70.8% of Earth’s surface is covered by oceans. The oceans comprise 97.2% of all of the water on or near the surface. The oceans supply as much as 70% of the oxygen that humans breathe.
The world ocean is a single interconnected body of water. It can be divided into four principal oceans – the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, and the Arctic Ocean – and one additional ocean, the Southern Ocean.
A sea is smaller and shallower than an ocean, composed of salt water, somewhat enclosed by land, and directly connected to the world ocean.
Oceanography, or marine science, is the scientific study of all aspects of the marine environment. It is an interdisciplinary science.
Oceanic crust underlies the ocean basins and is composed of the igneous rock basalt. The average thickness of the oceanic crust is only about 5 miles. Oceanic crust floats lower in the mantle than continental crust due to isostatic adjustment.
The origin of the oceans began with outgassing of water vapor that condensed and fell to Earth once Earth had cooled sufficiently. Rainfall on the surface dissolved compounds that, when carried to the ocean, made it salty. Some Kuiper Belt objects and asteroids may also have contributed to Earth’s oceans. The oceans were present by at least 4 billion years ago.
Life is thought to have begun in the oceans.
—July 2020
Lance Jones 