1. Earth Science Standard 3
Earth Science Standard 3.a - Students know features of the ocean floor (magnetic patterns, age, and sea-floor topography) provide evidence of plate tectonics.

2. Have you ever wondered why Earth has earthquakes, volcanoes, and mountains?
Many people throughout time have wondered the same thing.
Eventually, people sought to seek out scientific explanations to these problems.

3. What do you notice about the continents
What do you notice about the continents? Do any of the continents seem to fit together?

4. When we fit the continents of Africa and
South America “back together again”, we find that:
Similar rock types extend from one continent to the other
The rocks are also the same age.
CONCLUSION - perhaps the two continents were once part of a single larger continent?

5. Alfred Wegener and Continental Drift
In 1912, German meteorologist Alfred Wegener proposed the continental drift hypothesis.
Continental Drift – continents once joined as a single landmass and drifted apart over time

6. What type of Evidence did Wegener search for?
Wegener proposed – 200 mya the continents broke apart and have been moving ever since
Pangaea – supercontinent that joined all the landmasses on Earth
What type of Evidence did Wegener search for?

8. Wegener’s Evidence Fit of the Continents
Location of Past Glaciations 300 mya
Distribution of Tropical Climates 300 mya
Distribution of Fossils 300 mya
Matching Rocks and Mountain Belts

9. Fit of the Continents
Scholars and cartographers as early as the 1500’s saw that Africa appeared to tuck in neatly against the coasts of North and South America.
In fact, all of the continents can be arranged to join with nearly no overlap or gaps.
Coincidence? Wegener thought not.

11. Location of Past Glaciations 300 mya
The passage of a glacier causes deep grooves to be carved into underlying rocks, called STRIATIONS. These point along the direction of travel of the glacial ice.
Glaciers also leave behind a distinctive sediment type, called TILL.

13. Using the distribution of till and markings that were created ~300 million years ago, Wegener saw that South America, southern Africa, India, Australia and Antarctica all experienced a glaciation. Today, these places are widely separated and tropical.
The striations pointed to a common center of the glacial ice cap.

15. Distribution of Tropical Climates 300 mya
Rock types such as coals, minerals (salts), desert sand dunes and reefs built by coral-like organisms indicate a tropical climate.

16. Wegener’s reconstruction of Pangea, together with his evidence of glaciation, predicted that tropical climates should have extended in a belt across southern North America and Europe and northern South America and Africa about 300 million years ago.
These rock types are found exactly within the belt predicted by Wegener’s theory.

17. Distribution of Fossils 300mya
Today, certain plants and animals live only on certain continents. Many fossil animals and plants exhibit a similarly restricted geographic range.
If continents were all joined ~300 Ma (million years ago), then this would have provided an opportunity for plants and animals to disperse among joined continents.

18. Once continents drifted apart, similar species would then be separated by large expanses of uncrossable ocean.
This is seen in several animal (Cynognathus, Lystrosaurus) and plant (Glossopteris) species

21. Matching Rocks and Mountain Belts
Geoscientists can recognize particular and distinctive rocks that were created all at the same place and at the same time.
Wegener recognized several such assemblages of rock, now separated by ocean basins.
A) – in both South America and Africa.
B) – in a belt along the east coast of North America and Greenland, and along the west coast of Africa, Great Britain and Scandinavia.

23. Andean Mountain Range

24. Would you have supported Wegener’s idea of continental drift?
Why do you think scientists rejected his hypothesis?

25. Wegener’s Evidence Fit of the continents Location of past glaciations
Distribution of tropical climates
Distribution of fossils
Matching of rock units across ocean basins
Wegener hypothesized that
Continents may move because of Earth’s rotation.
Continents ‘plow’ through oceanic crust like a ship through water.

26. A Rejected Hypothesis…
But Was it Enough? . . No!
Studies showed that the forces from rotation of the Earth were ~1 million times to small to move large continents.
Continental crust (?) is weaker than oceanic crust (?)
Wegener could not explain what was causing the continents to move.