Geology - The Earth's Crust

• see also:
  • geologic ages and prehistoric times
  • faulting and earthquakes
  • volcanoes

Introduction:

• the Earth 's surface is ~6378km from the centre of it's core with the outer continental crust being about 35km thick and below which is the mantle to a depth of 2900km.
• rocks of the crust consist of three major groups:
  • igneous rocks:
    • those that have solidified from the molten state (ie. from magma)
    • some magma bodies (eg. large masses of coarsely crystalline rocks such as granite) completely solidify while they are still within the crust
      • such rocks that solidify in large masses with dimensions measured in kilometres are called plutonic rocks
      • granites have high proportion of silica which may be combined with alumina
    • if the magma is hot & rises rapidly to reach the surface whilst still liquid it forms lava such as basalts which are poor in silica but contain much combined iron and magnesium
  • sedimentary rocks: 
    • those which have been deposited in layers either in water or by the wind
    • examples are:
      • conglomerate - rounded pebbles, sandstone (grains of sand), mudstone, siltstone, and shale
      • lignite/coal/anthracite - from compressed stems, leaves which accumulated in swamps as peat
      • limestone - compressed marine organism shells, etc or from crystallisation of mineralised waters which may also form salt or gypsum deposits
  • metamorphic rocks: 
    • igneous or sedimentary rocks that have been altered from their original state by heat, pressure and shearing deformation.
    • examples are:
      • hornfels - hard fine-grained rock formed from granite (eg. Mt Ararat & Mt Tarrangower near Maldon)
      • slate
      • schists & gneisses - coarser grained (eg. north-eastern Victoria and Casterton district)
• the earth's crust is segmented into large tectonic plates which tend to move or be stressed at their junctions which are the main areas of earthquakes and volcanic activity
  • the edge of the Indo-Australian Plate includes NZ (forming the Alpine Fault), the Pacific Islands, Papua New Guinea & the southern aspect of Indonesia, mid-Indian Ocean and mid-Southern Ocean.
  • the Indo-Australian plate has been moving north-east at a rate of some 6cm/yr (India & Australia were once a super-continent with Antartica) resulting in mountain forming pressures, frequent earthquakes & vulcanicity in New Zealand, Indonesia & the Himalayas. This plate appears to be developing an east-west oriented stress fracture some 1000km south of India which in a few million years will complete fracture, relieving much of the compressive pressures on the Himalayas and thus their destruction due the lack of forces keeping them up. It is estimated that Australia currently receives some 10% of the forces released with each earthquake in Indonesia.
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• types of landforms:
  • tectonic:
    • result from earth movements
    • examples:
      • mountain ranges resulting from orogenic movements, faulting & uplift; troughs due to faulting, broad up- or down-warps
  • constructional:
    • result from building up of deposits
    • examples are:
      • volcanic mountains; volcanic plains; alluvial plains; sand dunes; coral reefs;
      • stalactites;
  • destructional:
    • result from erosion
    • examples:
      • river-cut & glacial valleys; coastal cliffs; surfaces of lower relief;
      • honey-comb weathering of Mesozoic feldspathic sandstones on the shore platform at Lorne
  • residual:
    • hills or mountains remaining after the removal of formerly surrounding rocks by erosion
    • examples:
      • rock stacks along the coasts (eg. the 12 Apostles); granitic hills (eg. You Yangs near Geelong; Mt Macedon)
      • exposure of residual granitic boulders (tors) eg. the Leviathan on Mt Buffalo measuring 33m x 12m
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