130-618: The Nautical Institute (NI) is an international professional organisation for maritime professionals, based in the United Kingdom . It was established in 1971 and has the status of a company limited by guarantee and is registered with the Charity Commission . It has over 7,000 members in over 110 countries. It publishes a members' newsletter "Seaways" and organizes meetings. There are four levels of membership for qualified maritime professionals; Sea A sea
260-517: A biodiverse range of larger and smaller animal life. Light is only able to penetrate the top 200 metres (660 ft) so this is the only part of the sea where plants can grow. The surface layers are often deficient in biologically active nitrogen compounds. The marine nitrogen cycle consists of complex microbial transformations which include the fixation of nitrogen , its assimilation, nitrification , anammox and denitrification. Some of these processes take place in deep water so that where there
390-683: A mediterranean sea ) or wholly (as inland seas ) enclosed by land . However, an exception to this is the Sargasso Sea which has no coastline and lies within a circular current, the North Atlantic Gyre . Seas are generally larger than lakes and contain salt water, but the Sea of Galilee is a freshwater lake . The United Nations Convention on the Law of the Sea states that all of the ocean
520-422: 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
650-592: A common impurity. Tin ores are rare, as reflected in the fact there were no tin bronzes in Western Asia before 3000 BCE. The Bronze Age forms part of the three-age system for prehistoric societies. In this system, it follows the Neolithic in some areas of the world. While copper is a common ore, deposits of tin are rare in the Old World , and often had to be traded or carried considerable distances from
780-400: 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
910-507: A large and multidisciplinary field: it examines the properties of 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 known as physical oceanography . Marine biology (biological oceanography) studies the plants , animals , and other organisms inhabiting marine ecosystems . Both are informed by chemical oceanography , which studies
1040-453: 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 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
1170-674: A much more evident Mesolithic era, lasting millennia. In Northern Europe , societies were able to live well on rich food supplies from the marshlands fostered by the warmer climate. Such conditions produced distinctive human behaviours that are preserved in the material record, such as the Maglemosian and Azilian cultures. These conditions also delayed the coming of the Neolithic until as late as 4000 BCE (6,000 BP ) in northern Europe. Remains from this period are few and far between, often limited to middens . In forested areas,
1300-534: A new world of creatures living on the seabed that scientists had not previously known to exist. 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. A dead whale sinking to
1430-536: A saline body of water and therefore solely a sea in the sense of the common use of the word, like all other saltwater lakes called sea. Earth is the only known planet with seas of liquid water on its surface, although Mars possesses ice caps and similar planets in other solar systems may have oceans. Earth's 1,335,000,000 cubic kilometers (320,000,000 cu mi) of sea contain about 97.2 percent of its known water and covers approximately 71 percent of its surface. Another 2.15% of Earth's water
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#17327731892641560-654: A single room. Settlements might have a surrounding stone wall to keep domesticated animals in and hostile tribes out. Later settlements have rectangular mud-brick houses where the family lived in single or multiple rooms. Burial findings suggest an ancestor cult with preserved skulls of the dead. The Vinča culture may have created the earliest system of writing. The megalithic temple complexes of Ġgantija are notable for their gigantic structures. Although some late Eurasian Neolithic societies formed complex stratified chiefdoms or even states, states evolved in Eurasia only with
1690-749: A single source. The emergence of metallurgy may have occurred first in the Fertile Crescent , where it gave rise to the Bronze Age in the 4th millennium BCE (the traditional view), although finds from the Vinča culture in Europe have now been securely dated to slightly earlier than those of the Fertile Crescent. Timna Valley contains evidence of copper mining 7,000 years ago. The process of transition from Neolithic to Chalcolithic in
1820-471: 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 impact of large meteorites . The seas have been an integral element for humans throughout history and culture. Humans harnessing and studying the seas have been recorded since ancient times and evidenced well into prehistory , while its modern scientific study
1950-414: A wide array of species including corals (only six of which contribute to reef formation). 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. About 45 percent of the sea's primary production of living material
