*information presented comes from Earth Science published by Glencoe McGraw-Hill

Plate Tectonics

I. Alfred Wegener proposed the theory of continental drift

A. Continental Drift - theory about the continents separating

1. Pangaea - the early land mass consisting of all current continents put together

2. Evidence

a. similar rocks have been found on different continents
b. fossils of similar plants and animals found on different continents (for example: pre-dinosaurs found in Africa and South America)
c. Glacier evidence in Africa, India, Australia and South America


3. Scientists at the time disputed Wegener's theory because he could not explain how the continents separated
 

B. Seafloor Spreading

1. Ocean ridges - separation in the seafloor causing a "gap"

a. Gap fills with magma
b. magma cools and hardens
c. gap reopens and pushes hardened magma to the side creating more seafloor



 

2. Missing link in Wegener's theory: seafloor spreading caused pangaea to separate

3. Analysis of the seafloor shows Earth's polarity has reversed several times throughout history

4. Seafloor closest to ocean ridges is younger than seafloor farther from the ridge
 

II. Theory of Plate Tectonics

A. Earth's crust is broken into slabs called PLATES


 

B. Plate boundaries - areas where plates interact

1. Divergent boundaries - where plates separate

a. Ocean ridges are caused by divergent boundaries between two oceanic plates
b. Rift Valley - divergent boundary on land, may lead to a new ocean basin (ex. Great Rift Valley in eastern Africa)


2. Convergent boundaries - where plates move together

a. Ocean to ocean: one plate will slide under the other, pushing the second plate up. This causes a subduction zone and volcanic islands
b. Ocean to continent: oceanic plate will slide under the continental plate, pushing the continental plate up. This causes a trench in the ocean and a mountain range on land
c. Continent to continent: both plates push together and uplift causing a mountain range, for example the Himalayas


3. Transform boundaries - plates slide horizontally against each other

a. causes shallow earthquakes
b. San Andreas fault in California is the best known transform boundary



C. Causes of Plate Motion: Scientists are not sure, but they have ideas of what causes the motion of the plates


 

1. Idea 1: Mantle convection currents: material near the mantle is warmer than at the surface, which causes it to rise and the cooler material to sink. As the cooler material gets lower, it warms up and rises and the material near the surface (now cooled) sinks, process continues

a. Ridge push - where the mantle rises - convergent boundary
b. Slap pull - where the material sinks - divergent boundary



2. Idea 2: Continental Divergent Boundary: continent acts as an insulating blanket - mantle heats beneath is - warm matter rises, splits the land mass.

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