Geography; Volcanism, Volcanoes, Weathering and Rocks

  • Volcanism –
    • Several integrated processes such as gradual increase in temperature with the depth, due to the heat generated by degeneration of radioactive elements inside the earth.
    • Origin of magma due to the lowering of the melting point caused by reduction in pressure of overlying rocks due to fractures caused by splitting of plates.
  • Classification of volcanoes –
    • On the basis of mode of eruption:
      • Central eruption or explosive type.
        • Example: Hawaiian type, Strombolian type, Pelean type, Visuvious type, volcanic type etc.
      • Fissure eruption or quiet eruption.
        • Successive flow of lava results in the growth of lava plateau. Example: Deccan plateau etc.
    • On the basis of periodicity of eruption:
      • Active volcano.
        • Erupts periodically.
        • Example: Etna, Stromboli, Mayon etc.
      • Dormant volcano.
        • Volcano which becomes quiet after its eruption for some time.
        • Example: Fujiyama, Krakatoa, Barren Island etc.
      • Extinct volcano.
        • No indication of future eruption.
  • Various volcanic belts –
    • Circum pacific belt.
      • Fire girdle of the pacific or the fire ring of the pacific.
      • It extends across the Kamchatka peninsula, Kurile Islands, the islands of Japan, Philippines, New Guinea, New Zealand and the soloman islands.
    • Mid continental belt.
      • Volcanic zones of convergent continental plate margins.
      • Includes volcanoes of alpine mountain chain, the Mediterranean sea and the fault zone of eastern Africa of Stromboli, Vesuvius, Etna, Kilimanjaro.
    • Mid Atlantic belt.
      • Fissure eruption type.
      • Example: Iceland, Canary Islands, Cape Verde, Azores etc.
  • Highest volcanic peaks are Cotopaxi (South America), Fujiyama (Japan) and Valley of Ten Thousand Smokes (Alaska).
  • Weathering –
    • Physical weathering.
      • Factors are temperature change, Crystallization of water into ice, the pressure release mechanism etc.
    • Chemical weathering.
      • Factors are solution, oxidation, carbonation, hydration, hydrolysis and chelation.
    • Biological weathering.
      • Three types Faunal, Floral and Anthropogenic.
  • Rocks (can be defined as an aggregate of minerals).Depending upon the process of their formation –
    • Igneous rocks / Primary rocks.
      • Formed due to the cooling, solidification and crystallization of hot and molten magma.
      • Believed that they are formed during each period of geological history of the earth.
      • Are hard, granular and crystalline rocks.
      • Less affected by chemical weathering.
      • It does not have any fossil or does not form any strata or layers of lava.
      • Classification of igneous rocks
        • On the basis of mode of occurrence –
          •  Intrusive rocks.
            • Formed due to solidification of rising magma below the surface of the earth.
            • Example: Granite, Lapolith, Batholiths, Sills etc.
          • Extrusive rocks.
            • Formed due to cooling and solidification of hot and molten magma at the earth surface.
            • Example: Basalt, Gabbro etc.
        • On  the basis of silica content –
          • Acidic.
            • Has more silica content. Example: Granite.
          • Basic.
            • Has less amount of silica content. Example: Gabbro.
    • Sedimentary rocks.
      • Formed due to the aggregation and compaction of sediments derived from the older rocks, plants, animals and contains fossils of plants.
      • Can be classified on the basis of the nature of sediments –
        • Mechanically formed rocks.
        • Chemically formed rocks.
        • And organically formed rocks.
    • Metamorphic rocks.
      • Changed form of igneous and sedimentary rocks.
      • Are the rocks which change either in form or composition without disintegration.
      • Are metamorphosed.
      • Agents of metamorphism are heat, compression and solution.
Original rocksMetamorphic rocks
Shale/ClaySlate, Phyllite, Schist
CoalDiamond/Graphite coal
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