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Oceans Part 1

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					DID YOU KNOW?
◆ The largest creatures in the ocean, blue whales, feed only on the smallest creatures in the ocean – microscopic plankton! ◆ The Atlantic ocean is growing wider by about 2.5 cm every year, as the MidAtlantic Ridge expands. ◆ Waves may travel great distances across the ocean, but the water in each wave stays in the same place, moving around in circles. ◆ There was once just one great ocean called Panthalassa, which surrounded the supercontinent Pangaea about 290-240 million years ago. ◆ A body is much more buoyant in salt water, making it easier to float.

Oceans

Part 1

Oceans and seas cover 71 per cent of the Earth’s surface – over 360 million square kilometres of the Earth’s surface. This contains about 1 370 million cubic kilometres of water. The Earth’s five oceans – the Pacific, Atlantic, Indian, Southern and Arctic) are constantly moving, subject to tides, the wind whipping up waves, helping to drive the currents of the oceans.
SALTY SEAS Sea water contains minerals washed into the sea from the land by rivers. These dissolved minerals are mainly sodium and chloride, which together make salt. Most oceans contain one part salt to 35 parts water. The saltiest sea on earth is the Dead Sea, which contains one part salt for every five parts water, which makes it seven times saltier than the rest of the oceans. OCEAN CURRENTS Ocean water moves around in huge circles called gyres. Regular winds blowing across the oceans start the currents at the water’s surface that may continue for thousands of kilometres. Warm surface currents are heated by the Sun. Some warm currents affect the climate of the land which they flow past. There are also cold currents deep in the oceans that flow from the poles and across the ocean floor towards the equator. OCEAN FLOOR The ocean floor has landscapes as dramatic and varied as those on land. Some areas have deep chasms, some cliffs and some volcanoes. Some areas are featureless plains. Many of these features are caused by movements of the tectonic plates that form the Earth’s crust.

WAVES the surface of the ocean is never completely still, even in calm weather. Wind moves the surface to form ripples. If the wind remains strong, the ripples grow into waves. As the waves approach land they get bigger until they break onto the shore, to build up beaches or wear away coasts. TSUNAMIS Tsunamis are enormous waves caused by undersea earthquakes or volcanic eruptions. Far out to sea tsumanis are not so noticeable, but as they near land they can reach massive proportions, towering up to 75 m high. Giant tsunamis have smashed ports and even drowned whole islands. Tsunamis are sometimes wrongly referred to as “tidal waves”, but they are not caused by tides. OCEAN ZONES The ocean can be divided into three zones: the sunny surface waters at the top, called the photosynthetic zone, which contains most of the ocean fishes as well as a floating community made up of billions of microscopic creatures called plankton. Below this is the twilight zone, which is less well lit, which leads to the dark zone. The dark zone contains fewer life forms, and those few being mainly carnivorous fish. The ocean is mainly at a constant temperature around 4˚C. As the depth increases so does the pressure, making it difficult to move quickly, it also gets colder to about 2˚C. The light gets less and less until at about 1 000 m there is no light at all. Here, in the darker areas, hunters can spot their prey silhouetted dimly against the light above them. Many fishes have silvery scales along their sides to reflect any light and disguise their shapes. Here too, many fish have huge mouths and can eat prey larger than themselves!
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