Nature of Oceanography

• Oceanography is the science of the oceans: it studies the ocean basins and their floor, the physical and chemical properties of sea water, its movements, the life it supports and the ways people use and alter it.
• The oceans are a single interconnected world ocean covering about 71% of the Earth’s surface. They hold about 97% of the planet’s water and reach a greatest depth of about 10,935 m in the Challenger Deep.
• Oceanography is ancient in origin but modern in development. Sailors, traders and philosophers gathered knowledge of the sea for thousands of years, but it became an organised science only in the nineteenth century, with the Challenger Expedition (1872–76) as its usual starting point.

Nature and Scope of Oceanography

Geography Optional Notes {PDF}

Complete Notes Package (Paper I & II) | 3000+ Pages | Instant Download

Discounted Price | Buy Now @ ₹1,180 →

Meaning and definitions

• The word comes from the Greek okeanos (the great stream believed to encircle the Earth’s disc) and graphein (to write or describe).
  • Taken literally it means only description of the ocean. A science must also explain causes and processes, so the stricter term oceanology is sometimes preferred. Oceanography remains the accepted name.
• Definitions differ mainly in what they stress:

• A working definition combines them. Oceanography is the science that investigates and explains the origin and relief of the ocean basins, the physical and chemical properties of sea water, its dynamics (waves, tides, currents, tsunamis), coastal processes, marine sediments, marine life and human interaction with the marine environment.
• William Abbott Herdman (1923) summed up its history in one line: oceanography is “a subject of modern development, though of ancient origin”.

A science of synthesis

• Oceanography has no single method of its own. It applies the methods of physics, chemistry, biology and geology to one natural system, the ocean, and then integrates the results.
  • In this sense it is an interdisciplinary, synthesising science rather than a pure science such as mathematics.
• Its character has changed over time.
  • It grew from geographical soil: early oceanography was largely description and mapping of coasts, depths, currents and the distribution of life.
  • From the twentieth century, mathematical models and physical laws came to explain each phenomenon by cause and effect. Modern physical oceanography is therefore treated as a branch of geophysics, and like meteorology it now develops largely as an independent discipline.
• The geographical element remains central. Almost every oceanographic variable has a spatial pattern: where trenches lie, how salinity changes from the equator to the poles, where sediments accumulate, how currents move heat across latitudes. Explaining these regional patterns is the geographer’s contribution.

Place within physical geography

• Physical geography is classically divided by the Earth’s spheres. Oceanography is the study of the hydrosphere, beside geomorphology (lithosphere), climatology (atmosphere) and biogeography (biosphere). The cryosphere (ice) is now often treated as a fifth field.
• The ocean cuts across all of them:
  • the crust beneath the sea, a lithospheric concern;
  • the water and its motion, the hydrosphere proper;
  • marine life, part of the biosphere;
  • sea ice and ice shelves, part of the cryosphere;
  • the air–sea interface, shared with the atmosphere.
• The ocean drives processes on land. Sea level is the ultimate base level of river erosion, so a fall of sea level makes rivers cut down and a rise drowns their mouths. Shoreline changes and sea-level changes are therefore basic to geomorphological analysis.

Scope: what oceanography studies

• The ocean system can be read as three parts of the hydrosphere:
  • the solid part: the crust and sediments of the ocean basins;
  • the liquid part: the water, its properties and its motion;
  • the living part: the organisms of the water column and the sea floor.
• The subject matter groups into eight fields:

