The Theory of Plate Tectonics


The plate tectonics theory says that the crust of the earth is broken up into huge slabs of rock called plates. These plates float on the heavy semi-molten rocks (magma) of the lower mantle and are


moved around by slow but powerful convection currents of magma beneath them. This movement causes plates to collide, to move apart and to slide past each other in different parts of the world.


Plate tectonics theory is a combination of two separate theories: 1. Continental drift theory. 2. Sea-floor spreading theory.


225 million years ago


1. Continental Drift In 1912 a scientist named Alfred Wegener made some interesting discoveries.


L a u r a s i a


l He noticed that the coastlines of Africa and South America matched as if they once fitted together.


l He discovered exact matches between seams of rocks and rare fossils in South Africa and Brazil.


60 million years ago


These discoveries led Wegener to come up with the theory of continental drift. He said that all the continents of the world once formed a single landmass (which he called Pangaea). About 200 million years ago, this landmass began to break up into two huge continents (which he called Laurasia and Gondwanaland). These, in turn, broke up and moved apart to form today’s landmasses, which continue to drift slowly on the earth’s crust. At first, most experts rejected the theory


Today 2


2 The continental drift of crustal plates over the past 225 million years. (The red dot shows the position of Ireland.) (a) Describe some of the main changes shown. (b) Describe the changing position of Ireland over the time shown.


of continental drift because Wegener could not explain how continents could move. The mystery of moving continents was later solved by the theory of sea-floor spreading.


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