Plate tectonics and physical hazards

CHAPTER 2

PLATE TECTONICS AND PHYSICAL HAZARDS

http://pubs.usgs.gov/publications/text/dynamic.html

http://www.ucmp.berkeley.edu/geology/tectonics.html

Video: Continental drift and plate tectonics: Tanya Atwater, Univ. of California, Santa Barbara.

Video: Plate tectonics and Continental drift: Gould Media, Inc., Mount Vernon, NY

Video: Earth Revealed #1: The living machine: The Theory of Plate Tectonics. Annenberg/CPC Collection; P.O. Box 1922, Santa Barbara, CA

Video: Earth Revealed #5: The birth of a theory – Continental drift and sea-floor spreading: Annenberg/CPB Collection.

Video: Earth Revealed #6: Plate dynamics. Annenberg/CPG Collection; P.O. Box 1922, Santa Barbara, CA

Video: Plate Tectonics and Continental Drift: Gould Media, Inc., Mount Vernon, NY
Reference:

Kious, W. J. and R.I. Tilling, 1995, This Dynamic Earth: the Story of Plate Tectonics, U.S. Geological Survey, 77 p.

CHAPTER 2: End of chapter answers

1. What direction is the Pacific Plate currently moving?

* west-northwest

* west-northwest

2. Before people understood plate tectonics, what evidence led some scientists to believe in continental drift?

* match of coastlines across the Atlantic Ocean

* match of rock formations across the Atlantic Ocean

* match of ages of continental rocks across the Atlantic Ocean

3. If the coastlines across the Atlantic Ocean are spreading apart, why isn’t the Atlantic Ocean deepest in its center?

• New ocean floor wells up and forms under the mid-Atlantic Ridge

4. What evidence confirmed seafloor spreading? Be brief but to the point

• Reversals of the Earth’s magnetic field are preserved in ocean-floor basalt, as matching patterns across the mid-oceanic ridge. The youngest ocean-floor basalt is at the ridge.

5. Distinguish between Earth’s crust, lithosphere, asthenosphere, and mantle.

• Earth’s crust overlies mantle – distinguished by composition: oceanic crust is basaltic composition and continental crust is “granitic” composition.

• Earth’s lithosphere overlies asthenosphere – distinguished by rock properties/behavior/strength: lithosphere is more-rigid; asthenosphere is more-plastic/easily deformed.

6. 
   What are the main types of lithospheric plate boundaries in terms of relative motions? Provide a real example of each (by name or location).
   

7. What does oceanic lithosphere consist of and how thick is it?

* Earth’s crust and uppermost mantle (basalt over peridotite)

* about 100 km

8. 
   Along which type of lithospheric plate boundary are earthquakes common?
   

* convergent, divergent, and transform

9. Along which type(s) of lithospheric plate boundary are andesite stratovolcanoes common? Provide an example.

* continent – ocean plate collision (Cascadia subduction zone/ Cascade volcanoes)

10. Why does oceanic lithosphere almost always sink beneath continental lithosphere at convergent plate boundaries?

* oceanic lithosphere is denser/heavier

1. 
   When did the direction of the Pacific plate change, and what direction was it moving before this change?
   

* changed at 43 million years.

* Before this change, it was moving north-northwest.

2. If the ocean floor is getting wider, why is the Earth not becoming larger?

* old ocean floor sinks at subduction zones (trenches)

3. Distinguish between Earth’s crust and mantle.

4. 
   Roughly how many tectonic plates move around on the surface of the Earth?
   

5. 
   What characteristics of tectonic plates distinguish them from deeper Earth materials?
   

6. 
   Do tectonic plates consist of crust, mantle, or some combination of crust and mantle, and if so, what part of parts of each?
   

7. 
   What keeps the Appalachians standing as a mountain range even though they have been continuously eroding since they formed hundreds of millions of years ago?
   

8. 
   What is the basis for the “scientific method?”
   

9. 
   Why do many oceanic volcanoes occur as long lines of volcanoes that are active at only one end? How does the process work?
   

10. 
    Distinguish between Earth’s lithosphere and asthenosphere in terms of both location and properties.
    

• The lithosphere overlies the asthenosphere. It is rigid and the asthenosphere deforms plastically.

