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Of The Sea

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In open water, when the wind blows continuously as happens in the Southern Hemisphere in the Roaring Forties , long, organised masses of water called swell roll across the ocean.

The size of the waves depends on the fetch , the distance that the wind has blown over the water and the strength and duration of that wind. When waves meet others coming from different directions, interference between the two can produce broken, irregular seas.

The top of a wave is known as the crest, the lowest point between waves is the trough and the distance between the crests is the wavelength.

The wave is pushed across the surface of the sea by the wind, but this represents a transfer of energy and not a horizontal movement of water.

As waves approach land and move into shallow water , they change their behavior. If approaching at an angle, waves may bend refraction or wrap rocks and headlands diffraction.

When the wave reaches a point where its deepest oscillations of the water contact the seabed , they begin to slow down.

This pulls the crests closer together and increases the waves' height , which is called wave shoaling.

When the ratio of the wave's height to the water depth increases above a certain limit, it " breaks ", toppling over in a mass of foaming water.

A tsunami is an unusual form of wave caused by an infrequent powerful event such as an underwater earthquake or landslide, a meteorite impact, a volcanic eruption or a collapse of land into the sea.

These events can temporarily lift or lower the surface of the sea in the affected area, usually by a few feet. The potential energy of the displaced seawater is turned into kinetic energy, creating a shallow wave, a tsunami, radiating outwards at a velocity proportional to the square root of the depth of the water and which therefore travels much faster in the open ocean than on a continental shelf.

A trigger event on the continental shelf may cause a local tsunami on the land side and a distant tsunami that travels out across the ocean. The energy of the wave is dissipated only gradually, but is spread out over the wave front, so as the wave radiates away from the source, the front gets longer and the average energy reduces, so distant shores will, on average, be hit by weaker waves.

However, as the speed of the wave is controlled by the water depth, it does not travel at the same speed in all directions, and this affects the direction of the wave front — an effect known as refraction — which can focus the strength of the advancing tsunami on some areas and weaken it in others according to undersea topography.

As a tsunami moves into shallower water its speed decreases, its wavelength shortens and its amplitude increases enormously, [46] behaving in the same way as a wind-generated wave in shallow water, but on a vastly greater scale.

Either the trough or the crest of a tsunami can arrive at the coast first. Much of the destruction may be caused by the flood water draining back into the sea after the tsunami has struck, dragging debris and people with it.

Often several tsunami are caused by a single geological event and arrive at intervals of between eight minutes and two hours. The first wave to arrive on shore may not be the biggest or most destructive.

Wind blowing over the surface of the sea causes friction at the interface between air and sea. Not only does this cause waves to form but it also makes the surface seawater move in the same direction as the wind.

Although winds are variable, in any one place they predominantly blow from a single direction and thus a surface current can be formed.

Westerly winds are most frequent in the mid-latitudes while easterlies dominate the tropics. There are five main gyres in the world's oceans: two in the Pacific, two in the Atlantic and one in the Indian Ocean.

Other smaller gyres are found in lesser seas and a single gyre flows around Antarctica. These gyres have followed the same routes for millennia, guided by the topography of the land, the wind direction and the Coriolis effect.

The surface currents flow in a clockwise direction in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. The water moving away from the equator is warm, and that flowing in the reverse direction has lost most of its heat.

These currents tend to moderate the Earth's climate, cooling the equatorial region and warming regions at higher latitudes.

Surface currents only affect the top few hundred metres yards of the sea, but there are also large-scale flows in the ocean depths caused by the movement of deep water masses.

A main deep ocean current flows through all the world's oceans and is known as the thermohaline circulation or global conveyor belt.

This movement is slow and is driven by differences in density of the water caused by variations in salinity and temperature. Both these factors make it denser, and the water sinks.

From the deep sea near Greenland, such water flows southwards between the continental landmasses on either side of the Atlantic. When it reaches the Antarctic, it is joined by further masses of cold, sinking water and flows eastwards.

It then splits into two streams that move northwards into the Indian and Pacific Oceans. Here it is gradually warmed, becomes less dense, rises towards the surface and loops back on itself.