2080-529: Is "sea". The law of the sea has at its center the definition of the boundaries of the ocean, clarifying its application in marginal seas . But what bodies of water other than the sea the law applies to is being crucially negotiated in the case of the Caspian Sea and its status as "sea", basically revolving around the issue of the Caspian Sea about either being factually an oceanic sea or only
2210-449: Is a continuous circulation of water in the oceans. Warm surface currents cool as they move 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 changes in the temperature and density of the water, before 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
2340-539: Is a large body of salt water . There are particular seas and the sea . The sea commonly refers to the Ocean , the interconnected body of seawaters that spans most of Earth. Particular seas are either marginal seas , second-order sections of the oceanic sea (e.g. the Mediterranean Sea ), or certain large, nearly landlocked bodies of water. The salinity of water bodies varies widely, being lower near
2470-399: Is a point of land jutting out into the sea and a larger promontory is known as a cape . 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 . Coastlines are influenced by several factors including the strength of the waves arriving on the shore, the gradient of the land margin,
2600-800: Is also a transition period between Stone Age and Bronze Age, the Chalcolithic or Copper Age. For the prehistory of the Americas see Pre-Columbian era . The notion of "prehistory" emerged during the Enlightenment in the work of antiquarians who used the word "primitive" to describe societies that existed before written records. The word "prehistory" first appeared in English in 1836 in the Foreign Quarterly Review . The geologic time scale for pre-human time periods, and
2730-516: Is an example. In archaeology, the Iron Age refers to the advent of ferrous metallurgy . The adoption of iron coincided with other changes, often including more sophisticated agricultural practices, religious beliefs and artistic styles, which makes the archaeological Iron Age coincide with the " Axial Age " in the history of philosophy. Although iron ore is common, the metalworking techniques necessary to use iron are different from those needed for
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#17327731892642860-487: Is an upwelling of cold waters, and also near estuaries where land-sourced nutrients are present, plant growth is higher. 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. Some vertebrates such as seabirds , seals and sea turtles return to
2990-488: Is anonymous. Because of this, reference terms that prehistorians use, such as " Neanderthal " or " Iron Age ", are modern labels with definitions sometimes subject to debate. The concept of a "Stone Age" is found useful in the archaeology of most of the world, although in the archaeology of the Americas it is called by different names and begins with a Lithic stage , or sometimes Paleo-Indian . The sub-divisions described below are used for Eurasia, and not consistently across
3120-556: Is called oceanography and maritime space is governed by the law of the sea , with admiralty law regulating human interactions at sea. The seas provide 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 seas include 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
3250-782: Is called the Lower Paleolithic (as in excavations it appears underneath the Upper Paleolithic), beginning with the earliest stone tools dated to around 3.3 million years ago at the Lomekwi site in Kenya. These tools predate the genus Homo and were probably used by Kenyanthropus . Evidence of control of fire by early hominins during the Lower Palaeolithic Era is uncertain and has at best limited scholarly support. The most widely accepted claim
3380-478: Is contributed by diatoms . Much larger algae, commonly known as seaweeds , are important locally; Sargassum forms floating drifts, while kelp form seabed forests. Flowering plants in the form of seagrasses grow in " meadows " in sandy shallows, mangroves line the coast in tropical and subtropical regions and salt-tolerant plants thrive in regularly inundated salt marshes . All of these habitats are able to sequester large quantities of carbon and support
3510-481: Is frozen, found in the sea ice covering the Arctic Ocean , the ice cap covering Antarctica and its adjacent seas , and various glaciers and surface deposits around the world. The remainder (about 0.65% of the whole) form underground reservoirs or various stages of the water cycle , containing the freshwater encountered and used by most terrestrial life : vapor in the air , the clouds it slowly forms,
3640-401: 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 each 1 °C of upper-ocean warming. The amount of light that penetrates the sea depends on the angle of
3770-530: Is near the Mariana Islands , a volcanic archipelago in the West Pacific. Its deepest point is 10.994 kilometres (nearly 7 miles) below the surface of the sea. 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. A headland
