tectonics

• Hazard


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

  • Hydro-meteorological (caused by climatic processes)
  • Geophysical (caused by land processes)
• Geophysical hazards


  • Occur near plate boundaries
  • Plates move at different speeds and directions which can cause collisions, earthquakes and volcanic activity

  
• Intraplate earthquakes


  • Occur near the middle of plates
  • Caused by pre-existing weaknesses in the plates that become reactivated

  
• Volcanic hotspots


  • Localised areas of the lithosphere which have an unusually high temperature due to the upwelling of hot molten material from the core
• Volcanic hotspots, such as the Ring of Fire, are situated amongst the centre of plates
• At hotspots, such as the Hawaii hotspot, magma rises as plume (hot rock)
• The most powerful earthquakes usually occur at convergent or conservative boundaries
• OFZ (Oceanic Fracture Zone)


  A belt of activity through the oceans and along the mid-ocean ridges through Africa, the Red Sea, the Dead Sea

  
• CFZ (Continental Fracture Zone)


  A belt of activity along the mountain ranges from Spain through the Alps to the Middle East and to the Himalayas
• Reporting disaster impacts


  • Depends on whether you look at direct deaths or indirect deaths
  • Location is important as rural and isolated areas are hard to reach
  • Different methods may be used by different organisations
  • The number of deaths quoted by a government could be subject to bias
• 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 composed of silicate rocks, rich in iron and magnesium
  • Semi-molten and a temperature gradient generates convection currents
• Outer Core


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


  Similar composition to the outer core, solid due to extreme pressures
• Core's high temperature


  • Primordial heat left over from the earth's formation
  • Radiogenic heat produced from radioactive decay
• Plate Boundaries


  1. Destructive (plates move towards each other)

  2. Constructive (plates move away from each other)

  3. Conservative (plates move parallel to each other)

  
• Destructive plate boundaries


  • Denser oceanic plate subducts below the continental plate
  • Leaves a deep ocean trench
  • Oceanic crust is melted as it subducts into the asthenosphere
  • Extra magma created causes pressure to build up
  • Pressurised magma forces through weak areas in the continental plate, causing explosive, high pressure volcanoes (composite volcanoes)
  • Fold mountains occur when sediment is pushed upwards during subduction

  
• Destructive plate boundaries (oceanic and oceanic)


  • Heavier plate subducts leaving an ocean trench
  • Built up pressure causes underwater volcanoes bursting through oceanic plate
  • Lava cools and creates new land called island arcs

  
• Destructive plate boundaries (continental and continental)


  • Both plates are not as dense as oceanic so lots of pressure builds
  • Ancient oceanic crust is subducted slightly, but there is no subduction of continental crust
  • Pile up of continental crust on top of lithosphere due to pressure between plates
  • Fold mountains formed from piles of continental crust

  
• Constructive plate boundaries (oceanic and oceanic)


  • Magma rises in between the gap left by the two plates separating, forming new land when it cools
  • Less explosive underwater volcanoes formed as magma rises
  • New land forming on the ocean floor by lava filling the gaps is known as seafloor spreading

  
• Constructive plate boundaries (continental to continental)


  • Any land in the middle of the separation is forced apart, causing a rift valley
  • Volcanoes form where the magma rises
  • Eventually the gap will most likely fill with water and separate completely from the main island
  • The lifted areas of rocks are known as horsts whereas the valley itself is known as a graben

  
• Ridge push


  The slope created when plates move apart has gravity acting upon it as it is at a higher elevation, pushing the plates further away
• Slab pull


  When a plate subducts, the plate sinking into the mantle pulls the rest of the plate (slab) with it, causing further subduction
• Conservative plate boundary


  • Parallel plates move in different directions or at different speeds
  • No plates are destroyed so no landforms are created
  • A lot of pressure is built up
  • On oceanic crust, this movement can displace a lot of water
  • On continental crust, fault lines can occur where the ground is cracked by the movement

  
• Oceanic crust


  Low density of rock, mainly basalt, thin, newly created

  
• Continental crust


  High density of rock, mainly granite, thick, old
• Mantle convection


  Radioactive elements in the core of the Earth decay which produce thermal energy, causing the lower mantle to heat up and rise, creating convection currents that push the plates
• Slab pull


  Old oceanic crust (which is the most dense plate) will submerge into the mantle, pulling the rest of the plate with it
• Slab pull is now believed to be the primary mechanism for plate movement, rather than mantle convection
• Earthquakes


  1. Plates become stuck due to friction between them

  2. Convection currents continue to push, building up pressure

  3. Pressure is eventually released in a sudden movement, causing seismic waves

  
• Focus (or hypocentre)


  The point underground where the earthquake originates from
• Epicentre


  The area above ground that is directly above the focus
• Seismic waves


  • Primary (travels through solids, compressional)
  • Secondary (vibrate at right angles to direction of travel, travel only through solid rocks)
  • Love (near to ground surface, rolling motion producing vertical ground movement)
  • Rayleigh (vertical and horizontal displacement)

  
• Seismic waves


  • Secondary and Love waves are the most destructive as they have large amplitudes
  • Intensity of waves decreases further from the epicentre, but other factors affect a location's vulnerability

  
• Soil liquefaction


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


  • The shaking caused by the earthquake can weaken or damage cliff faces, hills and snow material
  • Unconsolidated material or loose rocks can collapse
  • Landslides can travel several miles and accumulate material on the way

  
• Tsunamis


  • When an oceanic crust is jolted during an earthquake, all of the water above this plate is displaced, normally upwards
  • The water is then pulled back down due to gravity, transferring the energy into the water and travelling through it like a wave
  • As the wave gets closer to the coast, the sea level decreases so there is friction between the sea bed and the waves, causing the waves to slow down and gain height

  
• Tsunamis are generally generated in subduction zones at convergent plate margins, with the most vulnerable areas being along the Pacific ring of fire