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Distribution of Oceans and Continents – Class 11 Geography notes, important questions and MCQs

NCERT Class 11 Geography, chapter 4. Notes, key terms, important questions with answers and MCQs.

Chapter overview

The positions of continents and oceans have changed over geological time. Alfred Wegener proposed the continental drift theory in 1912, suggesting that all continents were once joined in a supercontinent called Pangaea surrounded by an ocean called Panthalassa. Later evidence of sea floor spreading led to the theory of plate tectonics, which explains the movement of lithospheric plates and the formation of mountains, earthquakes and volcanoes.

Continental drift

Wegener argued that Pangaea broke into Laurasia in the north and Gondwanaland in the south, which further split into the present continents.
Evidence includes the jig-saw fit of continents such as South America and Africa, similar rocks of the same age across oceans, tillite deposits showing past glaciation, placer gold deposits in Ghana linked to Brazil, and the distribution of fossils such as Mesosaurus.
Wegener suggested that pole-fleeing force and tidal force moved the continents, but these forces were considered too weak.

Sea floor spreading and plate tectonics

Studies of the ocean floor after the Second World War revealed mid-oceanic ridges, young rocks near the ridges and older rocks farther away, and thin sediments; Harry Hess proposed sea floor spreading in 1961.
The theory of plate tectonics (1967) states that the lithosphere is divided into seven major and several minor plates that move over the asthenosphere.
Plate boundaries are divergent (plates move apart and new crust forms), convergent (plates collide and crust is destroyed by subduction) and transform (plates slide past each other); convection currents in the mantle drive the movement.

The Indian plate

The Indian plate was part of Gondwanaland; it moved northward and collided with the Eurasian plate about 40–50 million years ago, forming the Himalayas.
The Deccan Traps formed from lava eruptions about 60 million years ago as the plate moved over a hotspot.
The Himalayas are still rising, and the region experiences earthquakes due to continued convergence.

Key terms

Pangaea: the supercontinent in which all continents were joined
Continental drift: the theory that continents have moved over time
Sea floor spreading: formation of new oceanic crust at mid-ocean ridges and its movement away
Subduction: one plate sinking beneath another at a convergent boundary
Transform boundary: a boundary where plates slide past each other

Important questions with answers

What is a transform boundary?2 marks

A transform boundary is a plate boundary where two plates slide horizontally past each other, so crust is neither created nor destroyed. Movement along it often causes earthquakes; the San Andreas Fault in California is an example.

What evidence supports the theory of continental drift?3 marks

The coastlines of South America and Africa fit like a jig-saw; rocks of the same age and type are found on opposite sides of oceans; tillite deposits show that areas now in the tropics were once glaciated; placer gold in Ghana is linked to source rocks in Brazil; and fossils of Mesosaurus are found only in southern Africa and Brazil.

Explain the theory of plate tectonics.5 marks

The theory of plate tectonics, developed in the 1960s from the ideas of continental drift and sea floor spreading, states that the Earth's lithosphere is divided into large rigid plates that move over the partly molten asthenosphere. There are seven major plates, including the Pacific, Eurasian, Indo-Australian, African, North American, South American and Antarctic plates, and several minor plates. Plates move at rates of a few centimetres a year, driven mainly by convection currents in the mantle caused by heat from the interior. Plate movements create three types of boundaries. At divergent boundaries, plates move apart and magma rises to form new crust, as at the Mid-Atlantic Ridge. At convergent boundaries, plates collide; an oceanic plate sinks beneath another plate by subduction, forming trenches and volcanoes, or two continental plates collide to form fold mountains such as the Himalayas. At transform boundaries, plates slide past each other, as along the San Andreas Fault. The theory explains the distribution of earthquakes, volcanoes, mountain chains and ocean trenches, and the movement of the Indian plate from Gondwanaland to its collision with Eurasia.

Quick revision

Wegener 1912: Pangaea, Panthalassa; Laurasia and Gondwanaland.
Evidence: jig-saw fit, rocks, tillite, placer gold, fossils.
Sea floor spreading (Hess); plate tectonics (1967).
Indian plate collision; Himalayas; Deccan Traps.

Exam tips

Learn the three types of plate boundaries with one example each.
List five pieces of evidence for continental drift.

Practice MCQs

The continental drift theory was proposed by
1. Harry Hess
2. Alfred Wegener
3. Arthur Holmes
4. Charles Darwin
Show answer
Answer: (b) Alfred Wegener. He proposed it in 1912.
The supercontinent of the past is called
1. Panthalassa
2. Tethys
3. Laurasia
4. Pangaea
Show answer
Answer: (d) Pangaea. It was surrounded by Panthalassa.
The southern part of Pangaea was
1. Laurasia
2. Gondwanaland
3. Eurasia
4. Panthalassa
Show answer
Answer: (b) Gondwanaland. India was part of it.
At a divergent boundary
1. plates move apart and new crust forms
2. plates collide
3. plates slide past
4. crust is destroyed
Show answer
Answer: (a) plates move apart and new crust forms. Mid-ocean ridges are examples.
The process of one plate sinking beneath another is called
1. sea floor spreading
2. faulting only
3. weathering
4. subduction
Show answer
Answer: (d) subduction. It occurs at convergent boundaries.
Sea floor spreading was proposed by
1. Wegener
2. Ritter
3. Humboldt
4. Harry Hess
Show answer
Answer: (d) Harry Hess. He proposed it in 1961.
Rocks near mid-ocean ridges are
1. the oldest
2. the youngest
3. the same age as continents
4. absent
Show answer
Answer: (b) the youngest. New crust forms at the ridges.
The Himalayas were formed by
1. collision of the Indian and Eurasian plates
2. divergence of plates
3. a transform fault
4. volcanic eruptions only
Show answer
Answer: (a) collision of the Indian and Eurasian plates. The Indian plate moved northwards.

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Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.