Chapter 4: Distribution of Oceans and ContinentsClass 11 Geography — summary, notes, extra questions & MCQ quiz

Summary

This chapter explains how the positions of oceans and continents have changed over time. Alfred Wegener, a German meteorologist, put forward the Continental Drift Theory in 1912, proposing that all continents once formed a single mass, Pangaea ("all earth"), surrounded by a mega-ocean, Panthalassa. Around 200 million years ago Pangaea split into Laurasia (north) and Gondwanaland (south), which broke into today's continents. Evidence for drift includes the jig-saw fit of Africa and South America, matching rocks of the same age across the Atlantic, tillite (glacial) deposits, placer deposits of gold, and the distribution of fossils such as Mesosaurus. Wegener's forces (pole-fleeing force and tidal force) were considered inadequate. Post-war ocean-floor mapping revealed mid-oceanic ridges, deep trenches, young oceanic crust and thin sediments, leading Hess (1961) to propose sea-floor spreading — new crust forms at the ridges and is consumed at the trenches. In 1967 McKenzie, Parker and Morgan developed plate tectonics: the lithosphere is divided into seven major and several minor plates that move over the asthenosphere. Plate boundaries are divergent (new crust), convergent (crust destroyed at subduction zones) and transform (crust neither created nor destroyed). The movement of the Indian plate, which collided with Asia about 40–50 million years ago, caused the uplift of the Himalayas and the outpouring of the Deccan Traps.

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