Hazardous environments

• Tectonic hazards


  Hazards caused by the movement of tectonic plates
• Tectonic hazards


  • 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
• 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
• At all boundaries, plates can become stuck due to the friction between plates
• Earthquake occurrence


  1. Plates become stuck

  2. Convection currents in the asthenosphere continue to push

  3. Pressure builds up

  4. Plates eventually give way

  5. Sudden movement causes seismic waves
• 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


  Affects the magnitude of the earthquake - conservative boundaries have the shallowest boundaries, convergent boundaries usually have deeper focuses
• Earthquakes are frequent around the world and occur every day at boundaries
• 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
• Shockwaves (seismic waves)


  1. Friction builds up and pressure increases

  2. Pressure becomes too much, plates move

  3. Energy is released as kinetic energy, vibrating throughout the ground

  4. Weaker the further away from the focus 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, becoming compressed and gaining height as it reaches the coast
• Liquefaction


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


  Movement in soil or snow will cause it to become unstable, sending large amounts of debris or snow tumbling downhill
• Hazards caused by earthquakes

  • Shockwaves (seismic waves)
  • Tsunamis
  • Liquefaction
  • Landslides and avalanches
• Effects of seismic hazards


  • Environmental
  • Economic
  • Social
  • Political
• Volcanoes


  Occur on plate boundaries where plates melt and lava erupts through a plate, or on hotspots
• Volcanoes on convergent plate boundaries

  Volcanic eruptions are usually explosive due to the high pressure the magma is under, forming composite volcanoes made from ash and lava
• Formation of volcanoes on continental and oceanic convergent boundaries

  1. Denser oceanic plate subducts below the continental

  2. Plate subducting leaves a deep ocean trench

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

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

  5. Extra magma created causes pressure to build up

  6. Pressurised magma forces through weak areas in the continental plate, causing explosive, high pressure volcanoes
• Formation of volcanoes on oceanic and oceanic convergent boundaries

  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 called island arcs
• Volcanoes on divergent plate boundaries

  Volcanic eruptions are usually effusive as the magma is under less pressure, forming shield volcanoes made mainly from lava
• Formation of 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
• Formation of 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, 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
• Hazards caused by volcanoes

  • Lava flows
  • Lahars
  • Mudflows
  • Glacial floods (jökulhlaups)
  • Tephra
  • Toxic gases
  • Acid rain
  • Volcanic landslides
• Mudflows


  Different to lahars, which are volcanic material, mudflows may be triggered by the violent shaking that an eruption brings, or meltwater from the volcanic heat
• Glacial floods (jökulhlaups)


  When temperatures are high from lava, glaciers or ice sheets at high temperatures 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 can replace oxygen as it is heavier
• 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