Major movements of ocean water can be categorised as follows:-

• Large mass of moving water from one part of the ocean to another in a definite direction is called as ocean current.
• The movement is produced due to earth’s rotation, temperature difference of ocean water, salinity, density , air pressure and winds.
• They circumnavigate the earth .

What Currents do

• Heat re-distribution – from  Equator to Poles.
  •  warm water from equator to poles .
  •  cold water from poles to equator.

Factors responsible for ocean current

The primary forces that influence the currents are:–

a. Wind

• Most dominated force.
• In most cases, direction of wind will be direction of underlying current .

b. Insolation

• Heating by solar energy cause the water to expand. That is why, near  equator the ocean water is about 8 cm higher in level than in the middle latitudes.
• This causes a very slight gradient and water tends to flow down the slope. 

c. Salinity

• Water will move from lower salinity to higher salinity on the surface .
• Reason : Water with higher salinity being denser sinks & as a result water with low salinity comes to fill the space. 
• Eg : Water from Atlantic flows to Mediterranean

d. Coriolis Force

Coriolis Effect : Due to rotation of earth – West to East .

• Object moving in Northern Hemisphere will deflect Clockwise
• Object moving in Southern Hemisphere will deflect Anti Clockwise

Nomenclature : Warm Current & Cold Current

• Warm current is one that carry water warmer than water found at that latitude .
• Water flowing from low latitude to higher latitude is warm.

Currents effect climate of bordering landmass eg wind blowing to land from warm current cause rainfall. This is the  reason why mostly east coasts of continents receive rainfall & west coasts are deserts.

General Pattern of Currents in all Oceans

• Due to rotation of earth from west to east  , two currents one on each side of equator are formed called North & South Equatorial Current.
• Water carried in form of Equatorial currents tend to accumulate in western part of ocean or eastern coast of continent.
• To balance this Counter Equatorial Current is formed between two Equatorial Currents from West to East.
• Water at eastern coast of continent start piling up & under influence of density gradient start moving towards middle latitudes
• At same time high density water at poles start flowing towards equator in form of currents along eastern coast of continent & two meet in middle latitudes where cold water sinks & warm water spreads on it .
• The water is pushed towards east by strong westerlies & water reaches eastern part of ocean or western coast of other continent.
• Obstruction at western part of continent divides water & one moves toward pole & other toward equator.
• The water sinked in middle latitude upwells along the western coast to make good removal of water at equator . As a result circulatory cells known as gyres are formed one in each major ocean basin & is situated between 20 to 30 degree in both hemisphere.
• This system is found in all oceans except Indian ocean where major changes occur due to presence of Indian peninsula &  seasonal reversal of winds producing double gyre moving in seasonally opposite directions.

Warm Currents in Atlantic Ocean

a. North Equatorial Current

• North equatorial current flows from east to west between 5– 20 N latitudes.
• After leaving the west coast of Africa, it attains its main characteristics.
• When it reaches the east coast of South America, it splits into two branches and one branch called Antilles current moves along the coast of West Indies and other branch is diverted into the Caribbean sea.

b. South Equatorial current

• It  flows south of equator within 0 – 12 S latitude in between the coast of Africa and South America.
• It is stronger than the North equatorial ocean current

c. Gulf Stream

• Gulf Stream starts from the Gulf of Mexico and carries warm waters into the colder latitudes.
• It bends with the coastline up to 40th parallel after which the direction is almost to the east, due to the force and the direction of the westerlies and the deflective force of the earth.
• It joins the Labrador cold current near New Found land, Canada  making it a good fishing ground.

d. Brazil Current

• Warm current moving south along the coast of Brazil.
• It joins the Falkland cold current near Bahia Blanca  making it a good fishing ground.

e. Norwegian Current /North Atlantic Drift

• Norwegian Current takes warm water towards the coast of Norway allowing it’s coast to be operational throughout the year.

