Chapter – 12

Water (Oceans)

In this post we have given the detailed notes of class 11 Geography Book 1 Chapter 12 (Water (Oceans)) in English. These notes are useful for the students who are going to appear in class 11 board exams.

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Chapter 12: Water (Oceans)

Introduction

• Water is a vital component for all life forms on Earth, playing a crucial role in biological processes.  
• Earth is called the “Blue Planet” due to its abundance of water, which distinguishes it from other planets in our solar system.  

Hydrological Cycle

• The hydrological cycle is the continuous movement and transformation of water within the Earth’s hydrosphere, encompassing various forms like liquid, solid (ice), and gas (water vapor).  
• It involves the dynamic exchange of water between oceans, the atmosphere, land surfaces, and even subsurface organisms.  
• Key processes in the hydrological cycle include:
  • Evaporation: The transformation of liquid water into water vapor, primarily from oceans and other water bodies.  
  • Evapotranspiration: The combined process of evaporation from open water bodies and transpiration from plants.  
  • Sublimation: The direct conversion of ice into water vapor.  
  • Condensation: The transformation of water vapor into liquid water, forming clouds.  
  • Precipitation: The release of water from clouds in various forms, such as rain, snow, sleet, or hail.  
  • Snowmelt Runoff: The flow of melted snow into streams and rivers.  
  • Streamflow: The movement of water in rivers and streams.  
  • Infiltration: The process of water seeping into the ground.  
  • Groundwater Discharge: The flow of groundwater into springs, rivers, or oceans.  

• Distribution of Water:

  • Oceans hold about 71% of the Earth’s total water.  
  • The remaining 29% is freshwater, distributed among glaciers, ice caps, groundwater, lakes, soil moisture, atmosphere, streams, and living organisms.  

• Water Scarcity and Pollution:

  • The increasing demand for renewable water resources, coupled with pollution, has led to water scarcity issues in many regions.  

Relief of the Ocean Floor

• The ocean floor is not a flat, featureless plain. It exhibits a variety of landforms, similar to those found on continents.  
• These features are formed by various geological processes, including:
  • Tectonic activity: Movement of Earth’s crustal plates, causing earthquakes, volcanic eruptions, and the formation of mountains and trenches.  
  • Volcanic activity: Eruptions of underwater volcanoes, creating seamounts and volcanic islands.  
  • Depositional processes: Accumulation of sediments carried by rivers, glaciers, and wind, forming continental shelves and plains.  

• Major Divisions of the Ocean Floor:

  • Continental Shelf:
    • The shallow, submerged extension of a continent, gently sloping away from the land.  
    • It is the shallowest part of the ocean, with an average depth of about 80 km.  
    • The width of the continental shelf varies significantly, from nearly absent to over 1,000 km wide (e.g., the Siberian shelf).  
    • It is covered with sediments brought down from the land by rivers, glaciers, and wind, and redistributed by waves and currents.  
    • These sediments can become sources of fossil fuels over time.  
  • Continental Slope:
    • The steeply sloping region that connects the continental shelf to the deep ocean basin.  
    • It begins at the shelf break, where the continental shelf abruptly ends.  
    • The gradient of the slope is typically between 2 and 5, and the depth varies from 200 to 3,000 meters.  
    • Canyons and trenches are often found in this region.  
  • Deep Sea Plain:
    • The flat and smooth area of the ocean basin, covering a vast portion of the ocean floor. 
    • It is one of the flattest regions on Earth, with depths ranging from 3,000 to 6,000 meters.  
    • Covered with fine-grained sediments like clay and silt.  
  • Oceanic Deeps or Trenches:
    • The deepest parts of the ocean, characterized by steep sides and narrow basins.  
    • They are typically 3 to 5 km deeper than the surrounding ocean floor.  
    • Trenches are associated with active volcanoes and strong earthquakes, making them important for studying plate tectonics.  
    • Most trenches are found in the Pacific Ocean.  

• Minor Relief Features:

  • Mid-Oceanic Ridges:
    • Underwater mountain ranges formed by plate tectonics, where new oceanic crust is created.  
    • They consist of two parallel chains of mountains separated by a large depression (rift valley).  
    • Some peaks rise above the ocean surface, forming islands like Iceland.  
  • Seamounts:
    • Underwater mountains with pointed summits, formed by volcanic activity.  
    • They rise from the seafloor but do not reach the ocean surface.  
  • Submarine Canyons:
    • Deep, narrow valleys cut into the continental shelf and slope.  
    • Often formed by the extension of large rivers underwater.  
  • Guyots:
    • Flat-topped seamounts, formed by the erosion of volcanic islands as they subside over time.  
  • Atolls:
    • Low-lying islands formed by coral reefs surrounding a central lagoon.  
    • Typically found in tropical oceans.  