It takes a thousand years for this circulation pattern to be completed. Besides gyres, there are temporary surface currents that occur under specific conditions.

When waves meet a shore at an angle, a longshore current is created as water is pushed along parallel to the coastline. The water swirls up onto the beach at right angles to the approaching waves but drains away straight down the slope under the effect of gravity.

The larger the breaking waves, the longer the beach and the more oblique the wave approach, the stronger is the longshore current. It may occur at a gap in a sandbar or near a man-made structure such as a groyne.

This cold water is often rich in nutrients and creates blooms of phytoplankton and a great increase in the productivity of the sea. Tides are the regular rise and fall in water level experienced by seas and oceans in response to the gravitational influences of the Moon and the Sun, and the effects of the Earth's rotation.

During each tidal cycle, at any given place the water rises to a maximum height known as "high tide" before ebbing away again to the minimum "low tide" level.

As the water recedes, it uncovers more and more of the foreshore , also known as the intertidal zone. The difference in height between the high tide and low tide is known as the tidal range or tidal amplitude.

Most places experience two high tides each day, occurring at intervals of about 12 hours and 25 minutes.

This is half the 24 hours and 50 minute period that it takes for the Earth to make a complete revolution and return the Moon to its previous position relative to an observer.

On the opposite side of the Earth, the lunar force is at its weakest and this causes another bulge to form. As the Moon rotates around the Earth, so do these ocean bulges move around the Earth.

The gravitational attraction of the Sun is also working on the seas, but its effect on tides is less powerful than that of the Moon, and when the Sun, Moon and Earth are all aligned full moon and new moon , the combined effect results in the high "spring tides".

Tidal flows of seawater are resisted by the water's inertia and can be affected by land masses. In places like the Gulf of Mexico where land constrains the movement of the bulges, only one set of tides may occur each day.

Inshore from an island there may be a complex daily cycle with four high tides. The island straits at Chalkis on Euboea experience strong currents which abruptly switch direction, generally four times per day but up to 12 times per day when the moon and the sun are 90 degrees apart.

Although tides are regular and predictable, the height of high tides can be lowered by offshore winds and raised by onshore winds.

The high pressure at the centre of an anticyclones pushes down on the water and is associated with abnormally low tides while low-pressure areas may cause extremely high tides.

The Earth is composed of a magnetic central core , a mostly liquid mantle and a hard rigid outer shell or lithosphere , which is composed of the Earth's rocky crust and the deeper mostly solid outer layer of the mantle.

On land the crust is known as the continental crust while under the sea it is known as the oceanic crust. The latter is composed of relatively dense basalt and is some five to ten kilometres three to six miles thick.

The relatively thin lithosphere floats on the weaker and hotter mantle below and is fractured into a number of tectonic plates.

Parallel to these ridges and nearer the coasts, one oceanic plate may slide beneath another oceanic plate in a process known as subduction.

Deep trenches are formed here and the process is accompanied by friction as the plates grind together. The movement proceeds in jerks which cause earthquakes, heat is produced and magma is forced up creating underwater mountains, some of which may form chains of volcanic islands near to deep trenches.

Near some of the boundaries between the land and sea, the slightly denser oceanic plates slide beneath the continental plates and more subduction trenches are formed.

As they grate together, the continental plates are deformed and buckle causing mountain building and seismic activity. It occurs where the oceanic Nazca Plate slides under the continental South American Plate and is associated with the upthrust and volcanic activity of the Andes.

The zone where land meets sea is known as the coast and the part between the lowest spring tides and the upper limit reached by splashing waves is the shore.

A beach is the accumulation of sand or shingle on the shore. The indentation of a coastline, especially between two headlands, is a bay , a small bay with a narrow inlet is a cove and a large bay may be referred to as a gulf.

Normally, waves roll towards the shore at the rate of six to eight per minute and these are known as constructive waves as they tend to move material up the beach and have little erosive effect.