Importance of the oceans

• Origin and support of life. Life almost certainly began in the sea. John Burdon Sanderson Haldane (late 1920s) proposed a “hot dilute soup” of organic compounds as its cradle, and by the Cambrian (about 540 million years ago) the oceans were broadly as they are now.
  • Marine phytoplankton produce roughly half of the oxygen generated by photosynthesis on Earth.
• Climate regulator. Water has a high specific heat, so the oceans warm and cool slowly. They moderate coastal climates and move heat poleward through currents.
  • Since the 1970s the oceans have absorbed over 90% of the excess heat trapped by greenhouse gases and about a quarter of human carbon dioxide emissions. This uptake slows atmospheric warming but drives ocean warming and acidification.
  • Evaporation from the sea surface starts the hydrological cycle.
• Record of the past. The remains of plankton settle as marine deposits, and cores of these sediments preserve a record of past climates across millions of years.
• Resources. Sea water contains, in dissolved form, nearly every element found in rocks; common salt, magnesium and bromine are already extracted from it.
  • The shelves hold large reserves of oil and gas (Mumbai High), and the deep floor holds polymetallic nodules, sulphides and crusts.
  • Fisheries are a major source of animal protein, and the oceans offer tidal, wave, wind and thermal energy. These ocean resources underpin the blue economy.
• Transport and strategy. Oceans, once barriers, became the cheapest highways of trade. About 80% of world trade by volume now moves by sea.
  • Control of sea lanes, chokepoints and naval bases gives the oceans great strategic weight, and maritime rights are now set by the UN Convention on the Law of the Sea.
• Hazards. Cyclones, storm surges, tsunamis and coastal flooding all originate at sea. The 2004 Indian Ocean tsunami (magnitude 9.1, about 2.3 lakh deaths across 14 countries) showed how much coastal safety depends on ocean science.
• India’s stake is large: a coastline recently re-measured at about 11,098 km, over 1,300 islands, an EEZ of more than 2 million km², and a monsoon that is itself an ocean–atmosphere system.

Branches of oceanography

• The subject first divided, like geography, into a physical half and a biological half (marine biology). It is now studied through four core branches, each with sub-branches, together with marine meteorology and several applied fields. The branches are distinct but closely interdependent.

Physical oceanography

• Studies the physical properties of sea water: temperature, density, pressure, transparency, light and sound, and the water masses they define.
• Its dynamic part studies the motions: waves, currents, tides, tsunamis, sinking and upwelling, and the deep circulation.
• It stresses energy exchange at the interfaces, between air and sea and between water layers. It gives marine biology its physical base, because the distribution of life follows that of heat, light and nutrients.
  • Example: vertical and horizontal temperature distribution sets the thermocline, which in turn controls mixing and nutrient supply.

Chemical oceanography

• Studies the composition of sea water: dissolved salts, gases, nutrients, trace elements and isotopes, and the properties that follow from them (conductivity, osmotic pressure, freezing point).
• Water samples drawn from known depths are analysed for salinity, oxygen, carbon dioxide and nutrients. The nutrient cycle, and through it the whole organic life of the sea, depends on this chemistry.
• Present-day priorities are pollution (oil, plastics, radioactive and industrial waste), ocean acidification, deoxygenation and desalination.

Biological oceanography (marine biology)

• Treats the sea as a biological environment: plankton (phytoplankton and zooplankton), nekton (swimmers) and benthos (bottom dwellers), from microbes to whales.
• Studies productivity, food chains, migration and ecology, all controlled by the physical and chemical character of sea water.
  • Organisms also alter the water: plankton draw down carbon dioxide and nutrients, and corals and shelled organisms remove calcium carbonate.
• Includes coastal ecosystems such as mangroves, estuaries and coral reefs, and the chemosynthetic communities of hydrothermal vents.

Geological oceanography (marine geology)

• Studies the origin and structure of the ocean basins, the relief of the sea floor, marine deposits, coral reefs and the origin of oceanic islands, heat flow, submarine earthquakes and magnetism.
• It includes geomorphological oceanography, the study of coastal and submarine landforms.
• Its greatest contribution is plate tectonics. Marine geologists mapped ridges, trenches and magnetic stripes on the sea floor. These findings turned continental drift into sea-floor spreading and then into the unified theory of plate tectonics.

