Oceanography

Oceanography is the science that studies oceans.

Oceans cover about 71% of the Earth’s surface. They form the largest and most prominent feature of the planet.

The world ocean is one interconnected body of saltwater, divided into five principal oceans:

• Pacific Ocean
• Atlantic Ocean
• Indian Ocean
• Southern Ocean
• Arctic Ocean

Oceanographic research has become important because oceans control climate, support marine life, provide resources and influence global heat balance.

One important international organisation related to oceanographic research is the International Council for the Exploration of the Sea, or ICES, headquartered in Copenhagen.

Marine Resources

Marine resources include oceans and seas, which contain a wide range of useful resources.

These resources include:

• Marine organisms
• Mineral resources
• Energy resources

Marine Organisms

Oceans are home to a large variety of organisms.

Many of these are harvested for human use.

Examples include:

• Fish
• Shellfish
• Seaweed

Mineral Resources

Oceans contain many mineral resources.

Important examples are:

• Oil reserves
• Natural gas reserves
• Sand and gravel
• Polymetallic nodules containing manganese, copper, cobalt and nickel

Energy Resources

Oceans also provide renewable energy resources.

Important marine energy resources include:

• Wind energy
• Wave energy
• Tidal energy
• Ocean thermal energy

Divisions of the Ocean Floor

The ocean floor has several major and minor divisions.

The five major divisions of the ocean floor are:

• Continental Shelf
• Continental Slope
• Continental Rise
• Deep Ocean Plain or Abyssal Plain
• Ocean Deeps or Trenches

Continental Shelf

The continental shelf is the gently sloping submerged extension of a continent from the shoreline to the shelf break.

The shelf break generally occurs around 200 metres depth, or about 100 fathoms.

An isobath is an imaginary or drawn line on a map that connects all points of equal depth below a water surface such as an ocean, sea or lake.

The continental shelf is the seaward extension of the continental margin.

These extended margins are occupied by shallow seas and gulfs.

The continental shelves of all oceans together cover about 7-8% of the total ocean area.

Features of Continental Shelf

• It has a very gentle slope, usually less than 1°.
• Its depth generally ranges from near sea level to about 200 metres.
• Its width varies greatly from region to region.
• It is wide in regions such as the Bay of Bengal and the east coast of North America.
• It is narrow or almost absent along the west coast of South America.
• It is covered with sediments brought by rivers, glaciers and marine processes.

Massive sedimentary deposits accumulated over long periods on continental shelves become important sources of fossil fuels such as petroleum and natural gas.

Examples include:

• Continental shelf of Southeast Asia
• Grand Banks near Newfoundland
• Submerged region between Australia and New Guinea

Formation of Continental Shelf

The continental shelf is formed mainly due to:

• Submergence of part of a continent
• Relative rise in sea level
• Deposition of sediments brought down by rivers

Types of Shelves Based on Sediment Origin

Important types include:

• Glaciated shelf around Greenland
• Coral reef shelf near Queensland, Australia
• Shelf near large river deltas such as Nile Delta
• Shelf with drowned river valleys such as Hudson River region
• Shelves along young mountain ranges

Continental Slope

The continental slope is the steep gradient between the outer edge of the continental shelf and the deep ocean basin.

It begins at the shelf break.

The continental slope is often cut by submarine canyons.

Features of Continental Slope

• Its gradient generally ranges between 2° and 5°.
• It extends from about 200 metres to nearly 3000-3500 metres depth.
• In some regions, such as off the coast of the Philippines, the continental slope descends to very great depths.

The slope marks the real boundary between continents and deep ocean basins.

Deep Ocean Plain or Abyssal Plain

The deep ocean plains, also called abyssal plains, cover a large portion of the ocean floor.

They are extensive, nearly level and gently sloping areas of ocean basins.

They are among the flattest and smoothest regions of the Earth.

This smoothness occurs because fine sediments of terrestrial and shallow-water origin bury irregular ocean-floor topography.

Features of Abyssal Plains

• They lie about 3000 to 6000 metres below sea level.
• They cover roughly 40% of the ocean floor.
• They are covered with fine-grained sediments such as clay and silt.
• They are commonly found between the continental rise and mid-ocean ridges.

