lect7 - Feb. 1, 2012 Geologic time (geochronology) Earth is...

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Unformatted text preview: Feb. 1, 2012 Geologic time (geochronology) Earth is ~4.6 Ga (billion years old) ..compressed into 1 year Jan 1: Earth formed Feb 21: life formed molluscs in the ocean Oct 25: complex organisms amphibians come out of water Dec 7: reptiles evolved Dec 25: dinosaurs extinct Dec 31, 11:00 pm: homo sapiens appear Dec 31, 11:59:59.97: Columbus discovers Americas • typical university course: 0.0000011% of Earthʼs history - Geologic time (geochronology) Historical perspective absolute dating Ussher, 1625: Sat evening, 22 Oct, 4004 BC (Bible using science genealogy; catastrophism) to find age Hutton, 1770-80’s: like, really 'old' - older than 5500 years Kelvin, 1890’s: 24-40 Ma (cooling molten earth model) Joly, 1890’s: 90-100 Ma (ocean salinity model) eo-1900’s: 3 Ma to 1600 Ma (sediment deposition model) Becquerel & Currie, 1896: radioactivity discovered; first age estimate = 1640 Ma James Ussher meaning that the Earth was only 5500 years old Hutton and Ussher didn't put measure on how old the Earth was - RELATIVE DATING thought that Earth was molten hot ball - constantly cooling 1 Geologic time (geochronology) 1) relative dating – compare 2 or more entities to determine which is older ex. decrepit apple is older than red, crisp one 2) numerical dating – specifying the actual number of years (± years) that have passed since an event occurred (also known as absolute age dating) We learned how to classify rocks.. now we need to classify time 2 Principles of relative dating Rules to compare rocks that are not located near one another: 1) Principal of Uniformitarism (Hutton) • "present is key to past" • same processes act throughout time, but possibly at different rates • more influential than catastrophes hurricanes, floods, impacts looked at sediment deposition in rivers - compare these rocks to current rocks Principles of relative dating 1) Law of Uniformitarism (Hutton) • for general cases: OK (rivers flow downhill; volcanoes erupt lava) • specific cases: ? (there has not always been rivers; volcanoes not always erupt same composition lava) e.g. ancient Earth spun more rapidly than does modern Earth (440 ʻdaysʼ v 365 days) But still useful to determine how rocks are 'positioned' relative to each other 3 2) Law of Superposition • most sediments settle from water / wind • young rock material (sediment) is deposited on top of earlier, older deposits 3) Principle of Horizontality • layers of sediment (& lava) are generally deposited in a horizontal position • when/if tilted, how identify upper or top surface? 4 formed by water movement ripple marks layer of mud that cracks as water evaporates mudcracks 4) Principle of Cross-cutting Relationships • intrusive formation must be younger than rock it cuts across; also used for faults fault is younger than the rocks - cuts across older rock the dike is younger than rock surrounding it - intrusive formation 5 lateral correlations - sequence of rocks that are over a great range of space vertical correlation • superposition • horizontality • x-cutting relations what about lateral correlations? Correlation of rock layers Matching of rocks of similar ages in different regions is known as correlation • often relies upon fossils • William Smith (late 1700s) noted that sedimentary strata in widely separated area could be identified & correlated by their distinctive fossil content • Lithostratigraphic - matching up continuous rock sequences - can use the different traits of the rock • Chronostratigraphic - matching up rocks of the same age; usually done with fossils using biostratigraphy - can assign dates depending on their fossil content 6 Principles of relative dating Correlation often relies upon fossils • fossils- remains of ancient organisms, or other evidence of their existence, that became preserved in rock • only 1% of all species ever existed are preserved as fossils • use superposition & horizontality to determine lot of soft tissue = no fossil age of rock/fossil within = relative age of fossils Principles of relative dating 5) Principle of Fossil Succession • fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content • short-lived, widespread organisms = index fossils - can assign a very definitive time to it ex. if we find a dinosaur fossil, it probably came from the Cretaceous or Jurassic period 7 can say that B is definitely older than A time Determining the ages of rocks using fossils Correlation of rock layers • Vertical and horizontal relationships of rocks • How about their regional rock unit boundaries? contact 8 Principles of relative dating used to describe contacts between rocks 1) Conformable sequences • layers of rock deposited w/o interruption - unrealistic - erosion, tectonics 2) Unconformity • a gap or break in the rock record produced by erosion and/or nondeposition of rock units • implies tectonic movements (uplift) 2) Unconformity types: a) Disconformity – strata on either side of the unconformity are parallel, but ages differ How determine? - composition change - deposition style change - paleosol (ancient soil) - conglomerate/breccia - colour change? signify some sort of tectonic upheaval 9 2) Unconformity types: b) Angular unconformity – tilted/folded rocks are overlain by flat-lying rocks deposition of sediments by river mountain formation erosion of mountains - flat again angular unconformity is here deposition of sediments by river Angular unconformity prob. big diff in time between upper and lower layers 10 