Presentation on theme: "Correlation and Dating of the Rock Record"— Presentation transcript:
1Correlation and Dating of the Rock Record Geologic TimeCorrelation and Dating of the Rock Record
2Time Relative Absolute Order of deposition of a body of rock based on positionAbsoluteA number representing the time a body of rock was deposited
3Relative TimeDetermining of sequence of eventsWhich came first?
4In what order did these events occur in American History?
5Relative Time Tools Smith Steno Lyell Fossil Succession Superposition Original horizontalityLyellCross-cutting relationshipsIntrusionsInclusions
6Principle of fossil succession Fossils occur in a consistent vertical order in sedimentary rocks all over the world. (William"Strata Bill" Smith, late 1700's, England).This principle is valid and does not depend on any pre-existing ideas of evolution. (In fact, Charles Darwin's ideas on evolution did not appear until 50 years later ).
7Principle of fossil succession Geologists interpret fossil succession to be the result of evolution - the natural appearance and disappearance of species through time."Fossil species succeed one another in a definite and recognizable order"Fossils at the base of a thick sequence of sediments (so older, by previous principles) are less like present-day species than those near the top
8Principle of fossil succession * Fossils unlike present-day species, but like each other, are found in widely separated sites* A fossil species which is observed to occur above (and so younger than, by previous principles) a second fossil species in one locality will always occur above that second species, wherever found.
10UnconformitiesUnconformities are buried surfaces of erosion or non-deposition
11Unconformities 1.Angular unconformities 2.Nonconformity Implies tectonic deformation and erosion of underlying strata.2.NonconformitySedimentary strata overlying igneous or metamorphic rocks (in an erosional - not intrusive- contact)3.DisconformityAn irregular surface of erosion between two units of parallel strata
22Intrusions vs Unconformities A xenolith is a fragment of country rocks which has been broken off during an intrusion, and has become surrounded by magma. The xenolith is older than the igneous rock which contains it.Through erosion and resedimentation, younger rocks will often have pieces of the older rock included (sedimentary).
26History of Geologic Time Geologic SystemsBody of rock that contains fossils of diverse animal lifeCorresponds to geologic periodSedgewickNamed CambrianMurchisonNamed Silurian
27StratigraphyStudy of stratified rocks, especially their geometric relations, compositions, origins, and age relationsStratigraphic unitsStrataDistinguished by some physical, chemical, or paleontological propertyUnits of time based on ages of strataGeologic SystemsCorrelationDemonstrate correspondence between geographically separated parts of a stratigraphic unitLithologicTemporal
28Units of Time Time-rock unit Time unit Boundary stratotype Chronostratigraphic unitAll the strata in the world deposited during a particular interval of timeErathem, System, Series, StageTime unitGeochronologic unitInterval during which a time-rock unit is formedEras, Period, Epoch, AgeBoundary stratotypeBoundary between two systems, series or stages, formally defined at a single locality
29Geologic Time Scale Chronologic units - Time/Age Eons (largest): Era PeriodsEpochsAges
30Geologic Time Scale Geochronologic Units = Place Eon (largest) = Eon Era = EraSystem = PeriodSeries = EpochStage = Age
31Biostratigraphy Biostratigraphic unit Stratigraphic range Defined and characterized by their fossil contentStratigraphic rangeTotal vertical interval through which that species occurs in strata, from lowermost to uppermost occurrence
32Biostratigraphy Index fossil Abundant enough in the stratigraphic record to be found easilyEasily distinguished from other taxaGeographically widespread and thus can be used to correlate rocks over a large areaOccurs in many kinds of sedimentary rocks and therefore can be found in many placesHas a narrow stratigraphic range, which allows for precise correlation if its mere presence is used to define a zone
33Magnetic Stratigraphy Use of magnetic properties of a rock to characterize and correlate rock unitsMagnetic fieldReversals in polarity of field are recorded in rocks when they crystallize or settle from water
34Magnetic Stratigraphy ChronPolarity time-rock unitPeriod of normal or reversed polarityNormal intervalSame as todayBlackReversed intervalOpposite to todayWhite
35LithostratigraphySubdivision of the stratigraphic record on the basis of physical or chemical characteristics of rockLithostratigraphic unitsFormationLocal three-dimensional bodies of rockGroupMemberStratigraphic sectionLocal outcrop of a formation that displays a continuous vertical sequenceType sectionLocality where the unit is well exposed, that defines the unit
36Lithologic Correlation Cross-sections of strataEstablish geometric relationshipsInterpret mode of origin
37Lithologic Correlation Grand CanyonMcKeeUsed Trilobite biostratigraphy to determine age relationshipsEastern portion of units is younger than western
38Facies Transgression Facies Facies changes Landward migration of shorelineGrand CanyonCambrian transgressionFaciesSet of characteristics of a body of rock that presents a particular environmentFacies changesLater changes in the characteristics of ancient strata
39Absolute Age 4.6 billion years old Early estimates Salts in the ocean 90 million years oldAccumulation of sediment100 m.y. or lessGaps in stratigraphic recordUnconformities represent large breaks in accumulationDidn’t include metamorphosed sedimentary rocksEarth’s temperatureKelvin20-40 million years old
40Absolute Ages How old is the Earth? 4.6 billion years (4,600,000,000 years)Radiometric dating (Uranium, Thorium). Mass spectrometer.