Abyssal plains may contain features such as:

• Submarine plateaus
• Ridges
• Trenches
• Seamounts
• Oceanic islands rising above sea level

Examples include:

• Azores
• Ascension Island

Ocean Deeps or Trenches

Oceanic deeps or trenches are deep, narrow and elongated depressions in the ocean floor.

They are mostly found along convergent plate margins.

They represent the deepest parts of the oceans.

Features of Oceanic Trenches

• They are steep-sided, narrow and elongated.
• They are of tectonic origin.
• They form during ocean-ocean or ocean-continent convergence.
• They are often several kilometres deeper than surrounding ocean floor.
• They usually lie along continental margins or island arcs.
• They commonly run parallel to fold mountain chains or island arcs.

Trenches are very common in the Pacific Ocean, forming an almost continuous belt known as the Pacific Ring of Fire.

The Mariana Trench near Guam in the Pacific Ocean is the deepest known trench, with a depth exceeding 11 km.

Oceanic trenches are associated with:

• Active volcanism
• Deep-focus earthquakes
• Plate subduction

As many as 57 oceanic deeps have been explored so far.

Out of these:

• 32 are in the Pacific Ocean
• 19 are in the Atlantic Ocean
• 6 are in the Indian Ocean

Important examples include:

• Mindanao Deep – 35,000 feet
• Tonga Trench – 31,000 feet
• Japanese Trench – 28,000 feet

All three are in the Pacific Ocean.

Continental Rise

The continental rise forms where the seaward edge of the continental slope gradually loses gradient.

In some regions, the continental rise is very narrow.

In other regions, it may extend up to hundreds of kilometres in width.

When the gradient decreases to about 0.5° to 1°, the region is called the continental rise.

With increasing depth, the continental rise becomes almost flat and merges with the abyssal plain.

Minor Relief Features of the Ocean Floor

Apart from the major divisions, the ocean floor has many minor relief features.

Important minor relief features include:

• Ridges
• Abyssal hills
• Seamounts
• Guyots
• Submarine canyons
• Fracture zones
• Island arcs
• Atolls
• Coral reefs
• Submerged volcanoes
• Sea-scarps

Ridges

Oceanic ridges are continuous underwater mountain ranges formed by tectonic activity and volcanic processes.

They are created when magma rises between diverging lithospheric plates.

This forms new crust.

Oceanic ridges usually consist of two mountain chains separated by a large depression.

This depression marks a divergent boundary.

A typical oceanic ridge has a depth of about 2600 metres and rises about 2000 metres above the deepest part of an ocean basin.

Abyssal Hills

Abyssal hills are smaller sea hills found on abyssal plains.

They rise less than 1000 metres from the ocean floor.

They are low, elongated volcanic features formed by faulting and volcanism within oceanic crust.

They are among the most abundant geological features on Earth.

They are found near mid-ocean ridges and on abyssal plains.

Seamounts

Seamounts are underwater hills on abyssal plains that rise more than 1000 metres from the ocean floor.

They are usually volcanic in origin.

A seamount does not reach the ocean surface.

They may be 3000-4500 metres tall.

The Emperor Seamount, an extension of the Hawaiian Islands in the Pacific Ocean, is a good example.

Guyots

A guyot, also called a tablemount, is an isolated underwater volcanic mountain with a flat top.

Its flat top lies more than 200 metres below sea level.

Guyots show evidence of gradual subsidence.

They were once higher, then were flattened by wave action and later submerged.

It is estimated that more than 10,000 seamounts and guyots exist in the Pacific Ocean alone.

Submarine Canyons

Submarine canyons are deep concave gorges found on the continental shelf, slope or rise.

They often extend from the mouths of large rivers.

A submarine canyon is a steep-sided valley cut into the seabed of the continental slope.

Some may extend well onto the continental shelf.

They can have nearly vertical walls.

In some cases, canyon wall height may reach up to 5 km, as seen in the Great Bahama Canyon.

The Hudson Canyon is the best-known submarine canyon in the world.

Fracture Zones

Fracture zones are long, linear scars on the ocean floor.

They are commonly found perpendicular to mid-ocean ridges.

They are formed due to tectonic plate movement.

They represent inactive or fossil traces of transform faults where seafloor spreading has been offset.