3900-520: Is not generally used in those parts of the world where the working of hard metals arrived abruptly from contact with Eurasian cultures, such as Oceania , Australasia , much of Sub-Saharan Africa , and parts of the Americas . With some exceptions in pre-Columbian civilizations in the Americas, these areas did not develop complex writing systems before the arrival of Eurasians, so their prehistory reaches into relatively recent periods; for example, 1788
4030-541: 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
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4160-558: Is provided by a wide variety of natural and social sciences, such as anthropology , archaeology , archaeoastronomy , comparative linguistics , biology , geology , molecular genetics , paleontology , palynology , physical anthropology , and many others. Human prehistory differs from history not only in terms of its chronology , but in the way it deals with the activities of archaeological cultures rather than named nations or individuals . Restricted to material processes, remains, and artefacts rather than written records, prehistory
4290-551: Is seen as a transition period between the Stone Age and Bronze Age. An archaeological site in Serbia contains the oldest securely dated evidence of copper making at high temperature, from 7,500 years ago. The find in 2010 extends the known record of copper smelting by about 800 years, and suggests that copper smelting may have been invented independently in separate parts of Asia and Europe at that time, rather than spreading from
4420-500: Is that H. erectus or H. ergaster made fires between 790,000 and 690,000 BP in a site at Bnot Ya'akov Bridge , Israel . The use of fire enabled early humans to cook food, provide warmth, have a light source, deter animals at night and meditate. Early Homo sapiens originated some 300,000 years ago, ushering in the Middle Palaeolithic . Anatomic changes indicating modern language capacity also arise during
4550-595: Is the interconnected system of all the Earth's oceanic waters, including the Atlantic , Pacific , Indian , Southern and Arctic Oceans . However, the word "sea" can also be used for many specific, much smaller bodies of seawater, such as the North Sea or the Red Sea . There is no sharp distinction between seas and oceans , though generally seas are smaller, and are often partly (as marginal seas or particularly as
4680-419: 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/h) and usually have
4810-446: Is usually taken as the end of the prehistory of Australia . The period when a culture is written about by others, but has not developed its own writing system, is often known as the protohistory of the culture. By definition, there are no written records from human prehistory, which can only be known from material archaeological and anthropological evidence: prehistoric materials and human remains. These were at first understood by
4940-516: Is when the first signs of human presence have been found; however, Africa and Asia contain sites dated as early as c. 2.5 and 1.8 million years ago, respectively. Depending on the date when relevant records become a useful academic resource, its end date also varies. For example, in Egypt it is generally accepted that prehistory ended around 3100 BCE, whereas in New Guinea
5070-653: The Copper Age or Bronze Age ; or, in some geographical regions, in the Iron Age ). The term Neolithic is commonly used in the Old World ; its application to cultures in the Americas and Oceania is complicated by the fact standard progression from stone to metal tools, as seen in the Old World, does not neatly apply. Early Neolithic farming was limited to a narrow range of plants, both wild and domesticated, which included einkorn wheat , millet and spelt , and
5200-651: 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
5330-487: The Indus Valley Civilisation , and ancient Egypt were the first civilizations to develop their own scripts and keep historical records, with their neighbours following. Most other civilizations reached their end of prehistory during the following Iron Age . The three-age division of prehistory into Stone Age , Bronze Age , and Iron Age remains in use for much of Eurasia and North Africa , but
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5460-857: The Mediterranean and Red Sea with the Egyptian Hannu reaching the Arabian Peninsula and the African Coast around 2750 BC. In the first millennium BC, Phoenicians and Greeks established colonies throughout the Mediterranean and the Black Sea . Around 500 BC, the Carthaginian navigator Hanno left a detailed periplus of an Atlantic journey that reached at least Senegal and possibly Mount Cameroon . In
5590-531: The Paleolithic , by the Neolithic only Homo sapiens sapiens remained. This was a period of technological and social developments which established most of the basic elements of historical cultures, such as the domestication of crops and animals , and the establishment of permanent settlements and early chiefdoms. The era commenced with the beginning of farming , which produced the " Neolithic Revolution ". It ended when metal tools became widespread (in
