Numerical and Relative Age Dating Assignment (4) (1)

RELATIVE AGE DATING GEOL 1122: Lab #7 (Part I) When you arranged historic photos of GSU in order from oldest to youngest, you were applying principles of relative age dating, just as you would in determining the history of a sequence of rocks. You analyzed photos of GSU locations as older or younger in comparison to other photos and you looked for clues in the photos such as hairstyles, clothes, and cars from previous decades just as you might study a rock layer for fossils to provide insight as to a specific moment in history. Here you’ll learn about the principles of relative age dating in more depth and how to apply them to determine the order of events when studying a cross-section of rock layers. Just as you did in class for historic GSU photos, you’ll write the letters of rock layers and related events from oldest to youngest. PRINCIPLES OF RELATIVE AGE DATING Relative dating involves putting geologic events in order to determine the age of a rock relative to other rocks, that is, assigning whether a rock is older or younger than other rocks. The entire geologic time scale is actually based on the principles of relative dating because until recently, methods had not yet been developed to calculate numerical ages for rocks. Instead, ages of rocks were dated as “Jurassic” or “Devonian”, names which refer to certain periods in earth history when specific groups of fossils are known to have existed. The principles of relative dating are the rules that guide geologists when they interpret the geological history of a series of rocks. Principle of Superposition: Sedimentary rocks form when sediments such as sand, silt, or clay settle out of the water column and fall to the bottom of a basin, creating layers that become lithified. In any sequence of undeformed sedimentary rocks, layers at the bottom are going to be older than the ones near the top, because rocks at the bottom of a sequence must have been deposited first (in order for other rocks to have formed on top of them). See Figure 1a where A is the youngest rock and E is the oldest rock. P rinciple of Original Horizontality: Sediments under the influence of gravity, are almost always laid down in horizontal layers called beds (as they fall to the bottom of a basin when settling out of the water column). Thus, if you see rock layers that are tilted, it can be assumed that they have been changed from their original position (Figure 1b). Figure 1: Principles of relative dating show that the rocks in a) are horizontal compared to b) after having been tilted due to a tectonic event. Comprehension Check #1: a) Is rock C older than D in Figure 1? Explain. No, rock C is younger because it was deposited after rock D. Figure 2 : Principles of relative dating focusing on cross-cutting relationships. (a) Faulting of rocks. Rocks A-E are older than the fault as it cuts through the rock layers. (b) Igneous intrusions and lava flows. These igneous features are younger than rocks A-E. Principle of Cross-Cutting Relationships: Rock layers that are changed due cross-cutting phenomena include faults that break through rocks and igneous intrusions. The first scenario happens when a fault forms due to earthquake activity, and so rocks are then broken up. Thus, a fault that cuts through a rock is going to be younger that the rock itself as it is a newer event (Figure 2a). The second scenario is when molten rock (magma) pushes through (intrudes) a body of rocks, and so the resulting igneous rocks must be younger than those rocks which were intruded (Figure 2). Likewise, lava flows must be younger than rocks they cover up. Comprehension Check #2: a) Is rock B or D older in Figure 2a? Explain . Rock B is older because it is under rock D (which is younger). b) Is the lava flow or rock E younger in Figure 2b? Explain. The lava flow is younger than rock E because the lava flow is a new event that is covering rock E.

