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of the
Label as “Plate Tectonics and Continental Drift” on
your vocab template.
Contraction Theory
Plate Tectonics and Continental Drift
Sea floor spreading
Mid-ocean ridges
Convection Currents
Pangaea Ultima
People Used to Believe…
Contraction Theory- Continents were all connected
like they are now, but massive cooling in the Earth
collapsed land bridges between the continents.
Plate Tectonics
•proposed by Alfred Wegener in 1910.
•noticed that the shorelines of the continents seemed
• to ‘fit together’like the pieces of a giant jig saw
Wegener’s theory stated:
1) The continents were once all together in one
place forming a supercontinent, Pangea.
2) The continents broke apart and drifted to their
present locations.
Wegener’s theory was not taken seriously
because no one could believe that things as
large as continents could move and because
Wegener could not propose a mechanism
which could explain such motion.
-Continents and their tectonic plates
Plate Movement and Land Changes
Divergence (Separation)- When plates separate, they form sea floor
spreading and ocean basins.
Transformation- When plates slide past each other, they create fault
lines that are prone to earthquakes.
Convergence (when plates collide)- Mountains are pushed
Today plate tectonics and continental drift are accepted as facts.
Why? Let’s see the evidence!
First, there’s Wegener’s original observation. The shorelines of the
continents do fit together like the pieces of a puzzle.
But that could just be a
coincidence. For a theory
to be accepted as fact, there
must be more.....................
Wegener’s Evidence
• Fossils of the same animals, rocks, plants, etc were found to
correlate (match up to eachother) in different parts of the globeparts that fit together like puzzle pieces. (Rainforest tree fossils
found in the Arctic).
(Ex: Mesosaurus)
• Evidence of climates having at one time been the same in these
puzzle piece areas. (Ex: The Karoo Desert in Africa shows marks
that indicate the presence of Glaciers! Very cold in very hot?)
• Resources not produced in certain biomes are mysteriously found
there (Ex: coal found in the Artic when it is produced in Tropical
Same fossils found on different continents
 These
are the pictures on the puzzle pieces.
Copyright © 2010 Ryan P. Murphy
Example of fossil/rock similarities. Also findings that show similar past
Further Evidence
•First, a series of under-water
mountains called mid-ocean
ridges are found throughout the
•These mountains are
formed as new sea floor is created
from magma that wells
up from the mantle below.
As new sea floor is added
it pushes the old sea floor
Apart (Sea Floor Spreading)
New sea floor is found at the mid ocean ridge where material is
constantly being added. As distance from the ridge in either
direction increases, so does the age of the rock.
The Ultimate Proof
1992- the Laser Geodynamics/GPS Satellites
(Lageos I and II) found that the continents are still
drifting apart a few centimeters per year.
Let’s review the evidence for continental drift................................
1) The continents seem to fit together like the pieces of a jig saw
2) Same rocks from opposite shores.
3) Same fossils from opposite shores.
4) Sea floor spreading.....
The existance of mid ocean ridges (mountain ranges)
5) Direct satellite measurement of the drifting continents.
Continental drift is now a well supported
theory, but why is it happening???
How could continents, giant slabs of rock, possibly move?
•Earth’s crust is a thin outer
•Beneath the crust
is the mantle. Hot rock in the
mantle actually moves/flows
•The deeper we go
into the Earth, the
hotter it gets.
•Heat Rises and Cold
Air sinks
•This forms
convection currents
that rotate!
•These convection currents in the mantle circulate and
cause the continents above them to move.
Continental Drift: The gradual movement
of the continents across the earth.
Copyright © 2010 Ryan P. Murphy
• Wegener’s ideas were not accepted until the
Copyright © 2010 Ryan P. Murphy
• Same fossils found on different continents
– These are the pictures on the puzzle pieces.
Copyright © 2010 Ryan P. Murphy
• Current Day
In hundreds of millions of years, it is believed that the continents
will create a “super continent” again by drifting together. This has
been called Pangaea Ultima.