Copyright © 2007 Dorling Kindersley
Dry land covers just under one-third of Earth’s surface. It is made up mostly of seven huge landmasses called continents, plus many smaller islands. The largest continent, Asia, is 16,838,000 sq miles (43,608,000 sq kilometers). The smallest continent, Australia, is 2,968,124 sq miles (7,686,850 sq kilometers).
The world looked very different millions of years ago, when all the continents were joined in one huge block of land, or supercontinent, which we call Pangaea. This was surrounded by a vast ocean, called Panthalassa. Over millions of years, Pangaea split into smaller continents, which drifted across Earth’s surface.
The continents (and oceans) rest on top of giant slabs called TECTONIC PLATES which make up Earth’s outer crust. These plates float like rafts on the hot, semi-liquid mantle below the crust. Slow-moving currents deep inside Earth send the plates (and the land or ocean that rests on them) slowly moving across the surface of the planet.
Scientist Alfred Wegener put forward the idea of drifting continents in 1919. He noticed that the shapes of continents fitted together like a giant jigsaw puzzle, which suggested they had once been joined. His ideas were not generally accepted until the 1960s.
Earth’s outer crust is split into seven large tectonic plates and about twelve smaller ones. Studying plate tectonics (plate movement) helps scientists to understand why earthquakes strike and volcanoes erupt and how mountains form.
As the plates slowly drift across the planet’s surface, they may slide past each other, grind against one another, pull apart, or all three. The boundary between two tectonic plates is called a plate margin. Mountains, earthquakes, and volcanoes usually occur at plate margins, where Earth’s crust is thinner than in the center of the plates.
When plates carrying continents collide, the land may crumple up and form a massive mountain range. If one plate is forced under the other, oceanic crust sinks into the mantle and melts. Where plates pull apart, molten rock rises up from inside Earth. This cools and adds new material to the plates.