© 2008 Kendall Hunt Publishing Company and Brent Zaprowski RELATIVE AGE DATING GEOL 1122: Lab #7 (Part I) Principle of Inclusions: Rocks that have broken apart from their original source rock and are later preserved in a different rock layer are called inclusions. The layer with the inclusions is younger than the inclusions (having originated from older rock). Thus, inclusions are older than the surrounding rock in which they are preserved. Unconformities: The surface between distinct rock units is called a contact. Sometimes, contacts between layers are what are called unconformable, which indicate a gap in sedimentary deposition. There are 3 major types of unconformities. Angular unconformities are when newer sedimentary layers are situated on top of older tilted sedimentary rocks (Figure 4). Disconformities are when sedimentary deposition is on an erosional surface that is roughly parallel to the underlying undeformed sedimentary layers such as seen in Figure 3 whereby D and E are separated by a wavy surface indicating erosion. Nonconformities are when sedimentary rocks overlie a surface under which are older igneous or metamorphic rocks. Hint: Each of the following geologic cross-sections features one or more unconformities demonstrating a ‘change’ in between the expected layering of rock units over time. 1) Of the rocks A-F, which of the rocks was deposited first and is the oldest ? Rock D 2) Of the rocks A-F, which of the rocks is the youngest and was deposited last? Rock F 3) Which occurred first: The tilting of A, B, C and D or the deposition of E? The tilting of A, B, C, and D 4) Which occurred first: The erosion of F or deposition of A, B, C, D? The deposition of A, B, C, D 5) Write the order of events for the entire sequence including any tilting, erosion, or unconformities. Start with the oldest rock. Deposition of D, deposition of C, deposition of B, deposition of A, tilting of A, B, C, and D, deposition of E, deposition of F, erosion of F Figure 4 Earth’s surface F E A B C Figure 3: Principles of relative dating. The inclusions in rock layer E are visible above the erosional surface between D and E. This indicates that the inclusions originated from rock layer D and so are older than the layer E in which they are preserved. G F E D C B A Comprehension Check #3: a) Is rock F or the inclusions in rock E older? Explain. The inclusions in rock E are older than rock F because the inclusions originated from rock D which is older than rock F. b) Are the inclusions in rock E or rock D older? Explain. The inclusions in rock E and rock D are the same age. This is because the inclusions originated from rock D. Use Figure 4 to answer the following questions now that you have learned principles of relative age dating:

© 2008 Kendall Hunt Publishing Company and Brent Zaprowski RELATIVE AGE DATING GEOL 1122: Lab #7 (Part I) Figure 5 Answer the below questions based on Figure 5: Note: E is an unconformity. 6) Which of the rocks was formed first and is the oldest ? Rock M was formed first and is the oldest. 7) Which of the rocks is the youngest and was formed last? Rock P was formed last and is the youngest. 8) Which occurred first: The folding of M, F, P, X or the deposition of F and G? The folding of M, F, P, X occurred first. 9) Which occurred first: The erosion that led to the surface E or the intrusion by dike H? The erosion that led to the surface E. 10) Write the order of events for the entire sequence including any folding or erosion. Start with the oldest rock. Deposition of M, deposition of F, deposition of P, deposition of X, folding of M, F, P, X, unconformity E, deposition of W, deposition of G, and intrusion by dike H. M F W H P X P

(Appendix 1) Age range Age range Age range 14) Using fossils as a guide and indicator of different periods by assigning ages, you can correlate different sets of rocks across distances to develop a detailed understanding about the history of a region. Using your skills gained in this lab, which is the oldest outcrop in Figure 8? Which is the youngest outcrop? Explain. RELATIVE AGE DATING GEOL 1122: Lab #7 (Part I) Figure 6 11) Write the order of events for this cross-section. Include any tilting, faulting, or erosion. Start with the oldest rock. Deposition of E, deposition of A, deposition of F, deposition of I, tilting of E, A, F, I, deposition of C, erosion of C, deposition of K, fault, deposition of G, deposition of D, deposition of J, erosion of G, D, J, deposition of H, erosion of H. PRINCIPLE OF FAUNAL SUCCESSION Throughout earth history, organisms have evolved and succeeded each other in a definite and determinable order, a concept known as the principle of faunal succession. By knowing what fossils within a rock, you can determine the age of the rock. The principle of faunal succession is the primary basis for the geologic time scale. All of the divisions within the geologic time scale are based in large part by the appearance of, the dominance of or disappearance of key fossil groups. These fossils are known as index fossils , and index fossils have a very narrow age range , and so are key indicators of specific periods in earth’s history. An age range is the part of geologic time during which certain fossil species are known to have existed (Figure 7). As is shown in Figure 7, fossils V, W, X, Y, and Z each existed during a specific time, and the arrow next to the species indicates the extent of the age range for when that organism existed in earth history. The age range is always described starting with the oldest age first and then finishing with the end of the species range. So for fossil V, it’s age range is Silurian-Triassic. For fossil W, it is Triassic-Cretaceous instead. For fossil X, it’s Ordovician- Jurassic, and then Jurassic- Quaternary for fossil Y, and finally, for fossil Z it’s age range is Cambrian-Devonian. T Figure 7: An example of age ranges. Quaternary T ertiary Fossil Y Rock A Rock B Cretaceous Jurassic Triassic Permian Pennsylvanian Mississippian Devonian Silurian Ordovician Cambrian Fossil W Fossil V Fossil X Fossil Z Rock C Rock D Figure 8: An example of biostratigraphy. Based on the age ranges in Figure 6, Rock A is Ordovician-Devonian, as it is the only time during which both species lived. Rock B is Triassic following the same logic that the rock had to have formed when both species co-existed in order for them to end up in the same rock. Figure 8 Use Figure 7 to answer the following questions: 12) If a rock has fossils W and X, what is the age of the rock? Triassic-Jurassic 13) If a rock has fossils Y and W, what is the age of the rock? Jurassic-Cretaceous Comprehension Check #4: a) What is the age of Rock C? Silurian- Devonian b) What is the age of Rock D? Jurassic

