Geology 110: Earth and Space Science
Chapter 8 (Geologic Time)
Homework
SELF-REFLECTION AND COMPREHENSION SURVEYS
Checkpoint 8.1, p. 216
#1: Place the following events that were described in the earlier chapters of the book in
the correct relative chronological order, from earliest to the most recent.
a. Tsunami struck Japan.
b. Ice sheet was present in India.
c. Asteroid collided with Yucatan Peninsula.
d. Mount Pinatubo erupted in the Philippines.
e. Wegener developed the continental drift hypothesis
Checkpoint 8.2, p. 219
#2: Examine the following image of rock layers and answer Questions 1 and 2 about
relative time.
1. Which statement is most accurate?
a. D is older than B
b. E is older than A
c. F is older than C
2. When did the tilting of the layers occur?
a. After A was deposited
b. Between deposition of layers E and A
c. Before B was deposited
d. Between deposition of layers C and E
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Examine the following image of layers (below) and answer Questions 3 and 4 about
relative time.
(NOT REQUIRED, NOT EC)3. Which sequence of letters best represents the order in
which the layers were formed (from oldest to youngest)?
a. C, D, B, A
b. C, B, D, A
c. B, C, D, A
d. A, B, D, C
(NOT REQUIRED, NOT EC) 4. An unconformity is present between layers
a. C and D.
b. B and D. c. C and B.
d. A and B.
Checkpoint 8.3, p. 221
#3: Use the principles of original horizontality, superposition, cross-cutting relationships,
and inclusions to determine the order of events for the idealized location shown in the
following diagram.
a) Place the rock units in their order of formation, oldest to youngest.
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Youngest
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Oldest
b) Examine the rock types identified by the symbols in the diagram, and determine which
rock units best match the following descriptions.
Letter
Characteristics
Interbedded layers of rocks that indicate alternating shallow marine
environments and freshwater swamps in tropical conditions
Coarse-grained clastic sedimentary rocks overlying an erosional surface
(unconformity surface)
Granite
A rock containing a foliation
The most recently deposited sedimentary rock
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Sedimentary bed that has undergone contact metamorphism on its uppermost
surface
Basalt
Checkpoint 8.4, p. 224 (NOT REQUIRED, NOT EXTRA CREDIT)
#4: Construct a diagram that illustrates a cross section of rock units that would account
for the features listed below (not in order). Draw a relative time diagram that illustrates
the correct order for these features. Clearly label your units. Remember: These events are
not in order. You must determine the order of events based on the descriptions.
A: Rhyolite crosscuts and covers all units except sandstone.
B: Dark, fine-grained igneous rock crosscuts and covers conglomerate and older units.
C: Oldest rocks are made of black, biochemical layers that were later tilted.
D: Course-grained clastic rock is deposited immediately over coal.
E: Opaque chemical sedimentary rock deposited directly over basalt.
F: River cuts partially into limestone.
G: Medium-grained clastic rock deposited over small-grained, high-silica extrusive rock.
Checkpoint 8.5, p. 225(EXTRA CREDIT)
#5: Geologists look for similar rock types or fossils to tell them that geologic
environments were similar between two widely spaced locations. Can we do the same
kind of thing? What are some examples of modern environments that have characteristic
assemblages of organisms?
Checkpoint 8.6, p. 225 (EXTRA CREDIT)
#6: Outcrops of rock are examined in four different locations in a state. The rock types
and the fossils they contain are illustrated in the following diagram. Which fossil would
be the best choice to use as an index fossil for these rocks? Which fossil is least
characteristic of a specific set of geological conditions?
a) Fossil 1
b) Fossil 2
c) Fossil 3
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Checkpoint 8.7, p. 226
#7: Examine the following illustration and predict which rock unit in the Grand Canyon
is most likely to have formed in a depositional environment like the one pictured.
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Checkpoint 8.8, p. 226 (NOT REQUIRED, NOT EXTRA CREDIT)
#8: Limestone is present in multiple rock units at different elevations in the Grand
Canyon. If you were handed six large samples of limestone, each containing fossils,
which group of fossils would be most helpful in identifying the different units? Which
would be least helpful? Explain your choices.
