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About 71 per cent of our planet’s surface is covered
by oceans and seas. In order of size, the five great oceans are the Pacific,
Atlantic, Indian, Southern, and Arctic oceans. Seawater contains dissolved
minerals, mostly sodium chloride (table salt), which make the oceans salty. The
oceans are never still, but are stirred by powerful currents, WAVES, and TIDES. Coral reefs in warm, coastal waters are the ocean’s richest
habitat. These reefs are built by organisms such as corals with mineral
skeletons. The oceans can be divided into two main biomes – the deep
open ocean and the coastal, or neritic, zone. The shallow waters of the coastal
zones occupy just 10 per cent of the total ocean area, but are home to 98 per
cent of marine life. Jellyfish, fish such as herring, mackerel, and sharks, and
crustaceans such as this lobster all inhabit shallow and surface waters. The
open ocean can be divided into several layers, each with different levels of
light, oxygen, and water temperature. The upper waters, or euphotic zone, down
to 200 m (660 ft) are warm (up to 25°C or 77°F), sunlit, and rich
in oxygen. Creatures such as this brittlestar, squid, and hatchetfish inhabit
the bathyal zone, or mid-depths, between 200–2,000 m (660–6,600
ft). Some sunlight penetrates the upper bathyal zone down to about 1,000 m
(3,300 ft). Here the water temperature may be about 5°C (41°F).
No light reaches the dark zone beyond, where temperatures fall to -2°C
(28°F). The pitch-black, ice-cold abyssal zone below 2,000m (6,600 ft) is
home to fish such as the fangtooth. Animals that live in these vast abyssal
waters have to be able to cope with immense water pressure and freezing
temperatures. Some parts of the ocean drop off to depths of 10,000 m (33,000
ft) or more; this region is called the hadal zone. The water in the oceans is never still, but moves continually in
strong currents that flow both near the surface and at great depths. This helps
to distribute the Sun’s heat around the globe. Winds create surface
currents, which are then bent by Earth’s rotation and by land masses to
flow in great circles, called gyres. Warm surface currents coming from the
tropics warm the lands they flow past. Cool deep currents flowing from polar
waters have the opposite effect. The changes in the oceans’ water levels, called
tides, are caused by the tug of the Moon’s gravity on Earth, the
Sun’s gravity, and Earth’s spin. As the Moon orbits Earth, its
gravity causes a bulge of water to build up on the ocean. The force of
Earth’s spin produces a matching bulge on the opposite side. Twice a
day, these bulges form high tides. Tides vary from a few centimetres to 15 m (50 ft) or more. Very
high tides, called spring tides, occur every two weeks, when the Sun and Moon
line up so that their gravitational pulls combine. Weak tides, called neap
tides, occur in the weeks in between, when the Sun and Moon lie at right angles
to the Earth. Their pulls cancel one another out. Except in very calm weather, waves ruffle the surface of
the oceans. They are caused by winds blowing over the surface, which creates
friction. Winds blowing over vast expanses of open ocean form unbroken waves
called rollers. Waves break into foamy crests as they reach shallow coastal
waters, and finally smash onto coasts. Waves may travel huge distances across oceans, yet surprisingly,
the water in a wave stays in roughly the same place. As the wave passes, the
water particles move around in a circle and return to their original position.
As a wave moves in to the shore, its strength and size increases. The sea bed
disrupts the pattern, causing the wave to break. Giant tidal waves, called tsunamis, are caused by earthquakes or
volcanic eruptions on the ocean bed. As waves ripple outward from an undersea
earthquake or volcano, they are barely noticeable. However in shallow waters
they rear up to great heights and crash onto coasts with huge destructive
force. Copyright © 2007 Dorling Kindersley
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