2 - Tectonic hazards

• Tectonic plate


  Major fragment of the Earth's rigid outer shell that moves very slowly over the upper mantle
• Earth's internal structure


  • Core (mainly iron and nickel, extremely hot and under a lot of pressure)
  • Inner core (solid)
  • Outer core (liquid)
  • Mantle (solid material that can flow very slowly)
  • Asthenosphere (weak layer that can deform like plastic)
  • Crust (oceanic and continental)
• Oceanic crust


  Thinner and more dense (5-10 km), formed of basaltic rock, sinks when it meets continental plates, usually less than 200 million years old
• Continental crust


  Thicker (20-200 km), composed mainly of granite rock, up to 3.8 billion years old
• Lithosphere


  Crust and upper mantle together, a rigid shell at the surface of the Earth, broken into several major fragments called tectonic plates
• Plate margin


  Where two plates meet
• Types of plate margins

  • Constructive
  • Destructive
  • Conservative
• Convection


  Core's high temperature causes magma to rise in the mantle, cool and sink back towards the core, carrying plates with the convection currents
• Ridge push and slab pull

  1. At constructive margins, molten magma rises and cools to form new plate material, causing plates to move apart (ridge push)

  2. At destructive margins, denser plate sinks back into mantle, pulling the rest of the plate along (slab pull)
• Earthquakes and volcanoes are not randomly distributed, they match the locations of plate margins
• Earthquakes are found at all three types of plate margins, volcanoes are found at constructive and destructive margins
• Earthquakes and volcanoes occur both on land and in the sea
• Not every earthquake and volcano lies along a plate margin, some occur in the middle of plates at 'hot spots'
• Constructive plate margin


  Tectonic plates move apart from each other
• Plate movement at constructive margins

  Mantle melts, molten magma rises and cools to form new plate material, causing plates to move further apart
• Constructive margins form mid-ocean ridges and shield volcanoes
• Iceland is located on the Mid-Atlantic Ridge, a constructive plate margin
• Rift valleys form at constructive plate margins as the plates move apart
• Destructive plate margin


  Tectonic plates move towards each other and collide
• Plate movement at destructive margins

  Denser oceanic plate subducts under continental plate, causing earthquakes, fold mountains and volcanoes
• Japan has many active volcanoes because it lies on the margin of four tectonic plates
• The Mariana Trench is the deepest known part of the Earth's oceans, formed at a destructive plate margin
• Subduction zone


  Where tectonic plates move towards each other and one plate is pushed under the other
• Figure 2.7 shows Destructive plate margins
• Destructive plate margins


  • Earthquakes and volcanoes are found at them
• How plates at destructive margins move

  1. Plates move towards each other

  2. One plate is pushed under the other
• Figure 2.8 is a satellite image of Japan, with darker blue indicating greater ocean depth

  
• Describing the direction of plate movement at destructive plate margins

  1. Plates move towards each other

  2. One plate is pushed under the other
• Destructive plate margins


  • Eurasian and Philippine plates
  • Nazca and Pacific plates
  • Nazca and South American plates
  • North American and Eurasian plates
• Landforms found at destructive plate margins
• How earthquakes and volcanoes are formed at destructive plate margins

  1. One plate is pushed under the other

  2. Friction causes the plates to become stuck

  3. Pressure builds up

  4. Rock fractures and pressure is released, causing an earthquake

  5. Magma rises to fill the gap between the plates, forming volcanoes
• Conservative plate margin


  Where tectonic plates move parallel to each other
• How plates move at conservative margins

  1. Plates can move side by side in the same direction at different speeds

  2. Plates can move in opposite directions
• Why earthquakes are found at conservative plate margins

  • Pressure builds up as plates are pulled along behind a subducting plate
  • Friction causes the plates to become stuck
  • Pressure builds up until the rock fractures and is released, causing an earthquake
• Volcanoes are not found at conservative plate margins
• Conservative plate margin


  • San Andreas Fault between North American and Pacific plates
• The North American plate moves at 6cm/year, the Pacific plate moves at 10cm/year
• The 1906 San Francisco earthquake measured 8.3 on the Richter scale
• The US Geological Survey predicts a similar earthquake on the San Andreas Fault every 200 years, with a 2% chance in the next 30 years
• Figure 2.10 shows a map of the San Andreas Fault in California