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p109, 1-20:
1. Wegener used the following lines of evidence: puzzle-like fit of continents; matching rock layers (types, ages,
structures) from one continent to another across intervening oceans; distribution of rocks and fossils indicating particular
past climate conditions (e.g., glacier scarring in low latitude areas); distribution of land fossils without intervening land
bridges. Scientists doubted that continents could plow their way through rigid ocean crust.
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2. Convincing diagrams (you’d hope) would be any diagram(s) that show(s):
- change of age of oceanic crust from mid-ocean ridge to continental margins
- Earth’s layered structure (especially with lithosphere and asthenosphere), and convection of heat in upper
mantle.
- earthquake/volcano distribution
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3. The lines of magnetic force of Earth’s magnetic field go outward from the surface of Earth at the South Pole
and inward at the North Pole (see Figure 3-7). Magnetic particles are aligned with the lines of force of the magnetic
field. Thus, the angle of dip of the magnetized particle indicates latitude. At the equator, the lines of magnetic force are
parallel to Earth’s surface. The angle of dip increases from 0o at the equator to 90o at the magnetic poles.
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4. Earth’s magnetic field periodically reverses its polarity. These periodic reversals produce a mirror-image
zebra-stripe pattern of magnetism shifts on either side of the mid-ocean ridge. The symmetry of the pattern means
that ocean crust is produced by the cooling of magma that upwelled at the crest of the mid-ocean ridge system. Once the
magma cools below the Curie point (600 deg C), the magnetic field at that time is permanently recorded in rocks. These
rocks are then transported away from the crest of the mid-ocean ridge. If we know the age of a particular magnetic
anomaly, then we can calculate rate of spreading.
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5. According to (Doctor/Admiral) Harry Hess, seafloor spreading means that ocean crust is created by cooling
magma at crest regions of mid-ocean ridges. Older seafloor is destroyed at oceanic trenches. Hess’s idea was
important because continents could move with respect to each other by creation/destruction of ocean floor.
Continents did not have to “plow” through rigid ocean crust. Seafloor spreading provided a mechanism for continents
to move apart or together.
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6. With increasing distance from the spreading center of a ridge:
- the amount of heat flowing through seafloor decreases
- the age of the ocean floor increases
- the lithosphere cools
- the ocean gets deeper
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7. Earthquakes usually occur at plate boundaries. Thus, a map showing the distribution of earthquakes will also
delineate plate boundaries.
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8. Dividing the Earth into layers based on chemical composition results in three layers--crust, mantle, core--with
pronounced differences in chemical composition (or mineral makeup) of these layers. In contrast, dividing the Earth into
layers based on how the rocks respond to increased temperature and pressure (physical properties) results in five
layers--Lithosphere, asthenosphere, mesosphere, outer core, inner core. For example, the core is made up of
predominantly iron and nickel, but the outer core is liquid, whereas the inner core is solid (because of the very high
pressure at the center of Earth). Similarly, the lithosphere (brittle) encompasses both the crust and the uppermost mantle,
even though crust and mantle have different chemical compositions.
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9. The lithosphere is made up of the crust and uppermost mantle, and it behaves as a rigid and brittle solid. The
lithosphere extends from the surface to about 100 km/60 mi below the surface of Earth. The asthenosphere extends from
about 100 km to about 700 km/420 mi below the surface. It is made up of partially molten rocks, and it behaves like a
plastic and is capable of slow flow.
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10. Continental crust is made up of granite, a light-colored rock comprised of less dense minerals (2.7 g/cm3).
Oceanic crust is made up of basalt, a rock composed of more dense minerals (3.0 g/cm3). Granite is more buoyant than
basalt because granite is less dense. Granite continental plates “float” higher in the asthenosphere than basalt
oceanic plates because of isostacy.
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11. Divergent plate boundary: plates move away from each other; mid-ocean ridge crest, example: Mid-Atlantic Ridge
Convergent plate boundary: plates move toward each other; subduction occurs at oceanic trenches; example: PeruChile Trench
Transform plate boundary: plates slide pass each other at mid-ocean ridge; example: San Andreas Fault, California
(Cross-section, see Figure 3-10.)