5720-584: The early Mediaeval period , the Vikings crossed the North Atlantic and even reached the northeastern fringes of North America. Novgorodians had also been sailing the White Sea since the 13th century or before. Meanwhile, the seas along the eastern and southern Asian coast were used by Arab and Chinese traders. The Chinese Ming Dynasty had a fleet of 317 ships with 37,000 men under Zheng He in
5850-614: The hundred-year wave ) they are designed 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 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
5980-422: 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
6110-436: 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
6240-421: The rain falling from them, and the lakes and rivers spontaneously formed as its waters flow again and again to the sea. The scientific study of water and Earth's water cycle is hydrology ; hydrodynamics studies the physics of water in motion. The more recent study of the sea in particular is oceanography . This began as the study of the shape of the ocean's currents but has since expanded into
6370-412: 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
6500-606: The three-age system for human prehistory, were systematised during the nineteenth century in the work of British, French, German, and Scandinavian anthropologists , archaeologists , and antiquarians . The main source of information for prehistory is archaeology (a branch of anthropology), but some scholars are beginning to make more use of evidence from the natural and social sciences. The primary researchers into human prehistory are archaeologists and physical anthropologists who use excavation, geologic and geographic surveys, and other scientific analysis to reveal and interpret
6630-420: The water , carbon , and nitrogen cycles . The surface of water interacts with the atmosphere, exchanging properties such as particles and temperature, as well as currents . Surface currents are the water currents that are produced by the atmosphere's currents and its winds blowing over the surface of the water, producing wind waves , setting up through drag slow but stable circulations of water, as in
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#17327731892646760-475: The "Chalcolithic", "Eneolithic", or "Copper Age" refers to a transitional period where early copper metallurgy appeared alongside the widespread use of stone tools. During this period, some weapons and tools were made of copper. This period was still largely Neolithic in character. It is a phase of the Bronze Age before it was discovered that adding tin to copper formed the harder bronze . The Copper Age
6890-569: The Austronesian " Lapita " peoples displayed great feats of navigation, reaching out from the Bismarck Archipelago to as far away as Fiji , Tonga , and Samoa . Their descendants continued to travel thousands of miles between tiny islands on outrigger canoes , and in the process they found many new islands, including Hawaii , Easter Island (Rapa Nui), and New Zealand. The Ancient Egyptians and Phoenicians explored
7020-705: The Azilian cultures, before spreading to Europe through the Iberomaurusian culture of Northern Africa and the Kebaran culture of the Levant . However, independent discovery is not ruled out. "Neolithic" means "New Stone Age", from about 10,200 BCE in some parts of the Middle East, but later in other parts of the world, and ended between 4,500 and 2,000 BCE. Although there were several species of humans during
7150-405: The Earth , clarified the process of sedimentation , and assisted the study of volcanism and earthquakes . A characteristic of seawater is that it is salty. Salinity is usually measured in parts per thousand ( ‰ or per mil), and the open ocean has about 35 grams (1.2 oz) solids per litre, a salinity of 35 ‰. The Mediterranean Sea is slightly higher at 38 ‰, while the salinity of
7280-420: 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 working on the seas, but its effect on tides is less powerful than that of
7410-411: The Earth to make a complete revolution and return the Moon to its previous 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 effect on tides as the Sun. A bulge is formed in the ocean at the place where
7540-460: The Iron Age, often through conquest by empires, which continued to expand during this period. For example, in most of Europe conquest by the Roman Empire means the term Iron Age is replaced by "Roman", " Gallo-Roman ", and similar terms after the conquest. Even before conquest, many areas began to have a protohistory, as they were written about by literate cultures; the protohistory of Ireland
7670-694: The Middle East is characterized in archaeological stone tool assemblages by a decline in high quality raw material procurement and use. North Africa and the Nile Valley imported its iron technology from the Near East and followed the Near Eastern course of Bronze Age and Iron Age development. The Bronze Age is the earliest period in which some civilizations reached the end of prehistory, by introducing written records. The Bronze Age, or parts thereof, are thus considered to be part of prehistory only for
