lab 9asssn
docx
keyboard_arrow_up
School
Metropolitan Community College, Kansas City *
*We aren’t endorsed by this school
Course
115
Subject
Geology
Date
Dec 6, 2023
Type
docx
Pages
6
Uploaded by JudgeKoala3560
Lab 9: Geologic Time
Geologists use two very different “clocks” when assigning ages to rocks – an absolute age and a relative age. An absolute age is the quantitative (measurable) age of a rock, while a relative age is a qualitative age of a rock. Relative dating orders geologic events in the sequence they occurred but does not place quantitative constraints on when they actually happened. For example, I can tell you that this morning I first woke up, got out of bed, I then took a shower, sat down for breakfast, read the newspaper, and finally drove to work. Notice that I told you the sequence of my morning routine but did not tell you the exact time each event occurred. Absolute dating does place quantitative constraints on when an event occurred. For example, if I
tell you I woke up at 6:00 am, got in the shower at 6:05 am, sat down for breakfast at 6:20 am, and left for work at 7:00 am, this time I have given you the absolute times of the events.
Geologists use several basic principles for determining relative age relationships among bodies of rock:
•
Principle of Original Horizontality
– Sedimentary layers and lava flows were originally deposited as relatively horizontal sheets. If they are no longer horizontal, it is because they have been displaced by subsequent movements of Earth’s crust.
•
Principle of Superposition
– In an undisturbed sequence of sedimentary layers or lava flows, the oldest layer is at the bottom of the sequence and the youngest is at the top.
•
Principle of Inclusions
– Any piece of rock that has become included in another rock or body of sediment must be older than the rock or sediment into which it has been incorporated. Such a rock is called an inclusion.
•
Principle of Cross-Cutting Relationships
– Any feature that cuts across a rock or body of sediment must be younger than the rock or sediment it cuts across. Such cross-cutting features include fractures (cracks in rocks), faults, or masses of magma (igneous intrusions) that cut across preexisting rocks before they cooled. When a body of magma intrudes preexisting rocks, a zone of contact metamorphism (baked zone) usually forms in the preexisting rocks adjacent to the intrusion.
•
Principle of Unconformities
– Most contacts between adjacent strata or formations are conformities, meaning that rocks on both sides of the contact formed at about the same time. An unconformity is a rock surface that represents a gap in the geologic record. It is like the place where pages are missing from a book. An unconformity can be a buried surface where there was a pause in sedimentation, or a surface that was eroded before more sediment was deposited on top of it.
•
Faunal Succession
– Geologists have studied sedimentary rocks around the globe and have seen a clear pattern of relative ordering of species through time. All extremely ancient species were sea-dwelling single-celled organisms. Then fossil arthropods and fish appear in the rock layers, followed by fossil amphibians and then reptiles. Only more recently do mammal fossils appear in the rock record. From these patterns, it appears species succeed one another in a regular pattern, so their fossils allow geologists to use the principle of fossil succession to determine their relative age.
Relative Dating
In each of the following cross sections, each rock formation, intrusion, fault, and erosion surface is labeled with a letter. List these letters from the oldest to the youngest. You may find it easier
if you start with the oldest layers (bottom) and work your way up to the top at the surface of the Earth. The cross sections will increase in difficulty as you go. I have done the first one to get you
started. Look closely at the example and see how I listed the proper sequence of events. Remember to take your time, use the principals listed above, and have fun!
