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GEOL101
Study Questions for Lesson 2: Geologic Time
Use these questions to evaluate your knowledge of the material to better target concepts you should study further. They can provide a personalized road map of what concepts you need to focus on. The following questions will help you review the MAJOR concepts and relationships you should understand and may not comprehensively cover every question on the exam for this unit. Many of the questions require synthesis and integrate several concepts; they will take you some time to complete. Therefore, you work through these questions as we cover them in class to allow enough time for you to identify areas you need to study in more detail. Trying to complete this study guide hours before the exam will be overwhelming and is not the best approach for concept mastery. You do not have to submit this for a grade. In addition to these questions, you
should review all assigned readings and videos, assignments, quizzes, and handouts.
Geologic Time: Relative Time
1.
How old is Earth?
4.543 billion years 2.
What is the difference between absolute and relative age? Give an example of each. How do they each play a role in the construction of the geologic time scale?
Relative age is the age of a rock layer compared to other layers, and can be determined by looking at the postion of rock layers. Absolute age is the numeric age of a layer of rocks or fossils 3.
Discuss the relevance of Hutton’s observations at Siccar Point, Scotland to the development of the principle of uniformitarianism. Known as the birthplace of modern geology, were unconformities there, which led to the principle of uniformitarianism. This principle says the present is key to the past, and that the processes that are acting today were operating in the past at the same rate. 4.
You will need to know the definitions of the following as well as have a working knowledge of how to apply
them to decipher a geologic history from a cross-section (consult the practice cross-sections done in lecture, the Geologic Time Worksheet, the Group Evaluation, and the examples at the end of this guide):
The principle of original horizontality
States that layers of sediment are originally deposited horizontally under the action of gravity – sloping of layers must have occurred after layers were formed “ “ of superposition
The oldest layer is at the base and the layers are progressively younger with ascending order in sequence “ “ cross-cutting relations (for igneous intrusions and faults)
Geologic feature which cuts another is younger of the two features “ “ fossil succession
is
a technique used to define the relative age of the fossil
. It identifies rock layers where the fossils are found and can place them in a chronological order according to rock strata.
“ “ of baked contacts
states that
the heat of an intrusion will bake (metamorphose) the rocks in close proximity to the intrusion
. Hence the presence of a baked contact indicates the intrusion is younger than the rocks around it.
5.
Describe how unconformities form. List 3 types of unconformities and what defines each of them. Draw each of them. You should also know how to identify them to decipher a geologic history from a cross-
section (consult the in-class assignment and the example at the end of this study guide).
Unconformities form by weathering and erosion which happens with a rise and fall in sea levels. Nonconformities(sedimentary over non sedimentary), Disconformities(sed over sed), and angular unconformities(Sed over sed but rocks below are at an angle) 6.
How did geologists reconstruct Earth’s history if there is not a single
stratigraphic section that represents it? How do fossils and lithology aid in this? Incorporate the terms marker bed and index fossil into your discussion. 7.
What characteristics distinguish an index fossil from a typical fossil?
every fossil tells us something about the age of the rock its found in but index fossils are those that are used to define periods of geologic time 8.
Describe how the geologic column was created and how it later became the geologic time scale
. You should incorporate relative and absolute time concepts in your answer.
Absolute Time
9.
Explain how relative and absolute dates are used in tandem to discern a geologic history of an area. They used relative dating to divide Earth's past in several chunks of time when similar organisms were on Earth. Later, scientists used absolute dating to determine the actual
number of years ago that events happened. The geologic time scale is divided into eons, eras, periods, and epochs.
10. Explain in your own words how isotopes allow geologists to calculate the numeric age of a mineral/rock. Incorporate the following terms correctly: parent isotope, daughter product, radioactive decay, half-life. Isotopes are important to geologists because
each radioactive element decays at a constant rate, which is unique to that element
. These rates of decay are known, so if you can measure the proportion of parent and daughter isotopes in rocks now, you can
calculate when the rocks were formed. Half-lives are how long it takes for half of a group of parent atoms to decay 11. Apply your understanding of radioactive decay and half-lives to plot the decay curves for parent and daughter isotopes on the following graph.
