1. Continental Drift & Seafloor Spreading
PLATE TECTONICS
Continental Drift
&
Seafloor Spreading

3. Continental Drift
German scientist Alfred Wegener proposes the theory of Continental Drift in 1912

4. What does Continental Drift mean?
It means exactly what is sounds like!
Continental Drift = continents (land masses) drifting (moving)
How many continents are there?
There are 7 continents.

5. Wegener hypothesized that all the present continents once formed a single landmass, which he named Pangaea.
Pangaea means “all lands”
Panthalassa was the huge ocean surrounding Pangaea.

6. The break up of Pangaea

7. Evidence of Continental Drift
1. Similarities in the coastlines

8. Evidence of Continental Drift
2. Fossils of the same plants and animals found in areas that had been adjoining part of Pangaea.

9. Mesosaurus Small extinct land reptile Lived 270 million years ago
Fossils found in eastern South America and western Africa.

10. Glossopteris Fossilized fern found on 4 continents
Continents with very different modern climates, led Wegener to believe these continents were once connected having a similar climate

11. Evidence of Continental Drift
3. Geologic Evidence
Age and types of rocks
Mountain age and structure
Climatic pattern
Debris form glaciers
Coal deposits

12. Did people accept Wegener’s theory?
NO!
The evidence Wegener had to support his theory was not enough to convince many people during his lifetime.
Not until the 1960’s were his beliefs accepted by the scientific community.
The truth was out there!

13. Seafloor Spreading
In the 1960’s, Harry Hess developed his now famous and accepted theory of Seafloor Spreading.
Hess proposed that hot, less dense material in the mantle is forced upward to the surface at a mid-ocean ridge.
Example: Mid-Atlantic Ridge

14. Harold Hess

16. Evidence of Seafloor Spreading
1. Age evidence
Studies by the Glomar Challenger in 1968 discovered that the youngest rocks were found at the mid-ocean ridges and rocks became increasingly older farther from the ridges on both sides.

18. Evidence of Seafloor Spreading
2. Paleomagnetism (Magnetic clues)
Important evidence that led to the absolute belief in seafloor spreading
Scientist found that rocks on the ocean floor showed many magnetic reversals.
Reverse back and forth in strips parallel to the mid-ocean ridge.
Evidence needed to solidify Wegener’s original hypothesis.

21. Theory of Plate Tectonics
By the 1960’s Continental Drift was widely accepted and this led to the rise of another theory known as Plate Tectonics.
The term tectonic comes from the Greek work tektonikos meaning “construction”
Plate tectonics not only describes continental movement but also proposes a possible explanation of how and why the continents move.

22. The theory of Plate Tectonics states Earth’s crust and upper mantle are broken into sections.
These sections, called plates, move around on the mantle.

24. About 30 lithospheric plates have been identified
Some of these plates are moving towards each other, some are moving away from each other, and some are sliding past one another.

25. How fast do plates move per year? Plate movements vary, one example:

26. Lithospheric Plate Boundaries
The boundaries or edges of the plates have been divided into three types of plate boundaries. They are:
1. Divergent Plate Boundaries
2. Convergent Plate Boundaries
3. Transform Fault Boundaries

27. 1. Divergent Plate Boundaries
2 plates moving away from each other
Examples:
Mid-Atlantic Ridge
The Great Rift Valley in eastern Africa

28. 2. Convergent Plate Boundaries
Plate colliding (pushing in to) with neighboring plates.
There are 3 types of Convergent Plate Boundaries, They are as follows:

29. 1. Ocean-Continent
The area where the oceanic plate descends into the upper mantle is called a subduction zone.

30. The oceanic crust subducts (moves under another) because it is less dense than the continental crust.
Oceanic crust is generally basaltic in nature as continental crust generally granitic.
Ocean trenches generally form along subduction zones.
Many volcanic mountains also form along subduction zones.
Examples: The Andes Mountains in South America where Nazca and South American Plates are colliding.
Where does this happen in the U.S.?

31. 2. Continent-Continent
When two colliding plates have the same density neither plate is subducted, the plates experience uplifting instead.

32. Example:
The Himalayan Mountains
Formed when the Indo-Australian Plate crashed into the Eurasian Plate.
This is still going on today as the Himalayas are growing.

34. 3. Ocean-Ocean
Part of the subducted plate melts, and the resulting molten rock rises to the surface along the trench to form a chain of volcanic islands, called an island arc.

35. Example:
The islands of Japan are volcanic island arcs formed when two oceanic plates collided.

36. Review The Three types of Convergent Boundaries
Ocean-Ocean (Subduction occurs)
Ocean-Continent (Subduction occurs)
Continent-Continent (Uplifting occurs)

37. 3. Transform Fault Boundaries
Forms when two plates are sliding past one another.
Scrape and move in series of sudden spurts of activity separated by periods of little or no movement.
Example: San Andreas Fault where the Pacific Plate is sliding past the North American Plate

40. Causes of Plate Tectonics
Convection Currents
The entire cycle of heating, rising, cooling, and sinking is called a convection current.
The same process occurring in the mantle is thought to be the force behind plate tectonics.

41. Convection occurs in a heated water filled beaker.

42. The same process is thought to happen in the mantle as it is also experiencing heat gain rising from the mantle.

43. Effects of Plate Tectonics
1. Tension forces
Rift Valleys form where plates are diverging.
Examples:
Great Rift Valley of Africa
Earthquakes
East Pacific Rise
Tension also forms normal faults.

44. Effects of Plate Tectonics
2. Compression Forces
Mountain Building
Island Arcs and Volcanoes form
Reverse Faults also occur.
Earthquakes

45. Effects of Plate Tectonics
Shearing forces create strike-slip faults
Example:
San Andreas Fault, California
Earthquakes