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
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3. 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
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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?
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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? 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?

15. 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

16. 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

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