7800-739: The Middle Palaeolithic. During the Middle Palaeolithic Era, there is the first definitive evidence of human use of fire. Sites in Zambia have charred logs, charcoal and carbonized plants, that have been dated to 180,000 BP. The systematic burial of the dead , music , prehistoric art , and the use of increasingly sophisticated multi-part tools are highlights of the Middle Paleolithic. The Upper Paleolithic extends from 50,000 and 12,000 years ago, with
7930-413: 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 coast in a shallow area and this, coupled with
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#17327731892648060-556: The Southern Hemisphere in the Roaring Forties , long, organised masses of water called swell roll across the ocean. If the wind dies down, the wave formation is reduced, but already-formed waves continue to travel in their original direction until they meet land. 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
8190-534: The Spanish Magellan-Elcano expedition which would be the first to sail around the world. Prehistory Prehistory , also called pre-literary history , is the period of human history between the first known use of stone tools by hominins c. 3.3 million years ago and the beginning of recorded history with the invention of writing systems . The use of symbols, marks, and images appears very early among humans, but
8320-517: The added CO 2 is absorbed by the oceans, forming carbonic acid and lowering the pH (now below 8.1 ) through a process called ocean acidification . The extent of further ocean chemistry changes, including ocean pH, will depend on climate change mitigation efforts taken by nations and their governments. 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,
8450-411: The behavior of elements and molecules within the oceans: particularly, at the moment, the ocean's role in the carbon cycle and carbon dioxide 's role in the increasing acidification of seawater. Marine and maritime geography charts the shape and shaping of the sea, while marine geology (geological oceanography) has provided evidence of continental drift and the composition and structure of
8580-529: The bottom of the ocean provides food for an assembly of organisms which similarly rely largely on the actions of sulphur-reducing bacteria. 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 . By c. 3000 BC, Austronesians on Taiwan had begun spreading into maritime Southeast Asia . Subsequently,
8710-467: 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. The Earth's deepest trench is the Mariana Trench which extends for about 2,500 kilometres (1,600 miles) across the seabed. It
8840-569: The case of Indigenous Australian "highways" known as songlines . The Mesolithic, or Middle Stone Age (from the Greek mesos , 'middle', and lithos , 'stone'), was a period in the development of human technology between the Palaeolithic and Neolithic . The Mesolithic period began with the retreat of glaciers at the end of the Pleistocene epoch, some 10,000 BP, and ended with
8970-516: The case of the ocean sustaining deep-sea ocean currents . Deep-sea currents, known together as the global conveyor belt , carry cold water from near the poles to every ocean and significantly influence Earth's climate. Tides , the generally twice-daily rise and fall of sea levels , are caused by Earth's rotation and the gravitational effects of the Moon and, to a lesser extent, of the Sun . Tides may have
9100-437: 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. Dredging removes material and deepens channels but may have unexpected effects elsewhere on the coastline. Governments make efforts to prevent flooding of
9230-463: The collection of folklore and by analogy with pre-literate societies observed in modern times. The key step to understanding prehistoric evidence is dating, and reliable dating techniques have developed steadily since the nineteenth century. The most common of these dating techniques is radiocarbon dating . Further evidence has come from the reconstruction of ancient spoken languages . More recent techniques include forensic chemical analysis to reveal
9360-446: The composition and hardness of the coastal rock, the inclination of the off-shore slope and the changes of the level of the land due to local uplift or submergence. 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
9490-598: 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 0.1 percent of the world's ocean surface, yet their ecosystems include 25 percent of all marine species. The best-known are tropical coral reefs such as Australia's Great Barrier Reef , but cold water reefs harbour
9620-522: 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 zooplankton feed on phytoplankton and on each other and form a basic part of the complex food chain that extends through variously sized fish and other nektonic organisms to large squid , sharks , porpoises , dolphins and whales . Some marine creatures make large migrations, either to other regions of
9750-410: 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
