• The exploration of oceans, encompassing the study of structures, features, and the dynamic interplay between the ocean and the atmosphere, is termed oceanography. This field delves into all facets of the world's oceans and seas, including their physical and chemical attributes.

Oceans:

• Approximately 71% of Earth's surface is veiled by seas. The Pacific Ocean dominates with 35.4%, followed by the Atlantic Ocean at 18.4%, the Indian Ocean at 14.5%, and the Arctic Ocean at 3%. The remaining 29% is land surface.
• The average depth of oceans reaches 2.3 miles or 3.7 km. The Pacific Ocean boasts the highest average depth, trailed by the Indian Ocean, Atlantic Ocean, and Arctic Ocean.
• Ocean water is primarily composed of chloride, succeeded by sodium, sulfate, magnesium, and others.
• The Northern Hemisphere is labeled the continental hemisphere, housing around 40% of continents and 60% ocean. In contrast, the Southern Hemisphere is termed the oceanic hemisphere, as oceans envelop approximately 81% of the total surface.
• With an abundant water presence, Earth is aptly named the Water Planet and the Blue Planet, owing to its azure hue when viewed from space.
• Oceans play a pivotal role in the hydrological cycle, making their study crucial.

Hydrological Cycle:

• The Hydrological Cycle involves the perpetual circulation of water in Earth's hydrosphere, transitioning between liquid, solid, and gaseous phases.
• It also denotes the perpetual interchange of water among the oceans, atmosphere, land surface, sub-surface, and living organisms.

Distribution of Water:

• The distribution of water across Earth is notably uneven. Some areas boast abundant water resources, while others grapple with scarcity. Oceans harbor about 71% of the planetary water.
• The remaining portion exists as freshwater, encompassing glaciers, icecaps, groundwater reservoirs, lakes, soil moisture, atmosphere, streams, and within living organisms. 
• Nearly 59% of the water that descends on land reenters the atmosphere through evaporation over oceans and other regions. 
• The remaining portion either runs off on the surface, infiltrates into the ground, or contributes to the formation of glaciers.
• Renewable water on Earth remains constant, yet the escalating demand is causing a substantial increase.
• This surge in demand has led to water crises, both spatially and temporally, in various parts of the world. The predicament is further compounded by the pollution of river water.

Distribution of Water on the Earth's Surface

Oceans of the World

The world's five major oceans are described below:

(i) Pacific Ocean

• The Pacific Ocean holds the title of the world's largest ocean, covering an area of 165,200,000 sq km. Named by the Portuguese explorer Ferdinand Magellan, it boasts an average depth of 4,280 m, excluding adjacent seas.
• The Pacific Ocean is subdivided into the North Pacific and South Pacific by the Equator and is connected to the Atlantic Ocean via the Bering Strait. Major trenches include the Mariana Trench, Middle America Trench, Japan Trench, Kermadec Trench, Kuril-Kamchatka Trench, Tonga Trench, Aleutian Trench, Peru-Chile Trench (Atacama Trench), and Philippine Trench.

(ii) Atlantic Ocean

• The Atlantic Ocean, the world's second-largest ocean, spans an area of 106,400,000 sq km. Its name is derived from the Greek mythology of Atlantis Thalass, meaning the 'Sea of Atlas.' 
• Resembling an S-shape, it boasts an average depth of 3,646 m. The Atlantic Ocean plays a pivotal role in economic activities, providing grounds for fishing and hosting significant oil and natural gas production.
•  Major trenches include the Puerto Rico Trench, South Sandwich Trench, and Hellenic Trench.

(iii) Indian Ocean

• The Indian Ocean, the third-largest ocean globally, covers an area of 73,556,000 sq km and is named after India. 
• It constitutes 19.8% of the Earth's surface water and is situated between Africa, the Southern Ocean, Australia, and Asia. 
• Known as the warmest ocean, India has the right to explore polymetallic nodules in the Indian Ocean as per the United Nations Convention Law of the Sea (UNCLOS). 
• Islands in the Indian Ocean include Andaman and Nicobar Islands, Sri Lanka, Madagascar, Zanzibar, Lakshadweep (coral islands), and Maldives. Major trenches consist of the Diamantina Trench and Java or Sunda Double Trench.