Storm waves arrive on shore in rapid succession and are known as destructive waves as the swash moves beach material seawards. Under their influence, the sand and shingle on the beach is ground together and abraded.

Around high tide, the power of a storm wave impacting on the foot of a cliff has a shattering effect as air in cracks and crevices is compressed and then expands rapidly with release of pressure.

At the same time, sand and pebbles have an erosive effect as they are thrown against the rocks. This tends to undercut the cliff, and normal weathering processes such as the action of frost follows, causing further destruction.

Gradually, a wave-cut platform develops at the foot of the cliff and this has a protective effect, reducing further wave-erosion.

Material worn from the margins of the land eventually ends up in the sea. Here it is subject to attrition as currents flowing parallel to the coast scour out channels and transport sand and pebbles away from their place of origin.

Sediment carried to the sea by rivers settles on the seabed causing deltas to form in estuaries. All these materials move back and forth under the influence of waves, tides and currents.

Governments make efforts to prevent flooding of the land by the building of breakwaters , seawalls , dykes and levees and other sea defences.

For instance, the Thames Barrier is designed to protect London from a storm surge, [69] while the failure of the dykes and levees around New Orleans during Hurricane Katrina created a humanitarian crisis in the United States.

Land reclamation in Hong Kong also permitted the construction of Hong Kong International Airport through the leveling and expansion of two smaller islands.

Over most of geologic time, the sea level has been higher than it is today. For at least the last years, sea level has been rising at an average rate of about 1.

Additional contributions, as much as one quarter of the total, come from water sources on land, such as melting snow and glaciers and extraction of groundwater for irrigation and other agricultural and human needs.

The sea plays a part in the water or hydrological cycle , in which water evaporates from the ocean, travels through the atmosphere as vapour, condenses , falls as rain or snow , thereby sustaining life on land, and largely returns to the sea.

In central Asia and other large land masses, there are endorheic basins which have no outlet to the sea, separated from the ocean by mountains or other natural geologic features that prevent the water draining away.

The Caspian Sea is the largest one of these. Its main inflow is from the River Volga , there is no outflow and the evaporation of water makes it saline as dissolved minerals accumulate.

The Aral Sea in Kazakhstan and Uzbekistan, and Pyramid Lake in the western United States are further examples of large, inland saline water-bodies without drainage.

Some endorheic lakes are less salty, but all are sensitive to variations in the quality of the inflowing water. Oceans contain the greatest quantity of actively cycled carbon in the world and are second only to the lithosphere in the amount of carbon they store.

The deep layer's concentration of dissolved inorganic carbon is about 15 percent higher than that of the surface layer [79] and it remains there for much longer periods of time.

Carbon enters the ocean as atmospheric carbon dioxide dissolves in the surface layers and is converted into carbonic acid , carbonate , and bicarbonate : [81].

It can also enter through rivers as dissolved organic carbon and is converted by photosynthetic organisms into organic carbon. This can either be exchanged throughout the food chain or precipitated into the deeper, more carbon rich layers as dead soft tissue or in shells and bones as calcium carbonate.

It circulates in this layer for long periods of time before either being deposited as sediment or being returned to surface waters through thermohaline circulation.

Seawater is slightly alkaline and had an average pH of about 8. One important element for the formation of skeletal material in marine animals is calcium , but calcium carbonate becomes more soluble with pressure, so carbonate shells and skeletons dissolve below its compensation depth.

Affected planktonic organisms will include the snail-like molluscs known as pteropods , and single-celled algae called coccolithophorids and foraminifera.

All of these are important parts of the food chain and a diminution in their numbers will have significant consequences. In tropical regions, corals are likely to be severely affected as it becomes more difficult to build their calcium carbonate skeletons, [91] in turn adversely impacting other reef dwellers.

The current rate of ocean chemistry change appears to be without precedent in Earth's geological history, making it unclear how well marine ecosystems will be able to adapt to the shifting conditions of the near future.

The oceans are home to a diverse collection of life forms that use it as a habitat. Since sunlight illuminates only the upper layers, the major part of the ocean exists in permanent darkness.

