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• Tectonic hazards


  • Hazards caused by the movement of tectonic plates
  • Usually occur on plate boundaries where two tectonic plates meet each other
• The global pattern of tectonic hazards are very clear on tectonic plate maps
• Volcanoes and earthquakes occur on plate boundaries on average
• Volcanoes


  • Occur on convergent boundaries (aside from when two continental plates move towards each other) and divergent boundaries
  • Earthquakes occur on all types of boundaries (divergent, convergent, or conservative)
• Plates


  Do not perfectly fit into each other, meaning they do not move in fluid motions
• Earthquake occurrence


  1. Plates become stuck due to friction between plates

  2. Convection currents in the asthenosphere continue to push, building pressure

  3. Pressure builds so much that it cannot be sustained and the plates eventually give way

  4. Sudden movement causes a jolting motion in the plates

  5. Jolting motion spreads seismic waves (or shock waves) throughout the ground
• Focus


  The point underground where the earthquake originates from
• Epicentre


  The area above ground that is directly above the focus
• Richter Scale


  A logarithmic scale that measures the strength of seismic waves
• Modified Mercalli Intensity Scale


  A scale that rates the destruction caused by an earthquake (originally the Mercalli scale when developed in 1884, but the name was changed after 1931 when it was modified)
• The Mercalli scale is subjective, meaning sometimes it is disputed as it is dependent on human development being present rather than the strength of the seismic waves
• Depth of focus


  Conservative boundaries have the shallowest boundaries, meaning they are closer to the epicentre and the seismic waves are stronger. Convergent boundaries usually have deeper focuses, meaning the seismic waves are spread over a larger area before they reach the epicentre
• Earthquakes are frequent around the world and occur every day at boundaries
• Hundreds of smaller magnitude earthquakes that cannot be felt by humans occur every day, whereas the larger earthquakes are less frequent
• Earthquakes follow no pattern and are random so there is irregularity between events
• Earthquakes are almost impossible to predict. Microquakes may give some indication but the magnitude cannot be predicted as how strong they are is random
• Earthquake shockwaves (seismic waves)

  1. Friction builds up and pressure increases, stored as potential energy

  2. Pressure becomes too much, plates move

  3. Energy is transferred into kinetic energy, released and vibrates throughout the ground

  4. Further away from the focus, the weaker the shockwaves as energy is transferred into the surroundings
• Tsunamis


  • Caused when an oceanic crust is jolted during an earthquake, displacing the water above the plate
  • The water travels fast but with a low amplitude (height)
  • As it gets closer to the coast, the water becomes shallower, forcing the waves to become compressed into a smaller area
  • 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
• Liquefaction


  When soil is saturated, the vibrations of an earthquake cause it to act like a liquid, becoming weaker and more likely to subside when it has large weight on it
• Landslides and avalanches


  Movement in soil or snow will cause it to become unstable, causing large areas to give way and send large amounts of debris or snow tumbling downhill, damaging infrastructure and buildings, the environment, and posing a huge threat to life
• Types of seismic hazards and their effects


  • Primary hazards: Fault lines, liquefaction
  • Secondary hazards: Radioactive materials and other dangerous substances leaked, saltwater flooding freshwater ecosystems, soil salinisation, economic decline, high cost of rebuilding and insurance payout, gas pipes rupturing and starting fires, water supplies contaminated, tsunamis causing damaging flooding, political unrest from food/water shortages, borrowing money for international aid, initial chaos and lawlessness
• Volcanoes


  • Occur on plate boundaries where plates melt and lava erupts through a plate, or on hotspots
  • Volcanoes on convergent plate boundaries are usually explosive due to the high pressure the magma is under, forming composite volcanoes made from ash and lava
  • Volcanoes on divergent plate boundaries are usually effusive as the magma is under less pressure, forming shield volcanoes made mainly from lava
  • Hotspots are areas of volcanic activity not related to plate boundaries, where hot magma plumes rise and burn through weaker parts of the crust
• Volcanoes on continental and oceanic convergent boundaries

  1. Denser oceanic plate subducts below the continental plate

  2. Subducting plate leaves a deep ocean trench

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

  4. Oceanic crust is melted as it subducts into the asthenosphere, creating extra magma that builds up pressure

  5. Pressurised magma forces through weak areas in the continental plate, causing explosive, high pressure eruptions
• Volcanoes on oceanic and oceanic convergent boundaries

  1. Heavier plate subducts leaving an ocean trench, fold mountains also occur

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

  3. Lava cools and creates new land called island arcs
• Volcanoes on oceanic and oceanic divergent boundaries

  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 sea floor spreading
• Volcanoes on continental to continental divergent boundaries

  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 whereas the valley itself is known as a graben
• Hotspots


  • Areas of volcanic activity not related to plate boundaries, where hot magma plumes from the mantle rise and burn through weaker parts of the crust, creating volcanoes and islands
  • The plume stays in the same place but the plates continue to move, sometimes causing a chain of islands
• Volcanic hazards


  • Lava flows
  • Lahars
  • Mudflows
  • Glacial floods (jökulhlaups)
  • Tephra (any type of rock ejected by a volcano)
  • Toxic gases
  • Acid rain
  • Volcanic landslides
• Lahar


  Mudflow of volcanic material
• Mudflows


  Different to lahars, may be triggered by violent shaking from an eruption or meltwater from volcanic heat
• Glacial floods (jökulhlaups)


  When temperatures are high from lava, glaciers or ice sheets quickly melt and a large amount of water is discharged
• Tephra


  Any type of rock that is ejected by a volcano
• Toxic gases


  Released during some eruptions, even CO₂ can be toxic as it is heavier than oxygen
• Acid rain


  Caused when gases such as sulfur dioxide are released into the atmosphere
• Volcanic landslides


  High velocity flows of debris caused when the energy from the eruption blows apart rocks and other material, sending it down the volcanic slope
• Nuées ardentes/pyroclastic flows


  Clouds of burning hot ash and gas that collapses down a volcano at high speeds, average speeds of around 60 mph but can reach 430 mph
• Volcanic Explosivity Index (VEI)

  Measure of vulcanicity, the more powerful and explosive the eruption, the higher the VEI
• Intense high magnitude eruptions are explosive whereas calmer, lower magnitude eruptions are effusive
• Active, dormant, extinct volcanoes


  Volcanoes are classed based on their frequency of eruptions
• An estimated 50-60 volcanoes erupt each month, meaning volcanic eruptions are always frequent