These zones separate lithospheric plates of different ages and may be thousands of kilometres long.

They help explain the process of plate tectonics.

Island Arcs

Island arcs are curved chains of volcanic islands.

They form at convergent plate boundaries where one oceanic plate subducts beneath another.

As the subducting plate sinks into the mantle, it releases volatiles.

These volatiles melt the overlying mantle wedge and produce magma.

This magma rises to the surface and forms a volcanic arc.

Examples include:

• Japanese Archipelago
• Aleutian Islands
• Lesser Antilles

Atolls

An atoll is a ring-shaped coral reef.

It includes a coral rim that partly or completely surrounds a lagoon.

There may be coral islands or cays on the rim.

Atolls are found in warm tropical or subtropical oceans and seas where corals can grow.

Coral Reefs

Coral reefs are marine structures formed mainly by coral organisms in warm tropical and subtropical waters.

They are important for marine biodiversity and are closely associated with shallow, clear and warm ocean waters.

Submerged Volcanoes

Submerged volcanoes, also called submarine volcanoes, are volcanic vents on the seafloor.

They are responsible for much of Earth’s volcanic activity.

They are most commonly found at mid-ocean ridges where tectonic plates spread apart.

These volcanoes usually produce passive, effusive eruptions.

Because of high water pressure, they commonly produce pillow lava.

Some submerged volcanoes grow large enough to form volcanic islands such as the Hawaiian Islands.

Sea-Scarps

Sea-scarps may refer to:

• Beach scarps formed on beaches by wave action
• Tectonic scarps along coastlines formed by tectonic plate movement

Beach scarps are usually temporary and are influenced by wave energy and beach nourishment.

Tectonic scarps are large and ancient features that define continental margins.

They may be hundreds or thousands of metres high.

Ocean Deposits

Ocean deposits are materials that accumulate at the bottom of oceans due to natural processes.

These materials include:

• Biological remains such as dead marine plants and animals
• Inorganic materials such as sand, clay and rock particles
• Chemically dissolved minerals that later precipitate from seawater

Over long geological periods, these materials form layered sediments on the ocean floor.

They preserve records of Earth’s geological and climatic history.

Some deposits come from continental sources through rivers, winds and glaciers.

Others are marine in origin, formed by marine organisms or by chemical precipitation within ocean water.

Types of Ocean Deposits

Ocean deposits are classified on the basis of:

• Location
• Depth
• Sediment composition

Ocean Deposits Based on Location

Shelf Deposits

Shelf deposits are found on the continental shelf.

This is the shallowest and most biologically productive region of the ocean.

Clastic Deposits

Clastic deposits are formed from fragmented rock particles or sediments.

They are transported into the ocean by:

• Rivers
• Glaciers
• Winds

Non-Clastic Deposits

Non-clastic deposits are formed through chemical precipitation.

Examples include:

• Salts
• Phosphates
• Carbonate deposits

Continental Shelf Deposits

These occur in shallow marine waters along the continental shelf region.

Slope Deposits

Slope deposits are found in the steep gradient zone of the continental slope.

They often consist of comparatively coarser materials transported downslope.

Oceanic Basin Deposits

Oceanic basin deposits are found in the deepest parts of ocean basins.

They mainly contain:

• Fine-grained sediments
• Clay
• Organic oozes

Ocean Deposits Based on Sediment Type

Terrigenous Deposits

Terrigenous deposits are formed from materials transported from land into the ocean.

They are carried by:

• Rivers
• Winds
• Glaciers
• Coastal erosion

Biogenous Deposits

Biogenous deposits originate from the remains of marine organisms.

Examples include:

• Shells
• Corals
• Microscopic plankton skeletons

Hydrogenous Deposits

Hydrogenous deposits consist of minerals chemically precipitated directly from seawater.

Examples include:

• Manganese nodules
• Phosphorite deposits

Cosmogenic Deposits

Cosmogenic deposits are rare.

They form from extraterrestrial materials such as micrometeorites settling on the ocean floor.

Significance of Ocean Deposits

Ocean deposits are important scientifically, economically and environmentally.