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12. Although some plates are wholly oceanic (e.g., Pacific Plate), most plates contain both continental and oceanic
crust. Figure 3-14, for example, shows that earthquakes generally mark the plate boundaries. These boundaries delineate
plates that are most frequently made up of continents and oceans (e.g., North American Plate).
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13. Oceanic ridges and rises both mark divergent plate boundaries. Oceanic RIDGES have steeper slopes, are
narrow, and tend to have large, well-developed central rift valleys. Oceanic ridges develop with slower spreading
rates. Under slower spreading rates, lithosphere has more opportunity for thermal contraction and subsidence. In
contrast, oceanic RISES have gentler slopes, are broader, and tend to have small, less well-developed central rift
valleys. Oceanic rises develop with faster spreading rates. Under faster spreading rates, lithosphere has less time for
thermal contraction and subsidence. So much magma is spewn out, oceanic rises are broader and smoother than
oceanic ridges.
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14. - Oceanic-continental plate convergence: denser oceanic plate subducts; andesitic volcanism; continental arc: PeruChile trench
- Oceanic-oceanic plate convergence: denser oceanic plate subducts; basaltic volcanism; island arc; deepest oceanic
trenches; Mariana Trench
- Continental-continental plate convergence: neither plate is dense enough to subduct; high mountain range
Himalayas, where India collides with southern Asia
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15. Divergent plate boundary: earthquake intensity is less (<6.0 Mw)
Convergent plate boundary: earthquake intensity is greater (max Mw = 9.5)
Transform plate boundary: earthquake intensity is greater at continental transform (where crust is thicker = more
friction = more stored energy = more energy released)
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16. A seamount is an underwater volcano with a conical top. A tablemount (guyot) is an underwater volcano with
an eroded, flat top. A tablemount is an oceanic island that was eroded to near flatness and later subsided. Plate tectonics
explains that flanks of mid-ocean ridges are deeper than the crest region of mid-ocean ridges. Seafloor cools and sinks as
it moves away from the crest of the mid-ocean ridge.
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17. The Hawaiian hotspot has remained nearly stationary. The Pacific Plate has moved over it. Magma from the
hotspot produced volcanoes (and ocean islands) within the Pacific Plate. The systematic increase of age of the
Hawaiian Islands from the current location toward the northwest indicates that the Pacific Plate moved northwest
over the hotspot. The Emperor Seamounts become systematically older toward the north. The bend or kink between the
two chains of islands and seamounts indicates a change in the direction of motion of the Pacific Plate (from northerly
motion to later northwesterly motion).
NOTE: The Hawaiian islands get noticeably larger as you move southeast toward the hotspot. (see fig 3-32, p97) In other
words, the newest islands are the biggest. What might this tell us about the Pacific Plate’s northwestern motion? Or does
it perhaps tell us something about that hotspot?
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18. The Aleutian Islands are an island arc that resulted from the subduction of the Pacific Plate into the Aleutian
Trench. Island arcs are made up of basalts, but these basalts are different in composition from the basalts that make up
the Hawaiian Islands. Hotspot volcanoes produce basalts with higher concentrations of potassium, rubidium, cesium,
uranium, and thorium compared with basalts produced at mid-ocean ridges. In addition, island arc volcanoes DO NOT
show a systematic variation in age along the length of the island arc, whereas nemataths (like the Hawaiian Islands)
do show a systematic variation in age.
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19. Mid-ocean ridge basalts have chemical compositions that indicate that the magma derived from the upper
mantle. Hotspot basalts, in contrast, have much deeper source areas of magma, perhaps as deep as the mantle/core
boundary. Mid-ocean ridges mark divergent plate boundaries and are much larger features with much greater outputs of
magma. Hotspots are localized columnar areas of magma that are most frequently within plates. Mid-ocean ridges are
much larger than hotspots. Mid-ocean ridges move through time; hotspots are nearly stationary.
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20. Using fig 3-38, when do the following first appear in the geologic record:
a. N. America on the equator. 480 mya
b. Continents merge, form Pangaea. 300 mya
c. North Atlantic Ocean opens. 120 mya
d. India separates from Antarctica. 120 mya