Marine meteorology

• Studies the ocean–atmosphere interface: exchange of heat, moisture and momentum, sea-surface pressure and winds.
• The two-way link is close:
  • Winds drive surface currents and waves.
  • Warm and cold currents reduce the latitudinal heat imbalance and steer weather.
  • Sea-surface temperature anomalies set off El Niño and La Niña and influence the Indian monsoon and tropical cyclones.
• Air–sea interaction, long neglected, is now one of the most active research fields.

Applied branches

• Economic oceanography: the ocean as a resource base for food, minerals and energy, and as a highway for transport.
• Environmental oceanography: human impact on the sea, from oil spills, dredging and coastal construction to warming, acidification and plastics. It is closely tied to the study of ocean pollution.
• Strategic and legal oceanography: sea power, maritime boundaries, the EEZ and the law of the sea.
• Newer fields: satellite oceanography, operational oceanography (daily forecasts of ocean state, as for weather) and palaeoceanography (past oceans from sediment cores).

Links with other sciences

• Oceanography both draws on and feeds neighbouring sciences:

• Bridging fields include geochemistry, biophysics, biochemistry, ocean engineering, marine archaeology, maritime law and disaster management.

History of Ocean Exploration

• The history moves through three broad stages: exploration of the oceans’ shape and extent (to about 1800), measurement by systematic expeditions (1800–1950), and continuous observation by drilling, satellites, floats and submersibles (since 1950).
• In oceanographic history, ships are often better remembered than their leaders. Expedition reports were published under the ship’s name: the Challenger report, the Meteor volumes.

Ancient and classical seafaring

• Early seafaring was coastal and practical. Sailors stayed within sight of land, steered by the stars and fished for food.
  • Lothal, a Harappan port on the Gulf of Khambhat with a basin widely read as a tidal dock, shows that Indus traders used sea routes in the third millennium BC.
• Phoenicians of the eastern Mediterranean were the first great seafaring traders of the West. They sailed the Mediterranean and Red Sea and, by tradition, circumnavigated Africa around 600 BC.
• The Greeks laid the scientific foundations:
  • Aristotle (Meteorologica, 4th century BC) described evaporation, condensation and precipitation, the elements of the hydrological cycle.
  • Pytheas (about 325 BC) sailed round Britain to the far north (Thule), worked out latitude from the Sun and was the first to link the tides to the Moon.
  • Eratosthenes (3rd century BC) measured the Earth’s circumference from the Sun’s angle at Syene and Alexandria, with a result close to the true value.
• Ptolemy (about AD 150) drew a world map on a grid of latitude and longitude, but showed the Indian Ocean as an enclosed sea bounded by an unknown southern land.

The medieval period

• In post-Roman Europe, religious cosmology replaced Greek science; maps showed the world as a disc centred on Jerusalem. Knowledge of the sea was kept and extended elsewhere.
• Arab and Indian sailors mastered the monsoon. They sailed east across the Indian Ocean with the south-west summer monsoon and returned with the north-east winter monsoon, using an ocean–atmosphere pattern long before it was explained.
  • The Chola navy of Rajendra Chola I crossed the Bay of Bengal to strike Srivijaya (1025).
• Bede (725) described the monthly rhythm of the tides and their link to the Moon, and noted that tides vary along a coast.
• The Vikings used the Medieval Warm Period (about 950–1250) to cross the North Atlantic:
  • Iceland was settled in the late ninth century, and Erik the Red reached Greenland in the 980s;
  • Leif Erikson reached North America (Vinland, Newfoundland) about 1000;
  • cooling and sea ice after about 1250 cut the routes off.
• Zheng He led seven great Chinese fleets across the Indian Ocean to East Africa (1405–33).

