1. Earth Science- The Earth’s Moving Plates
The Theory of Continental Drift

3. The Earth’s Moving Plates
In 1912 a scientist named Alfred Wegner proposed the hypothesis of continental drift
Wegner proposed that the continents had drifted apart after forming Pangaea- a super continent

4. The Hypothesis of Continental Drift
He supported his theory with the available evidence:
Continental shape
Fossil evidence
Landform evidence
Ancient ice age evidence

5. Continental shape
Wegner noted how the East coast of South America fit Africa’s West coast
He found other coasts could be fit as well

6. Fossil evidence
Fossils of identical species of plant and animal are found on different continents
Since the organisms could not have crossed the ocean, Wegner proposed the continents were joined

7. Landform evidence Joining the continents lines up ancient landforms
Mountain ranges match between:
Argentina and South Africa
E. USA and Scottish highlands
Unusual rock formations are shared between Brazil and South Africa

8. Ancient Ice Age Evidence
Glaciers carve striations into the Earth
South Africa and South America have matching striations
Glacial remains on South America and Africa support a position of Pangaea around the South Pole

9. The Theory of Continental Drift
Scientists of the day rejected Wegner’s ideas
They could not believe continents could move

10. The Theory of Plate Tectonics
30 years after his death, Harry Hess proposed explanation of more recent evidence supporting a new theory that explains continental drift and ocean movement:
The theory of plate tectonics

11. Review of Continental Drift
Alfred Wegner proposed the continents had been joined in a super-continent he called Pangaea
He supported this with fossil, continent shape, land form and ice age evidence
His theory was rejected because scientists of his day did not believe continents could “drift”  

12. Earth Science 7- The Earth’s Moving Plates
The Theory of Plate Tectonics

13. Moving Continents
New evidence explaining continental movement came from:
Mapping of the ocean floor
Age of Rocks on the ocean floor
Magnetic reversals
Locations of earthquakes and volcanoes

14. Mapping of the ocean floor
Mapping of the ocean floor revealed deep trenches and huge mountains
Trenches run along the edge of the ocean
Mountains create a ridge in the middle of the ocean
An ancient ocean would be expected to be flat

15. Age of Ocean Floor Rocks
Sediment layers in mid ocean are thin
If ocean had remained unchanged for millions of years sediment would be thick
Young rocks are on the mid ocean ridges
Old rocks are closer to the trenches
Conclusion: new ocean crust forms at the ridge

16. Sea Floor Spreading
Scientist Harry Hess named this formation of new crust “sea floor spreading”
Magma rises at the mid ocean ridges
Magma cools to form new crust
Crust expands at mid ocean ridge and moves
Oceanic crust sinks back down to the mantle at the trenches

17. Magnetic Reversals Magnetite is a magnetic mineral
In molten rock it aligns with the Earth’s magnetic field
When rock solidifies the alignment is fixed

18. Magnetic Reversals
The Earth’s magnetic field reverses, and the pattern of magnetite alignment reverses
Strips with alternating patterns of magnetic alignment are found running parallel to the mid ocean ridge
This suggests the mid ocean ridge is the strips origin

19. Volcanoes and Earthquakes
Modern technology allows earthquakes to be recorded in the ocean
These observations led to the theory of plate tectonics
This theory explains the way oceans and continents move, Wegner’s didn’t explain oceans

20. The Theory of Plate Tectonics
The surface of the Earth is composed of ~ a dozen plates
The plates are continually moving
Oceanic plates have ocean floor on them
Continental plates have continents on them

21. The Theory of Plate Tectonics
Plates move at different speeds
It is still a mystery why plates move
It is believed that plates move on the currents of the mantle below
It is also suggested the plates moves because of pull at the subduction zone

22. Review of Plate Tectonics
Plate tectonics is based on evidence from the last 40 years
Mapping of the ocean floor
Sea floor sediment
Magnetic reversals
Undersea volcanoes and earthquakes
Plate tectonics provides evidence that both the continents and oceans are moving