Tectonics Key

• 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


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


  • Localised area of the lithosphere (Earth's crust and upper mantle) which has an unusually high temperature due to the upwelling of hot molten material from the core
• Volcanic hotspot


  A plume (hot rock) rises as magma
• The most powerful earthquakes usually occur at convergent or conservative boundaries
• Tectonic hazard belts


  • OFZ (Oceanic Fracture Zone)
  • CFZ (Continental Fracture Zone)
• Reporting disaster impacts is very difficult and controversial for several reasons
• Earth's structure


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


  Uppermost layer of the Earth which is thinnest, least dense and lightest
• Mantle


  • Largely composed of silicate rocks, rich in iron and magnesium
  • Semi-molten and a temperature gradient (towards the core) 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 the extreme pressures it experiences
• 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
  • Oceanic crust is melted as it subducts into the asthenosphere
  • Pressurised magma forces through weak areas in the continental plate, causing explosive, high pressure volcanoes
  • Fold mountains occur when sediment is pushed upwards during subduction
• Constructive plate boundaries


  • 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 sea floor spreading
• Conservative plate boundaries


  • No plates are destroyed so no landforms are created
  • A lot of pressure is built up, which can displace water or cause fault lines
• Oceanic crust is low density, mainly basalt, thin and newly created
• Continental crust is high density, mainly granite, thick and old
• Mantle convection


  Radioactive elements in the core of the Earth decay which produce a lot of thermal energy, causing the lower mantle to heat up and rise, and the magma to cool and sink
• Slab pull


  Old oceanic crust (which is the most dense plate) will submerge into the mantle, dragging the rest of the plate with it
• Earthquake generation


  1. Plates become stuck due to friction between them

  2. Convection currents continue to push, building up pressure

  3. Pressure is suddenly released, causing a jolt and 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 (compressional, vibrate in direction of travel)
  • Secondary (vibrate at right angles to direction of travel)
  • Love (rolling motion producing vertical ground movement)
  • Rayleigh (vertical and horizontal displacement)
• Secondary and Love waves are the most destructive as they have large amplitudes
• 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
• Tsunami


  • 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, and the energy is transferred into the water and travels through it like a wave
• Tsunamis are generally generated in subduction zones at convergent plate margins, with the most vulnerable countries often located in Asia or Oceania
• The impact of a tsunami depends on various human and physical factors, including population density of the area hit
• 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. The energy is transferred into the water and travels through it like a wave
  • The water travels fast but with a low amplitude (height)
  • As it gets closer to the coast, the sea level decreases so there is friction between the sea bed and the waves. 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 generally generated 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
• Volcanoes - Primary hazards

  • Lava flows - Streams of lava that have erupted onto the Earth's surface. Fast flowing lava can be very dangerous which depends on the lava's viscosity (the explosivity and viscosity depends on silicon dioxide content)
  • Pyroclastic flows - A mixture of hot dense rock, lava, ash and gases which move very quickly along the surface of the Earth. Due to their high speeds, pyroclastic flows are extremely dangerous and can cause asphyxiation for anyone unfortunately caught by the flow
  • Tephra and ash flows - When pieces of volcanic rock and ash are blasted into the air. This can cause serious damage to buildings, which can collapse under the weight of ash or tephra
  • Volcanic gases - Gases like sulphur dioxide and carbon monoxide are released into the atmosphere. Due to their potency, volcanic gases can travel long distances
• Volcanoes - Secondary hazards

  • Lahars - Combination of rock, mud and water which travel quickly down the sides of volcanoes. These can occur when the heat of the eruption causes snow and ice to melt or alternatively when an eruption coincides with heavy rainfall
  • Jokulhlaup - Snow and ice in glaciers melt after an eruption which causes sudden floods that are very dangerous
  • Acid rain - caused when gases such as sulfur dioxide are released into the atmosphere
• Disaster


  A serious disruption of the functioning of a community or society involving human, material, economic and environmental losses which exceeds the ability of the affected community or society to cope using its own resources