Asia

Pacific Ocean

Aleutian Islands

Alaska

Hawaii

Asia

CHAPTER 2PLATE TECTONICS AND PHYSICAL HAZARDS

Web Sites:http://pubs.usgs.gov/publications/text/dynamic.htmlhttp://wrgis.wr.usgs.gov/docs/parks/animate/ - U.S.G.S. Plate tectonic animationshttp://www.fourmilab.ch/earthview/vplanet.htmlhttp://www.ucmp.berkeley.edu/geology/tectonics.html

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

Barbara.Video: Plate tectonics and Continental drift: Gould Media, Inc., Mount Vernon, NYVideo: Earth Revealed #1: The living machine: The Theory of Plate Tectonics.

Annenberg/CPC Collection; P.O. Box 1922, Santa Barbara, CAVideo: 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, CAVideo: 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

1A. (alternative). What direction is the Pacific Plate currently moving based on the figure below?

* west-northwest

Correct answers are indicated by an asterisk, both in short answer and multiple choice questions.

True or False questions can be easily prepared from multiple choice questions

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).

* rift (or spreading) zones: Mid-Atlantic ridge* subduction zones: Cascadia subduction zone* continental collision zones: India against Asia (Himalayas)* transform faults: San Andreas Fault

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

Short answer questions:

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.* Crust overlies mantle. It is basalt composition under the ocean basins, granitic

composition in the continents

4. Roughly how many tectonic plates move around on the surface of the Earth?* 10 or 12

5. What characteristics of tectonic plates distinguish them from deeper Earth materials?* They are rigid.

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

* all of the crust and part of the upper mantle.

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?

* Isostacy. As material is eroded off the range, it floats higher, just as removing ice from the top of an iceberg causes it to float higher.

8. What is the basis for the “scientific method?”* tentative hypotheses are tested using new observations and experiments

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

* They are hotspot volcanoes. A hot plume of magma rising through the mantle burns through the overlying lithosphere to erupt as a volcano. Since the lithosphere moves over the deeper mantle, new volcanoes form progressively in a line over the deep plume.

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

13. 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

14. 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

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

* spreading (Iceland/oceanic ridge)

16. 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

17. 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

18. 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.

* mantle plume or hotspot* An example in an ocean basin is Mauna Loa, Hawaii* An example on continent is Yellowstone.

19. The lithosphere is composed of what two layers?* Crust and upper mantle

20. With which type or types of boundary are active volcanoes generally associated?* divergent (or spreading), convergent (or subduction)

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

* He had no mechanism for moving continental crust through strong basaltic oceanic crust.

22. The “Ring of Fire” refers to what and where is it?* the ring of active volcanoes surrounding the Pacific Ocean.

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

* earthquakes and volcanoes +/- tsunami

24. 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.

25. 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?

a. to the northeastb. * to the northwestc. to the southeastd. to the southweste. 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?

a. Rocks on the ocean floor are oldest in the center, becoming progressively younger towards each continent.

b. Rocks on the continents can be traced through the ocean floor to the other side.c. Anthropologists have found human carvings in Africa that match those in Brazil.d. Glaciers near the mouth of the Amazon River in Brazil carried distinctive rocks

into South Africa, demonstrating that those countries were once connected.e. * Ages of bedrock formations match across the Atlantic Ocean

3. Which of the following was not used as early evidence for continental drift?a. Ages of bedrock formations match across the Atlantic Oceanb. match of coastlines across the Atlantic Ocean c. match of rock formations across the Atlantic Oceand. match of ages of continental rocks across the Atlantic Oceane. * the fact that the magnetic pole shifts from North to South

4. Which of the following is true?a. The lithosphere and asthenosphere consist entirely of mantle material.b. The lithosphere is the mantle under the oceans; the asthenosphere is the mantle under

the continents

c. The asthenosphere is the upper part of the mantle.d. * The asthenosphere is more plastic in behavior.e. The lithosphere is more plastic in behavior.

5. Which of the following is true?a. The mantle is denser than the lithosphere.b. The lithosphere is denser than the mantle.c. The asthenosphere is the more-rigid equivalent of the mantle.d. * The asthenosphere is the more-plastic part of the mantlee. The mantle is everywhere more rigid than the crust.

