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CHAPTER 12 ANSWERS
Reading Check Answers, p. 509
1. Wegener noticed how continents had shapes that seemed to fit together.
2. All the continents joined together.
3. The pattern of mountain ranges makes sense when continents are connected.
4. The pattern of the fossil locations seemed to match Wegener’s idea that the continents were once together.
5. Paleoglaciation
Reading Check Answers, p. 513
1. A tectonic plate is a solid but moving piece of the lithosphere (crust and upper mantle).
2. The Mid-Atlantic Ridge is a diverging plate boundary (spreading centre) where two plates are moving apart.
3. The magnetic field aligns northward.
4. A hot spot is a weak part of the lithosphere where mantle material breaks through, forming a chain of
volcanoes.
12.1: Check Your Understanding Answers p. 517
1. A super continent is a single continent composed of all the continents.
2. The jigsaw puzzle fit of the continents, similar fossils discovered on distant continents, patterns of mountain
ranges, glacial evidence
3. Any two of the following: Antarctica has dinosaur fossils, tropical plant fossils, and coal deposits (the remains
of tropical swamps), all of which require warm climates.
4. Sea floor spreading occurs where two tectonic plates are moving away from each other.
5. Note: As an alternative question, ask “What is the relationship between plate tectonics and earthquakes?”
Earthquakes tend to occur at the boundaries between tectonic plates.
6. Earth’s surface is broken into large, solid but moveable rock plates. These plates move around the surface
on convection currents in the asthenosphere.
7. The presence of coal in these areas suggests that these areas were once closer to the equator (and
therefore were much warmer) than they are today.
8. Wegener could not explain how the continents moved.
9. Ocean floor is older the farther one is away from an ocean ridge. Therefore, there has been more time for
sediments from the ocean to accumulate.
10. Sea floor spreading causes plates to move apart. These plates will then slide past, collide with, or subduct
under other plates, causing mountains, volcanoes, and earthquakes.
11. (a) The newer area. Ridges are where magma reaches the surface, cools, and solidifies into new rock. The
older rock is farther away from the ridge.
(b) The geologist can conclude Earth’s magnetic field has reversed between the time the old rock formed and
the time the new rock formed.
12. Paleoglaciation suggests continents were once farther from the equator than they are now. Pale magnetism
suggests Earth’s magnetic field reverses from time to time. These reversals are recorded in the rock as it cools
at a spreading ridge.
Reading Check Answers, p. 522
1. Inner core, outer core, mantle, crust
2. Solid rock (crust and upper mantle)
3. Any three of: sea floor spreading, slab pull, ridge push, convection currents
4. Subduction occurs when one plate collides with another plate and is forced beneath that plate.
Reading Check Answers, p. 526
1. Transform boundary (diagram should show plates sliding past each other), diverging boundary (diagram
should show plates pulling apart), converging boundary (diagram should show plates pushing together)
2. A dense plate is forced beneath another plate. At continental/continental convergence, two plates collide and
crumple rather than subduct.
3. When an oceanic plate collides with another oceanic plate, one plate subducts under the other, forming a
chain of volcanic islands.
4. Himalayas, Alps, etc.
Reading Check Answers, p. 531
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1. The focus of an earthquake is the point in the ground where the earthquake actually originated. The
epicentre is a location on the surface directly above the focus.
2. Primary (P) waves, secondary (S) waves, surface (L) waves
3. A seismograph/seismometer measures the amount of ground shaking (magnitude).
Check Your Understanding Answers p.537
1. Transform boundary, diverging boundary, and converging boundary
2. Ridge push occurs at a spreading centre, or ridge, where two plates are being pushed apart by rising
magma.
3. The pattern of earthquakes and volcanoes follows the plate boundaries around the world.
4. (a) Convection currents are currents of magma rising and falling in the mantle (asthenosphere).
(b) Mantle (asthenosphere)
(c) Convection currents help drive the motion of plates.
5. Magma bubbles up through weak parts of the lithosphere, forming large shield volcanoes.
6. Lava plateaus (large, flat areas of cooled lava)
7. P-waves
8. Seismometers measure the amount of ground shaking in an earthquake (i.e., magnitude)
9. Time-distance graphs show the amount of time earthquake waves take to travel certain distances.
10. P-waves
11. (a) Plates move apart.
(b) Plates move together.
(c) Plates slide past one another.
12. At subduction zones, the subducting plate material melts, forming material that creates volcanoes. At
transform boundaries, there is no source of magma.
13. The ground motion of a P-wave is a squeezing and stretching in the direction of travel. For an L-wave, the
ground tends to ripple and roll.
