1. GEOLOGY Chapter 10 Plate Tectonics 10.4 Causes of Plate Motions

2. 10.4 Causes of Plate Motions Objectives • Explain the process of convection. • Summarize how convection in the mantle is related to the movements of tectonic plates. • Compare and contrast the processes of ridge push and slab pull.

3. Mantle Convection • Convection is the transfer of thermal energy by the movement of heated matter. • The heating of matter causes it to expand and decrease in density. • The warmed matter then rises as a result of buoyancy. • The cooler part of the matter sinks as a result of gravity. • The resulting up and down flow produces the pattern of motion that we call a convection current.

4. Convection Currents

5. Mantle Convection • Convection currents in the mantle are thought to be the driving mechanism of plate movements. • Hot mantle material is less dense than cool mantle material, so the hot material is slowly forced toward the crust and the cooler material sinks back toward the core. • The resulting convection currents can be thousands of kilometers across but flow at rates of only a few centimeters per year.

6. Mantle Convection

7. Mantle Convection • The rising part of a convection current spreads out as it reaches the upper mantle and causes both upward and lateral forces. • These forces lift and split the lithosphere at divergent plate boundaries. • As the plates separate, material rising from the mantle supplies the magma that hardens to form the new ocean crust. • The downward part of a convection current occurs where a sinking force pulls tectonic plates downward at convergent boundaries.

8. Mantle Convection • As ocean ridges form, forces in the mantle cause the asthenosphere to rise. • The weight of this massive ridge that is lifted is thought to push the oceanic plate toward the trench formed at the opposing end of the plate at the subduction zone. • This process is known as ridge push. • The horizontal flow of a convection current could also create drag on the lithosphere and contribute to plate motion.

9. Mantle Convection

10. Mantle Convection • Mantle convection in a sinking region of the mantle may also serve to suck an oceanic plate downward into a subduction zone. • Slab pull is the process in which the weight of a subducting plate helps to pull the trailing lithosphere into the subduction zone. • Even though scientists agree that convection currents in the mantle are related to plate movement, their exact role still requires further study.

11. Mantle Convection