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Practice Questions for Lectures 8
Geologic Time
Geology 1200
A. Short answer:
1. The principles of relative dating were published in 1669 by Nicolaus
____________.
2. Tilted outcrops of turbidites, the deposits of sediment avalanches that fall
off of the continental slope, can be oriented right-side-up using observations
of their sedimentary structures called __________ bedding.
3. Preparing descriptive diagrams of isolated rock outcrops and their fossils
is called measuring section. These are useful in determining equivalence in
age of sediments, a process called _________________.
4. A fault that cuts across sediments is ____________ than the sediments.
5. Whenever we see folded sediments, we know that the sediments were
deposited _____________ the folding occurred.
6. Paradoxides is a Cambrian trilobite which lived for a relatively short
amount of time but was widespread and abundant. It is a good example of an
_________ fossil, one which is useful in correlation.
7. Whenever we see Mesozoic dinosaur skeletons in sediments underneath
sediments containing Paleozoic trilobites, we know that these beds are
________________.
8. Whenever we see Ordovician trilobites in a horizontal layer just beneath
another horizontal layer containing Devonian trilobites, we know that some
time is missing, and we call the boundary between them a _____________.
9. The lake deposits up in Pompton Lakes, NJ would be a good place to
study the chronology of changes in local plants using fossil pollen in the
muds, since the _________ give a reliable and inexpensive way to date the
changes.
10. The peats and wood in a Quaternary bog deposit could be dated using
___________-______ radiometric methods.
B. Match the Terms.
1. half-life _____
a. youngest on top
2. index fossil ______
b. Gave us principles of relative dating
3. absolute date ____
c. it is younger than sediments
4. superposition____
d. determined radiometrically
5. tilted turbidites____
e. widespread and brief geologic span
6. Steno _____
f. determine equivalence in age
7. dike cross cuts shale _____
g. better use graded bedding
8. unconformity ___
h. fossils have some original tissue
9. correlation ______
i. half of parent isotope decays
10. amber _____
j. lakes with cold winters
11. varves
k. some time is missing
_____
C. True or False? Circle the correct answer.
1. A dike is older than a stack of sediments it cross-cuts. True or
False?
2. Most sediments are deposited in horizontal layers. True or False?
3. The Principle of Superposition says that the oldest sediments are on
top. True or False?
4. Comparing fossils in rocks is part of the processes of relative dating
and correlation. True or False?
5. Determining an actual age for a rock unit using radiometric dating
is called absolute dating. True or False?
6. Sediments tend to be deposited in horizontal layers. True or False?
7. According to the principle of cross-cutting relationships, if a fault is
found cutting across sedimentary strata, the reasonable conclusion
would be that the fault is younger than the sedimentary strata. True or
False?
8. According to the principle of inclusions, if a rock is found
embedded within an igneous pluton, the reasonable conclusion would
be that the rock is older than the pluton. True or False?
9.Sedimentary rocks are more likely to contain fossils than
metamorphic rocks. True or False?
10.A marine clam is less likely to be fossilized than a marine worm.
True or False?
D. Multiple choice:
1. Of the following organisms, the best index fossils would be:
(a) cockroaches, because they have existed in widespread geographic
areas almost since the first appearance of insects.
(b) bacteria, because they have existed in widespread geographic areas
since life first began.
(c) a specific type of clam that existed for only a short geologic time in
very limited geographic areas.
(d) a specific type of trilobite that existed for only a short geologic time in
widespread geographic areas.
2. All of the following examples illustrate preservation of some original
parts in fossils EXCEPT:
(a) preservation in tar pits.
(b) preservation in frozen permafrost.
(c) preservation in marine shales.
(d) preservation in hardened tree sap.
3. Which of the following statements about unconformities is NOT true?
(a) No single place on Earth contains all of the rock strata composing the
entire geologic record.
(b) Unconformities occur when plate-tectonic forces cause sediments to
be deposited at an angle instead of horizontally.
(c) Virtually every sequence of sedimentary rock layers contains one or
more unconformities.
4. All of the following represent examples of unconformities EXCEPT:
(a) horizontal sedimentary strata overlying crystalline rocks.
