Detailed

• 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, 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

  
• 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) - belt of activity through the oceans and along the mid-ocean ridges
  • CFZ (Continental Fracture Zone) - belt of activity along the mountain ranges

  
• Reporting disaster impacts


  • Depends on whether you look at direct deaths or indirect deaths
  • Location is important as rural and isolated areas are hard to reach
  • Different methods may be used by different organisations
  • The number of deaths quoted by a government could be subject to bias

  
• Earth's structure


  • Crust (lithosphere) - thinnest, least dense and lightest layer
  • Mantle (asthenosphere) - semi-molten, temperature gradient generates convection currents
  • Outer Core - dense, semi-molten rocks containing iron and nickel alloys
  • Inner Core - solid due to extreme pressures

  
• Core's high temperature


  • Primordial heat left over from the earth's formation
  • 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 boundary (continental and oceanic)

  1. Denser oceanic plate subducts below the continental plate

  2. Subducting plate leaves a deep ocean trench

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

  4. Extra magma created causes pressure to build up

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

  6. Fold mountains occur when 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 called 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 there is 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 between 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 whereas 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. Gravity pushes the plates further away, widening the gap (gravitational sliding)
• 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 can displace a lot of water on oceanic crust, and cause fault lines on continental crust where the ground is cracked

  
• Plate types


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

  
• Plate movement mechanisms


  • Mantle convection - radioactive elements in the core decay, producing thermal energy that causes convection currents which 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 convection currents
• Earthquakes


  1. Plates do not perfectly fit into each other, meaning they do not move in fluid motions

  2. Plates can become stuck due to friction between them

  3. Convection currents continue to push, building up pressure until the plates eventually give way, causing a jolting motion that spreads seismic waves

  4. The focus (or hypocentre) is the point underground where the earthquake originates from, and the epicentre is the area above ground that is directly above the focus

  
• Seismic wave types


  • 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

  
• Seismic wave intensity


  • Secondary and Love waves are the most destructive as they have large amplitudes
  • Intensity of waves decreases further from the epicentre as waves lose energy, but other factors like geology, location, education, and building durability also affect a location's vulnerability

  
• Soil liquefaction


  1. Affects poorly compacted sand and silt

  2. Water moisture within the soil separates from the soil particles and rises to the surface, causing the soil to behave like a liquid
• Landslides


  1. The shaking caused by the earthquake can weaken or damage cliff faces, hills and snow material

  2. Unconsolidated material or loose rocks can collapse, and landslides can travel several miles accumulating material
• Tsunamis


  1. When an oceanic crust is jolted during an earthquake, the water above it is displaced, normally upwards

  2. The water is then pulled back down due to gravity, transferring the energy into the water and travelling through it like a wave

  3. As the wave approaches the coast, the sea level decreases causing friction with the sea bed, slowing the wave down and increasing its height, creating a wall of water

  
• Tsunamis are generally generated in subduction zones at convergent plate margins, with the Pacific ring of fire being the most vulnerable area

  
• Risk


  Varies with topography, rainfall, soil and land use
• Tsunamis


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

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

  
• Primary hazards caused directly from a volcano

  • Fast speed of onset

  
• Primary volcanic hazards

  • Lava flows
  • Pyroclastic flows
  • Tephra and ash flows
  • Volcanic gases

  
• 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