Name: __________________ Date: ____________ Grade: _________ Physical Geology 111 Laboratory Relative Dating of Rocks Using Stratigraphic Principles I. Introduction & Purpose: The purpose of this lab is to learn and apply the concepts of relative dating to rocks, fossils and geologic events. The history and concepts of stratigraphy and the use of fossils for relative dating will be discussed. You will learn about the geologic timescale, how to determine relative ages, and the methods 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. Define the seven basic laws of physical stratigraphy: Stratigraphic Law Definition 1) Superposition ____________________________________________________________ ___________________________________________________________________________ 2) Cross-Cutting ___________________________________________________________ ___________________________________________________________________________ 3) Inclusion _______________________________________________________________ ___________________________________________________________________________ 4) Fossil Succession ________________________________________________________ ___________________________________________________________________________ 5) Lateral Continuity ________________________________________________________ ___________________________________________________________________________ 6) Original Horizontality ______________________________________________________ ___________________________________________________________________________ 7) Unconformity ____________________________________________________________ ___________________________________________________________________________ 1 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: Type of Unconformity Definition 1) _______________________ _________________________________________________ 2) _______________________ __________________________________________________ 3) _______________________ __________________________________________________ III. Determining Relative Ages of Rocks and Geologic Events Based on Stratigraphic Order Directions: Complete the analysis and evaluation of the 2 geologic cross-sections below. 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: SP = superposition, IN = inclusions; CC = cross-cutting, UN = unconformity. 3. Determine and name (by type) all the lettered unconformities found in each cross-section. 2 3 Geologic Cross-Section #1 Age Sequence (Youngest) _____ Stratigraphic Law _____ Grand Canyon Cross-Section #2 Age Sequence (Youngest) ____ Stratigraphic Law _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ (Oldest) _____ _____ _____ _____ (Oldest) _____ _____ Type of Unconformities – X-Section #1 Type of Unconformities – X-section #2 R ___________________________ R ______________________________ S ___________________________ S ______________________________ O ___________________________ P____________________________ Questions: 1) Which stratigraphic principle did you primarily use for dating the sedimentary layers? 2) Which stratigraphic principle did you primarily use for dating intrusions and faults? 3) Which other stratigraphic principle did you use for dating rocks directly above and below an unconformity? 4 IV. Index Fossils and Their Usefulness in Geologic Dating 1. What is a fossil? Answer. _____________________________________________________________ Biostratigraphy is based on the Principle of Fossil Succession and the identity of time-constrained rock units called range zones, which contain a unique assemblage of fossilized plant and animal species useful for dating. 2. Define the Principle of Fossil Succession (from lecture and lab text): Answer. ____________________________________________________________________________________ 3. Fossils used for dating rocks are called ___________________ fossils. 4. What are the important attributes that makes for a good index fossil? 1) _______________, 2) ______________, 3) _______________, and 4) _______________ 5. Index Fossils According to Eras. Directions: Use the Geologic Timescale diagram below to find and list the names of some of the most common, useful groups of index fossils for each of the three eras. Cenozoic Examples: 1) _____________________ 2) ______________________________ Mesozoic Examples: 1) ______________________2) _____________________________ Paleozoic Examples: 1) _________________2) ___________________3) __________________ 5 6 V. Using Index Fossils to Establish Rock Age Directions: Refer to the figures below. Use the chart above 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), 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. 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 7 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 other stratigraphic principle is fundamental to the logic of using fossils (principle of Fossil Succession) for dating? Answer: _______________________________________________ 8 VI. 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 Principles and Techniques 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 Half-life Constant Suitable Minerals Potassium 40 Argon 40 1.3 billion years K-spar, hornblende, biotite, muscovite Rubidium 87 Strontium 87 47 billion Feldspars, hornblende, biotite, muscovite Thorium 232 Lead 208 Uranium 235 Lead 207 713 million years Zircon, monazite,. sphene, apatite Uranium 238 Lead 206 4.5 billion years Zircon, monazite,. sphene, apatite Carbon 14 Nitrogen 14 5730 years Organics 14 billion years Zircon, monazite, titanite, apatite 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 the two illustrations below) Radioactive decay occurs by releasing particles and energy. Uranium decays producing subatomic particles, energy, and lead. 9 10 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 be 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: ✓ Potassium feldspar (orthoclase) ✓ Muscovite ✓ Amphibole Rubidium 87 (parent) – Strontium 87 (daughter) are found in: ✓ Feldspar (orthoclase) ✓ Muscovite ✓ Hornblende ✓ Biotite Uranium 235 and 238 (parents) – Lead 207 and 206 (daughters, respectively) are found in: ✓ Zircon ✓ Urananite ✓ Monazite ✓ Apatite ✓ Sphene VII. Determining “Absolute” Dates of Rocks by Radiometric Dating Introduction: Below is a 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 contains Potassium-40. 11 Directions: Using the principles of relative dating to arrange the geologic units in the above cross section, what is the proper age sequence? Include unconformities B-D and A-C in your list. You’ll calculate the Absolute Age in the section below and back fill the answers. Youngest Absolute Age myr myr myr Oldest myr 12 Directions: A mass-spectrometer was used to count the isotopic ratios of uranium-235 (U-235) and lead-207 (Pb-207) from zircons in both the rhyolite and the granite. It was also used to count the isotopic ratios of potassium-40 (K-40) and argon-40 (Ar-40) in both the andesite and basalt. Rhyolite Lava Flow: Zircon crystals in yielded the following isotopic analyses: ✓ 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 = _______ myr Andesite Lava Flow: Amphibole crystals yielded the following isotopic analyses: ✓ 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: Andesite Lava Flow Age = _______ myr Basalt Dike: Amphibole crystals yielded the following isotopic analyses: ✓ 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 = _________ myr Granite Intrusion: Zircons crystals yielded the following isotopic analyses: ✓ 50% of the atoms were Uranium-235 and 50% of the atoms were Lead-207. 13 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 = _______ myr Stratigraphic Sequence Dating Using Igneous Rock Absolute Ages Directions: Use the absolute age information above to answer questions 1 through 5 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. 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? GEO-DATING LABORATORY REFLECTION Directions: Write a reflection of the lab activity, explaining its purpose, the methods used, the results obtained, and a brief personal reflection of what you enjoyed and learned about doing this lab 1) What was the purpose of this lab? What did you actually discover and learn during this lab? 2) What did you enjoy most about this lab? Also, what was challenging or thought-provoking? 14

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