(iv) Southern Ocean

• Also recognized as the Antarctic Ocean or Austral Ocean, the Southern Ocean is the fourth-largest ocean, spanning an area of 20,327,000 sq km. 
• It encircles the continent of Antarctica and extends northwards to 60° South latitude. This frigid ocean features a persistent easterly current.
• The Southern Ocean interacts with the Atlantic Ocean, Indian Ocean, and Southern Pacific Ocean.
• Prominent trenches in the Southern Ocean include the Factorian Deep and South Sandwich Trench.

(v) Arctic Ocean

• The Arctic Ocean, smaller and shallower than its counterparts, spans an area of 13,986,000 sq km. Situated within the Arctic Circle, it surrounds the North Pole.
• Linked to the Pacific Ocean through the narrow Bering Strait, the Arctic Ocean is bordered by Eurasia and North America. It becomes completely ice-covered during winter.
• With the lowest average salinity among the five major oceans, the deepest point in the Arctic Ocean is the Molloy Deep (5551 m).
• Significant trenches in the Arctic Ocean consist of the Molloy Deep and Litke Deep.

Oceanic Relief Features

• Oceanic relief features differ from continental features due to the relatively younger age of the oceanic crust, which is less than 70 million years old. In contrast, continental features are over 1 billion years old.

• Oceanic landforms significantly influence seawater movement and have substantial implications for human activities such as fishing and navigation.
• The diagram below illustrates various relief features, including:
  • Continental Shelf
  • Continental Slope
  • Continental Rise
  • Deep Sea Plain (Ocean Basin)
  • Ocean Deeps (Trenches)
  • Oceanic Ridge
  • Guyots
  • Submarine Canyons

Continental Shelf

• The continental shelf refers to the submerged margin of the continent, extending up to 100 fathoms (600 feet) under ocean water.
• Constituting 8.6% of the total ocean basin, its slope ranges from 1° to 3°, determined by coastal relief. Narrow shelves occur when a high mountain runs parallel to the coast, such as in the Atlantic Ocean, where it spans 2 km to 80 km wide.
• In the Pacific Ocean, the width of continental shelves varies from 160 km to 1600 km, influenced by the coastal land's relief characteristics.
• On average, the continental shelf of the Indian Ocean is 640 km wide in the West, narrows to 160 km near Java and Sumatra in the East, and further reduces along the coast of Antarctica.

Shelf Break

• The point where there is a marked increase in the slope angle is known as the shelf break. Approximately 20% of oil and gas production is extracted from these areas, supporting productive fishing grounds and serving as a valuable source of marine food (e.g., Dogger Bank and Grand Bank).
• The submergence and emergence of the continent play a crucial role in the origin of the continental shelf.
• These areas are not part of the oceanic crust but are composed of continental crust, namely granite and sediment derived from the erosion of land.

Continental Slope

• The continental slope is formed due to erosion, tectonic movement, and aggravations (an increase in land elevation).
• The continental slope has a steep incline beyond the continental shelf toward the ocean.
• It varies from 2° to more than 5° at different locations, occupying 8.5% of the total area of the ocean basin.
• The depth of water over continental slopes varies from 200 m to 2000 m.
• Continental slopes are indented by numerous submarine canyons and mounds.
• The boundary line between the continental slope and the shelf is called the andesite line.

Continental Rise

• It is a sudden rise of the bad land, which gives a V-shaped notch. The bottom of the V is known as the trench, and the slope is continental.
• The continental rise is generally absent in regions where deep-sea trenches exist and subduction zones are active.
• Continental rises feature deep-sea fans. In appearance, they are much like alluvial fans on land found along the fronts of mountain ranges.

Deep Sea Plain or Ocean Basin

• The ocean basin is a vast submarine region that collectively covers nearly three-quarters of Earth's water on the planet and has an average depth of almost 4 km (about 2.5 miles).
• The basins also contain a variable amount of sedimentary fill that is thinnest on the ocean ridges and usually thickest near the continental margins.
• Some places have less than 1 cm per 1000 years. The dominant geologic process is Planktonic rain, which blankets the seafloor with organic sediments.

