2. Plate Tectonics The theory of plate tectonics was first proposed by
Alfred Wegener in Wegener noticed that the
shorelines of the continents seemed to ‘fit together’
like the pieces of a giant jig saw puzzle.
Wegener’s theory stated:
The continents were once all together in one
place forming a supercontinent, Pangea.
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.

3. In 1915, The German geologist Alfred Wegener ( ) proposed continental drift. Not accepted until 1950’s!
“I say.” “Africa and South America fit strangely like two puzzle pieces.”
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4. 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

5. Evidence for continental drift.-
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6. The shapes do match.
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7. If the continents were once together, we ought to be able to find
similar kinds of rocks in places that were once together but have
since drifted apart.
If we believe that area A and
area B were once together, then
we should find similar rock types
in these two locations even though
today they are thousands of miles
apart.
Rocks are a lot like fingerprints. The mineral content and structure
of rock outcrops often identify them as unique.
Wegener himself investigated this possibility and found that, yes,
he was able to correlate rock types from opposite shorelines.
Correlating rock types is a second piece of evidence.

9. If we examined rocks from area A and found in them fossils that are
also found in area B but nowhere else, this would be powerful evidence
that these two areas were once together forming a single habitat.
It is important to choose our
fossils carefully. Finding fossils
of organisms that lived all over
the world would mean nothing.
But if we could find the same fossils in areas A and B, fossils that
exist nowhere else because they had a very limited range, then this
is even more powerful evidence that areas A and B were once
together.
Correlating fossils from opposite shores is a third piece of evidence.

10. Same fossils found on different continents
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12. What is this a fossil of?
Where would you expect to find a specimen like this on the planet?
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13. Answer! This is a fossilized tropical plant found on Antarctica.
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14. Answer! This is a fossilized tropical plant found on Antarctica.
Remember, the continents have moved and Antarctica use to be in a warmer climate.
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15. “Darn you continental Drift.”
Answer! This is a fossilized tropical plant found on Antarctica.
Remember, the continents have moved and Antarctica use to be in a warmer climate.
“Darn you continental Drift.”
“Darn you.”
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16. Sea Floor Spreading
If the continents are moving apart then the seas between them must be
getting wider. This is called sea floor spreading. How do we know that
this is true?
First, a series of under-water mountains called mid-ocean ridges is found throughout the world. These mountains are
formed as new sea floor (basalt) is created from magma that wells
up from the mantle below.

17. As new sea floor is added
it pushes the old sea floor
apart.
Proof that this occurs can be
found by looking at the age
of the sea floor itself.
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.

18. The Ultimate Proof This is the Laser Geodynamics Satellite
(Lageos). It was launched in 1976 and
was joined in 1992 by Laegos II. They
orbit the earth at an altitude of 3,700 miles
and carry no electronic equipment.
Laser beams are bounced from one point on the earth, off the
satellite, to a second point on the ground. Scientists can then
measure the distance between the two points with great accuracy.
Geo positioning satellites enable scientists to
use GPS devices to make accurate measurements
of positions on the earth.
These systems show conclusively that the
continents are still drifting at a rate of a few
centimeters a year.

19. The speed at which the plates move is about the speed at which your fingernails grow.
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20. Behold the Supercontinent! Pangea?
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22. Applause! Behold the Supercontinent! Pangea
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23. End Applause! Behold the Supercontinent! Pangea
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24. Gondwondaland and Laurasia were two mega continents before Pangea.
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26. Current Day + or – 4mm

28. Continental drift is not a theory. It’s a fact.
Let’s review the evidence for continental drift
The continents seem to fit together like the pieces of a jig saw
puzzle.
2) Same rocks from opposite shores.
3) Same fossils from opposite shores.
4) Sea floor spreading.....
a) The existance of mid ocean ridges (mountain ranges)
b) Age of the sea floor.....
c) Paleomagnetism
5) Direct satellite measurement of the drifting continents.
Continental drift is not a theory. It’s a fact.
Wegener was right! But why......?

29. BRAIN BREAK!

30. How could continents, giant slabs of rock, possibly move? The key
is to understand the internal structure of the earth
We live on a very thin, cool
crust which averages about
50 km thick under the con-
tinents and 10 km under the
oceans. Beneath the crust
is the mantle. Rock in the
mantle is very hot and is
under high pressure. Rock
here is described as plastic
which means that, like
silly putty, it can flow very
slowly over inimaginably
long periods of time.

31. Because temperature
increases with depth,
the inside of the earth
is very hot. Rock near
the core is heated and
becomes less dense. It
slowly rises while
cooler rock nearer the
surface is more dense
and sinks. This forms
convection currents
just like those in our
atmosphere. As these convection currents in the mantle circulate, they
cause the continents above them to move. What Wegener had no way
of knowing is that the force that is driving plate tectonics is convection
currents in the mantle.

32. Plate Tectonics
Lithosphere (the crust and the solid upper part of the mantle) is not one solid mass but is broken into small sections called tectonic plates
Each plate floats independently on the asthenosphere (the part of the mantle that acts like a liquid)
Explains why earthquakes and volcanoes happen; where they occur; and, why mountain ranges, ocean trenches and faults have formed.

33. Boundaries The location where two plates are floating side by side
Plates interact with each other at this boundary.
Convergent boundary
plates are running into each other
Divergent boundary
plates are moving away from each other
Transform boundary
one plate is sliding past the other plate

34. Divergent Plates

35. Continental Convergence
The Himalayan mountain range provides a spectacular example of continent vs. continent collision.
When two huge masses of continental lithosphere meet head-on, neither one can sink because both plates are too buoyant.

36. Oceanic Plate Convergence
When two oceanic plates collide, the plate that is older, therefore colder and denser, is the one that will sink.
Volcanoes rise out of the ocean
volcanic mountain chains island arcs.
The Aleutian Peninsula of Alaska is an excellent example of a very volcanically-active island arc.

37. Transform Plates

38. What causes continental drift and plate tectonics?
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39. Answer! Convection currents (Remember heat rises) move the plates
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40. Answer!
Divergent plates
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41. Answer!
Convergent plates
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42. Which colored arrows are incorrect based on the convection current patterns and plate movements below?
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43. Answer! The blue arrows. The plates should be moving toward each other.
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44. Answer! The blue arrows. The plates should be moving toward each other.
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45. “Oh-no!” “We are trying it again.”

46. Try Again! Which colored arrows are incorrect based on the convection current patterns and plate movements below?
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47. Answer! The purple arrows should be diverging instead of converging.
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48. Answer! The purple arrows should be diverging instead of converging.
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49. “Oh-no!” “We are trying it one more time.”

50. Try Again! Which colored arrows are incorrect based on the convection current patterns and plate movements below?
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51. Answer! The light blue arrows should be diverging instead of converging.
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52. Answer! The light blue arrows should be diverging instead of converging.
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53. “Oh-no!” “We are trying it one more time.”

54. Try Again! Which colored arrows are the only correct on in the entire group.
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55. Try Again! Which colored arrows are the only correct on in the entire group.
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56. Let's sum up everything we've learned:
1) The earth's crust is divided into many pieces called tectonic
plates which move around over long periods of time.
2) Evidence for crustal movement includes:
a) The 'fit' of the continents.
b) Correlation of rock types from opposite shores.
c) Correlation of fossils from opposite shores.
d) Age of the sea floor (youngest near mid-ocean ridges
and oldest near the continents).
e) Paleomagnetism: zones of oppositely magnetized
rock on opposite sides of the mid ocean ridge.
f) Direct satellite and GPS measurement of continental
movement.
3) The force driving the plates are convection currents (density
differences) in the earth's mantle.