____G____ youngest
____E_(cuts across H)
____H_(cuts across F)
____F____
____A____
____B____
____C____
____D_(on bottom) oldest
___
M
_____
youngest
__
D
______
____
E
____
____
L
____
____
A
____
__
B
______
__
J
______
___
C
_____
__
H
______
________
________ oldest
__
M
_____
youngest
___
B
____
__
H
_____
__
P
_____
__
K
_____
__
S
_____
_
J
______
___
N
____
_
G
______
__
C
_____
_
A
______
___
F
____
___
E
____
___
R
____
___
D
____
_______ oldest
_______ youngest
____
M
___
___
R
____
___
D
____
__
N
_____
____
P
___
_
H
______
__
G
_____
__
E
_____
____
B
___
___
X
____
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help
__
Z
_____
__
A
_____
___
L
____
__
K
_____
_
J
______
__
T
_____
___
S
____
__
F
_____
__
C
_____ oldest
Absolute Dating
Absolute dating relies on radioactive decay of isotopes. Remember isotopes are atoms of a particular element that have a different number of neutrons in the nucleus. Some isotopes are stable and some are unstable. Geologists are interested in unstable isotopes because those are the ones that undergo radioactive decay or the breakdown of an atom to a new atom of a different element. Radioactive decay is characterized by the half-life, which is a measure of the time it takes for one-half of the radioactive parent atoms to decay into the stable daughter element. In a second half-life, the number of radioactive parents is again reduced by one-half. Each stage in the process decreases the amount of radioactive parent by one-half. As the amount of the parent decreases, the daughter element increases by the same amount. Provided
there is no loss of daughter or parent from a mineral or rock, the sum of these two isotopes must equal the number of parent atoms originally present. See the graph below:
A few different isotopes, their half-life, effective dating range, and materials commonly dated are
provided in the table below.
Method
N
p
N
d
Half-life*
Effective Dating
Range*
Materials Commonly Dated
Thorium-lead
Th-232
Pb-208
14.1 billion
10 my – 4.6 by
Zircon minerals
Potassium-argon
K-40
Ar-40
1.3 billion
100,000 – 4.6 by
Potassium rich minerals; volcanic rocks
Carbon-14
C-14
N-14
5,730 yrs
100 – 70,000 yrs
Carbon rich materials like bones, wood, charcoal, paper…
*my = millions of years; by = billions of years
N
p
= parent isotope
N
d
= daughter product
Questions
1. A mineral forms with 100 atoms of isotope A. After 5 half-lives, how many atoms of isotope A
remain?
3.125
2. A mineral forms with 1,000 atoms of isotope K. I analyze the mineral some time later and find 125 atoms of isotope K and 875 atoms of the daughter isotope L. How many half-lives have past? (Assume that the mineral does not “leak” any atoms.)
3
3. I have measure the half-life of isotope Y to be 10,000 years. I then analyze a mineral and find 31 atoms of isotope Y and 469 atoms of the daughter element, isotope Z. How many half-
lives have past? How old is the mineral? (Assume that the mineral does not “leak” any atoms.)
4.0116 halflives 40116 years
4. I have collected some volcanic rocks and would like to use radiometric dating to determine their age. I choose to use carbon-14. Is this a wise choice on my part? Can you use carbon-
14 to date volcanic rocks? Why or why not? Explain your answer.
No theres not a lot of carbon in these rocks
5. If I have a pretty good intuition that my volcanic rocks were formed less than 5 million years ago, could I use thorium-lead isotopes to date them? Why or why not?
No thorium lead isnt effective at 5 million years
6. I have gathered three samples from an archeological dig near the Kansas River. The samples are charcoal collected from ancient fire rings and have been removed from three different layers in the soil. Analysis of isotopes of carbon-14 yielded ages for the three samples as:
Sample R: 1,565 years Sample H: 10,340 years
Sample C: 4,780 years Which sample was likely collected from the bottom most layer? How do you know?
H because it is the oldest
7. I assumed the charcoal from the site was collected from a fire ring and were therefore evidence of human activity. However, that may not necessarily be the case. What are some other possible sources of charcoal that may not be human related? What other lines of evidence would you look for to be sure of early human habitation at this site?
wildlife could turn wood to charcoal, human bones 8. Pieces of organic wood have been collected from permafrost deposits in Alaska that are Pliocene in age. Can I use carbon-14 dating to determine the ages of the wood? Why or why not?
Yes, the wood will still be usable and have a lot of carbon
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help