GLG 103 Lab 9: Geologic Time | Questions
and Answers | 2026/2027 Updated | Arizona
State University.
Practice Question Bank & Answer Rationale
THIS PACK CONTAINS
50 exam-style multiple-choice questions. Each question includes the correct answer and a complete rationale
for focused revision.
COVERAGE
This lab exam assesses mastery of relative and absolute dating, stratigraphic principles, unconformities,
fossils, and the geologic time scale. Students interpret rock sequences and radiometric data to reconstruct
Earth history. Aligned with the 2026/2027 ASU GLG 103 Lab Manual and ICS 2026 standards.
STUDY GUIDE
Recommended duration: 1.5 hours. Passing target: 70%.
HIGHP - Page 1 of 18
,1. In a sequence of sedimentary rocks, a dike cuts across both a sandstone layer and an
overlying shale layer. The dike is truncated by an erosional surface overlain by
conglomerate. Which principle best establishes that the dike is older than the
conglomerate?
[ ] A. Principle of superposition
[ ] B. Principle of cross-cutting relationships
[ ] C. Principle of original horizontality
[ ] D. Principle of faunal succession
CORRECT: B. Principle of cross-cutting relationships
The dike cross-cuts the sandstone and shale, so it must be younger than both. Since the dike is truncated by
the erosion surface, the conglomerate deposited above must be younger than the dike. Cross-cutting
relationships establish the relative timing of the dike and the conglomerate.
2. A zircon crystal from an ash bed yields a 206Pb/238U ratio of 0.35. Assuming no initial
Pb and a 238U half-life of 4.47 Ga, what is the approximate age of the ash bed? (ln(1.35) =
0.300)
[ ] A. 1.93 Ga
[ ] B. 2.58 Ga
[ ] C. 3.22 Ga
[ ] D. 4.47 Ga
CORRECT: A. 1.93 Ga
Using the decay equation t = (1/) ln(1 + D/P), where = ln2/4.47 Ga = 0.155 Ga¹. t = (1/0.155) × ln(1.35) =
6.45 × 0.300 1.93 Ga. Other options misapply the half-life or use incorrect ratios.
3. Which unconformity type is characterized by sedimentary rocks overlying crystalline
igneous or metamorphic rocks, with no weathered horizon preserved?
[ ] A. Angular unconformity
[ ] B. Disconformity
[ ] C. Nonconformity
[ ] D. Paraconformity
CORRECT: C. Nonconformity
A nonconformity is a surface separating sedimentary rocks from underlying igneous or metamorphic rocks.
Angular unconformity involves tilted layers, disconformity is between parallel sedimentary layers, and
paraconformity is a subtle disconformity with no visible erosion.
HIGHP - Page 2 of 18
, 4. A rock unit contains fossils of the trilobite genus Olenellus. Based on the ICS 2026 time
scale, this unit is most likely from which period?
[ ] A. Cambrian
[ ] B. Ordovician
[ ] C. Silurian
[ ] D. Devonian
CORRECT: A. Cambrian
Olenellus is a key index fossil for the Early Cambrian. It is not found in Ordovician or later periods. The ICS
2026 time scale places Olenellus in the Cambrian, making it a reliable indicator.
5. A sequence shows horizontal limestone overlain by horizontal shale, with a distinct
erosional surface between them. Which term best describes this contact?
[ ] A. Angular unconformity
[ ] B. Nonconformity
[ ] C. Disconformity
[ ] D. Conformity
CORRECT: C. Disconformity
A disconformity is an erosional surface between parallel sedimentary layers. Angular unconformity involves
tilted layers, nonconformity involves igneous/metamorphic rocks, and conformity implies continuous
deposition.
6. Which of the following best explains why the principle of faunal succession is useful for
correlating rock units across wide distances?
[ ] A. Fossils are always preserved in the same rock type.
[ ] B. Organisms evolve over time, so fossils change in a predictable order.
[ ] C. Fossils can be dated directly by radiometric methods.
[ ] D. Fossils are only found in marine rocks.
CORRECT: B. Organisms evolve over time, so fossils change in a predictable order.
Faunal succession relies on the fact that organisms evolve and change through time, allowing fossils to be
used as time indicators. This enables correlation even when rock types differ. The other options are incorrect
generalizations.
7. A radiometric date from a lava flow yields 1.2 Ga. The flow lies between two
sedimentary layers. What can be concluded about the age of the sedimentary layers?
[ ] A. Both sedimentary layers are older than 1.2 Ga.
[ ] B. Both sedimentary layers are younger than 1.2 Ga.
[ ] C. The lower sedimentary layer is older than 1.2 Ga, and the upper is younger.
