1 ES202 Geologic Time Lab Key Your task is to complete portions of Lab 8 in your lab manual (p. 128-139) Part 1. Short Answer. Read the lab materials and define the following terms and concepts / answer the questions. 1. Discuss the difference between relative age dating and absolute age dating, as pertaining to the geologic rock record. Relative age dating simply describes the age of something relative to other things. So a rock layer that has layers above and below it is older than the layers above it and younger than the layers below it. Absolute age dating describes the age of something in exact units, like years. It is more specific than relative age dating. For example, one rock layer could be 425 million years old. 2. Law of Original Horizontality – Sedimentary layers (strata) and lava flows were originally deposited as relatively horizontal sheets. 3. Law of Lateral Continuity – Lava flows and strata extend laterally in all directions until they thin to nothing (pinch out) or reach the edge of their basin of deposition. 4. Law of Superposition – In an undisturbed sequence of strata or lava flows, the oldest layer is at the bottom of the sequence and the youngest is at the top. 5. Law of Inclusions – Any piece of rock (clast) 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. 6. Law of Cross-Cutting Relations – Any feature that cuts across a rock or body of sediment must be younger than the rock or sediment that it cuts across. 7. Unconformity – A surface that represents a gap in the geologic record that formed wherever layers were not deposited for a time or else layers were removed by erosion. 8. Angular Unconformity – An unconformity between two sets of strata that are not parallel to one another. 9. Disconformity – An unconformity between parallel strata or lava flows. Part 2. Lab Activities. Complete the following lab questions. Lab Question 1 (p.130), Lab Question 3 (p.130), Lab Question 4 (p.135), Lab Question 5 (p.135), Lab Question 8 (p.139). 1. Oldest to Youngest: K, J, M, E, F, L, D, C, B, A, I , H, G2 3. For Fig. 8.11, Oldest to Youngest: F (parts of F included in P, so it came first), P (part of P included in B, so it came before B), G (doesn’t cut layer B), B, R, M (doesn’t intrude layer H), H, A, E, X, D, K (doesn’t cut J or S), J, S. For Fig. 8.12, Oldest to Youngest: V, C, O, M, X, K, E, B, S, Z, J, G, L, F, T, A, D, H, P, N (I interpret that N occurred during faulting of P. Since the garage is dipping into the river channel where H has formed, H must have formed before faulting P occurred), R. 4. a.) Ordovician. The fossils present appear to be Flexicalymene and Strophomena, only present during this period. b.) 490 to 443 million years. These fossils could have been deposited together at any time during this period. They are no younger than 443 m.y. 5. a.) Devonian. The fossils present appear to be Phacops and Mucrospirifer. Although Phacops existed during the late Silurian, the presence of Mucrospirifer restricts the age of the rock to Devonian. b.) 417 to 354 million years – no younger than 354 m.y. 8. a.) According to the chart on page 138, about ½ of a half-life has elapsed when there is 70.7% parent (Uranium), and 29.3% daughter (Lead). b.) 713 m.y x 0.500T½ = 356.5 m.y. 1T½ c.) The lava flow was deposited about 357 m.y.a, so any layer beneath it is older than that (Devonian and older). d.) Any layers above the lava flow are younger than 357
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