Plate Tectonics

Continental Drift Wegener’s Hypothesis The continents once formed part of a single landmass called a supercontinent. The supercontinent began breaking up into smaller continents about 250 million years ago. Over millions of years, these continents drifted to their present locations.

Wegener’s Evidence Fossil Evidence: identical fossils found in both South America and western Africa Coastline Evidence: similarities in the coastlines of the continents Geologic Evidence: ages and types of rocks in the coastal regions of separate continents matched. Coal found in cold regions. Climatic Evidence: layers of debris from ancient glaciers on separate continents

Mid-Ocean Ridge Undersea mountain chain that forms as magma rises and creates new sea floor as tectonic plates move apart http://www.pmel.noaa.gov/vents/nemo/education/curr_p1_09.html

Sea-Floor Spreading New sea floor forms as magma rises to Earth’s surface and solidifies at a mid-ocean ridge http://arizonaenergy.org/EarthEnergy/sea_floor_spreading.htm

Magnetic reversals  Geologic evidence suggest that Earth’s magnetic field has not always pointed north. http://www.sonoma.edu/users/f/freidel/global/372chapt4platetectonics.htm

Magnetic Symmetry The stripes in the sea floor illustrate Earth’s alternating magnetic field http://thesymbiont.blogspot.com/2011/11/geomagnetism-how-it-works-and.html

Plate Boundaries

Divergent Boundaries Plates move away from each other. Magma from the asthenosphere rises to the surface, cools and forms new oceanic lithosphere. http://tectonicplatefun.wordpress.com/2012/04/03/divergent-boundary-diagram/

Convergent Boundaries Plates collide with each other Convergent Boundaries Plates collide with each other. Three types of collisions can happen at convergent boundaries. First Type: Dense oceanic lithosphere collides and sinks or subducts under the less dense continental lithosphere. http://skywalker.cochise.edu/wellerr/students/subduction-earthquakes/project.htm

Convergent Boundaries Second Type: Two continental lithosphere plates collide. Neither subducts instead the colliding edges crumple and thicken, which causes uplift that forms large mountains ranges. http://geography-student.blogspot.com/2012/02/convergent-boundaries.html

Convergent Boundaries Third Type: Two oceanic lithosphere plates collide. One plate subducts under the other plate, and a deep-ocean trench forms. Magma rises to the surface to form an island arc, which is a chain of volcanic islands. http://pubs.usgs.gov/gip/dynamic/understanding.html

Transform Boundaries Two plates slide past each other horizontally. They scrape against each other in a series of sudden spurts of motion that are felt as earthquakes. Short segments of mid-ocean ridge are connected by transform boundaries called fracture zones. http://mohumes.wikispaces.com/How+Does+An+Earthquake+Occur%3F

Causes of Plate motion Mantle Convection: Earth’s core and radioactivity within the mantle heat mantle material. The hot material rises, the cooler material sinks. As the mantle moves it drags the plates along with it. http://funwithteachingrocks.wordpress.com/2011/07/14/convection-cells-in-earths-mantle-and-plate-tectonics/ http://www.sciencelearn.org.nz/Contexts/Earthquakes/Sci-Media/Images/Earth-structure

Plate Motion Ridge Push: Warm and less dense than older rock the newly formed rock rises and pushes the older denser rock away from the ridge. http://www.umich.edu/~gs265/tecpaper.htm

Plate Motion Slab Pull: As the lithosphere moves away from the mid-ocean it cools and becomes denser. It begins to subduct, as the leading edge begins to sink it pulls the rest of the plate along behind it. http://www.umich.edu/~gs265/tecpaper.htm