Checkpoint 8.9, pp. 228-229: Tracks from a Primordial Sea
#9: Read the following abbreviated version of a newspaper article and then answer the
questions at the end concerning the difference between observations, hypotheses, and
predictions. (Note that the dates of periods quoted have been modified by more recent
interpretations.)
Scientists investigating an abandoned quarry in Canada have found what appear to be
the oldest known footprints of terrestrial creatures--foot-long critters resembling modern
bugs that crawled from the sea onto land and left tracks in sandy dunes. The sandstone is
480 million to 500 million years old. Scientists believe the discovery region was a sandy
beach on a primordial sea. The find, the scientists say, pushes back the colonization of
land by about 40 million years and puts it in or near the late Cambrian period, when the
seas were starting to boil with large creatures. In the past decade or so, specialists
studying old rocks have steadily pushed back the time when sea animals are believed to
have first come ashore. The date has gone from the Silurian period, generally accepted to
have started 440 million years ago, to the Ordovician, which started 490 million years
ago, and to the Cambrian, which started 544 million years ago.
The scientists said the Canadian find includes more than 25 trackways that crisscross an
area roughly the size of a small basketball court. Typically, the trackways have a central
area, where the body and tail made impressions as the animal moved forward, and
parallel areas where rows of legs left multiple footprints. Trackways of several sizes
indicate the presence of several individual animals over an extended period, which
scientists interpret as a “group exodus from the water.”
What kind of creatures made the marks? The scientists suspect they might have been
euthycarcinoids--rare fossil organisms whose segmented bodies included protective outer
shells and long legs. “There were at least eight pairs of walking legs,” Dr. MacNaughton
[a scientist at the Geological Survey of Canada who led the research team] said. “We see
the drag marks left behind but can’t say for sure” what animal made the sandy imprints.
Theories of why the sea creatures came ashore include hunting for food, fleeing
predators, and searching for safe places to reproduce. It is well known that insect-like
arthropods first came out of the sea, followed much later by limbed vertebrates whose
descendants include humans.
The scientists said the abundance of trackways at the old quarry raised the prospect of
similar finds elsewhere. “The same rocks occur in northern New York,” Dr.
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MacNaughton said, adding that examinations of sandstone beds there might yield
discoveries of ancient tracks.
Source: The New York Times, June 4, 2002, Page 3, William Broad.
1. What were the key observations in this article?
a) Trackways of footprints are present in Cambrian age rocks.
b) Trackways are common in sandstone that represented a beach environment.
c) Euthycarcinoids had eight legs and segmented bodies.
Fossil remains of insect-like arthropods were found in the rocks.
2. What hypothesis was developed from the observations?
a) Rocks dated as Silurian in age were really Cambrian.
b) Euthycarcinoids are associated with sandstone quarries.
c) Organisms may have colonized the land much earlier than previously thought.
d) Life evolved earlier in Canada than elsewhere.
3. What prediction did scientists intend to test in an attempt to support the hypothesis?
a) Fossils of insect-like arthropods were found in the rocks.
b) Rocks previously identified as Silurian can be dated by radioactive decay methods
to discover if they are actually Cambrian in age.
c) Trackways of footprints may be found in similar Cambrian-age rocks in New
York.
Checkpoint 8.10, p. 229 (NOT REQUIRED, NOT EXTRA CREDIT)
#10: Carefully examine the relative positions of the lettered arrows in the following
diagram and assign a key geologic event to each arrow.
Which letter corresponds most closely to the first appearance in the rock record of
abundant fossils?
a. A
b. B
c. C
d. D
Which letter corresponds most closely to the extinction of the dinosaurs?
a. A
b. B
c. C
d. D
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.
Checkpoint 8.11, p. 231: Ancient Leaves and Insect Extinctions(EXTRA CREDIT)
When a 6-mile-wide asteroid slammed the Earth 65 million years ago, it wiped out the
dinosaurs, about 80 percent of the world’s plant species, and all animals bigger than a
cat. But what happened to the bugs?
It’s been tough for scientists to determine how the insects fared because they rarely leave
behind fossils, but a Denver paleontologist and his Smithsonian Institution colleagues
found a way around the problem. By studying insect damage etched into thousands of
fossil leaves, they determined that many plant-eating bugs perished in the big impact.