9880-413: The direction of the wind. The friction between air and water caused by a gentle breeze on a pond causes ripples to form. A strong blow over the ocean causes larger waves as the moving air pushes against the raised ridges of water. The waves reach their maximum height when the rate at which they are travelling nearly matches the speed of the wind. In open water, when the wind blows continuously as happens in
10010-506: The earliest known writing systems appeared c. 5,200 years ago. It took thousands of years for writing systems to be widely adopted, with writing having spread to almost all cultures by the 19th century. The end of prehistory therefore came at different times in different places, and the term is less often used in discussing societies where prehistory ended relatively recently. In the early Bronze Age , Sumer in Mesopotamia ,
10140-571: The early fifteenth century, sailing the Indian and Pacific Oceans. In the late fifteenth century, Western European mariners started making longer voyages of exploration in search of trade. Bartolomeu Dias rounded the Cape of Good Hope in 1487 and Vasco da Gama reached India via the Cape in 1498. Christopher Columbus sailed from Cadiz in 1492, attempting to reach the eastern lands of India and Japan by
10270-506: The end of the Bronze Age large states, whose armies imposed themselves on people with a different culture, and are often called empires, had arisen in Egypt, China, Anatolia (the Hittites ), and Mesopotamia , all of them literate. The Iron Age is not part of prehistory for all civilizations who had introduced written records during the Bronze Age. Most remaining civilizations did so during
10400-639: The end of the prehistoric era is set much more recently, in the 1870s, when the Russian anthropologist Nicholai Miklukho-Maklai spent several years living among native peoples, and described their way of life in a comprehensive treatise. In Europe the relatively well-documented classical cultures of Ancient Greece and Ancient Rome had neighbouring cultures, including the Celts and the Etruscans , with little writing. Historians debate how much weight to give to
10530-446: The few mines, stimulating the creation of extensive trading routes. In many areas as far apart as China and England, the valuable new material was used for weapons, but for a long time apparently not available for agricultural tools. Much of it seems to have been hoarded by social elites, and sometimes deposited in extravagant quantities, from Chinese ritual bronzes and Indian copper hoards , to European hoards of unused axe-heads. By
10660-493: The first organized settlements and blossoming of artistic work. Throughout the Palaeolithic, humans generally lived as nomadic hunter-gatherers . Hunter-gatherer societies tended to be very small and egalitarian, although hunter-gatherer societies with abundant resources or advanced food-storage techniques sometimes developed sedentary lifestyles with complex social structures such as chiefdoms, and social stratification . Long-distance contacts may have been established, as in
10790-497: The first signs of deforestation have been found, although this would only begin in earnest during the Neolithic, when more space was needed for agriculture . The Mesolithic is characterized in most areas by small composite flint tools: microliths and microburins . Fishing tackle , stone adzes , and wooden objects such as canoes and bows have been found at some sites. These technologies first occur in Africa, associated with
10920-422: 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. The oceans are home to a diverse collection of life forms that use it as a habitat. Since sunlight illuminates only
11050-580: 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
11180-570: The globe. Seawater with a typical salinity of 35 ‰ has a freezing point of about −1.8 °C (28.8 °F). When its temperature becomes low enough, ice crystals form on the surface. These break into small pieces and coalesce into flat discs that form a thick suspension known as frazil . In calm conditions, this freezes into a thin flat sheet known as nilas , which thickens as new ice forms on its underside. In more turbulent seas, frazil crystals join into flat discs known as pancakes. These slide under each other and coalesce to form floes . In
11310-442: 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. When water moves in this way, other water flows in to fill
11440-473: The introduction of agriculture , the date of which varied by geographic region. In some areas, such as the Near East , agriculture was already underway by the end of the Pleistocene , and there the Mesolithic is short and poorly defined. In areas with limited glacial impact, the term " Epipalaeolithic " is preferred. Regions that experienced greater environmental effects as the last ice age ended have
11570-474: The keeping of dogs , sheep , and goats . By about 6,900–6,400 BCE, it included domesticated cattle and pigs, the establishment of permanently or seasonally inhabited settlements, and the use of pottery . The Neolithic period saw the development of early villages , agriculture , animal domestication , tools , and the onset of the earliest recorded incidents of warfare. Settlements became more permanent, some with circular houses made of mudbrick with
