Oceans

Five large bodies of salt water that cover most of the earth's surface.

Oceans cover about 71% of the Earth's surface and contain 97% of all the water on Earth. From space, oceans give the Earth a blue appearance, so Earth has been called the “Blue Planet.” Oceans are a key element for the existence of life on Earth. The majority of animal life on Earth is found in the oceans. The air we breathe, the food we eat, and the climate we live in are all connected to the ocean. The ocean is home to the earth’s longest mountain range, deepest canyon, and largest living animal. Yet, as important as oceans are, less than 10% of the global ocean has been searched and mapped. Let’s explore the earth’s oceans. 

Water in the ocean is salty. Much of the salt comes from minerals that have been dissolved from rocks that lie on the ocean floor. Another source of salts is heated water that is released through vents in the seafloor. Underwater volcanoes also contribute minerals into the water. Animals that live and die in the ocean leave behind minerals from their bodies. In addition, rainwater from the water cycle also washes additional minerals from the land into the water. Some of these minerals are used by organisms in the ocean, while others are left behind when water evaporates. The salts going into the ocean keep a balance with the salt being removed or deposited.

Some areas of the ocean are more salty than others. The degree of saltiness is known as the water's salinity. The salinity of water affects its temperature and the types of life that live there. Salinity also makes water denser. In general, seawater is less salty at the polar regions due to melting ice and at the equator where the tropics receive an abundance of fresh rainwater. The saltiest ocean water is found in regions where evaporation is high and there is little fresh water flowing in, such as the Red Sea and the Persian Gulf.

For most of us land dwellers, ocean water is not drinkable. Because of the salty composition of the water, only certain plants and animals can exist in the ocean. But for these ocean dwellers, salt water is a necessity. They cannot live in fresh water.

Scientists are still studying the reasons behind the amount of salt in ocean water. The National Oceanic and Atmospheric Administration (NOAA) studies the ocean and how it impacts our weather and climate. Learn more about the salinity of the ocean from NOAA.

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 of the earth: North and South America, Africa, Europe, Asia, Antarctica and Australia. The ocean surrounds the continents. Humans have given separate names to the ocean areas depending upon where they are in relation to the continents, 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 southernmost area of the Pacific, Atlantic, and Indian Oceans, and was known at one time as the Antarctic Ocean. Each ocean has unique qualities and features, even though the water is largely the same. The Pacific Ocean is the largest and deepest ocean, covering about one-third of the earth.

While the ocean may sometimes be called the sea, the earth has other bodies of water that are geographically identified as seas. Seas are surrounded by land but 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 other bodies of water on the earth that are called seas, although not all scientists agree that they are truly seas. Learn more about the Earth’s seas.

The word “marine” refers to things related to the ocean. So, the term “marine life” refers to “ocean life,” and “marine habitat” refers to the living environment in the ocean.

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.

Underwater mountain ranges are often formed by the movement of the earth’s tectonic plates. When these plates spread apart, molten rock from deep within the earth rises to the seafloor, creating ridges. The longest mountain range on Earth is known as the mid-oceanic ridge. This huge range is 40,390 miles (65,000 km) long, zig-zagging across the globe like the stitching on a baseball. While 90% of the range is underwater, a portion known as the Mid-Atlantic Ridge rises above sea level near Iceland. Learn more about tectonic plates at Science Trek’s Earth and Earthquakes pages.

Underwater mountains can rise above 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. Sometimes, underwater 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 tallest mountain on Earth from base to peak is an underwater volcano called Mauna Kea in the Hawaiian Islands. Mauna Kea is 33,481 feet (10,205 meters) in height, which is taller than Mount Everest (29,032 feet/8,849 meters.) However, over half of Mauna Kea is underwater in the Pacific Ocean.

The Mariana Trench in the Pacific Ocean is the deepest point in the ocean at 36,201 feet (11,034 meters) deep. It is so deep that it is beyond most people's imagination. If Mount Everest were placed into the trench, there would still be over 7,000 feet of water above the mountain!

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 geological events, marine life, and human existence. Everything from shipwrecks to remnants of ancient civilizations to prehistoric 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 different-sized grains. Beaches can stretch out towards the ocean at varying distances. Much of this difference is due to currents and wave action.

Sandbars can form when waves move sand from the beach out further into the ocean. This sand can pile up and form a sort of wall of sand. This affects 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 and darker the water is. Sunlight does not penetrate very deep, so most ocean life lives in upper areas where food can be produced. Some animals do live in the deeper parts of the ocean, but they have specific adaptations that allow them to live without sunlight. Learn more about the layers of the ocean from NOAA. The diagram below shows the different zones of light below the surface of the ocean.

