Student Name(s): Grade:
Physical Geology 101 Lab #6
Scientific Dating of Rocks, Fossils, and
Geologic Events
Pre-lab Worksheet – To be completed
prior to Lab Meeting #6
I. Introduction & Purpose:
The purpose of this lab is to learn and
apply the concepts of relative and absolute dating to rocks, fossils and
geologic events. The history and
concepts of stratigraphy, the use of fossils for relative dating, and the
techniques of radiometric dating will be discussed. You
will learn about the geologic timescale, how to determine relative and absolute
time, and the techniques used by geologists to date events in Earth history.
You will also get some practice in using the principles and techniques.
II. Knowing and Understanding
the Six Principles of Stratigraphy:
A. List and Define the six basic laws
of physical stratigraphy (see page 152 in your lab manual):
Stratigraphic Law Definition
1)
_______________________
__________________________________________________
2) _______________________
___________________________________________________
3) _______________________
___________________________________________________
4) _______________________ ___________________________________________________
5) _______________________
___________________________________________________
6) _______________________
___________________________________________________
B.
Unconformities represent gaps in the time-rock record where non-deposition
and/or erosion were occurring over a significant period of time in between
periods of deposition. They typically
appear as obvious irregularity surfaces between two sets or groups of rock
units, termed formations. Note that an
unconformity can also record other geologic events such as tilting, folding,
faulting, intrusion, and uplift.
Therefore, unconformities provide important rock-dating information.
List and define the three kinds of
stratigraphic unconformities (examine Figure. 8.1, page 153):
Type of Unconformity Definition
1)
_______________________
__________________________________________________
2) _______________________
___________________________________________________
3) _______________________ ___________________________________________________
III. Knowing and
Understanding the Principle of Fossil Succession:
A.
Define this very important stratigraphic rock-dating principle (from lecture
and lab text):
Answer.
____________________________________________________________________________________
____________________________________________________________________________________________
B. Biostratigraphy is based on the identity of time-constrained
rock
units called range zones,
which contain unique index fossils. Question: What makes index fossils useful for dating
rocks?
Answer.
____________________________________________________________________
___________________________________________________________________________________________
IV. Principles of Radiometric Absolute Dating
A. How do we determine the age of a rock?
1. Relative Dating - "A is older than B" → Use the Principles of Stratigraphy
2. Absolute Dating - Quantify the date in years → Use the Principles of Radiometric Dating
B. Principles of Radiometric Dating
Naturally-occurring radioactive materials break down into other materials at known rates. This is known as radioactive decay. Radioactive parent elements decay to stable daughter elements. Henri Becquerel discovered radioactivity in 1896. In 1905, Rutherford and Boltwood used the principle of radioactive decay to measure the age of rocks and minerals (using Uranium decaying to produce Helium. In 1907, Boltwood dated a sample of urnanite based on uranium/lead ratios. Amazingly, this was all done before isotopes were known, and before the decay rates were known accurately. The invention of the MASS SPECTROMETER after World War I (post-1918) led to the discovery of more than 200 isotopes. Many radioactive elements can be used as geologic clocks.
Each radioactive element decays at its own nearly constant rate. Once this rate is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent element and the amount of stable daughter elements.
Examples: Radioactive parent isotopes and their stable daughter products
|
Radioactive Parent |
Stable Daughter |
|
Potassium 40 |
Argon 40 |
|
Rubidium 87 |
Strontium 87 |
|
Thorium 232 |
Lead 208 |
|
Uranium 235 |
Lead 207 |
|
Uranium 238 |
Lead 206 |
|
Carbon 14 |
Nitrogen 14 |
In the above table, note that the number is the mass number (the total number of protons plus neutrons). Note that the mass number may vary for an element, because of a differing number of neutrons. Elements with various numbers of neutrons are called isotopes of that element. Each radioactive isotope has its own unique half-life. A half-life is the time it takes for half of the parent radioactive element to decay to a daughter product.
Radioactive decay occurs at a constant exponential or geometric rate. The rate of decay is proportional to the number of parent atoms present.
The proportion of
parent to daughter tells us the number of half-lives, which we can use to find
the age in years. For example, if there are equal amounts of parent and
daughter, then one half-life has passed. If there is three times as much
daughter as parent, then two half-lives have passed. (see
graph, above) Radioactive decay occurs by releasing particles and energy.
Uranium decays producing subatomic particles, energy, and lead.
