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The NCERT Class 11 Geography Chapter 12: Water (Oceans) Notes provide a comprehensive overview of the significance of oceans, their features, and properties. These concise notes cover the hydrological cycle, ocean floor relief, temperature distribution, and salinity variations, simplifying complex concepts from the Fundamentals of Physical Geography textbook. Designed for exam preparation and quick revision, they enhance understanding of oceanic processes and their role in sustaining life. Ideal for students, these notes are available as a free PDF for effective study and exam readiness.
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Explore Notes of Class 11 Fundamentals of Geography
Introduction
Water is essential for life, making Earth, with its abundant surface water, a unique ‘Blue Planet’ in the solar system. Oceans cover about 71% of Earth’s surface, holding most of the planet’s water. The remaining water exists as freshwater in glaciers, groundwater, lakes, soil moisture, atmosphere, streams, and organisms. The uneven distribution and increasing demand for water, coupled with pollution, lead to water crises globally.
Hydrological Cycle
The hydrological cycle describes the continuous movement of water within the Earth’s hydrosphere in liquid, solid, and gaseous forms. It involves the exchange of water between oceans, atmosphere, land surface, subsurface, and organisms. About 59% of water falling on land returns to the atmosphere via evaporation, while the rest runs off, infiltrates, or forms glaciers. Water is a renewable but constant resource, and its increasing demand and pollution exacerbate water scarcity.
Relief of the Ocean Floor
Oceans occupy major depressions in Earth’s crust, with complex topography shaped by tectonic, volcanic, and depositional processes. The oceanic part is divided into five oceans: Pacific, Atlantic, Indian, Southern, and Arctic. The ocean floor, found mostly between 3–6 km below sea level, features mountain ranges, trenches, and plains.
Divisions of the Ocean Floors
The ocean floor is divided into four major regions:
- Continental Shelf: Shallow, gently sloping margins of continents with an average gradient of 1° or less. Width varies (e.g., 80 km average, up to 1,500 km in the Siberian shelf). Depth ranges from 30 m to 600 m. Covered with sediments from rivers, glaciers, and waves, it is a source of fossil fuels.
- Continental Slope: Steep region (2–5° gradient) connecting the shelf to ocean basins, with depths of 200–3,000 m. Features include canyons and trenches.
- Deep Sea Plain: Gently sloping, flat regions at 3,000–6,000 m depth, covered with fine-grained clay and silt sediments.
- Oceanic Deeps or Trenches: Steep-sided, narrow basins 3–5 km deeper than the surrounding floors, associated with volcanoes and earthquakes. There are 57 deeps, with 32 in the Pacific, 19 in the Atlantic, and 6 in the Indian Ocean.
Also Read:
- NCERT Class 6 Geography: Chapter 5 Major Domains of the Earth
- NCERT Class 7 Geography Chapter 3 ‘Our Changing Earth’: Notes and Solutions (Free PDF)
Minor Relief Features
Here are the minor relief features of the oceans described:
- Mid-Oceanic Ridges: Mountain chains with peaks up to 2,500 m, separated by depressions (e.g., Mid-Atlantic Ridge, part of Iceland).
- Seamounts: Volcanic mountains with pointed summits, rising 3,000–4,500 m but not reaching the surface (e.g., Emperor seamount).
- Submarine Canyons: Deep valleys cutting across shelves and slopes, often linked to river mouths (e.g., Hudson Canyon).
- Guyots: Flat-topped seamounts, submerged due to subsidence, are common in the Pacific.
- Atolls: Low coral reef islands in tropical oceans, surrounding a central lagoon.
Temperature of Ocean Waters
Ocean water temperature varies spatially and vertically, influenced by solar heating, which is slower in water than on land.
Factors Affecting Temperature Distribution
Here we have mentioned the factors that affect the temperature distribution in the ocean:
- Latitude: Surface water temperature decreases from the equator to the poles due to decreasing insolation.
- Unequal Distribution of Land and Water: Northern hemisphere oceans are warmer due to greater land contact.
- Prevailing Winds: Offshore winds cause upwelling of cold water, lowering temperatures; onshore winds pile up warm water, raising temperatures.
- Ocean Currents: Warm currents (e.g., Gulf Stream) raise temperatures in cold areas; cold currents (e.g., Labrador) lower temperatures in warm areas.
