Continental drift

Continents cover about 29 per cent of the earth's surface; the rest is ocean. Their positions were different in the past and will change in future. Look at the Atlantic: its coastlines on both sides match remarkably.

• In 1596, the map maker Abraham Ortelius first suggested that the Americas, Europe and Africa were once joined. Antonio Pellegrini drew a map showing them together.
• In 1912, the German meteorologist Alfred Wegener put forward the continental drift theory. All continents formed one supercontinent, PANGAEA ('all earth'), surrounded by one mega-ocean, PANTHALASSA ('all water').
• About 200 million years ago, Pangaea began to split, first into Laurasia (north) and Gondwanaland (south), which later broke into today's continents.

Evidence for drift:

• Jig-saw fit: the facing shores of Africa and South America match. In 1964, Bullard used a computer to find the best fit, matching at the 1,000-fathom line (the edge of the continental shelf), not the present shoreline. It was almost perfect.
• Rocks of the same age across oceans: radiometric dating shows a belt of 2,000-million-year-old rocks on the Brazil coast matching rocks in western Africa. The earliest marine deposits on both coasts are Jurassic, so the ocean did not exist before then.
• Tillite, a sedimentary rock formed from glacier deposits: India's Gondwana sediments, with thick tillite at the base, have counterparts in Africa, the Falkland Islands, Madagascar, Antarctica and Australia. Such matching histories, and the evidence of old glaciation, point to drift.
• Placer deposits: rich gold placers on the Ghana coast have no source rock nearby; the gold-bearing veins are in Brazil.
• Fossils: the same land or freshwater species on both sides of an ocean. Lemurs in India, Madagascar and Africa once led people to imagine a joining landmass, 'Lemuria'. Mesosaurus, a small reptile of shallow brackish water, is found only in South Africa's southern Cape and in Brazil, now 4,800 km apart.

Wegener's forces: the pole-fleeing force (from the earth's rotation and its equatorial bulge) and the tidal force (the pull of the moon and sun). Most scholars found these totally inadequate, and the theory was set aside.

Example (new): tear a newspaper page into two pieces and move them apart. You can prove they were once one sheet not only because the edges fit, but because the lines of print run straight across the join. Wegener's rocks, fossils and tillites are the 'lines of print' across the Atlantic.