The Rock RecordThe Rock RecordChapter 8Chapter 8

James HuttonJames Hutton

1818thth Century Scottish physician Century Scottish physician Observed geologic changes that took Observed geologic changes that took

place on his farmplace on his farm By studying the present, people By studying the present, people

could learn about the pastcould learn about the past

UniformitarianismUniformitarianism

The present is the key to the pastThe present is the key to the past Geologic processes that occurred in Geologic processes that occurred in

the past can be explained by current the past can be explained by current geologic processesgeologic processes

Same processes, but rates can Same processes, but rates can changechange

Volcanism, erosionVolcanism, erosion Opposite of catastrophism – changes Opposite of catastrophism – changes

occur abruptly occur abruptly

Earth’s AgeEarth’s Age

4.6 billion years4.6 billion years Hutton reasoned that change was Hutton reasoned that change was

very slowvery slow Observed changes must have taken Observed changes must have taken

place over billions of years, not place over billions of years, not thousandsthousands

Supported Darwin’s Theory of Supported Darwin’s Theory of EvolutionEvolution

Relative AgeRelative Age

Age of an object related to anotherAge of an object related to another Object 1 is older than object 2Object 1 is older than object 2 Don’t know the actual age, just that Don’t know the actual age, just that

one is older than the otherone is older than the other

Original HorizontalityOriginal Horizontality– Sedimentary rocks left undisturbed will Sedimentary rocks left undisturbed will

remain in horizontal layersremain in horizontal layers

Law of SuperpositionLaw of Superposition

Sedimentary layer of rock is older Sedimentary layer of rock is older than the layer above it, but younger than the layer above it, but younger than the layer below itthan the layer below it

This is true as long as layers are NOT This is true as long as layers are NOT disturbeddisturbed

Determining Original Rock PositionDetermining Original Rock Position

Graded BeddingGraded Bedding– Large particles settle first, followed by Large particles settle first, followed by

smaller particlessmaller particles– If particle size doesn’t match up, there If particle size doesn’t match up, there

must have been some geologic activitymust have been some geologic activity

Determining Original Rock PositionDetermining Original Rock Position

Cross bedsCross beds– Sand beds can form at angles as sand Sand beds can form at angles as sand

blows down a duneblows down a dune– Older ones erode as newer ones are Older ones erode as newer ones are

formedformed– Get strange anglesGet strange angles– Can violate Original Horizontality RuleCan violate Original Horizontality Rule

Determining Original Rock PositionDetermining Original Rock Position Ripple MarksRipple Marks

– Smalls waves on rock surfaces (like at Smalls waves on rock surfaces (like at the shore on sand)the shore on sand)

– Can match up ripple marksCan match up ripple marks– Ripple marks should point upRipple marks should point up

UnconformitiesUnconformities

Break in geologic record when rock Break in geologic record when rock layers erode or sediment is not layers erode or sediment is not deposited for a long timedeposited for a long time

Three types of unconformitiesThree types of unconformities1.1. NonconformityNonconformity

– Stratified rock sitting on a nonstratified Stratified rock sitting on a nonstratified rockrock

– Sedimentary on igneous or Sedimentary on igneous or metamorphicmetamorphic

UnconformitiesUnconformities

2.2. Angular unconformityAngular unconformity– Tilted layers set on horizontal layersTilted layers set on horizontal layers

3.3. DisconformityDisconformity– New layers deposited on old layer after New layers deposited on old layer after

erosionerosion

Disconformity

1)Sediment deposition2)Sediment erosion3)More sediment deposition

Law of Crosscutting RelationshipsLaw of Crosscutting Relationships

A fault or rock intrusion is younger A fault or rock intrusion is younger than the rock it cuts throughthan the rock it cuts through

Intrusions can be dikes, sills, or Intrusions can be dikes, sills, or magma (plutons)magma (plutons)

Absolute AgeAbsolute Age

Numeric age of a rock formationNumeric age of a rock formation Absolute dating methodsAbsolute dating methods

– Geological methodsGeological methods– Chemical methodsChemical methods

Age – Geological MethodsAge – Geological Methods

1.1. Rates of erosionRates of erosion– Practical only for features that have Practical only for features that have

formed in the past 10,000 to 20,000 formed in the past 10,000 to 20,000 yearsyears

– Won’t work for older things because Won’t work for older things because rates of erosion change over timerates of erosion change over time

