Chapter 12 – Water (Oceans) | CBSE Notes

GEOGRAPHY | CLASS XI | NCERT

Book: Fundamentals of Physical Geography | Chapter 12

Water (Oceans)

⭐ Topper Level 💬 Easy Language 📌 Point-Wise 🌊 Ocean Floor Covered

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1. Learning Objectives

After reading these notes, you will be able to:

Understand the Hydrological Cycle — its components, processes and importance.

Know the four major divisions of the ocean floor — Continental Shelf, Continental Slope, Deep Sea Plain and Oceanic Deeps.

Describe minor relief features of the ocean floor — ridges, seamounts, guyots, submarine canyons, atolls.

Understand the temperature distribution of ocean waters — horizontal, vertical and the thermocline concept.

Understand ocean salinity — definition, factors, horizontal and vertical distribution, halocline.

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2. Hydrological Cycle

Water is a cyclic resource — it can be used and re-used. Earth is called the ‘Blue Planet’ because it has an abundant supply of water on its surface. Water is a rare commodity in our solar system — there is no water on the Sun or anywhere else in the solar system. Next to air, water is the most important element required for the existence of life on Earth.

📖 Definition — Hydrological Cycle

The hydrological cycle is the circulation of water within the Earth’s hydrosphere in different forms — liquid, solid and gaseous phases. It also refers to the continuous exchange of water between the oceans, atmosphere, land surface, subsurface and organisms. The water cycle has been working for billions of years.

91%

Of planetary water is found in the oceans

59%

Of water that falls on land returns to the atmosphere through evaporation

5 Oceans

Pacific, Atlantic, Indian, Southern and Arctic — divided by geographers

3–6 km

Depth at which a major portion of the ocean floor is found below sea level

🔄 Components and Processes of the Water Cycle

Component	Processes Involved
Water storage in oceans	Evaporation, Evapotranspiration, Sublimation
Water in the atmosphere	Condensation, Precipitation
Water storage in ice and snow	Snowmelt runoff to streams
Surface runoff	Stream flow, freshwater storage, infiltration
Groundwater storage	Groundwater discharge, springs

📌 Important Note

The renewable water on Earth is constant while the demand is increasing tremendously. This leads to water crisis in different parts of the world — spatially and temporally. Pollution of river waters has further aggravated the crisis.

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3. Relief of the Ocean Floor

The ocean floors exhibit complex and varied features — just like those observed over the land. The floors are rugged with the world’s largest mountain ranges, deepest trenches and the largest plains. These features are formed by tectonic, volcanic and depositional processes.

🗺️ Four Major Divisions of Ocean Floor

🌊 Mind Map — Ocean Floor Divisions

Ocean Floor

🏖️ Continental Shelf
Shallowest part
Avg gradient 1°
Avg width 80 km

📉 Continental Slope
Connects shelf
to ocean basin
Gradient 2–5°

🏜️ Deep Sea Plain
Flattest region
Depth 3000–6000 m
Clay and silt

🕳️ Oceanic Deeps
Deepest parts
3–5 km deeper
57 deeps explored

🏖️ 1. Continental Shelf

The extended margin of each continent occupied by relatively shallow seas and gulfs. Shallowest part of the ocean — average gradient of 1° or even less. Ends at a very steep slope called the shelf break.

Average width: 80 km. Narrowest: coasts of Chile, west coast of Sumatra. Largest: Siberian shelf in Arctic Ocean — 1,500 km wide.
Depth: 30 m to 600 m. Covered with sediments brought by rivers, glaciers, wind. Massive sedimentary deposits → source of fossil fuels.

📉 2. Continental Slope

Connects the continental shelf and the ocean basins. Begins where the continental shelf sharply drops off into a steep slope. Gradient: 2–5°. Depth: 200–3,000 m. The slope boundary indicates the end of the continents. Canyons and trenches are observed in this region.

🏜️ 3. Deep Sea Plain

Gently sloping areas of the ocean basins. These are the flattest and smoothest regions of the world. Depths: 3,000–6,000 m. Covered with fine-grained sediments like clay and silt.

🕳️ 4. Oceanic Deeps / Trenches

Deepest parts of the oceans. Relatively steep-sided, narrow basins. 3–5 km deeper than the surrounding ocean floor. Occur at bases of continental slopes and along island arcs. Associated with active volcanoes and strong earthquakes — significant for study of plate movements.

