Theory of Plate Tectonics

The theory states that the Earth’s crust is broken up into plates that are in motion, driven by convection currents in the mantle.

Three types of plate boundary Divergent Convergent Transform

Divergent Boundaries Spreading ridges As plates move apart new material is erupted to fill the gap

Ocean – Ocean Divergence P.112 Rising convection currents in the asthenosphere push the plates apart forming a mid-ocean ridge in the oceanic crust. Magma erupts out of the mid-ocean ridge

p. 112 Mid-Ocean Ridges Areas of high heat flow and elevation. Forms new crust where the plates separate. Sea floor spreading. Black smokers or vents form near the ridge.

Pillow lavas form as the lava cools quickly in the water.                                                  

P. 112 Iceland is an O-O divergent boundary The Mid-Atlantic Ridge is going through the country The country is spreading 1 inch a year The country is above sea level More active volcanoes than anywhere else in the world..

Iceland: An example of continental rifting Iceland has a divergent plate boundary running through its middle

C-C Divergent p. 113 Rift Valleys form where the two continental plates are separating.

East African Rift System P. 113 The Great Rift Valley of East Africa East African Rift System

Convergent Boundaries There are three styles of convergent plate boundaries Continent-continent collision Continent-oceanic crust collision Ocean-ocean collision

C-C Convergent p. 114 Plates of similar density are converging creating folded mountains. No volcanoes are created.

Continent-Continent Collision European Alps Himalayas

Himalayas

p. 114 The Appalachian Mtns. in the eastern U. S p. 114 The Appalachian Mtns. in the eastern U.S. is a folded mountain range.

O-C Convergent p. 115 An oceanic and continental plate are colliding, the denser oceanic plate subducts (dives) beneath the less dense continental plate.

Continent-Oceanic Crust Collision SUBDUCTION

p. 115 Trenches Trenches form deep valleys at the point where the plate dives down. These are the deepest points on Earth.

Subduction Oceanic lithosphere subducts underneath the continental lithosphere Oceanic lithosphere heats and dehydrates as it subsides The melt rises forming volcanism E.g. The Andes

p. 115 Subduction As the plate subducts, friction generates heat which melts the subducting plate, magma rises to create volcanic mtns that parallel the boundary. Cascade Mtns (Wash, Oregon) Andes Mtns. (S. America)

p. 115 Subduction Crust is destroyed in the process of subduction. Creates the world’s most explosive volcanic eruptions.

O-O Convergence p. 116 The denser older of the two oceanic plates subducts. Marianas Trench – 33,035 feet below sea level.

p. 116 Subduction Trenches form at the boundary. Volcanic islands (called island arcs) form at the surface where the magma rises. Philippines Japan Aleutian Islands (off Alaska) Indonesian Islands

Ocean-Ocean Plate Collision When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone. The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench. The worlds deepest parts of the ocean are found along trenches. E.g. The Mariana Trench is 11 km deep!

p. 117 Transform Boundaries Plates slide past each other Shearing : Stress that pushes a mass of rock in opposite directions. Above: View of the San Andreas transform fault

What will happen in the future? The Atlantic Ocean will continue to expand. The Pacific Ocean will close. The Mediterranean will disappear connecting Africa with Europe. India will continue to push into Eurasia pushing the Himalaya Mtns. higher. Los Angeles will continue to the northwest joining with San Francisco.

Age The age of the ocean crust. Courtesy of www.ngdc.noaa.gov