Almost a third of the surface of the earth is land. There are seven vast pieces of land, called continents, which make up most of this area. The rest consists of islands which are much smaller land masses completely surrounded by water. The 7 continents are crowded into almost one half of the globe; the huge Pacific Ocean occupies most of the other half. The largest continent is Asia, which has an area of more than 44 million square km (17 million square miles).
Most scientists now agree that, about 200 million years ago, the Q were joined together in one huge land mass. Over millions of years they drifted around and changed shape, and they are still moving today. The continents lie on vast pieces of solid rock, called plates, which collide and move against one another. These movements cause volcanoes and earthquakes, push up mountains, and create huge trenches in the Earth's crust.
Plate tectonics: The continents and ocean lie on top of several huge plates of rock about 100 km (60 miles) deep. These plates float on the hot, molten rock in the mantle underneath. Heat from the earth's interior makes the plates move, carrying the continents with them. Mountains and undersea ridges, deep trenches, and huge valleys form at the edges of the plates as they move and collide.
Moving plates: The plates move about 2.5 cm (1 inch) every year ― about ad fast as your finger-nails grow. The Atlantic Ocean is widening at this speed as the America's drift apart from Europea and Africa.
The 7 continents:
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