57 deeps explored so far: Pacific = 32, Atlantic = 19, Indian = 6.

⛰️ Minor Relief Features of Ocean Floor

Feature	Description	Example
Mid-Oceanic Ridge	Two chains of mountains separated by a large depression. Peaks can be 2,500 m high — some reach above ocean surface.	Iceland (part of mid-Atlantic Ridge)
Seamount	Mountain with pointed summits rising from seafloor — does not reach surface. Volcanic in origin. 3,000–4,500 m tall.	Emperor Seamount (extension of Hawaiian Islands, Pacific)
Submarine Canyon	Deep valleys comparable to the Grand Canyon. Found cutting across continental shelves and slopes. Often extend from mouths of large rivers.	Hudson Canyon — best known in the world
Guyot	Flat-topped seamount. Evidence of gradual subsidence. More than 10,000 seamounts and guyots exist in Pacific Ocean alone.	Found across Pacific Ocean
Atoll	Low islands in tropical oceans consisting of coral reefs surrounding a central depression. Central part may be sea (lagoon) or enclosed freshwater/saline water body.	Tropical ocean islands

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4. Temperature of Ocean Waters

Ocean waters get heated up by solar energy just as land. However, the process of heating and cooling of oceanic water is slower than land. The maximum temperature of oceans is always at their surfaces — they directly receive heat from the Sun; heat is transmitted to lower sections through convection.

📋 Factors Affecting Temperature Distribution of Ocean Water

🌐 1. Latitude

Temperature of surface water decreases from equator towards poles because the amount of insolation decreases poleward.

🗺️ 2. Unequal Distribution of Land and Water

Oceans in the Northern Hemisphere receive more heat due to their contact with a larger extent of land than the oceans in the Southern Hemisphere.

💨 3. Prevailing Wind

Winds blowing from land towards oceans drive warm surface water away from coast → upwelling of cold water from below → longitudinal temperature variation. Onshore winds pile up warm water near coast → raise temperature.

🌊 4. Ocean Currents

Warm currents raise temperature in cold areas; cold currents decrease temperature in warm ocean areas. Example: Gulf Stream (warm) raises temperature near eastern coast of N. America and west coast of Europe. Labrador current (cold) lowers temperature near NE coast of N. America.

📊 Key Temperature Data — Ocean Surface

Location / Latitude	Average Surface Temperature
Overall ocean average	~27°C
20° latitude	~22°C
40° latitude	~14°C
Near Poles	~0°C
Northern Hemisphere (annual avg)	~19°C
Southern Hemisphere (annual avg)	~16°C

📌 Important Facts

The highest temperature is NOT recorded at the equator but slightly towards the north of it. Northern Hemisphere records relatively higher temperature than Southern Hemisphere — due to unequal distribution of land and water. Enclosed seas in low latitudes record relatively higher temperature than open seas; enclosed seas in high latitudes have lower temperature than open seas.

📉 Vertical Distribution — Three Layers

🔴 First Layer (Top — Warm)

Top layer of warm oceanic water. About 500 m thick. Temperature: 20°–25°C. Present throughout the year in tropical regions; develops only during summer in mid-latitudes.

🔵 Second Layer (Thermocline)

Lies below the first layer. Characterised by rapid decrease in temperature with increasing depth. Thermocline is 500–1,000 m thick. Boundary usually begins around 100–400 m below sea surface.

❄️ Third Layer (Cold — Deep)

Very cold; extends up to the deep ocean floor. Temperature approaches 0°C. About 90% of total volume of ocean water is found below the thermocline in this zone.

📖 Thermocline — Definition

The thermocline is the boundary region between the surface waters of the ocean and the deeper layers — from where there is a rapid decrease of temperature. The temperature falls very rapidly up to 200 m depth and thereafter, the rate of decrease is slowed down. Rate of decrease: generally 0.5°C per latitude.

📌 Arctic and Antarctic Regions

In Arctic and Antarctic circles, the surface water temperatures are close to 0°C — so temperature change with depth is very slight. Here, only one layer of cold water exists — extends from surface to deep ocean floor.