41Early Attempts1654 Archbishop Usher (Ireland), genealogy in Bible Earth was created October 22, 4004 BC,9:00 am was added laterEarth was 6000 years old.Led to the Doctrine of Catastrophism:Earth was shaped by series of giant disasters.Many processes fit into a short time scale.
42Early Attempts 1770's, 1780's "Revolution" James Hutton, Father of Geology (Scotland)Published Theory of the Earth in 1785.
43HuttonHadrian's Wall built by Romans, after 1500 years no change. Suspected that Earth was much older.Slow processes shape earth.Mountains arise continuously as a balance against erosion and weathering
44Hutton Doctrine of Uniformitarianism: "Present is key to the past". The physical and chemical laws that govern nature are uniformUnconformity at Siccar Point, Scotland"No vestige of a beginning, no prospect of an end"
45Charles LyellCharles Lyell 1800's compared amount of evolution shown by marine mollusks in the various series of the Tertiary System with the amount that had occurred since the beginning of the Pleistocene.Estimated 80 million years for the Cenozoic alone.
46Various GeologistsThickness of total sedimentary record divided by average sedimentation rates (in mm/yr).In 1860, calculated to be about 3 million years old.In 1910, calculated to be about 1.6 billion years old.
47Lord KelvinIn 1897, Lord Kelvin assumed that the Earth was originally molten and calculated a date based on cooling through conduction and radiation.Age of Earth was calculated to be about million years.
48Lord KelvinProblem: Earth has an internal heat source (radioactive decay)Discovery of radioactivity by Henri Becquerel in 1896.
49John JolyIn , John Joly (Irish) calculated the rate of delivery of salt to the ocean. River water has only a small concentration of salts. Rivers flow to the sea.Evaporative concentration of salts.Age of Ocean = Total salt in oceans (in grams) divided by rate of salt added (grams per year)Age of Earth = million years.
50John JolyProblems: no way to account for recycled salt, salt incorporated into clay minerals, salt deposits.
51von Helmholtz and Newcomb The German physicist Hermann von Helmholtz and the American astronomer Simon Newcomb joined in by independently calculating the amount of time it would take for the Sun to condense down to its current diameter and brightness from the nebula of gas and dust from which it was born.100 million years, consistent with Thomson's calculations. However, they assumed that the Sun was only glowing from the heat of its gravitational contraction. They knew of no other ways for it to produce its energy.
52Rutherford and Boltwood In 1905, they used radioactive decay to measure the age of rocks and minerals.Uranium decay produces He, leading to a date of 500 million years.
53Rutherford and Boltwood In 1907, Boltwood suspected that lead was the stable end product of the decay of uranium. Published the age of a sample of urananite based on Uranium-Lead dating.Date was 1.64 billion years.
54Age of Earth So far, oldest dated Earth rocks are 3.96 billion years. Canadian Shield. (NW Territories near Great Slave Lake, 3.96 byr).Detrital Zircons in sedimentary rocks are byrOlder rocks include meteorites and moon rocks with dates on the order of 4.6 billion years.
55Geologic Time Scale. The age for the base of each division is in accordance with recommendations of the International Commission on Stratigraphy for the year 2000.
63Absolute Age Radioactive decay Becquerel, 1895 Uranium undergoes spontaneous decayAtoms release subatomic particles and energyChange to another elementParent isotope decays/daughter isotope produced
64Principles of Radiometric Dating Naturally-occurring radioactive materials break down into other materials at known rates. This is known as radioactive decay.Radioactive parent elements decay to stable daughter elements.
