Geologic Time
Basic
Standard 1.1.2
1. The geologic time scale is a record of
a. the thickness of sedimentary rock layers. b. the rate of fossil formation. c. the life forms and
geologic events in Earth’s history. d. the time since the evolution of dinosaurs.
2. The Mesozoic Era is divided into the Triassic, the Jurassic, and the Cretaceous. These
divisions of the geologic time scale are called
a. periods. b. eras. c. epochs. d. decades.
Advanced
3. Define the geologic time scale and explain why geologists use it to show Earth’s history.
Basic
Standard 1.2.1
4. What is a fossil?
a. any dead organism b. the preserved remains or traces of an organism c. a perfect copy of a
rock d. an unusual type of rock
5. Which type of fossil can provide clues about the activities of ancient organisms?
a. trace fossils b. remains preserved in amber c. carbon films d. molds and casts
Advanced
6. A fossil formed when minerals replace all or part of an organism is called a
a. mold. b. petrified fossil. c. cast. d. trace fossil.
b.
7. Describe what scientists can learn about the past by studying fossils.
Basic
Advanced
Standard 1.2.3
8. The relative age of a rock is
a. its age compared to the ages of other rocks. b. less than the age of the fossils the rock
contains. c. the number of years since the rock formed. d. its age based on how much
carbon-14 the rock contains.
9. The law of superposition states that, in horizontal layers of sedimentary rock, each layer is
a. younger than the layer above it and older than the layer below it. b. neither older nor younger
than the other layers. c. older than the layer above it and younger than the layer below it.
d. always older than any vertical layers.
10. What is the age of an intrusion of igneous rock in relation to the sedimentary rock layers
through which it passes?
a. sometimes younger, sometimes older b. always older c. the same age as the other rock layers
d. always younger
11. The fossils of organisms that were widely distributed but only lived during a short period of
time are called
a. trace fossils. b. petrified fossils. c. index fossils. d. carbon film fossils.
12. Why are index fossils useful to geologists?
a. They tell the absolute age of the rock in which they occur. b. They tell the ages of many
different rock layers. c. They tell the age of the rock at one location only. d. They tell the
relative age of the rock in which they occur.
13. Which sedimentary rock layer is older, A or B? Explain your answer and name the scientific
law that you applied to reach your conclusion.
14. List layers E, D, and C in order from oldest to youngest.
15. G began as magma that hardened underground to form igneous rock. What is the age of
G in relation to C? What term applies to bodies of rock such as G?
Standard 4.1.2
16. Contrast the relative and absolute age of a rock.
17.
17. Radioactive dating enables geologists to determine
a. the age of the atoms in a rock. b. the half-life of a fossil organism. c. the relative ages of
rocks. d. the absolute ages of rocks.
18. The time it takes for half of the radioactive atoms in a sample of a radioactive element to
decay is the element’s
a. relative age. b. potassium-argon date. c. absolute age. d. half-life.
19. Radioactive decay occurs when atoms of an unstable element
a. become part of a fossil. b. join with atoms of another element. c. break down to form atoms
of another element. d. are exposed to chemical weathering.
Use the diagram to answer each question.
20. Strontium-90 is a radioactive form of the element strontium that undergoes radioactive decay.
The graph shows the decay of strontium-90 over time. What is the half-life of strontium-90?
Explain your answer.
21. Strontium-90 is a radioactive form of the element strontium that undergoes radioactive decay.
The graph shows the decay of strontium-90 over time. Would strontium-90 be useful for
determining the age of fossils? Explain your answer.
Standard 4.1.3
22. How do most fossils form?
a. Living things die and their remains are buried by sediments. b. The hard parts of an organism
dry out in the air. c. The soft parts of an organism change to stone. d. Freezing preserves
the remains of an organism.
Geologic Time
Answer Section
MULTIPLE CHOICE
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
C
A
B
C
D
A
B
A
C
D
D
D
C
A
SHORT ANSWER
15. B is older. According to the law of superposition, each layer of horizontal sedimentary rock is older than
the layer above it. A is above B, therefore, B is older.
16. C, E, D
17. G is younger than C; intrusion
18. The half-life is 28 years, or the time it takes for one half (50%) of the original amount of strontium-90 to
decay.
19. No. Fossils can be many thousands or millions of years old. Strontium-90’s half-life is so short that the
amount left undecayed in a fossil would be too small to measure accurately.
ESSAY
20. Scientists can learn what organisms were like in the past, including information about their appearance,
food, and activities. Fossils can be used to determine the relative ages of the rock layers in which they
occur. By studying fossils, scientists also can learn how organisms, Earth’s surface, environments, and
climate have changed over time.
21. The geologic time scale is a record of the life forms and geologic events in Earth’s history. The units that
normally are used to measure time are months, years, and centuries. But Earth has existed for billions of
years. The geologic time scale is broken into large units that span Earth’s long history.
22. The relative age of a rock layer is its age compared to that of another rock layer. The absolute age is the
number of years ago that the rock layer formed.
Basic
Advanced