Cold Currents of Atlantic Ocean

a. Canary Island

• It flows south  along the Western coast of North Africa between Maderia and Cape verde
• Gradually , it merges with the North equatorial current.

b. Labrador Current

• In the north Atlantic, it flows from the Baffin Bay and Davis Strait towards south.
•  It brings cold waters from polar zone and moves along the coast of green land.
• It meets with warm Gulf Stream near New Found Land making it a good fishing ground.

c. Benguela Current

• It flows northward along the western coast of Africa
• It carries cold waters from sub-Antarctica surface water and mixes with south equatorial current

Side note : Newfound Land Island

• Meeting point of Gulf Stream( warm current) & Labrador current (cold Current)
• Amount of planktons increases resulting in large number of fishes. Hence, it is Good fishing ground.

Side Note: North Atlantic Drift / Norwegian Current

• It provides warming effect to western Europe.
• Due to this, ports of Norway –  Narvik,  Hammerfest , Murmansk etc keep on operating in winter but at same latitude ports in Russia freeze during winter .

Side Note : Sargasso Sea

• It is the only sea in the world without a land boundary.  Its boundaries are defined only by ocean currents. It is encircled by the Gulf Stream in the west, north Atlantic drift in the north, Canary current in the east and North Equatorial and Antilles current in the south
• Due to confinement , Sargassum ie a sea weed grow in this region which obstructs navigation. Plants have become adapted to life in the open sea, need no roots or holdfasts for attachment, and are able to propagate by breaking off bits to grow into new plants through vegetative propagation.
• It is very saline. Reasons for this are
  • Prevention of mixing of fresh water from surrounding oceans due to virtual boundary created by the ocean currents.
  • High Evaporation due to high temperature owing to its subtropical location
  • Gulf Stream brings in high salinity water to the sea.
• It is devoid of nutrients & hence, poor marine life .

Side Note : Bahia Blanca & Falkland Island

• Meeting point of warm Brazilian & cold Falkland Current .
• Important for fishing activity.

Side Note : Benguela Current & Namib Desert

• Benguela Current is a Cold Current.
• It gives  desiccating effect leading to low rainfall on the south-west coast of Africa and formation of  Namib Desert.

Cold Currents of Pacific Ocean

a. Oyashio current

• It originates from the Bering Strait and flows towards south carrying cold water.
• It meets with Kuroshio warm current 

b. California Current

• Californian current is flowing towards south along the west coast of U.S.A between 48 N and 23 N latitudes.
• Has desiccating effect which leads to formation of Mohave and Sonoran desert.

c. Peru Current

• Peru Current is the best studied ocean current of the Pacific Ocean.
• Alexander Von Humboldt in 1802 noted the details of the Peru Current. Hence, it is also known as Humboldt Current.
• It flows north along the west coast of South America carrying cold water  

d. Alaska Current

• Cold current originating near Alaska and flowing south to meet North Pacific Current

e. West Wind Drift

• It is an easterly moving drift in the Pacific Ocean extending from Tasmania to the South American coast. It is a cold current.
• The speed of the drift is greater under the influence of Roaring Forties.

Warm Currents of Pacific Ocean

a. North Equatorial Current

• North equatorial current originates from Revilla Gigedo island west of Mexico due to trade winds and flows towards the Philippines Island covering a distance of about 12,000 km
• It gets divided into two and the northern branch joins the Kuroshio Current and the southern branch abruptly turns and forms the Pacific counter current.

b. South Equatorial Current

• South equatorial current is originated due the action of the trade winds from east to west
• It is stronger than the North equatorial current.
• It is further divided into many branches due to the presence of many islands and uneven surface topography.

c. Kuroshio Current

• Warm ocean current flowing in north easterly direction up to 30 N latitude
• It meets Oyashio cold current off the Kuril Islands.

d. East Australian Current

• Warm current flowing southward along the east coast of Australia

Side Note : Japan is good Fishing Ground

• Warm Kuroshio Current & cold Oyashio Current meets near Southern Japan. Hence, it becomes rich fishing ground.
• Along with that  Japan has very shallow continental Shelf  leading to  more photic zone and  more marine organisms.
• Indented coastline of Japan leads  to formation of  natural harbors .
• Furthermore, Japan is a hilly region with less developed agriculture . Hence, there is main role of fishes in Japanese Diet.

