3How Old is the Earth? Historical Records, written word 5000 yearsModern View, ~ 4.6 billion years oldHutton (1795)Uniformitarianism“deep” time very oldEarthBishop Usher (1664)9:00 AM, Oct. 23, 4004 BCEarth is ~6000 years oldGeologic eventscatastrophism
4How Old is the Earth Relative Dating Geochronology is the study of time in relation to earth’s existenceRelative DatingDetermines how old a rock is in relation to its surroundingAbsolute (Numerical) DatingDetermines actual age in years
5Geologic Time and the Rock Record Rocks record the processes and events and help us measure geologic time.By studying outcrops with the scientific method, we can figure out the relative order of events! Even absolute ages!How?Geometric relationships Stratigraphy.Fossils Biostratigraphy.Radiometric dating Geochronology.
6Stratification (Strata) Layering of Sedimentary Rocks Distinct layeringBedsSedimentary RockBed 1Bed 2Different thicknessesColor, & other characteristics
7Relative Age Inferences Original Horizontality Sedimentary rocks are formed in layers (strata) which were originally horizontal.If layers are inclined at an angle, then something tilted them - they didn’t form that way
9Relative Age Inferences Superposition If one layer is on top of another, then it came later (it’s younger).Note that layers can be completely upside down, and you need something like ripple marks to tell which way the layers are “facing”YoungestOldestYoungestOldest
10Relative Age Inferences Cross-cutting relationships Faults are younger than what they cut.Crosscutting igneous rocks are younger than what they intrude.
11Relative Age Inferences Inclusion Units that include bits of another came later (are younger)youngerolderolderyounger
12Relative Age Inferences Assumptions / Geometric Principles:Sediments deposited horizontallyYounger sediments on top of olderUnits that cross-cut (e.g. faults or intrusions) came after (i.e., are younger than) those that they cutUnits that include bits of another came later (are younger)
13Let’s practiceList events from oldest to youngest (including faulting and erosion)Deposition of Abo Formation, Yeso Formation, Moenkopi Formation, Agua Zarco FormationFault (covered) offsets the four sedimentary unitsErosion (especially of Moenkopi)Emplacement of Bandelier Rhyolite (as hot ash flow)Erosion
14Missing Time-Gaps Happen Buried and tilted erosional surface
15Conformable ContactLayers of rock that have been deposited without any interruption.No gaps in time.No missing record due to erosion, non-deposition, etc.
16Unconformity 3 types of break in the rock record. Such surfaces represent:A hiatus in deposition and/or…A period of erosion.“Missing time”Significant events.Popostosa Fm. Playa deposits & post-Santa Fe Group (Pl) alluvium.N of San Lorenzo Cyn (Socorro, NM) – P. A. Scholle (1999).
17Angular UnconformityA sharp discontinuity in the rock record separating strata that are not parallel.Indicates that during the break, a period of deformation occurred.
18DisconformityA break in the rock record across which there is little change in orientation of strata.Often just a pause in deposition (subtle).May also be obvious erosion surface.River Road
19NonconformityHorizontal sedimentary rocks on top of eroded crystalline rocks (metamorphic or igneous).Requires erosion to bring crystalline rocks to the surface.
24Relative Dating faunal succession groups of fossil animals and plants occur in the geologic record in a definite chronological orderperiods of time recognized by characteristic fossilsdinosaurstrilobites
25Correlation of Rock Units: Index Fossils Common occurrenceWide geographic distributionVery restricted age range
29The Geologic Column and the Geologic Time Scale In 19TH Cent., geologists began to assemble a geologic columncomposite column containing, in chronological orderthe succession of known strata, fitted together on the basis of their fossils or other evidence of relative age.The corresponding column of time is the geologic time scale.
30Geologic Column Catalog of all known strata Not one physical locality but a chronological compilation of all localities
32Eon: largest interval into which geologic time is divided. Hadean EonSome moon samples were formed during the Hadean Eon.Archean EonArchean rocks, which contain primitive microscopic life forms are the oldest rocks we know of on the Earth.Proterozoic EonPhanerozoic Eon
33Relative Time Scale Worldwide changes in fossils give break points When did dinosaurs go mostly extinct?
50Relative Time Scale Worldwide changes in fossils give break points The relative time scale doesn’t give us numerical ages.Where do these numbers come from?
51Early Attempts to Measure Geologic Time Numerically Quantity of SomethingTime =Rate Quantity changes with timeFor example, Rates of sedimentation & thickness of sedimentary rocksProblem: did not account for past erosiondifferences in sedimentation rates
52Early Attempts to Measure Geologic Time Numerically Saltiness of Seawater (date the ocean)Edmund Halley (1715)John Joly (1889)SaltriversAnswer: ~ 90 million yearsOceansIncorrect!!!Salts are added both by erosion and by submarine volcanism, but salts are also removed by solution.
53Early Attempts to Measure Geologic Time Numerically Lord Kelvin (1870’s), a physicist, attempted to calculate the time Earth has been a solid body.