11. 
    What is the difference between the Earth’s lithosphere and asthenosphere?
    

* lithosphere is more-rigid; asthenosphere is more-plastic/easily deformed.

* It is on top of the asthenosphere.

12. What does the oceanic crust consist of and how thick is it?

* basalt, about 7 km thick

12. What does the continental crust consist of and how thick is it?

• “granite” composition (whole range of continental rocks, especially granite, gneiss, schist) 30 to 60 or 70 km thick

12. What are the main types of lithospheric plate boundaries, in terms of relative motions? Provide a real example of each (by name or location)

* convergent (continent – ocean plate collision): Cascadia subduction zone (Pacific Ocean floor against North America)

* convergent (continent- continent collision): Himalayas

* divergent (rift or spreading, or extension): mid-oceanic ridge

* transform (lateral motion): San Andreas Fault

12. Along which type(s) of lithospheric plate boundary are basalt-flow eruptions abundant? Provide a real example (name or location).

* spreading (Iceland/oceanic ridge)

12. In what tectonic environment that is not at a plate boundary, are major volcanoes found? Identify one of these in an ocean basin and one on a continent.

* hotspots or mantle plumes

* in ocean basin: Hawaii;

* on continent: Yellowstone

12. List the three main tectonic environments for large earthquakes in western North America and name a specific example of each.

Tectonic environment Name or specific location

* subduction zone Pacific Coast trench (Cascadia trench)

* spreading zone Basin and Range

* transform fault San Andreas Fault

12. 
    In what tectonic environment that is not at a plate boundary are major volcanoes found? Identify one of these in an ocean basin and one on a continent.
    

12. 
    The lithosphere is composed of what two layers?
    

12. 
    With which type or types of boundary are active volcanoes generally associated?
    

12. 
    What was the primary reason why Alfred Wegener’s theory of continental drift rejected?
    

12. 
    The “Ring of Fire” refers to what and where is it?
    

12. 
    Convergent plate boundaries are especially noted for which two types of natural hazards?
    

12. In the line of arc volcanoes, like the Cascades of Washington and Oregon, over an active subduction zone, a series of events leads to the magmas that erupt at the surface. What magma forms first and where and how does it form?

* Basalt magma forms at (just above) the subduction zone below the volcano, when water formed by dehydration of hydrous minerals in the descending slab causes melting of the mantle peridotite above.

12. How is the magma formed that feeds the volcanoes in Hawaii?

* The lithosphere carrying Hawaii slowly moves over a mantle hotspot causing melting to form basalt magma..

Multiple Choice Questions

1. 
   What direction is the Pacific Plate currently moving, based on the chain of Hawaiian islands with only the easternmost island active?
   

1. 
   to the northeast
   

2. * to the northwest

3. to the southeast

4. to the southwest

5. 
   It is not moving; the chain of islands is not related to the active one.
   

2. 
   Before people understood plate tectonics, what evidence led some scientists to believe in continental drift?
   

1. 
   Rocks on the ocean floor are oldest in the center, becoming progressively younger towards each continent.
   
2. 
   Rocks on the continents can be traced through the ocean floor to the other side.
   
3. 
   Anthropologists have found human carvings in Africa that match those in Brazil.
   
4. 
   Glaciers near the mouth of the Amazon River in Brazil carried distinctive rocks into South Africa, demonstrating that those countries were once connected.
   
5. 
   * Ages of bedrock formations match across the Atlantic Ocean
   

3. 
   Which of the following was not used as early evidence for continental drift?
   

1. 
   Ages of bedrock formations match across the Atlantic Ocean
   
2. 
   match of coastlines across the Atlantic Ocean
   
3. 
   match of rock formations across the Atlantic Ocean
   
4. 
   match of ages of continental rocks across the Atlantic Ocean
   
5. 
   * the fact that the magnetic pole shifts from North to South
   

4. 
   Which of the following is true?
   

1. 
   The lithosphere and asthenosphere consist entirely of mantle material.
   
2. 
   The lithosphere is the mantle under the oceans; the asthenosphere is the mantle under the
   

3. 
   The asthenosphere is the upper part of the mantle.
   
4. 
   * The asthenosphere is more plastic in behavior.
   