Side Note : Peru Current and Atacama Desert

Peru / Humboldt current is cold current. Hence, it provides desiccating effect to the South American coast leading to formation of Atacama Dessert . Atacama desert is one of the driest places on Earth.

Current Topic : Pacific Garbage Patch

Pacific Ocean Garbage Patch is an enormous collection of marine debris (mainly plastic) that is collected and deposited by ocean currents in the middle of the North Pacific Ocean. 

Reasons of formation

• Geographical : Circular motion of the ocean currents draws debris into the center which is calm & stable, where it becomes trapped.
• Almost 80% of the debris in the Great Pacific Garbage Patch comes from land-based activities in North America and Asia& the remaining 20% from boaters, offshore oil rigs, and large cargo ships. The amount of debris in the patch accumulates since most of it is non-biodegradable, mainly microplastics.

Impact

• Destruction of marine life due to ingestion of toxic and harmful substances .
• These toxic substances also enters the food chain resulting in biomagnification
• Presence of the garbage blocks the sunlight from reaching the planktons and algae, thereby reducing productivity of marine ecosystem.
• It affects the free flow of traffic through oceans.
• Impacts the tourism and economic livelihood of small Pacific Ocean island countries 

Current system in Indian Ocean is unique & different from other current systems .

Why it is different than other currents?

• In northern hemisphere, it is confined to tropical zone only & that too is divided by Indian peninsula to form Arabian sea & Bay of Bengal.
• Complete reversal in  direction of winds as monsoon winds also impacts the movement of current.

(wind is important factor – direction in which winds flow determine direction of currents ).

Side Note : Navigation in Indian Ocean.

• In Summer ,   Wind & current flows toward India . Hence, sea vessel can move from Arabia to India .
• In Winter,  Wind & current flows toward Arabia . Hence, sea vessel can move from India to Arabia .

Important for sea trade & cultural interaction since Ancient times.

• Ocean Currents play an important role in earth’s climate by distributing the energy within the ocean. Along with that, they impact the local climate as
  • Warm ocean current increases the temperature of an area
  • Cold ocean current decreases the temperature of an area.

• Currents also effect the amount of rainfall.
  • Warm water currents increase the rainfall because they bring winds along with them, laden with water vapours.  North America, Ireland, Britain and south India receive rainfall because of this process.
  • On the other hand Atacama desert remains dry because cold water currents don’t play any role in occurrence of rainfall.  Due to this various deserts are situated on the western coast of Australia, South America and Africa.

• Currents helps in free navigation navigation as Norweigian Current keeps ports of Scandinavia navigable throughout the year and Kuroshio Current keeps ports of Japan navigable during winter.

• Conjugation of warm and cold water currents helps in increasing the development rate of ‘Plankton’, which is a diet of fish.  This conjugation takes place of eastern coast near New Found land in North America, Bahia Blanca in South America, Japan coast in Asia etc. But on the  other hand conjugation of currents forms dense fog which creates problem for marine navigation. Sometimes ships are not able to move because of this fog.  This whole situation effects the economic activities of respective areas.

Vertical circulation of ocean water takes place due to difference in salinity and temperature between the surface and the water deep below.

Upwelling

Upwelling is an oceanographic phenomenon that involves movement of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water.

UPWELLING & FISHING

• Cool & nutrient rich water come at surface.
• Phytoplankton reproduce rapidly in this condition => Consequently, fishes  increase as well.
• All Upwelling Regions in World  are  good fishing ground. Eg : Canaries, Benguela and Peru Coast

Downwelling

• Down welling is the process of accumulation and sinking of cold high saline water beneath warmer or fresher water.
• Not good for fishing .