11700-547: 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, while the failure of the dykes and levees around New Orleans during Hurricane Katrina created a humanitarian crisis in the United States. The sea plays a part in the water or hydrological cycle , in which water evaporates from
11830-523: The land to breed but fish, cetaceans and sea snakes have a completely aquatic lifestyle and many invertebrate phyla are entirely marine. In fact, the oceans teem with life and provide many varying microhabitats. One of these is the surface film which, even though tossed about by the movement of waves, provides a rich environment and is home to bacteria, fungi , microalgae , protozoa , fish eggs and various larvae. The pelagic zone contains macro- and microfauna and myriad zooplankton which drift with
11960-648: The levels of salinity in different seas, the relative composition of the dissolved salts is stable throughout 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 remains 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 formation (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
12090-508: 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 creating underwater mountains, some of which may form chains of volcanic islands near to deep trenches. Near some of
12220-416: 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
12350-401: The nature and behavior of pre-literate and non-literate peoples. Human population geneticists and historical linguists are also providing valuable insight. Cultural anthropologists help provide context for societal interactions, by which objects of human origin pass among people, allowing an analysis of any article that arises in a human prehistoric context. Therefore, data about prehistory
12480-539: The northern Red Sea can reach 41‰. In contrast, some landlocked 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 ) make up about 85 percent of the solids in solution, there are also other metal ions such as magnesium and calcium , and negative ions including sulphate , carbonate , and bromide . Despite variations in
12610-730: 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 1497 and 1502, discovering the mouth of the Amazon River . In 1519 the Portuguese navigator Ferdinand Magellan led
12740-484: 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. 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
12870-440: The ocean on a seasonal basis or vertical migrations daily, often ascending to feed at night and descending to safety by day. Ships can introduce or spread invasive species through the discharge of ballast water or the transport of organisms that have accumulated as part of the fouling community on the hulls of vessels. The demersal zone supports many animals that feed on benthic organisms or seek protection from predators and
13000-471: 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. Even in the Atacama Desert , where little rain ever falls, dense clouds of fog known as the camanchaca blow in from the sea and support plant life. In central Asia and other large land masses, there are endorheic basins which have no outlet to
13130-423: The open pelagic zone. The organisms living in the sea range from whales 30 metres (98 feet) long to microscopic phytoplankton and zooplankton , fungi, and bacteria. 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
13260-430: The process of freezing, salt water and air are trapped between the ice crystals. Nilas may have a salinity of 12–15 ‰, but by the time the sea ice is one year old, this falls to 4–6 ‰. Seawater is slightly alkaline and had an average pH of about 8.2 over the past 300 million years. More recently, climate change has resulted in an increase of the carbon dioxide content of the atmosphere; about 30–40% of
13390-418: 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 oceans' surface layer holds large amounts of dissolved organic carbon that is exchanged rapidly with the atmosphere. The deep layer's concentration of dissolved inorganic carbon is about 15 percent higher than that of
13520-658: The regions and civilizations who developed a system of keeping written records during later periods. The invention of writing coincides in some areas with the beginnings of the Bronze Age. After the appearance of writing, people started creating texts including written records of administrative matters. The Bronze Age refers to a period in human cultural development when the most advanced metalworking (at least in systematic and widespread use) included techniques for smelting copper and tin from naturally occurring outcroppings of ores, and then combining them to cast bronze . These naturally occurring ores typically included arsenic as
13650-502: The rise of metallurgy, and most Neolithic societies on the whole were relatively simple and egalitarian. Most clothing appears to have been made of animal skins, as indicated by finds of large numbers of bone and antler pins which are ideal for fastening leather. Wool cloth and linen might have become available during the later Neolithic, as suggested by finds of perforated stones that (depending on size) may have served as spindle whorls or loom weights. In Old World archaeology,
13780-417: 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
