9.3 Actions at Plate Boundaries Textbook pp 258-264

Different Kinds of Boundaries Remember that there are three kinds of plate boundaries. At each kind of boundary there are different movements. Divergent – plates move apart Convergent – plates move together Transform Fault – plates move past each other

Are there any plates here with only one kind of border?!??

Divergent Boundaries Most divergent plate boundaries are located along the crests of oceanic ridges (mountain ridges at the ocean floor).

These can be thought of as constructive plate margins.

The Mid-Atlantic Ridge

The Mid-Atlantic Ridge is just one part of the Ocean Ridge, which winds its way over 70,000 km around the entire planet.

Divergent Boundaries create Rift Valleys Rift valleys are deep faulted structures found along the edges of divergent boundaries. Rift valleys can develop on land, too.

As plates are pulled apart at divergent boundaries, they are pushed together at convergent boundaries.

Convergent Boundaries As new lithosphere is being added on one edge of a plate, old lithosphere is being destroyed at another. Convergent boundaries are where the edges of tectonic plates are “eaten.” Because of this destruction, convergent boundaries are called destructive boundaries.

Subduction Zones A subduction zone occurs when one oceanic plate is forced down into the mantle beneath a second plate.

Different Types of Convergent Boundaries The outcome of convergent boundary movement is controlled by which two types of convergent boundaries come together: Oceanic – Continental Plates Oceanic – Oceanic Plates Continental – Continental Plates

Oceanic-Continental When the leading edge of a continental crust converges with an oceanic plate, the less dense continental plate remains floating. The denser oceanic slab sinks into the asthenosphere. This rock will melt and become new magma.

Oceanic-Continental Convergence Boundary

Some of this magma may reach the surface and cause volcanic eruptions. The volcanoes of the Andes are examples of this continental volcanic arc.

The Andes Mountains of Peru… and friend.

Oceanic-Oceanic When two oceanic plates converge, one descends beneath the other. This causes volcanic activity to form on the ocean floor (instead of on a continent, like at the oceanic- continental boundary).

The Aleutian Islands are an example of a volcanic island arc.

Look at these two examples of convergent boundaries. Which one shows ocean- ocean? Which shows ocean-continental? How can you tell?

Continental-Continental When two continental plates converge, the subduction eventually forces one plate beneath the other. Since both plates are lighter in density than oceanic plate, a mountain range will form as the lower plate pushes the upper plate…. UP!

The Himalayas are an example of a mountain range formed by Continental-Continental Convergent Boundaries.

The Himalayas are still growing taller, about 5 cms per year.

Remember… Each tectonic plate has both divergent and convergent boundaries (as well as transform fault boundaries). Every plate boundary moves because of the convection currents underneath it!