Category: Geology

GEOL101 WEEK 6 LAB: GEOLOGIC TIME

PRE-LAB LECTURE: Listen to instructors explanation of the lab (provided in Modules on Canvas).

Refer to LAB 1, pages 2-8, and 12 in the ONLINE DELINE MANUAL:

PART I: RELATIVE DATING & GEOLOGIC TIME SCALE

Refer to Figures 8.2 8.8 in this lab (provided below) and answer the following questions about some of the figures:

1. Questions on Figure 8.3 (above):

1a. Are the inclusions the same age, older, or younger than the surrounding sandstone B? ____________________; why? ___________________________________________________

1b. What kind of rock are the inclusions made of (hint: see legend for Fig. 8.2)? __________________

1c. Are the inclusions the same age, older, or younger than the underlying rock A? _________________

2. Questions on Figure 8.4 (above):

2a. What kind of fold is present on the block diagram: _______________________

2b. What type of unconformity is shown by E? ___________________________________

2c. What is the OLDEST rock (letter)? ____________________; why? _________________________

2d. What is the YOUNGEST rock (letter)? _________________________; why? ____________________

3. Questions on Figure 8.5 (above):

3a. What type of unconformity is shown by E? ______________________________

3b. Is the fault D the same age, older or younger than rock A? ___________________

3c. Is the fault D the same age, older or younger than rock C? ___________________

3d. Is the fault D the same age, older or younger than unconformity E? ___________________

Questions on Figure 8.8 (above):

4a. What kind of igneous rock is A (hint: see legend in Fig. 8.2)? _____________

4b. What kind of igneous rock is B (hint: see legend in Fig. 8.2)? _____________

4c. Which one of these igneous rocks is younger? _________; why? _______________________

4d. Is it the nonconformity C the same age, younger or older than both igneous rocks (A and B)? ______________; why? ________________________________

4e. What is the youngest rock on this block diagram? ___________; why? ________________________

5. Please examine Cross Section 1 (below) and answer the following questions:

5a. How many angular unconformities are there? __________

5b. How many disconformities? ____________

5c. How many nonconformities?

5d. how many faults? ____________

5e. Is the igneous intrusion (granite A) the same age, older, or younger than S? _____________

5f. Is the igneous intrusion (granite A) the same age, older, or younger than H? _____________; why do you think so (explain) __________________________________

5g. Write down letters (from LEFT to RIGHT) to indicate the sequence of events (represented by rocks, unconformities, intrusions, faults) from the OLDEST to the YOUNGEST: _________________

6. Please examine Cross Section 2 (below) that represent the rock formations of the Grand Canyon area, and answer the following questions:

6a. What is the name (not letter) of the OLDEST rock formation? _________________ and what is its relative age according to the geologic time scale shown on on Cross Section 2? ____________________

6b. What is the name (not letter) of the YOUNGEST rock formation? _________________ and what is its relative age according to the geologic time scale shown? ____________________

6c. What is R (be specific)? _____________________________________

6d. What is S? (be specific)? ______________________________________

6e. What is the letter AND name of the youngest rock of Precambrian age?_____________________

6f. Write down letters (from LEFT to RIGHT) to indicate the sequence of events (rocks, unconformities, intrusions) from OLDEST to YOUNGEST. _______________________________________________________

6g. Why is Muav Limestone not present on the other side of the Grand Canyon? ___________________

6h. What geological process is dominant in the Grand Canyon area at present? ___________________

7. Please examine the photograph below and answer the following questions:

7a. What would you call the contact between two rock formations? _____________________________

7b. What kind of structure do you see below that contact? (be specific) _________________________

7c. What happened here? (hint: please list events from the oldest to the most recent in order, including erosion, deformation, uplift, deposition of rocks): ______________________________

USING FOSSILS AND CORRELATION TO DETERMINE AGE RELATIONSHIPS

Refer to the Geologic Correlation Chart (below or in handout) and answer the following questions:

8. The following questions are based on Photograph A (below) from Delaware.

8a. What two fossils are shown in both Photograph A (below) and the Correlation Chart (above)?

_______________________________ AND ______________________________

8b. Based on the overlap of the stratigraphic ranges (black vertical lines on the Correlation Chart (above) of these index fossils, what is the RELATIVE AGE (Era and Period) of the rock. Use expressions such as the early, middle or late part of a period.

______________________________ Era ____________________________ Period

8c. Using the overlap of their stratigraphic ranges, what is the NUMERICAL AGE of the rock in which both of these fossils are found in Ma (millions of years), expressed as a RANGE from the oldest to youngest possible age of the rock.