Abyssal Plain

• The abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between 4500 m to 6000 m. It extends from the continental rise (continental lithogenous sediments accumulate along continental margins) to the distant deep ocean basin, where continental-derived sediment deposition is not significant.
• Abyssal plains are large horizontal seafloor regions, typically some of the flattest places on the Earth's surface.
• Abyssal plains are underlain by oceanic crust that formed and moved away from spreading centers associated with mid-ocean ridges and rises because they are so far from land. They have very slow sedimentation rates.

Ocean Deeps or Trenches

• Deep ocean trenches are formed during a process known as subduction. In subduction, one tectonic plate (a large portion of the Earth's crust) slides under another.
• The plate that is slipping under the other plate bends and forms an ocean trench. The subduction zones, where ocean trenches form, become long and deep valleys.
• These are the deepest parts of the ocean and are relatively steep-sided, narrow basins. They occur at the bases of continental slopes and along island arcs and are associated with active volcanoes and strong earthquakes.
• Major trenches in the Pacific Ocean, such as the Mariana Trench, Japan Trench, and Peru-Chile Trench, as well as the Puerto Rico Trench in the Atlantic Ocean and the Java Trench in the Indian Ocean, hold significance. The Pacific Ocean boasts around 32 trenches, the Atlantic Ocean has 19, and the Indian Ocean features 6.

Oceanic Ridge

• The oceanic ridge presents itself as a broad, fractured swell, spanning over 1,400 km in width, with peaks rising up to 3,000 m above the ocean floor.
• Encompassing approximately 22% of the ocean surface, oceanic ridges extend continuously from the Arctic basin through the center of the Atlantic Ocean into the Indian Ocean and across the South Pacific.
• The Mid-Atlantic Ridge, stretching about 14,450 km from Iceland in the North to Bouvet Island in the South, is a prominent example of an oceanic ridge. These formations are created by underwater lava eruptions that solidify into rocks.
• The age of seafloor rocks increases in parallel bands outward from central ocean ridges.

Other Oceanic Relief Features

• Seamount: An isolated mountain-sized feature rising above the seafloor, possibly a large tectonic block separated from a continental landmass or an ancient or active submarine volcano. An exposed portion above the ocean surface is termed an island.
• Atoll: A ring-shaped reef or island formed of coral, usually on the foundation of an extinct volcano in the ocean. Over time, the volcano may erode or sink, while the limestone ring continues to grow.
• Bank: A flat-topped elevation in continental margins, shallow enough for navigation, formed by erosional and depositional activities.
• Shoal: A detached elevation with shallow depth, projecting above the water and posing navigation risks.
• Guyots: Submarine volcanic mountains (seamounts) with flat tops, often ancient formations shaped by wave action before submerging into ocean depths. They may lack the fringing limestone reefs associated with atolls.
• Submarine Canyons: Narrow, steep-sided valleys originating within continental slopes or on a continental shelf. Examples include the Congo Canyon, the largest river canyon, and the Hudson Canyon, the world's best-known submarine canyon.
• The largest submarine canyon globally is the Zhemchug Canyon.

Movement of Continents and Oceans

• Scientists propose that the current configuration of continents and oceans has changed throughout Earth's history and will continue evolving.
• Currently, 71% of the Earth is covered by water, with the remaining 29% as landmass.
• Major theories explaining the formation of continents and oceans include the Continental Drift Theory, Convectional Current Theory, Seafloor Spreading Theory, and Plate Tectonic Theory.

