physical geography

• EQ1 Causes of intra plate earthquakes and plume volcanoes

  
• Volcanoes are a geophysical hazard. They are created by processes in the earth and most are caused by processes at plate boundaries

  
• Volcanoes on a convergent (destructive) boundary

  
• Plate Earthquakes


  Earthquakes that occur near the middle of plates, associated with ancient faults

  
• The solid crust cracks as it travels over millions of years, creating newer faults like the East African Rift Valley which may create a new plate boundary

  
• Intra-plate earthquakes


  • Associated with smaller magnitude earthquakes

  
• Volcanoes are a geophysical hazard, most are caused by processes at plate boundaries

  
• Volcanoes on a convergent (destructive) boundary

  1. Oceanic crust subducts under continental crust

  2. Plate burns in the hotter asthenosphere

  3. Magma chamber forms

  4. Magma breaks through the lithosphere

  
• Volcanoes on a divergent boundary


  1. Plates diverge, creating a gap in the lithosphere

  2. Magma rises to fill the gap

  
• Volcanoes on a plume


  1. Magma plume breaks through the lithosphere

  2. Volcano is created

  
• Stratovolcanoes (or composite volcanoes)

  • Built up by many layers of ash, tephra and lava
  • Characterised by steep profile with a summit crater, tall and 'classic' shape
  • Rhyolitic lava, highly viscous, slightly cooler and likely to be explosive
  • Greatest threat to human civilisations due to high speed eruptions
  • Pyroclastic flows, travelling at over 100mph and 700°C can occur
  • High energy eruptions can dense clouds of ash
  • Does not get significant lava flows due to high viscosity therefore moves slowly
  • Volcanic bombs can affect areas up to 12 miles away and be as big as cars

  
• Shield Volcanoes


  • Low profile, made from effusive (non-violent) eruptions
  • Least viscous lava flows, highly fluid that travel long distances creating shallow slopes
  • Many have near continuous eruptions
  • Many have 'Hawaiian' eruptions, which erupt in a calm way with basaltic lava
  • Some erupt in long fissures, not through vents, and can create plateaus

  
• Cinder Cone Volcanoes


  • Steep conical hill of loose pyroclastic fragments
  • Consist of loose pyroclastic debris formed by explosive eruptions or lava fountains from a single vent
  • Slopes between 30–40°; and a nearly circular ground plan
  • Most have a bowl-shaped crater at the summit
  • Mildly eruptive, low viscosity lava

  
• Caldera Volcanoes


  • Large cauldron-like hollow that forms when the magma chamber is emptied
  • Eruptions mean structural support for the crust above the magma chamber is lost
  • The ground surface then collapses downward into the partially emptied magma chamber
  • Can range from 1 to 40km wide
  • If volcanic activity continues, pressure can build underneath, creating a resurgent dome
  • Some calderas are non-explosive, e.g. on shield volcanoes when the magma chamber is drained from lava flows

  
• Basaltic lava


  • Very hot, iron rich, silica poor
  • Low gas content and very hot runny lava
  • Can erupt almost continuously
  • Not very explosive

  
• Andesitic lava


  • Hot, iron poor, silica rich
  • Sticky can take decades or centuries between eruptions
  • Can be very explosive
• Rhyolitic lava


  • Cooler lava, iron poor, silica very rich
  • The high silica content and low temperatures mean a very high gas content which makes it very combustible
  • They erupt rarely, but can be devastating

  
• Earthquake


  • Caused by a sudden slip on a fault
  • Plates are always moving but they get stuck, this pressure is built up and when released, creates seismic energy waves
• Subduction earthquake


  1. Oceanic plate subducts, there is significant pressure in the Benioff zone

  2. This can produce the highest magnitude earthquakes

  
• Collision earthquake


  1. As two continental plates collide, pressure is built between them in the creation of fold mountains

  2. These can create thrust faults

  
• Strike Slip earthquake


  On conservative boundaries, the land tends to shift horizontally

  
• Not all fault movements result in violent earthquakes, some faults move slowly and continuously, a movement called fault creep

  
• Primary (P) Waves


  Fastest and reach the surface first, travel through mantle and core all around the world
• Secondary (S) Waves


  Half as fast as P Waves, high frequency but shake like a skipping rope, travel through mantle but not core
• Love (L) Waves


  Slower than P and S Waves but faster that R Waves, cause the most damage
• Rayleigh (R) Waves


  Radiate from the epicentre in a rolling motion on the surface, slowest Wave
• All earthquakes release all waves but the amount depends on geology, depth of focus, presence of water, and boundary type

  
• Tsunami formation


  1. Earthquake releases significant energy into the waters above it, creating a 360° wave

  2. As the wave approaches the coast, it slows down and grows in height, creating a wall of water
• Tsunami


  Not a wave, but a significant rise in sea level
• Factors affecting tsunami size

  • Magnitude of earthquake
  • Depth of earthquake
  • Depth of sea above epicentre
  • Proximity to land
  • Coastal landscape
• Pyroclastic flows (nuees ardentes)


  • Fast moving currents of hot gas and volcanic matter, moving at ~400kmh (up to 700kmh) and are ~400°C (up to 1000°C)
  • Can travel over water, boiling it and actually gaining speed as steam is added and heavy material drops out
  • Most are 1-10 cubic km and incinerate all living organisms instantaneously
• Poisonous gases


  • Known as the 'silent killer' of volcanoes
  • Can be released from under lakes or in calderas, causing limnic eruptions and suffocating people
• Lava flows


  • Generally slow but can reach temperatures of 2000°C, destroying vegetation and property
  • Create a great deal of pressure, crushing anything in their path
• Volcanic fallout


  • Tephra can range from a few millimetres up to a metre in diameter
  • Can be injected into the atmosphere and then fall globally, the larger pieces falling closer to the eruption which can injure people
  • Smaller fragments can fall, almost as snow, and put pressure on infrastructure such as housing
  • Can also contaminate water supplies and impact on global air travel and even global temperatures

  
• Mudflows and lahars


  • Extremely powerful, created when ash, soil and rock have been changed to clays by acids in volcanic gases and hot-spring waters
  • Can be hot or cold and can be caused by eruptions directly under lakes, melting of snow on volcanoes, heavy rainfall mixing with volcanic material or pyroclastic flows entering bodies of water
  • Destroy more property than any other volcanic activity and are part of the distal hazard zone
• Flooding


  • Sediment from fallout can block rivers and cause local flooding
  • Can also lead to snow and ice melt on volcanoes, causing a sudden rise in the discharge levels of rivers
• Tsunamis


  Small tsunamis can be triggered by a submarine eruption, collapse of a volcano into a sea or if a landslide travels into nearby waters
• Long Term Issues


  • Health - tephra can cause respiration issues, contamination of water supplies can lead to spread of diseases
  • Lifestyles - Pyroclastic flows and lava flows can destroy large areas of land and may mean people need to relocate
  • Economics - low income countries may struggle to rebuild after impact on infrastructure
  • Environment - large scale eruptions can change global temperatures as the volcanic ash blocks sunlight
• Secondary Impacts


  • Infrastructure collapse
  • Building and home collapse
  • Damage to transport networks
  • Injury and death from glass and debris
  • Evacuation causing stress and health issues
  • Liquefaction
  • Fires from broken power lines
  • Aftershocks causing further damage
  • Spread of disease from lack of aid
• Tsunamis are a secondary impact, caused by the energy of the earthquake