Ocean water is always moving. Some forms of ocean movement include currents, waves, and tides.

Currents

Within the oceans are streams that move like rivers through the water. Temperature, wind and the turning of the earth on its axis cause these ocean streams to flow in constant motion along the coasts of the continents. These rivers of ocean water are called currents. In the northern hemisphere, the water moves clockwise, and in the southern hemisphere, it moves counter-clockwise. This is known as the Coriolis Effect. Currents effect weather trends and climate, impact bird and whale migration, and have been used by sailors to direct their ships for travel to distant locations. You may have heard of the Gulf Stream, a strong ocean current that brings warm water from the Gulf of Mexico up into the Atlantic Ocean, warming the east coast of the United States and western Europe. See an animation of ocean currents from NASA.

Waves

Another type of ocean movement is waves. Waves happen because 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 such as a boat sits on the surface of the water, it will move up and down with each wave, but it won't really go anywhere because the water isn't actually moving -- just bobbing! It is energy that is moving with the wave, not water.

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 and pushes the energy upwards. The water is forced to climb higher than the wavelength and then break because of gravity. These waves then crash down to create surf.

Some unusual waves are formed not by wind but by underwater earthquakes or volcanoes. These are known as tsunamis and they travel from deep in the ocean. Storm surges are long waves caused by the pressure of a hurricane or other severe weather. These kinds of waves can be very destructive when they reach land.

Over time, waves batter and change the shoreline. Erosion of the rocks and sand can reshape the coastal land. Most of the sand found on a beach is a result of this process and came from rocks that have been ground down through weathering and erosion.

Tides

Oceans are subject to another type of movement known as tides. If you have ever been to the beach, you may have noticed that there are times of the day when the ocean water moves further into the land (high tide), and times when the water retreats further away from the land (low tide.) Tides are caused by the gravity force from the moon and sun on the oceans.

When the moon's gravity pulls on the ocean, it causes the 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. As the Earth rotates, a particular region of the Earth passes through both of these bulges each day. When a region is in one of the bulges, it experiences a high tide. When it is not, it experiences a low tide. This cycle of daily tides occurs on most of the world’s coastlines. Twice a month, the sun and the moon are in line with each other, which increases the gravity force. At these times, the high and low tides are more extreme. Watch this NOAA animation of the causes and effects of tides. 

When you look out over the vast ocean, it may seem to be nothing but water. But the ocean teems with plant and animal life! In fact, although hundreds of thousands of species have been discovered in the ocean, scientists estimate that 91% of ocean species have not yet been identified. All ocean life is interconnected, from the smallest algae to the largest whale.

Plants
Most ocean plant life is found 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 allow photosynthesis to take place in ocean plants. Kelp, which are long, brown algae plants, attach to the ocean floor and can even make up forests that support large populations of animals. 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 microscopic algae that drift in the upper surface of the ocean water. Phytoplankton are the foundation of most ocean food chains and are critical to the health of oceans. 

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 include mammals, fish, crustaceans, mollusks, reptiles, and zooplankton – microscopic animals that drift in the ocean. Some ocean animals are herbivores, feeding on plant life, while others are carnivores who eat other animals. Some eat both plant and animal life and are called omnivores. Just like land biomes, the ocean also has food chains and food webs. In many cases, phytoplankton are eaten by zooplankton, which are in turn eaten by larger animals. For example, krill is a type of zooplankton that is the primary food of some whales. Learn more about ocean food webs.

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 kinds of life exists. Let's take a look at 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 into the area. When the tide is low,  organisms are exposed to the elements and land predators. Anemone, starfish, and barnacles often make their homes in tide pools. As the tide changes, food sources can also come and go, so 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 it is in reality an animal. Coral is often found clustered together in what is known as a coral reef. Reefs sometimes form on atolls, or remnants of ancient volcanoes. Coral reefs form ecosystems for many ocean creatures, including seahorses, clownfish, and sea turtles. To learn more about coral reef ecosystems, visit the Smithsonian’s Ocean Portal.

Kelp forests found along coastlines form an ecosystem that provides food and shelter for a wide variety of marine life, including sea urchins, crabs, rockfish, seals, and sea otters.

The deep ocean ecosystem is home to life that is unusual in form and behavior. Because it is so cold and dark in the deep ocean, many of the animals create their own light to attract food or to find mates. This self-generated light is known as bioluminescence. Because the deep ocean is so difficult to explore, there are still many forms of life yet to be discovered there. 

