2. This is a story of 1 Hypothesis and the theory that replaced it
1. In 1915 a German scientist named Alfred Wegener proposed the hypothesis of continental drift
Even though he had collected much evidence, this idea wasn’t well accepted
2. By 1968 new discoveries lead to the development of the Theory of Plate Tectonics.
These new discoveries gave the scientific community the evidence they needed to understand how Wegener’s hypothesis could be possible.

3. 9.1 Continental Drift An Idea Before Its Time
Wegener’s continental drift hypothesis: continents once formed a single supercontinent.
• Proposed Pangaea began to break apart 200 mya.
-Pangaea means: “All land”

4. Breakup of Pangaea

5. 9.1 Continental Drift
 Evidence:
1) The Continental Puzzle

6. 9.1 Continental Drift 2) Matching Fossils - several organisms found on
different landmasses.

7. 3) Rock Types & Structures
- several mountain belts end at one coastline, then reappear on a landmass across the ocean.

8. Glacier Evidence
4) Ancient Climates

9. Glaciation

10. 9.1 Continental Drift Rejecting the Hypothesis  A New Theory Emerges
• Wegener - no explanation of what made continents move.
New technology lead to findings - new theory called plate tectonics.

11. 9.4 Testing Plate Tectonics
Evidence for Plate Tectonics
1) Paleomagnetism - natural remnant magnetism in rocks; Shows Earth’s magnetic field as it existed in the past.
Normal polarity - rocks show same magnetism as present magnetic field
Reverse polarity - rocks show opposite magnetism as the present magnetic field

12. Paleomagnetism on the Ocean Floor
Discovery of strips of alternating polarity
Lie as mirror images across the ocean ridges
Among the strongest evidence of seafloor spreading

13. Evidence For Plate Tectonics
2) Earthquake Patterns
Link between deep-focus earthquakes and ocean trenches.
Absence of deep-focus quakes along oceanic ridge was consistent with the new theory.
3) Ocean Drilling
Ages of seafloor sediment confirmed what seafloor spreading hypothesis predicted.
Youngest oceanic crust is at the ridge crest; oldest at continental margins.

14. 4) Hot Spots
Hot spot (mantle plumes) - concentration of heat in mantle capable of producing magma, which rises to Earth’s surface
Hot spot evidence supports that the plates move over the Earth’s surface. (Hot spots hold still, and plates move)
Ex: Pacific plate moves over a hot spot, producing the Hawaiian Islands.

15. Hot Spot

16. Hot Spot

17. Section 9.4 Review
What feature produces volcanoes that do not occur at plate boundaries?
Hot spots