1. Causes of Plate Motions
Earth Science
Chapter 17
Section 4

2. Convection
Transfer of thermal energy by movement of heated material from one place to another. Cool material more dense than warm. Warm rises and then descends when it cools.

3. Convection Currents Lava lamp example: Warm material rises
As it rises it cools and then contracts slightly
Become more dense than the warm material and falls

4. Convection Currents
Convection currents aid in thermal energy transfer of energy from areas of warm regions to cool regions.
Earth’s mantle

5. Convection in the Mantle
Considered to be the driving force in plate movement.
Mantle just beneath crust is too cold to flow
Crust is attached to it
Moves with the plates
Mantle cooled near the crust sinks- more dense
Displaces warmer material – less dense and rises

6. Convection in the Mantle
Convection currents in mantle move slowly
Centimeters per year
Thousands of kilometers across
Transfer energy form hot interior to cooler crust
Scientists believe they are set in motion by subduction slabs

7. Plate Movement Rising movement of warm mantle
Spreads when reaches crust
Divergent plate boundaries
Supplies magma that hardens into oceanic crust

8. Plate Movement Downward portion of convection current
Sinking forces pulls tectonic plates downward
Convergent boundaries

9. Plate Movement

10. Push and Pull Two Theories
Ridge Push
Slab Pull
Older portion of seafloor sinks
Weight of uplifted ridge pushes ocean plate toward trench formed at subduction zone
Weight of the subducting plate pulls the trailing slab into the subduction zone