____________________ to _________________________ Ma

10. Evaluate the geologic cross section on the block diagram in Fig 8.2 (provided in the very first part of this lab) and using the Correlation Chart answer the following questions.

10a. Which one of the contacts shown in Fig. 8.2 between lettered layers is a disconformity (hint: identify the fossils and determine their stratigraphic ranges and relative age using the correlation chart above)?

between layer ____________ and layer __________

10b. Rocks of what Period are completely missing at this disconformity? _____________________

10c. What (numerical) amount of time in Ma (millions of years) is missing at this disconformity? ________________ Ma

11. Evaluate the geologic cross section in the block diagram in Fig 8.6 provided in the first part of the lab) and use the correlation chart (above) answer the following questions (hint: identify the fossils and determine their stratigraphic ranges and relative age).

11a. How old is the oldest fossil in Fig. 8.6? ____________________ Era; _________________ Period

11b. How old is the youngest fossil in Fig. 8.6? __________________ Era; ________________ Period

11c. What kind of unconformity is shown on this diagram? ________________________________

11d. Is there a change in the depositional environment across that unconformity? ________; why? _______________________

12.**In-class hands-on exercise (for F2F GEOL101-01 lab section only)

PART II: DETERMINING NUMERICAL AGES BY RADIOMETRIC DATING

Listen to instructors explanation of principles of Numerical Dating and

refer to pages 218-220 in the Cronin lab manual

and pages 14-16 in the online Deline lab manual (link provided at the beginning of this lab)

Study the charts and tables (provided below) very carefully.

13a. How much (in %) of the parent isotope would be remaining after 5 half-lives have passed? ______

13b. Based on your graph above, approximately how much of the parent isotope would be remaining after 3.5 half-lives? ____________

13c. Based on your graph above, how many half-lives have passed, when only 35% of the parent isotope is remaining in a sample? __________________

14. Question A (based on all the diagrams and tables above in Part II of this lab).

A solidified lava flow containing zircon mineral crystals is present in a sequence of rock layers that are exposed in a hillside. A mass spectrometer analysis was used to count the atoms of uranium-235 and lead-207isotopes in zircon samples from the lava flow. The analysis revealed that 71% of the atoms were uranum-235 and 29% of the atoms were lead-207. Answer the following questions.

14a. About how many half-lives of the uranium-235 to lead-207 decay pair have elapsed in the zircon crystal? __________________________

14b. What is the NUMERICAL AGE of the lava flow based on its zircon crystals? __________________________

15. Astronomers think that Earth probably formed at the same time as all of the other rocky materials in our solar system, including the oldest meteorites. The oldest meteorites ever found on Earth contain nearly equal amounts of both uranium-238 and lead-206. Based on Fig 8.14, what is Earths age? ______________Explain your reasoning _______________________________________

16. If you assume that the global amount of radiocarbon (formed by cosmic ray bombardment of atoms in the upper atmosphere and then dissolved in rain and seawater) is constant then decaying carbon-14 continuously replaced in organisms while they are alive. However, when an organism dies, the amount of its carbon-14 decreases as it decays to nitrogen-14.

16a. The carbon in a buried peat bed has about 6% of the carbon-14 left. What is the age of the peat bed (hint: use the table above for half-life of Carbon-14)? ____________________________

16b. In sampling the buried peat bed, you must be careful to avoid any young plant roots or old limestone. Why? ________________________

17. An authentic dinosaur bone is being offered for sale on the internet. The seller claims that she/he had it analyzed by scientists who confirmed that it is a dinosaur bone and used carbon dating to determine that it is 400 million years old. Give two reasons why you should be suspicious of this bones authenticity.

Reason 1: ___________________________________

Reason 2: ___________________________________

18. AGE ANALYSIS OF TWO OUTCROPS. The figure below shows an outcrop of surface mine (strip mine in northern New Mexico). Answer the following questions.

8a. What is the relative age of the sedimentary rocks (layered shale) in this rock exposure? (Hint: use fossils found in them and the correlation chart above in the same way you did in previous exercises): ___________________ Era; _______________ Period

18b. What is the numerical age of the sill in Ma? (Hint: use percentages of isotopes found in it)? _______________________ Ma

18c. Locate the fault zone. How much displacement (in meters) has occurred along this fault? (Hint: Use the linear map scale near the bottom of the photo above). ____________________