Summary
NCERT Class 11 Geography Chapter 4 Distribution of Oceans and Continents explains how continents and ocean floors have shifted over millions of years through continental drift, sea floor spreading, and plate tectonics. It traces the break-up of the supercontinent Pangaea and the journey of the Indian plate up to the formation of the Himalayas.
Chapter 4 of Fundamentals of Physical Geography covers the evolution of the positions of continents and ocean basins. It begins with Alfred Wegener's 1912 continental drift theory—Pangaea (all earth) surrounded by Panthalassa (all water) split around 200 million years ago into Laurasia and Gondwanaland. Evidence includes the jig-saw fit of Atlantic coastlines, matching rock belts of 2,000 million years between Brazil and western Africa, glacial tillite found across six Southern Hemisphere landmasses, placer gold deposits linking Ghana to Brazil, and shared fossils such as Mesosaurus. Post-World War II ocean-floor mapping led to Hess's 1961 sea floor spreading hypothesis and, in 1967, the unified theory of plate tectonics describing seven major lithospheric plates and their divergent, convergent, and transform boundaries.
Key points & formulas
- 01Continents cover 29 per cent of Earth's surface; the rest is under oceanic waters.
- 02Abraham Ortelius (1596) first proposed that continents were once joined; Alfred Wegener formalised the continental drift theory in 1912.
- 03Pangaea (the supercontinent) broke first into Laurasia (north) and Gondwanaland (south) around 200 million years ago.
- 04Five lines of evidence support continental drift: jig-saw fit of coastlines, matching ancient rock belts, glacial tillite, placer deposits, and fossil distribution (e.g., Mesosaurus found in South Africa and Brazil, 4,800 km apart).
- 05Harry Hess proposed sea floor spreading (1961): volcanic eruptions at mid-oceanic ridge crests push the ocean floor apart; old crust is consumed at deep trenches. No oceanic crust is older than 200 million years.
- 06Plate tectonics (McKenzie, Parker, and Morgan, 1967) identifies seven major plates—Antarctica, North American, South American, Pacific, India-Australia-New Zealand, Africa, and Eurasia—plus several minor plates including Cocos, Nazca, Arabian, Philippine, and Caroline.
- 07Plate movement rates range from less than 2.5 cm/yr (Arctic Ridge) to more than 15 cm/yr (East Pacific Rise near Easter Island).
- 08The Indian plate collided with the Eurasian plate 40–50 million years ago, uplifting the Himalayas; the Deccan Traps outpouring began around 60 million years ago while India was still near the equator.
Frequently asked questions
01What is the continental drift theory?
Alfred Wegener proposed in 1912 that all continents once formed a single landmass called Pangaea (meaning 'all earth') surrounded by Panthalassa (meaning 'all water'). Around 200 million years ago Pangaea split into Laurasia in the north and Gondwanaland in the south, which further broke into today's continents.
02Who first suggested that continents were once connected?
Abraham Ortelius, a Dutch map maker, first proposed in 1596 that Europe, Africa, and the Americas may have been joined. Antonio Pellegrini later drew a map showing three continents together. Alfred Wegener then gave the comprehensive argument as the continental drift theory in 1912.
03What evidence supports the continental drift theory?
Five main lines of evidence are: (1) the jig-saw fit of Africa and South America's Atlantic coastlines, verified by Bullard's computer programme at the 1,000-fathom line in 1964; (2) matching rock belts of 2,000 million years between Brazil and western Africa; (3) glacial tillite of the Gondwana system found in India, Africa, Falkland Island, Madagascar, Antarctica, and Australia; (4) gold placer deposits in Ghana derived from gold-bearing veins in Brazil; and (5) identical fossils like Mesosaurus found in South Africa and Brazil, now 4,800 km apart.
04What forces did Wegener propose for continental movement?
Wegener suggested two forces: the pole-fleeing force related to the rotation of the Earth (linked to the equatorial bulge), and the tidal force due to the gravitational attraction of the Moon and Sun. Most scholars considered both forces totally inadequate to move continents.
05What is sea floor spreading?
Proposed by Hess in 1961, sea floor spreading states that constant volcanic eruptions at the crest of mid-oceanic ridges rupture the oceanic crust and push it apart on either side. The displaced ocean floor sinks into deep trenches and is consumed. This explains why no oceanic crust is older than 200 million years.
06What are the three types of plate boundaries?
Divergent boundaries—where plates pull apart and new crust is generated (e.g., Mid-Atlantic Ridge); convergent boundaries—where one plate dives under another at a subduction zone (three subtypes: oceanic-continental, oceanic-oceanic, continental-continental); and transform boundaries—where plates slide horizontally past each other and crust is neither created nor destroyed.
07What are the seven major tectonic plates?
The seven major plates are: (1) Antarctica and its surrounding oceanic plate, (2) North American plate, (3) South American plate, (4) Pacific plate, (5) India-Australia-New Zealand plate, (6) Africa with the eastern Atlantic floor, and (7) Eurasia and the adjacent oceanic plate.
08What drives the movement of tectonic plates?
The driving force is convection currents in the mantle. Heat from radioactive decay and residual heat causes hot mantle material to rise, spread, cool, and sink—forming convection cells. Arthur Holmes first considered this idea in the 1930s, and it later influenced Hess's thinking on sea floor spreading.
09How fast do tectonic plates move?
Plate movement rates vary considerably. The Arctic Ridge has the slowest rate at less than 2.5 cm per year. The East Pacific Rise, near Easter Island in the South Pacific about 3,400 km west of Chile, has the fastest rate at more than 15 cm per year.
10How did the Himalayas form according to plate tectonics?
The Indian plate began its northward journey around 200 million years ago when Pangaea broke up. The Tethys Sea separated India from Asia until about 225 million years ago. India collided with the Eurasian plate about 40–50 million years ago, causing rapid uplift of the Himalayas. Scientists believe this process is still continuing and the height of the Himalayas is rising even today.
11What is tillite and why is it important for continental drift?
Tillite is a sedimentary rock formed from deposits of glaciers. The Gondwana system of tillite found in India has counterparts in Africa, Falkland Island, Madagascar, Antarctica, and Australia. This glacial tillite provides unambiguous evidence of past climates (palaeoclimates) and the drifting of continents, since these landmasses must have been joined to share the same glacial history.
12Is the NCERT Class 11 Geography Chapter 4 PDF free to download?
Yes, it is free to download with no sign-up.
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