Causes of Plate Motions Earth Science Chapter 17 Section 4

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.

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

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

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

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

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

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

Plate Movement

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