As the different depth and temperature zones each provide habitat for a unique set of species, the marine environment as a whole encompasses an immense diversity of life.

Marine life plays an important part in the carbon cycle as photosynthetic organisms convert dissolved carbon dioxide into organic carbon and it is economically important to humans for providing fish for use as food.

Life may have originated in the sea and all the major groups of animals are represented there. Scientists differ as to precisely where in the sea life arose: the Miller-Urey experiments suggested a dilute chemical "soup" in open water, but more recent suggestions include volcanic hot springs, fine-grained clay sediments, or deep-sea " black smoker " vents, all of which would have provided protection from damaging ultraviolet radiation which was not blocked by the early Earth's atmosphere.

Marine habitats can be divided horizontally into coastal and open ocean habitats. Coastal habitats extend from the shoreline to the edge of the continental shelf.

Most marine life is found in coastal habitats, even though the shelf area occupies only 7 percent of the total ocean area. Open ocean habitats are found in the deep ocean beyond the edge of the continental shelf.

Alternatively, marine habitats can be divided vertically into pelagic open water , demersal just above the seabed and benthic sea bottom habitats.

A third division is by latitude : from polar seas with ice shelves, sea ice and icebergs, to temperate and tropical waters. Coral reefs, the so-called "rainforests of the sea", occupy less than 0.

Marine primary producers — plants and microscopic organisms in the plankton — are widespread and very essential for the ecosystem. It has been estimated that half of the world's oxygen is produced by phytoplankton.

The marine nitrogen cycle consists of complex microbial transformations which include the fixation of nitrogen , its assimilation, nitrification , anammox and denitrification.

This means that the most productive areas, rich in plankton and therefore also in fish, are mainly coastal. There is a broader spectrum of higher animal taxa in the sea than on land, many marine species have yet to be discovered and the number known to science is expanding annually.

In fact, the oceans teem with life and provide many varying microhabitats. The pelagic zone contains macro- and microfauna and myriad zooplankton which drift with the currents.

Most of the smallest organisms are the larvae of fish and marine invertebrates which liberate eggs in vast numbers because the chance of any one embryo surviving to maturity is so minute.

The demersal zone supports many animals that feed on benthic organisms or seek protection from predators and the seabed provides a range of habitats on or under the surface of the substrate which are used by creatures adapted to these conditions.

The tidal zone with its periodic exposure to the dehydrating air is home to barnacles , molluscs and crustaceans. The neritic zone has many organisms that need light to flourish.

Here, among algal encrusted rocks live sponges , echinoderms , polychaete worms, sea anemones and other invertebrates.

Corals often contain photosynthetic symbionts and live in shallow waters where light penetrates. The extensive calcareous skeletons they extrude build up into coral reefs which are an important feature of the seabed.

These provide a biodiverse habitat for reef dwelling organisms. There is less sea life on the floor of deeper seas but marine life also flourishes around seamounts that rise from the depths, where fish and other animals congregate to spawn and feed.

Close to the seabed live demersal fish that feed largely on pelagic organisms or benthic invertebrates. Some like the detrivores rely on organic material falling to the ocean floor.

Others cluster round deep sea hydrothermal vents where mineral-rich flows of water emerge from the seabed, supporting communities whose primary producers are sulphide-oxidising chemoautotrophic bacteria, and whose consumers include specialised bivalves, sea anemones, barnacles, crabs, worms and fish, often found nowhere else.

Such places support unique biomes where many new microbes and other lifeforms have been discovered. Humans have travelled the seas since they first built sea-going craft.

Mesopotamians were using bitumen to caulk their reed boats and, a little later, masted sails. Christopher Columbus sailed from Cadiz in , attempting to reach the eastern lands of India and Japan by the novel means of travelling westwards.

He made landfall instead on an island in the Caribbean Sea and a few years later, the Venetian navigator John Cabot reached Newfoundland.

The Italian Amerigo Vespucci , after whom America was named, explored the South American coastline in voyages made between and , discovering the mouth of the Amazon River.