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5. Salinity of Ocean Waters

All waters in nature — whether rain water or ocean water — contain dissolved mineral salts. Salinity is the term used to define the total content of dissolved salts in sea water. It is calculated as the amount of salt (in grams) dissolved in 1,000 gm (1 kg) of seawater. Expressed as parts per thousand (‰ or ppt). Salinity of 24.7‰ is the upper limit to demarcate ‘brackish water’.

330‰

Lake Van, Turkey — Highest salinity water body

238‰

Dead Sea — Second highest salinity

220‰

Great Salt Lake — Third highest salinity

41‰

Red Sea — Highest salinity among open seas (land-locked)

📋 Factors Affecting Ocean Salinity

Evaporation and Precipitation: Salinity of surface layer depends mainly on evaporation and precipitation. High evaporation → high salinity; high precipitation → low salinity.
Fresh water from rivers (coastal) and freezing/thawing of ice (polar): Surface salinity greatly influenced — river influx reduces salinity; freezing increases salinity.
Wind: Influences salinity of an area by transferring water to other areas.
Ocean Currents: Contribute to salinity variations. Salinity, temperature and density of water are interrelated — any change in temperature or density influences salinity.

🌍 Horizontal Distribution of Salinity

Water Body	Salinity	Reason
Normal open ocean	33‰ – 37‰	Standard range for open seas
Red Sea (land-locked)	Up to 41‰	High evaporation, limited freshwater influx
Estuaries and Arctic	0‰ – 35‰ (seasonal)	Seasonal variation
Hot and dry regions	Up to 70‰	Very high evaporation
Atlantic Ocean (avg)	~36‰	Max 37‰ between 20°N–30°N
Indian Ocean (avg)	35‰	—
Bay of Bengal	Low	Influx of river water (large rivers)
Arabian Sea	Higher	High evaporation + low freshwater influx
North Sea	Higher (despite high latitude)	Saline water brought by North Atlantic Drift
Baltic Sea	Low	Large influx of river water
Mediterranean Sea	Higher	High evaporation
Black Sea	Very low	Enormous freshwater influx by rivers

📉 Vertical Distribution of Salinity

📖 Halocline — Definition

Salinity generally increases with depth. There is a distinct zone called the halocline where salinity increases sharply. Lower salinity water rests above the higher salinity dense water. High salinity seawater generally sinks below the lower salinity water → leads to stratification by salinity. Salinity at depth is very much fixed — no way water is ‘lost’ or salt is ‘added’ at depth.

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Summary — Quick Revision

Blue Planet: Earth has abundant water. Next to air, water = most important for life. Water = rare in solar system. 91% of planetary water is in oceans. 59% of water that falls on land returns to atmosphere through evaporation.

Hydrological Cycle: Continuous circulation of water in liquid, solid and gaseous forms between oceans, atmosphere, land surface, subsurface and organisms. Has been working for billions of years.

4 Major Ocean Floor Divisions: Continental Shelf (shallowest, avg 80 km wide, gradient 1°), Continental Slope (gradient 2–5°, depth 200–3000 m), Deep Sea Plain (3000–6000 m, flattest region), Oceanic Deeps/Trenches (57 explored — Pacific 32, Atlantic 19, Indian 6).

Minor Features: Mid-Oceanic Ridge (two mountain chains with depression — Iceland example), Seamount (pointed, volcanic, 3000–4500 m), Submarine Canyon (deep valleys — Hudson Canyon), Guyot (flat-topped seamount), Atoll (coral reef + lagoon in tropics).

Ocean Temperature: Avg surface = 27°C. Decreases equator to poles — rate 0.5°C per latitude. NH avg 19°C; SH avg 16°C. Highest temperature NOT at equator but slightly north. Temp falls rapidly up to 200 m depth, then slows.

3 Thermal Layers: First (surface, 500 m, 20–25°C), Second (thermocline, 500–1000 m, rapid temperature drop), Third (very cold, deep, ~0°C — 90% of ocean volume). Arctic/Antarctic = only ONE cold layer.

4 Factors Affecting Ocean Temperature: Latitude, Unequal distribution of land and water, Prevailing wind (onshore/offshore), Ocean currents (warm raises temp, cold lowers temp — Gulf Stream, Labrador current).

