A sea is a large body of salt water that is surrounded in whole or in part by land. More broadly, the sea is the system of Earths salty. The sea moderates Earths climate and has important roles in the cycle, carbon cycle. Although the sea has been traveled and explored since prehistory, the scientific study of the sea—oceanography—dates broadly to the British Challenger expedition of the 1870s. Owing to the present state of continental drift, the Northern Hemisphere is now equally divided between land and sea but the South is overwhelmingly oceanic. Salinity in the ocean is generally in a narrow band around 3. 5% by mass, although this can vary in more landlocked waters, near the mouths of large rivers. About 85% of the solids in the sea are sodium chloride. Deep-sea currents are produced by differences in salinity and temperature, surface currents are formed by the friction of waves produced by the wind and by tides, the changes in local sea level produced by the gravity of the Moon and Sun.
The direction of all of these is governed by surface and submarine land masses, former changes in sea levels have left continental shelves, shallow areas in the sea close to land. The most diverse areas surround great tropical coral reefs, whaling in the deep sea was once common but whales dwindling numbers prompted international conservation efforts and finally a moratorium on most commercial hunting. Life may have started there and aquatic microbial mats are generally credited with the oxygenation of Earths atmosphere, the sea is an essential aspect of human trade, mineral extraction, and power generation. It is the scene of activities including swimming, surfing. However, population growth, industrialization, and intensive farming have all contributed to marine pollution. Atmospheric carbon dioxide is being absorbed in increasing amounts, lowering its pH in a known as ocean acidification. The shared nature of the sea has made overfishing an increasing problem, both senses of sea date to Old English, the larger sense has required a definite article since Early Middle English.
Seas are generally larger than lakes and contain salt water, while the defining elements of size and being bounded are generally used, there is no formally accepted technical definition of sea among oceanographers. In international law, the United Nations Convention on the Law of the Sea states that all the ocean is the sea. Earth is the known planet with seas of liquid water on its surface, although Mars possesses ice caps
Sea in culture
The role of the sea in human culture has been important for centuries, as people experience the sea in contradictory ways, as powerful but serene, beautiful but dangerous. Human responses to the sea can be found in artforms including literature, poetry, theatre, the earliest art representing boats is 40,000 years old. Since then, artists in different countries and cultures have depicted the sea, the sea has been perceived as a hostile environment populated by fantastic creatures, the Leviathan of the Bible, Isonade in Japanese mythology, and the kraken of late Norse mythology. In the works of the psychiatrist Carl Jung, the sea symbolises the personal, the sea and ships have been depicted in art ranging from simple drawings on the walls of huts in Lamu to seascapes by Joseph Turner and Dutch Golden Age painting. The Japanese artist Katsushika Hokusai created colour prints of the moods of the sea, the sea has appeared in literature since Homers Odyssey. The sea is a theme in the Haiku poems of the Japanese Edo period poet Matsuo Bashō.
In the Middle Ages, the sea appears in such as the Tristan legend, with motifs such as mythical islands. Pilgrimage is a theme in stories and poems such as The Book of Margery Kempe. Burial at sea has been practised in various ways by countries around the world since the ancient civilisations of Egypt and Rome. Human reactions to the sea are found in, for example, art, film and classical music, as well as in mythology and exchange of ideas with neighbouring nations is one of the means by which civilizations advance and evolve. This happened widely among the ancient peoples living in lands bordering the Mediterranean Sea, as well as in India, petroglyphs depicting boats made of papyrus are among rock art dating back 40,000 years on the shores of the Caspian Sea. The Egyptians placed figures of birds on the prow while the Phoenicians used horses representing speed. The Ancient Greeks used boars heads to symbolise acute vision and ferocity while Roman boats often mounted a carving of a centurion representing valour in battle.
In northern Europe, bulls and dragons were used to decorate ships prows and by the thirteenth century. The Greek mythology of the sea includes a pantheon of gods. The god of the sea, Poseidon, is accompanied by his wife, the Tritons, sons of Poseidon, who were variously represented with the tails of fish or seahorses, formed Poseidons retinue along with the Nereids. The mythic sea was further peopled by dangerous sea monsters such as Scylla, Poseidon himself had something of the shifting character of the sea, presiding not only over the sea, but earthquakes and horses. Neptune occupied a position in Roman mythology
An ocean is a body of saline water that composes much of a planets hydrosphere. On Earth, an ocean is one of the major divisions of the World Ocean. These are, in descending order by area, the Pacific, Indian, the word sea is often used interchangeably with ocean in American English but, strictly speaking, a sea is a body of saline water partly or fully enclosed by land. The ocean contains 97% of Earths water, and oceanographers have stated that less than 5% of the World Ocean has been explored, the total volume is approximately 1.35 billion cubic kilometers with an average depth of nearly 3,700 meters. As the world ocean is the component of Earths hydrosphere, it is integral to all known life, forms part of the carbon cycle. The world ocean is the habitat of 230,000 known species, but because much of it is unexplored, the origin of Earths oceans remains unknown, oceans are thought to have formed in the Hadean period and may have been the impetus for the emergence of life. Extraterrestrial oceans may be composed of water or other elements and compounds, the only confirmed large stable bodies of extraterrestrial surface liquids are the lakes of Titan, although there is evidence for the existence of oceans elsewhere in the Solar System.
Early in their histories and Venus are theorized to have had large water oceans. The Mars ocean hypothesis suggests that nearly a third of the surface of Mars was once covered by water, compounds such as salts and ammonia dissolved in water lower its freezing point so that water might exist in large quantities in extraterrestrial environments as brine or convecting ice. Unconfirmed oceans are speculated beneath the surface of many planets and natural satellites, notably. The Solar Systems giant planets are thought to have liquid atmospheric layers of yet to be confirmed compositions. Oceans may exist on exoplanets and exomoons, including surface oceans of water within a circumstellar habitable zone. Ocean planets are a type of planet with a surface completely covered with liquid. The concept of Ōkeanós has an Indo-European connection, Greek Ōkeanós has been compared to the Vedic epithet ā-śáyāna-, predicated of the dragon Vṛtra-, who captured the cows/rivers. Related to this notion, the Okeanos is represented with a dragon-tail on some early Greek vases, though generally described as several separate oceans, these waters comprise one global, interconnected body of salt water sometimes referred to as the World Ocean or global ocean.
This concept of a body of water with relatively free interchange among its parts is of fundamental importance to oceanography. The major oceanic divisions – listed below in descending order of area and volume – are defined in part by the continents, various archipelagos, Oceans are fringed by smaller, adjoining bodies of water such as seas, bays and straits. The Mid-Oceanic Ridge of the World are connected and form the Ocean Ridge, the continuous mountain range is 65,000 km long, and the total length of the oceanic ridge system is 80,000 km long