Continental Drift Theory

• Proposed in 1912 by German climatologist Alfred Wegener, the theory hypothesizes the distribution of oceans and continents. 
• It suggests lateral movement of continents or continental drift, stating that continents float over oceans.
• Approximately 250 million years ago, the Earth featured a solitary supercontinent called Pangaea and an encompassing super-ocean named Panthalassa.
• Pangaea comprised North America (with Greenland attached) and Eurasia (excluding Arabia and India) in the extreme North. Further down, South America and Africa (with Arabia attached), and even further down, Antarctica, Australia, and India.
• Around 200 million years ago, Pangaea underwent fragmentation into two parts, one floating Northward and the other Southwards. The Northern part was referred to as Angara land (Laurasia), and the Southern part as Gondwana land. 
• The space between these two continental blocks was filled by a narrow, shallow water body known as the Tethys Sea.
• Laurasia included present-day North America, Greenland, and Eurasia (sans India and Arabia), while Gondwanaland encompassed Africa with Arabia, South America, Australia, and India. The South Pole was located near Durban in South Africa.
• Later, during the Jurassic period, further disruption of Gondwana land occurred. The Northward movement, driven by gravitational forces and the intense pull of the poles, caused the crumpling and deformation of sediments deposited in the Tethys, resulting in the formation of the Alpine and Himalayan mountain ranges.
• Westward movement, caused by tidal forces, led to the formation of the Andes and Rockies, along with the Caribbean islands in the Americas.
• The forces responsible for the drifting of continents, as suggested by Wegener, included polar-fleeing forces related to Earth's rotation and tidal forces caused by the gravitational attraction of the Moon and the Sun.
• Many scholars considered these forces insufficient for continental drifting. Nonetheless, the evidence presented by the continental drift theory establishes a foundation for future research.

Evidences Supporting the Theory

• Jigsaw fit of continents
• Glacial evidence
• Fossil similarities
• Similarities in mountain ranges
•  Structural similarities
• Coal deposits
• Evidence of glacial striations
• Criticisms of the theory centered around the insufficiency of forces responsible for continental drift, but the provided evidence offers a basis for further exploration.

Diagrammatic Representation of Drift of Continents

Convectional Current Theory

• Arthur Holmes explained the possibility of forces for drifting of continents. He stated that there is a possibility of permanent convection current operating in the whole mantle portion generated due to radioactive elements that drive the continents continuously.
• On this premise, scientists dismissed the Continental Drift Theory. The theory proposed that where convection currents collide, the area weakens, and over time, these currents break the lithosphere, leading to the fragmentation of continents.
•  Holmes elucidated that convection currents form a cell-like structure, and a system of such cells develops in the Earth's mantle.

Seafloor Spreading Theory

• The Seafloor Spreading hypothesis, introduced by geophysicist Harry H. Hess in 1961, focused on the ocean floor.
• It posits that the age of seafloor rocks varies. According to Hess, ocean floors, propelled by volcanic eruptions at the crust of oceanic ridges, sink at oceanic trenches and are consumed. 
• Therefore, the spreading of one ocean does not cause the shrinkage of another. The youngest rocks are concentrated in the central parts of oceans, while older rocks are found towards the margins.
• New crust forms in the central ocean, spreading towards the margins and pushing older rocks towards them—a concept known as Seafloor Spreading.

Plate Tectonics Theory

• Following the evolution of the Continental Drift Hypothesis and Seafloor Spreading, a broader concept, Plate Tectonics, emerged in 1967, termed Global Plate Tectonics. 
• Coined by J.T. Wilson in 1965, influential thinkers like Morgan, McKenzie, Parker, and Holmes contributed independently to this concept. The theory elucidates the large-scale motions of Earth's lithosphere due to tectonic plates.
• A tectonic plate, a massive irregularly shaped slab of solid rock, comprises both continental and oceanic lithosphere. Earth's lithosphere is divided into 7 Major plates and about 20 Minor plates, with thickness ranging between 5-100 km in oceanic areas and about 200 km in continental regions. 
• The classification of a plate as continental or oceanic depends on its large occupancy, where the Pacific plate stands as the largest oceanic plate, and the Eurasian plate is the largest continental plate.

• Major and minor plates encompass:
  • Eurasia and the adjacent oceanic plate.
  • Antarctica and the surrounding oceanic plate.
  • North American plate.
  • South American plate.
  • Pacific plate (the largest, covering 1/5th of Earth's surface).
  • India-Australia, New Zealand plate.
  • Africa with the Eastern Atlantic plate.
  • Cocos plate between Central America and the Pacific plate.
  • Nazca plate between South America and the Pacific plate.
  • The Arabian plate is mostly associated with the Saudi Arabian landmass.
  • Philippines plate between the Asiatic and Pacific plate.
  • Caroline plate between the Philippines and the Indian plate.
  • Fuji plate North-East of Australia.

