Earth Structure


The Structure of the Earth

  • The core is Iron-nickel which creates the magnetic field 
  • The inner layer is solid and is 1,250km thick 
  • The outer layer is liquid and is 2,200km thick 
  • The mantle accounts for 80% of the earths volume 
  • It is semi-solid rock that is 2,900km deep
  • The upper mantle is the asthenosphere, it has plastic properties that allow it to flow under pressure
  • There is oceanic crust - 5km composed of dense basalt or continental crust - 30 to 100km composed of less dense granite 
  • The crust and the upper mantle form the lithosphere
  • Pressure and temperature increase with depth reaching over 3000degrees in the core
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Plate Tectonic Theory

  • Earths crust and Lithosphere is broken into 7 large peices and several smaller ones known as tectonic plates 
  • Plate boundaries are zones of violent tectonic activity
  • Convection currents move tectonic plates across the asthenosphere constantly
  • New crust is added by volcanic activity at mid-oceanic trenches at constructive plate margins and is moved by sea floor spreading 
  • It is then destroyed at subduction zones or destructive plate margins  
  • The outcome is tectonic activity along plate boundaries and the movement of the continents known as continental drift
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Evidence for Plate Tectonics

  • Continental drift is the most powerful evidence 
  • Matching shapes of the continents on opposite sides of the ocean
  • The same species of fossil plants and animals found in modern Africa and South America 
  • The crucial evidence is palaeomagnetism - iron particles in the sea floor show the earths polarity at the time they solidified 
  • The earth reverses polarity every 400,000 years the ocean floor shows mirror patterns of polarity either side of an ocean trench 
  • This proved new lithospeheric crust was being formed by volcanic activity and convection currents were puching old crust away
  • This is called sea floor spreading - as ocean crust moves, the continents 'ride' the natural conveyor thus creating continental drift. 
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Constructive Plate Boundaries

  • New crust forms at Constructive plate boundaries where rising plumes of magma stretch the crust and lithosphere
  • This results in intense volcanic activity, most often, on the ocean floor
  • Volcanic eruptions build submarine volcanoes and mountain ranges and mid ocean ridges
  • Parallel faults associated with tension in the crust form rift valleys that seperate the submarine mountain chains 
  • Mid ocean ridges are offset by large transverse faults  
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Destructive Plate Boundaries

  • Subduction zones where oceanic crust is destroyed when two tectonic plates converge 
  • The older, denser plate is subducted 
  • The plate including water and sea floor sediments descends into the upper mantle 
  • Magma, which is less dense than surrounding rocks, then rises towards the surface 
  • Lava, gases and ash erupt at the surface through volcanoes and fissures 
  • Oceanic-Oceanic subduction forms island arcs and Oceanic-Continental subduction forms fold mountains 
  • Destructive plate boundaries include volcanoes, earthquakes and ocean trenches
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Fold Mountains + Ocean Trenches

  • Fold mountain ranges include the Himalayas and the Andes
  • The Andes were formed when the Nasca oceanic plate converged with the South American plate 
  • Sedimetary rocks are squeezed against the South American plate and crumple to form the Andes - The Nasca plate is subducted and creates huge intrusions of magma benath the mountains
  • The Himalayas were formed by the convergence of the Indo-Australian plate and the Eurasian plate - the crust is so thick there is no volcanic activity 
  • Ocean trenches are narrow trenches, hudreds of km long - the zone of subduction
  • They occur parrallel to to island arcs and fold mountain ranges  
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Conservative Plate Boundaries

  • The two plates slide past each other 
  • This can be violent and results in powerful earthquakes 
  • Volcanism is absent 
  • The boudary between the Pacific and North American plate is a conservative margin known as the San Andreas Fault 
  • Earthquakes occur frequently and present major hazards to metropolitan areas such as San Francisco and Los Angeles 
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