Geography; Reefs, Salinity, Ocean currents and Tides

  • Coral reefs –
    • Formed due to the accumulation and compaction of skeletons of lime secreting organisms (Coral polyps).
    • Found mainly in the tropical oceans and seas as they require high mean annual temperature. (200 C to 250 C)
    • Do not lies in deep sea water, due to lack of sunlight and oxygen.
    • On the basis of nature, mode of occurrence and shape, Coral reefs are classified into three types:
      • Fringing reefs.
        • Develops along continental margins or along the islands.
        • Seawards slope is steep and vertical, while landwards slope is gentle.
        • Runs as a narrow belt of about 0.5 km to 2.5 km width.
        • Usually attached to the coastal land but occasionally they are separated from the shore by a shallow and narrow lagoon (Boat channel).
        • Example: Near Rameshwaram in the Gulf of Mannar, etc.
      • Barrier reefs.
        • Largest coral reefs off the coastal platform, but parallel to them.
        • A broad lagoon develops in between the reef and the shore.
        • Are hundreds of km long and a few km wide.
        • Example: Great Barrier Reef of Australia (largest), etc.
      • Atoll.
        • A reef of narrow growing corals of horse shoe shape and crowned with palm trees.
        • Partly or completely encloses a lagoon.
        • Formed around an island or in an elliptical form on a submarine platform.
        • Example: Funafuti atoll of Ellice Islands, etc.
  • Coral bleaching – When chorals are stressed by changes in conditions such as temperature, light or nutrients, they expel the symbiotic algae living in there tissues causing them to turn colourless.
  • Salinity –
    • Total amount of salts and minerals in grams contained in 1 kg of sea water, expressed as part per thousand.
    • Affects marine organism and plant community, also physical properties of ocean such as temperature, pressure, density, waves and currents.
    • Average salinity: 35 mg / kg. (35 ppt)
    • Average salinity in southern hemisphere > average salinity of the northern hemisphere.
    • Iso halines: Represents the salinity distribution on the surface of the sea. (lines joining places having an equal degree of salinity)
    • Due to dissolution of rocks of oceanic crust.
    • Variation of salinity causes vertical circulation of water.
    • More saline water freezes slowly and the boiling point is higher than the fresh water.
    • Decreases from equator to poles.
    • Highest salinity is near the tropics, because of heavy precipitation in the equatorial region.
Controlling factorRelation with salinity
EvaporationGreater evaporation higher the salinity
PrecipitationHigher the precipitation lower the salinity
Influx of river waterSalinity is reduced at the mouth
Atmospheric pressureAnti cyclonic conditions with stable air and high temperature Increases the salinity of the surface water of the ocean
Circulation of oceanic waterOcean currents affects the spatial distribution of salinity by mixing sea waters
  • Ocean currents –
    • Due to the action of breaking waves, wind, Coriolis effect, temperature, salinity, shape and configuration of coast line and the tides (caused by sun and the moon).
    • Clockwise in northern hemisphere and anticlockwise in the southern hemisphere.
    • Are of two types:
      • Warm currents.
        • Flows from lower latitude to the higher latitude.
      • Cold currents.
        • Flows from higher latitude to the lower latitude.
  • Tides –
    • Rises at an interval of approx 12 hrs 26 min.
    • The two major types of tides are:
      • Spring tides.
        • Occurs when earth, moon and sun are in line i.e straight line configuration of the three celestial bodies.
        • Sun enhances the gravitational pull of the moon, creating the condition of higher high tides and lower low tides.
      • Neap tides.
        • When the sun and the moon are at right angles to the earth.
        • The sun partially contracts the pull of the moon, producing lower high tides.
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