Oceans

Two-thirds of our planet is covered with oceans. The Pacific, the Atlantic, the Indian, and the Artic are the world's major oceans. They were formed by a series of geological processes that continue to affect the Earth. Seven main parts, called lithospheric plates, make up the Earth's mantle layer and crust. The plates fit together millions of years ago, and are constantly moving (at a slow pace-like a fingernail grows) over a layer of squishy, soft rock called asthenosphere that lies beneath the crust. Magma rises to fill the space when the two plates move away from each other to form an ocean. This is the way the ocean grows over millions of years.

The ocean floor has an amazing landscape similar to dry land with huge mountains and deep valleys, slopes and plains, trenches and ridges. Through today's technology advances we are able to study the ocean deep and learn about it.

The ocean is consistently moving. Its surface can change from calm and mirror like to wild and treacherous. Most waves at sea are caused by wind. The waves created the gales that blow during a tropical cyclone are 46ft and higher. The largest wave known to have been caused by the wind was 112ft high. Waves can also be created by volcanic eruptions. These waves are known at tsunamis. They are wide columns of water that reach down to the sea floor and can travel for great distances, at the speed of a jet plane. Colliding currents can also change the surface of the ocean. When the tide turns, the opposing currents meet and may create a whirlpool.

Ocean currents are massive bodies of water that travel long distances around the world. Wind is the major force that creates currents. There are seven main ocean currents that move in large, circular streams at a walking pace. In the Northern Hemisphere currents move in a clockwise direction; in the Southern Hemisphere they are counterclockwise. Warm or cold water currents are carried by winds along the shorelines. This affects the climate of the various continents on the way. The current that carries warm water from the Caribbean Sea, up the east coast of the United Stated and then to the west coasts of Britain and Northern Europe is called the Gulf Stream. These areas would be much colder without the Gulf Stream. The "pull" of the moon and the sun also affect the ocean by causing the tides. These occur when the level of the sea rises and falls each day and then rises and falls again. Each high tide and the following low tide are about six hours apart. The difference in height between high tide and low tide is called the tidal range. The largest tidal ranges are found in bays and estuaries.

Many different kinds of life are found in estuaries. This is the gateway where fresh river water and salty seawater meet. Salmon travel through them to lay their eggs in rivers before their journey back to the ocean. The abundant sea grasses that grow there provide camouflage for newly hatched fish. Birds search at low tide for worms and crabs to eat. People come there to fish and collect shellfish.

The richest areas of the ocean are the coastal seas. Bountiful sea life are found in these coastal waters where most of the fish and shellfish are caught that we eat. Coastal seawater is alive with plankton, making the water appear green.

Thousands of beautiful fish and sea creatures live in coral reefs. Polyps or coral animals with soft bodies and mouths build these marine homes in warm shallow seas. They build tube-shaped skeletons of limestone around themselves. They keep dividing in two as the polyps grow upwards. The coral reef is made when they leave their skeletons fused to each other. A film of flesh always forms on top of the skeletons of the living mass of growing polyps. Sunlight, water, and carbon dioxide are the ingredients that the polyps use to make their food. Plant cells provide the food for these coral reefs to grow quickly.

The ocean is the perfect place to observe the food chain in action. It is like a giant puzzle, where all the pieces fit together. The largest animals devour the smaller ones. Killer whales eat seals and sea lions, which feed on fish and squid. Fish like salmon eat smaller fish that eat plankton. Plankton are tiny plants that float near the surface of the water. It is the basic source of food in the ocean.

In the darkness of the ocean floor it is cold and still. The temperature never rises much above freezing. Plants can't grow without sunlight, so there is not much food. Tiny pieces of food that have fallen from the surface of the ocean provide the nutrition for these deep-sea dwellers that filter and sift the ocean floor. There are no waves, so these sea creatures don't need strong skin and bones. Many