5. 
   The lithosphere is more plastic in behavior.
   

5. 
   Which of the following is true?
   

1. 
   The mantle is denser than the lithosphere.
   
2. 
   The lithosphere is denser than the mantle.
   
3. 
   The asthenosphere is the more-rigid equivalent of the mantle.
   
4. 
   * The asthenosphere is the more-plastic part of the mantle
   
5. 
   The mantle is everywhere more rigid than the crust.
   

6. 
   Which of the following is true?
   

1. 
   oceanic rift zones are found only in the center of the oceans
   
2. 
   rift zones are found only in the ocean basins
   
3. 
   * rift zones are areas where oceanic crust is formed
   
4. 
   rift zones are the oldest parts of the oceanic crust
   
5. 
   rift zones mark the boundaries between oceanic and continental crust.
   

7. 
   Which of the following is not true?
   

1. 
   subduction zones are areas where ocean floor descends into the mantle
   
2. 
   subduction zone activity includes very large earthquakes
   
3. 
   subduction zone activity leads to active volcanoes
   
4. 
   * subduction zones are areas where ocean floor rocks are formed
   
5. 
   subduction zones are marked the deepest parts of the oceans
   

8. 
   Which of the following is true?
   

1. 
   transform faults show dominantly vertical movement
   
2. 
   transform faults are only in the ocean basins.
   
3. 
   transform faults are areas of spreading and new crustal generation
   
4. 
   transform faults change with time from horizontal to vertical motion
   
5. 
   * transform fault motion typically ends abruptly at both ends.
   

9. 
   What does oceanic lithosphere consist of?
   

1. * basalt on top of peridotite

2. 
   only basalt
   
3. 
   only peridotite
   
4. 
   basalt and peridotite in layers of variable thickness, in some places above, in others below.
   
5. 
   partly basalt and partly granite
   

10. 
    Along which type of lithospheric plate boundary are earthquakes common?
    

1. 
   only convergent (subduction zones)
   
2. 
   only divergent (spreading zones)
   
3. 
   only transform
   
4. 
   only divergent and transform
   

5. * convergent, divergent, and transform

11. 
    Near which type(s) of lithospheric plate boundary are andesite stratovolcanoes most common?
    

1. 
   rift zones on continents
   
2. 
   collision zones between continental plates
   
3. 
   * subduction zones between oceanic and continental plates
   
4. 
   subduction zones between two continents
   
5. 
   transform fault boundaries between oceanic and continental plates
   

12. 
    Why does oceanic lithosphere almost always sink beneath continental lithosphere at convergent plate boundaries?
    

1. 
   oceanic lithosphere moves so slowly that it can only sink.
   
2. 
   oceanic lithosphere is at the bottom of the ocean so it can’t float high enough to ride over a continent.
   
3. 
   oceanic lithosphere almost twice as dense as the underlying mantle.
   

4. * oceanic lithosphere is denser than continental lithosphere

5. 
   oceanic lithosphere is partly liquid so it floats the solid continental lithosphere.
   

13. 
    If the Atlantic Ocean floor is getting wider, why is the Earth not becoming larger?
    

1. actually the Atlantic Ocean floor is not getting wider

2. * old ocean floor sinks at subduction zones (trenches)

3. 
   It is becoming denser so it takes up no more space.
   
4. 
   It becomes part of the edge of the adjacent continent.
   
5. 
   It melts at oceanic transform faults
   

14. 
    Which of the following is true?
    

1. 
   Earth’s crust is more dense than the mantle.
   
2. 
   Earth’s crust is thicker than the mantle.
   
3. 
   Earth’s crust is part of the asthenosphere and equivalent to it in composition.
   
4. 
   Earth’s crust is part of the mantle and forms the upper part of it.
   
5. 
   * Earth’s crust is less dense than the mantle
   

15. 
    Roughly how many tectonic plates move around on the surface of the Earth?
    

1. 
   none; they don’t mover around on the surface, they mostly move up and down.
   
