Plate Tectonics

picture from http://www.ngdc.noaa.gov/mgg/image/relief_slides2.html


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Volcano data and names of plates http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/Maps/map_plate_tectonics_world.html
world earthquake data http://www.ruf.rice.edu/~leeman/seis_world.gif
Paleomap reconstructions of Pangea http://www.scotese.com/earth.htm
Types of plate boundaries http://www.pbs.org/wgbh/aso/tryit/tectonics/

more background information

Animations
http://vimeo.com/14258924
Pangaea Break up
http://www.scotese.com/sfsanim.htm Pangaea Break up
http://bcs.whfreeman.com/thelifewire/content/chp22/22020.html Pangaea break up
http://www.learner.org/interactives/dynamicearth/slip3.html rift valley formation
http://emvc.geol.ucsb.edu/forteachers/flashmovies/Spreading.swf Sea floor spreading
http://www.wwnorton.com/college/geo/egeo2/content/ch02/animations.htm Plate boundaries and hot spot volcanoes


Theory of Continental Drift

-Theory was first propsed by Alfred Wegener that the continenets were once a Supercontinent named Pangaea with one ocean called Panthalassa. As it broke apart it first split into two chunks. South America and Africa togther made up Gondwanaland. Europe, Asia, and North America made up Laurasia. Eventually the continents moved to their present day positions.

Evidence:
Continents fit together like a jigsaw puzzle
fossils of terrestrial species match for different continents
rock layers/mountain ranges match up from continent to continent
glacial features match up
and appear in places near the equator
coal deposits (indicating tropical environments) match and appear in cold locations

Why wasn't it accepted?
Wegener could not explain how the continents moved. He suggested that the continents pushed their way through the dense seafloor which did not make sense. Without a mechanism to explain how the continents could move, it was ignored.

Seafloor Spreading

-Idea that the oceanic crust is spreading apart at the midocean ridge. New material forms at the ridge and spreads outwards pushing older sea floor away. Old floor eventually sinks back into the mantle at subduction zones.

Evidence:
Pattern of rocks on either side of the midocean ridge go from youngest near the ridge to oldest away from th risge. This is a mirror image on both sides of the ridge
A similar mirror image pattern results from magnetic reversals. Heated up rocks record the magnetic direction of earth's magnetic field for certain time periods. This magnetic field changes direction. The pattern of reversals is recorded by special sonar devices which show a mirror image pattern on either side of the midocean ridge.

 

Plate Tectonics

-built on previous ideas of Continental Drift and Seafloor spreading. The world is broken up into a series of plates. These plates move on the asthenosphere by convection currents. Plates move apart at midocean ridges where ocean floor is generated. Other places plates collide or sink back down into the mantle at subduction zones.

Evidence
-volcanic activity along plate boundaries where dense oceanic crust is sinking and melting
-earthquakes a result of friction and movement of plates which mark locations of plate boundaries. Deep earthquakes can only uccur at subduction zones; earhquake locations are found by differences between S (shear) wave and P (compression) wave arirval times

-location of moutain ranges
-location of trenches
-location of mid ocean ridges


(illustration by José F. Vigil from This Dynamic Planet , USGS, http://pubs.usgs.gov/publications/text/Vigil.html)

TYPES OF BOUNDARIES
1. divergent--two plates separate, form rift valley and new sea floor, eventually become mid-ocean ridges (ie--Mid Atlantic Ridge)

2. convergent--two plates collide;
- if two ocean plates collide: subduction zones and ocean trenches form. As the plate sinks, it melts and the magma rises to the surface forming a chain of volcanic islands called an island arc
- if two plates of continental crust collide, neither plate can sink so they buckle and push up to form non volcanic mountain ranges.
-if plates of continental crust collide with oceanic, the thinner, denser oceanic crust sinks at a subduction zone creating a trench. As the oceanic crust melts, hot magma reaches the surface and forms a volcanic arc (ie--Andes Mountains)

3. transform--two plates slide past each other (ie--San Andreas)

 

POTENTIAL CAUSES OF PLATE MOVEMENT:

1)Heat from the core causes convection currents in the asthenosphere. Plates glide on the asthenosphere forming divergent boundaries where the convection currents rise and subduction zones where it sinks.

2) ridge-push is caused by new warm ocean floor pushing older away at midocean ridges. These plates are at a slight angle so they are highest at the ridge. As the warm seafloor cools it becomes denser and gravity helps to move the material away

3) slab pull- at subduction zones where the angle of the plate is steep and is sinking into the mantle under its own weight. Gravity pulls on the heavy, cold, dense plate. The plate pulls the rest of the plate with it like covers sliding off the edge of a bed.