As for the history of navigational instrument , a compass was first used by the ancient Greeks and Chinese to show where north lies and the direction in which the ship is heading.

The longitude a line on the globe joining the two poles could only be calculated with an accurate chronometer to show the exact time difference between the ship and a fixed point such as the Greenwich Meridian.

In , John Harrison , a clockmaker, designed such an instrument and James Cook used it in his voyages of exploration.

With regards to maps that are vital for navigation, in the second century, Ptolemy mapped the whole known world from the "Fortunatae Insulae", Cape Verde or Canary Islands , eastward to the Gulf of Thailand.

This map was used in when Christopher Columbus set out on his voyages of discovery. Scientific study has continued with the depth recordings of the Tuscarora , the oceanic research of the Challenger voyages — , the work of the Scandinavian seamen Roald Amundsen and Fridtjof Nansen , the Michael Sars expedition in , the German Meteor expedition of , the Antarctic survey work of Discovery II in , and others since.

Scientific oceanography began with the voyages of Captain James Cook from to , describing the Pacific with unprecedented precision from 71 degrees South to 71 degrees North.

This was proven wrong by the British biologists W. Carpenter and C. Wyville Thomson , who in discovered life in deep water by dredging.

At great depths, no light penetrates through the water layers from above and the pressure is extreme. For deep sea exploration it is necessary to use specialist vehicles, either remotely operated underwater vehicles with lights and cameras or manned submersibles.

They have viewing ports, 5,watt lights, video equipment and manipulator arms for collecting samples, placing probes or pushing the vehicle across the sea bed when the thrusters would stir up excessive sediment.

Bathymetry is the mapping and study of the topography of the ocean floor. Methods used for measuring the depth of the sea include single or multibeam echosounders , laser airborne depth sounders and the calculation of depths from satellite remote sensing data.

This information is used for determining the routes of undersea cables and pipelines, for choosing suitable locations for siting oil rigs and offshore wind turbines and for identifying possible new fisheries.

Ongoing oceanographic research includes marine lifeforms, conservation, the marine environment, the chemistry of the ocean, the studying and modelling of climate dynamics, the air-sea boundary, weather patterns, ocean resources, renewable energy, waves and currents, and the design and development of new tools and technologies for investigating the deep.

It stresses freedom to navigate the oceans and disapproves of war fought in international waters. Article 87 1 states: "The high seas are open to all states , whether coastal or land-locked.

Its objectives include developing and maintaining a regulatory framework for shipping, maritime safety, environmental concerns, legal matters, technical co-operation and maritime security.

Within this area, the coastal nation has sole exploitation rights over all natural resources. The "continental shelf" is the natural prolongation of the land territory to the continental margin 's outer edge, or nautical miles from the coastal state's baseline, whichever is greater.

Here the coastal nation has the exclusive right to harvest minerals and also living resources "attached" to the seabed.

Control of the sea is important to the security of a maritime nation, and the naval blockade of a port can be used to cut off food and supplies in time of war.

Battles have been fought on the sea for more than 3, years. In about B. Battle of Salamis , the Greek general Themistocles trapped the far larger fleet of the Persian king Xerxes in a narrow channel and attacked vigorously, destroying Persian ships for the loss of 40 Greek vessels.

With steam and the industrial production of steel plate came greatly increased firepower in the shape of the dreadnought battleships armed with long-range guns.

Submarines became important in naval warfare in World War I, when German submarines, known as U-boats , sank nearly 5, Allied merchant ships, [] including however the RMS Lusitania , so helping to bring the United States into the war.

Some of these are kept permanently on patrol. Sailing ships or packets carried mail overseas, one of the earliest being the Dutch service to Batavia in the s.

Later, scheduled services were offered but the time journeys took depended much on the weather. When steamships replaced sailing vessels, ocean-going liners took over the task of carrying people.

By the beginning of the twentieth century, crossing the Atlantic took about five days and shipping companies competed to own the largest and fastest vessels.

The Blue Riband was an unofficial accolade given to the fastest liner crossing the Atlantic in regular service.