Age – Geological MethodsAge – Geological Methods

2.2. Rates of depositionRates of deposition– Can estimate average rates of Can estimate average rates of

depositiondeposition– About 30 cm of sedimentary rock About 30 cm of sedimentary rock

formed every 1000 yearsformed every 1000 years– Rates can change though (Floods) so Rates can change though (Floods) so

age is estimated age is estimated

Age – Geological MethodsAge – Geological Methods

3.3. Varve countVarve count– Varves – banded layer of sand and silt Varves – banded layer of sand and silt

that is deposited annually in lakes that is deposited annually in lakes – Each band represents one yearEach band represents one year– Annual events bring in the sand and Annual events bring in the sand and

silt to form the layerssilt to form the layers Snow melt runoff in springSnow melt runoff in spring Leaves in the fallLeaves in the fall

Age – Chemical MethodsAge – Chemical Methods

1.1. Radiometric datingRadiometric dating– Age determined by comparing relative Age determined by comparing relative

percentages of a radioactive isotope percentages of a radioactive isotope and a stable isotopeand a stable isotope

– Radioactive isotope = parentRadioactive isotope = parent– Stable isotope = daughterStable isotope = daughter– Using known decay rate, can calculate Using known decay rate, can calculate

absolute ageabsolute age

Radiometric DatingRadiometric Dating

Half lifeHalf life– Time it takes half the mass of a given Time it takes half the mass of a given

amount of a radioactive isotope to amount of a radioactive isotope to decay into its daughter isotopedecay into its daughter isotope

– Not affected by temperature, pressure Not affected by temperature, pressure or other environmental factorsor other environmental factors

– C-14 half-life = 5,730 yearsC-14 half-life = 5,730 years– U-238 half life = 4.5 billion yearsU-238 half life = 4.5 billion years

Radiometric DatingRadiometric Dating

LimitationsLimitations– No parent or daughter isotopes can be No parent or daughter isotopes can be

lost or gained through leaking or lost or gained through leaking or contaminationcontamination

– If sample is too old, not enough parent If sample is too old, not enough parent isotopeisotope

– If sample is too young, not enough If sample is too young, not enough daughter isotopedaughter isotope

– Must choose correct isotopeMust choose correct isotope

Carbon DatingCarbon Dating Young rock age determined by Young rock age determined by

dating organic matter in the rock by dating organic matter in the rock by means of Carbon – 14 means of Carbon – 14

Living organisms contain both C-14 Living organisms contain both C-14 (radioactive) and C-12 (not (radioactive) and C-12 (not radioactive)radioactive)

C-14 to C-12 ratio constant during C-14 to C-12 ratio constant during lifelife

C-14 decays to C-12 after deathC-14 decays to C-12 after death

The Fossil RecordThe Fossil Record

FossilsFossils– The remains of animals or plants that The remains of animals or plants that

lived in a previous geologic timelived in a previous geologic time– Most commonly found in sedimentary Most commonly found in sedimentary

rockrock PaleontologyPaleontology

– The scientific study of fossilsThe scientific study of fossils

The Fossil RecordThe Fossil Record How fossils formHow fossils form

– MummificationMummificationFound in hot and dry placesFound in hot and dry places

– Amber – hardened tree sapAmber – hardened tree sap– Tar seepsTar seeps

Animals fall in and can’t get outAnimals fall in and can’t get out

– Freezing – buried in frozen soilFreezing – buried in frozen soil– PetrificationPetrification

Minerals replace organic materialsMinerals replace organic materialsCreates nearly perfect replicaCreates nearly perfect replica

The Fossil RecordThe Fossil Record

Types of FossilsTypes of Fossils– ImprintsImprints– Molds & CastsMolds & Casts

formed from empty shellsformed from empty shells

– CoprolitesCoprolitesfossilized waste materialsfossilized waste materials

– GastrolithsGastrolithsStones in dinosaur digestive systemsStones in dinosaur digestive systems

The Fossil RecordThe Fossil Record Trace FossilsTrace Fossils

– Tracks, footprints, borings and burrows giving Tracks, footprints, borings and burrows giving evidence of past animal movementevidence of past animal movement

Index fossilsIndex fossils– Fossil used to establish age of rock layers Fossil used to establish age of rock layers

because it is distinct, abundant, and because it is distinct, abundant, and widespreadwidespread

– Only existed for a short span of geologic timeOnly existed for a short span of geologic time