(b) horizontal sedimentary strata overlying tilted sedimentary strata.
(c) tilted sedimentary strata offset by a fault.
(d) horizontal sedimentary strata overlying eroded horizontal sedimentary
strata.
5. Correlation of rock units is used to determine:
(a) which geographically distant rock units were formed under
approximately the same environments.
(b) which geographically distant rock units are the same type of rock.
(c) the ages of similar fossils found in geographically distant rocks.
(d) which geographically distant rock units are approximately the same
age.
6. The half-life of a radioactive isotope refers to:
(a) one-half of the total lifespan of a parent isotope.
(b) the time it takes for the ratio of parent to daughter isotopes to be 1:2
(half as many parent isotopes as daughter isotopes).
(c) the time it takes for the ratio of parent to daughter isotopes to be 2:1
(twice as many parent isotopes as daughter isotopes).
(d) the time it takes for half of the parent isotopes to decay to daughter
isotopes.
6. If the half-life of a radioactive isotope is 1 billion years, and the parent
isotope atoms are now 25% of the total, the age of the sample would be
about:
(a) 1 billion years old.
(b) 2 billion years old.
(c) 3 billion years old.
(d) 4 billion years old.
7. If a mineral sample is dated using the potassium-argon dating method,
and if some of the argon-40 gas has escaped, the mineral’s age:
(a) will appear to be older than it really is.
(b) will appear to be younger than it really is.
(c) will not be affected.
(d) can be determined by measuring the quantity of parent material
present.
8. All of the following radiometric dating methods are useful for rocks
older than 100,000 years EXCEPT:
(a) uranium-lead dating.
(b) potassium-argon dating.
(c) rubidium-strontium dating.
(d) carbon-14 dating.
9. Which of the following statements about carbon-14 dating is NOT
true?
(a) Cosmic bombardment changes the rate of production of carbon-14.
(b) Rocks dated by the carbon-14 method are generally in the 100- to
70,000-year age range.
(c) The bodies of organisms take in carbon-14 throughout their lives and
after death until they are isolated from the atmosphere by burial.
(d) Carbon-14 dates are determined by measuring the ratio of carbon-14
to nitrogen-14.
10. Which of the following statements about varve chronology is NOT
true?
(a) Varves are more apparent in lakes that freeze in winter than in those
that do not.
(b) Varve layers are examined in cores extracted from the bottom of a
lake.
(c) Summer sediment layers are dark and thin, while winter sediments are
light and thick.
(d) Varves can be used to date events near the lake, such as vegetation
changes recorded by pollen preserved in the lakes' muds.
11. The numerical (absolute) age of fossiliferous marine strata could best
be determined by using:
(a) radiometric dates from nearby igneous intrusions.
(b) the temperature-time relationships of nearby metamorphic rocks.
(c) radiometric dates from interbedded layers of volcanic ash.
(d) weathering rinds of fossil-bearing sandstone concretions.
12. The proper order of the divisions of the geologic time scale from
largest to smallest is:
(a) eon, epoch, period, era.
(b) eon, era, period, epoch.
(c) era, eon, epoch, period.
(d) epoch, period, eon, era.
13. When it was first developed, the divisions of the geologic time scale
were based primarily on:
(a) differences in types of rocks.
(b) differences in sedimentary rock structures.
(c) differences in minerals.
(d) differences in fossil assemblages.
14. Which of the following are believed to have been an important cause
of mass extinction?
(a) volcanic flood basalts.
(b) ice ages.
(c) meteorite impact.
(d) All of the above.
15. We believe the age of the Earth to be about 4.6 billion years old. This
theory is confirmed by all of the following EXCEPT:
(a) radiometric dating of meteorites that have struck the Earth.
(b) radiometric dating of rocks found on the Moon.
(c) radiometric dating of the oldest rocks found on Earth.
E. Short Answer (Answer all questions)
1. Discuss how unconformities develop and describe the three types
of unconformities. Use sketches.
2.
Explain the means by which correlation of geographically
separated sedimentary rock units is accomplished. Use
sketches to explain the use of overlapping ranges of index
fossils.
3.
Describe the basic principles of radiometric dating. Use
sketches.