Oceans cover about 70% of the Earth's surface and contain most of the water and life on our planet. The oceans give the Earth a blue appearance from space, so planet Earth has been called the “Blue Planet.” Oceans are sometimes called seas, and the word “marine” can refer to things related to the ocean.
Water in the ocean is salty. The reason behind this is complex and even somewhat difficult to identify. Much of the salt comes from minerals that have been dissolved from the rocks that lie on the ocean floor. Underwater volcanoes also contribute minerals into the body of water. Some areas of the ocean are more salty than others. This is known as the water's salinity. The salinity effects temperature and the types of life that live there. Salinity also makes water denser. Animals that live and die in the ocean, also give up minerals. Rain water from the water cycle also washes additional minerals from the continents into the water.
Another contributing factor to the salinity of the ocean, is that when water evaporates from the ocean, the existing mineral salts do not leave. They are too heavy and stay put in the ocean.
But even with all of these factors, scientists are still a bit puzzled about the reasons behind the amount of salt in ocean water. The National Oceanic and Atmospheric Administration or NOAA studies the ocean and how it impacts our weather and climate. They have some great information about the salinity of the ocean – check out NOAA's Ocean Facts.
For most of us land dwellers ocean water is not drinkable. But for all of the ocean life it is a necessity. Because of the salty composition of the water, only certain plants and animals can exist in the ocean.
The water of the ocean lies on top of the earth's crust which is the outer layer of rock material that makes up the earth. This crust layer also forms the continents or large land masses. The ocean surrounds the continents. Humans have given separate names to the ocean areas depending upon where they are in relation to these land masses, but the truth is, the water circulates all around the land and so is really one body of water.
The oceans, in order of size, are the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Southern Ocean and the Arctic Ocean. The Southern Ocean is really the southern-most area of the Pacific, Atlantic and Indian Oceans and was known, at one time, as the Antarctic Ocean. Many people don't recognize it as a separate ocean at all because of its connection to the other three. Each ocean has unique qualities and features even though the water is largely the same.
While the ocean is sometimes called a sea, the Earth has specific bodies of water that are geographically identified as seas. Seas are surrounded by land and connected to the ocean so they contain the same salt water with many of the same life forms living in them. The Caribbean Sea, the Red Sea, the Baltic Sea and the Mediterranean Sea are just a few of these bodies of salt water. There are many bodies of water on the Earth that are identified as seas, although not all people agree that all of them are truly seas. Here is a possible list of Earth's seas.
The ocean floor is that area beneath the water and on top of the Earth's crust. The area closest to the continents is known as the continental shelf. It is often a shallow area and slopes gradually from the beach area out to the continental slope. The continental slope drops off quickly – like a cliff – to the abyss where the deepest parts of the ocean lie. Here there are mountains and trenches and areas that no one has ever seen because it is too deep for humans to explore.
Sometimes mountains can rise up to the surface of the water to form islands. These are the result of volcanoes that build up layers of lava until the volcano is taller than the water's surface. Some of these mountains get worn down from weather and wave action making their surface flat. These are known as guyots. Other worn down volcanoes can form a ring of islands known as atolls.
The Mariana Trench in the Pacific Ocean is the deepest point in the ocean at 36,201 feet deep. It is so deep, that it is beyond most people's imagination. Consider putting Mount Everest into the trench – there would still be over 7000 feet of water above the mountain.
The tallest mountain in the ocean is part of the Mid Atlantic Ridge in the Atlantic ocean where tectonic plates are moving away from each other. Check out this animation about this movement. This ridge is about 6000 miles long.
Most of the ocean floor is covered in sediments created from wave action and wearing down of rocks and minerals. This sediment builds up year after year on the ocean floor. The layers of sediment can be a historical record of events including marine life and human existence. Everything from shipwrecks, to ancient civilizations, to ancient animal fossils have been found on the ocean floor under the sediment.
Beaches are a part of the ocean floor too. They are the beginning of the continental shelf. Most beaches are made of sand, although depending upon their location, the sand can be of different colors and even sized grains. Beaches can stretch out towards the ocean in varying sizes too. Much of this is due to currents and wave action.
Sandbars can form when the waves move the sand from the beach out further into the ocean. This sand can pile up and form a sort of wall of sand. This effects the waves and can even stretch the beach further out towards the ocean. Birds can often be seen standing on sandbars that are hiding just below the water's surface, which may make it look as if the birds are standing “ON” the water.
The ocean water exists in layers. The deeper below the surface, the colder the water is. It also get darker in the deeper waters. Sunlight does not penetrate very deep, leaving most of the ocean life to live in the shallow areas. Some life does live in the deeper parts of the ocean, but they also have specific attributes that allow them this ability. If you want to know more about the layers of the ocean, follow this link to NOAA. The diagram below shows the comparison of the water and land below the surface of the ocean.
