Global distribution

• There are around 500 active volcanoes globally and around 50 erupt every year
• the oceanic fracture zone (OFZ)

  belt of activity throughout the ocean along the mid ocean ridges
• the continental fracture zone (CFZ)

  belt of activity following the mountain ridges
• convergent plate boundary

  • aka destructive
  • lots of frequent volcanoes and earthquakes
  • often forms large mountain ranges like the Himalayas
  • deforming collision locations with plate material melting in the mantle

  
• conservative plate boundary

  • lithosphere is neither created nor subducted
  • no volcanic activity
  • extensive shallow focus earthquakes

  
• around 70% of earthquakes are found in the ‘ring of fire’

  
• Plate movement and earthquake type - divergent

  • plates moving apart (divergent) are in the oceans, new oceanic crust is created and is thinner and denser than continental crust. Earthquakes here are frequent, small and typically low hazard risk as they are in the ocean and don't trigger tsunamis
• Plate movement and earthquake type - conservative

  • where plates slide past eachother can present more risk than at divergent boundaries as a zone of friction is created eg the San Andreas Fault in California
• Plate movement and earthquake type - convergent

  • where two plates are moving towards each other and creates some of the largest and most damaging earthquakes. Typically, one plate starts sliding under the other, creating friction at the subduction zone that is released as energy. This can cause both earthquakes (like the tsunami of Tohoku 2011) and volcanoes as the magma is created by the hydration of oceanic crust - the one being subducted. This is what has caused the ring of fire
• Active subduction zones

  • are characterised by magmatic activity, a mountain belt with thicker continental crust and active seismicity
  • passive continental margins are the remaining boundaries where there is no collision or subduction and minimal tectonic activity
• Plate movement + volcanic activity - destructive boundary

  • where plates move towards each other and either form a subduction zone or continental collision
  • when its an oceanic and continental plate, the more dense oceanic is subducted. This can form surface volcanoes (on the ocean floor/ Earths' surface) as magma is pushed above subducting plate
  • two continental plates forms large mountain ranges like the Himalayas
  • Destructive boundaries cause the majority of active volcanoes and the most explosive type, composite cone shaped volcanoes that erupt less frequently but more destructively
• Plate movement + volcanic activity - divergent

  • where two plates are moving apart and can form rift volcanoes that are less explosive and more effusive (lots of gases and low viscosity magma) especially when they are under water like at the Mid-Atlantic Ridge
• Plate movement + volcanic activity - hotspot volcanoes

  • found away from plate boundaries and are thought to be fed by underlying mantle plumes
• Hotspot volcanoes and mantle plumes

  • a hotspot is in area in the mantle where heat rises as a hot thermal plume
  • this high heat, lower pressure at the base of the lithosphere allows rock to melt
  • the magma then rises through cracks and erupts to form active volcanoes
  • as the tectonic plate moves over the stationary hotspot, the volcanoes are moved with the plate and new ones form in their place
  • as oceanic volcanoes move away from the hotspot, they cool and subside, producing older islands, atolls and seamounts, they can also form chains of volcanoes like the Hawaiian Islands