Horizontal and Vertical Distribution of Temperature
Temperature in the ocean varies according to horizontal and vertical distribution. Here is what it means:
- Horizontal: Average surface temperature is 27°C, decreasing by 0.5°C per latitude from the equator (22°C at 20°, 14°C at 40°, 0°C near poles). Northern hemisphere oceans are warmer (19°C) than southern (16°C). Maximum temperature occurs slightly north of the equator.
- Vertical: Temperature decreases with depth. The thermocline (100–400 m to several hundred meters) marks a rapid temperature drop. Below it, 90% of the ocean water is near 0°C. A three-layer system exists in middle/low latitudes:
- Top layer (500 m, 20–25°C, year-round in tropics, summer-only in mid-latitudes).
- Thermocline layer (500–1,000 m, rapid temperature decrease).
- Deep layer (near 0°C, to the ocean floor). Arctic/Antarctic oceans have a single cold layer.
Also Read:
- NCERT Class 6 Geography: Chapter 3 Motions of the Earth
- NCERT Class 6 Geography: Chapter 1 The Earth in the Solar System
Salinity of Ocean Waters
Salinity is the total dissolved salt content in seawater, expressed as parts per thousand (ppt). Normal ocean salinity ranges from 33–37 ppt, with 24.7 ppt as the upper limit for brackish water.
Factors Affecting Ocean Salinity
Ocean Salinity depends on various factors. Some of these factors are mentioned below.
- Evaporation and Precipitation: High evaporation increases salinity; precipitation lowers it.
- Freshwater Inflow: Rivers and ice melting reduce salinity in coastal and polar regions.
- Winds: Transfer water, affecting salinity distribution.
- Ocean Currents: Influence salinity variations, interrelated with temperature and density.
Ocean Salinity also has two types: Horizontal and Vertical. Here, we have explained the Horizontal Distribution of Salinity and the Vertical Distribution of Salinity:
Horizontal Distribution of Salinity
- Pacific Ocean: Salinity ranges from 35 ppt to 31 ppt in the northern hemisphere due to Arctic meltwater; decreases to 33 ppt south of 15–20°S. The average is lower due to its large extent.
- Atlantic Ocean: Average salinity is 36 ppt, with a maximum of 37 ppt between 20°N–30°N and 20°W–60°W, decreasing northward. The North Sea has high salinity due to the North Atlantic Drift; the Baltic Sea has low salinity due to river inflow.
- Indian Ocean: Average salinity is 35 ppt. The Bay of Bengal has lower salinity due to river inflow; the Arabian Sea has higher salinity due to high evaporation.
- Other Regions: Red Sea reaches 41 ppt; hot, dry regions can hit 70 ppt; estuaries and Arctic vary seasonally (0–35 ppt).
Vertical Distribution of Salinity
Salinity varies with depth, increasing sharply in the halocline zone. Surface salinity rises with evaporation or ice formation and falls with freshwater input. Deep water salinity is stable. Higher salinity water sinks below lower salinity water, causing stratification.
Important Definitions in NCERT Notes Class 11 Geography Fundamentals of Physical Geography Chapter 12: Water (Oceans)
Here we have explained the key concepts and terms of this chapter to make it easy for you to understand.
- Hydrological Cycle: Continuous circulation of water in liquid, solid, and gaseous forms within the Earth’s hydrosphere.
- Continental Shelf: Shallow, gently sloping margin of continents, covered with sediments.
- Continental Slope: Steep region connecting the shelf to ocean basins.
- Deep Sea Plain: Flat, sediment-covered regions of ocean basins.
- Oceanic Deeps/Trenches: Deep, narrow basins associated with tectonic activity.
- Mid-Oceanic Ridges: Mountain chains on the ocean floor.
- Seamounts: Volcanic mountains not reaching the ocean surface.
- Submarine Canyons: Deep valleys on shelves and slopes.
- Guyots: Flat-topped, submerged seamounts.
- Atolls: Coral reef islands surrounding a lagoon.
- Thermocline: Boundary region with rapid temperature decrease with depth.
- Salinity: Total dissolved salt content in seawater, in parts per thousand (ppt).
- Halocline: Zone of sharp salinity increase with depth.
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FAQs
The hydrological cycle is the continuous movement of water within the Earth’s hydrosphere in liquid, solid, and gaseous forms, involving oceans, atmosphere, land, and organisms.
The ocean floor is divided into the continental shelf, continental slope, deep-sea plain, and oceanic deeps or trenches.
Ocean salinity is influenced by evaporation, precipitation, freshwater inflow from rivers and ice, winds, and ocean currents.
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