2. 
   4 or 5, depending on size
   
3. 
   * 10 or 12
   
4. 
   about 50
   
5. 
   more than 300
   

16. 
    What characteristics of lithospheric plates distinguish them from deeper Earth materials?
    

1. 
   They are partly molten
   
2. 
   They behave in a more plastic manner
   
3. 
   * They are rigid.
   
4. 
   They consist entirely of basalt
   
5. 
   They are generally overlain by continental crust.
   

17. 
    What keeps the Appalachians standing as a mountain range even though they have been continuously eroding since they formed hundreds of millions of years ago?
    

1. 
   They are actually becoming lower by the amount that material is eroded off the top.
   
2. 
   They are again being pushed up by collision between the Atlantic Ocean floor and the
   

3. 
   The subduction zone at the Atlantic continental margin is now pushing ocean floor
   

4. 
   * As material is eroded off the range, it floats higher on the mantle rocks below.
   
5. 
   The continents below a depth of 5 to 10 kilometers are molten basalt and they float on
   

18. 
    What is the basis for the “scientific method?”
    

1. 
   Scientists come up with a theory and then seek out data to prove that it is correct.
   
2. 
   Scientists experiment with data to observe the data that they like best.
   
3. 
   Scientists try to discredit ideas proposed by others.
   
4. 
   Scientists prove their hypotheses with data that demonstrates their correctness.
   
5. 
   * Scientists test their hypotheses with new observations and experiments
   

19. 
    Why do many oceanic volcanoes occur as long lines of volcanoes that are active at only one end?
    

1. 
   * The lithosphere moves over a stable hotspot in the mantle.
   
2. 
   The mantle convection cell under the crust carries the magma source from one end of the
   

3. 
   The mantle plume that feeds the volcano rotates around the Earth’s core, tracing new
   

4. 
   The spreading lithosphere pushes the underlying magma source across the ocean floor.
   
5. 
   The active volcanoes are gradually moving away from the oceanic ridge.
   

20. 
    What is the difference between the Earth’s lithosphere and asthenosphere?
    

1. Lithosphere is below the asthenosphere.

2. * Lithosphere is more-rigid; asthenosphere is more-plastic/easily deformed.

3. Lithosphere behaves in a more plastic manner so it moves over the Earth’s interior.

4. Asthenosphere is almost 100 percent melt so it moves easily towards the oceanic ridges.

5. 
   Asthenosphere consists of basalt with lesser amounts of peridotite.
   

21. 
    What does the oceanic crust consist of and how thick is it?
    

1. peridotite about 7 km thick

2. peridotite between 60 and 100 km thick

3. * basalt, about 7 km thick

4. 
   basalt over peridotite, a total of about 60 km thick
   
5. 
   basalt over peridotite about 15 km thick
   

22. 
    What does the continental crust consist of and how thick is it?
    

1. soil of variable thickness / generally between 1 and 100 meters

2. 
   “granite” composition / about 10 km thick over peridotite 5 to 10 km thick
   
3. 
   shale, sandstone, and limestone / about 10 km thick
   

4. *“granite” composition / 30 to 60 km thick

5. 
   “granite” composition / 100 to 300 km thick
   

23. 
    The San Andreas Fault is:
    

1. 
   a normal fault
   
2. 
   the subduction zone bordering the Pacific Plate
   
3. 
   a rift zone running the length of California
   
4. 
   a reverse fault
   
5. 
   * a transform fault
   

24. 
    Along which type(s) of lithospheric plate boundary are basalt-flow eruptions abundant?
    

25. 
    What type of volcano is not associated with a plate boundary?
    

1. 
   * a hotspot volcano
   
2. 
   a stratovolcano
   
3. 
   a rift zone flood basalt
   
4. 
   a rhyolite volcano over a subduction zone
   
5. 
   a basalt volcano over a subduction zone
   

26. 
    In western North America, very large earthquakes are not associated with which of the following:
    

1. 
   The Cascadia subduction zone
   
2. 
   * The Cascade stratovolcanoes
   
3. 
   The San Andreas Fault
   
4. 
   The Seattle Fault
   
5. 
   The eastern margin of the Basin and Range
   

27. 
    Which of the following is or was not produced by plate tectonics?
    

1. mid-ocean ridges

2. 
   rift valleys
   
3. 
   “Ring of Fire”
   
