Tectonics

• Hazard


  A potential threat to human life and property
• Types of natural hazards

  • Hydro-meteorological
  • Geophysical
• Types if tectonic hazards

  • Earthquakes
  • Volcanic eruptions
  • Secondary hazards
• Seismic hazard


  Generated when rocks within 700km of the earth's surface come under stress that they break and become displaced
• Volcanic hazard


  A landform that develops around a weakness in the earth's crust from molten magma, volcanic rock and gases are ejected or extruded
• The global distribution of tectonic hazards is concentrated along plate boundaries, with the 'Ring of Fire' in the Pacific Ocean being particularly significant
• Powerful earthquakes mostly occur at convergent or conservative plate boundaries
• Intra-plate earthquakes are rare, they mainly occur near plate boundaries
• Main areas for earthquakes

  • Continental fracture zone (runs mountain ranges in Spain/France to East Indies)
  • Oceanic fracture zone
• Volcanoes mainly occur around the ring of fire
• Volcanoes occur on plate boundaries at convergent and divergent boundaries
• Tsunamis are a secondary hazard of earthquakes so occur on oceanic plate boundaries
• Types of plate boundaries

  • Divergent
  • Convergent
  • Conservative
• Divergent plate boundary


  Where plates are moving apart, typically oceanic
• Divergent plate boundary


  • Rarely cause tsunamis
  • Generally low magnitude earthquakes
  • Shield volcanoes usually
• Convergent plate boundary


  Plates are pushing together, dense plate subducts (usually the oceanic plate)
• Convergent plate boundary


  • Composite volcanoes
  • More violent earthquakes
  • Can cause tsunamis
• Conservative plate boundary


  Plates are travelling parallel to each other either in the same direction or in opposite directions
• Conservative plate boundary


  • Earthquakes
• The plate boundary type depends on the motion of the plates and plate type (oceanic or continental)
• Intraplate earthquake


  Earthquakes that occur in the middle or interior of tectonic plates
• Intraplate earthquakes are caused by stresses within a plate. Since plates move over a spherical surface, zones of weakness are created. Intraplate earthquakes happen along these zones of weakness.
• Hot spots


  Form around the core of the Earth where radioactive decay is concentrated. This extreme heat creates magma plumes. These are upwellings of superheated rock that rise from deep within the Earth's mantle towards the surface
• Hot spots and magma plumes can lead to the creation of island chains such as Hawaii. The magma plume is stationary so when the tectonic plate moves over it, a chain of volcanoes is formed. The volcanoes are active when they are above the magma plume, but become extinct as the plate moves away.
• Tectonic plates


  • Earth's tectonic plates move at a speed of 2-5 cm per year
  • There are seven very large plates (e.g. African, Pacific) and smaller minor plates (Nazca, Philippine Sea) and dozens of small microplates
  • They all fit together in a constantly moving jigsaw of rigid lithosphere
• Sections of the Earth

  • Lithosphere (0-100km)
  • Mantle/Asthenosphere
  • Outer Core
  • Inner Core
• Lithosphere


  The uppermost layer of the Earth which is thinnest, least dense and lightest, two types: Oceanic - Low density of rock, mainly basalt, thin, newly created; Continental - High density of rock, mainly granite, thick, old
• Mantle/Asthenosphere


  Largely composed of silicate rocks, rich in iron and magnesium, semi-molten and vary in temperatures
• Outer Core


  Dense, semi-molten rocks containing iron and nickel alloys
• Inner Core


  Similar composition to the outer core, the inner core is solid due to the extreme pressures it experiences
• Reasons for the Earth's core being hot

  • Primordial heat left over from the earth's formation
  • Radiogenic heat produced from radioactive decay
• Mantle Convection


  Radioactive elements in the core of the Earth decay which produce a lot of thermal energy. This causes the lower mantle to heat up and rise, as the magma rises it cools down and becomes more dense and begins to sink back down to the core. These are convection currents. These convection currents push the plates.
• Slab Pull


  Oceanic crust (which is the most dense plate) will submerge into the mantle. This pulling action drags the rest of the plate with it.
• Sea floor spreading occurs at divergent plate boundaries, it is the continuous output of magma forming mid-ocean ridge. Paleomagnetism is evidence of this and can be used to date the age of the crusts creation.
• The alternating polarisation of new land created. As magma cools, the magnetic elements within will align with the Earth's magnetic field, which can alternate over thousands of years. Depending on the direction of the magnetic element it can be used to work out when the rock was created.
• Divergent plate boundary


  Convection current moving plates apart, two different directions, slab pull at a different part of the plates
• Constructive plate boundary


  Slab pull, convection currents moving them together
• Conservative plate boundary


  Convection currents moving plates in same direction, slab pull happening to one/both of plates
• Impact of plate boundary types on volcanic eruptions and earthquakes

  • Divergent: Eruptions are small and effusive, earthquakes are shallow and low magnitude
  • Constructive (Oceanic-Oceanic): Frequent earthquakes, and violent eruptions from curving chain of volcanic islands
  • Constructive (Continent-Continent): Infrequent major earthquakes distributed over a wide area
  • Constructive (Oceanic-Continent): Frequent large earthquakes and violent eruptions
  • Conservative: Frequent, shallow earthquakes, but no volcanic activity
• Benioff zone


  An area where friction is created between colliding tectonic plates, resulting in intermediate and deep earthquakes