Temperature of Ocean Waters

• Factors Affecting Temperature Distribution:

  • Latitude: The primary factor, as the amount of solar energy received decreases towards the poles.  
  • Distribution of Land and Water: The Northern Hemisphere, with more landmass, has warmer oceans compared to the Southern Hemisphere.  
  • Prevailing Winds: Winds blowing from land to ocean cause upwelling of cold water, while onshore winds pile up warm water near the coast.  
  • Ocean Currents: Warm currents raise temperatures in cold regions, while cold currents lower temperatures in warm regions.  

• Temperature-Depth Profile:

  • The ocean exhibits a three-layer temperature structure in middle and low latitudes:
    • Surface Layer: Warmest layer, about 500 meters thick, with temperatures between 20°C and 25°C.  
    • Thermocline Layer: A zone of rapid temperature decrease with depth, 500 to 1,000 meters thick.  
    • Deep Ocean Layer: Extends to the ocean floor, with very cold temperatures near 0°C.  
  • In polar regions, the water is uniformly cold from surface to bottom.  

• Average Temperature:

  • The average surface temperature of the oceans is about 27°C.  
  • It decreases towards the poles at a rate of about 0.5°C per latitude.  
  • The Northern Hemisphere has slightly warmer oceans than the Southern Hemisphere due to the distribution of land and water.  

Salinity of Ocean Waters

• Salinity is the total amount of dissolved salts in seawater, measured in parts per thousand (°/₀₀) or ppt.  

• Factors Affecting Salinity:

  • Evaporation: Increases salinity by removing freshwater.  
  • Precipitation: Decreases salinity by adding freshwater.  
  • Freshwater Flow from Rivers: Reduces salinity in coastal areas.  
  • Freezing and Thawing of Ice: Affects salinity in polar regions.  
  • Wind: Influences salinity by transferring water.  
  • Ocean Currents: Contributes to salinity variations by mixing water masses.  

• Salinity Variation:

  • The average salinity of the open ocean is between 33°/₀₀ and 37°/₀₀.  
  • Salinity varies across different oceans and regions due to factors like evaporation, precipitation, and freshwater influx.  

• Vertical Distribution of Salinity:

  • Salinity generally increases with depth.  
  • Halocline: A zone where salinity increases sharply with depth.  
  • High salinity water is denser and tends to sink below lower salinity water, leading to stratification.  

Movements of Ocean Water

Ocean water is in constant motion, driven by various forces. These movements are crucial for regulating Earth’s climate, distributing nutrients, and supporting marine life. The major types of ocean water movements are:

Waves:

• Definition: Waves are disturbances that travel along the water’s surface, characterized by their crests (high points) and troughs (low points).  
• Causes: Most waves are generated by wind blowing over the water surface. The stronger the wind and the longer it blows, the larger the waves become.  
• Types: There are various types of waves, including:
  • Wind waves: Generated by local winds.  
  • Swells: Waves that have traveled away from their source and become more regular and rounded.  
  • Tsunamis: Large waves caused by earthquakes, volcanic eruptions, or underwater landslides.  
• Wave Characteristics:
  • Wave height: The vertical distance between the crest and trough.  
  • Wavelength: The horizontal distance between two successive crests or troughs.  
  • Wave period: The time it takes for two successive crests or troughs to pass a fixed point.  
  • Wave Energy: Waves carry energy, which can be harnessed for power generation.  
• Tides:
  • Definition: Tides are the periodic rise and fall of sea level caused by the gravitational pull of the Moon and the Sun on Earth’s oceans.  
  • Tidal Cycle:
    • High tide: The highest level reached by the sea during a tidal cycle.  
    • Low tide: The lowest level reached by the sea during a tidal cycle.  
    • Tidal range: The difference in height between high tide and low tide.  
  • Types of Tides:
    • Spring tides: Occur when the Sun, Moon, and Earth are aligned, resulting in higher high tides and lower low tides.  
    • Neap tides: Occur when the Sun and Moon are at right angles to each other, resulting in lower high tides and higher low tides.  
  • Importance: Tides play a significant role in coastal ecosystems, navigation, and fishing.  

Ocean Currents:

• • Definition: Ocean currents are continuous, directed movements of seawater generated by various forces, including wind, density differences, and the Earth’s rotation.  
  • Types of Currents:
    • Surface currents: Driven by wind and influenced by the Earth’s rotation.  
    • Deep currents: Driven by density differences caused by variations in temperature and salinity.  
  • Major Ocean Currents:
    • Gulf Stream: A warm current in the North Atlantic Ocean that flows from the Gulf of Mexico towards Europe.  
    • Kuroshio Current: A warm current in the North Pacific Ocean that flows from the Philippines towards Japan.  
    • California Current: A cold current in the North Pacific Ocean that flows southward along the coast of California.  
    • Peru Current: A cold current in the South Pacific Ocean that flows northward along the coast of South America.  
  • Importance: Ocean currents play a vital role in heat transfer, climate regulation, and the distribution of nutrients and marine organisms.  

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