NCERT Class 11 Geography Part 1 Chapter 13: Water (Oceans) YouTube Lecture Handouts

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Importance of Water

  • Water is essential for life

  • Blue Planet

Hydrological Cycle

  • Water is found in solid, liquid and gaseous phases

  • Evaporation

  • Transpiration

  • Condensation

  • Ground water and infiltration

  • Continuous exchange between ocean, atmosphere and land

  • 71% water in oceans – rest is freshwater in glaciers, lakes, groundwater and streams

  • 59% water falling on ground returns to atmosphere

  • Renewable water is constant and demand is constantly increasing (pollution aggravating the crisis)

Ocean Relief

  • 4 Oceans – Pacific, Atlantic, Indian, Arctic and now Southern Ocean

  • Most ocean floor between 3-6 km BSL

Ocean Floor Divisions

  • Continental Shelf – extended with gradient of 1 degree; ends with steep slope called shelf break; width varies with average with of 80 km. Siberian Shelf in Arctic Ocean are largest with 1500 km. Depth can vary from 30 to 600m. Sedimentary deposits received becomes source of fossil fuels.

  • Continent Slope – gradient is 2 to 5 degree; depth varies from 200 to 3000 m. It indicates end of continents – have canyons and trenches

  • Deep Sea Plain – gentle slope and are smoothest regions with 3000 to 6000 m with fine grained clay and silt

  • Ocean Deeps or Trenches – deepest, steep sided with narrow basin, 3-5 km deeper than nearby areas – associated with volcano and earthquake (32 deeps in Pacific, 19 in Atlantic, 6 in Indian Ocean)

Minor Relief Features

  • Mid-Oceanic Ridge – 2 mountain chain separated by depression, Iceland

  • Seamount – pointed mountain summit, rising from seafloor but does not reach ocean surface (Emperor Seamount, extension of Hawaiian Islands in Pacific)

  • Submarine Canyons – Deep valley comparable to Grand Canyon in Colorado (Hudson Canyon)

  • Guyots – Flat-topped seamounts (lots in Pacific)

  • Atoll – low islands with coral reef and depression

Temperature

  • Heating and cooling by solar energy

  • Factors affecting temperature distribution

  • Latitude – temperature decrease from equator to poles

  • Unequal distribution of and – in north and south hemisphere

  • Prevailing wind – winds blowing from land towards ocean drive warm surface water away form the coast

  • Ocean Currents – warm ocean currents raise temperature in cold areas while cold current decrease temperature (Gulf stream is warm current while Labrador current is cold current)

  • Enclosed sea in low latitude record higher temperature than open sea while enclosed sea in high latitude have lower temperature

Temperature – Horizontal and Vertical Distribution

  • Temperature decreases with increasing depth

  • Thermocline – boundary where there is a rapid decline in temperature (90% of the volume is found below the thermocline in deep oceans)

  • 3 Layers

  • 1st Layer – warm oceanic water – 500 m thick with 20-25 degree Celsius (in tropics it is throughout the year while in mid latitudes only in summers)

  • 2nd layer – thermocline – rapid decrease in temperature with depth – 500-1000m thick

  • 3rd Layer – cold and extends upon deep ocean floor; temperature change with depth is very less – layers of cold water exists to deep ocean floors

  • Average temperature of ocean surface water is 27 degree Celsius and decrease from poles to equator. Rate of decline is 0.5 degree Celsius with latitude

  • Oceans of North Hemisphere record higher temperature than south hemisphere (due to unequal distribution of land and sea)

  • Maximum temperature of ocean near surface as it directly receives heat from sun and it is transmitted to lower sections by conduction. It results in decrease in temperature with depth

Salinity

  • Salinity is total dissolved salt in sea water. It is calculated as salt dissolved in 1000 gm of seawater

  • Salt of 24.7 parts per thousand is explained as upper limit to demarcate brackish water. Highest salinity Lake Van (330); Dead Sea (238) and Great Salt Lake (220)

  • Salinity depends on

  • Evaporation and precipitation

  • Freshwater inflow from rivers

  • Wind transfers water to other areas and affect salinity

  • Ocean current contribute to salinity variation

Salinity – Horizontal and Vertical Distribution

  • Horizontal Distribution – ranges from 33 and 37 with Red sea with 41 and Arctic with 0 to 35. In hot areas it can reach 70.

  • Salinity in Western Pacific decrease due to influx of melted water from Arctic north (decreases from 35 to 31)

  • Average salinity of Atlantic is 36 with maximum between 15 to 20 degree latitudes and decreases to the north.

  • North sea despite of its location in north hemisphere records higher salinity due to influx from North Atlantic Drift

  • Baltic Sea records low salinity due to influx of river water in large quantity (same in Black Sea)

  • Mediterranean Sea has higher salinity due to high evaporation

  • Salinity in Indian Ocean is 35 – low due to influx of river Ganga and low salinity in Bay of Bengal

  • Salinity in Arabian sea is high due to higher evaporation and low influx of water

  • Vertical Distribution- surface salinity increases due to loss of water to ice and evaporation or decreases due to fresh water input. At depth it remains fixed. Low salinity water rest on higher salinity dense waters.

  • Halocline – salinity increases sharply with depth. (Salinity increases with depth) – higher salinity leads to higher density. High salinity water sinks below low salinity water and leads to stratification by salinity.

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