2) Unconformity types: c) Nonconformity – metamorphic or igneous rocks in contact with sedimentary strata very diff rock types hanging out together = nonconformity sedimentary metamorphic rock Unconformities in Grand Canyon disconformity - Muav should be same angular unconformity metamorphic sediments noncomformity 11 F is the oldest - oldest tends to be flat - G is some sort of igneous rock that intruded B and C- angular unconformity occurs H oldest - then G horizontally - F igneous - C is younger than D, and older than B 12 H is the oldest - G and F deposited on top then E, D, C, and then disconformity and B and A J is the youngest C, D, E are older than J while A, B are younger than C,D,E Absolute / numerical dating Sometimes relative dating is enough.. but other times we need to know the age (in years) of a rock or geologic event Common numerical dating relies on the rate of decay of radioactive isotopes w/in minerals 13 Using radioactivity in dating Reviewing basic atomic structure • Nucleus: Protons – particles+ with mass Neutrons – neutral particles with mass • Electrons: particles- that orbit the nucleus Using radioactivity in dating Radioactivity • spontaneous changes (decay) in the structure of atomic nuclei nucleus unstable Rutherford-Soddy Law number of radioactive atoms decaying at any time "t" is proportional to the number "n" of atoms present !n if lots of radioactive "n atoms, we'll have !t more decay • produces isotope- new form of element; same # protons, different # neutrons thru emission or capture of subatomic particles 14 Types of decay 1) Alpha emission α particle = 2 protons & 2 neutron (i.e. He) • nucleus decays (expulsion of) alpha particle • mass number is reduced by 4 • atomic number is lowered by 2 (U238 -> Th234) 2) Beta emission β particle = electrons originate from breakdown of neutron in nucleus • increases # protons • increases atomic # (K40 -> Ca40) 3) Electron capture nucleus steals electron from atomʼs own orbiting cloud • creates new neutron; decreases atomic # (Ca40 -> K40) proton becomes a neutron by stealing an electron Using radioactivity in dating • Parent – an unstable (decaying) radioactive isotope (U238, K40, Rb87) - unstable decays until it is a daughter • Daughter – the isotopes resulting from the decay of a parent (Pb206, Ar40, Sr87) • Half-life – defined by the decay constant λ for each isotope – the time required for one-half of the radioactive nuclei in a sample to decay and become daughter equal parent to daughter ratio 15 Isotopes used in geochronology and produced by supernova explosions before our solar system formed (explosive nucleosynthesis) geochronology for igneous & metamorphic rocks stable daughter isotope 16 Using radioactivity in dating 6) Principle of radiogenic dating • percentage of radioactive atoms that decay during one half-life is always the same (50%) • however, actual number of atoms that decay continually decreases Radioactive decay curve Counting half-lives: • Half-lives: 1 2 3 4 • Parent: 1/2, 1/4, 1/8, 1/16, etc. • Daughter: 1/2, 3/4, 7/8, 15/16, etc. • P:D: 1:1, 1:3, 1:7, 1:15 Ratios of 1:3, 1:7, 1:15, etc. are for whole half lives, but any ratios can be measured; e.g. 1:4.2, or 8.6:1 17 • Do not know exactly when the individual (kernel, atom) will convert (pop, decay) • Do know the probability that it will occur in the next time interval age equation t = ln (P+D)/P λ • only two measurements are needed: 1) parent : daughter (measured w/ mass spectrometer) 2) decay constant (λ , measured w/ scintillometer) don't need to know absolute amounts for 1) 18 Using radioactivity in dating 6) Principle of radioactive dating • rates of decay have been measured and do not vary through time • assume closed system (initial mineral has only parent material; no loss of daughter after mineral forms) • radiometric clock in minerals starts when daughter product begins to be trapped in mineral (usually controlled thermally) - we're not losing any daughter or parent forming - stagnant system - ex. nothing except uranium forming into lead Radiogenic dating (constraining) of sedimentary sequences 19 Other chronometers: • varve chronology (layered lake seds) annual deposition • dendrochronology (tree rings) " • lichenometry (lichen diameters) less craters, younger • crater impact density (surfaces of planets) • surface exposure age dating; intergalactic cosmic-ray bombardment of Earth’s surface • C14 dating (bones, wood, paper, cloth); Carbon-14 is produced by cosmic ray bombardment of Nitrogen-14 in the atmosphere; dating is useful for about 10 half lives, or only about 57,000 years - doesn't really have applications in geology compared to archaeology • C14 occurs in organic remains rather than in minerals • C14 is absorbed by all living organisms from the atmosphere or the food they eat • Clock is 'set' when an organism dies 20 geologic divisions in cream •Archean Eon (>2.5 Ga): Ancient primitive crust preserved some unicellular microorganisms •Proterozoic Eon (2.5 Ga-543 Ma): More evolved rocks, oxygenation of atmosphere & development of more advanced life forms •Phanerozoic Eon (543 Ma- ): Advanced life forms; modern tectonics •Paleozoic Era (543-251 Ma): From marine invertebrates to dinosaurs •Mesozoic Era (251-65 Ma): Dinosaurs flourish & become extinct; reptiles evolve •Cenozoic Era (65 Ma- ): mammals flourish 21 ...
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This note was uploaded on 03/26/2012 for the course GEO 1111 taught by Professor Dumas during the Winter '09 term at University of Ottawa.

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