chapter 8: geological times

Geochronology is the study of time in relation to earth’s existenceRelative Dating

Determines how old a rock is in relation to its surrounding

Numerical DatingDetermines actual age in years

Chapter 8: Geological TimesChapter 8: Geological Times

Geologic Time - PerspectiveGeologic Time - Perspective

Differing Views of the Earth’s AgeJames Ussher (1664): 5,668 years oldBiblical Perspective Modern Scientific View

Relative DatingRelative DatingRelies on Key Principles such as Uniformitarianism- the present is key to the past Original horizontality

Sediments deposited in horizontal layers Superposition

Youngest rocks are on top (assuming no tectonic activity) Cross-cutting relationships

Cut layer is older than ‘cutting’ rock Faunal succession

Organisms succeed one another in recognizable reproducible pattern

Unconformity Represents a break (gap) in the rock record

Earth HistoryEarth History

Sedimentary StructuresSedimentary Structures

An organism buried in sedimentAn organism buried in sediment

3 Types of unconformities3 Types of unconformities

3 Types of unconformities-contd3 Types of unconformities-contd

3 Types of unconformities-contd.3 Types of unconformities-contd.

Hypothetical view of early earthHypothetical view of early earth

Grand CanyonGrand Canyon

Sedimentary Rock SequenceSedimentary Rock Sequence

Hypothetical LandscapeHypothetical Landscape

Relative Dating-contd.Relative Dating-contd. INDEX FOSSILS: Species that had wide geographical

distribution, but lived for only a brief period of time Unconformmities: Boundary between 2 layers that are of

different age Reasons:

– Rock layers removed by erosion– No rock Layers deposited

CORRELATION METHODS

METHODS OF CORRELATION:

Paleontological Similarities

Mineralogical Similarities

Numerical Dating MethodNumerical Dating Method Isotope Dating relies on the rate of decay of

radioactive isotopes within a rock Radioactive isotopes have nuclei that

spontaneously decay emitting or capturing a variety of subatomic particles

Decaying radioactive isotope- parent isotopes decay to form daughter isotopes

Half-life- is the time it takes for half the atoms of parent isotope to decay

Some radioactive isotopes with daughter products U-238 => Pb-206; K-40 => Ar-40; C-14 => N-14

Unstable atomic nuclei decayUnstable atomic nuclei decay

Beta Decay – unstable atomic Beta Decay – unstable atomic nucleinuclei

Electron Capture – unstable nucleiElectron Capture – unstable nuclei

Radioactive DecayRadioactive Decay

Bracketing AgesBracketing Ages

Factors affecting Isotope Dating Results Factors affecting Isotope Dating Results

Isotope dating is more useful for igneous rocksClock is set when igneous rock crystallizes locking the

radioactive isotopes within its crystal lattice

Rock/Mineral must be a closed systemAtoms of parent and daughter are still present in

rock/mineral being dated

Condition of parent MaterialFracture, weathering and migrating ground water

Age of SubstanceEnough measurable daughter isotope, use appropriate

radioactive isotope

Enough Number of Atoms??Enough Number of Atoms?? Case Study: Half-life = 10 days Time (d) Number of atoms: 0 1024 80 d 4 10 512 90 d 2 20 256 100 d 1 30 128 1000 atoms----- 1

atom 40 64 in 10 half-life 50 32 60 16 70 8

Loss of daughter isotopesLoss of daughter isotopes

Loss of daughter isotopes-contd.Loss of daughter isotopes-contd.

Loss of daughter isotopes-Loss of daughter isotopes-contd.contd.

Loss of daughter isotopes-contd.Loss of daughter isotopes-contd.

Carbon-14 MethodCarbon-14 Method

Other Numerical dating methodOther Numerical dating method Fission Track

High speed particles emitted during radiation may pass through crystal leaving ‘tears’ within the crystal- the older the rock, the more fission tracks

Dendrochronology (Tree-Ring dating)Annual growth rings

Varve- deposited layers of lake-bottomPaired layers of sediments

LichenometryLichens grow at a fairly constant rate

Cosmogenic isotopesUsed in dating land features

Fission-Track Dating methodFission-Track Dating method

Correlation of tree ring sectionCorrelation of tree ring section

Origin of Lake VarvesOrigin of Lake Varves

Origin of Lake varves-contd.Origin of Lake varves-contd.

Age of LichenAge of Lichen

Cosmogenic Isotopes-surface exposure datingCosmogenic Isotopes-surface exposure dating

Geology at a GlanceGeology at a Glance

Geological Time ScaleGeological Time Scale Contrasting several dating techniques chronicling

Earth’s history to produce a geologic Time ScaleGeologic Time Scale- divided into Eons, Eras,

Periods, and EpochesPhanerozoic Eon (evidence of life began) divided

into three erasPaleozoic (ancient life) dominated by marine

invertebratesMesozoic (middle life) dominated by reptilesCenozoic (recent life) dominated by mammals

Geological Time ScaleGeological Time Scale

Sequence of Time scaleSequence of Time scale

Eons--- Eras---Periods---EpochsLife on earth ~4.6 byr oldLack of 4.6 byr old on earth – however,

moon rocks and meteorite provide a handle on this

Extinction GraphExtinction Graph

METAMORPHISM & METAMORPHIC METAMORPHISM & METAMORPHIC ROCKSROCKS

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