The entire earth is dependent upon the ocean for its weather. First of all, the ocean is an essential part of the water cycle, the most basic component of weather. 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 pushes the clouds over the land surface, where the moisture will eventually become too heavy to stay in the clouds and falls 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.

Ocean water is constantly evaporating, increasing the temperature and humidity of the surrounding air to form clouds, rain, and storms that are then carried by trade winds. Most of the rain that falls on land starts off in the ocean. The tropics are particularly rainy because heat absorption from the sun and ocean evaporation are highest there.

The ocean absorbs and stores much of the sun’s heat and distributes it around the globe. Like a great conveyer belt, ocean currents move warm water from the equator to the poles, and cold water from the polar regions back toward the tropics. In this way, ocean currents help regulate the global climate. Without ocean currents, temperatures in different regions on land would be more extreme – super hot or super cold -- and less able to support life.

Rainstorms over the ocean affect weather on land as well. Wind from ocean storms can become extremely violent, as in the case of hurricanes. Hurricanes, also known as cyclones or typhoons, form over the ocean and may travel many miles with wind speeds that can reach 150 miles per hour or more. See NOAA’s hurricane scale and animation that explain the effects of hurricane-strength winds.

The ocean also influences much of the wind on land. When the sun heats up the land along the coasts, warm air rises upwards. This rising air creates what is known as a low pressure area. Cooler air from the ocean moves into the space, causing wind and sea breezes. In southern Asia winds that blow seasonally – both wet and dry – are known as monsoons. These too, come from over the ocean.

What do humans obtain from the ocean? The most important thing all humans get from the ocean is oxygen. Half of the oxygen on our planet is produced by plants in the ocean. Oceans are also responsible for absorbing 50% of the carbon dioxide released into the atmosphere. We literally need the ocean to survive.

The simple fact that it covers so much of the earth means that humans have always looked to the ocean for materials and resources. One obvious resource that comes from the ocean is food. Seafood is harvested and eaten all over the world and is a significant part of the world economy. Oceans provide an important means of transporting people and materials throughout the world by ships and boats. Oil is pumped from below the ocean floor for use in manufacturing fuel, plastics, medications, and other materials. Salt, gravel, copper, manganese, and other minerals are mined from the ocean floor. Engineers are developing technology that could one day be used to generate electricity from the oceans.

Many medicines come from the sea, and researchers continue to look for answers to medical questions 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 do this, it would greatly benefit humans who have lost an arm or leg due to accident or disease.

Oceans provide recreation for people who enjoy swimming, sailing, fishing, surfing, boating or just enjoying the beach.

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 much 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 materials can become hazardous to life. For example, sea turtles have been known to get caught in the plastic loops used for carrying packs of soda pop. They are unable 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 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. Trash on the surface can block sunlight from reaching algae and plankton below, threatening entire food webs.

Oil that escapes from drilling or from ships can get caught in the feathers of birds that inhabit beaches or in the fur of seals and otters. This oil can poison these animals or prevent them from flying or swimming. Discarded fishing nets, chemicals from factories, mud from erosion, and debris from airplane and shipping accidents also add unwanted materials to the oceans.

Ocean currents can carry garbage long distances from where the trash entered the water. This trash tends to gather together and form “garbage islands” composed of tiny pieces of plastic. Much of this marine debris comes from plastic bags, caps, and water bottles. The largest of these garbage islands is known as the Great Pacific Garbage Patch and is estimated to cover about 600,000 square miles (1.6 million square kilometers.) Learn more about these garbage islands and what can be done to reduce them.

Today, oceans are threatened by climate change. Warmer waters in the oceans endanger coral reefs and other kinds of ocean life. As oceans absorb more carbon dioxide from the air, the water becomes more acidic, which causes problems for plants and animals. Sea levels are rising due to melting glaciers and expansion of warmer seawater, putting many coastal communities in danger. Warming waters could also affect the movement of ocean currents, which in turn can affect climate on land. Learn more from NASA about how climate change affects the ocean.

Being able to swim below the ocean's surface and explore the ocean depths has always intrigued humans. But going very deep into the ocean requires 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. But 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 pressure of the water over their heads can become too much for divers’ bodies to handle. Breathing air from a tank can also cause a painful condition called the bends when divers go too deep or come up to quickly from a dive.

Scuba is one way that people go below the ocean surface. Highly specialized equipment allows some divers to go as deep as 1,000 feet. Submersibles can carry explorers to depths of up to 15,000 feet. Underwater robots and remote vehicles can send back scientific data from the deepest parts of the ocean. To see how deep humans have gone, take a look at this interactive chart from NASA.

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.