Plate Tectonics—part 2 Alfred Wegener did not know the earth’s composition, but by the 1960’s scientists were able to revise Wegener’s hypothesis into a theory now known as Plate Tectonics. Alfred Wegener did not know the earth’s composition, but by the 1960’s scientists were able to revise Wegener’s hypothesis into a theory now known as Plate Tectonics.

Post 1950’s evidence Convection currents Convection currents Mid-ocean ridge & trenches Mid-ocean ridge & trenches Age of continent rocks vs. seafloor rocks Age of continent rocks vs. seafloor rocks Earthquake/volcano and satellite data Earthquake/volcano and satellite data

Directions and rates of plate motions

Plate Tectonics The lithosphere is composed of 7 major and 13 minor plates. The lithosphere is composed of 7 major and 13 minor plates. A plate can be composed of continental and/or oceanic crust. A plate can be composed of continental and/or oceanic crust. Continental crust- felsic Continental crust- felsic Oceanic crust-mafic Oceanic crust-mafic

14.3 Differences Between Continental & Oceanic Lithosphere Oceanic – Oceanic –  younger (180 million years or less)  more dense  ~7 km thick Continental – Continental –  older (4 billion years old)  less dense  ~35-40 km thick

Plate Tectonics The areas where plates are in contact with each other are called plate boundaries. There are three types of plate boundaries.

Plate Boundaries (type 1) Divergent-two plates are moving away from each other. New crust is formed. Most divergent boundaries are on the ocean floor. These are marked by mid-ocean ridges. Divergent-two plates are moving away from each other. New crust is formed. Most divergent boundaries are on the ocean floor. These are marked by mid-ocean ridges. Tensional force is applied to rocks at a divergent boundary. (tension is what pulls them apart) Tensional force is applied to rocks at a divergent boundary. (tension is what pulls them apart)

Geographic Examples

The East African rift – a divergent boundary on land

Plate Boundaries (type 2) Convergent- two plates are moving towards each other. (which applies a “compressional force”)  Oceanic/continental oceanic crust has a higher density than Continental crust, therefore it sinks below the continental crust. This is called a subuction zone.  Volcanoes, mountains and trenches form.

Plate Boundaries (type 2)  Continental/continental neither plate is subducted. In this case, both edges are crumpled and uplifted producing large mountain ranges.  Himalayan Mountains (India is colliding with Asia)

Plate Boundaries (type 2)  Oceanic/oceanic These are rare and create deep trenches.  Marianas Trench deepest trench in the world.

Plate Boundaries (type 3) Transform Fault Boundaries- two plates grind past each other. (which applies a “shearing” force) Transform Fault Boundaries- two plates grind past each other. (which applies a “shearing” force)  They do not slide easily-they stick and slip.  When these plates slip they create earthquakes.  San Andreas Fault in California

MOUNTAIN BUILDING

Deformation Deformation  The bending, tilting and breaking of Earth’s Crust. MOUNTAIN BUILDING

Stress Stress  Force applied to rock material from Isostasy. Strain Strain  Change in shape or volume of rocks from rocks being squeezed, twisted or pulled apart. MOUNTAIN BUILDING

Folding Folding  Permanent deformation or bending of a rock under stress. Faulting Faulting  Break in rock along which rocks on either side of the break have moved. MOUNTAIN BUILDING

Faulted Faulted  Faulting breaks Earth’s crust into large blocks and those blocks become tilted.  Ex. Sierra Nevada Mountains

MOUNTAIN BUILDING Folded Folded  Plates collide (Convergent boundary)  Ex. Appalachian Mtns, Himalayas

MOUNTAIN BUILDING Dome Dome  Molten rock pushes up rock layers on Earth’s surface creating a dome. Over time this top layer erodes away leaving the igneous rock exposed.  Ex. Black Hills, South Dakota

MOUNTAIN BUILDING Volcanic Volcanic  Molten rock that has erupted onto Earth’s Surface.  Cascade Mtns., Mid-Atlantic Ridge