What are tides

• Rhythmic rise & fall in level of  ocean  water.

Causes of tide formations (Prelims-2015) .

• Moon’s gravitational pull is the most important factor.
• Sun’s gravitational pull ( although not as prominent factor as Moon’ pull but still a major factor)
• Centrifugal force due to earth’s rotation ( mainly responsible for causing bulges).

On the side of Earth facing the moon , a tidal bulge occurs while on the opposite side although the gravitational attraction of the moon is less as it is farther away, the centrifugal force causes tidal bulge on the other side 

Stages of Tides

Syzygy

• Alignment of three or more celestial bodies of same gravitational system in a line.
• Effects tides in form of variation between high tides & low tides.

a. Spring tide

• Sun , moon & earth are in line.
• Occur on full moon or new moon (no moon day)
• When there is greatest variation between high tide & low tide .
• High tide is higher than normal high tide & low tide is lower than normal low tide.

b. Neap tide

• Sun , Earth & Moon are at right angle.
• Occur when moon is at quarter phase.
• High tide lower than normal high tide & low tide higher than normal low tides.

Daily frequency of Tides

Earth rotates  & moon revolve in same direction,  hence in relative term wrt to moon earth take 24 hrs & 52 minutes to complete one rotation.

• 2 high tides & two low tides in a day (24.52 hours to be exact)=Semidiurnal
• Most enclosed water bodies & bodies away from open ocean experience only one high & one low tide .

Tidal Current

• Movement of ocean water as a result of tidal action is known as a tidal current.
• In places of narrow coastal inlet these tidal currents flow rapidly through the mouth with greater height and velocity. For example in the Bay of Fundy, between Nova Scotia and New Brunswick of Canada, the difference between high and low tides is as high as 14m. Ports which utilize the tidal current for entry and exit of ships from the harbour are known as tidal ports.

• Oceanic Crust is the most fundamental crust.
• It is Basaltic in nature .        
• Oceanic crust is more denser & darker  than Continental Crust.
• Age of continental shelfs is 1 billion year, oceanic crust is younger & nowhere aged more than 200 million years .
• Deepest point in ocean is Mariana Trench (in Pacific ocean near Northern Mariana Islands with depth 10,971 m )
• Oceanic Crust is created at Mid Oceanic Ridge & destroyed at Trenches.
• Ocean floor is not flat & more diverse than continents & quite different from continental ones.
• Oceans consist of  71% of earth’s crust. 

To study ocean bottom properly it should be divided into two divisions

• Ocean Continent Margin.
• Deep Ocean Plains.

1 . 1 Coast

Coast can be of two types

• Submergent Coast : Estuarine , Ria , Fiord and Dalmatian
• Emergent Coast

1 . 1 . 1 Submergent Coast

a. Fiord

• Initially mountains are perpendicular to the coast . Glaciers come down to the coast forming u-shaped valley submerging the area. Hence, coastline which was initially straight becomes indented
• Eg  : Norwegian Coast

b. Ria

• If valleys were submerged by rivers (instead of Glaciers) , it is called Ria
• Eg : South Hampton, Wales, British Isles etc

c. Dalmatian

• If mountains are parallel to the coast (instead of perpendicular)
• If ships are entering this coast, they face barriers
• Eg : Adriatic Coast (Italy)

d. Estuary

• If normal low land region (not mountainous) is submerged at the mouth of river .
• Eg : Thames, Buenos Aires