The Age of Discovery (1450–1600)

• Motives: the Ottoman capture of Constantinople (1453) blocked the land routes to Asia. Europeans also sought trade, colonies, the spread of Christianity and geographical knowledge. Prince Henry the Navigator of Portugal sponsored the early voyages.
• Key voyages:
  • Bartolomeu Dias rounded the Cape of Good Hope (1488);
  • Christopher Columbus crossed the Atlantic (1492), reaching the Caribbean while seeking the Indies;
  • Vasco da Gama reached Calicut (1498), opening the sea route to India;
  • Vasco Núñez de Balboa sighted the Pacific from Panama (1513);
  • Ferdinand Magellan found the strait at the tip of South America and named the Pacific (1520). He was killed in the Philippines (1521), and Juan Sebastián Elcano completed the first circumnavigation (1522). Magellan’s weighted line failed to reach the Pacific floor.
• Early ocean science of the period:
  • Juan Ponce de León recorded the strength of the Gulf Stream (1513);
  • Leonardo da Vinci studied waves and currents and read marine fossils in Italian hills as signs of former seas;
  • Gerardus Mercator‘s projection (1569) let navigators plot courses of constant compass direction as straight lines.

Early scientific study (17th–18th centuries)

• Robert Boyle (1674) studied the saltness of the sea and its variation with depth and temperature.
• Isaac Newton (Principia, 1687) gave the gravitational theory of the tides, which Daniel Bernoulli, Leonhard Euler and Colin Maclaurin developed further (1740).
• Luigi Ferdinando Marsigli wrote the first book devoted to ocean science, Histoire physique de la mer (1725). It covered the relief, temperature, salinity and currents of the Mediterranean, and he is regarded as a founder of regional oceanography.
• Benjamin Franklin, with the whaling captain Timothy Folger, printed the first chart of the Gulf Stream (1769–70) to speed mail ships.
• Captain James Cook made three voyages (1768–71, 1772–75, 1776–79) whose main purpose was scientific.
  • He charted New Zealand and eastern Australia and the Great Barrier Reef, and was the first to cross the Antarctic Circle (1773).
  • He showed that no habitable southern continent existed outside the ice, measured surface temperatures, winds and currents, took soundings and observed coral reefs.
  • With a copy of John Harrison‘s chronometer, he fixed longitude accurately and produced the first reliable charts of much of the Pacific.

The nineteenth century: foundations

• By 1800 the distribution of land and water was known except in polar regions. Attention turned to the depths, the chemistry and the life of the sea.
• Deep soundings:
  • John Ross brought up worms and a starfish from about 1,800 m in Baffin Bay (1818);
  • James Clark Ross made the first true deep-ocean sounding, about 4,400 m in the South Atlantic (1840), and noted that deep waters were uniformly cold.
• Edward Forbes (1843) divided the sea into depth zones of life and proposed the azoic hypothesis: no life below about 550 m (300 fathoms). It was influential, and wrong.
• Charles Darwin, on HMS Beagle (1831–36), gathered the observations behind his theory of evolution and his subsidence theory of coral reefs (1842).
• The United States Exploring Expedition under Charles Wilkes (1838–42) surveyed the Pacific and Antarctic coasts.
• Matthew Fontaine Maury, of the US Navy’s Depot of Charts and Instruments, compiled wind and current charts from thousands of ships’ logs.
  • He organised the Brussels conference (1853), which standardised observations at sea.
  • He wrote The Physical Geography of the Sea (1855), the first modern textbook of oceanography.
• Charles Wyville Thomson and William Benjamin Carpenter, dredging from HMS Lightning (1868) and Porcupine (1869), found animals at depths of over 4,000 m. This disproved the azoic hypothesis and led to the Challenger voyage.

The Challenger Expedition (1872–76)

