Plate Tectonics

Types of plate boundaries http://www.pbs.org/wgbh/aso/tryit/tectonics/
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
more background information

Theory of Continental Drift

-Theory was first propsed by Alfred Wegener that the continenets were once a Supercontinent named Pangea. South America And Africa togther made up Gondwanaland and Eurpoe, Asia, and North America made up Laurasia.

Evidence:
Continents fit together like a jigsaw puzzle
fossils of terrestrial species match for different continents
rock layers match up from continent to continent
glacial features match up

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

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

-combined idea of Continental Drift and Seafloor spreading. The world is broken up into a series of plates. These plates move on the asthenosphere. Plates drift apart at ridges where ocean floor is generated. Other places plates collide or sink backj 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

(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, everntually become mid-ocean ridges (ie--Mid Atlantic Ridge)

2. convergent--two plates collide; if two ocean plates collide: island arcs form and ocean trenches. Two continental plates form mountain ranges. When continental crust and oceanic crust collide, the oceanic crust sinks forming 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) Slab-push is caused by gravity pulling down on the plates which are at a slight angle, sending them into the mantle. Slap pull is due to the plates becoming colder and therefore denser than the asthenosphere material. This enables the end of the plate to sink into the asthenosphere at subduction zones