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GEO1111 Lecture 4- GeochronologyWhich occurred first? Are they the same age?
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How do we measure these ages?Learning ObjectivesUnderstand principals of:Relative datingRadioactive datingApply principals to specific earth systemsLearn age of earth and strata and key periods of geologic time Geologic time (geochronology)Why does age matter???Who cares how about the age of the formation or the rock or the ice or the fossils???Sometimes it’s academic other times it is for economic output, ie. Mineral resources such as oil, coal,uranium, etc.An animated view of Earth HistoryIf 4.6 Ga is compressed into 1 year …Jan 1: Earth formedFeb 21: life formedOct 25: complex organismsDec 7: reptiles evolvedDec 25: dinosaurs extinctDec 31, 11:00 pm: homo sapiens appearDec 31, 11:59:59.97: Columbus discovers AmericasTypical university course: 0.0000011% of Earth’s historyHistorical PerspectiveUssher, 1625: Sat evening, 22 Oct, 4004 BC (based on Bible’s chronology and assuming catastrophism).Ussher believed this was the starting point of the universe.Hutton, 1770-80 few million years old based on erosion calculations …Kelvin,1890’s: 24-40 Ma (cooling molten earth model) he tried to say the earth was really hot and itthen cooled he calculated this using the rate of cooling based on the temperature the earth was when hewas alive
Joly,1890’s: 90-100 Ma (ocean salinity model)- used the amount of salt added to water each year thenfrom there and calculated the relative age it would take for that amount of salt to be there.Becquerel & Currie, 1896: radioactivity discovered; first age estimate = 1640 MaGeologic time1) relative dating– compare 2 or moreentities to determine which is older2) numerical dating– specifying the actual number of years (± years) that have passed since an eventoccurred (also known as absolute age dating)Geologic maps of rock type can be complicated!Principles of relative dating Rules to compare rocks to one another:1) Principal of Uniformitarism (Hutton)• "present is key to past"•same processesact throughout time, but at different rates• E.G. Sediments have been deposited in oceans which cause more or less organisms within an area• Ongoing processes can be more influential than catastrophes such as hurricanes and floods etcPrinciples of relative dating1) Law of Uniformitarism (Hutton)• General cases: OK (rivers flow downhill; volcanoes erupt lava) Specific cases: ? (there has not alwaysbeen rivers; volcanoes not always erupt same composition lava). Good for concept and relativitye.g. ancient Earth spun more rapidly than does modern Earth (440 ‘days’ v 365 days) But still useful todetermine how rocks are 'positioned'2) Law of Superposition• most sediments settle from water / wind• youngrock material (sediment) is depositedon top of earlier,olderdeposits the question is how do