Oceans and Seas

The oceans are the vast bodies of salt water that cover most of the Earth's surface and together form a single connected world ocean, conventionally divided into the Pacific, Atlantic, Indian, Southern and Arctic oceans. A sea is generally a smaller area of salt water, often partly enclosed by land or forming part of an ocean. Lakes, by contrast, are inland bodies of water, usually fresh. Together these waters dominate the planet's surface and play a central role in its climate and life.

Seawater is salty because rivers and other processes have carried dissolved minerals into the oceans over immense spans of time. The oceans are layered and constantly moving, driven by winds, tides, temperature and differences in salt content, which produce currents that circulate water and heat around the globe. Physical oceanography studies these movements and properties, while marine ecology examines the organisms that inhabit the sea. On the deep ocean floor, hydrothermal vents, including dark mineral-rich plumes called black smokers, support unusual communities that draw energy from chemicals rather than sunlight, a process of interest to geochemistry.

Oceans and seas teem with life, from microscopic plankton to the largest animals on Earth, organised into ecosystems that depend on light, nutrients and temperature. They also regulate climate by absorbing heat and carbon dioxide, generate much of the planet's oxygen through marine plants and algae, and sustain fisheries and trade. Many vulnerable marine areas are now protected as underwater sanctuaries to conserve their biodiversity.

Frequently asked questions

What is the difference between an ocean and a sea?

Oceans are the largest bodies of salt water and form the connected world ocean, while seas are smaller, often partly enclosed by land or forming part of an ocean.

Why is the sea salty?

Rivers and weathering carry dissolved minerals, including salts, into the oceans over long periods. As water evaporates the salts remain, so they accumulate in seawater.

What is physical oceanography?

It is the branch of ocean science that studies the physical properties and movements of seawater, such as currents, tides, temperature, salinity and waves.