Download Ch 3 Homework Answers

PSc 201
Chapter 3 Homework Answers
Critical Thinking Questions
1.
The ship will sink farther into the fresh water since the ship will need to displace a larger volume of
this less dense fresh water to equal its own weight.
2.
a) See Fig. 3.3 b in your text
b) See Fig 3.3 a in your text
c) A P-wave is a longitudinal wave.
3.
The lower part of the wave will travel faster since sound travels faster in hotter air, so the wave will
curve upward away from the ground.
4.
a) Your author didn’t get this quite right in Fig. 3.4 c. He drew a P-wave which just missed the
outer core and ended up at 103º from the epicenter of the earthquake, and I agree with this. The other
P-wave he should have drawn would be one which just entered the outer core (but almost missed it).
The outer core refracts the wave and it ends up at 143º as indicated in Fig 3.4 c, but the path is not
quite right. This shadow zone is a band all the way around the Earth from 103º to 143º, symmetric
with the earthquake epicenter. Seismographs in this zone will see no direct P-waves (and no direct Swaves either).
b) The S-wave shadow zone extends from 103º all the way around the Earth. Seismographs in this
region will see no direct S-waves, but they will see direct P-waves beyond 143º. Fig. 3.4 d shows the
S-wave shadow zone correctly.
5.
S-waves cannot travel through liquids, so this shows that the outer core is liquid (actually mostly
molten metals)
6.
a) These regions differ primarily by chemical composition.
b) These regions differ primarily by physical properties, so they behave differently in terms of
flowing or bending and breaking, and the speed of seismic waves is different in the different regions.
7.
The planet or moon must have a liquid electrically conducting region in the interior, such as molten
metal or a salty ocean.
The planet or moon must be rotating on its axis at a reasonable rate, fast enough to set up swirls in
the liquid.
8.
a) Continental lithosphere is thicker.
b) Oceanic lithosphere is denser.
9.
The lithosphere was weighed down by an ice sheet several miles thick during the last ice age, so it
slowly sank lower into the asthenosphere just as a piece of floating wood will sink lower if you put a
weight on it. The ice age ended and the ice sheets melted about 10,000 years ago. With the weight
removed, the lithosphere is slowly rising to its former height.
10. He didn’t know about radioactive elements and the heat they release when they decay. This heat has
kept the interior of the Earth so hot that the outer core is still molten.
11. They thought that the Earth had contracted as it cooled, and that would make the hard surface (the
crust) too large so the crust would wrinkle into mountains. This is exactly what happens to a plump
grape as it dries out and contracts and shrivels into a raisin. But the Earth has not cooled very much
yet so it has not contracted.
12. a) Uniformitarianism.
b) Catastrophism.
13. Even though maps were not very accurate yet by the 1500’s, Leonardo noticed a suggestive similarity
between coastlines.
14. a) Fossil evidence such as messosaurus and glossopteris fossils at corresponding locations on the
coasts of South America and Africa.
b) Similar rock structures at corresponding locations, such as the Appalachian Mountains.
c) Evidence of glaciers moving over the surface (polished and grooved rock) in South Africa,
southern South America, Australia, and India - all places where glaciers at sea level could not exist
today.
d) Paleoclimatic evidence through fossils of tropical plants in regions which are now very cold,
showing that these regions must have been near the equator long ago - glacier evidence is similar to
paleoclimatic evidence.
15. The true edge of a continent is at the edge of the ‘continental shelf’ where the surface changes from
being relatively flat to sloping downward fairly steeply. The edge is called the ‘shelf break’.
16. It seemed like the continents should have left wakes or trails across the rocks of the ocean bottom if
they had somehow pushed their way through the rock, and no such trails had been found.
