Tectonics and hazards

• 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
• Earthquakes can also occur near the middle of plates (called intra-plate)
• Intra-plate earthquakes


  Caused by plates having pre-existing weaknesses which become reactivated, forming seismic waves
• Volcanic hotspots


  Localised areas of the lithosphere (Earth's crust and upper mantle) which have an unusually high temperature due to the upwelling of hot molten material from the core
• 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
• Trends in tectonic hazards since 1960

  • Total number of recorded hazards has increased
  • Number of fatalities has decreased, but some spikes during mega disasters
  • Total number of people affected is increasing due to population growth
  • Economic costs have increased significantly
• Reporting disaster impacts


  Depends on whether looking at direct or indirect deaths, location, methods used, and potential government bias
• Sections of the Earth

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


  • Uppermost layer, 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 has a temperature gradient that generates convection currents
• Outer Core


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


  • Similar composition to outer core, but solid due to extreme pressures
• The core's high temperature is a result of primordial heat left over from the earth's formation and radiogenic 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 boundaries


  1. Denser oceanic plate subducts below continental plate

  2. Subducting plate leaves deep ocean trench

  3. Oceanic crust is melted as it subducts into the asthenosphere

  4. Pressurised magma forces through weak areas in the continental plate, causing explosive, high pressure volcanoes (composite volcanoes)

  5. Fold mountains occur when sediment is pushed upwards during subduction
• Constructive plate boundaries (oceanic and oceanic)

  1. Magma rises in the gap left by the separating plates, 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 boundaries (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 and slab pull are further forces influencing how convergent boundaries occur
• 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 can displace a lot of water on oceanic crust, and cause 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 that 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 until the plates suddenly give way, causing a jolting motion that spreads seismic waves

  3. 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
  • 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
• Seismic wave intensity


  Decreases further from the epicentre, but other factors like geology, location, education, and building durability affect a location's vulnerability
• Secondary earthquake hazards


  • Soil liquefaction
  • Landslides
  • Tsunamis
• Soil liquefaction


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


  Shaking can weaken or damage cliff faces, hills and snow material, causing unconsolidated material or loose rocks to collapse
• Tsunamis


  1. Caused by the displacement of water above a jolted oceanic crust, the energy is transferred into the water and travels through it like a wave

  2. As the wave approaches the coast, it slows down and gains height, creating a wall of water
• Most tsunamis are found along the Pacific ring of fire, at subduction zones in convergent plate margins
• Risk varies with topography rainfall, soil and land use
• Tsunamis


  When an oceanic crust is jolted during an earthquake, all of the water above this plate is displaced, normally upwards
• Tsunamis


  1. The water is then pulled back down due to gravity

  2. The energy is transferred into the water and travels through it like a wave

  3. The water travels fast but with a low amplitude (height)

  4. As it gets closer to the coast, the sea level decreases so there is friction between the sea bed and the waves

  5. This causes the waves to slow down and gain height, creating a wall of water that is on average 10 feet high, but can reach 100 feet
• Tsunamis are generated generally in subduction zones at convergent plate margins. Most tsunamis are found along the Pacific ring of fire, hence the most vulnerable countries are often located in Asia or Oceania
• Factors affecting the impact of a tsunami

  • Population density of area hit
  • Coastal defences
  • Duration of the event
  • Wave amplitude and distance travelled
  • Gradient of the continental shelf
  • The shape of the land - bays will funnel and concentrate tsunami waves
  • Warning & Evacuation Systems
  • Level of economic and human development