A sea is a large body of water that is connected with another sea or with an ocean, which can also be referred to as a sea, albeit that an ocean forms an independent whole with its own circulation (ocean current). A sea, on the other hand, has a bottom that belongs to a continent (continental shelf). If the connection between seas is narrow and consists of one or more straits, it is called an inland sea.
Seas are fed by rivers and by rainwater.
The sea as a natural phenomenon
The geology of the seabed is largely determined by plate tectonics. New rock is surfacing in the mid-ocean ridges, which are interconnected worldwide and cover about a third of the ocean floor. The bottom of the deep sea is geologically relatively young: the oldest deposits are about 150 million years old, so less than 5% of the age of the oldest continental rocks. Any pre-Jurassic deep sea deposits are believed to have subducted into troughs. The entire ocean floor is slowly sliding away from the mid-ocean ridge. At the Atlantic Ocean, this results in the continents drifting apart; at the Pacific Ocean, the ocean floor disappears in troughs along the Ring of Fire, an active zone of volcanoes and deep earthquakes along most of the coast. So the Pacific Ocean is slowly getting smaller, while the Atlantic Ocean is getting bigger. Another consequence of the drift of the ocean floor is the formation of chains of volcanic islands above so-called hotspots.
The water on Earth is constantly in motion. Due to the tidal action, the water in the oceans and their marginal seas moves back and forth every day. Due to differences in density (as a result of, among other things, temperature and salinity), ocean currents are created that ensure water transport across our planet. Due to the Coriolis effect, these currents usually rotate counterclockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere. Ocean currents are one of the most important factors in Earth's climate. The Gulf Stream, for example, carries heat from the Caribbean to Europe, which is why Europe has a milder climate than, say, Canada or Siberia. Because the sea warms up and cools more slowly than land, the influence of the sea on the climate is moderate everywhere.
The seas on Earth have played a crucial role in the development of life on Earth. Until about 400 million years ago, life only took place in water. It was not until the Devonian that the first plants land and produce oxygen on a large scale, after which the amphibians can land in the same period. The seas of life still teem with life, only a portion of which has yet been described. Well-known animal groups that live in and around the sea are fish, marine mammals, sea turtles and sea birds. But of at least as much importance are lesser-known groups such as molluscs, echinoderms (such as starfish and sea urchins), crustaceans and cnidarians (such as disc jellyfish and flower animals). The basis of the food pyramid in the sea is formed by unicellular organisms such as the foraminifera, dinoflagellates, radiolaria, diatoms and the phytoplankton. Some marine ecosystems are coral reefs and kelp forests.
Sea and man
The seas have played an important role for mankind since ancient times. They are an important source of food for us and ancient shipbuilding techniques made it possible for humans to transport large amounts of mass over great distances for the first time. Seas became transport arteries in trade and took on military importance. The mild climate that the Gulf Stream gives Europe has undoubtedly been a critical factor in the development of European civilization and thanks to the sea, European civilization has been preserved since the Renaissance.