14. P-wave = 5000 km, S-wave = 2250 km, L-wave = 1750 km
15. (a) Composite cone (strato)
(b) Rift eruption
(c) Cinder cone. Note: This is a challenge question for students.
CHAPTER 12 REVIEW ANSWERS p.538
1. Pangaea means “all world.”
2. Sample answers:
(a)
Asthenosphere
Both
– Partly-molten layer of upper mantle
– Involved in
– Flows
plate tectonics
– Found below lithosphere
(b)
Shield
Both
– Produces a large cone
– Not explosive
with gentle slopes
– Produces lava
(c)
P wave
Both
– Compressional wave
– Travel through ground
– Fastest wave
– Caused by earthquake
– Travels through anything
– Cause damage
– Make ground shake
(d)
Seismometers
Both
– Instrument to measure earthquakes
– Record ground
– Measures ground shaking
shaking in an
– Produces seismogram
earthquake
(e)
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Lithosphere
– Solid, rocky layer of crust and upper mantle
– Rigid
– Found above asthenosphere
Rift Eruption
– Lava erupts from long,
narrow cracks
S wave
– Shear wave
– Slower than P
– Cannot travel through
liquid
Seismogram
– Paper record of an earthquake
– Displays ground shaking of P-,
Sand L-waves
Focus
– Point in Earth
where earthquake
actually begins
Both
Epicentre
– Focus and epicentre can be same location on Earth’s
– Point on Earth’s surface
surface.
directly above the focus
– Both give the specific location of an earthquake.
– Used for geographical
– Both are used to describe an earthquake.
location of an earthquake
3. (a) Matching continental margins suggest the continents were once part of a single large continental mass.
(b) Similar animal fossils suggest the continents were once together, because it is unlikely the same organism
that produced the fossil would develop identically on continents several thousands of kilometres apart.
(c) Rocks of the same age and matching mountain ranges suggest the forces that produced them were acting
on a single large continental mass that has since broken apart.
(d) Note: You may wish to have students delete this question.
4. Note: As an alternative question, ask “Why do earthquakes occur at tectonic plate boundaries?” Earthquakes
occur at plate boundaries because large slabs of rock are trying to slide past each other or into each other. The
rock resists this motion, and stress (pressure) builds up. When that pressure is released, an earthquake
occurs.
5. Rocks increase in age as distance increases from a spreading ridge.
6. Subduction zones experience the deepest earthquakes, because one plate is diving deep beneath another.
7. Magma rises and breaks through the lithosphere at spreading ridges. The magma solidifies into rock. New
magma rising through the ridge pushes the new rock material away.
8. Shield volcanoes occur over hot spots.
9. The magma that forms composite volcanoes traps gas, which increases the pressure. When the pressure
becomes too great, the volcano erupts violently.
10. Sample answer: Similarities: Both are caused by earthquakes; both cause the ground to vibrate/shake;
both start at the same focus; both cause damage to buildings/structures; and both can travel through solids.
Differences: P-waves squeeze and stretch the ground in the direction they travel, and are faster than S-waves;
S-waves squeeze and stretch the ground at 90 degrees to the direction they travel; S-waves are slower than Pwaves; and Swaves cannot travel through liquids.
11. Earthquakes are caused by the build-up of stress between tectonic plates, caused by friction.
12. Continental drift
13. (a) Transform plate boundary
(b) Convergent plate boundary
(c) Divergent plate boundary
14. There is no source of magma at these types of plate boundaries.
15. Rocks of the same age and matching mountain ranges suggest the forces that produced them were acting
on a single large continental mass that has since broken apart.
16. If mantle convection stopped, there would be no plate movement and therefore no earthquakes or
volcanoes.
17. Material from volcanoes (lava from magma) originated deep in Earth.
18. Volcanoes occur at subduction plate boundaries and diverging plate boundaries, where magma from the
mantle rises and breaks through the lithosphere.
19. A rift eruption may produce a great deal of lava, which could cause damage to buildings, bridges, rivers,
and agriculture. Ash released from the eruption may affect climate.
20. Earthquakes are difficult, if not impossible, to predict.
21. A. Trench
B. Volcanic island arc
C. Upper mantle
D. Oceanic crust
E. Mantle
22. She can conclude that the rock in the middle layer formed at a time when Earth’s polarity was reversed.
23. (a) Convergent
(b) Toward each other
(c) 0 m to –35 m
(d) Note: As an alternative question, ask “Would you expect to find volcanoes in this area? Explain.” Answer:
Yes, the depth of foci indicates that one plate is subducting beneath another plate. The results would be
volcanoes in the area of the Subduction zone.
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