4. 
   Appalachian Mountains
   
5. 
   * Earth’s magnetic field
   

28. 
    Earth’s lithospheric plates move at speeds of:
    

1. 
   a few millimeters per year
   
2. 
   * a few centimeters per year
   
3. 
   a few meters per day
   
4. 
   a few centimeters per day
   
5. 
   a few millimeters per century
   

29. 
    An example of a divergent boundary is a:
    

1. 
   volcanic island arc
   
2. 
   * mid-ocean ridge
   
3. 
   transform fault
   
4. 
   hotspot
   
5. 
   subduction zone
   

30. 
    In terms of movement of large parts of the Earth, the term “plates” refers to:
    

1. 
   rocks that form the visible parts of the continents
   
2. 
   rocks that form the ocean basins
   
3. 
   large moving sections of Earth’s crust
   
4. 
   * large moving sections of Earth’s crust plus part of the mantle
   
5. 
   large moving sections of Earth’s mantle
   

31. 
    A transform fault is characterized by:
    

1. 
   *lithospheric plates sliding past each another
   
2. 
   the movement of one plate over another
   
3. 
   the movement of plates away from each other
   
4. 
   the movement of one plate down against another
   
5. 
   earthquakes but no plate movement
   

32. 
    The youngest seafloor rocks are found:
    

1. 
   nearest the continental slopes
   
2. 
   * near the rift valleys of the mid-ocean ridges
   
3. 
   beneath the deep sea trenches
   
4. 
   evenly distributed over the ocean floors
   
5. 
   underlying the continental shelves
   

33. 
    The magnetic stripes of the seafloor are considered evidence of seafloor spreading and :
    

1. 
   subduction in the rift valleys
   
2. 
   spreading centers in the trenches
   
3. 
   changes in the Earth’s axis of rotation
   
4. 
   * periodic reversals in the polarity of Earth’s magnetic field
   
5. 
   periodic collapses of Earth’s gravitational field
   

34. 
    New crust is being generated:
    

1. 
   in the deep trenches
   
2. 
   in submarine canyons
   
3. 
   * in the rift valleys of the mid-ocean ridges
   
4. 
   at the centers of large continents
   
5. 
   in all of these places
   

35. 
    The rigid outermost layer of the Earth is called:
    

1. 
   the asthenosphere
   
2. 
   * the lithosphere
   
3. 
   the mantle
   
4. 
   the Mohoroviçic discontinuity
   
5. 
   the outer core
   

36. How does rhyolite magma form in the line of arc volcanoes like the Cascades?

1. Rhyolite is not found in the Cascades volcanoes.

2. * Basalt magma rising from above the subducted slab rises to melt continental crust to form the rhyolite

3. The subducted slab gets hot enough at depth that it melts to form rhyolite

4. Friction at the top of the subducted slab heats the mantle to form rhyolite

5. Water rising from the subducted slab causes melting of the Earth’s mantle to form rhyolite

37. Why do the Hawaiian Islands form a chain of volcanoes?

1. The mantle below flows slowly to the east, creating new volcanoes as it does

2. The Hawaiian Islands are not part of a chain. They are over a stationary hot spot in the lithosphere

3. The crack in the lithosphere is progressively splitting eastward, permitting magma to rise along a line

4. The top of the basalt plume in the deep mantle is dragged eastward by the eastward-moving lithosphere .

5. * The lithosphere carrying Hawaii slowly moves over a hotspot feeding basalt magma to the overlying volcano

38. How is the magma formed that feeds the volcanoes in Hawaii?

1. The spreading rift under Hawaii causes decrease in pressure on the mantle to produce basalt magma

2. Frictional heating of the mantle under the lithosphere causes melting to form basalt magma

3. * The lithosphere moves over a mantle hotspot causing melting to form basalt magma.

4. Melting above the subduction zone in Hawaii produces basalt magma that rises.

5. Convective flow in the deep mantle carries the upper mantle hotspot with it.