17. a) Seismographs mapped the epicenters of earthquakes and showed that earthquakes occurred in
lines, not scattered just anywhere. One of these lines of earthquakes was down the center of the
Atlantic Ocean and was shaped like the coastlines of the continents on either side of the Atlantic.
b) Radioactive dating had found that some surface rocks were more than 3 billion years old (even
older rocks would be found later on), but the oldest ocean floor rocks were less than 200 million
years old which was very very young.
c) Echo-sounding was beginning to show the shape of the Mid-Atlantic Ridge, which had also been
seen to some extent through the location of earthquakes, and this ridge matched the shape of the
coastlines of South America and Africa pretty well.
d) Seismograph data began to show that there was a layer in the upper mantle which was less stiff
than the lithosphere and might be able to flow (later named the asthenosphere), and some thought the
continents might float on this layer and be able to move around on it.
18. Mattthews and Vine published paleomagnetic data for a section of the seafloor along the MidAtlantic Ridge in the north Atlantic Ocean. This data showed that there were strips of seafloor which
were magnetized in opposite directions and the pattern on one side of the ridge was the mirror image
of the pattern on the other side.
19. Wegener’s continental drift hypothesis described the continents as pushing their way through the
solid rock of the seafloor - this does not happen. Plate tectonics explains that it is the tectonic plates
(pieces of the lithosphere) which move around as they float on the asthenosphere, and the continents
move along with the plates they are a part of.
20. a) Slab push refers to a plate sliding down the slope of a ridge or upwarp because of the force of
gravity, somewhat like a child sliding down a slide at a playground.
b) Slab pull refers to a plate subducting down into the asthenosphere and pulling the rest of the plate
along with it. The plate has cooled and become dense enough to sink deep into the asthenosphere
fairly easily, and as it angles downward gravity pulls it along. I usually demonstrate this by pushing
my class notes part way over the edge of the front desk and letting gravity pull the the rest of the way.
21. The Pacific Ocean seafloor is the largest, and the trip from the East Pacific Rise where new Pacific
seafloor is produced to the Aleutian Islands where the Pacific seafloor subducts takes a little less than
200 million years. The oldest ocean floor is found near the Aleutian Islands just before the
subduction zone.
22. The locations of earthquake epicenters show the edges of the plates most clearly.
23. The Juan de Fuca plate is subducting under the Pacific Northwest.
24. a) convergent
b) divergent
c) transform
25. Oceanic lithosphere is created at divergent boundaries and is destroyed at convergent boundaries.
26. Continental lithosphere is created, but once created it is never destroyed (other than some loss by
erosion) because it is too thick and its density is too low for subduction to occur.
27. Upwarping and stretching due to a plume of hot mantle material rising underneath a continent, rifting
and the formation of a rift valley, and ocean water finally flowing in turning the rift valley into a
narrow ‘linear sea’, slowly the linear sea will widen into an ocean.
28. In East Africa where the rift valley has formed and continues to widen and deepen.
29. This divergent boundary is called the East Pacific Rise.
30. Pangaea began to break apart about 210 million years ago.
31. a) The Mid-Atlantic Ridge is an excellent example of a divergent oceanic-oceanic boundary.
b) The rift valley in East Africa is an excellent example of a divergent continental-continental
boundary.
c) The west coast of South America is an excellent example of a convergent oceanic-continental
boundary.
d) The Aleutian Island chain is an excellent example of a convergent oceanic-oceanic boundary.
e) The Himalayan Mountains are an excellent example of a convergent continental-continental
boundary.
f) The San Andreas fault is an excellent example of a transform boundary.
32. Deep sea trenches are always found at convergent boundaries where oceanic seafloor is subducting.
33. a) Hot spots.
b) Subduction zones, which are convergent boundaries where an oceanic plate is subducting.
34. Seismic evidence shows that subducting oceanic lithosphere slabs can survive all the way down to the
boundary between the mantle and the outer core.
35. The Earth’s magnetic field is generated in the outer core.
36. a) The magnetic pole nearest the north geographic pole is called the ‘north magnetic pole’.
b) If we pretend that there is a giant bar magnet buried inside the Earth, the pole of this bar magnet
which is nearest the north geographic pole is a south magnetic pole.
37. If a compass needle points to one end of a bar magnet, that end of the bar magnet is a south magnetic
pole.
38. A horizontally magnetized rock sample must have cooled and magnetized somewhere on the Earth’s
magnetic equator.
