1. Chapter 17
Plate Tectonics

2. I. Drifting Continents
A. Early Observations
1. Abraham Ortelius proposed, in the late 1500’s, that the Americas were separated from Europe and Africa by Earthquakes and floods

3. 2. Eduard Suess hypothesized that Southern Continents had once been joined as a landmass called “Gondwanaland”

4. 3. Alfred Wegener presented his ideas seriously to the Scientific Community.

5. B. Continental Drift
1. Alfred Wegener presented continental drift in 1912
2. Earth’s continents were once joined as the supercontinent Pangaea

6. 3. The Continents started to break up 200 million yrs. Ago
4. Landmasses would have fractured, similar rock types were found on different continents
5. Fossils of same animals were found on different continents

8. 6. Coal deposits were found in Antarctica, suggesting it once had a wet climate and was once located near the equator

9. 7. Glacial deposits were found in Africa, India, Australia, and South America, indicating that they were once covered by thick ice caps and were located near the South Pole

10. 8. Wegener’s Theory was not widely accepted in the early 1900’s because he couldn’t explain the force that was moving the continents.

11. 9. Another problem is that granite could not move through more dense basalt without breaking up!
Continental rock is granite with a density of ______
Ocean floor is
basalt with a
density of ______

12. II. Seafloor Spreading A. Help from Technology
1. Sonar - Used to map seafloor

14. 2. Magnetometer - Used to record
magnetic strength of a rock on the
seafloor

17. B. Ocean Floor Topography
1. Underwater mountain chains called ocean ridges were discovered, The East Pacific Rise forms largest mountain chain on Earth

18. 2. Counterparts to Ocean Ridges are Deep-Sea Trenches - Narrow, elongated depressions in seafloor with very steep sides, the Mariana trench is the deepest deep-sea trench

20. FYI: Movie Director James Cameron goes into a small sub to explore the depths of the Mariana trench. The pressure exerted on the sub is over 16,000 psi!

21. C. Ocean Rocks and Sediments
1. Rocks near ocean ridges were younger than rocks near trenches
2. Age of the seafloor crust increases with distance from ridges
3. The oldest part of the seafloor is only 180 million years old, much less than expected

23. 4. Seafloor sediments are typically only a few hundred meters thick, continental sediments are typically 3-20 kilometers thick
5. Thickness also increases with distance from ridges
D. Magnetism
1. Paleomagnetism - The study of Earth’s magnetic field
2. The mineral basalt provides an accurate record of ancient Earth’s magnetism

24. 3. Cooled basaltic lava (now rocks) are effective compasses that record the magnetic field at the time of hardening.
4. Continental basalt flows show patterns of magnetic reversals over geologic time.
5. Magnetic Reversal - Change in Earth’s magnetic field
6. Magnetic field with same orientation as Earth has normal polarity
7. Magnetic field with opposite orientation as Earth has reversed polarity

25. 8. In some places, seafloor magnetism was greater than normal, and in other areas of seafloor, magnetism was lower
9. Positive and negative areas of seafloor formed stripes parallel to ridges
10. Patterns on opposite sides of ridges are mirrored
11. Magnetic data collected from seafloor matched basaltic patterns on land.
12. Scientists were now able to create isochron maps of seafloor

27. 13. Isochron - A line on a map that connects points of equal age
14. Older ocean crust was found along trenches, younger crust along ridges

28. E. Seafloor Spreading
1. Harry Hess proposed his theory, called Seafloor Spreading
2. Seafloor Spreading - A theory which states that seafloor crust is formed at ridges and destroyed at trenches
3. Magma is forced toward the crust along a ridge, hardens and creates new crust.

29. Iso-chron map

30. 4. Wegener’s theory was proven correct, the continents did move!
5.Seafloor spreading was the mechanism that moved the continents.
5. The continents do not move by themselves, they just “ride on top” of ocean crust, moving away from ridges.

31. III. Theory of Plate Tectonics
87 Boundaries
1. Tectonic plates touch at plate boundaries
2 .Plates can come together, move away or move horizontally past each other at plate boundaries

33. B. Divergent boundaries
1. Boundaries where plates move apart
2. Widens ocean and form ridges on ocean floor
3. When plates separate they form rift valleys

34. C. Convergent Boundaries 1. Where Plates move together
2. There are three types of convergent boundaries
a. Ocean crust combining with ocean crust
b. Ocean crust combining with continental crust
c. Continental crust combining and colliding with continental crust
3. Subduction when one plate goes under another plate

35. 4. Ocean -Ocean: a. Subduction zone and trench is created
b. As plate melts magma rises -> Island arc forms
c. Island arcs made from basaltic and andesitic magma
d. Island arcs may then hit a coast and form a range

36. 5. Ocean -Continental: a. Subduction zone and trench is created
b. Ocean crust uplifts continental crust
c. Compression causes crust to fold and thicken
d. Magma intrudes -> granitic intrusions volcanoes
e. Sediments fill low areas-> folded, faulted + meta’ed

37. a. Tallest mountain ranges
6. Continental -Continental:
a. Tallest mountain ranges
b. Less dense crust therefore no subduction zone
c. Compression causes crust to fold and thicken
d. Magma that forms as a result creates granite batholiths
e. Presents of marine sediments at peak!

38. D. Transform Boundaries
1.At transform boundaries crust is deformed or destroyed
2. Usually characterized as long faults and small earthquakes
3. Most transform boundaries offset ocean ridges
4. Best known
exception is the San
Andreas Fault

39. E. Mantle Convection
1.thought to move the plates
2. Convection currents put in motion by transfer of energy from the hot interior to cold exterior
3. Hot mantle is less dense than cold mantle so the hot mantle rises
4.The upward force lifts and splits at divergent boundaries
5. The downward force pulls tectonic plates together at convergent boundaries

40. F. Push and Pull
1. Ridge push the weight of an uplifted ridge that pushes on an oceanic plate
2. Slab pull the weight of a subducting plate helps pull the lithosphere

41. G. Unanswered questions
1. Is convection only taking place in the upper mantle?
2. Images suggest that convection currents associated with plate movements might include the lower mantle.

43. 3. What causes an upward convection current to form under a continent?
4. Large continental masses caused their own breakup by acting as insulating blankets.
5. The mantle got warmer and forced the mantle convection upward.

44. Which is not evidence that Wegener used for his theory of continental drift?
A. Coal was found in Antarctica
B. Continents seemed to fit together like a puzzle
C. Matching fossils and rocks across the ocean
D. Sonar showing the seafloor was not flat.
2. Which is not true about Seafloor Spreading?
A. Sonar and the magnetometer are two technologies that helped prove it.
B. The age of the seafloor decreases as distance from the ridge increases.
C. Basalt keeps a record of Earth’s magnetic field and it’s reversals.
D. Sediment thickness increases as distance from the ridge increases.

45. Which is not a tectonic plate boundary?
A. Thrust Fault
B. Divergent
C. Convergent
D. Transform
What is thought to drive the plate motion?
A. Conduction of heat energy in the Earth’s outer core.
B. Convection currents in the mantle
C. Radiation from the Sun
Transference energy from the Earth’s core
Bonus: Describe how Ridge Push and Slab Pull work.