1Geologic time (geochronology)Earth is ~4.6 Ga (billion years old)Jan 1: Earth formedFeb 21: life formedOct 25: complex organismsDec 7: reptiles evolvedDec 25: dinosaurs extinctDec 31, 11:00 pm:homo sapiensappearDec 31, 11:59:59.97: Columbus discovers Americas• typical university course: 0.0000011% of Earthʼs history..compressed into 1 yearHistorical perspectiveUssher, 1625: Sat evening, 22 Oct, 4004 BC (Biblegenealogy;catastrophism)Hutton, 1770-80’s: like, really 'old'Kelvin, 1890’s: 24-40 Ma (cooling molten earthmodel)Joly, 1890’s: 90-100 Ma (ocean salinitymodel)eo-1900’s: 3 Ma to 1600 Ma (sediment depositionmodel)Becquerel & Currie, 1896:radioactivitydiscovered; firstage estimate = 1640 MaGeologic time (geochronology)
21) relative dating–compare 2 or moreentities to determine which is older2) numerical dating–specifying theactual number of years (± years) thathave passed since an event occurred(also known asabsolute age dating)Geologic time (geochronology)We learned how toclassify rocks..now we need toclassify time
3Principles of relative datingRules to compare rocks that are not located nearone another:1) Principal of Uniformitarism(Hutton)• "present is key to past"•sameprocessesact throughout time,but possibly at different rates• more influential than catastrophes -hurricanes, floods, impactsPrinciples of relative dating1) Law of Uniformitarism(Hutton)• for general cases: OK(rivers flow downhill; volcanoes erupt lava)• specific cases: ?(there has not always been rivers; volcanoes not alwayserupt same composition lava)e.g. ancient Earth spun more rapidly than does modern Earth (440ʻdaysʼv 365 days)But still useful to determine how rocks are 'positioned'