1. Earth’s Changing Surface

2. Continental Drift
In 1915, Alfred Wegener (VEG nur) proposed a hypothesis that suggested that Earth’s continents once were part of a large super-continent, Pangaea.
About 200 million years ago, the super-continent broke into pieces that drifted over the surface of Earth like rafts on water.

3. Matching Coastlines
The most apparent match of continents is the eastern coastline of South America with the western coastline of Africa.
Wegener argued that you could match rock types, fossils, erosion features, and mountain ranges.
If you found similar formations and structures on each continent then the continents could have been joined together in that place.

4. Matching Fossils
Large land animals provided better evidence because they could not have crossed oceans.

5. Matching Rocks and Mountains
Mountain ranges were shown to be continuous in Pangaea.
Once Pangaea broke apart, the mountain ranges became separated.
Wegener was able to show that continents that were joined shared unique rocks and minerals.

6. Matching Rocks and Mountains
Wegener’s hypothesis was not accepted by his contemporaries because he was unable to conceive of a force or mechanism that could drive continents apart.

7. Pangea

8. Seafloor Spreading Hypothesis
Dr. Harry Hess (1962) used sonar, intended to detect submarines, to obtain accurate maps of the seafloor.
A mid-ocean ridge system, or MOR, was continuous and wrapped around Earth.

9. Seafloor Spreading Hypothesis
Hess proposed a hypothesis of seafloor spreading, or divergence.
Magma from the mantle is forced upward because of its low density.
This causes the crust to crack (fault) and move apart.
The faulting causes twin mountain ranges with a down-dropped rift valley between.

10. Ages of Sediment and Rocks
When the ages of rocks are measured, the continental rocks are billions of years old, while seafloor rocks are less than 200 million years of age.

11. Theory of Plate Tectonics
This system consists of about a dozen major plates and many minor ones.
Plates are composed of a rigid layer of uppermost mantle and a layer of either oceanic or continental crust above.

12. Plate tectonics

13. Divergent Plate Boundaries
At a mid-ocean ridge (MOR), magma rises along a faulted rift valley, spreads, and cools to form new oceanic crust. This spreading apart is what happens at divergent boundaries.

14. Divergent Plate Boundaries
A MOR represents divergence that is well-developed.
Divergent boundaries exist as rift valleys, where no mature ocean basins exist yet.

15. African Rift Valley

16. Convergent Plate Boundaries
Where plates collide, they come together to form convergent boundaries.
Less-dense, thick continental lithosphere moves toward denser thin oceanic lithosphere.

17. Convergent Plate Boundaries
The ocean side is forced downward beneath the continental slab in a process called subduction.
The region of collision also has a deep-sea trench that parallels the zone.

18. Western Coast of South America

19. Convergent Plate Boundaries
Convergent plate boundaries also exist between two slabs of oceanic lithosphere. In this case, the
oceanic lithosphere that is colder, and therefore denser, subducts.
Magma erupted here produces chains of volcanic islands called island arcs.

20. Japan

21. Convergent Plate Boundaries
Two continental slabs of low density collide and tend not to subduct.
The plates collide and buckle upward to form a high range of folded mountains.
Volcanic activity is noticeably absent and there is no trench.

22. Himalayan Mountains

23. Transform Plate Boundaries
No new lithosphere is forming, as along a divergent boundary.
Old lithosphere is not being recycled, as along a subduction zone.
The main result of transform boundaries is horizontal motion of lithosphere.

24. San Andreas Fault

25. Thermal Energy
Internal convection of mantle material is the driving force for all mechanisms of plate motion.
The main source of thermal energy comes from the decay of radioactive elements in Earth.