• Organisation: HMS Challenger, a converted warship under Captain George Nares, with Charles Wyville Thomson as scientific director and a staff of six scientists including John Murray. It was sponsored by the Royal Society and the Admiralty.
  • It sailed on 21 December 1872 and returned on 24 May 1876, covering about 127,600 km across the Atlantic, Pacific and Southern Oceans.
• Aims: to study the distribution of life at all depths, the physical conditions of the deep sea, the chemistry of sea water at various depths, and the nature and origin of bottom deposits.
• Method: at each of some 360 stations it followed the same routine: sounding, bottom sampling, trawling, water sampling and temperature at standard depths, and weather records. This repeatable protocol made it the first global systematic survey.
• Main results:
  • 492 deep soundings, 133 bottom dredges, 151 open-water trawls and 263 serial temperature observations;
  • 4,717 new species, with life found at every depth sampled;
  • a sounding of about 8,180 m near the Mariana Islands (1875), in the trench whose deepest point is now named the Challenger Deep;
  • the first world maps of deep-sea deposits (oozes and red clay), and the discovery of manganese nodules across the deep floor;
  • a rise in the middle of the Atlantic, the first glimpse of the Mid-Atlantic Ridge;
  • water samples from which William Dittmar (1884) confirmed the principle of constant proportions: total salinity varies, but the ratios of the major dissolved salts are nearly constant everywhere.
• Publication: the results filled 50 volumes (1880–95), edited by John Murray after Thomson’s death.
• Limitations: with no physicist aboard, ocean circulation was not studied. The northern Indian Ocean and the Arctic were not surveyed.

After the Challenger

• John Murray went on to identify the Wyville Thomson Ridge between Scotland and the Faroes (1880), studied plankton and deep-sea deposits, and wrote The Depths of the Ocean (1912) with Johan Hjort.
• Alexander Agassiz surveyed the Caribbean and Pacific aboard the Blake and the Albatross (from 1877). He disputed Darwin’s subsidence theory of atolls.
• Fridtjof Nansen let the specially built Fram freeze into the Arctic pack and drift across it (1893–96).
  • The drift proved that no northern polar continent exists. The Arctic is a deep ocean basin covered by moving pack ice, with a layer of warmer Atlantic water below the fresher surface water.
  • Nansen noticed that the ice drifted 20°–40° to the right of the wind. Vagn Walfrid Ekman (1905) explained this mathematically as the Ekman spiral.
  • The Nansen bottle became the standard deep-water sampler.
• The International Council for the Exploration of the Sea (1902) was the first intergovernmental body for ocean science.

The twentieth century: institutions and technology

• Echo sounding (Alexander Behm’s patent, 1913) replaced the weighted line.
  • The German Meteor expedition (1925–27) made some 67,000 echo soundings across the South Atlantic and revealed the rugged ocean floor and the Mid-Atlantic Ridge.
• Institutions made the science permanent: the Scripps Institution of Oceanography (1903), the Woods Hole Oceanographic Institution (1930), the Lamont Geological Observatory (1949) under Maurice Ewing, and the US National Oceanic and Atmospheric Administration (NOAA, 1970).
• War accelerated research. Submarine warfare brought sonar, the bathythermograph and large naval funding. Sverdrup and Walter Munk developed wave forecasting for the Allied landings in the Second World War.
• Deep-sea expeditions:
  • the Swedish Albatross (1947–48) took long sediment cores;
  • the Danish Galathea (1950–52) found life below 10,000 m in the Philippine Trench;
  • Jacques Piccard and Don Walsh reached the floor of the Challenger Deep in the bathyscaphe Trieste (1960).
• The theoretical breakthrough:
  • Bruce Heezen and Marie Tharp mapped the global ridge and rift system;
  • Harry Hammond Hess (1960, published 1962) proposed sea-floor spreading;
  • the magnetic stripes of the ocean floor confirmed it (1963), which led to plate tectonics.
• International cooperation:
  • the International Geophysical Year (1957–58);
  • the International Indian Ocean Expedition (1959–65), with dozens of research ships from more than 20 nations;
  • UNESCO’s Intergovernmental Oceanographic Commission (1960);
  • the Monsoon Experiment (MONEX, 1979).
• Drilling and submersibles:
  • the Deep Sea Drilling Project (from 1968, drill ship Glomar Challenger) showed that sea-floor sediments grow older away from the ridges;
  • the submersible Alvin found hydrothermal vents and their chemosynthetic life on the Galápagos Rift (1977).
• Satellites: Seasat (1978), the first ocean satellite, worked for about 100 days. TOPEX/Poseidon (1992) began precise measurement of sea-surface height, mapping currents, tides and sea-level rise.
• Global programmes followed: GEOSECS (1972–78) on the chemistry of ocean circulation, TOGA (1985–94) on El Niño, and the World Ocean Circulation Experiment (1990–2002).

