Download Ch. 7 -2 Restless Continents

Ch. 7 -2
Restless Continents
Continental Drift
Sea Floor Spreading
Wegner’s Continental Drift
Hypothesis
 Alfred
Wegener wrote a hypothesis
about continental drift that states
they were once a single landmass,
broke up and then drifted to their
present location.
 It explained why fossils of the same
plant and animal species are found
on continents on different sides of
the Atlantic Ocean.
Pangaea

Pangaea is Greek for “all earth.” It existed about
245 million years ago. We know that Pangaea
further split into 2 huge continents: Laurasia and
Gondwana.
Sea Floor Spreading
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From the calculated strength of rocks, it seem
possible that crust could move this way – spread
out! See p. 200 figure 3.
Mid Ocean Ridges are underwater mountain
chains that run through Earths ocean basins.
This means that they are ALSO where sea floor
spreading takes place!
Sea floor spreading is the process by which new
oceanic lithosphere forms as magma rises toward
the surface and solidifies.
Sea Floor Spreading con’t.
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As the tectonic plates move away from each
other the sea floor move away and the magma
fills in the gap.
The older crust is farther away from the mid
ocean ridge that the younger crust is.
Magnetic Reversals
– Evidence: when Earth’s magnetic poles change places
this change, is called a magnetic reversal.
– Since the molten rock at the mid ocean ridges contain
tiny grains of magnetic minerals (contain iron), they act
like compasses. They will align when they cool and this
creates a record or proof that sea floor spreading occurs.