Oceanic Plate:

• Oceanic plates, primarily beneath the ocean, form at divergent plate boundaries. Composed of mafic basaltic rocks, they are denser than continental plates due to their heavy ferromagnesian elements. 
• An example is the Pacific plate, stretching from the East Pacific Rise to the deep-sea trenches bordering the Western part of the Pacific basin.

Continental Plate:

• Containing continents, continental plates form mainly at convergent plate boundaries. 
• Felsic in nature, dominated by granitic rocks, a notable example is the North American Plate, encompassing North America and the oceanic crust between it and a portion of the mid-Atlantic ridge.

Plate Boundaries:

Plate boundaries typically fall into three types:

• Constructive margin or divergent plate boundary.
• Conservative margin or parallel boundary.
• Destructive margin or convergent plate boundary.

Constructive Margin or Divergent Plate Boundary:

• When two plates move apart, a crack forms between them, allowing molten lava from the asthenosphere to continuously emerge and create a new crust. 
• These plate boundaries are termed constructive plate margins, leading to the formation of a new crust and submarine mountains or ridges, such as the mid-Atlantic ridge, East-Pacific ridge, and Chagos-Laccadive ridge in the Indian Ocean.

(ii) Destructive Plate Margin or Convergent Plate Boundary:

• When two plates converge, forming a destructive plate margin, the denser plate is subducted below the lighter one. 
• This collision results in a region known as the Benioff zone or Subduction zone, also referred to as a Consuming plate margin. 
• Convergent plates interact in various ways, with an example being the Oceanic-Continental Plate Boundary, where an oceanic plate subducts below a continental plate, forming a trench, and often leading to the crumpling, folding, and uplift of the continental plate to create fold mountains like the Rockies and Andes.

• Oceanic-Oceanic Convergent Plate When the margins of one oceanic plate collide with another, one plate descends under the adjacent plate and melts down to form the part of the mantle. This may lead to the formation of oceanic trenches. Convergence of the Pacific and the Philippines plates explains the formation of islands, festoons and chain of volcanoes in that region e.g., Philippines and Indonesian islands.
• Continent-Continent Plate Boundary When both the plates are continental, the subduction at the Benioff zone is not so effective that it can give rise to vulcanicity in that region. 
• But, these regions are geographically unstable and long chains of fold mountains are formed here. The Himalayas are a striking example of the convergence of two continental plates. Other examples are Alps, Atlas and Ural mountains,

(iii) Conservative Margin or Parallel Boundary

• A conservative plate margin involves two tectonic plates sliding past each other. 
• Example of a conservative plate margin is the San Andreas Fault in California.
• Here, the Pacific plate and the North American plate are moving in the same direction but at different speeds.

Prelims Facts

• The surface of the Earth covered with water is approximately Which sea is the deepest sea in the world?

- South-China Sea (UPPSC (Mains) 2015]

• The fresh water available for use amounts to what percentage of total water found

- Less than 1% (UPPSC (Pre) 2010

• On the sea level, nearest place to the centre of the Earth is

- North Pole (BPSC (Pre) 1992]

• A Horizontal line to the sea level from where heights and depths are measured is called

Datum Line [UPPSC (Pre) 1991)

• The largest part of our hydrosphere is

-Pacific Ocean [UKPSC (Pre) 2010]

• The largest amount of freshwater on our planet is in

Glaciers and Mountains [UPPSC (Pre) 2015]

• The largest proportion of fresh water after ice caps and glaciers is

Groundwater [IAS (Pre) 2013]

• Which sea lies towards the North of Turkey?

Black Sea (MPPSC (Pre) 1991, IAS (Pre) 2014]

• Sargasso is related to the ocean, that ocean is

-Northern Atlantic (RAS (Pre) 1999, MPPSC (Pre) 2022]

• The Aral sea is lying between which countries?

-Kazakhstan and Uzbekistan [UPPSC (Pre) 2012]

• Red sea is an example of

- Axial Trough (UPPSC (Pre) 2017

• How many oceans are there on the planet Earth ?