1 . 1.2 Emergent Coast

a. Lowland Emergence

• It usually results in lagoon, salt marshes, long beaches
• Eg : Eastern Coast of India

b. Emergent Upland Coast

• No natural harbors are found here
• Eg : Scotland

• Continental shelf is the seaward extension of land that lies under the sea water. It occupies 7% of the sea floor.
• It is quite shallow (approx 200m ).
• Region is called PHOTIC REGION ie receives large amount of sunlight . Marine organisms are found here (all floating planktons  , benthos(crabs) & nektons(fishes)).
• It has gentle slope with an average gradient of 1° or even less. The shelf typically ends at a very steep slope, called the shelf break.
• Width varies – The shelves are almost absent or very narrow along some of the margins like the coasts of Chile, the west coast of Sumatra, etc. On the contrary, the Siberian shelf in the Arctic Ocean, the largest in the world, stretches to 1,500 km in width . (average width is 80 km)
• The depth of the shelves also varies. It may be as shallow as 30 m in some areas while in some areas it is as deep as 600 m (Less depth means it will have more photic zone).
• Made up of Granite rock ( because it is part of continental crust).
• The continental shelves are covered with variable thicknesses of terrigenous sediments brought down by rivers, glaciers, wind, from the land and distributed by waves and currents. Massive sedimentary deposits received over a long time by the continental shelves, become the source of fossil fuels.

Continental shelves are well known for oil, natural gas, mineral deposits, sulphur, marine fishery  etc. Large number of resources are found in continental Shelf.

a. Petroleum

• 90% of petroleum reserves of world are found on continental shelves.
• Because petroleum is formed from submergence of marine living organisms which get converted to petroleum under high temperature & pressure (marine organisms like fish etc are maximum on Continental Shelf because of Photic Zone)
• Examples : Bombay High, Gulf of Cambay, Persian Gulf, North Sea, Barents Sea(Russia) , Gulf of Mexico, Norwegian Sea  .

b. Sulphur

• Rarely found on land .
• Available in sea because of  marine volcanism .
• Gulf of Mexico is rich source of Sulphur.

c. Placer Deposit

• Wave action erode beach rocks but there are some materials in rock which don’t disintegrate easily . Disintegrated  material of lower weight is moved very swiftly by waves but  heavy non-disintegrated material keeps on concentrating on shelf . These are called Placer Deposit.
• Example are as follows
  1. Monazite Sand found on Kerala Coast which is source of Thorium . It is important for  Indian Nuclear Program
  2. Gold in Alaska .
  3. Zircon on coast of Brazil & Australia.
  4. Diamond on South African Coast .

d. Calcium

• Least soluble in ocean water & can be easily extracted  .
• Peruvian coast  is rich in Calcium & Phosphate .

e. Fishes

• Due to photic zone and availability of planktons , fishes are also found in the wasters of Continental Shelf.
• World famous fishing grounds like Grand Bank are situated here

f. Pearls

• Pearls of ornamental value are also found here.

a. Internal waters

• All the water that is inside the coastline is called Internal Waters
• State has complete sovereignty over this and  it is equivalent to territory of country

b. Territorial Sea

• Upto 12 Nautical Miles (1 NM = 1.85 Km)  (provided there is no other country on the other side. If any other country is present, it becomes midway)
• Here too almost all of the laws of the land apply. It is almost as good as territory
• Vessel of any other nation need permission to enter into territorial waters of other country & submarine has to come on surface & declare its identity while navigation .

c. Contiguous Zone

• Upto 24 Nautical Miles .
• Some dilution of political laws
• Navy & Coast Guard cant guard this but if criminals like smugglers after  doing some crime try to run then that country can follow them till this point .

d. Exclusive Economic Zone (EEZ)

• Upto 200 Nautical Miles.
• All the economic laws of country apply but political laws don’t apply here
• Country can explore both biotic and abiotic resources of the water and ocean floor .
• Other countries have right to peaceful passage .

