Causes of Plate Motion

– Mantle Convection Current– ridge push– slab pull

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.

Vocabulary

Causes of Plate Motions

• HYPOTHESIS: large-scale motion in the mantle drives the movement of tectonic plates.

• MANTLE CONVECTION = transfer of thermal energy by the movement of heated matter between Earth’s hot interior (Mantle) and its cooler exterior Crust).

• * causes the plates to MOVE… similar to a boat floating on an ocean current

The rising part of a convection current spreads out as it lifts and splits the crust at a divergent plate boundary .

The sinking force pulls tectonic plates downward at convergent boundaries.

Push and Pull Ridge push = the weight of the uplifted ridge pushes

an oceanic plate toward the trench formed at the subduction zone.

Push and Pull

– A sinking region of a mantle convection current

could suck an oceanic plate downward into a subduction zone.

Slab pull = the weight of a subducting plate helps pull the trailing lithosphere into the subduction zone.

Mantle ConvectionPush and Pull

Unanswered Questionsset of questions surround the formation of plate boundaries :

1.Are mantle convection currents permanent features?

2.How does a convection current start?

3. Is convection occurring only in the upper mantle?

4.Do convection currents shift their position in the mantle over time?

Great Rift Valley in AfricaThe Great Rift Valley lies on a continental divergent plate boundary. Divergent plate boundaries are where the earth's rigid lithosphere is being separated due to activity in the flexible asthenoshere and is creating new lithosphere. Because of this rifting process, the Great Rift Valley is constantly changing and "pulling from East and West to cause the North-South divide"

Section Assessment1. How might a convection current cause a

divergent boundary?

Causes of Plate Motions

An upward flow in the mantle causes the asthenosphere to rise. This force causes the lithosphere to rise and split. As the plates separate, material rising from the mantle supplies the magma that hardens to form new ocean crust.

Section Assessment2. How might a convection current cause a

convergent boundary?

Causes of Plate Motions

A sinking region of a mantle convection current could suck an oceanic plate downward into a subduction zone. The weight of a subducting plate helps pull the trailing lithosphere into the subduction zone in a process called slab pull.

Section Assessment

Causes of Plate Motions

3. How are slab push and slab pull related processes?Slab pull is thought to be the most important process driving tectonic plate motions. The material that is subducted through slab pull enters the convection current that drives slab push.