Formation of Earthquakes & Volcanoes (Cambridge International IGCSE Environmental Management)

Convection currents

• The heat within the Earth's core (geothermal energy), which is estimated to reach up to between 5000 and 6000°C, is generated by radioactive decay
• Heat rises from the Earth's core and creates convection currents in the mantle
• The tectonic plates which make up the crust move on top of the semi-molten mantle below
• The current theory of tectonic plate movement is called slab pull theory

Convection currents and slab pull

• This means that plate movement is caused by more than just convection currents
• The heavier oceanic plates subduct and pull the lighter plates along with them

Plate boundaries

• Volcanic eruptions and earthquakes most commonly occur at or near plate boundaries

Divergent (constructive) boundaries

• Plates move apart

• Magma rises to fill the gap, creating new crust

• Produces:

  • Mid-ocean ridges
  • Volcanoes (often gentle lava eruptions)
  • Shallow earthquakes

• Example: Mid-Atlantic Ridge (Iceland)

Constructive (divergent) plate boundary

Convergent (destructive) boundaries

• Plates move towards each other

• One plate subducts beneath another

• The subducting plate melts, forming magma that rises to create a volcanic arc

• Produces:

  • Powerful earthquakes
  • Explosive volcanoes
  • Ocean trenches

• Example: Andes (Nazca Plate beneath South American Plate)

Destructive (convergent) plate boundary

Conservative (transform) boundaries

• Plates slide past one another horizontally
• No crust is created or destroyed
• Friction locks plates together until stress is released as sudden, violent earthquakes
• No volcanoes because there is no magma generation
• Example: San Andreas Fault, California

Conservative (transform) boundary