“These little insects are leaving their calling cards on the fossil leaves, and we have an
excellent fossil record of leaves,” said Kirk Johnson, curator of paleontology at the
Denver Museum of Nature & Science. “So by looking at the insect damage on the leaves
before and after the dinosaur extinctions, we can make a pretty good educated guess of
what happened to the insects.”
Johnson and his collaborators estimate that 55% to 60% of plant-eating insects were
exterminated. Over the past 20 years, Johnson has collected 13,441 plant fossils from
quarries in southwestern North Dakota. When the asteroid hit Mexico’s Yucatan
Peninsula, it threw up clouds of dust that traveled around the globe. Johnson pulled the
fossils from rock layers directly above and below those sediments. At the time,
southwestern North Dakota was a warm, forested plain with lots of broad-leafed trees.
Some leaves, now stored at the Denver museum and at Yale University, are up to a foot
long. Individual leaf veins are visible, as are the diagnostic chomp marks, tunnels, and
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holes left by prehistoric beetles, grasshoppers, butterflies, and moths. Some insects are
specialists, rely on a single species of plant for sustenance; others are generalists that
feed on several plant types. By analyzing insect-damaged leaves before and after the
impact, the researchers determined that the generalists survived, while 70% of the
specialists did not.
Source: Rocky Mountain News (Denver, CO), February 22, 2002, Page 7A: Jim
Erickson.
Read the following abbreviated version of a newspaper article (above) and answer these
questions.
a. What was the question being investigated by the scientists?
b. What observations did the scientists make during their investigations?
c. What was the principal conclusion of their research?
Checkpoint 8.12, p. 232: Geologic Time Metaphor
(TAKE A DEEP BREATH, IT’S NOT AS HARD AS IT LOOKS…EVERYTHING IS
HERE, YOU ONLY HAVE TO MAKE ONE CALCULATION TO FILL IN THE
TABLE BELOW)
#12: Suppose that all of geologic time is proportional to the length of a football field (100
yards). Earth would have formed at the opposing team’s goal line (100 yards) and present
day would represent the home team’s goal line (0 yards).
Metaphor Equation
Metaphor value = (years before present / age of Earth) x metaphor maximum
Example
Oldest fossil bacteria = 3,800 million years old
Age of Earth = 4,600 million years
Metaphor maximum = 100 yards
Metaphor value = (3,500,000,000/4,600,000,000) x 100 = 76 yards
Key metaphor dimensions:
1 inch = 1.3 million year
100 yards = 4,600 million years
10 yards = 460 million years
1 yard = 46 million years
1 foot = 15.3 million years
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Calculate the yardage of the extinction at the end of the Paleozoic era. Then fill in the blank cell
in the table and label the following figure.
Distance from home
goal line
Time
Event
76 yards
3,800
Oldest fossil bacteria
26 yards
1,200
Oldest known animal fossil (jellyfish)
12 yards
540
Hard skeletons become common (fossils)
10 yards
458
First land plants (ferns, mosses)
251
Widespread extinction ends Paleozoic Era
1.4 yards
66
Dinosaurs become extinct
0.00036 inches
0.00051
Columbus landed, 1492
(million years)
(The next part of the question associated with this checkpoint is EXTRA CREDIT)
NOW, develop your own metaphor for geologic time and describe it. Choose some of the
most significant geologic events from the geologic time scale and convert them into your
own metaphor equation.
(note: don’t try to be too detailed in your analysis. The intention here is to recognize the
length of the geologic timescale and the relative positions of key events. Approximate
lengths, distances, heights, widths, depths, sizes, time periods, etc., are okay as long as
you recognize the relative proportions of the time intervals).
Examples of metaphors you could use are height of a building, distance on a golf
course hole, a ladder…there are many!...
Checkpoint 8.13, p. 233 (NOT REQUIRED, NOT EXTRA CREDIT)
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#13: Between 1860 and 1920, geologists attempted to estimate Earth’s age by how long it
would take for the thickest sequences of sedimentary rocks to form. Geologists examined
sequences of rocks for each geologic period. From the estimated rates for the formation
of these units, different scientists estimated ages for Earth ranging from 3 million years to
15 billion years. Explain why these estimates varied over such a wide range.