6. Which of the following is true?a. oceanic rift zones are found only in the center of the oceansb. rift zones are found only in the ocean basinsc. * rift zones are areas where oceanic crust is formedd. rift zones are the oldest parts of the oceanic cruste. rift zones mark the boundaries between oceanic and continental crust.

7. Which of the following is not true?a. subduction zones are areas where ocean floor descends into the mantleb. subduction zone activity includes very large earthquakesc. subduction zone activity leads to active volcanoes d. * subduction zones are areas where ocean floor rocks are formede. subduction zones are marked the deepest parts of the oceans

8. Which of the following is true?a. transform faults show dominantly vertical movementb. transform faults are only in the ocean basins.c. transform faults are areas of spreading and new crustal generationd. transform faults change with time from horizontal to vertical motione. * transform fault motion typically ends abruptly at both ends.

9. What does oceanic lithosphere consist of? a. * basalt on top of peridotiteb. only basaltc. only peridotited. basalt and peridotite in layers of variable thickness, in some places above, in others

below.e. partly basalt and partly granite

10. Along which type of lithospheric plate boundary are earthquakes common?a. only convergent (subduction zones)b. only divergent (spreading zones)c. only transformd. only divergent and transforme. * convergent, divergent, and transform

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

a. rift zones on continentsb. collision zones between continental platesc. * subduction zones between oceanic and continental platesd. subduction zones between two continentse. transform fault boundaries between oceanic and continental plates

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

a. oceanic lithosphere moves so slowly that it can only sink.b. oceanic lithosphere is at the bottom of the ocean so it can’t float high enough to ride

over a continent.c. oceanic lithosphere almost twice as dense as the underlying mantle.d. * oceanic lithosphere is denser than continental lithospheree. 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?

a. actually the Atlantic Ocean floor is not getting widerb. * old ocean floor sinks at subduction zones (trenches)c. It is becoming denser so it takes up no more space.d. It becomes part of the edge of the adjacent continent.e. It melts at oceanic transform faults

14. Which of the following is true?a. Earth’s crust is more dense than the mantle.b. Earth’s crust is thicker than the mantle.c. Earth’s crust is part of the asthenosphere and equivalent to it in composition.

d. Earth’s crust is part of the mantle and forms the upper part of it.e. * Earth’s crust is less dense than the mantle

15. Roughly how many tectonic plates move around on the surface of the Earth?a. none; they don’t mover around on the surface, they mostly move up and down.b. 4 or 5, depending on sizec. * 10 or 12d. about 50e. more than 300

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

a. They are partly moltenb. They behave in a more plastic mannerc. * They are rigid.d. They consist entirely of basalte. 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?

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

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

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

that.

18. What is the basis for the “scientific method?”a. Scientists come up with a theory and then seek out data to prove that it is correct.b. Scientists experiment with data to observe the data that they like best.c. Scientists try to discredit ideas proposed by others.d. Scientists prove their hypotheses with data that demonstrates their correctness.e. * 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?

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

the line of volcanoes to the other.

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

volcanoes as it does.d. The spreading lithosphere pushes the underlying magma source across the ocean

floor.e. The active volcanoes are gradually moving away from the oceanic ridge.

20. What is the difference between the Earth’s lithosphere and asthenosphere?a. Lithosphere is below the asthenosphere.b. * Lithosphere is more-rigid; asthenosphere is more-plastic/easily deformed. c. Lithosphere behaves in a more plastic manner so it moves over the Earth’s interior.d. Asthenosphere is almost 100 percent melt so it moves easily towards the oceanic

ridges.e. Asthenosphere consists of basalt with lesser amounts of peridotite.