Their study is significant for:

• Historical record of past oceanic and climatic conditions
• Study of ocean currents through sediment distribution
• Climate reconstruction using marine sediments
• Understanding carbon and water cycles
• Oceanographic studies linking atmosphere, lithosphere and hydrosphere

Ocean deposits also help in climate change studies.

Marine sediments such as microfossils, coral structures and deep-sea deposits help scientists trace past climatic events such as ice ages and warming periods.

They also help understand how oceans absorb atmospheric carbon dioxide and moderate global warming.

Marginal Seas

Marginal seas are parts of oceans partially enclosed by landforms such as islands, peninsulas or archipelagos.

They are usually connected with the main ocean through broad openings or submarine ridges.

Because of their semi-enclosed nature, they have distinct physical and climatic characteristics.

Marginal seas are important for:

• Marine biodiversity
• Fisheries
• Navigation
• Regional climate influence

Examples of marginal seas include:

• Arabian Sea
• Persian Gulf
• Red Sea
• Gulf of Oman
• Gulf of Aden
• Gulf of Kutch
• Gulf of Khambhat
• Bay of Bengal
• Andaman Sea
• Strait of Malacca
• Mozambique Channel
• Great Australian Bight
• Gulf of Mannar
• Laccadive Sea

Ocean Temperature

The study of ocean temperature is important for understanding:

• Ocean currents
• Distribution of marine organisms
• Coastal climatic conditions

The main source of heat for ocean water is insolation.

Oceans play an important role in regulating Earth’s heat balance because water has high specific heat capacity.

This means water heats and cools slowly.

The average surface temperature of ocean water is about 26°C-27°C, though it varies from region to region.

Factors Affecting Ocean Temperature Distribution

Latitude

Surface water temperature decreases from the equator toward the poles due to decreasing intensity of insolation.

The highest temperature is found in the tropics.

Hemispheric Variation

The Northern Hemisphere is generally warmer than the Southern Hemisphere because of larger landmass in the north.

Prevailing Winds

Offshore winds drive warm surface water away from the coast.

This causes upwelling of cold water.

Onshore winds raise coastal temperatures by piling up warm water near the coast.

Ocean Currents

Warm ocean currents increase temperatures in colder areas.

Cold ocean currents lower temperatures in warmer regions.

Enclosed and Open Sea

Enclosed seas have higher temperatures at lower latitudes.

Open seas have higher temperatures at higher latitudes.

Physical Characteristics of Sea Surface

Salinity affects the boiling point of seawater.

Higher salinity increases the boiling point.

Diurnal Range of Temperature

Tropical waters have a higher diurnal temperature range than equatorial waters because of less cloud cover.

Variation in Ocean Temperature

The Pacific Ocean has a lower annual temperature range than the Atlantic Ocean.

The equatorial region receives much higher solar energy than polar regions.

Solar radiation can penetrate several metres below the ocean surface because of the transparency of water.

Short-wavelength radiation penetrates deeper.

Heat is further distributed through water mixing processes.

Seasonal and daily temperature variations in oceans are much smaller than on land because water has high heat capacity.

Most solar energy is absorbed near the surface layer, which supports marine photosynthesis.

Vertical Variation in Oceanic Temperature

Vertical temperature distribution is influenced by:

• Density differences
• Vertical water movement

Maximum temperature is found near the ocean surface where direct solar heating occurs.

Heat transfer to deeper layers occurs mainly through:

• Convection
• Water mixing

Conduction plays only a minor role.

Thermal Layer Distribution in the Ocean

Ocean water has three broad thermal layers.

First Layer: Surface Layer

The upper warm layer has a thickness of about 400-500 metres.

Its average temperature ranges from 20°C to 25°C.

This layer exists throughout the year in tropical regions.

In mid-latitudes, it mainly develops during summer.

Second Layer: Thermocline Layer

Between about 200 metres and 1000 metres depth, temperature decreases rapidly.

This forms the thermocline.

A thermocline is a transition zone separating warm surface water from colder deep water.

It acts as a barrier to vertical mixing.

Nearly 90% of ocean water volume lies below the thermocline, where temperatures approach 0°C-2°C.

The thermocline is weak or absent in polar regions because surface waters are already very cold.

Third Layer: Deep Ocean Layer

Beyond 1000 metres depth, temperature remains nearly constant around 2°C.

