Ch 8-Rock Record

1. Ch 8-Rock Record • Objectives • Principle of uniformitarianism • Law of superposition and how it is used to determine relative age of rock • Compare types of unconiformities • Apply law of crosscutting relationships to determine relative age of rocks • Summarize limitation of using rates of erosion and deposition for determining absolute age • Describe formation of varves • Radioactive decay • How entire organisms can be preserved as fossils • Examples of fossilized traces of organisms • Describe how index fossils can be used to determine age of rocks

2. Sec 1-Determining Relative Age • Uniformitarianism-principle that geologic processes that occurred in past can be explained by current geologic processes • One of basic foundations of the science of geology • James Hutton • Welcome to Discovery Education Player

3. Relative Age • Age of an object in relation to ages of other objects • Strata-layers of rock that shows sequence of events that took place in the past • Law of Superposition • Sedimentary rock layer is older than the layers above it and younger than layers below, if layers are not disturbed • Beds-sedimentary rock layers • Bedding plane-boundary between 2 beds • Principle of Original Horizontality • Sedimentary rocks left undisturbed remain in horizontal layers • Graded bedding, cross beds, ripple marks • Welcome to Discovery Education Player

5. Unconformities • Breakage in geologic record created when rock layers are eroded or when sediment is not deposited for long period of time • Types • Nonconformity=stratified rock rests upon unstratified rock • Angular=boundary between set of tilted layers and set of horizontal layers • Disconformity=boundary between horizontal layers of old sedimentary rock and younger, overlying layers deposited on eroded surface

8. Law of Crosscutting • Faults and intrusions make it difficult to determine rocks age • Fault=break or crack in Earth’s crust along which rocks shift their position • Intrusion=mass of igneous rock that forms when magma is injected into rock and cools then solidifies • Law of crosscutting • Fault or igneous intrusions are always younger than rock layers it cuts through • Cuts through unconformity then the fault or intrusion is younger than all rocks above and below

9. Sec 2-Determining Absolute Age • Numeric age=absolute age • Limitations using erosion and deposition • Rate of erosion is only useful if geologic feature was formed w/in past 10,000-20,000 yrs. • Ex. Niagara Falls, What about Grand Canyon? • Rate of Deposition calculates rate of sediment deposition • Collect data for long time-limestone, shale, sandstone • 30 cm of sedimentary rock=1,000 yrs

10. Varves • What is a varve count? • Summer time=coarse particles, winter=fine particles. Fine settles on coarse at bt of lake • Varve-banded layer of sand and silt that is deposited annually in a lake • Ice sheets or glaciers • Used to determine absolute age • Light colored=coarse, dark=fine

11. Radioactive Decay • Use natural breakdown of isotopes to measure absolute age of rocks • Radiometric dating-method of determining absolute age of object by comparing relative percentages of radioactive isotope and stable isotope • Radioactive isotopes’ nuclei emit particle and energy at constant rate • Why is it used? How is it used? • Welcome to Discovery Education Player

12. Half-Life • Radioactive decay happens at relatively constant rate-not changed by temperature, pressure, or other environmental conditions • Half-life-time required for half of a sample of radioactive isotope to break down by radioactive decay to form a daughter isotope

13. Carbon Dating • Isotope Carbon 14 combines w/ oxygen to form radioactive CO2 • Ratio of C 14 and C 12 in living organisms is relatively common • Organism dies, amount of 14 C decreases steadily • Half life of 14 C=5,730 yrs • Used to date ages of wood, bones, shells, and other organic remains that are less than 70,000 yrs old

14. Sec 3-Fossil Record • What are fossils? • Remains of animals or plants that lived in previous geologic time • Provides clues to past • Paleontology-study of fossils • Most found in sedimentary rock. Why?

15. How fossils form • Mummification • Ancient civilizations extracted internal body organs and wrapped body in prepared strips of cloth • Found in dry places • Amber • Insects get trapped in sticky amber from tree. DNA can be recovered • Tar seeps • Animals drink water that covers petroleum, get stuck, remains preserved in tar • Freezing • Low temps of frozen soil and ice can protect and preserve organisms • Bacteria cannot survive • Petrification • Mineral solutions replace original organic material, results in formation of mineral replica of original organism • Silica, calcite, pyrite

16. Types of Fossils • Imprints • Carbonized imprints of leaves, stems, flowers, and fish in soft mud or clay in sed rock • Molds • Shells leave empty cavity w/in hardened sediment. Shells decay and leave empty space • Casts • Sand or mud fills mold, cast forms, replica of original organism • Coprolites • Fossilized dung or waste materials from ancient animals. Dinosaurs • Gastroliths • Stones in digestive system of dinosaurs to help grind food, survive as fossils • Trace fossils-fossilized mark formed in sed rock by movement of animal on or w/in soft sediment

17. Index Fossils • Fossil used to establish age of rock layers. Why? • Distinct, abundant, widespread, existed for short span • Use them to date rock layers in separate areas • Use them to locate rock layers likely to contain oil and natural gas deposits