Enquiry question 1:

• Tectonic hazards


  Caused by the Earth's internal heat engine
• Generation of Earth's internal heat

  1. Radioactive decay of isotopes such as uranium-238 and thorium-232 in the Earth's core and mantle

  2. Heat flow generates convection currents in the plastic mantle
• The interior of the Earth is dynamic rather than static
• Tectonic plate boundaries


  Locations of ascending (divergent plate boundaries) and descending (convergent plate boundaries) arms of mantle convection cells
• Most tectonic hazards occur at or near tectonic plate boundaries
• Most earthquakes occur at, or close to, tectonic plate boundaries
• Most volcanic eruptions occur at tectonic plate boundaries
• Some plate boundary earthquakes

  Cause a secondary tectonic hazard, tsunami
• Not all tectonic plate boundaries are the same and this has an important impact on the type and magnitude of tectonic hazards. Plate boundary types depends on two factors:

  • Motion - whether plates are moving apart, colliding, or sliding past each other)
  • Plate type - Whether the tectonic plates are continental. Continental plates make up Earth's landmasses and are much thicker at 25-70km but made of less dense granitic rock. Whereas Oceanic plates make up the ocean floor and are high-density, basaltic rocks but only 7-10km thick.
• Plate boundary types


  • Divergent
  • Convergent
  • Conservative or transform
• Oceanic plates are high-density, basaltic rock but only 7-10 km thick
• Continental plates are much thicker at 25-70 km but made of less dense granitic rock
• The vast majority of earthquakes and eruptions occur at plate boundaries
• Intra-plate volcanoes


  Volcanic eruptions distant from a plate boundary at locations called mid-plate hotspots
• Formation of intra-plate volcanoes

  1. Isolated plumes of convecting heat, called mantle plumes, rise towards the surface, generating basaltic volcanoes that tend to erupt continually

  2. A mantle plume is stationary, but the tectonic plate above moves slowly over it

  3. Over millennia, this produces a chain of volcanic islands, with extinct ones most distant from the plume location
• Earthquakes can occur in mid-plate settings, usually associated with major ancient fault lines being reactivated by tectonic stresses or areas of crustal weakness and thinning. For example, The New Madrid Seismic Zone on the Mississippi River generates earthquakes up to magnitude 7.5 but is thousands of miles from the nearest plate boundary
• Earthquakes at subduction zones


  • Occur at a range of focal depths from 10 km to 400 km, following the line of the subducting plate
  • Called a Benioff Zone
  • Can yield very large earthquakes up to magnitude 9.0
• Divergent (Constructive) Oceanic-Oceanic:

  Example of this is the Mid-Atlantic Ridge in Iceland.