Ch. 21 - Fossils & the Rock Record Study Guide
Modified True/False - Indicate whether the statement is true or false.
If false, change the identified word or phrase to make the statement true.
____ 1. Eons are longer units of time than Eras.
____ 2. The Precambrian consists of the three earliest eons of geologic time.
____ 3. Intrusions are younger than the rocks they cut across.
____ 4. An unconformity between two sedimentary layers is classified as a disconformity.
____ 5. The radioactive decay scale divides the history of Earth into time units that help geologists analyze the rock record.
____ 6. The remains of once living plants or animals are called varves.
____ 7. After one half life, the ratio of parent atoms to daughter atoms in a sample is 1:2.
____ 8. According to the principle of superposition, the oldest rock layers are closest to the surface.
____ 9. Dendrochronology is the science of comparing annual growth rings in trees to date changes in Earth's environments and past events.
____ 10. Present time is part of the Mesozoic era.
Multiple Choice - Identify the choice that best completes the statement or answers the question.
____ 11. Index fossils are useful to geologists if the fossils ____.
a. / have lived over a short period of timeb. / are not easily recognized
c. / are not widely distributed geographically
d. / are scarce
____ 12. The amount of time it takes for one-half of the original amount of an isotope to decay is known as its ____.
a. / correlation / c. / periodb. / half-life / d. / radioactivity
____ 13. What classification of unconformity occurs when sedimentary rock overlies igneous rock?
a. / disconformity / c. / angular unconformityb. / nonconformity / d. / conformity
____ 14. An example of a trace fossil includes a ____.
a. / cast of a clam / c. / mud crackb. / worm trail / d. / raindrop impression
____ 15. Radiocarbon dating can be used to determine the age of what type of materials?
a. / igneous rock / c. / organic materialsb. / living organisms / d. / lava flows
____ 16. What characteristic of radioactive decay makes it useful for determining the absolute age of samples?
a. / occurs at a constant rateb. / changes based on temperature
c. / speeds up under high pressure
d. / slows down in very dense samples
____ 17. Which of the following is used to determine the absolute age of a rock?
a. / relative-age dating / c. / permineralizationb. / correlation / d. / radiometric dating
____ 18. Which of the following is marked by the appearance of organisms with hard parts?
a. / the beginning of the Cenozoic / c. / the end of the Mesozoicb. / the end of the Precambrian / d. / the beginning of the Cretaceous
____ 19. The principle of superposition can be used to determine .
a. / the actual age of these rock layersb. / the relative densities of these rock layers
c. / the actual temperatures of these rock layers
d. / the relative age of these rock layers
____ 20. The oldest rock layer in an undisturbed rock sequence occurs ____.
a. / at the bottom of the sequence / c. / below the unconformityb. / below the sedimentary rock layer / d. / at the top of the sequence
____ 21. Molds, casts, coprolites, and petrified wood are all example of ____.
a. / fossils / c. / unconformitiesb. / radioactive isotopes / d. / disconformities
____ 22. Which of the following lists the units of geologic time in order from shortest to longest?
a. / era, eon, period, epoch / c. / eon, era, period, epochb. / epoch, eon, period, era / d. / epoch, period, era, eon
____ 23. On the geologic time scale, the smallest unit of time is called a(n) ____.
a. / era / c. / periodb. / eon / d. / epoch
Completion - Complete each statement using the words below
Angular unconformity Correlation Cross-cutting relationships
Dendrochronology Nonconformity Superposition
24. The principle of ______states that, in an undisturbed sequence, the oldest rocks are at the bottom of the sequence and successive layers are younger than those below them.
25. The matching of rock layers from one geographic area with those of another area is known as ______.
26. The science of ______uses the annual growth of tree rings to date events and environmental changes.
27. You can use the principle of ______to infer that a fault or an intrusion is younger than the rock it cuts across.
28. A buried erosional surface between a nonsedimentary rock and a sedimentary rock is called a(n) ______.
29. The gap in the rock record that occurs between folded or uplifted rock layers and a sedimentary rock layer on top of them is called a(n) ______.
