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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

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.

[pic]

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 |

| |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

[pic]

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.

[pic]

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. 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

[pic]

.

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 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:

100 yards = 4,600 million years

10 yards = 460 million years

1 yard = 46 million years

1 foot = 15.3 million years

1 inch = 1.3 million year

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 |

| |(million years) | |

|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 |

(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)

#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 238U/206Pb isotopes.

[pic]

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

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, low- magnitude processes? Justify your choice.

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 NEOs bombard early Earth, cause extinction events

B

C

D Water evaporated to form gypsum

E

F

G

H Humans use rocks to build pyramids

I

J

K Salt in ocean from dissolved rocks; Pangaea reduced shallow marine conditions

L

M

N

-----------------------

Youngest

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

Oldest

5 billion

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