1. Continental Drift
Pangaea
** German meteorologist Alfred Wegener proposed the theory of continental drift.
** According to the theory of continental drift, continents have moved slowly to their current locations.

2. Continental Drift
Pangaea
** Wegener suggested that all continents were once connected as one landmass that broke apart about 200 million years ago.
** He called this large landmass Pangaea which means “all land.”

3. Fossil Clues ** Fossils provided support for continental drift.
**Fossils of the reptile Mesosaurus have been found in South America and Africa.

4. How could continents drift?
Continental Drift
How could continents drift?
Because other scientists could not provide explanations either, Wegener’s idea of continental drift was initially rejected.
** The main types of evidence for continental drift are:
1. Continent shape
2. Rock structure
3. Fossil clues
4. Climate clues

5. Theory of Plate Tectonics
** The hypothesis of continental drift was developed before the present theory of plate tectonics.
**The Theory of Plate Tectonics states that the Earth is divided into plate sections that include the crust and ocean floor) that float move) on the asthenosphere.

6. Theory of Plate Tectonics3
Plate Movements
**According to the theory of plate tectonics, Earth’s crust and part of the upper mantle are broken into sections.
**These sections, called lithospheric plates, move on a plasticlike layer of the mantle.

7. Theory of Plate Tectonics3
**Plate Boundaries**
**Movement along any plate boundary means that changes must happen at other boundaries.
What is happening to the Atlantic Ocean floor between the North American and African Plates?

8. Theory of Plate Tectonics3
Plates Moving Apart
**1. Divergent boundary- The boundary between two plates that are moving apart.
In the Atlantic Ocean, the North American Plate is moving away from the Eurasian and the African Plates.

9. Theory of Plate Tectonics3
Plates Moving Apart
That divergent boundary is called the Mid-Atlantic Ridge.
**1-A
Most divergent
Boundaries are
Located along
The mid-ocean
Ridge (sea-floor
Spreading).

10. ** 2 actions can occur on the Earth’s crust at a convergent boundary:
Theory of Plate Tectonics 3
Plates Moving Together- Convergent Boundary
2. **Convergent boundary- occurs where two plates move together and collide
** 2 actions can occur on the Earth’s crust at a convergent boundary:
Mountain building
subduction

11. Mountains and Volcanoes
Theory of Plate Tectonics 3
Mountains and Volcanoes
**As continental plates collide, the forces that are generated cause massive folding and faulting of rock layers into mountain ranges such as the Himalaya.

12. Plates Moving Together
Theory of Plate Tectonics 3
Plates Moving Together
2-A **When an oceanic plate comes together with a less dense continental plate, the more dense oceanic plate sinks under the continental plate.
2-B **The area where an oceanic plate subducts, or goes down, into the mantle is called a subduction zone.

13. Plates Moving Together
Theory of Plate Tectonics 3
Plates Moving Together
2-B **Some volcanoes form above subduction zones.
This type of convergent boundary creates a deep-sea trench where one plate bends and sinks beneath the other.

14. Where Plates Slide Past Each Other
Theory of Plate Tectonics 3
Where Plates Slide Past Each Other
3. **Transform boundaries- occur where two plates slide past one another.
3-A **They move in opposite directions or in the same direction at different rates.

15. Where Plates Slide Past Each Other
Theory of Plate Tectonics 3
Where Plates Slide Past Each Other
3-B **When one plate slips past another suddenly, earthquakes occur.
The San Andreas Fault is part of a transform plate boundary. It has been the site of many earthquakes.

16. Moving Mantle Material
Theory of Plate Tectonics 3
Moving Mantle Material
In one hypothesis, convection currents occur throughout the mantle.
** Convection currents can cause the astenosphere to flow slowly carrying with it the plates of the lithosphere (plate tectonics).