2. Continental Drift TheoryContinental Drift Theory
• First proposed by Alfred Wegener in 1912:
– 250 million years ago, all of the continents
were combined into one super-continent
called “Pangaea”
– The continents gradually drifted apart to
where they are today
• Wegner didn’t make up this theory out of
the blue – like all scientists, he based it on
evidence
3. Evidence SummaryEvidence Summary
• Geographic fit of South America and
Africa
• Fossils match across oceans
• Rock types and structures match across
oceans
• Ancient glacial features
4. Geographic FitGeographic Fit
• Continents look like
they could be part
of a giant jigsaw
puzzle
• Here’s how they moved apart
6. Fossil Fuel in AntarcticaFossil Fuel in Antarctica
• Tropical plant remains (coal deposits) found in
Antarctica
– this is evidence that Antarctica was once much warmer
and much closer to the equator, since tropical plants
don’t grow in Antarctica today
7. Rock Structures Match Across OceansRock Structures Match Across Oceans
• Same rock patterns found in South America,
India, Africa, Antarctica and Australia
11. Evidence but no MethodEvidence but no Method
• While Wegener presented compelling
evidence, there was still no explanation for
HOW the continents drifted.
• The question remained: “If continents drift,
what is making them move?”
12. Sea Floor DiscoveriesSea Floor Discoveries
• WW II: Military Spending
• U.S. Navy mapped seafloor with sonar in
order to help ships and submarines
navigate.
• They expected to find that the ocean floor
was a vast, flat plain. What they found was
shocking.
13. Sea Floor DiscoveriesSea Floor Discoveries
• Instead of miles and miles of flat surface,
they found that the ocean floor had:
– oceanic ridges - submerged mountain ranges
– fracture zones - cracks perpendicular to ridge
– trenches - narrow, deep gashes
– seamounts - drowned undersea islands
14. Sea Floor DiscoveriesSea Floor Discoveries
• In addition, they discovered that the rocks of the
seafloor included only basalt, gabbro, and
serpentinite - no continental materials.
• This suggested that the sea floor was not simply
“covered up” continental crust, but was made of
different materials and at a different time
15. Sea Floor DiscoveriesSea Floor Discoveries
• Further study of these rocks led scientists
to even more surprising information:
• The sea floor’s youngest rocks were
located right at the ocean ridge – and as
you moved away from the ridge in either
direction, the rocks got progressively
older.
17. Sea Floor DiscoveriesSea Floor Discoveries
• What scientists discovered was that the sea floor was
being constantly “recycled.” The youngest rocks were
created from magma rising to the surface, hardening and
pushing aside the older rock.
• Scientists called this process “sea floor spreading.”
18. Sea Floor DiscoveriesSea Floor Discoveries
• So now we know:
– sea floor is being created at the mid-
ocean ridges
– sea floor is spreading
– the oldest ocean floor occurs at the
coastlines of continents…
• Why doesn’t the earth get
bigger? Where does the
ocean floor go? Why
doesn’t it get any older?
19. Sea Floor DiscoveriesSea Floor Discoveries
• The ocean floor is pushed against the
continental crust – and because it is
denser, it dives under the crust.
• This process is
called
subduction
21. The Rise of Plate TectonicsThe Rise of Plate Tectonics
• In the late 1950’s, the United States was
engaged in The Cold War with the Soviets
• To keep an eye on Soviet nuclear tests, the
U.S. military developed new, advanced
seismometers
• These seismometers were deployed in over 40
allied countries and were recording 24 hrs/day,
365 days/year
22. The Rise of Plate TectonicsThe Rise of Plate Tectonics
• Besides nuclear tests, the seismometers
recorded every moderate to large earthquake
on the planet.
• Scientists mapped the earthquake data and
found something they weren’t expecting:
– Armed with this high-precision earthquake data,
seismologists found that activity happens in narrow
bands.
23. The Rise of Plate TectonicsThe Rise of Plate Tectonics
24. The Rise of Plate TectonicsThe Rise of Plate Tectonics
• The discovery of these bands led scientists to
understand that the earth’s outer shell is broken
into thin, curved plates that move laterally atop
a weaker underlying layer.
• Think of it like a hard-boiled egg: you can put
cracks all over the shell of a hard-boiled egg,
but the egg is still “whole”
25. Types of Plate BoundariesTypes of Plate Boundaries
• The interaction of the plate edges with each other
can be classified as one of three main types of
boundaries:
• Convergent boundaries
• Divergent boundaries
• Transform boundaries
26. Types of Plate BoundariesTypes of Plate Boundaries
• Convergent: areas of plates that are moving
toward each other
– there are three sub-types of convergent boundaries:
• oceanic to continental
• continental to continental
• oceanic to oceanic
27. Types of Plate BoundariesTypes of Plate Boundaries
• Divergent: areas of plates that are moving away
from each other
28. Types of Plate BoundariesTypes of Plate Boundaries
• Transform: areas of plates that are sliding past
each other
29. Types of Plate BoundariesTypes of Plate Boundaries
• Here’s an animation of each type of plate boundary
33. Continental Drift 1 ptContinental Drift 1 pt
• The scientist who first proposed the
theory of continental drift
34. Continental Drift 1 pt AnswerContinental Drift 1 pt Answer
• Who was Alfred Wegner?
35. Continental Drift 2 ptContinental Drift 2 pt
• fossils match across continents and
oceans, same rock patterns found on
five different continents, mountain
ranges match across the Atlantic
ocean
36. Continental Drift 2 pt AnswerContinental Drift 2 pt Answer
• What is some of the evidence
Wegner used to support his theory
that the continents were once joined?
37. Continental Drift 3 ptContinental Drift 3 pt
• Because he had no answer to the
question, “If the continents are
drifting, what mechanism is causing
them to move?”
38. Continental Drift 3 pt AnswerContinental Drift 3 pt Answer
• Why was Wegner’s theory of
continental drift widely ignored?
40. Sea Floor 1 pt AnswerSea Floor 1 pt Answer
• What did Harry Hess and the US Navy
discover when they mapped the sea floor
using sonar?
41. Sea Floor 2 ptSea Floor 2 pt
• The process of ocean crust “diving” under
other crust into the mantle
42. Sea Floor 2 pt AnswerSea Floor 2 pt Answer
• What is subduction?
43. Sea Floor 3 ptSea Floor 3 pt
• The process by which the newest ocean
floor is created at mid-ocean ridges,
pushing older crust outward
44. Sea Floor 3 pt AnswerSea Floor 3 pt Answer
• What is sea floor spreading?
45. Plate Tectonics 1 ptPlate Tectonics 1 pt
• What the earth’s outer shell is broken into
46. Plate Tectonics 1 pt AnswerPlate Tectonics 1 pt Answer
• What are plates?
47. Plate Tectonics 2 ptPlate Tectonics 2 pt
• Instrument deployed by US Military to spy
on soviet nuclear tests which also detected
medium-to-large earthquakes.
48. Plate Tectonics 2 pt AnswerPlate Tectonics 2 pt Answer
• What is a seismometer?
49. Plate Tectonics 3 ptPlate Tectonics 3 pt
• The three ways plate boundaries interact
50. Plate Tectonics 3 pt AnswerPlate Tectonics 3 pt Answer
• What are convergent, divergent, &
transform?
51. Review – Exit SlipReview – Exit Slip
• Choose what you believe to be the strongest piece of
evidence Wegner uncovered to support the idea of
continental drift and explain why you think it is the most
convincing.
• If new sea floor is constantly being created, why isn’t the
earth growing in size?
• How did earthquakes help scientists to understand the
structure of the earth’s crust?
