The theory that states that pieces of Earth’s lithosphere are in constant, slow motion, driven by convection currents in the mantle.

The theory that states that pieces of Earth’s lithosphere are in constant, slow motion, driven by convection currents in the mantle.

• The theory of plate tectonics explains the formation, movement, and subduction of Earth’s plates.

Forces that causes Earth’s plates to move.

• The plates of the lithosphere float on top of the Asthenosphere.

• Convection currents rise in the Asthenosphere and spread out beneath the lithosphere.

• Convection current forces drag the overlying plates along. The currents cools and sinks deeper into the mantle.

• Scientist think that the downward movement may provide the force that causes the subduction of plates carrying oceanic crust.

Forces that causes Earth’s plates to move.

Slab Push Hypothesis

• Magma rising along the mid oceanic ridge exerts a force that pushes an oceanic plate away from the ridge.

• The force of gravity causes plate movement by pulling cooler, denser oceanic plates down toward the mantle.

• Slab push and pull work together with convection currents to move the plates.

The earth’s surface is made up of about nine large plates & several smaller plates.

Places where two plates meet.

There are three types of plate boundaries.

1. Transform Boundary

2. Divergent Boundary

3. Convergent Boundary

Two plates slip past each other, moving in opposite directions.

Earthquakes occur frequently along this boundary.

The San Andreas fault in California is an example of a transform boundary.

Two plates move away from each other.

• When divergent boundary develop on land, two of Earth’s plates slide apart.

• A rift valley forms along the divergent valley.

• Ex. The Great Rift Valley in east Africa is about 3,000 kilometers long.

Two plates move towards each other.

• When plates converge, it results in a collision.

• Collisions happen between:

1. Oceanic crust to oceanic crust

2. Oceanic crust to continental crust

3. Continental crust to continental crust

Plate Movement

• The plates move at about 1-10 centimeters per year.

• The North American and Eurasian plates move at about 2.5 cm/year.

Plate Movement

• Pangaea existed about 260 million years ago.

• Approximately 225 million years ago Pangaea began to break apart.

Plate Movement