1. 17.1 Drifting Continents
Plate Tectonics

2. Early Observations
In the late 1500s, mapmakers noticed the apparent “fit” of the continents on either side of the Atlantic Ocean.
But what could be the cause?

3. Continental Drift- Pangea
Proposed in 1912 by German scientist Alfred Wegener
States that the continents were once joined as a single landmass and broke apart 200 mya

4. Continental Drift- Evidence
Similarities on either side of the Atlantic Ocean
Coastal Shapes
Rock Formations
Fossils
Ancient Climates
Glaciation

5. Continental Shapes
The coastal shapes of the continents fit together like pieces in a puzzle.

6. Rocks
Rock formations in Africa and South America are of the same age and structure.

7. Ancient Plants and Animals
Fossil evidence suggests landmasses were once joined.

8. Past Environments
Ancient Climates including swamps, deserts, ice caps, and bodies of water line up.
As evidenced by coal, sand, and evaporite (gypsum and salt) deposits

9. Glaciation
Rocks in South America, Africa, India, Australia and Antarctica possess similar deep scratches explainable by glacial movement.

10. Hypothesis Rejected?
Wegener could not prove his hypothesis of continental movement during his lifetime.
Many scientists maintained their view of a static, flat Earth surface.
However…
New evidence found on the seafloor was revealed in the 1960s.

11. 17.2 Seafloor Spreading
Plate Tectonics

12. Studying the Ocean Floor
Technology
Sonar- uses sound waves
Magnetometer- detects small changes in magnetic fields

13. Ocean Floor Topography
Not flat as scientists once thought!
Ocean Ridges- longest mountain range on Earth
Earthquakes and volcanism are common
Deep-Sea Trenches- deepest places on Earth
Marianas Trench in the Pacific Ocean is > 11 km deep!

14. Ocean Rocks and Sediment
Rocks near ridges are younger than those near trenches.
Age of rocks and depth of sediment increases with distance from ridges.

15. Magnetism
Paleomagnetism- study of the Earth’s magnetic record using iron-bearing minerals that record changes in the magnetic field through time.

16. The Geomagnetic Time Scale
Magnetic reversal- change in Earth’s magnetic field
North and South Pole swap
Magnetic symmetry- parallel lines on either side of ridges
Isochrons- connects points of the same age

17. Seafloor Spreading
New ocean crust is created at ocean ridges and destroyed at deep-sea trenches
Hotter and less dense, magma rises at the ridges and hardens to form new oceanic crust
Old crust plunges and is recycled in the deep-sea trenches

18. Hypothesis to Theory!
Seafloor spreading finally answered the “How” question looming over Wegener’s Continental Shift Hypothesis.

19. 17.3 Theory of Plate Tectonics

20. Theory of Plate Tectonics
States that the Earth’s crust and rigid upper mantle are broken into enormous slabs called plates.

21. Plate Boundaries- Types
Converge- come together
Diverge- move apart
Transform- slide past

22. Divergent Boundaries
NOT FINISHED