Page 562 - 9780077418427.pdf

/Volume/201/MHDQ233/tat78194_disk1of1/0073378194/tat78194_pagefiles
tiL12214_ch21_521-540.indd Page 539 9/23/10 11:10 AM user-f465
tiL12214_ch21_521-540.indd Page 539 9/23/10 11:10 AM user-f465 /Volume/201/MHDQ233/tat78194_disk1of1/0073378194/tat78194_pagefile

Group A—Continued Group B—Continued
4. The absolute age of a rock is its age in years. The absolute age of 4. The absolute age of a rock is its age in years. The absolute age of
sedimentary rocks can be determined by dating igneous rocks sedimentary rocks can be determined by dating igneous rocks
that have a cross-cutting relationship with sedimentary rocks. that have a cross-cutting relationship with sedimentary rocks.
Suppose the basalt in Figure 21.13 is 210 million years old and Suppose there is additional evidence that indicates the basalt in
the lava flow is 189 million years old. What is the possible Figure 21.13 was intruded before the limestone and shale were
absolute age of the flat-lying sandstones? tilted. The basalt is 55.8 million years old, and the granite is
41.2 million years old. What is the possible absolute age at which
the limestone and shale were tilted?
5. The absolute age (age in years) of sedimentary rocks can be 5. The absolute age (age in years) of sedimentary rocks can be
determined by dating igneous rocks that have a cross-cutting determined by dating igneous rocks that have a cross-cutting
relationship with sedimentary rocks. Suppose the basalt in relationship with sedimentary rocks. Suppose the basalt in
Figure 21.13 is 56.3 million years old and the lava flow is Figure 21.13 is 73.2 million years old and the lava flow is
37.3 million years old. There are fossil shells in the sandstone. 64.9 million years old. There are dinosaur footprints in the
What is the age of the fossils in terms of the geologic time scale? sandstone. During what geologic period did the dinosaur leave
the footprints in the sandstone?
6. A sequence of rocks consists of a flat-lying shale, a basalt lava 6. A sequence of rocks consists of a flat-lying limestone, a
flow, then sandstone, shale, and limestone. The entire sequence is basalt sill, then sandstone, shale, and a lava flow. The lava flow
cut across by vertical basalt dike. The lava flow is 64.5 million is 22.9 million years old. The basalt sill is 19.8 million years old.
years old. The basalt dike is 61.2 million years old. What is the What is the possible absolute age of the sandstone and shale?
possible absolute age of the limestone and shale?
7. A student examining a roadcut recorded the following 7. A student examining a roadcut excavated through a mountain
observations: ridge recorded the following observations:

Event in Event in
Sequence Description Sequence Description

A Basalt dike that cuts events C and D A Normal fault that cuts across D and E
B Angular unconformity that cuts youngest layer of C B Basalt dike that cuts across A
C Tilted sequence of sandstone, shale, and C Erosion that cuts across B, A, D, and E
limestone that is 100 m thick D Reverse fault that cuts across E
D Flat-lying sequence of conglomerate, sandstone, E Sequence of sandstone, shale, and limestone
and shale that overlies B folded in a syncline
E Erosion of D that cuts across A
By applying the principles of reading history from rocks, arrange
By applying the principles of reading history from rocks, arrange the events, including the folding, in the correct order from oldest
the events, including the tilting, in the correct order from oldest to youngest.
to youngest.
8. A canyon has the following sequence exposed: 8. A cliff has the following sequence exposed:

Event in Event in
Sequence Description Sequence Description

A Shale and siltstone overlies B, contains Triassic A Normal fault that cuts across C and E
dinosaur bones
B Unconformity that cuts C and D
B Unconformity that cuts C and D
C Folded sedimentary rocks, Pennsylvanian ferns in
C Granite intrusion, 375 million years old, cuts the uppermost layer
across D
D Reverse fault that cuts across C
D Flat-lying sequence of conglomerate, sandstone, E Flat sedimentary rocks with Jurassic dinosaur
and shale
footprints over B
E Basalt dike, 240 million years old, cuts across C,
D, and A
Arrange the events in the correct order from oldest to youngest,
Arrange the events in the correct order from oldest to youngest, and determine the oldest possible geologic time scale age for the
and determine the geologic period(s) missing from the record normal fault.
due to the unconformity.
9. Laboratory analysis of a crystal of the mineral zircon that formed 9. Analysis of a stony-iron meteorite indicated 2 moles of
when a rock was metamorphosed indicated 2 moles of uranium-235 uranium-238 and 2 moles of lead-206. What is the age of the
and 6 moles of lead-207 in the crystal. When did metamorphism meteorite? (Refer to chapter 10 for the definition of a mole.)
occur? (Refer to chapter 10 for the definition of a mole.)

21-19 CHAPTER 21 Geologic Time 539