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 (Grand Canyon cross section)

       Age Sequence           Stratigraphic Law                          Age Sequence               Stratigraphic Law

 

(Youngest) _____                     _____                                 (Youngest) ____               _____

               _____                      _____                                                 _____                         _____

               _____                      _____                                                 _____                         _____

               _____                      _____                                                 _____                         _____

               _____                      _____                                                 _____                         _____

               _____                      _____                                                 _____                         _____

   _____                      _____                                                 _____                         _____            

   _____                      _____                                                 _____                         _____

   _____                      _____                                                 _____                         _____

   _____                      _____                                                 _____                         _____

   _____                      _____                                                 _____                         _____

   _____                      _____                                                 _____                         _____

               _____                      _____                                                 _____                         _____

   _____                      _____                                                 _____                         _____            

   _____                      _____                                     (Oldest) _____                         _____

   _____                      _____            

    (Oldest) _____                      _____                                  

 

Geologic Cross section in Figure 8.9                       Grand Canyon section in Figure 8.10 

           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                       Age Range:  (in million years ago = mya)

 

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                       Age Range:  (in million years ago = mya)

 

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:  

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