Currents- The ocean is always moving. Water temperature, wind and the turning of the earth on its axis cause the oceans to move in a river-like motion along the coasts of the continents. In the northern hemisphere the water moves clockwise and counter-clockwise in the southern hemisphere. This is known as the Coriolis Effect. These “rivers of ocean water” are known as currents. Currents effect weather trends and climates, impact bird and whale migration, and have been used by sailors to direct their ships for travel to distant locations. See an interactive about ocean currents from Exploring Earth.
Waves- Another type of ocean movement is waves. Waves happen because the wind blows over the surface of the water and the friction pushes the waves upwards. This causes the surface to bob up and down. If an object sits on the surface of the water, such as a boat, it will move up and down with each wave, but it won't actually go anywhere because the water isn't actually moving. Just bobbing!!
Each wave makes a rolling motion, up, over and down. However, as the waves come in contact with the shoreline something changes. With each up, over, and down motion the wave hits the bottom of the land surface at the shore or on rocks. This compresses each wave into a shorter space vertically. This causes the energy to be pushed upward and forces the water to climb higher than the wavelength and then break because of gravity. These waves then crash down to create surf. See an animation of wave motion from Exploring Earth.
Waves batter the shoreline and change it over time. Erosion of the rocks and sand can reshape the coastal land. Most of the sand found on a beach is from this erosion process and came from rocks that have been ground down through weathering and erosion.
Tides- Monthly, the oceans are subject to a third type of movement known as the tides. Tides are caused by the gravity force from the moon and the sun on the oceans. When the moon's gravity pulls on the water, it causes the entire body of water to bulge on the side of the Earth closest to the moon. On the opposite side of the Earth a bulge also takes place because of Earth's spinning. This bulge pulls water away from shorelines at some times of the month and then returns at other times of the month. These are the high and low tides. When the sun and the moon are in line with each other the strength of the gravity increases and the tides are more severe. Watch this NASA animation of the tides and the Earth over the course of the moon's orbit.
High Tide
Low Tide
Tidal Waves - Tidal waves are really not related to the tides at all and are really called
Plants - Most ocean plant life lives in the shallow portion of the continental shelf. It is called the sunlit zone where the sun is able to shine through the water to provide photosynthesis for the ocean plants. Algae – mostly kelp, attach to the ocean floor and can even make up forests. Seagrass is another ocean plant that grows thick and can be a great place for animals to hide. Phytoplankton make up the largest population of ocean plant life. Phytoplankton are one celled and drift in the upper surface of the ocean water. They can be so numerous that they make the water take on a color.
Animals - Plants provide food and shelter for animal life. Because of this, a great number of animals also live in the sunlit zone. The animals of the ocean range from mammals, to fish, to crustaceans, to reptiles, and zooplankton – a one celled animal. Some of the animals are herbivores; feeding on the plant life while others are carnivores; eating other animals. Some eat both plant and animal life and are called omnivores. The ocean also has food chains and food webs. To learn about how food chains and webs function on land, visit Science Trek's Food Chains site.
While it might seem that the ocean is just one big ecosystem, plants and animals have specific needs as to water temperature, depth, and nutrients. Because of this, there are various places in the ocean where only certain life exists. Here are just a few of these ecosystems.
Tide pools form on beaches, often in rocky settings, where the tide moves in and out. When the tide is high, water and life move in to the area. When the tide is low, the life is exposed to the elements and land predators. Anemone, starfish, barnacles, and algae can make their homes in tide pools. As the tide changes, food can also come and go. Living in a tide pool can be challenging for these sea creatures.
Coral is an animal that anchors itself to rock structures and provides food and shelter to other plants and animals. Coral looks more like a plant, but in reality is an animal. Coral is often found clustered together in what is known as a coral reef. Reefs sometimes form on atolls – or the remnant of old fallen volcanoes. Coral reefs form ecosystems for many creatures of the ocean. To learn more about coral reef ecosystems go to Ocean Portal from the Smithsonian.
The deep ocean is home to life that is unusual in form and behavior. Because it is so deep, scientists are continually discovering new forms of life here. It is cold and dark in the deep parts of the ocean, so many of the animals create their own light to attract food or to find mates.
There are additional ecosystems found in the ocean that are home to many forms of life. Over 90% of the earth's living creatures make their home in the ocean. Some life migrates and moves from ecosystem to ecosystem.
The entire earth is dependent upon the ocean for its weather. Weather begins with the water cycle. The sun heats the ocean's surface which causes water to evaporate. The evaporated water rises and forms clouds when the water condenses on dust particles in the air. Wind will push the clouds over the land surface where the moisture will eventually become too heavy to stay in the clouds and will fall from the clouds as rain, snow, sleet or hail. This is called precipitation. Fallen water becomes streams, rivers, lakes, and ground water. Most of it eventually finds its way back to the ocean again. This is the water cycle.
