Geography - Tectonics

• Tectonic hazard


  Anything that threatens human life or infrastructure because of the processes of plate boundaries and the movement of them
• Most earthquakes occur on plate boundaries
• The most powerful earthquakes occur at conservative plate boundaries
• Ocean fracture zone (OFZ)


  Earthquake activity along mid-ocean ridges
• Continental fracture zone (CFZ)


  Earthquake activity along mountain ranges
• The Pacific Ring of Fire is a very active area for volcanoes and earthquakes
• Intraplate earthquakes


  Earthquakes that occur inside the plate margins, usually weaker
• Volcanic hotspots


  Fractures in the crust where a magma plume is rising, creating islands over time
• Constructive margins


  1. Plates move apart, allowing magma to rise

  2. Can be oceanic-oceanic or continental-continental

  3. Eruptions are usually effusive and not very damaging
• Destructive margins


  1. Oceanic plate subducts under continental plate

  2. Forms mountain ranges, large earthquakes, and violent volcanic eruptions

  3. Can also be oceanic-oceanic or continental-continental
• Conservative margins


  Plates slide past each other, causing frequent shallow earthquakes
• Earth structure


  Crust, lithosphere, asthenosphere, mantle, outer core, inner core
• Mantle convection


  Convection currents in the mantle that drive plate movement
• Alfred Wegener's theory of continental drift proposed that all continents were once joined in a supercontinent called Pangea
• Arthur Holmes proposed the idea of mantle convection as the driving force behind plate movement
• Harry Hess developed the theory of seafloor spreading and mid-ocean ridges
• Paleomagnetism provides evidence for seafloor spreading and plate movement
• Subduction


  Denser oceanic plate sinks under lighter continental plate, causing earthquakes and volcanoes
• Gravitational sliding


  Plates move apart at constructive margins due to rising magma creating a slope
• Slab pull


  Denser oceanic plate pulls itself into the mantle as it subducts
• Constructive margin example


  • Mid-Atlantic Ridge between Eurasian and North American plates
• Constructive margins have shallow, low-magnitude earthquakes and small, effusive volcanoes
• Destructive margin (oceanic-continental)


  1. Oceanic plate subducts under continental plate

  2. Forms mountain ranges, large earthquakes, and violent volcanic eruptions
• Subduction


  • Plate is pulled into the mantle
• Plate boundary activities and hazards

  • Constructive plate margin
  • Destructive plate margin (oceanic-continental)
  • Destructive plate margin (oceanic-oceanic)
  • Collisional plate margin
  • Conservative plate margin
• Constructive plate margin


  • Shallow earthquakes less than 60km deep
  • Low magnitude earthquakes under 5
  • Small, slow volcanoes with low gas content and high viscosity
• Constructive plate margin


  • Mid-Atlantic ridge between Eurasian and North American plates
• Destructive plate margin (oceanic-continental)


  • Large earthquakes up to magnitude 9
  • Frequent, violent volcanic eruptions of composite volcanoes with high gas and silica content
• Destructive plate margin (oceanic-continental)


  • Nazca and South American plate boundary (2010 Chile earthquake)
• Destructive plate margin (oceanic-oceanic)


  • Frequent earthquakes but not as large as oceanic-continental
  • Violent volcanic eruptions creating volcanic islands
• Destructive plate margin (oceanic-oceanic)

  • 2018 Hawaii earthquake, 1984 Mauna Loa eruption
• Collisional plate margin


  • Large shallow fault line earthquakes, no volcanic eruptions
• Collisional plate margin


  • Andes in South America (4.9 magnitude earthquake in 2024)
• Conservative plate margin


  • High magnitude shallow earthquakes, very destructive
• Conservative plate margin


  • Pacific and North American plates (San Andreas Fault, 1989 near 7 magnitude earthquake)
• Earthquake hazards


  • Seismic waves (P-waves, S-waves, L-waves)
  • Crustal fracturing
  • Liquefaction
  • Landslides
• P waves

  • Fastest seismic waves, cause least damage
• 1. S Waves

  • Arrive after P-waves, shake ground violently and cause damage
• 1. L waves

  • Arrive last, travel horizontally, cause significant damage including crustal fracturing
• Volcanic hazards


  • Pyroclastic flows
  • Ash fall
  • Lava flows
  • Gas eruptions
  • Lahars
  • Jökulhlaups