C. Minerals That You Can Date Isotopically to Get Rock Age
Many of the common rock-forming minerals contain radioactive isotope parent-daughter pairs, which can used for absolute dating. Igneous rocks are, by far, the superior rock for isotopic dating because the vast majority of minerals in an igneous rock are formed at the time the magma cooled, hence the isotopic age closely matches the rock-forming age. The following minerals are some of the most useful for the three most common types of isotopic-pair radiometric dating systems:
Potassium 40 (parent) – Argon 40 (daughter) are found in:
ü Potassium feldspar (orthoclase)
ü Muscovite
ü Amphibole
ü Glauconite (greensand; found in some sedimentary rocks; rare)
Rubidium 87 (parent) – Strontium 87 (daughter) are found in:
ü Feldspar (orthoclase)
ü Muscovite
ü Hornblende
ü Biotite
Uranium 235 and 238 (parents) – Lead 207 and 206 (daughters, respectively) are found in:
ü Zircon
ü Urananite
ü Monazite
ü Apatite
ü Sphene
Question 1) Discuss in a
few sentences whether or not you think that radioactive isotopes would tell the
age of sedimentary rocks (the actual age of the depositional event leading to
rock formation). Hint: When were the minerals that make up a sedimentary rock (for example, the
minerals that make up the sand in sandstone) created? Same time as when the sediments deposited?
______________________________________________________________________________________________________________________________________________________________
_______________________________________________________________________________
Question 2) Discuss in a
few sentences whether or not you think that radioactive isotopes would tell the
age of metamorphic rocks (the actual age of the metamorphism event). Hint:
When the minerals
that make up the parent rock recrystallize or neocrystallize into minerals that
become the metamorphic rock (for example, the minerals in a shale that change
to become a schist), does the isotopic “clock” in those minerals get completely
reset from their original pre-metamorphic rock age to its new metamorphic age
(time of metamorphism)? Parent mineral age or metamorphic mineral age?
______________________________________________________________________________________________________________________________________________________________
_______________________________________________________________________________
Student Name: Grade: Geology 101 Laboratory
Scientific Dating of Rocks, Fossils, and
Geologic Events
I. Determining Relative Ages of
Rocks and Geologic Events Based on Stratigraphic Order
Directions: Complete the analysis and evaluation of the geologic cross sections in Figures 8.9 and 8.10, and the geologic cross sections in Figures 8.11 and 8.12 (included in the
handout).
For each geologic cross section, do the
following:
1. Determine the relative ages for the rock bodies and other geologic
features/events, including
tilting, uplift, faulting,
and erosional unconformities.
2. List
the sequence of geologic events (each one is labeled with a letter) in
chronologic order by
writing down the
letters from oldest (bottom of list) to youngest (top of list) in the column of
blanks. For each dated event you must also indicate
which stratigraphic law was used to place
the event in its
proper time slot. Use the following
initials for the stratigraphic laws: OH =
original
horizontality; LC = lateral continuity; SP = superposition, IN
= inclusions; CC = cross-
cutting, UN
= unconformity.
3. Determine and name (by type) all the lettered unconformities found in each cross-section.
Fig. 8.9 (Geologic cross section#1) Fig. 8.10 (
Age Sequence Stratigraphic Law Age
Sequence Stratigraphic
Law
(Youngest) _____ _____ (Youngest) ____
_____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ (Oldest)
_____ _____
_____ _____
(Oldest) _____ _____
Geologic Cross section in Figure 8.9
Type of Unconformity
Type of Unconformity
R
___________________________ R ______________________________
S ___________________________ S
______________________________
O
___________________________
P____________________________
.Figure 8.11 (Geologic cross section#3) Fig. 8.12 (Geologic cross section#4)
Age Sequence Stratigraphic Law Age
Sequence Stratigraphic
Law
(Youngest) _____ _____ (Youngest) _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
_____ _____ _____ _____
(Oldest) _____ _____ _____ _____
_____ _____
_____ _____
_____ _____
_____ _____
_____ _____
_____ _____
_____ _____
_____ _____
(Oldest) _____ _____
Geologic cross section in Figure8.11 Geologic cross section in Figure. 8.12
Type of Unconformity
Type of Unconformity
N _____________________ 1
__________________
O _____________________ 2
___________________
C _____________________ 3
___________________
4 ___________________
5 ___________________
II. Using Fossils to Determine Age Relationships
Directions: Refer to Figures 8.14 and 8.15. Use the chart in Figure 8.13 and the geologic time scale to help you determine the relative age and the absolute age of the sample in each figure. Note: If, for example, you identified your fossils as dinosaurs (relative age Early Triassic through Cretaceous Periods, absolute age ca. 240–66 Ma) and mammals (Jurassic through Quaternary Periods, absolute age ca 208–66 Ma) from Fig. 8.13, the concurrent or Overlapping Age Range, or Resolved Age, of the two groups of organisms is Jurassic through Cretaceous, which equates to a numeric age range of 208 Ma to 66 Ma. Therefore, the resolved age of rock is the age range in which both fossil species were simultaneously alive.
