The Ocean

Almost three-quarters of the Earth is Ocean

Aerial view, coral reef off Hurghada, Red
Andrey Nekrasov/Imagebroker/Getty Images

Within the world's oceans, there are many different marine habitats. But what about the ocean as a whole? Here you can learn facts about the ocean, how many oceans there are and why they're important.

Basic Facts About the Ocean

From space, Earth has been described as a "blue marble." Know why? Because most of the Earth is covered by ocean. In fact, almost three-quarters (71%, or 140 million square miles) of the Earth is ocean.

With such an enormous area, there's no argument that healthy oceans are vital to a healthy planet.

The ocean is not divided evenly between the Northern Hemisphere and Southern Hemispheres. The Northern Hemisphere contains more land than ocean - 39% land versus the 19% land in the Southern Hemisphere.

How Did the Ocean Form?

Of course, the ocean dates back long before any of us, so nobody knows for sure how the ocean originated, but it is thought that it came from water vapor present in the Earth. As the Earth cooled, this water vapor eventually evaporated, formed clouds and caused rain. Over a long time, the rain poured into low spots on the Earth's surface, creating the ​first oceans. As the water ran off the land, it captured minerals, including salts, which formed salt water.

The Importance of the Ocean

What does the ocean do for us? There are many ways the ocean is important, some more obvious than others.

The ocean:

  • Provides food.
  • Provides oxygen through the photosynthesis of tiny plant-like organisms called phytoplankton. These organisms provide an estimated 50-85% of the oxygen we breathe and also have the ability to store excess carbon.
  • Regulates climate.
  • Is a source of important products such as medicines, and things that we use in food such as thickeners and stabilizers (which may be made from marine algae).
  • Provides recreational opportunities.
  • Contains natural resources such as natural gas and oil.
  • Provide "highways" for transportation and trade. More than 98% of U.S. foreign trade occurs via the ocean (Source).

How Many Oceans Are There?

The saltwater on the Earth is sometimes just referred to as "the ocean," because really, all of the world's oceans are connected. There are currents, winds, tides and waves that circulate water around this world ocean constantly. But to make geography a bit easier, the oceans have been divided and named. Below are the oceans, from largest to smallest. Click here for more details on each of the oceans.

  • Pacific Ocean: The Pacific Ocean is the largest ocean, and the largest single geographic feature on Earth. It is bound by the western coast of North and South America to the east, the coasts of Asia, and Australia to the west, and the more newly-designated (2000) Southern Ocean to the south.
  • Atlantic Ocean: The Atlantic Ocean is smaller and shallower than the Pacific Ocean, and is bound by North and South America to the west, Europe and Africa to the east, the Arctic Ocean to the north and the Southern Ocean to the south.
  • Indian Ocean: The Indian Ocean is the third-largest ocean. It is bound by Africa to the west, Asia and Australia to the east, and the Southern Ocean to the south.
  • Southern, or Antarctic, Ocean: The Southern Ocean was designated from parts of the Atlantic, Pacific and Indian Oceans in 2000 by the International Hydrographic Organization. This is the fourth largest ocean, and surrounds Antarctica. It is bounded to the north by parts of South America, Africa, and Australia.
  • Arctic Ocean: The Arctic Ocean is the smallest ocean. It lies mostly north of the Arctic Circle and is bounded by Europe, Asia and North America.

What Is Sea Water Like?

Sea water might be less salty than you'd imagine. Salinity (the salt content) of the sea differs across different areas of the ocean, but on average is have about 35 parts per thousand (about 3.5 % salt in salt water). To recreate the salinity in a glass of water, you'd need to put about a teaspoon of table salt into a glass of water.

The salt in sea water is different from table salt, though. Our table salt is made up of the elements sodium and chlorine, but the salt in sea water contains more than 100 elements, including magnesium, potassium, and calcium.

Water temperatures in the ocean can vary greatly, from about 28-86 degrees F.

Ocean Zones

When learning about marine life and their habitats, you'll learn that different marine life may live in different ocean zones. Two major zones include:

The ocean is also divided into zones according to how much sunlight they receive. There is the euphotic zone, which receives enough light to permit photosynthesis. The disphotic zone, where there is just a small amount of light, and also the aphotic zone, which has no light at all.

Some animals, like whales, sea turtles and fish may occupy several zones throughout their lives or in different seasons. Other animals, like sessile barnacles, may stay in one zone for most of their lives.

Major Habitats in the Ocean

Habitats in the ocean range from warm, shallow, light-filled waters to deep, dark, cold areas. Major habitats include:

  • Intertidal Zone, where land and sea meet. This is an area subject to great challenges for its marine life, as it is covered with water at high tide and water is largely absent at low tide. Therefore, its marine life must adapt to sometimes great changes in temperature, salinity and moisture throughout the day.
  • Mangroves: Mangroves are another salt water habitat along the coast. These areas are covered by salt-tolerant mangrove trees and are important nursery areas for a variety of marine life.
  • Seagrasses, or seagrass beds: Seagrasses are flowering plants and live in a marine or brackish environment, usually in protected areas such as bays, lagoons and estuaries. Seagrasses are another important habitat to a number of organisms and provide nursery areas for tiny marine life.
  • Reefs: Coral reefs are often described as the "rainforest of the sea" because of their great biodiversity. The majority of coral reefs are found in warm tropical and sub-tropical areas, although deep-water corals do exist in some colder habitats.
  • Pelagic Zone: The pelagic zone, also described above, is where some of the biggest marine life, including cetaceans and sharks, are found.
  • Reefs: Coral reefs are often referred to as the "rainforests of the sea" because of their great diversity. Although reefs are most often found in warm, shallow tropical and sub-tropical waters, there are also deep-water corals that live in cold water. One of the most well-known coral reefs is the Great Barrier Reef off Australia.
  • The Deep Sea: Although these cold, deep and dark areas of the ocean may appear inhospitable, scientists are realizing that they support a wide variety of marine life. These are also important areas to study, as 80% of the ocean consists of waters greater than 1,000 meters in depth.
  • Hydrothermal Vents: While they are located in the deep sea, hydrothermal vents provide a unique, mineral-rich habitat for hundreds of species, including bacteria-like organisms called archaea that turn chemicals from the vents into energy using a process called chemosynthesis, and other animals such as tubeworms, clams, mussels, crabs and shrimp.
  • Kelp Forests: Kelp forests are found in cold, productive, and relatively shallow waters. These underwater forests include an abundance of brown algae called kelp. These giant plants provide food and shelter for a variety of marine life. In the U.S., the kelp forests that may most readily come to mind are those off of the west coast of the U.S. (e.g., California).
  • Polar Regions: Polar habitats are areas near the Earth's poles, with the Arctic in the north and the Antarctic to the south. These areas are cold, windy and have wide fluctuations in daylight throughout the year. While these areas seem uninhabitable for humans, marine life thrives there, with many migratory animals traveling to these areas to feed on abundant krill and other prey. They are also home to iconic marine animals such as polar bears (in the Arctic) and penguins (in the Antarctic). Polar regions have been subject to increasing attention due to concerns about climate change - as it is in these areas where a warming of Earth's temperatures would likely be most detectable and significant.


  • CIA - The World Factbook. Accessed December 29, 2011.
  • National Marine Sanctuaries. 2007. Ecosystems: Kelp Forests. Accessed December 29, 2011.
  • WHOI. Polar Discovery. Woods Hole Oceanographic Institution. Accessed December 29, 2011.
  • Tarbuck, E.J., Lutgens, F.K. and Tasa, D. Earth Science, Twelfth Edition. 2009. Pearson Prentice Hall: New Jersey.