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

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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

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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

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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?– 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

– 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– 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