HIGHP - Page 3 of 18
and Answers | 2026/2027 Updated | Arizona
State University.
Practice Question Bank & Answer Rationale
THIS PACK CONTAINS
50 exam-style multiple-choice questions. Each question includes the correct answer and a complete rationale
for focused revision.
COVERAGE
This lab exam assesses mastery of relative and absolute dating, stratigraphic principles, unconformities,
fossils, and the geologic time scale. Students interpret rock sequences and radiometric data to reconstruct
Earth history. Aligned with the 2026/2027 ASU GLG 103 Lab Manual and ICS 2026 standards.
STUDY GUIDE
Recommended duration: 1.5 hours. Passing target: 70%.
HIGHP - Page 1 of 18
,1. In a sequence of sedimentary rocks, a dike cuts across both a sandstone layer and an
overlying shale layer. The dike is truncated by an erosional surface overlain by
conglomerate. Which principle best establishes that the dike is older than the
conglomerate?
[ ] A. Principle of superposition
[ ] B. Principle of cross-cutting relationships
[ ] C. Principle of original horizontality
[ ] D. Principle of faunal succession
CORRECT: B. Principle of cross-cutting relationships
The dike cross-cuts the sandstone and shale, so it must be younger than both. Since the dike is truncated by
the erosion surface, the conglomerate deposited above must be younger than the dike. Cross-cutting
relationships establish the relative timing of the dike and the conglomerate.
2. A zircon crystal from an ash bed yields a 206Pb/238U ratio of 0.35. Assuming no initial
Pb and a 238U half-life of 4.47 Ga, what is the approximate age of the ash bed? (ln(1.35) =
0.300)
[ ] A. 1.93 Ga
[ ] B. 2.58 Ga
[ ] C. 3.22 Ga
[ ] D. 4.47 Ga
CORRECT: A. 1.93 Ga
Using the decay equation t = (1/) ln(1 + D/P), where = ln2/4.47 Ga = 0.155 Ga¹. t = (1/0.155) × ln(1.35) =
6.45 × 0.300 1.93 Ga. Other options misapply the half-life or use incorrect ratios.
3. Which unconformity type is characterized by sedimentary rocks overlying crystalline
igneous or metamorphic rocks, with no weathered horizon preserved?
[ ] A. Angular unconformity
[ ] B. Disconformity
[ ] C. Nonconformity
[ ] D. Paraconformity
CORRECT: C. Nonconformity
A nonconformity is a surface separating sedimentary rocks from underlying igneous or metamorphic rocks.
Angular unconformity involves tilted layers, disconformity is between parallel sedimentary layers, and
paraconformity is a subtle disconformity with no visible erosion.
HIGHP - Page 2 of 18
, 4. A rock unit contains fossils of the trilobite genus Olenellus. Based on the ICS 2026 time
scale, this unit is most likely from which period?
[ ] A. Cambrian
[ ] B. Ordovician
[ ] C. Silurian
[ ] D. Devonian
CORRECT: A. Cambrian
Olenellus is a key index fossil for the Early Cambrian. It is not found in Ordovician or later periods. The ICS
2026 time scale places Olenellus in the Cambrian, making it a reliable indicator.
5. A sequence shows horizontal limestone overlain by horizontal shale, with a distinct
erosional surface between them. Which term best describes this contact?
[ ] A. Angular unconformity
[ ] B. Nonconformity
[ ] C. Disconformity
[ ] D. Conformity
CORRECT: C. Disconformity
A disconformity is an erosional surface between parallel sedimentary layers. Angular unconformity involves
tilted layers, nonconformity involves igneous/metamorphic rocks, and conformity implies continuous
deposition.
6. Which of the following best explains why the principle of faunal succession is useful for
correlating rock units across wide distances?
[ ] A. Fossils are always preserved in the same rock type.
[ ] B. Organisms evolve over time, so fossils change in a predictable order.
[ ] C. Fossils can be dated directly by radiometric methods.
[ ] D. Fossils are only found in marine rocks.
CORRECT: B. Organisms evolve over time, so fossils change in a predictable order.
Faunal succession relies on the fact that organisms evolve and change through time, allowing fossils to be
used as time indicators. This enables correlation even when rock types differ. The other options are incorrect
generalizations.
7. A radiometric date from a lava flow yields 1.2 Ga. The flow lies between two
sedimentary layers. What can be concluded about the age of the sedimentary layers?
[ ] A. Both sedimentary layers are older than 1.2 Ga.
[ ] B. Both sedimentary layers are younger than 1.2 Ga.
[ ] C. The lower sedimentary layer is older than 1.2 Ga, and the upper is younger.
HIGHP - Page 3 of 18