Exceptional Case

• If Continental shelf is beyond 200 Nautical Miles , country can claim EEZ upto that point with maximum limit of 350 NM
• Within this extended zone, state has exclusive right of exploiting and exploring non-living resources of the sea bed as well as sedentry species on the sea bed. Fishing rights arent inclusive in 200 to 350 Nautical Miles.

d. International waters / High Sea

• Beyond EEZ
• Here resources belong to everyone. No nation has sovereign rights over this area.

• The zone of steep slope extending from the continental shelf to the deep sea plain or abyssal plain is called continental slope.
• It has very steep slope .The slope angle varies from 5° to 60°.
• No deposition is possible because of steep slope . Hence,  no resources are  found in this zone. 
• But some unique landforms are created specially when there is mouth of very large river at continental shelf like  Indus, Ganga, Mississippi, Yukon & Congo. Example of such landform are
  • SUBMARINE CANYONS in which flow of water would be so great that it will vertically erode both continental shelf & slope . In Indian Ocean, submarine canyons are found at mouths of river Ganga and Indus.
  • Underwater Waterfalls – Highest waterfall on earth is under the Denmark strait.

An island is a landmass surrounded by water on all sides.

They are of four types

a. Continental Island

• Once part of large continent.
• Examples : Sri Lanka, Madagascar, Vancouver, Baffin islands.
• Note : Madagascar Island  formed after separation from Indo- Australian Plate
• Generally bigger in size than all other categories of Islands.
• Rock composition and all of continental island is same as plate from which it is separated .

b. Volcanic Island

• Ocean – Ocean plate collision or  Hotspot  activity.
• Examples : Andaman & Nicobar Islands, Japan, Philippines, Kurile, Aleutian Islands, Hawaii etc

c. Coral Island

• Marine organisms, the coral polyps colonize the tropical warm water and form islands known as coral islands. 
• Examples include Lakshadweep, Maldives, Mauritius, Bahama & Bermuda

d. Sandbar Island

• Formed by accumulation of sand near coast => Sand which is eroded from main land is again settled near coast & if continental shelf is shallow nearby then sandbar island is formed .
• Example : Long Island (New York)

Side Topic : Archipelago

• Cluster of islands  .
• (GK) Largest archipelago.

• Corals are indicative of health of Marine Ecosystem ie  if coral reef exists at any place in healthy way , entire ecosystem is healthy.
• Wherever there is coral reef , you will find great biodiversity at that point.
• Most diverse ecosystem of world => 10% area but 25% marine species . They can be said to be Rain Forests of the Ocean.

What are corals?

• Corals are tiny fleshy Sea Anemones Polyps.
• They extract Calcium from sea water for their skeletons to protect their body.When one generation die other generation comes on it.
• Corals live in symbiotic relationship with microscopic algae- Zoo-xanthalae which have photosynthesis capabilities.
• Zoo-xanthalae provide food to corals and corals provide protection to zooxanthalae.

Conditions conducive for  corals 

• Water  temperature  in  the  range  of    22-29  degrees  Celsius.  Thus,  areal  distribution  of  corals  is  limited to  tropical  and  sub-tropical  zones. 
• They  will  not  flourish  in  areas  of  cold  currents  but  thrive  in  warm current  regions.  Therefore,  they  are  generally  absent  on  western  coasts  of  continents.  
• Depth  of  water  should  not  exceed  180  feet  as  the  sunlight  is  too  faint  for  photosynthesis  which  is essential  for  survival  of  microscopic  Zoo-xanthalae  on  which  coral  polyps  depend.  However,  there  should  be plenty  of  water  as polyps cannot  survive for  too long  out  of  water.
• Water should  be  salty  and  free  of  sediments.  Corals  are  best  developed  on  seaward  side  of  reef,  where constantly  moving  waves, tides  and currents maintain  an abundant  supply  of  clear, oxygenated water

Spatial Distribution of corals in the world

Spatial distribution of Corals in India

In India they are found in

• Gulf of Mannar.
• Andaman &Nicobar
• Lakshadweep
• Gulf of Kutch
• Sindudurg coast ( near Malvan Coast) in Maharashtra  ( only here in India, bleaching hasn’t happened yet)

While the Lakshadweep reefs are atolls, the others are all fringing reefs.