Checkpoint 8.14, p. 235(NOT REQUIRED, NOT EXTRA CREDIT)
#14: Radioactive isotopes in clastic sedimentary rocks always predict an age that is
a. older than the sedimentary rock.
b. younger than the sedimentary rock.
c. correct for the sedimentary rock.
The isotope of element X has 15 protons, 17 neutrons, and 15 electrons. The element has
an atomic number of ____ and a mass number of ____.
a. 15; 32
b. 17; 15
c. 17; 47
d. 15; 30
This is a comprehension-level question.
If radioactive decay began with 400,000 parent atoms, how many would be left after
three half-lives?
a. 200,000
b. 100,000
c. 50,000
d. 25,000
Checkpoint 8.15, p. 235
#15: The half-life of a radioactive isotope is 500 million years. Scientists testing a rock
sample discover that the sample contains three times as many daughter isotopes as parent
isotopes. What is the age of the rock?
a. 500 million years
c. 1,500 million years
b. 1,000 million years
d. 2,500 million years
Checkpoint 8.16, p. 236
#16: The following diagram represents three rock exposures containing fossils. Each
exposure contains a layer of volcanic ash (in red) that has been dated by the analysis of
238 206
U/ Pb isotopes.
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a) Place the fossils in the correct order according to their relative ages, from oldest to
youngest:
b) Explain how you would estimate the potential age ranges of the C, G, and K fossils
based on the ages determined for the three volcanic ash layers.
(Question adapted from J. Dodick and N. Orion, “Measuring student understanding of
geological time,” Science Education, 2003, vol. 87, pp. 708-731.)
Checkpoint 8.17, p. 237(NOT REQUIRED, NOT EXTRA CREDIT)
#17: We daily encounter evidence of things that have changed over time. For example, an
instructor finds a stick of chalk that has become too small to use, or a student might find
that their jeans have become so worn that a hole has formed in the fabric. Identify three
examples of everyday objects that change over time but at different rates. For example,
something that is used up or worn out in a matter of days (e.g., chalk), or months (e.g.,
jeans), or years.
Checkpoint 8.18, p. 239: Rates Timeline
#18: Events happen on Earth over periods of time that vary from seconds to millions of
years. Place each of the following events in the appropriate location on the timeline
provided here, according to either its frequency (how often?) or the length of time over
which it occurs (how long?).
1. The time between large eruptions of the same volcano
2. A season (e.g., spring)
3. Time between great earthquakes on the San Andreas Fault
4. Period required to form the Atlantic Ocean
5. Formation and decay of a tornado
6. Earth’s orbit around the sun
7. Length of orbit for a long-period comet
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8. Time between mass extinctions
9. Time required to carve the Grand Canyon
10. Growth of major U.S. cities
11. Formation and decay of a hurricane
YOU CAN USE THE CHART BELOW TO ANSWER THE QUESTION ABOVE:
TIME
NUMBERED ANSWER(S)
ONE SECOND
to
ONE MINUTE
to
ONE DAY
to
ONE YEAR
to
ONE HUNDRED YEARS
to
ONE THOUSAND YEARS
to
ONE MILLION YEARS
to
ONE HUNDRED MILLION YEARS
to
ONE THOUSAND MILLION YEARS
(OR ONE BILLION YEARS)
Checkpoint 8.19, p. 239 (NOT REQUIRED, NOT EXTRA CREDIT)
#19: We have just provided some examples of rare, high-magnitude events and common,
low- magnitude events. Review the previous chapters, and identify other examples of
high- and low-magnitude events.
Checkpoint 8.20, p. 240 (EXTRA CREDIT)
#20: We have presented an Earth history stretching back 4.6 billion years. Has the history
of life on Earth been more affected by rare, high-magnitude events or frequent, lowmagnitude processes? Justify your choice.
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Geologic Time Concept Map, p. 241 (NOT REQUIRED, NOT EXTRA CREDIT)
#21: Complete the following concept map to evaluate your understanding of the
interactions between the Earth system and geologic time. Label as many interactions as
you can, using information from this chapter.
A
B
C
D
E
F
G
H
I
J
K
L
M
N
NEOs bombard early Earth, cause extinction events
Water evaporated to form gypsum
Humans use rocks to build pyramids
Salt in ocean from dissolved rocks; Pangaea reduced shallow marine conditions
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