Copyright © 2007 Dorling Kindersley
Scientists believe that the Earth’s outer crust is made up of about huge fragments, called tectonic plates, that fit together like a cracked eggshell. According to the theory of plate tectonics, devised in the 1970s, these plates ride like rafts on the softer, red-hot rock below and very move slowly over the globe, carrying the continents with them. Past arrangements of tectonic plates created one vast SUPERCONTINENT.
Earth’s crust is a giant jigsaw of seven enormous plates and about twelve smaller ones. Many scientists believe plate movement is driven by slow-churning currents deep in the mantle beneath. As the plates drift, they converge (move towards each other) and collide, or grind past one another at transform margins, or diverge (pull apart).
The edges of the plates that make up the lithosphere are called boundaries or margins. New crust is mainly created at plate boundaries in mid-ocean, where the SEA-FLOOR IS SPREADING. Older crust is destroyed near the edges of oceans, where plates collide and one subducts (dives) below the other and melts. This causes the plates to move very slowly over the softer asthenosphere, below.
The shapes of continents such as eastern South America and western Africa would fit neatly if pushed together. The discovery of matching fossils and rock layers on land separated by wide oceans provided further evidence that landmasses were once united. Scientists call this supercontinent Pangaea. The slow movement of Earth’s plates caused Pangaea to split apart.
Some 300 million years ago, plate movement drove Earth’s landmasses together to form Pangaea (All-Earth). This was surrounded by the vast ocean Panthalassa. About 100 million years later Pangaea began to break up.
An arm of the Tethys Sea, an ancient ocean, opened to split Pangaea in two. To the north lay Europe, North America, Greenland, and Asia, with South America, Africa, India, Australia, and Antarctica to the south.
As plate movement continued, these large fragments split into smaller continents, which slowly came to their present positions. They continue to move at a rate of a few centimetres per year.
Climate expert and geophysicist Alfred Wegener pioneered the theory of continental drift in 1915. He became convinced that the continents were once joined, and put forward the idea of Pangaea. On the Arctic island of Spitzbergen, Wegener found fossils of tropical ferns, which suggested that the island had once lain in the tropics. His ideas were not taken seriously until the 1960s.
Mountain chains, longer and mightier than any on land, run down the centre of the oceans. At these mid-ocean ridges, where tectonic plates diverge, molten magma erupts to bridge the gap. Rock samples taken from the Atlantic floor in the 1960s showed that the youngest rocks lay in the centre of the ridges, with older rocks to either side. As the new rock forms, older rock is pushed aside, and the sea floor widens, or spreads.