0
1
2
3
4
5
6
7
8
9
10
0
10
20
30
40
50
60
70
80
90
100
Parent and Daughter Isotopes in Radioactive Decay
Parent
Daughter
Number of Half Lives
% of Isotope
12. Examine the plot from the previous question and answer the following questions: why is radioactive decay exponential? Does this apply to both the parent and daughter isotopes? Why or why not?
Each atom has the same chance of decaying in a fixed time interval and this leads to an exponential decay
13. Discuss how a numeric age could be artificially too old or too young. 14. Describe how an isotopic system’s closure temperature can reflect the age of the mineral/rock. Discuss how multiple closure temperatures of different minerals and different isotopic systems in a single rock can yield information about the rock’s thermal evolution. Consult the assigned readings in our textbook for support.
The Anthropocene
15. What is the Anthropocene?
A new proposed division in geologic time scale 16. How do geologists (stratigraphers) define a new division on the geologic time scale? A new time scale must reflect global long term change and be marked by a identifiable boundary 17. What are some options for defining the start of the Anthropocene based on the assigned readings? Which
of these options has been agreed upon by the Anthropocene Working Group and is currently being searched for to designate the Global boundary Stratotype Section and Point (GSSP) a.k.a. “the golden spike”?
Does it make sense to define it at the 1945 event when we've been altering earth’s system many years before that
Issues
Finding an appropriate marker preserved
Scientific process
The effects of humans on the earth system prior to 1945
18. Discuss the issue social scientists have with the scientific use of “the Anthropocene” and how it’s at odds with how geologists (stratigraphers) propose to scientifically define “the Anthropocene”.
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Practice
19. Use the relative and absolute age principles to determine the geologic history of the cross section below. Remember periods of weathering and erosion should be included as an event. There are 16 events. Start with the oldest event and work forward. Use the clues below:
All units are sedimentary rocks except for J which is a granite and K which is a mafic dike (gabbro).
Unit B has fossils that are ~600 Ma.
Units C-G are Permian in age based on trilobite and brachiopod fossils.
Unit H has bivalves that are no older than 56 Ma (million years). _____ ____ ____ _____ _____ _____ ____ _____ _____ _____ _____ _____ ____ _____ _____ _____
20. Outline any angular unconformities in the above block diagram in red, any disconformities in blue, and any
nonconformities in green. 21. What principle did you use to determine if unit J or the succession of units A-G came first?
22. Was unit E deposited before K? What principle did you apply to determine this?
23. Do units A-G obey the rules of original horizontality and superposition? What happened to make them look the way they do now?
24. Based only
on relative age relationships, what are the age constraints of K? 25. You isolate zircons from unit K (gabbro) to run 235
U-
207
Pb analysis. In one zircon you measure 25 grams of 235
U and 75 grams of 207
Pb. Assuming no isotopes were lost or added to the zircon, how old is K? (Hints: consult Figure 10.12 and Table 10.1 in the textbook; if this is still difficult, first complete the #3 Practice Question below and check your answer in the answer key.) Does this age make sense with your geologic history, the relative age relationships, and the fossil ages? Why or why not?
26. Shade the areas that have baked contacts.
Vocabulary Practice
Practice Questions
Multiple Choice: More than one answer can be correct.
1.
In the previous cross section, which rock is best to isotopically date to yield its age of formation
?
A. J (granite)
B. F (shale)
C. G (sandstone)
D. D (shale)
E. A (limestone)
2.