Short Answer
30. Identify the two types of unconformities in the diagram to the right and describe how they formed.
31. Identify the eon, era, period, and epoch in which you live.
Eon / Era / Period / EpochPhanerozoic / Cenozoic / Neogene / Holocene
Pleistocene
Pliocene
Miocene
Paleogene / Oligocene
Eocene
Paleocene
Mesozoic / Cretaceous
Jurassic
Triassic
Paleozoic / Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
Problem
The table lists commonly used radioactive isotopes and their half-lives. Study the table and answer the following questions.
Half-Lives of Selected Radioactive IsotopesRadioactive Isotope / Approximate Half-Life / Decay Product
Rubidium-87 / 48.6 billion years / Strontium-87
Thorium-232 / 14.0 billion years / Lead-208
Potassium-40 / 8.4 billion years / Argon-40
Uranium-238 / 4.5 billion years / Lead-206
Uranium-235 / 0.7 billion years / Lead-207
Carbon-14 / 5730 years / Nitrogen-14
32. Which isotope would be most helpful in dating bones thought to be about 6000 years old?
33. Which stable element does Uranium-238 decay into?
34. Which isotope has the longest half-life?
Open Response Question – ANSWER ALL 4 PARTS OF THE QUESTION
USING COMPLETE SENTENCES
Fossils provide evidence of the past existence of a wide variety of life-forms, most of which are now extinct. Diversity of fossils provides evidence of how species have evolved. Evolution is simply a change in species over time. Rocks are about the same age as the fossils within them. Geologists can infer that same fossils found elsewhere are also the same age
A. What are fossils and how are they used to interpret Earth’s history?
B. Compare and contrast index fossils and trace fossils.
C. Describe how geologists use index fossils. List the characteristics that make an index fossil.
D. A scientist discovered a new variety of fossil that was very different from other fossils. The organism seems to have been very rare, and it fossil records indicate that it existed for a short period of time. Would this make a good index fossil? Why or why not?
Answer Open Response Question on back of ANSWER SHEET
Answer Section
Ch. 21 - Fossils & the Rock Record Study Guide
MODIFIED TRUE/FALSE
1. ANS: T
2. ANS: T
3. ANS: T
4. ANS: T
5. ANS: F, geologic time scale
6. ANS: F, fossils
7. ANS: F, 1:1
8. ANS: F, youngest
9. ANS: T
10. ANS: F, Cenozoic
MULTIPLE CHOICE
11. ANS: A
12. ANS: B
13. ANS: B
14. ANS: B
15. ANS: C
16. ANS: A
17. ANS: D
18. ANS: B
19. ANS: D
20. ANS: A
21. ANS: A
22. ANS: D
23. ANS: D
Ch. 21 - Fossils & the Rock Record Study Guide
COMPLETION
Ch. 21 - Fossils & the Rock Record Study Guide
24. ANS: superposition
25. ANS: correlation
26. ANS: dendochronology
27. ANS: cross-cutting relationships
28. ANS: nonconformity
29. ANS: angular unconformity
Ch. 21 - Fossils & the Rock Record Study Guide
SHORT ANSWER
30. ANS: An angular unconformity occurs below layer H. It formed when the layers below were uplifted and the surface was eroded, followed by the deposition of H. A disconformity occurs between layers J and K. It formed when J was eroded, followed by the deposition of K.
31. ANS:
Phanerozoic eon; Cenozoic era; Neogene period; Holocene epoch
32. ANS: Carbon-14
33. ANS: Lead-206
34. ANS: Rubidium-87
OPEN RESPONSE
A. Fossils are the remains or evidence of once-living plants or animals. They provide clues about Earth’s past environmental conditions and evolutionary changes in organisms over time. They also help to correlate rock layers from one area to another.
B. Index fossils are the remains of organisms that can be used by geologists to correlate rock layers over large geographic areas or to date rock layers. Trace fossils are not remains but indirect evidence of organisms, such as imprints, trails, and burrows.
C. Geologists use index fossils to correlate rock layers over large geographic areas or to date a particular rock layer. To be useful, an index fossil must be easily recognized, abundant, and widely distributed geographically. It must also have lived over a short period of time.
D. This fossil would not be a good index fossil, because the organism was rare. Index fossils must be abundant in order to be useful.