Salinity: Amount of salt (gm) per 1,000 gm of seawater. Expressed in ‰ (ppt). Brackish water limit = 24.7‰. Normal open ocean = 33–37‰. Highest: Lake Van Turkey (330‰), Dead Sea (238‰), Great Salt Lake (220‰), Red Sea (41‰).

Factors affecting salinity: Evaporation and precipitation (main), river influx (coast), freezing/thawing (polar), wind, ocean currents. Salinity, temperature and density are interrelated.

Vertical Salinity: Generally increases with depth. Halocline = zone where salinity increases sharply. Lower salinity water rests above denser higher salinity water → stratification by salinity. Deep salinity is very much fixed.

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Important Terms to Remember

Hydrological Cycle: The continuous circulation of water within Earth’s hydrosphere in liquid, solid and gaseous phases — between oceans, atmosphere, land surface, subsurface and organisms.
Blue Planet: Earth is called the Blue Planet because of its abundant water supply. Water is rare in the solar system — not found on the Sun or elsewhere.
Continental Shelf: The extended margin of each continent occupied by relatively shallow seas and gulfs. Average gradient 1° or less. Average width 80 km. Ends at the shelf break. Depths: 30–600 m. Largest: Siberian shelf (1,500 km wide).
Shelf Break: The very steep slope at which the continental shelf typically ends, marking the beginning of the continental slope.
Continental Slope: Connects the continental shelf and the ocean basins. Gradient 2–5°. Depth 200–3,000 m. Marks the end of continents. Canyons and trenches found here.
Deep Sea Plain: Gently sloping areas of ocean basins. The flattest and smoothest regions of the world. Depth 3,000–6,000 m. Covered with fine-grained clay and silt.
Oceanic Deeps / Trenches: Deepest parts of the oceans. Steep-sided narrow basins. 3–5 km deeper than surrounding ocean floor. Associated with active volcanoes and earthquakes — significant for plate movement study. 57 explored so far.
Mid-Oceanic Ridge: Two chains of mountains separated by a large depression on the ocean floor. Peaks can be up to 2,500 m. Some reach above ocean surface (e.g., Iceland — part of mid-Atlantic Ridge).
Seamount: A mountain with pointed summits rising from the seafloor but not reaching the ocean surface. Volcanic in origin. Height 3,000–4,500 m. Example: Emperor Seamount (Pacific Ocean).
Guyot: A flat-topped seamount. Shows evidence of gradual subsidence. More than 10,000 seamounts and guyots exist in the Pacific Ocean alone.
Submarine Canyon: Deep valley on the ocean floor, sometimes comparable to the Grand Canyon. Found cutting across continental shelves and slopes. Often extend from mouths of large rivers. Best known: Hudson Canyon.
Atoll: Low islands found in tropical oceans consisting of coral reefs surrounding a central depression (lagoon). May enclose sea water, fresh water, brackish or saline water.
Thermocline: The boundary region between the surface waters of the ocean and the deeper layers — where there is a rapid decrease of temperature with depth. Usually begins 100–400 m below sea surface. Thickness: 500–1,000 m.
Three Thermal Layers of Ocean: First layer (warm, 500 m, 20–25°C), Second layer (thermocline, rapid cooling), Third layer (very cold, ~0°C, 90% of ocean volume).
Upwelling: The process where winds blowing from land drive warm surface water away from coast, causing cold water from below to rise to the surface.
Salinity: The total content of dissolved salts in sea water. Calculated as amount of salt (gm) dissolved in 1,000 gm of seawater. Expressed in parts per thousand (‰ or ppt).
Brackish Water: Water with salinity between that of fresh water and seawater. Salinity of 24.7‰ is the upper limit to demarcate brackish water.
Halocline: A distinct zone in the ocean where salinity increases sharply with depth. Lower salinity water rests above higher salinity dense water — leading to stratification by salinity.
Stratification by Salinity: The layering of ocean water based on salinity — high salinity (denser) water sinks below low salinity (lighter) water, forming distinct layers.
Gulf Stream: A warm ocean current in the Atlantic Ocean — raises temperature near the eastern coast of North America and the west coast of Europe.
Labrador Current: A cold ocean current — lowers temperature near the north-east coast of North America.