65What is an Isotope?Nuclide of an element with different masses
66Absolute Age Three modes of decay Loss of alpha particle Convert parent into element that has nucleus containing two fewer protonsLoss of beta particleConvert parent into element whose nucleus contains one more proton by losing an electronCapture of beta particleConvert parent into element whose nucleus has one less proton
67Absolute Age Radiometric dating Half-life Radioactive isotopes decay at constant geometric rateAfter a certain amount of time, half of the parent present will survive and half will decay to daughterHalf-lifeInterval of time for half of parent to decay
68Absolute Age Useful isotopes Uranium 238 and thorium 232 Zircon grainsUranium 238 and lead 206Fission track datingRubidium-StrontiumPotassium-Argon, Argon-ArgonRadiocarbon datingProduced in upper atmosphereHalf life = 5730 yearsMaximum age for dating: 70,000 yearsBone, teeth, wood
69Absolute Age Fission-Track Dating Measure decay of uranium 238 by counting number of tracksTracks formed by subatomic particles that fly apart upon decay
70Radioactive parent isotopes and their stable daughter products Each radioactive isotope has its own unique half-life.A half-life is the time it takes for half of the parent radioactive element to decay to a daughter product.
71Radioactive parent isotopes and their stable daughter products Radioactive Parent Stable Daughter Half LifePotassium 40 Argon billion yrsRubidium 87 Strontium billion yrsThorium 232 Lead billion yearsUranium 235 Lead million yearsUranium 238 Lead billion yearsCarbon 14 Nitrogen years
73Radioactive DecayRadioactive decay occurs by releasing particles and energy.Alpha particlesBeta particlesNeutronsGamma rays (high energy X-rays) are also produced.
74Radioactive Decay Alpha particles (He) large, easily stopped by paper charge = +2mass = 4
75Radioactive Decay Beta particles penetrate hundreds of times farther than alpha particles, but easily stopped compared with neutrons and gamma rays.charge = -1mass = negligible
76Radioactive decay series of uranium-238 (238U) to lead-206 (206Pb).
77Datable MineralsMost minerals which contain radioactive isotopes are in igneous rocks. The dates they give indicate the time the magma cooled.Potassium 40 is found in:potassium feldspar (orthoclase)muscoviteamphiboleglauconite (greensand; found in some sedimentary rocks; rare)
78Datable RocksRadioactive elements tend to become concentrated in the residual melt that forms during the crystallization of igneous rocks. More common in SIALIC rocks (granite, granite pegmatite) and continental crust.
79Datable RocksRadioactive isotopes don't tell much about the age of sedimentary rocks (or fossils). The radioactive minerals in sedimentary rocks are derived from the weathering of igneous rocks. If the sedimentary rock were dated, the age date would be the time of cooling of the magma that formed the igneous rock. The date would not tell anything about when the sedimentary rock formed.
81How does 14Carbon dating work? Cosmic rays from the sun strike Nitrogen 14 atoms in the atmosphere and cause them to turn into radioactive 14C, which combines with oxygen to form radioactive CO2.
82How does 14Carbon dating work? Living things are in equilibrium with the atmosphere, and the radioactive CO2 is absorbed and used by plants. The radioactive CO2 gets into the food chain and the carbon cycle.
83How does 14Carbon dating work? All living things contain a constant ratio of 14C to 12C (1 in a trillion).At death, 14C exchange ceases and any 14C in the tissues of the organism begins to decay to Nitrogen 14, and is not replenished by new 14C.
84How does 14Carbon dating work? The change in the 14C to 12C ratio is the basis for dating.The half-life is so short (5730 years) that this method can only be used on materials less than 50,000 years old.Assumes that the rate of 14C production (and hence the amount of cosmic rays striking the Earth) has been constant.
85Deviation of carbon-14 ages to true ages from the present back to about 5000 B.C. Data are obtained from analysis of bristle cone pines from the western United States. Calculations of carbon-14 are based on half-life of 5730 years. (Adapted from Ralph, E. K., Michael, H. N., and Han, M. C Radiocarbon dates and reality. MASCA Newsletter 9:1.)
86Absolute Age Best candidates for most radiometric dating are igneous Not necessarily useful for sedimentsError in age estimate can be sizable
87Absolute Age Absolute ages change Error increases in older rocksTechniques changeBiostratigraphic correlations are usually more accurateRadiometric dates used when fossils not present
88How old is the Old Red Sandstone? Older than 425 myrYounger than 370 myrBetween 425 and 370 myrHave no idea