Anthropocene Working Group maintains the following 2 criteria must be satisfied to define the base of the Anthropocene. The boundary must be:
A. identified by both natural scientists and social scientists.
B. pre-1945 and reflect the gradual impact of humans on the Earth System.
C. a global, identifiable boundary.
D. result from global, geologically long-term change.
E. supported using a deductive approach (“top down” vs. “bottom up”). 3.
You’re doing 235
Ur-
207
Pb analysis on zircons from a felsic lava flow. The half-life of the system is 713 Ma. You measure 12.5 grams of 245
Ur and 87.5 grams of 207
Pb. What’s the age of the lava flow? A. 237.7 Ma
B. 356.5 Ma
C. 713 Ma
D. 1.43 Ga
E. 2.14 Ga
4.
A radiogenic isotope of 40
K has a half-life of 1.3 Ga. A sill cooled at 3.9 Ga and originally had 1000 grams of 40
K. How much 40
K is now left? A. 500 grams
B. 250 grams
C. 125 grams
D. 62.5 grams
E. There’s not enough data to determine an accurate answer.
5.
Which of the following data are used for stratigraphic correlation? A. lithology (rock type)
B. index fossils
C. marker beds
D. fossil assemblages
E. unconformities
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True/False:
If the statement is FALSE correct it so that it’s TRUE.
6.
The principle of baked contacts only applies if an igneous rock has cooled against another pre-
existing rock. T
7.
Beyond the relative age relationships observed between different rock types, nonconformities usually require numeric ages of rock units, fossil evidence, and/or other tangible clues like inclusions to identify them. F
8.
After two half-lives, the parent-daughter ratio will always be 1:3.
T
9.
The half-life for 87
Rb in the rubidium-strontium isotopic system changes through time from 48.8 Ga to
24.4 Ga to 12.2 Ga over 2 half-lives. F
10. The geologic time scale is a composite of globally collected relative age relationships, fossil data, and absolute ages. T
A Practice Problem
11. Using the relative age principles to determine the geologic history of the cross section below. Remember periods of weathering and erosion should be included as an event. There
are 14 events. Start with the oldest event and work forward through time.
All rock units are sedimentary except for C which is a gabbro.
G is a fault.
Outline all the unconformities and identify each of them.
Which units/features does the principle of cross-cutting relations apply to?
Which units/features does the principle of baked contacts apply to? Shade in the baked contacts.
Answers
1. A Igneous rocks where minerals cool below their Curie temperatures, setting the geochronological clock ticking. Dating the minerals in sedimentary rocks likely won’t reflect when the sedimentary rocks formed but when the original mineral cooled below its Curie temperature. Dating the minerals in metamorphic rocks may reflect when they formed ONLY IF metamorphism heats the mineral(s) ABOVE their Curie temperature(s) to “reset” the clock(s). 2. C, D
3. E
The parent/daughter ratio = 12.5/87.5 which reduces to 1/7 and indicates 3 half-lives have passed
Age = (half-lives that have passed)*(duration of the half-life of the isotopic system) 3*713 Ma = 2.14 Ga
4. C
3.9/1.3 = 3 half lives have passed. For the first half life, 1000 grams of 40
K/2 = 500 grams. For the second half
life 500 grams/2 = 250. For the third half life, 250 grams/2 = 150 grams of 40
K remaining. 5. A, B, C, D
6. T
7. F
8. T
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9. F
10. T 11. Deposition of D, deposition of H, deposition of A, formation of folds in D/H/A, intrusion of C and contact metamorphism of D/H/A (the gabbro had to happen after the folding because the gabbro isn’t folded), formation of fault G, weathering and erosion, deposition of F (and the formation of the angular unconformity at the interface between A/H/D and F), deposition of I, deposition of B, tilting of ALL previously
deposited/intruded units, weathering and erosion, deposition of E (and the formation of the angular unconformity at the interface between A/F/I/B and E), present weathering and erosion.
The principle of cross-cutting relations applies to unit C and fault G.
The principle of baked contacts applies to units D/H/A in the proximity of C.
The baked contact exists in the rocks older than C (D H, A) where C touches them (the perimeter of C but not
C itself).