Causes of Plate Motion
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
– Mantle Convection Current
– ridge push
– slab pull
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 sinking
force pulls
tectonic
plates
downward at
convergent
boundaries.
The rising
part of a
convection
current
spreads out
as it lifts
and splits
the crust at
a divergent
plate
boundary .
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
Slab pull = the weight of a subducting plate helps pull
the trailing lithosphere into the subduction zone.
– A sinking region of a mantle convection current
could suck an oceanic plate downward into a
subduction zone.
Mantle Convection
Push and Pull
Unanswered Questions
set 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 Africa
The 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 NorthSouth divide"
Causes of Plate Motions
Section Assessment
1. How might a convection current cause a
divergent boundary?
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.
Causes of Plate Motions
Section Assessment
2. How might a convection current cause a
convergent boundary?
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.
Causes of Plate Motions
Section Assessment
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.