 66 million years ago, the Indian landmass was located in the southern hemisphere and was moving northwards towards the Eurasian Plate.
3. Answer the following questions in about 150 words.
(i) What are the evidences in support of continental drift theory?
Answer:
Alfred Wegener supported the Continental Drift Theory with several important evidences. The first evidence was the jigsaw fit of continents, especially the coastlines of South America and Africa. Secondly, identical fossils of plants and animals such as Mesosaurus and Glossopteris were found on continents now separated by oceans. Thirdly, similar rock formations and mountain ranges of the same age were discovered on different continents, indicating that they were once connected. Wegener also used palaeoclimatic evidence. Glacial deposits found in present-day tropical regions such as India, Africa and Australia suggested that these continents were once located near the South Pole. Similarly, coal deposits found in cold regions indicated that those areas had once experienced warm climates. These evidences strongly suggested that all continents were once joined together as a single supercontinent called Pangaea, which later broke apart and drifted to their present positions.
(ii) Bring out the basic difference between the drift theory and Plate Tectonics.
Answer:
Continental Drift Theory, proposed by Alfred Wegener, states that the continents were once joined together as a supercontinent called Pangaea and later drifted apart. However, Wegener could not satisfactorily explain the force responsible for this movement. Plate Tectonic Theory provides a more complete explanation. It states that the Earth's lithosphere is divided into several rigid tectonic plates consisting of both continental and oceanic crust. These plates move over the semi-molten asthenosphere due to convection currents in the mantle. Plate movements cause earthquakes, volcanic eruptions, mountain building and the formation of ocean basins. Unlike Continental Drift Theory, Plate Tectonics explains not only the movement of continents but also the creation and destruction of oceanic crust and the different types of plate boundaries. Therefore, Plate Tectonic Theory is considered more comprehensive and scientifically accepted.
(iii) What were the major post-drift discoveries that rejuvenated the interest of scientists in the study of distribution of oceans and continents?
Answer:
Several discoveries made after Wegener's Continental Drift Theory renewed scientific interest in the study of continents and oceans. The discovery of the Mid-Oceanic Ridges showed that new oceanic crust is continuously formed at divergent boundaries. The concept of Sea Floor Spreading, proposed by Harry Hess, explained how magma rises through these ridges and creates new ocean floor. Studies of palaeomagnetism revealed symmetrical magnetic stripes on both sides of mid-oceanic ridges, providing strong evidence for sea floor spreading. Scientists also discovered that the age of ocean-floor rocks increases away from the ridges, proving that new crust forms at the ridges and moves outward. These discoveries, along with improved knowledge of earthquakes and volcanic activity, led to the development of the modern Plate Tectonic Theory, which successfully explains the movement of continents and the evolution of oceans.