NUMERICAL AGE DATING GEOL 1122: Lab #7 (Part II) 1 half-life 2 half- lives 3 half- lives 4 half-lives Origin al Parent 100% Daughter 50% Original 50% Daughte r 75% Origina l 25% Daughter 87.5% Original 12.5% Daughter 93.75% Original 6.25% U U U U U U U U U U U U U Pb Pb Pb U Pb U U Pb Pb U U Pb Pb Pb Pb Pb Pb U U Pb Pb U Pb Figure 2. Example mineral crystals showing the proportion of parent material (uranium = U) to daughter product (lead = Pb). The first image (at left) shows no daughter product yet (only 100% parent material). The second image shows that half of the uranium has changed into lead (i.e., one half-life has elapsed). The final image shows that another half of the uranium (parent material) decayed into lead (daughter product), and so another half-life has elapsed. produced). Notice that after two half-lives, only 25% of the parent is remaining in the crystal. This process continues as the parent decays further into the daughter product. Comprehension Check #3: a) How much daughter product is produced from the parent material after 1 half-life? 50% b) How much parent material remains after 2 half-lives have occurred? 25% c) How many half-lives have elapsed if you have 87.5% daughter product? 3 half-lives d) How many half-lives have elapsed if you have 6.25% of the parent material? 4 half-lives Figure 1. How a radioactive element decays. When the original parent element is solidified as part of a mineral crystal, it begins to decay into the daughter product within that mineral crystal. After one half-life has elapsed, 50% of the original parent is left (while 50% of the daughter is produced). After two half-lives, only 25% of the parent is left. This process continues as the parent decays further into the daughter product. 2 half-lives 1 half-life 25% Uranium 50% Lead 50% Uranium 100% Uranium 0% Lead Comprehension Check #4: a) What element is the parent material in the above example for Figure 2? Uranium b) What element is the daughter material in the above example for Figure 2? Lead c) What percentage of parent material remains after 1 half-life has elapsed? 50% d) What percentage of daughter material is produced after 2 half-lives have elapsed? 75% 75% Lead

RADIOACTIVE ELEMENTS Most natural elements on Earth are composed of both stable and radioactive isotopes. Isotopes are alternate versions of elements that have a different number of neutrons in the nucleus and correspondingly vary in atomic mass. Isotopes can be radioactive or stable. For example, 1 H and 2 H are stable isotopes of hydrogen whereas 3 H is radioactive. Table 1 below shows pairs of radioactive isotopes that are commonly used for radiometric dating. The half-lives (how long it takes for half of the parent to decay to the daughter product) is a constant for each pair of isotopes and so we can use this predictable decay rate to calculate rock ages. Table 1. Pairs of radioactive isotopes and representative half-lives as well as sample materials dated. Parent Isotope (P) Daughter Isotope (D) Half-Lives (T ½) Materials Dated Uranium-238 Lead-206 4.5 billion years zircon Uranium-235 Lead-207 713 million years zircon Potassium-40 Argon-40 1.3 billion years biotite, muscovite, whole volcanic rock Carbon-14 Nitrogen-14 5730 years shells, wood, bones, limestone How was this discovered? How do we know if an object is radioactive? The presence of radioactive atoms can be determined using a Geiger Counter as the energy and subatomic particles released during the decay of a radioactive parent to a daughter is detected. The early work of studying radioactivity showed that the amount of radioactive atoms seen or heard by the clicks on a Geiger Counter was proportional to the amount of radioactive atoms in the rock being measured. Now that we know the constant decay rates for different isotope pairs from Table 1 (as noted by T½ for the half-lives of these radioactive elements), we can determine the half-lives that have elapsed from parent to daughter ratios… and next calculate ages of rocks! HOW TO DETERMINE THE AGE OF ROCKS The number of parent atoms decrease as daughter atoms increase for each half-life that has elapsed. Thus, older rocks have more daughter product as more decay has occurred. The first step then is to determine the % parent compared to the % daughter as done before. Geologists can measure the amount of daughter atoms in a rock, and subtract from 100% to determine the proportion of parent atoms remaining, and then assess how many half-lives have elapsed using Table 2. Then the age equation can be used to calculate the age of a rock for a specific pair of isotopes given the decay constant and half-lives that have elapsed. For example, a rock that has 50% Carbon-14 and 50% Nitrogen-14 is 5730 years old (as it is 1 x T ½ for that isotope pair). If a rock instead had 75% Nitrogen-14, TWO half-lives have elapsed (parent material is divided in half 2x leaving only 25% Carbon-14), and so the rock is 5730 x 2 = 11,460 years old. Table 2. Decay parameters for all radioactive decay pairs and the age equations for half-lives elapsed. % Parent % Daughter Half-Lives Elapsed Age Equation 100 0 0 0 x T ½ 91.7 8.3 1/8 0.125 x T ½ 84.1 15.9 ¼ 0.250 x T ½ 70.7 29.3 ½ 0.500 x T ½ 50 50 1 1.0 x T ½ 35.4 64.6 1 ½ 1.5 x T ½ 25 75 2 2.0 x T ½ 12.5 87.5 3 3.0 x T ½ 6.2 93.8 4 4.0 x T ½ Comprehension Check #5:

Questions: 1) If a rock has 50% Potassium-40 (parent) to 50% Argon-40 (daughter) isotopes, what is the age of that rock? 1.3 billion years. Parent % ____ 50% ___ Daughter % ____ 50% _____ Age = (# Half-lives elapsed: __ 1 ___ ) x 1.3 billion yrs = ____ _ 1.3 _______ billion years 2) If a rock has 70.7% Uranium-235 (parent) to 29.3% Lead-207 (daughter) isotopes, what is the age of that rock? Parent % ____ 70.7% ______ Daughter % ____ 29.3%_ __ Age = (# Half-lives elapsed: _ ___ 1/2 ___ _) x ___713 million yrs ___ = _____ 356.5 ______ million years 3) If a rock contains a ratio of 12.5% Carbon-14 to 87.5% Nitrogen-14 isotopes, what is the age of that rock? Parent % _ ___ 12.5% ______ Daughter % _____ 87.5% ___ Age = (# Half-lives elapsed: __ 3 __) x (T ½: ___ ____ 5730 yrs ______ ___) = ___ ___ 17,190 ______ years 4) If a rock contained a ratio of 84.1% Uranium-238 to 15.9% Lead-206 isotopes, what is the age of that rock? Parent % __ 84.1% ___ Daughter % __ ___ 15.9% ______ Age = (# Half-lives elapsed: ____ 1/4 ____) x (T ½: _____ 4.5 billion yrs _______) = _____ 1.125 ______ billion years 5) a) If a rock has 6.2% Carbon-14, what is the proportion of Nitrogen-14 in the rock? ____ 93.8% ______ b) How many half-lives have elapsed? _____ 4 _____ c) What is the age of that rock? ____ __ 22,920 ________years 6) a) If a rock has 35.4% Argon-40, what is the proportion of Potassium-40 in the rock? ____ 64.6% _____ b) How many half-lives have elapsed? _____ 1 1/2 ________ c) What is the age of that rock? ______ 1.95 _______ billion years 7) a) If 1/8 half-lives elapsed in a rock containing Uranium-238 and Lead-206, what is the ratio of parent to daughter isotopes? ______ 91.7% ______ (Parent) vs. ______ 8.3% ______ (Daughter)

b) What is the age of that rock? Show your work. 4.5 billion yrs x 1/8 = 0.5625 billion years 8) a) If 2 half-lives elapsed in a rock containing Carbon-14 and Nitrogen-14, what is the ratio of parent to daughter isotopes? _______ __ 25% _ ________ (Parent) vs. _______ _ 75% _ _______ (Daughter) b) What is the age of that rock? Show your work. 5730 yrs x 2 = 11,460 years 9) What are the FOUR igneous rocks could be used for radiometric dating in the below diagram? ___Pegmatite___ ___Volcanic ash___ ___Granite___ ___Basalt___ 10) Complete the below for the sequence of events using the above diagram. Units marked by an asterisk are the four igneous rocks that could be dated using radiometric methods. ______H______ ______M______ * ______E______ * ______K______ ______B______ ______P______ * ______F______ A M H B F K Youngest