1. Part 1 Whose Idea Was This In The First Place?
Plate Tectonics
Part 1
Whose Idea Was This In The First Place?

2. The idea of continents in motion is hundreds of years old
In 1596, Dutch mapmaker Abraham Ortelius stated that “the Americas were torn away from Africa And Europe by earthquakes and floods.”

3. Alfred Wegener-
The idea was revived in the early 1900’s by German meteorologist, Alfred Wegener
He was inspired by the near perfect fit of Africa and South America.
He would spend years researching all areas of earth science to help support his theory.

4. Continental Drift
Wegener searched for evidence that supported his hypothesis that continents were once joined.
Evidence: Coastlines match with near perfection
Evidence: matching plant and animal fossils found on both continents
Impossible for the plants or animals to swim or be transported across ocean, the land must have been joined.

5. Matching Coastlines

6. Matching Mountain Ranges

7. Glacier Evidence

8. Fossil Evidence-Mesosaurus

9. More Evidence
Drifting continents explained dramatic climate changes on some continents.
Tropical plant fossils (coal) in frozen Antarctica
Tropical fern fossils (Glossopteris) in now colder climates.
Glacial deposits in South Africa.
Polar dinosaurs found in Australia

10. What has happened since Pangaea broke up?

12. Theory Rejected Geologists at the time rejected Wegener’s theory.
Wegener suggested the continents plowed through the ocean floor, or gravitational pull from the moon could tug the continents slowly into new positions
Other scientists argued correctly that this was physically impossible
Despite so much evidence, without a mechanism for moving the continents, the theory was not accepted

13. Developing the theory
Until new evidence was revealed, Wegener’s continental drift theory lay dormant.
Discoveries of ocean floor topography-trenches and mid-ocean ridges and rift valleys
Discovery of Earth’s changing magnetic field
Development of Seafloor Spreading model
Concentration of earthquakes and volcanoes at plate boundaries

14. Ocean Floor Mapping Timeline
Before WW1-believed ocean floor was flat and featureless
WW1-primitive sonar revealed a mid Atlantic ridge
1947-seismic survey showed thin sediments on ocean floor-expected to be thicker because ocean floor was believed to be very old
1950’s-mid ocean ridges discovered in all oceans

15. Paleomagnetism
1950’s-scientists discovered magnetism in ocean floor basaltic rocks
The magnetic pattern created a striped appearance when mapped
The pattern was a mirror image on either side of a mid-ocean ridge

16. Changing magnetic field recorded in iron bearing rocks

17. Seafloor spreading Idea came from the magnetic striping
Ages of rocks near mid-ocean ridges was youngest
Age increased away from ridge
Age of rocks with same magnetism is same
Therefore must have been formed at mid ocean ridge and been forced away by formation of new crust

18. Seafloor spreading cont’d
If new crust is formed at mid-ocean ridges, why isn’t earth getting larger?
Answer: it is being subducted and recycled into the mantle at ocean trenches at convergent boundaries
Idea was proposed by Harry Hess, a Princeton geologist and Navy Admiral

19. Ridge Push and slab pull
Gravity causes raised mid ocean ridges to push apart, causing seafloor spreading
Gravity pulls at subducting ocean slabs at convergent boundaries, dragging oceanic crust into mantle
The oldest ocean crust is 180 mya
The oldest continental crust is almost 4 bya (that’s 4000 mya)

20. Ridge push and slab pull driven by mantle convection

21. Earthquakes
Earthquakes are concentrated at plate boundaries