Water (Oceans) 11 Geography Chapter 12 CBSE Notes 2025-26

Earth is called the ‘Blue Planet’ because more than 71% of its surface is covered with water, making it unique in the solar system. Water is crucial for life and is not found in abundance anywhere else in our solar system. The presence of vast oceans and other water bodies forms the foundation for diverse life forms and maintains the earth’s environment.

Hydrological Cycle The hydrological (water) cycle is the process that governs the continuous movement of water on, above, and below the surface of the Earth. Water from the oceans evaporates due to solar heat, enters the atmosphere as water vapour, condenses to form clouds, and falls as precipitation. A significant portion of water returns directly to the oceans, while some run off over land, infiltrates the ground, or collects in glaciers. Despite the constant cycling of water, the total amount available remains unchanged, but increasing demand and pollution have led to water crises in several regions.

Major Components and Processes of Water Cycle A table summarises the key components and processes:

Components and Processes of the Water Cycle

Component	Processes
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

Relief of the Ocean Floor The ocean floor is not flat but displays varied features formed due to tectonic, volcanic, and depositional processes. Oceans are divided into the Pacific, Atlantic, Indian, Southern, and Arctic Oceans. Large sections of the ocean floor are situated 3 to 6 km below the sea surface, containing the deepest trenches, highest mountain ranges, and flattest plains on earth.

Main Divisions of Ocean Floors

• Continental Shelf: The shallowest part of the ocean, forming the extended margins of continents, with gentle slopes and depths ranging from 30 m to 600 m. Width can be up to 1,500 km (Siberian shelf).
• Continental Slope: Connects the shelf and deeper ocean basins, with steep gradients and depths between 200 m and 3,000 m. It marks the true boundary of continents and often includes submarine canyons and trenches.
• Deep Sea Plain: Extensive, flat regions found at depths of 3,000–6,000 m, covered by fine-grained sediments like clay and silt.
• Oceanic Deeps (Trenches): The deepest areas with steep sides and depths 3–5 km greater than surrounding floors. They are closely related to volcanic and earthquake activity. Most trenches are found in the Pacific Ocean.

• Minor Relief Features: These include mid-oceanic ridges (underwater mountain chains, some above sea level like Iceland), seamounts (submarine volcanic mountains), guyots (flat-topped underwater mountains), submarine canyons (steep valleys cut into the continental slope), and atolls (coral islands in tropical oceans encircling lagoons).

Temperature of Ocean Waters The sun heats the ocean water, but it both heats up and cools down more slowly than land. The temperature is highest at the surface and decreases with depth.

Factors Affecting Temperature Distribution

• Latitude: Surface temperature decreases from the equator to the poles because solar energy is less intense at higher latitudes.
• Land and Water Distribution: More land in the northern hemisphere makes those oceans warmer.
• Prevailing Winds: Winds blowing offshore can bring cold water to the surface, while onshore winds push warm water towards the shore.
• Ocean Currents: Warm and cold currents redistribute temperature by moving warm water to cold regions and vice versa (e.g., the Gulf Stream and Labrador Current).

Temperature Distribution by Depth A typical vertical profile shows that temperature falls sharply with depth, particularly in the thermocline zone between 100–400 m. The ocean is typically divided into three layers in mid- and low-latitude regions:

• Warm surface layer (up to about 500 m, with temperatures of 20–25°C).
• Thermocline layer (sharp temperature decrease, from about 500–1,000 m).
• Cold water layer (around 0°C), extending from the bottom of the thermocline to the ocean floor.

Polar oceans are dominated by the cold water layer from top to bottom. The average surface temperature is about 27°C at the equator, dropping to 0°C near the poles.

Salinity of Ocean Waters Ocean salinity is the measure of all the dissolved salts in seawater, usually expressed in parts per thousand (ppt or o/oo). It influences the physical properties of seawater and the marine ecosystem, affecting processes like circulation, density, and the type of marine life present.

Factors Affecting Ocean Salinity Salinity at the ocean surface can change depending on the balance of evaporation (which increases salinity) and precipitation (which decreases it). Freshwater input from rivers and melting ice also lowers salinity in coastal and polar areas. In contrast, freezing and ice formation can increase surface salinity. Winds and ocean currents play a major role in distributing salty and less salty waters across the oceans.

Salinity Variations

• Average open ocean salinity is between 33 and 37 o/oo.
• In the Red Sea, it may reach 41 o/oo due to intense evaporation.
• Polar seas and river estuaries have lower salinity, sometimes almost down to zero in seasons of intense river inflow or ice melt.
• In hot, dry areas, salinity can exceed 70 o/oo.

Regional Examples of Salinity

• Atlantic Ocean (~36 o/oo) is saltier than the Pacific Ocean (35–31 o/oo).
• The Mediterranean Sea has high salinity due to more evaporation, while the Black Sea and Baltic Sea are less salty because of large rivers.
• The Indian Ocean has an average salinity of around 35 o/oo, with the Arabian Sea being saltier than the Bay of Bengal due to differences in river inflow and evaporation rates.

Salinity by Depth (Vertical Distribution) Surface salinity may vary with weather conditions, evaporation, or freshwater inflow. Below the surface, salinity becomes more stable and is often marked by a zone called the halocline, where salinity rises quickly with increasing depth. High-salinity water tends to sink, creating different layers within the ocean.

Quick Revision Points

• Water is constantly recycled in the environment through the hydrological cycle, which supports all life on earth.
• Major features of the ocean floor include the continental shelf, continental slope, deep sea plain, and trenches.
• Ocean temperature decreases from the equator to the poles and from the surface downward into the deep ocean.
• Salinity is influenced by evaporation, rainfall, river inflow, wind, and currents, and affects the properties of seawater and marine life.
• Unique minor ocean features include mid-oceanic ridges, seamounts, submarine canyons, guyots, and atolls.

Class 11 Geography Chapter 12 Notes – Water (Oceans) | Fundamentals of Physical Geography

These CBSE Class 11 Geography Chapter 12 notes provide a structured overview of Water (Oceans) and cover key topics like the hydrological cycle, ocean floor relief, temperature, and salinity. Students can quickly revise important sub-topics using well-organized points, tables, and clear explanations.

With these fundamental notes, learners can easily understand the core concepts, data, and features of oceanic geography. These notes are a helpful guide for last-minute revision and ensuring complete coverage of the NCERT syllabus for board exam preparation.