Difference Between Ocean and Sea


Earth is distinctive in its abundance of water and there are several water bodies on the earth including rivers, lakes, seas, oceans, gulfs, streams, and more. Oceans are large water bodies and cover the majority of the area on the earth. These are significant in various aspects such as marine ecosystems, weather, aquatic trade, and world exploration, and two million species existing in the oceans that have specific adaptations and life. Oceans and seas play a vital role in weather and climate patterns and they contribute some ecological functions and supply food, medicine, and other needs for humans. They both share differences and few similarities.

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The ocean is a huge body of salt water and it is deep and wide. It covers nearly 71% of the surface of the earth and has a depth of approximately 3688 meters. The average temperature of the ocean is 80°F, however, it gradually decreases to the poles from the equator.


Earth had been formed around 4.6 billion years ago and it slowly began to separate into two layers as a lighter and heavier rock. Carbon dioxide and other gases formed the atmosphere of the earth. Earth’s crust has formed from lighter rock and Earth’s core has formed from heavy rock. The water for the ocean came from the inside of the rocks. The molten rocks get cooled and release gases and water vapour. This water vapour condensed and covered the crust by forming the oceans.


Pure water is ideally clear, however, the water appears as a deep navy blue in oceans. It is because there is a lot of water in the oceans and the water is so deep that there are no reflections off the seafloor. The blue colour is because of the absorption and scattering of light. The scattering of blue wavelengths of light is the same as the scattering of blue light in the sky and the absorption is a huge factor for the colour than scattering.


The global ocean is divided into five recognizable oceans such as Pacific, Atlantic, Indian, Arctic, and Southern. The first three oceans are commonly known oceans and the last is the newly named ocean. The Pacific Ocean is the largest in the world. The boundaries between these regions have developed for several causes such as geographical, historical, scientific, and cultural aspects. The living creatures in the oceans include seabirds, small invertebrates, sharks, shellfish, seahorses, etc.


Sea is a portion of the ocean that is a continuous body of salt water and it exists between areas where these are meeting oceans and land. The salinity of the seawater is based on precipitation, evaporation, and river discharge. The temperature of the surface of the seawater is 86°F and it is 30°F at the sea bed.

Major seas

There are above 50 seas on the earth such as the Arabian Sea, Red Sea, Yellow Sea, Aral Sea, Caspian Sea, Mediterranean Sea, Caribbean Sea, Black Sea, South China Sea, and more. The Philippine Sea is the largest in the world and there are three major types of seas −

Nearly enclosed seas

These seas are deep into continents, which are joined with the open ocean by narrow canals called straits and are almost landlocked with a small range of tides. It includes the Baltic Sea and the Mediterranean sea. There is a slight exchange of water between the open ocean and the enclosed sea. The Red sea is saltier than the Indian ocean and coral reefs such as sea turtles, sea horses, and clownfish are adapted to their life in this sea.

Partly or semi-enclosed seas

These seas are similar to the ocean and the Weddell sea of Antarctica is connected by a wide opening to the ocean. The organisms and waves of the Weddell sea are the same as the organisms and waves of the Atlantic ocean. The South China sea also comes under a partly enclosed sea.

Hypersaline lakes

Some seas have excessive high salt content in the water, therefore, it is called hypersaline lakes. The Caspian Sea is a landlocked sea that is situated between Asia and Europe. The southern part of the sea in Iran and Turkmenistan has excessive salt content. A hypersaline lake, the Dead Sea is located between Jordan and Israel.

Similarities between ocean and sea

  • Ocean and Sea are important for 50% of oxygen production on the earth.
  • These both are large bodies of saltwater.
  • These are breeding grounds for several marine animals and plants.

Differences between the ocean and sea

Ocean Sea
The ocean is larger Sea is smaller than the ocean
There are no landlocked oceans in the world There are landlocked seas in the world
Oceans do not have enclosed oceans Seas have enclosed seas
Deeper ocean Less deep than oceans
Marine creatures in the ocean are less dependent on sunlight such as whales, phytoplankton, etc. Marine creatures in the sea are more dependent on sunlight such as crabs, lobsters, etc.


Ocean and sea both are huge water bodies on the earth. The ocean is salty water and the sea is a part of the ocean. They are developed from molten rocks by the condensation of water vapour. The Pacific Ocean is the largest present on the earth. The colour of the sea or ocean water depends on the absorption and scattering of light. It is usually blue because of the scattering of blue light in the sky. Sea is categorised into three types depending on the salinity and enclosed area. The Philippine Sea is the largest and there are several seas present in the world.


Q1. When does a sea become an ocean?

Ans. A sea becomes an ocean when it reaches an opening where it is enclosed by land on most of its sides and connected with the larger water body.

Q2. What are enclosed seas?

Ans. A sea is connected to other seas or oceans by narrow waterways and surrounded by two or more states is called enclosed seas. The Baltic Sea, the Black Sea, the Mediterranean sea, and the Caspian sea are enclosed seas.

Q3. What factors play a major role in determining the salinity of seawater?

Ans.The salinity of the sea differs from one place to another place. Therefore, river discharge, precipitation, and evaporation are the important factors that determine the salinity of seawater.

Q4. What are the uses of oceans for humans?


  • Oceans are the depots of fish and minerals.
  • They are useful in transportation because they provide the cheapest route for trade and purposes.
  • They control climatic conditions.

Q5. Why is the ocean blue?

Ans. The ocean absorbs yellow, red, and orange by water rather than blue. It is due to the absorption and scattering wavelength of the light in the ocean. Therefore, the ocean turns blue when the white light from the sun meets the ocean.