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

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

Relative Age Welcome to Discovery Education Player 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

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

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

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

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

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

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

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

Sec 3-Fossil Record What are fossils? Paleontology-study of 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?

How fossils form Mummification Amber Tar seeps Freezing Petrification 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

Types of Fossils Imprints Molds Casts Coprolites Gastroliths 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

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