21. What does the oceanic crust consist of and how thick is it?a. peridotite about 7 km thickb. peridotite between 60 and 100 km thickc. * basalt, about 7 km thickd. basalt over peridotite, a total of about 60 km thicke. basalt over peridotite about 15 km thick

22. What does the continental crust consist of and how thick is it?a. soil of variable thickness / generally between 1 and 100 metersb. “granite” composition / about 10 km thick over peridotite 5 to 10 km thickc. shale, sandstone, and limestone / about 10 km thickd. *“granite” composition / 30 to 60 km thicke. “granite” composition / 100 to 300 km thick

23. The San Andreas Fault is:a. a normal faultb. the subduction zone bordering the Pacific Platec. a rift zone running the length of Californiad. a reverse fault e. * a transform fault

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

a. * oceanic rift zonesb. continental rift zonesc. continental collision zonesd. transform faultse. mantle plumes

25. What type of volcano is not associated with a plate boundary?a. * a hotspot volcanob. a stratovolcano

c. a rift zone flood basaltd. a rhyolite volcano over a subduction zonee. a basalt volcano over a subduction zone

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

a. The Cascadia subduction zoneb. * The Cascade stratovolcanoesc. The San Andreas Faultd. The Seattle Faulte. The eastern margin of the Basin and Range

27. Which of the following is or was not produced by plate tectonics?a. mid-ocean ridgesb. rift valleysc. “Ring of Fire”d. Appalachian Mountainse. * Earth’s magnetic field

28. Earth’s lithospheric plates move at speeds of:a. a few millimeters per yearb. * a few centimeters per yearc. a few meters per dayd. a few centimeters per daye. a few millimeters per century

29. An example of a divergent boundary is a:a. volcanic island arcb. * mid-ocean ridgec. transform faultd. hotspote. subduction zone

30. In terms of movement of large parts of the Earth, the term “plates” refers to:a. rocks that form the visible parts of the continentsb. rocks that form the ocean basinsc. large moving sections of Earth’s crustd. * large moving sections of Earth’s crust plus part of the mantlee. large moving sections of Earth’s mantle

31. A transform fault is characterized by:a. *lithospheric plates sliding past each anotherb. the movement of one plate over anotherc. the movement of plates away from each other

d. the movement of one plate down against anothere. earthquakes but no plate movement

32. The youngest seafloor rocks are found:a. nearest the continental slopesb. * near the rift valleys of the mid-ocean ridgesc. beneath the deep sea trenchesd. evenly distributed over the ocean floorse. underlying the continental shelves

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

a. subduction in the rift valleysb. spreading centers in the trenchesc. changes in the Earth’s axis of rotationd. * periodic reversals in the polarity of Earth’s magnetic fielde. periodic collapses of Earth’s gravitational field

34. New crust is being generated:a. in the deep trenchesb. in submarine canyonsc. * in the rift valleys of the mid-ocean ridgesd. at the centers of large continentse. in all of these places

35. The rigid outermost layer of the Earth is called:a. the asthenosphereb. * the lithospherec. the mantled. the Mohoroviçic discontinuitye. the outer core

36. How does rhyolite magma form in the line of arc volcanoes like the Cascades?a. Rhyolite is not found in the Cascades volcanoes.b. * Basalt magma rising from above the subducted slab rises to melt continental

crust to form the rhyolitec. The subducted slab gets hot enough at depth that it melts to form rhyolited. Friction at the top of the subducted slab heats the mantle to form rhyolitee. 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?a. The mantle below flows slowly to the east, creating new volcanoes as it doesb. The Hawaiian Islands are not part of a chain. They are over a stationary hot spot

in the lithospherec. The crack in the lithosphere is progressively splitting eastward, permitting

magma to rise along a lined. The top of the basalt plume in the deep mantle is dragged eastward by the

eastward-moving lithosphere . e. * 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? a. The spreading rift under Hawaii causes decrease in pressure on the mantle to

produce basalt magmab. Frictional heating of the mantle under the lithosphere causes melting to form basalt

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

magma.d. Melting above the subduction zone in Hawaii produces basalt magma that rises.e. Convective flow in the deep mantle carries the upper mantle hotspot with it.

Matching Question:

1. Match the appropriate type of plate boundary from list on the right with each of the locations on the left:a. the margin between India and Asia

*rift (or spreading)zonesb. in California between Los Angeles and San Francisco *subduction zonesc. off the East coast of the United States and Canada

*continental collision zoned. off the west coast of Washington, Oregon and

*transform fault British Columbia

e. off the west coast of South America*passive margin