Chapter 8: Geological Times
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Transcript of 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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