It shows very little seasonal variation.

This layer extends to the ocean floor.

It is influenced by cold, dense water formed in polar regions.

Horizontal Variation in Oceanic Temperature

The average surface temperature of ocean water is about 27°C.

It decreases gradually toward the poles.

The rate of temperature decrease is about 0.5°C per degree of latitude.

Horizontal distribution of ocean temperature is represented by isothermal lines.

Isothermal lines connect places having equal temperature.

Isotherms are closely spaced where the temperature gradient is high.

They are widely spaced where the temperature gradient is low.

Example:

In winter, isotherms bend northward near the western coast of Europe due to the warm Gulf Stream.

They bend southward near eastern Canada due to the cold Labrador Current.

Salinity

Salinity is the amount of dissolved salts present in seawater.

It is generally expressed as the number of grams of salt dissolved in 1000 grams, or 1 kg, of seawater.

Salinity is commonly measured in parts per thousand, or ppt / ‰.

The average salinity of ocean water is about 35 ppt.

A salinity value of about 24.7 ppt is considered the upper limit for brackish water.

Beyond this, water is considered saline.

Even small variations in ocean surface salinity can strongly influence:

• Ocean circulation
• Density distribution
• Global water cycle

Factors Affecting Salinity

Factors That Increase Salinity

• Evaporation from the ocean surface leaves salt behind.
• Ice formation concentrates salt in remaining water.
• Advection of more saline water increases salinity.
• Mixing with more saline deep water increases salinity.
• Solution of salt deposits increases salinity.

Factors That Decrease Salinity

• Precipitation adds freshwater to the ocean surface.
• Melting of ice dilutes salt concentration.
• Advection of less saline water decreases salinity.
• Mixing with less saline deep water decreases salinity.
• Inflow of freshwater from land decreases salinity.

Sources of Salts in Ocean Water

Ocean salts come from several sources.

Important sources are:

• Weathering of rocks on land
• Minerals transported into oceans by rivers
• Submarine volcanic eruptions
• Hydrothermal vents
• Chemical reactions between seawater and oceanic crust
• Erosion of coastal and submarine rocks

Distribution of Salinity

Salinity varies both vertically and horizontally.

Vertical Distribution of Salinity

In low latitudes, salinity generally decreases with increasing depth up to about 1000 metres and then becomes nearly constant.

In high latitudes, salinity may increase slightly with depth because surface waters are diluted by melting ice and precipitation.

The depth zone between about 300 metres and 1000 metres shows rapid change in salinity.

This zone is called the halocline.

Above the halocline, surface waters generally have higher salinity in subtropical regions.

Deeper layers show relatively uniform salinity.

Horizontal Distribution of Salinity

Salinity is generally highest in subtropical regions around 20°-30° latitude, where evaporation exceeds precipitation.

Near the equator, heavy rainfall lowers surface salinity.

Toward the polar regions, salinity decreases because of:

• Lower evaporation
• Melting ice

Relationship Between Salinity, Temperature and Density

Temperature and density have an inverse relationship.

When temperature increases, water expands and density decreases.

Salinity and density have a direct relationship.

Higher salinity increases water density.

Differences in temperature and salinity create density variations.

These density differences are important for thermohaline ocean circulation.

Warm, less dense seawater remains near the surface.

Cold and saline water sinks to deeper layers.

Variation of Density, Salinity and Temperature with Oceanic Depth

Rapid changes in temperature, density or salinity create transition zones in ocean water called clines.

Thermocline

A thermocline is the layer where temperature decreases rapidly with depth.

It separates warm surface water from cold deep-sea water.

It is well developed in tropical and temperate oceans.

It is weak in polar oceans.

Pycnocline

A pycnocline is the layer where density increases rapidly with depth.

It forms due to the combined effect of temperature and salinity variations.

This layer restricts vertical mixing of ocean water.

Halocline

A halocline is the layer where salinity changes sharply with depth.

In low latitudes, salinity generally decreases downward because of high surface evaporation.

In high latitudes, salinity may increase with depth because freshwater is added at the surface.

These layers together control:

• Vertical circulation of ocean water
• Distribution of marine organisms
• Transfer of heat within oceans

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FAQs On Oceanography