Coral Bleaching

• When corals are in stress they expel Zoo-xanthalae.
• Zoo-xanthalae give the colourful appearance to corals . In their absence, corals seems white (their original colour) .
• Ultimately without food corals will die out too.

Reasons of coral Bleaching

1. Global Warming : increase in sea surface temperature .
2. Ozone Depletion : increase in solar irradiation.
3. Ocean Acidification : because of increase in Carbon Dioxide in atmosphere . This Carbon Dioxide dissolves in ocean water & water becomes acidic . Corals can’t live in acidic water
4. Algal Bloom : excessive use of fertilisers go to ocean through rivers resulting in Algal Bloom . Sunlight cant go below & corals die.
5. Increase in sedimentation from rivers.
6. Marine pollution like oil spills . No sunlight & oxygen reach below.
7. Diseases.
8. Coastal construction and shoreline development: Results in heavy sedimentation
9. Coral mining: Live coral is removed from reefs for use as bricks etc.

Consequences of Coral Bleaching

• It will impact marine ecosystem, as coral reefs are some of the most bio diverse and productive ecosystems.
• Reefs act as natural barriers to shorelines, protecting them from the effects of moving water. As coral reefs die, coastlines become more susceptible to damage and flooding from storms, hurricanes, and cyclones.
• Ocean will not be able to absorb as much Carbon Dioxide because Zoo-xanthalae absorb Carbon Dioxide for photosynthesis leading to increased Global Warming.

Measures need to be taken

• Political measures: The global agreements combating climate change must keep the needs of coral islands as top priority.
• Switching to sustainable livelihood measures: It is high time to recognize the carrying capacity of each coral island ecosystem and limit population growth, tourist foot fall etc. at these islands.
• Sustainable development : Avoid major projects in threatened areas. Eg : Carmichael Coal Mine in Australia that threatens Great Barrier Reef should be avoided.

Formation of Coral Reef

Coral Reef can be developed both at margins of continents or around islands  .

3 stages of coral formation are as follows

Stage 1 : Fringing

• Coral reef  starts to develop along margin of an island & subduction of island just starts
• First stage of Fringing Reef

Stage 2 : Barrier Reef

• Corals keep on accumulating and keep on rising  upwards. As a result, large part of island is submerged in sea/ocean.
• Lagoon formation occurs

Stage 3 : Atoll

• Last Stage
• Island is submerged & only ring remains visible . 

Note : In India, while the Lakshadweep reefs are atolls, the others are all fringing reefs.

Side Topic : Great Barrier Reef , Australia

• Adani’s are investing in Coal Fields in Australia (Carmichael Coal Fields) . Project will create jobs for Australians and coal will be imported to India
• But due to large Carbon-dioxide emissions in the area  , Great Barrier Reef will be impacted heavily.

• The long, narrow, steep-sided depressions formed  along the convergent boundary where one plate subducts below the other is  called Ocean trenches. 
• The Challenger Deep in the Mariana Trench, (10,994 m) in the Pacific Ocean is the deepest part of the earth.
• Most trenches are in Pacific ocean & they are most active  geological feature. Great earthquakes leading to Tsunamis occur here.
• As many as 57 deeps have been explored so far; of which 32 are in the Pacific Ocean; 19 in the Atlantic Ocean and 6 in the Indian Ocean.

Main Trenches of the world

• Also known as Deep Sea Fans or Underwater Deltas or Submarine Fans .
• Delta like structure formed at  deep sea surfaces . Major reservoirs of biodiversity.

• Mid-ocean ridges are submarine mountains.
• Mid oceanic ridges are located on the divergent plate boundaries where magma flows through the fissure to form new oceanic crust. They form the longest mountain range in the world extending for more than 56,000 km long.