The Mauretania held the title with The great liners were comfortable but expensive in fuel and staff. The age of the trans-Atlantic liners waned as cheap intercontinental flights became available.

In , a regular scheduled air service between New York and Paris taking seven hours doomed the Atlantic ferry service to oblivion.

One by one the vessels were laid up, some were scrapped, others became cruise ships for the leisure industry and still others floating hotels.

Maritime trade has existed for millennia. The Ptolemaic dynasty had developed trade with India using the Red Sea ports and in the first millennium BC the Arabs , Phoenicians, Israelites and Indians traded in luxury goods such as spices, gold, and precious stones.

With the collapse of the Roman Empire, European trade dwindled but it continued to flourish among the kingdoms of Africa, the Middle East, India, China and southeastern Asia.

Nowadays, large quantities of goods are transported by sea, especially across the Atlantic and around the Pacific Rim. A major trade route passes through the Pillars of Hercules , across the Mediterranean and the Suez Canal to the Indian Ocean and through the Straits of Malacca ; much trade also passes through the English Channel.

Over 60 percent of the world's container traffic is conveyed on the top twenty trade routes. There are two main kinds of freight, bulk cargo and break bulk or general cargo, most of which is now transported in containers.

Commodities in the form of liquids, powder or particles are carried loose in the holds of bulk carriers and include oil, grain, coal, ore, scrap metal, sand and gravel.

Break bulk cargo is usually manufactured goods and is transported in packages, often stacked on pallets. Before the arrival of containerization in the s, these goods were loaded, transported and unloaded piecemeal.

Fish and other fishery products are among the most widely consumed sources of protein and other essential nutrients. Modern fishing vessels include fishing trawlers with a small crew, stern trawlers, purse seiners, long-line factory vessels and large factory ships which are designed to stay at sea for weeks, processing and freezing great quantities of fish.

The equipment used to capture the fish may be purse seines , other seines, trawls , dredges, gillnets and long-lines and the fish species most frequently targeted are herring , cod , anchovy , tuna , flounder , mullet , squid and salmon.

Overexploitation itself has become a serious concern; it does not only cause the depletion of fish stocks, but also substantially reduce the size of predatory fish populations.

Artisan fishing methods include rod and line, harpoons, skin diving, traps, throw nets and drag nets. Traditional fishing boats are powered by paddle, wind or outboard motors and operate in near-shore waters.

The Food and Agriculture Organization is encouraging the development of local fisheries to provide food security to coastal communities and help alleviate poverty.

About six hundred species of plants and animals were cultured, some for use in seeding wild populations. The animals raised included finfish , aquatic reptiles , crustaceans, molluscs, sea cucumbers , sea urchins , sea squirts and jellyfish.

Mesh enclosures for finfish can be suspended in the open seas, cages can be used in more sheltered waters or ponds can be refreshed with water at each high tide.

Shrimps can be reared in shallow ponds connected to the open sea. Oysters can be reared on trays or in mesh tubes.

Sea cucumbers can be ranched on the seabed. In the s, disease wiped out China's farmed Farrer's scallop and white shrimp and required their replacement by other species.

Use of the sea for leisure developed in the nineteenth century, and became a significant industry in the twentieth century. Humans enjoy venturing into the sea; children paddle and splash in the shallows and many people take pleasure in bathing and relaxing on the beach.

This was not always the case, with sea bathing becoming the vogue in Europe in the 18th century after Dr. William Buchan advocated the practice for health reasons.

Other marine water sports include kite surfing , where a power kite propels a manned board across the water, [] windsurfing , where the power is provided by a fixed, manoeuvrable sail [] and water skiing , where a powerboat is used to pull a skier.

Beneath the surface, freediving is necessarily restricted to shallow descents. Other useful equipment includes fins and snorkels , and scuba equipment allows underwater breathing and hence a longer time can be spent beneath the surface.

Deeper dives can be made with specialised equipment and training. The sea offers a very large supply of energy carried by ocean waves , tides , salinity differences, and ocean temperature differences which can be harnessed to generate electricity.