13910-409: 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. In the former case, the sea draws back and leaves subtidal areas close to 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
14040-479: 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
14170-436: 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
14300-537: The sea level has been higher than it is today. The main factor affecting sea level over time is the result 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 present times (2012). For at least the last 100 years, sea level has been rising at an average rate of about 1.8 millimetres (0.071 in) per year. Most of this rise can be attributed to an increase in
14430-399: 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, usually by a few feet. The potential energy of the displaced seawater
14560-671: 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
14690-657: 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
14820-427: 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. Exploration of the deep sea by submersibles revealed
14950-481: The seas, which offers 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 warm waters of coral reefs in tropical regions . Many of the major groups of organisms evolved in the sea and life may have started there. The ocean moderates Earth's climate and has important roles in
15080-418: 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. 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
15210-635: The sometimes biased accounts in Greek and Roman literature, of these protohistoric cultures. In dividing up human prehistory in Eurasia, historians typically use the three-age system, whereas scholars of pre-human time periods typically use the well-defined geologic record and its internationally defined stratum base within the geologic time scale . The three-age system is the periodization of human prehistory into three consecutive time periods , named for their predominant tool-making technologies: Stone Age , Bronze Age and Iron Age . In some areas, there
15340-429: 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 light reach greater 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,
15470-430: 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
15600-495: The surface and the mouths of large rivers and higher in the depths of the ocean; however, the relative proportions of dissolved salts vary little across the oceans. The most 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. A wide variety of organisms , including bacteria , protists , algae , plants, fungi , and animals live in
15730-473: The surface layer and it remains there for much longer periods of time. Thermohaline circulation exchanges carbon between these two layers. Carbon enters the ocean as atmospheric carbon dioxide dissolves in the surface layers and is converted into carbonic acid , carbonate , and bicarbonate : 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
15860-453: The temperature of the sea due to climate change , and the resulting slight thermal expansion of the upper 500 metres (1,600 ft) of water. 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. Wind blowing over the surface of a body of water forms waves that are perpendicular to
15990-432: The two can produce broken, irregular seas. Constructive interference can cause individual (unexpected) rogue waves much higher than normal. Most waves are less than 3 m (10 ft) high and it is not unusual for strong storms to double or triple that height; offshore construction such as wind farms and oil platforms use metocean statistics from measurements in computing the wave forces (due to for instance
16120-431: 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 habitats range from surface water to the deepest oceanic trenches , including coral reefs, kelp forests , seagrass meadows , tidepools , muddy, sandy and rocky seabeds, and
16250-587: The use and provenance of materials, and genetic analysis of bones to determine kinship and physical characteristics of prehistoric peoples. The beginning of prehistory is normally taken to be marked by human-like beings appearing on Earth. The date marking its end is typically defined as the advent of the contemporary written historical record. Both dates consequently vary widely from region to region. For example, in European regions, prehistory cannot begin before c. 1.3 million years ago, which
16380-460: 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
16510-426: 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. This rushes in a sheet up the beach before retreating into
16640-460: The whole area. "Palaeolithic" means "Old Stone Age", and begins with the first use of stone tools . The Paleolithic is the earliest period of the Stone Age . It extends from the earliest known use of stone tools by hominins c. 3.3 million years ago, to the end of the Pleistocene c. 11,650 BP (before the present period). The early part of the Palaeolithic
16770-445: The world ocean, so global climate modelling makes use 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
16900-407: 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
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