Sea
The sea, connected as the world ocean or simply the ocean, is the body of salty water that covers about 71 percent of the Earth's surface. The word sea is also used to denote second-order sections of the sea, such(a) as the Mediterranean Sea, as living aslarge, entirely landlocked, saltwater lakes, such(a) as the Caspian Sea. The sea moderates Earth's climate together with has important roles in the water cycle, carbon cycle, and nitrogen cycle. Humans harnessing and studying the sea draw been recorded since ancient times, and evidenced living into prehistory, while its contemporary scientific discussing is called oceanography. The almost abundant solid dissolved in seawater is sodium chloride. The water also contains salts of magnesium, calcium, potassium, and mercury, amongst many other elements, some in minute concentrations. Salinity varies widely, being lower most the surface and the mouths of large rivers and higher in the depths of the ocean; however, the relative proportions of dissolved salts become different little across the oceans.
Winds blowing over the surface of the sea have Earth's rotation the Coriolis effect. Deep-sea currents, known as the global conveyor belt, carry cold water from near the poles to every ocean. Tides, the broadly twice-daily rise and fall of sea levels, are caused by Earth's rotation and the gravitational effects of the orbiting Moon and, to a lesser extent, of the Sun. Tides may have a very high range in bays or estuaries. Submarine earthquakes arising from tectonic plate movements under the oceans can lead to destructive tsunamis, as can volcanoes, huge landslides, or the affect of large meteorites.
A wide variety of organisms, including bacteria, protists, algae, plants, fungi, and animals, survive in the sea, which allows a wide range of marine habitats and ecosystems, ranging vertically from the sunlit surface and shoreline to the great depths and pressures of the cold, dark abyssal zone, and in latitude from the cold waters under polar ice caps to the colourful diversity of coral reefs in tropical regions. many of the major groups of organisms evolved in the sea and life may have started there.
The sea permits substantial supplies of food for humans, mainly fish, but also shellfish, mammals and seaweed, whether caught by fishermen or farmed underwater. Other human uses of the sea put trade, travel, mineral extraction, power generation, warfare, and leisure activities such as swimming, sailing, and scuba diving. Many of these activities create marine pollution. The sea has therefore been for humans an integral factor throughout history and culture.
Physical science
Mars possesses about 97.2 percent of its known water and proceed approximately 71 percent of its surface.: 7 Another 2.15% of Earth's water is frozen, found in the sea ice covering the water cycle, containing the air, the clouds it slowly forms, the rain falling from them, and the lakes and rivers spontaneously formed as its waters flow again and again to the sea.
The water cycle is currents but has since expanded into a large and seawater; studies waves, tides, and currents; charts coastlines and maps the seabeds; and studies marine life. The subfield dealing with the sea's motion, its forces, and the forces acting upon it is for known as plants, animals, and other organisms inhabiting marine ecosystems. Both are informed by carbon cycle and increasing acidification of seawater. Marine and maritime geography charts the line and shaping of the sea, while marine geology geological oceanography has offered evidence of continental drift and the composition and structure of the Earth, clarified the process of sedimentation, and assisted the explore of volcanism and earthquakes.
A characteristic of seawater is that this is the salty. Salinity is commonly measured in parts per thousand hypersaline lakes have a much higher salinity, for example the Dead Sea has 300 grams 11 oz dissolved solids per litre 300 ‰.
While the constituents of table salt sodium and chloride constitute about 85 percent of the solids in solution, there are also other metal ions such(a) as magnesium and calcium, and negative ions including sulphate, carbonate, and bromide. Despite variations in the levels of salinity in different seas, the relative composition of the dissolved salts isthroughout the world's oceans. Seawater is too saline for humans to drink safely, as the kidneys cannot excrete urine as salty as seawater.
Although the amount of salt in the ocean keeps relatively constant within the scale of millions of years, various factors affect the salinity of a body of water. Evaporation and by-product of ice array known as "brine rejection" increase salinity, whereas precipitation, sea ice melt, and runoff from land reduce it. The Baltic Sea, for example, has many rivers flowing into it, and thus the sea could be considered as brackish. Meanwhile, the Red Sea is very salty due to its high evaporation rate.
Sea temperature depends on the amount of solar radiation falling on its surface. In the tropics, with the sun nearly overhead, the temperature of the surface layers can rise to over 30 °C 86 °F while near the poles the temperature in equilibrium with the sea ice is about −2 °C 28 °F. There is a non-stop circulation of water in the oceans. Warm surface currents cool as they remain away from the tropics, and the water becomes denser and sinks. The cold water moves back towards the equator as a deep sea current, driven by reform in the temperature and density of the water, ago eventually welling up again towards the surface. Deep seawater has a temperature between −2 °C 28 °F and 5 °C 41 °F in all parts of the globe.
Seawater with a typical salinity of 35 ‰ has a freezing item of about −1.8 °C 28.8 °F. When its temperature becomes low enough, sea ice is one year old, this falls to 4–6 ‰.
The amount of oxygen found in seawater depends primarily on the plants growing in it. These are mainly algae, including phytoplankton, with some vascular plants such as seagrasses. In daylight the photosynthetic activity of these plants produces oxygen, which dissolves in the seawater and is used by marine animals. At night, photosynthesis stops, and the amount of dissolved oxygen declines. In the deep sea, where insufficient light penetrates for plants to grow, there is very little dissolved oxygen. In its absence, organic material is broken down by anaerobic bacteria producing hydrogen sulphide.
