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Continental Drift
Alfred Wegner’s theory
What was Wegner’s Theory?
The continents
moved
What was his evidence?
•Jigsaw puzzle fit of the continents
•Matching rocks & mountain chains
•Similar fossils on different continents
•A single glacier covering several
continents
•Paleomagnetism (continents aligned in
different directions, not northward)
Theory of Plate Tectonics
Crust is made of 12
moveable lithospheric
plates sitting and
moving along the molten
asthenosphere
What scientific evidence
do we have to prove the
cracks exist?
Last 30 Days of Earthquakes
Earthquakes occur along the plate
boundaries!!
Evidence:
1. volcanoes and earthquakes occur in
predictable regions (eg. Ring of Fire)
We know that the Earth has a
magnetic field
(North on top, South at the bottom)
Any iron found in newly made rock
will point North
Once in a while, the Earth’s
magnetic field reverses
Suppose North is now down
“South”
…and the magnetism in the newly formed
rocks will reverse
Exploring the Magnetic “stripes” on sea floor
But why are the magnetic stripes
symmetrical?
• There must be a crack in the ocean
where the rocks are spreading outward
Evidence:
2. symmetrical magnetic stripes
occur on either side of spreading
mid-ocean ridges
Hawaiian Islands are not on the plate
boundary. How did they form?
Hot Spot
The plate must be moving over
the hot spot
Each island was formed one after the other
Evidence:
3. formation of the Hawaiian
islands as the Pacific Plate moves
over a central hot spot