Copyright © 2007 Dorling Kindersley
Earth’s five oceans (the Pacific, Atlantic, Indian, Southern, and Arctic) are constantly moving as tides rise and fall and winds whip up WAVES and help to drive ocean currents. The oceans are major sources of minerals and food.
The oceans are home to a huge variety of plant and animal life. Microscopic plants drift in the sunlit surface waters, forming the basis of most of the ocean food chain. They provide food for tiny animal plankton, which are eaten by fish, which, in turn, are eaten by larger predators, such as sharks.
0–650 ft (0–200 m)
The oceans’ sunlit waters, just below the surface, are home to most plant and animal life, including plankton, jellyfish, flying fish, shoaling fish, tuna, swordfish, and sharks.
650–6,500 ft (200–2,000 m)
Below the sunlit waters, the light begins to fade until, at 3,280 ft (1,000 m), it is completely dark. Marine life includes lantern fish, squid, shrimp, and deep-diving sperm whales.
6,500–33,000 ft (2,000–10,000 m)
The deepest parts of the ocean are near freezing and pitch-black. Marine life includes gulper eels, anglerfish, and rattail fish.
Sea water contains traces of minerals washed from the land by rivers. These dissolved minerals are mainly chloride and sodium, which together make salt. Most oceans contain about one part salt for every 35 parts water. The world’s saltiest sea, the Dead Sea, contains around one part salt for every five parts water, making it seven times saltier than the rest of the world’s oceans.
Water in the oceans is constantly moving in huge, slow circles called gyres. Prevailing (regular) winds blowing across the oceans start currents near the water’s surface, which may flow for thousands of miles. Warm surface currents are heated by the Sun. Some warm currents affect the climate of the land that they flow past. For example, the Gulf Stream keeps northern ports ice-free in winter. There are also cold currents deep in the oceans that flow from the poles and across the OCEAN FLOOR toward the equator.
The ocean floor has landscapes as dramatic and varied as those of Earth’s continents. Some parts have deep chasms, or towering cliffs, or volcanoes. Other places are vast, featureless plains. Many features of the ocean floor are caused by movements of the tectonic plates that form Earth’s crust.
Some of the oceans are expanding, as molten rock wells up at the edges of tectonic plates to make new crust. In the Atlantic Ocean, new crust is forming along the Mid-Atlantic Ridge, which runs down the ocean’s center. The Atlantic is growing about 1 in (2.5 cm) wider each year.
The surface of the sea is never completely still, even in calm weather. Winds ruffle the surface to form ripples. If the wind keeps blowing strongly, the ripples grow into waves. As the waves approach land, their size and strength increase until they break onto the shore, to build up beaches or wear away coasts.
Tsunamis are giant waves usually caused by undersea earthquakes or volcanic eruptions. Far out to sea, tsunamis are not so noticeable. However, as they reach land, they can tower up to 250 ft (75 m) high. Giant tsunamis have smashed ports and even drowned whole islands. Tsunamis are sometimes wrongly called “tidal waves,” but they are not caused by tides.