Earth Science 11; Geologic Time Unit Review
Multiple Choice
1) An unconformity is a buried ________.
A) fault or fracture with older rocks above and younger rocks below
B) surface of erosion separating younger strata above from older strata below
C) fault or fracture with younger strata above and older strata below
D) surface of erosion with older strata above and younger strata below
Answer: B
2)
The white arrow in this cross-section points to an unconformity (black line). What kind of
unconformity is it?
A) disconformity
B) angular unconformity
C) nonconformity
D) dike
Answer: B
3) A datable mineral crystal contains 40% parent isotope. How many half-lives have passed
since the crystal formed?
A) 1
B) more than 1 but less than 2
C) more than 2 but less than 3
D) more than 3
Answer: B
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4) Which of the following geologic observations would NOT bear directly on working out the
sequence of geologic events in an area?
A) inclusions of sandstone in a granite pluton
B) a well-exposed dike of basalt in sandstone
C) an unconformity between a granite and sandstone
D) the feldspar and quartz contents of a granite
Answer: D
5) The principle of ________ is the concept that ancient life forms evolved in a definite order
and therefore their fossils can help determine the geologic ages of strata.
A) superposition
B) cross-cutting relationships
C) fossil succession
D) fossil assemblage
Answer: C
6) Which of the following is an essential characteristic of an index fossil?
A) The organism lived in a very specific environment or two, such as tropical volcanic islands.
B) The organism lived for a very limited period of geologic time.
C) The fossils of the organism are exceptionally well preserved.
D) The organism lived on the land, not in the ocean.
Answer: B
7) Using the known rates of radioactive decay to date the crystallization of certain minerals is
known as ________ dating.
A) conventional
B) radiometric
C) relative
D) fossil assemblage
Answer: B
8)
Use the geological cross-section to determine
the age of rock layer D as precisely as
possible.
A) It is 40 million years old.
B) It is between 39 million years old and 25
million years old.
C) It is between 39 million years old and 17
million years old.
D) It is older than 17 million years old.
Answer: C
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9) Sandstone strata and a mass of granite are observed to be in contact. Which of the following
statements is correct?
A) The sandstone is younger if the granite contains sandstone inclusions.
B) The sandstone is younger if it shows evidence of contact metamorphism.
C) The granite is older if the sandstone contains pebbles of the granite.
D) The granite is older if it contains inclusions of sandstone.
Answer: C
10) Which is most useful for determining the age of the raisins in raisin bread, relative to the
bread in which they are found?
A) principle of superposition
B) principle of original horizontality
C) principle of cross-cutting relationships
D) principle of inclusions
Answer: D
True or False
1) The process by which geologists identify and match sedimentary strata and other rocks of the
same ages in different areas is called superposition.
Answer: FALSE
2)
Assuming that the image shows an undisturbed sedimentary sequence
(that is, superposition is valid), and that unit G is Cambrian in age, unit
F could be Ordovician.
Answer: TRUE
3) A granite dike cuts across a shale. The dike gives a radiometric date
of 84 million years old. The shale was therefore deposited more than
84 million years ago.
Answer: TRUE
4) Sedimentary layers can be important indicators of past
environmental conditions that existed when the sediments were
deposited.
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5)
According to the figure, the age of the Dakota Sandstone must be less than 160 million years,
and greater than 66 million years old.
Answer: TRUE
Fill in the Blank
1) The idea of ________ suggests that the physical, chemical, and biological laws that operate
today have also operated in the geologic past.
Answer: uniformitarianism
2) All else being equal, in a horizontal sequence of sedimentary rock layers, each higher bed is
younger than the bed below it. This idea is the principle of ________.
Answer: superposition
3) A lava flow includes fragments of limestone in it. Which rock unit is older, the lava flow or
the limestone? ________
Answer: limestone
4) The process by which geologists identify and match sedimentary strata and other rocks of the
same ages in different areas is ________.
Answer: correlation
5) The amount of time it takes for half of the remaining radioactive isotopes to decay is the
________ of a isotope system.
Answer: half-life
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6)
Examine the figure that shows the age ranges of some fossil groups. If trilobites ranged from the
Cambrian to the Permian, and ferns ranged from the Devonian until the present day, then rock
unit B must have been deposited ________.
Answer: between the Devonian and the Permian
7) A sandstone is found to overlie a shale. The shale contains a volcanic ash layer that has been
radiometrically dated to 84 million years ago. Both the shale and the sandstone are cut by a
pluton of gabbro that has been dated to 60 million years old. How old is the sandstone layer?
________
Answer: between 84 and 60 million years old
8) Would a pigeon or a penguin make a better index fossil for the present day? ________
Answer: pigeon
9) After three half-lives, ________ of an original, radioactive, parent isotope remains and the rest
has decayed into the daughter isotope.
Answer: 12.5% or one-eighth (1/8)
10) A(n) ________ is an erosional unconformity with parallel beds or strata above and below.
Answer: disconformity
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Labelling
1) Identify and label the three types of unconformities shown in this cross-section of the Grand
Canyon.
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Answer:
Short Answer
1)
What sequence of events would have to occur in order to produce the unconformity shown here?
Answer: First the lower units (purple/white) would have had to have been deposited as
horizontal layers. Then they got folded. The top got eroded off by some amount. A layer of
pebbles was deposited atop this eroded surface (making it an angular unconformity), and then
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some shale or other fine-grained sediment was deposited on top of that. Finally, I suppose,
something had to erode or cut into all three layers and the unconformity to expose the crosssectional view we see here.
2)
Examine the geological cross-section and describe the order of all the geological events that can
be depicted from this image. Don't limit yourself to merely the lettered rock units; include
episodes of folding and erosion, too. Use descriptive geologic verbs like "deposition" and
"intrusion" rather than simply saying the "rock unit J was formed."
Answer: The blue rock units labeled C are the oldest. These strata would have originally been
deposited horizontally, and then were folded, presumably during a compressional episode of
mountain building. A period of erosion followed, and the ancient mountain belt was ground
down to its roots. Deposition of the strata of unit B followed next, so the contact between C and
B is an angular unconformity. Sometime later, igneous unit A intruded into both C and B as
magma. The aureole of contact metamorphism that surrounds A shows that A must be younger
than the units it cuts across. The surface of B was subject to erosion, and unit D filled a large
concave gouge that was cut downward into B, perhaps an ancient river channel? (Technically,
we cannot tell whether unit D was incised into B first, or whether unit A intruded first, because
they have no direct or indirect relationship to one another.) Unit igneous F then cut across all
units on its way up to the surface as a dike. Note that dike F has baked the units it cuts across (A,
B, and D) with contact metamorphism. When it got to the surface, it erupted as a volcano. This
volcano, as well as units B and D, was eventually buried underneath another batch of strata. The
various layers of E were deposited next, covering everything that came before. Finally, the fault
marked "G" cut through, offsetting portions of B, D, and E with apparent normal dip-slip motion.
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3)
i) Is rock unit F likely to be sedimentary, metamorphic, or igneous?
Answer: igneous
ii) Which rock unit is most likely to have undergone regional metamorphism?
Answer: C
iii) If numerical dating of rock unit A gave a crystallization age of 1.2 billion years, and
numerical dating of rock unit F gave a crystallization age of 500 million years, then how old is
rock unit E?
Answer: Rock unit E would have to be younger than F, so it's younger than 500 million years
old. The information about A's age is superfluous.
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