39. A rock sample which is magnetized vertically upward must have cooled and magnetized near the
Earth’s ‘south magnetic pole’, which is the magnetic pole near the south geographic pole.
40. A rock sample which is magnetized at about a 45º angle downward must have cooled and magnetized
in the northern magnetic hemisphere about half way between the magnetic equator and the north
magnetic pole.
41. Two things protect the surface of the Earth from most of the radiation from outer space (called
cosmic radiation). These two things are the Earth’s atmosphere and the Earth’s magnetic field.
Earth’s magnetic field becomes almost zero during a magnetic reversal, and this will allow more
radiation from space to reach the surface of the Earth. This radiation can do genetic damage to living
things and thus can cause mutations, and these mutations could cause evolution to proceed more
rapidly.
42. Sometimes a volcano will have lava flows periodically for a period of thousands of years. As each
flow cools the magnetite in the lava will be magnetized by the Earth’s magnetic field at that time.
Paleomagnetism researchers look for lava flows where a stream has eroded through the lava flows so
they can take samples from many flows, one on top of the next. If these lava flows covered the time
period of the last magnetic reversal, the top flows will be magnetized in the same direction as the
Earth’s present magnetic field, but eventually they will find older deeper flows which are magnetized
in the opposite direction because they cooled before the last magnetic reversal. If the flows can be
dated by radioactive dating it is possible to determine when the Earth’s magnetic field reversed,
approximately how long it took, and how the field behaved while it was reversing (for example,
whether or not the field became very weak for a while).
43. As magma (molten rock) flows out along a divergent boundary, such as the Mid-Atlantic Ridge, the
new rock cools and becomes magnetized in the direction of the Earth’s magnetic field at that time.
So seafloor near the divergent boundary will be magnetized in the same direction as Earth’s present
magnetic field, but farther from the boundary the seafloor will be magnetized in the opposite direction
because it cooled before the last magnetic reversal. Even farther from the boundary the seafloor will
again be magnetized in the same direction as Earth’s present magnetic field because it cooled during
a ‘normal’ magnetic period, and farther still from the boundary the seafloor will be magnetized in the
opposite direction because it cooled during a ‘reversed’ magnetic period, and so on.
44. The magnetometer will register a total magnetic field which is lower than average because the
magnetic field from the reverse magnetized seafloor will subtract a small amount from the Earth’s
present magnetic field since it points in the opposite direction.
45. Paleomagentism can track the wandering of a continent because if the magnetization of a rock sample
is analyzed it is possible to judge where the north magnetic pole would have been relative to the
location of the rock sample. That probably is not where the north magnetic pole actually is now, so
you need to mentally move the continent to a latitude which puts the magnetic pole at the present
location. Radioactive dating will show the age of the rock. If this is done for many rock samples of
different ages on all the major continents it is possible to figure out where each continent was at any
particular date, and thus the past movement of the tectonic plates (and the continents which move with
their plates) can be determined.
46. a) The best example of a major hot spot under an ocean is the hot spot which has created the
Hawaiian Islands.
b) The hot spot under Yellowstone National Park is a good example of a major hot spot under a
continent.
47. Kauai is farthest from the hot spot and is thus the oldest of the islands between Kauai and Hawaii.
Radioactive dating confirms this.
48. The chain makes a bend dated to about 40 million years ago and this tells us that the Pacific Plate
changed it direction of motion significantly at that time.
49. The sediment will be thicker farther from the divergent boundary simply because the seafloor is older
and there has been more time for more sediment to fall to the bottom and accumulate on it.
50. The materials which were pasted onto the North American Plate are called ‘terranes’.
Analytical Thinking Questions
51. density = mass/volume = 488g/168 cm3 = 2.91 g/cm3, so this rock sample is oceanic lithosphere
(basalt).
52. a) speed = distance/time = 5,800 km/ 210x106 y = 5.80x106 m/ 210x106 y
= 0.0276 m/y = 0.0276 (100 cm)/y = 2.76 cm/y
b) d = vt = (0.0276 m/y)(200,000y) = 5.52x103 m = 5.52 km