Modern observing programmes

• The ocean is now observed continuously, not just visited by ships:

• Crewed deep diving has revived. James Cameron made the first solo dive to the Challenger Deep (2012), and Victor Vescovo reached the deepest point of all five oceans (2019).

India’s ocean programmes

• Indian oceanography began with the International Indian Ocean Expedition, which led to the National Institute of Oceanography (NIO), Goa (1966).
• A Department of Ocean Development was set up in 1981. Its work passed to the Ministry of Earth Sciences (MoES) in 2006.
• India in Antarctica and the Arctic:
  • the first Indian expedition to Antarctica reached it in January 1982, led by S. Z. Qasim;
  • the stations are Dakshin Gangotri (1983–84), Maitri (1989) and Bharati (2012), with Himadri in Svalbard in the Arctic (2008).
• Main institutions:

• INCOIS runs the Indian Tsunami Early Warning Centre (operational since 2007, set up after the 2004 tsunami). It also serves as a regional tsunami service provider for the Indian Ocean.
  • INCOIS leads India’s part in Argo, deploying floats in the Indian Ocean since the early 2000s. It also issues daily ocean-state forecasts for fishermen and shipping.
• Research vessels:
  • ORV Sagar Kanya (1983) is the flagship of the research fleet;
  • ORV Sagar Nidhi (in service since 2008), an ice-class vessel operated by NIOT, supports deep-sea and polar work;
  • ORV Sagar Manthan, built in India by GRSE for NCPOR, was launched in September 2026 to replace Sagar Kanya.
• Satellites: Oceansat-1 (1999), Oceansat-2 (2009), the Indo-French SARAL-AltiKa (2013) altimeter, and EOS-06, or Oceansat-3 (2022).
• Deep-sea minerals:
  • India was registered as the first pioneer investor for polymetallic nodules in the Central Indian Ocean Basin (1987);
  • it holds International Seabed Authority contracts for nodules there and for polymetallic sulphides on the South-West Indian Ridge (2016) and the Carlsberg Ridge (2025).

Deep Ocean Mission and Samudrayaan

• The Deep Ocean Mission (DOM) was approved by the Union Cabinet in June 2021 at ₹4,077 crore over five years, under MoES. It is a pillar of India’s blue economy policy.
• Six components:
  1. deep-sea mining and a crewed submersible;
  2. ocean climate-change advisory services;
  3. deep-sea biodiversity exploration and conservation;
  4. deep-ocean survey for hydrothermal sulphides on Indian Ocean ridges;
  5. energy and fresh water from the ocean (an OTEC-powered desalination plant);
  6. an advanced marine station for ocean biology.
• Samudrayaan is the crewed part of the mission, built around the submersible Matsya-6000, developed by NIOT with ISRO.
  • Design: it carries three people to 6,000 m in a titanium-alloy sphere about 2.2 m across. It runs 12 hours on a normal dive, with life support for up to 96 hours in an emergency.
  • Tests: wet trials were held at Kattupalli port near Chennai in early 2025. In August 2025 two Indian aquanauts from NIOT dived to 5,000 m in the Atlantic aboard the French submersible Nautile in preparation.
  • Next steps: crewed trials to 500 m are planned for late 2026, ahead of 6,000 m dives targeted for 2027–28. A crewed 6,000 m dive would put India among the few nations (with the US, France, Russia, Japan and China) able to send people that deep.
• The uncrewed vehicle OMe 6000 surveyed the nodule field of the Central Indian Ocean Basin at about 5,270 m (2022), operating from Sagar Nidhi.
• IIOE-2, the second International Indian Ocean Expedition, was launched in 2015 with INCOIS as a coordinating centre.