It can rain over the ocean too. Storms over the ocean can contribute to wave formation. Wind from the storms can become extremely violent, too. Hurricanes are one such storm. Wind in a hurricane can reach hundreds of miles an hour and can last for days; traveling many miles over water and land. NOAA has a scale that will explain hurricane damage that includes a great animation to show what happens to plants and structures. Hurricanes are called cyclones in the South Pacific and Indian Oceans. They call them typhoons in the Atlantic ocean or the northeast Pacific Ocean.
The ocean is also responsible for much of the wind on the Earth. When the sun heats up the land it causes the air above it to heat up and rise upwards. This rising air creates what is known as a low pressure area. Cooler air over the ocean moves into the space created by the low pressure. This moving air is what we call wind.
The Coriolis Effect, which influences currents, can influence winds too. The turning of the Earth causes winds to shift towards the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
The jet stream impacts weather as it moves weather systems across the surface of the Earth. The jet stream is a ribbon of wind that flows from west to east at very high altitudes and can move at 200 mph.
In southern Asia winds that blow seasonally – both wet and dry – are known as monsoons. These too, come from over the ocean.
The simple fact that the ocean covers so much of the earth means that it is a natural location to obtain materials and resources for use in industry and for consumption. Food is one of the most obvious of resources that come from the ocean. Seafood and kelp are eaten all over the world. The sea itself helps with transporting materials by ships and boats. Oil is pumped from below the ocean floor for use in creating fuel for cars and airplanes, plastics, medications, and other materials. Salt, sand, gravel, copper, manganese and other minerals are mined from the ocean floor. Researchers continue to look for answers in medicine through studying the plant and animal life in the ocean. For example, starfish have the ability to regrow arms that have been lost due to injury or attack. If scientists can identify how they are able to grow back their arms, imagine how this could affect humans who have lost an arm or a leg due to accident or disease. Technology is being developed to use the ocean to generate electricity.
The oceans of the world have, for a long time, been used for dumping garbage that humans no longer want or use. Some of this trash will eventually decompose, but most of it is plastic, metal, or glass. These objects will stick around for hundreds of years which is a serious pollution problem. Some of these can become hazardous to life. For example, sea turtles have been known to get caught in those plastic loops used for carrying packs of soda pop. They do not know how to free themselves and the plastic can restrict their movement, their growth, their ability to gather food or their ability to escape predators. Sometimes animals will eat the garbage thinking it is food. Plastic, glass and metal will not digest and the animal will die because the objects get stuck in their internal organs.
Oil that escapes from drilling or from ships can get caught in the feathers of birds that inhabit beaches or the fur of seals and otters. This oil can poison these animals or prevent them from flying or swimming.
Ocean currents can carry garbage long distances from where the trash went into the water. This junk tends to gather together and form “garbage islands.” There are several of these islands in the Pacific Ocean. National Geographic has a slide presentation of these formations.
These are just a few of the ways that the oceans are being polluted. Chemicals from factories, mud from erosion, fishing nets and equipment, along with airplane and shipping accidents also add unwanted materials to the oceans.
Being able to swim below the ocean's surface and explore the ocean depths has always intrigued humans. But going very deep would require being able to take air along so that breathing is possible. In the late 1400s, Leonardo da Vinci was the first person to come up with the idea of special equipment for diving below ships to fight enemies. His idea was never used during his lifetime.
It wasn't until many centuries later that people began using Self Contained Underwater Breathing Apparatus or SCUBA for the purpose of diving. Air is carried in tanks and hoses are used to supply air to the nose and mouth of the diver. With this equipment, divers can go to depths of about 130 feet below the surface. Going deeper becomes dangerous because the weight of the water over their heads can become too much for their bodies to handle. Breathing air from a tank also causes something called the bends when divers go too deep or stay too long underwater. The bends is caused when bubbles form in the blood – it is very similar to the bubbles that are released in a soda pop bottle when you unscrew the cap. This can be very painful and even deadly.
Scuba is one way that people go below the ocean surface. Submarines and other equipment allow explorers to see what lives even deeper than 130 feet. For a look at how deep humans have gone, take a look at this chart from Ocean Planet.
Scientists who study the ocean are known as oceanographers. Oceanographers study plant and animal life in the oceans and seas. They also study waves, currents, weather, minerals of the ocean and the ocean floor. Oceanographers are also interested in earthquakes, volcanoes and the movement of the tectonic plates that occurs under the ocean. Pollution, temperature changes, salinity, animal migration, resources, and the ocean's impact on climate are also topics studied by oceanographers. Check out NOAA's site for additional information on the job of an oceanographer.
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