Why do Plates Move?

• Convection Current


  Hot, lower density mantle rises in places and cools, becomes more dense and sinks in other places
• Evidence Against Convection Driving Plate Motion

  1. Some plates are too wide and convection cells are too small
  2. Likely multiple cells under Pacific plate, some would help it others would slow it down - would expect it to move slow and it doesn’t
  3. Asthenosphere is weak - convection would not apply enough force to move plates
• What Moves the Plates?

  Ridge Push, Slab Pull
• Ridge Push


  Thinner Lithosphere due to tension, lower pressure so mantle upsells and creates ridge

  MOR is 2-3km higher than abyssal plain - gravity acts to push the plate away from the ridge

  New lithosphere cooling and thickening provides an additional force away from the ridge
• Slab Pull


  Occurs a subduction zones

  Cold, dense lithosphere is denser than the asthenosphere so sinks downwards at ocean trenches and pulls the rest of the oceanic plate with it
• Slab pull is thought to be the main mechanism - those plates with more of their edge being subducted are those that are moving faster