Tidal power uses generators to produce electricity from tidal flows, sometimes by using a dam to store and then release seawater. The Rance barrage, 1 kilometre 0.

The large and highly variable energy of waves gives them enormous destructive capability, making affordable and reliable wave machines problematic to develop.

It was soon damaged by waves, then destroyed by a storm. Offshore wind power is captured by wind turbines placed out at sea; it has the advantage that wind speeds are higher than on land, though wind farms are more costly to construct offshore.

Electricity power stations are often located on the coast or beside an estuary so that the sea can be used as a heat sink. A colder heat sink enables more efficient power generation, which is important for expensive nuclear power plants in particular.

The seabed contains enormous reserves of minerals which can be exploited by dredging. This has advantages over land-based mining in that equipment can be built at specialised shipyards and infrastructure costs are lower.

Disadvantages include problems caused by waves and tides, the tendency for excavations to silt up and the washing away of spoil heaps.

There is a risk of coastal erosion and environmental damage. Seafloor massive sulphide deposits are potential sources of silver , gold , copper , lead and zinc and trace metals since their discovery in the s.

They form when geothermally heated water is emitted from deep sea hydrothermal vents known as "black smokers". The ores are of high quality but prohibitively costly to extract.

There are large deposits of petroleum , as oil and natural gas , in rocks beneath the seabed. Offshore platforms and drilling rigs extract the oil or gas and store it for transport to land.

Offshore oil and gas production can be difficult due to the remote, harsh environment. Animals may be disorientated by seismic waves used to locate deposits, and there is debate as to whether this causes the beaching of whales.

The infrastructure may cause damage, and oil may be spilt. In the Pacific these may cover up to 30 percent of the deep ocean floor. The minerals precipitate from seawater and grow very slowly.

Their commercial extraction for nickel was investigated in the s but abandoned in favour of more convenient sources. In deeper waters, mobile seafloor crawlers are used and the deposits are pumped to a vessel above.

In Namibia, more diamonds are now collected from marine sources than by conventional methods on land. The sea holds enormous quantities of valuable dissolved minerals.

Bromine , accumulated after being leached from the land, is economically recovered from the Dead Sea, where it occurs at 55, parts per million ppm.

Desalination is the technique of removing salts from seawater to leave fresh water suitable for drinking or irrigation. The two main processing methods, vacuum distillation and reverse osmosis , use large quantities of energy.

Desalination is normally only undertaken where fresh water from other sources is in short supply or energy is plentiful, as in the excess heat generated by power stations.

The brine produced as a by-product contains some toxic materials and is returned to the sea. Many substances enter the sea as a result of human activities.

Combustion products are transported in the air and deposited into the sea by precipitation. Industrial outflows and sewage contribute heavy metals , pesticides , PCBs , disinfectants , household cleaning products and other synthetic chemicals.

These become concentrated in the surface film and in marine sediment, especially estuarine mud. The result of all this contamination is largely unknown because of the large number of substances involved and the lack of information on their biological effects.

Much floating plastic rubbish does not biodegrade , instead disintegrating over time and eventually breaking down to the molecular level.

Rigid plastics may float for years. Turtles and whales have been found with plastic bags and fishing line in their stomachs. Microplastics may sink, threatening filter feeders on the seabed.

Most oil pollution in the sea comes from cities and industry. It can clog the feathers of sea birds, reducing their insulating effect and the birds' buoyancy, and be ingested when they preen themselves in an attempt to remove the contaminant.

Marine mammals are less seriously affected but may be chilled through the removal of their insulation, blinded, dehydrated or poisoned.

Benthic invertebrates are swamped when the oil sinks, fish are poisoned and the food chain is disrupted. In the short term, oil spills result in wildlife populations being decreased and unbalanced, leisure activities being affected and the livelihoods of people dependent on the sea being devastated.

In the Gulf of Mexico , where oil-eating bacteria are already present, they take only a few days to consume spilt oil. Run-off of fertilisers from agricultural land is a major source of pollution in some areas and the discharge of raw sewage has a similar effect.