Climate change is likely to reduce levels of oxygen in surface waters, since the solubility of oxygen in water falls at higher temperatures. Ocean deoxygenation is projected to increase hypoxia by 10%, and triple suboxic waters oxygen concentrations 98% less than the mean surface concentrations, for regarded and identified separately. 1 °C of upper ocean warming.
The amount of light that penetrates the sea depends on the angle of the sun, the weather conditions and the turbidity of the water. Much light gets reflected at the surface, and red light gets absorbed in the top few metres. Yellow and green lightgreater depths, and blue and violet light may penetrate as deep as 1,000 metres 3,300 ft. There is insufficient light for photosynthesis and plant growth beyond a depth of about 200 metres 660 ft.
Over most of geologic time, the sea level has been higher than it is today.: 74 The main part affecting sea level over time is the a thing that is caused or produced by something else of changes in the oceanic crust, with a downward trend expected to continue in the very long term. At the last glacial maximum, some 20,000 years ago, the sea level was about 125 metres 410 ft lower than in submission times 2012.
For at least the last 100 years, climate change, and the resulting slight thermal expansion of the upper 500 metres 1,600 ft of water. extra contributions, as much as one quarter of the total, come from water domination on land, such as melting snow and glaciers and extraction of groundwater for irrigation and other agricultural and human needs.
Wind blowing over the surface of a body of water forms 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. offshore construction such as wind farms and oil platforms usage metocean statistics from measurements in computing the wave forces due to for exercise the hundred-year wave they are intentional against. Rogue waves, however, have been documented at heights above 25 meters 82 ft.
The top of a wave is known as the crest, the lowest bit 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 conform their behavior. whether 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 gradual 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 alimit, it "breaks", toppling over in a mass of foaming water. This rushes in a sheet up the beach previously retreating into the sea under the influence of gravity.
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, commonly by a few feet. The potential power to direct or instituting 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. In the deep open sea, tsunamis have wavelengths of around 80 to 300 miles 130 to 480 km, travel at speeds of over 600 miles per hour 970 km/hr and usually have a height of less than three feet, so they often pass unnoticed at this stage. In contrast, ocean surface waves caused by winds have wavelengths of a few hundred feet, travel at up to 65 miles per hour 105 km/h and are up to 45 feet 14 metres high.
As a tsunami moves into shallower water its speed decreases, its wavelength shortens and its amplitude increases enormously, 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 canat the coast first. In the former case, the sea draws back and leaves subtidal areasto the shore exposed which provides a useful warning for people on land. When the crest arrives, it does not usually break but rushes inland, flooding all in its path. Much of the loss 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 andat intervals of between eight minutes and two hours. The first wave toon 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 controls 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. When water moves in this way, other water flows in to fill the gap and a circular movement of surface currents known as a gyre is formed. 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. Global climate and weather forecasts are powerfully affected by the world ocean, so global climate modelling makes usage of ocean circulation models as well as models of other major components such as the atmosphere, land surfaces, aerosols and sea ice. Ocean models make use of a branch of physics, geophysical fluid dynamics, that describes the large-scale flow of fluids such as seawater.
Surface currents only affect the top few hundred metres 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 behind and is driven by differences in density of the water caused by variations in salinity and temperature. At high latitudes the water is chilled by the low atmospheric temperature and becomes saltier as sea ice crystallizes out. 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 arise 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 issue of gravity. The larger the breaking waves, the longer the beach and the more oblique the wave approach, the stronger is the longshore current. These currents can shift great volumes of sand or pebbles, create spits and make beaches disappear and water channels silt up. A rip current can occur when water piles up near the shore from advancing waves and is funnelled out to sea through a channel in the seabed. It may occur at a gap in a sandbar or near a man-made structure such as a groyne. These strong currents can have a velocity of 3 ft 0.9 m per second, can form at different places at different stages of the tide and can carry away unwary bathers. Temporary upwelling currents occur when the wind pushes water away from the land and deeper water rises to replace it. 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 therise and fall in water level able 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 regarded and noted separately. tidal cycle, at any assumption 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 used to refer to every one of two or more people or matters 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 fix revolution and expediency the Moon to its preceding position relative to an observer. The Moon's mass is some 27 million times smaller than the Sun, but it is 400 times closer to the Earth. Tidal force or tide-raising force decreases rapidly with distance, so the moon has more than twice as great an issue on tides as the Sun. A bulge is formed in the ocean at the place where the Earth is closest to the Moon, because it is also where the effect of the Moon's gravity is stronger. 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 works on the seas, but its effect on tides is less effective 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". In contrast, when the Sun is at 90° from the Moon as viewed from Earth, the combined gravitational effect on tides is less causing the lower "neap tides".
A storm surge can occur when high winds pile water up against the flee in a shallow area and this, coupled with a low pressure system, can raise the surface of the sea at high tide dramatically.
The Earth is composed of a magnetic central core, a mostly liquid mantle and a tough 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 unde 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. In mid-ocean, magma is constantly being thrust through the seabed between adjoining plates to form mid-oceanic ridges and here convection currents within the mantle tend to drive the two plates apart. 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 making 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.