Plate TectonicsCauses of Plate Tectonics

Plate Boundaries

The lithosphere is broken into separate sections called plates. These plates “float” on top of the Asthenosphere.

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

The crust and part of the upper mantle = lithosphere•100 km thick •Less dense than the material below it so it “floats”

The semi-molten layer below the lithosphere =Asthenosphere

The plates of the lithosphere float on the Asthenosphere

Ocean plates – lithospheric plates below the oceans

Continental plates – lithospheric plates below the continents

Hot magma in the Earth moves toward the surface, cools, then sinks again. This creates convection currents beneath the plates that cause the plates to move.

At the locations where two tectonic plates interact, a boundary between these plates exist.

There are three types of boundaries: divergent boundaries, convergent boundaries, and transform boundaries.

Boundary between two plates that are moving apart or rifting

RIFTING causes SEAFLOOR SPREADING

Mid-ocean ridges (2 oceanic plates pulling apart)

Rift valleys (2 continental plates pulling apart)

Fissure volcanoes (either type)

In sea-floor spreading, the two oceanic plates spread apart. Magma reaches the surface, cooling to form new crust. As a result, the ocean floors move like conveyor belts, carrying the continents along with them.

Evidence includes:eruptions of molten material, magnetic

stripes in the rock of the ocean floor, and the ages of the rocks themselves (youngest closest to the mid-ocean ridge).

Boundaries between two plates that are colliding

There are 3 types…

Ocean –Continental Convergent

More dense oceanic plate colliding with a less dense continental plate

Subduction Zone: where the more dense plate (oceanic) slides under the less dense plate (continental)

Mountains and possibly volcanoes Examples: Cascade Mtns in Oregon,

Washington and Andes Mtns in South America.

Trenches (underwater canyons) on ocean floor

Examples: Marianas Trench in Pacific Ocean

Ocean-Ocean Convergent

The less dense ocean plate slides under the more dense ocean plate creating a subduction zone.

Island Arcs may form due to volcanic activity at this boundary. Examples: Caribbean Islands, Japan, Indonesia, Philippines, Aleutian Islands.

Trenches also form on ocean floor

Continent-Continent Convergent

Continental plate colliding with another continental plate.

Plates have similar density so both plates thrust (push) up.

Fault Block and Folded Mountain Ranges are formed.

Examples: Himalayan Mtns, Appalachian Mtns.

Boundary between two plates that are sliding past each other

EARTHQUAKES occur along these type of boundaries

Examples: San Andreas Fault in California