54Early Attempts to Measure Geologic Time Numerically Lord Kelvin (1897’s), a physicist, attempted to calculate the time Earth has been a solid body.Time=0Time=TodayCooling off by conductionEarthSolidEarthmoltenNo more heatingTheory of heat conductionExperimental data (melting temp. of rocks, size of Earth)Answer: million yearsToo Young for Geologists!
56Radioactivity: A Little History H. Becquerel (1896)discovers radioactivity in UraniumMarie Curie (1900)discovers radium & heat is given off as byproduct of radioact.E. Rutherford (1905)Radioactive elements transform from one chemical element to anotherB. Boltwood (1907)Radiometric dating of minerals ( million years)
57Radioactive Atoms Atoms contain Protons, Electrons & Neutrons Carbon: Atomic number =6 (6 protons)StableIsotopeUnstableIsotopeIsotope: atoms of the same element containing different # neutrons
58Radioactive DecayIt turns out that some elements will spontaneously turn into other elements. This is called radioactivity
59Half-life (T1/2)Time needed for ½ of parent atoms to decay (rate of decay)T1/2= 1 hourTime# of Parent atoms# of daughter atoms10001 hr5002 hr2507503 hr125875
61Decay rate is a non-linear process All radioactive elements follow the same lawBut, each element will have different decay rates (half-life)1 nanosecond 49 billion years
62Decay RatesDecay rates are unaffected by geological processes (mainly chemical)Once radioactive atoms are created they start to act like ticking clockKnow the decay rateAABABCount the daughter atomsCount the Parent atomsAABACalculate the time since the atomic clock started ticking
63Mass spectrometerA Minnesota ConnectionAlfred Nier
64Potassium-Argon Dating 40K-40Ar half-life = 1.3 billion yearsCrystallization40K40Ar40K40K40Ar40K40ArK-mineralClosed systemMagmaClock is tickingOpen systemRock clock is resetAge of crystallization
65Closed system no leakage or addition of K or Ar K-mineralClosed systemClosed system no leakage or addition of K or ArGeological processes can allow material to be added or lost date will be incorrectRock clock is resetAge of crystallizationCross-check with other radiometric systems using different minerals
66Carbon-14 Method Atmosphere Neutron + 14-Nitrogen 14-Carbon CO2 After DeathTime=014-C decays 14-NT1/2= 5730 yrsSurfaceCO2Water, Plants, &Animals
67Isotopic Systems Used for Radiometric Dating Rubidium-Strontium t1/2= 47 billion yrUranium-lead t1/2=4.5 billion yrPotassium-Argon t1/2=1.3 billion yrCarbon-14 t1/2=5730 yrLong t1/2 useful for dating old materialShort t1/2 useful for dating young material
68Dating the Geologic Time Scale Sedimentary and Igneous Rocks Granite is older than OLD RED SANDSTONEVolcanic is younger than OLD RED SANDSTONE
69Absolute Geologic Time Scale EonEraPeriodStarting Age (Ma)PhanerozoicCenozoicQuaternary65TertiaryMesozoicCretaceous248JurassicTriassicPaleozoicPermian540PennsylvanianMississippianDevonianSilurianOrdovicianCambrianPrecambrian - Proterozoic2500Precambrian - Archean3800Precambrian - Hadean4500
70Age of Earth Oldest dated rocks 3.94 by Oldest dated material 4.2 by Moon Rocks & Meteorites by
71A Year of Geologic Time 1second ≈ 200 years 0:00 AM, Jan 1 Formation of EarthLate JanuaryFormation of Core-Mantle-CrustMid FebruaryLife Begins, Oldest Know RocksLate MarchFirst Photosynthetic OrganismsMid JulyEvolution of Cells with NucleusMid NovemberFirst Organisms with ShellsLate NovemberFirst Land Plants/FishMid DecemberDinosaurs became DominantDec 26Extinction of DinosaursEvening of Dec 31Human-like Animals11:59:45-11:59:50Rome Ruled the Western World11:59:59Modern Geology Started with Hutton
73Clair Patterson & the Age of the Earth In early 1950’s, Clair Patterson was a graduate student at the University of Chicago.Wanted to use lead isotope ratios to determine the Earth’s age,but the background level of lead contamination was too highLead used in gasoline, paints, plumbing, solder (cans for food) and pesticides.
74Clair Patterson & the Age of the Earth To accurately measure very low lead concentrations, Patterson created the modern laboratory ‘clean room’.in 1953, published estimate of Earth’s age as 4.55 BY (previously estimated at 3.3 BY)
75By 1960’s, Patterson began to worry about the extent of lead contamination in our environment. Patterson discovered that modern humans had 700 to 1,200 times as much lead in their bones as pre-Columbian Incas. over 99% of the northern hemisphere atmospheric lead originated from human activity.The average atmospheric lead levelswere 10 to 200 times higher than in pre-industrial times and up to 1,000 to 10,000 times higher in urban areas!
76First recognition of the global scale and early history of lead pollution First recognition that essentially EVERYONE in 1950’s-60’s society suffered from low-level lead poisoning.Patterson campaigned extensively for lead removal, but was vigorously opposed by industry labs and some other scientists.Eventually, scientific data accumulated by Pattersonand others led to the 1970 Clean Air ActBy 1991, lead levels in Greenland snow had fallen by a factor of 7.5