The extra nutrients provided by these sources can cause excessive plant growth. Nitrogen is often the limiting factor in marine systems, and with added nitrogen, algal blooms and red tides can lower the oxygen level of the water and kill marine animals.

Such events have created dead zones in the Baltic Sea and the Gulf of Mexico. The Irish Sea was contaminated by radioactive caesium from the former Sellafield nuclear fuel processing plant [] and nuclear accidents may also cause radioactive material to seep into the sea, as did the disaster at the Fukushima Daiichi Nuclear Power Plant in The dumping of waste including oil, noxious liquids, sewage and garbage at sea is governed by international law.

The London Convention is a United Nations agreement to control ocean dumping which had been ratified by 89 countries by 8 June Several nomadic indigenous groups in Maritime Southeast Asia live in boats and derive nearly all they need from the sea.

The indigenous peoples of the Arctic such as the Chukchi , Inuit , Inuvialuit and Yup'iit hunt marine mammals including seals and whales, [] and the Torres Strait Islanders of Australia include ownership of the Great Barrier Reef among their possessions.

They live a traditional life on the islands involving hunting, fishing, gardening and trading with neighbouring peoples in Papua and mainland Aboriginal Australians.

The sea appears in human culture in contradictory ways, as both powerful but serene and as beautiful but dangerous.

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.

Music too has been inspired by the ocean, sometimes by composers who lived or worked near the shore and saw its many different aspects.

Sea shanties , songs that were chanted by mariners to help them perform arduous tasks, have been woven into compositions and impressions in music have been created of calm waters, crashing waves and storms at sea.

As a symbol, the sea has for centuries played a role in literature , poetry and dreams. Sometimes it is there just as a gentle background but often it introduces such themes as storm, shipwreck, battle, hardship, disaster, the dashing of hopes and death.

From Wikipedia, the free encyclopedia. For other uses, see Sea disambiguation and The Sea disambiguation. Not to be confused with Ocean or World Ocean.

Please do not move this article until the discussion is closed. Large body of salt water. Further information: List of seas.

Main articles: Oceanography and Physical oceanography. Main article: Seawater. Play media. Main article: Wind wave. Main article: Tsunami.

Main article: Ocean current. Main article: Tide. Main article: Sea level. Main article: Water cycle. Main article: Oceanic carbon cycle. Main article: Ocean acidification.

Main article: Marine life. Main article: Marine habitats. Main articles: History of navigation , History of cartography , Maritime history , Ancient maritime history , and Ocean exploration.

Main article: Deep-sea exploration. Main article: Naval warfare. Main articles: Shipping and Trade. Main articles: Fishing , Whaling , Aquaculture , and Seaweed farming.

Main articles: Cruising maritime , Sailing , and Recreational boat fishing. Main articles: Marine energy and Offshore wind power. Main articles: Offshore drilling and Deep sea mining.

Main article: Marine pollution. Main article: Sea in culture. Oceans portal Water portal Geography portal. One definition is that a sea is a sub-division of an ocean, which means that it must have oceanic basin crust on its floor.

This definition accepts the Caspian as a sea because it was once part of an ancient ocean. Experiments to recreate the conditions of the lower mantle suggest it may contain still more water as well, as much as five times the mass of water present in the world's oceans.

Gradually, they fall in with other waves travelling at similar speed—-where different waves are in phase they reinforce each other, and where out of phase they are reduced.

Eventually, a regular pattern of high and low waves or swell is developed that remains constant as it travels out across the ocean. Ocean Power Magazine.

Archived from the original on 13 July Retrieved 28 February Retrieved 13 March Ocean facts. National Oceanic and Atmospheric Administration.

Retrieved 19 April Encyclopedia of the Oceans. Oxford University Press. Chemical Geology. Martinus Nijhoff Publishers. View All Results. Your vacation starts in sun-soaked Miami, where days begin with hot shots of Cuban cafecito and end in neon nights filled with incredible global flavors, glitzy Art Deco sights, and plenty of dancing.

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