The World Ocean: Facts and Origin

Extent and distribution

• The Earth’s surface (about 510 million km²) is about 71% water (some 361 million km²) and 29% land (about 149 million km²).
  • The world ocean holds about 1.335 billion km³ of water, about 97% of all water on Earth. About 2% is ice in the Antarctic and Greenland ice sheets, and less than 1% is fresh water on and under the land.
  • Its mean depth is about 3,700 m, against a mean land height of about 840 m. If the solid surface were levelled, water would cover the whole Earth to a depth of about 2.7 km.
• Water dominates the Southern Hemisphere. About 81% of the Southern Hemisphere is ocean, against about 61% of the Northern. The water hemisphere, centred near New Zealand, is about 89% ocean.
  • The water share rises from about 29% in the belt at 65°–70°N to almost 100% at 55°–60°S. These land and water hemispheres and the antipodal arrangement of continents and oceans raise questions about the origin of the basins.
• The floor is far from flat. Shelves, slopes, abyssal plains, ridges and trenches give the relief of the sea floor a greater range than that of the land, and the mid-ocean ridge is the longest mountain system on Earth.

Oceans, seas and their boundaries

• The world ocean is one connected body. The divisions are conventions, set mostly by continents and, in the open south, by meridians.
  • Atlantic–Indian: the meridian of Cape Agulhas (20°E).
  • Indian–Pacific: the meridian of South East Cape, Tasmania (about 147°E), and the island chains of Indonesia.
  • Pacific–Atlantic: the meridian of Cape Horn (about 67°W) across the Drake Passage, and the Bering Strait in the north.
• Five oceans are now commonly recognised.
  • The Southern Ocean, south of 60°S, was proposed by the International Hydrographic Organization in 2000; the proposal is not formally ratified, but the ocean is widely used in science and atlases. It is defined by the Antarctic Circumpolar Current rather than by continents.
  • The Arctic Ocean is small, shallow and ice-covered, and is often treated as a mediterranean sea of the Atlantic.
• Seas are parts of the oceans partly cut off by land or island chains:

• Area and depth of the oceans:

• The Pacific is larger than all the land on Earth put together and holds about half the ocean’s water. It is ringed by trenches and volcanic arcs, and its ridge, the East Pacific Rise, lies well east of centre.
• The Atlantic is long and S-shaped, with the Mid-Atlantic Ridge down its middle. It has the most marginal seas and receives the largest share of the world’s river drainage.
• The Indian Ocean is closed to the north by Asia, so its northern part has the world’s strongest monsoon reversal of winds and currents. An inverted-Y ridge system divides its floor.

Origin of ocean water

• The classical view came from the planetesimal hypothesis of Thomas Chrowder Chamberlin and Forest Ray Moulton (1905).
  • Water vapour and other gases escaped from the growing Earth by volcanic outgassing, the same process that built the early atmosphere.
  • As the surface cooled, the vapour condensed and rained out, collecting in hollows of the crust as scattered water bodies that merged into oceans.
  • Erosion moved material from the high continents into the basins. Isostatic adjustment lowered the basins further, letting them hold more water.
• The modern view keeps outgassing at the centre but adds where the water came from:
  • Most of it was probably part of the Earth’s rocky building blocks, material similar to water-bearing chondrite meteorites. It was released from the interior and from an early magma ocean as the planet formed.
  • Late impacts of water-rich asteroids may have added more.
  • Comets supplied little. Most comets measured so far, including 67P studied by the Rosetta probe (2014), have a deuterium-to-hydrogen ratio well above that of sea water.
• Timing: zircon crystals from the Jack Hills of Western Australia, about 4.4 billion years old, show that liquid water existed within about 150 million years of the Earth’s formation. Oceans were probably established by 4.4–4.0 billion years ago, and they have kept roughly their present volume since the Archaean.
• The salt came from two sources:
  • chemical weathering of continental rocks, carried in by rivers (sodium, calcium, magnesium, potassium);
  • volcanic gases and hydrothermal vents (chlorine, sulphate).
  • Removal by sediments and at vents now roughly balances supply, so salinity is in long-term steady state.