Tectonics

• A hazard is a potential threat to human life and property
• Types of natural hazards


  • Hydrometeological (caused by climate processes)
  • Geophysical (caused by land processes)
• Geophysical hazards occur near plate boundaries
• Plates move at different speeds and directions which cause collisions, earthquakes and volcanic activity
• Earthquakes can also occur near the middle of plates called intraplate, the cause of these are not fully understood but it is assumed that plates have pre-existing weaknesses which become reactivated forming seismic waves
• Volcanic hot spots such as the ring of fire are situated along the centre of plates, this is a localized area of the lithosphere which has an unusually high temperature due to the upwelling of hot moulds and magma material from the core
• The total number of recorded hazards has increased since 1960, the number of fatalities has decreased but there are some spikes during mega disasters
• The total number of people being affected by tectonic hazards is increasing due to population growth
• The economic costs associated with hazards and disasters has increased significantly, partly due to the development of infrastructure in more developed countries costing more to repair and an increasing number of insurance policies especially in developed countries
• Reporting disaster impact is very difficult and controversial for several reasons
• Sections of the Earth's structure


  • Crust (lithosphere)
  • Mantle (asthenosphere)
  • Outer core
  • Inner core
• Crust


  • Uppermost layer of the Earth, thinnest, least dense and lightest
  • Oceanic crust is 7km thick, continental crust can be up to 70km thick
• Mantle


  • Largely compiled of silicate rocks rich in iron and magnesium, semi-molten, temperature gradient towards the core generates convection currents which may contribute to plate tectonic movement
• Outer core


  • Dense semi-molten rocks containing iron and nickel alloys
• Inner core


  • Similar composition to outer core, solid due to extreme pressures, high temperature is a result of primordial heat and radioactive decay
• Types of plate boundaries


  • Destructive (convergent)
  • Constructive (divergent)
  • Conservative (parallel)
• Destructive plate boundaries (continental and oceanic)


  1. Oceanic plate subducts below continental plate

  2. Oceanic crust melted as it subducts

  3. Pressurized magma forces its way up through weak areas in continental plate, forming explosive high pressure volcanoes (composite volcanoes)

  4. Fold mountains occur when sediment is pushed up during subduction
• Destructive plate boundaries (oceanic and oceanic)

  1. Heavier plate subducts leaving an ocean trench

  2. Pressure causes underwater volcanoes to burst through the oceanic plate, lava cools to form new land (island arcs)
• Destructive plate boundaries (continental and continental)


  Pile up of continental crust on top of the lithosphere due to pressure between plates, forming fold mountains
• Constructive plate boundaries (oceanic and oceanic)


  Magma rises between the gap left by the two plates separating, forming new land as it cools (seafloor spreading)
• Constructive plate boundaries (continental and continental)


  1. Land in the middle of the separation is forced apart, causing a rift valley

  2. Volcanoes form where the magma rises

  3. Eventually the gap will most likely fill with water and separate completely from the main island
• Ridge push - the slope created when plates move apart has gravity acting upon it, pushing the plates further away
• Slab pull - when a plate subducts, the plate sinking into the mantle pulls the rest of the plate with it, causing further subduction
• At conservative plate boundaries, the parallel plates move in different directions or at different speeds, no plates are created or destroyed so no landforms are created
• Differences between oceanic and continental crust

  • Oceanic crust is low density rock mainly basalt, thin and newly created
  • Continental crust is high density rock mainly granite, thicker and older
• The density of the plate will determine whether a plate subducts or is forced upwards, this will determine the landscape and the hazard the margin is vulnerable to
• Mechanisms that could cause plate movement


  • Mantle convection - radioactive elements in the core decay, producing thermal energy which causes the lower mantle to heat up and rise, creating convection currents that push the plates
  • Slab pull - old oceanic crust submerges into the mantle, pulling the rest of the plate with it
• Plates do not fit perfectly together, meaning they do not move in fluid motions at all boundaries
• Plates can become stuck due to the friction between them, building up pressure until it is released in a sudden movement, causing a jolting motion and seismic waves
• Types of seismic waves


  • Primary waves (compressional, vibrate in direction of travel, 4-8km/s)
  • Secondary waves (vibrate at right angles to direction of travel, 2.5-4km/s)
  • Love waves (rolling motion, producing vertical ground movement, 2-6km/s)
  • Rayleigh waves (vertical and horizontal displacements, 1-5km/s)
• Secondary and love waves are the most destructive as they have large amplitudes
• Intensity of waves will decrease further from the epicentre as waves lose energy, but this does not mean impacts or damage will always decrease as other factors affect a location's vulnerability
• Secondary hazards of earthquakes


  • Soil liquefaction
  • Landslides
• Soil liquefaction


  Affects poorly compacted sand and silt, water moisture separates from soil particles and rises to surface, causing soil to behave like a liquid
• Landslides


  Shaking caused by earthquakes can weaken or damage cliff faces, hills and snow material, unconsolidated material or loose rocks can collapse
• Tsunamis


  1. Oceanic crust is jolted during an earthquake, displacing the water above it normally upwards, which is then pulled back down by gravity, transferring energy into the water and travelling as a wave

  2. As the wave gets close to the coast, friction between the seabed and the waves causes the waves to slow down and gain height, creating a wall of water
• Factors affecting tsunami impact


  • Population density of area hit
  • Coastal defences
  • Duration of event, wave amplitude and distance traveled
  • Gradient of continental shelf
  • Shape of land
  • Warning and evacuation systems
  • Level of economic and human development
• Primary volcanic hazards


  • Lava flows
  • Pyroclastic flows (mixture of hot dense rock, lava, ash and gases)
  • Tephra and ash flows
• Volcanic gases released into the atmosphere can travel long distances and are very potent
• Secondary volcanic hazards


  • Lahars (combination of rock, mud and water)
  • Acid rain (caused by gases like sulfur dioxide released into atmosphere)