-5 (UPPSC (Pre) 2010]

• Bermuda triangle is located in

-North Atlantic Ocean [UPPSC (Mains) 2008)

• Grenada an island country located is

Caribbean Sea (UPPSC (Pre) 2011

• The Republic of Malta is located in which sea ?

-Mediterranean [UPPSC (Pre) 2010]

• The busiest oceanic route in the world?

-North Atlantic Ocean [UPPSC (Pre) 1998, MPPSC (Pre) 2009, 2014]

• Where is the Celebes sea located?

-South-East-Asia [IAS (Prc) 2002]

• Where is 'Ninety East Ridge' situated?

- Indian Ocean [BPSC (Pre) 2011]

• What is the most extensive part of the oceanic floor?

- Deep Sea Plain [UKPSC (Pre) 2012, 2014]

• What is the name of the deepest oceanic trench?

- Mariana [UPPSC (Mains) 2015]

•  In which ocean Diamantina trench is situated?

- Indian ocean [IAS (Pre) 2006]

• Sunda trench lies parallel to the island of

- Java [BPSC (Pre) 2022]

• The largest coral reef in the world is found near the coast of which country?

- Australia [IAS (Pre) 2007

Self Check

1. On the planet Earth, most of the fresh water exists as icecaps and glaciers. Out of the remaining fresh water, the largest proportion

IAS (Pre) 2013

(a) is found in the atmosphere as moisture and clouds.

(b) is found in freshwater lakes and rivers.

(c) exists as groundwater.

(d) exists as soil moisture..

2. In which of the following ocean the largest mid-oceanic ridge is located?

(a) Atlantic 

(b) Pacific 

(c) Arctic

(d) Indian

3. Which of the following statements with regard to the continental shelf is not correct?

(a) Shelves are absent close to plate boundaries.

(b) They are highly sedimented.

(c) They have abrupt falls towards the continental slopes.

(d) They are rich fishing areas,

4. The ocean floors are divided into the following major divisions. Select the correct option.

1. The continental shelves
2. The continental slope
3. The continental ridge and hills
4. The shallow-deep sea plateau
5. The deep sea plain
6. The oceanic deeps

Codes

(a) 1, 2, 3 and 5

(b) 1, 2, 5 and 6

(c) 1, 2, 4 and 5

(d) 2, 3, 4, 5 and 6

5. Match the List I with List II and select the correct answer by using the codes given below the lists.

UPPSC (Mains) 2017

Codes

(a) 4 3 2 1

(b) 34.12

(c) 1342

(d) 2 4 3 1

6. Which of the following is the busiest ocean (oceanic route) in the world?

UPPSC (Pre) 1998, MPPSC (Pre) 2014

(a) Indian ocean

(b) North Atlantic ocean

(c) South Atlantic ocean

(d) Pacific ocean

7. A ridge of 64,000 km length and 2,000 km to 2,400 km width passing through the North and South Atlantic oceanic basin enter into the Sour Pacific oceanic basin through Indian oceanic and then from the middle of Australia and This ridge is

(a) Socotra Lakshadweep Chagos ridge

(b) Pacific Antarctic ridge

(c) Dolphin Challenger ridge

(d) Mid-Oceanic ridge

9. Which of the following statement(s) is/are true? MPSC (Pre) 2016

1. Recent studies have shown that convergence of the Indian plate and Asian plate has caused a crustal shortening of about 500 km in the Himalayan region.
2. There has also been seafloor spreading along the oceanic ridge in the Indian ocean.

Codes

(a) 1 and 2 are true

(b) 1 is true and 2 is false

(c) 1 is false and 2 is true

(d) 1 and 2 are false

10. Plate tectonics is a scientific theory that describes the large-scale motions of Earth's lithosphere.

Which one among the following statement(s) regarding plate tectonics is/are not correct?

(a) Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere.

(b) Tectonic plates are able to move because the Earth's lithosphere has a higher strength than the underlying asthenosphere. 

(c) The Earth's lithosphere is tectonic plates. ere is broken up into

(d) Along divergent plate boundaries, subduction carries plates into the mantle.

Know Right Answer

1 (c)

2 ( a)

3 (a)

4 (b)

5 (b)

6 (b)

7 (d)

8 (c)

9 (c)

10 (b)