Question 4. Page 158 —Figure 8.14: Fossiliferous Rock Sample
for Age Analysis
Index Fossils
Present
1. _____________________________ _________ mya to
________ mya
2. _____________________________ _________ mya to
________ mya
Resolved age of sample: _________ mya to ________
mya
Question 5. Page 135—Figure 8.15: Fossiliferous Rock
Sample for Age Analysis
Index Fossils
Present
1. _____________________________ _________ mya to ________ mya
2. _____________________________ _________ mya to ________ mya
Resolved age of sample:
_________ mya to ________
mya
Question: Which stratigraphic principle is fundamental to the logic
of using fossils for dating?
Answer:
_______________________________________________
III. Determining “Absolute” Dates of Rocks by Radiometric Dating
Directions: Below is geologic
cross section consisting of sedimentary and igneous rock layers. There is a rhyolite lava surface flow, an
andesite lava flow, a basalt dike, and a granite intrusion. The
solidified rhyolite lava flow and granite intrusion both have zircon mineral
crystals, which contain Uranium-235. The
andesite lava flow and basalt dike both have abundant amphibole, which contain
Potassium-40.
A mass-spectrometer analysis was used to
count the isotopic ratios of the uranium-235 (U-235) and lead-207 (Pb-207) from
the zircons of both the rhyolite and the granite, and it was used to count the
isotopic ratios of potassium-40 (K-40) and argon-40 (Ar-40) in both the andesite
and basalt.
Part A – Radiometric Analysis
of Igneous Rock Units
Directions: Use
the below information to answer questions 1 through 8.
Rhyolite Lava Flow: Zircon crystals in yielded the
following isotopic analyses:
ü
98.9% of
the atoms were Uranium-235 and 1.1% of the atoms were Lead-207.
Question 1. About
how many half lives (t½) have elapsed since the zircon
crystals formed in the
rhyolitic lava flow? (time since it
became a closed system?) Number of
Half-lives = _____
Question 2: What
is the “absolute” (numeric) age of the zircon crystals and the lava flow?
You must show your calculations below
for full credit!
Calculation: Rhyolite
Lava Flow Age = _______ mya
Andesite Lava Flow: Amphibole crystals yielded the following isotopic analyses:
ü
97.9% of the atoms were Potassium-40 and 2.1% of the atoms were Argon-40.
Question
1. About how many half
lives (t½) have elapsed since the hornblende crystals formed in
the andesite lava flow? (time since it
became a closed system?) Number of
Half-lives = ____
Question 2. What
is the “absolute” (numeric) age of the amphibole crystals and the andesite flow?
You must show your calculations below
for full credit!
Calculation:
Diabase Lava Flow Age = _______ mya
Basalt Dike: Amphibole crystals yielded the following
isotopic analyses:
ü
84.1% of the atoms were Potassium-40 and 15.9% of the atoms were Argon-40.
Question
1. About how many half
lives (t½) have elapsed since the hornblende crystals formed in
the basalt dike? (time since it
became a closed system?) Number of
Half-lives = ______
Question 2. What
is the “absolute” (numeric) age of the amphibole crystals and the basalt dike?
You must show your calculations below
for full credit!
Calculation:
Basalt Dike Age = _________ mya
Granite Intrusion: Zircons crystals yielded the following isotopic analyses:
ü
50% of
the atoms were Uranium-235 and 50% of the atoms were Lead-207.
Question
1. About how many half
lives (t½) have elapsed since the zircon crystals formed in the granite intrusion? (time since it
became a closed system?) Number of
Half-lives = ______
Question 2: What is the “absolute” (numeric) age of the zircon
crystals and the granite intrusion?
You must show your calculations below
for full credit!
Calculation:
Granite Intrusion Age = _______ mya
Part B – Stratigraphic Sequence Dating Using Igneous Rock
Absolute Ages
Directions: Use
the absolute age information above to answer questions 1 through 6 below.
Question
1. Tightest
constrained age range of nonconformity
B-D? _____ mya to
_____ mya
Question
2. Tightest
constrained age range of limestone unit? ______ mya to
______ mya
Question
3. Tightest
constrained age range of nonconformity
A-C? ____ mya to ____ mya
Question
4. Tightest
constrained age range of shale unit? ______ mya to
______ mya
Question 5. Using the
principles of relative dating to arrange the geologic units the above cross
section, what is the proper age sequence?
Include unconformities B-D and A-C in your list.
Youngest __________________
__________________
__________________
__________________
__________________
__________________
__________________
__________________
__________________
Oldest __________________
Question
6. Do the absolute ages agree with the relative
ages of all the units, based on the stratigraphic principles? Yes or No? _____. If
not, what is the best explanation for the discrepancy?
Laboratory
Reflection
Directions: Write
a short reflection (100 to 200 words) about your experience in doing the
activities in lab today. Include the
following: 1) The purpose of the lab; 2) What you learned from this laboratory;
3) What was interesting; 4) The problems and challenges you encountered; and 5)
How this lab was designed (the good and/or bad).
Response: