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
• Belts of tectonic activity


  • OFZ (Oceanic Fracture Zone) - through the oceans and along the mid-ocean ridges
  • CFZ (Continental Fracture Zone) - along the mountain ranges

  
• Direct deaths


  Those killed in the disaster straight away
• Indirect deaths


  People who died of diseases that spread after the disaster
• Reporting disaster impacts is very difficult and controversial due to factors like location, data collection methods, and government bias
• Sections of the Earth


  • 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
  • Temperature gradient generates convection currents
• Outer Core


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


  • Similar composition to outer core, solid due to extreme pressures
• Primordial heat


  Heat left over from the Earth's formation
• Radiogenic heat


  Heat produced from radioactive decay
• Plate boundary types


  • Destructive (plates move towards each other)
  • Constructive (plates move away from each other)
  • Conservative (plates move parallel to each other)

  
• Destructive plate boundary (continental and oceanic)


  1. Denser oceanic plate subducts below continental plate

  2. Leaves deep ocean trench

  3. Oceanic crust melted as it subducts

  4. Extra magma created causes pressure to build up

  5. Pressurised magma forces through weak areas in continental plate, forming explosive high pressure volcanoes (composite volcanoes)

  6. Fold mountains occur as sediment is pushed upwards during subduction

  
• Destructive plate boundary (oceanic and oceanic)


  1. Heavier plate subducts leaving an ocean trench

  2. Built up pressure causes underwater volcanoes bursting through oceanic plate

  3. Lava cools and creates new land (island arcs)

  
• Destructive plate boundary (continental and continental)


  1. Both plates are not as dense as oceanic so lots of pressure builds

  2. Ancient oceanic crust is subducted slightly, but no subduction of continental crust

  3. Pile up of continental crust on top of lithosphere due to pressure between plates

  4. Fold mountains formed from piles of continental crust

  
• Constructive plate boundary (oceanic and oceanic)


  1. Magma rises in the gap left by the two plates separating, forming new land when it cools

  2. Less explosive underwater volcanoes formed as magma rises

  3. New land forming on the ocean floor by lava filling the gaps is known as seafloor spreading

  
• Constructive plate boundary (continental to continental)

  1. Any 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

  4. The lifted areas of rocks are known as horsts, 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


  1. Parallel plates move in different directions or at different speeds

  2. No plates are destroyed so no landforms are created

  3. Movement builds up pressure, which can displace water or create fault lines on continental crust

  
• Types of crust

  • Oceanic - low density, mainly basalt, thin, newly created
  • Continental - high density, mainly granite, thick, old

  
• 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
• Slab pull is now believed to be the primary mechanism for plate movement, rather than mantle convection
• Earthquakes

  1. Plates do not perfectly fit into each other, causing friction and sticking

  2. Convection currents continue to push, building up pressure

  3. Pressure is eventually released in a sudden jolt, causing seismic waves to spread through the ground

  4. The focus (hypocentre) is the point underground where the earthquake originates, the epicentre is the area above ground directly above the focus
• Types of seismic waves

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


  • They have large amplitudes
  • They are the most destructive
• Due to their different speeds


  These different waves will hit a location at different times
• Aftershocks


  The different types of waves arriving after each other that survivors feel
• Intensity of waves


  Decreases further from the epicentre
• Waves lose energy as they travel
• Impacts felt or damage caused will not always decrease further from the epicentre