2 time is critical for geologic processes Rockies and Alps are ~3000 m tall-- mountains grow at ~1 meter per 5000 yrs (0.2 mm/yr)m x 5000 yr/m = 15,000,000 (yrs necessary)Atlantic Ocean is ~5000 km across-- today, seafloor spreading in Atlantic is ~4 cm/yrkm = 6000 km x 1000 m/km x 100 cm/m= 600,000,000 cm-- 600,000,000 cm / 4 cm/yr = 150,000,000 yearsfor comparison: fingernail grows at 1 cm/yr
3 age of the Earthprior to 19th century, accepted age from religious beliefs-- 6,000 years for Western culture (Christian)…Bishop Usher from geneology in the Bible-- old beyond comprehension (Hindu/Buddhist/Chinese)-- age not certain (Islam)during 19th century, length of time required forgeologic processes to occur was recognized-- fundamental contribution of geologyto scientific knowledge
4 historical developments James Hutton ( ) “Father of Modern Geology”native of Edinburgh, Scotlandeducated as a medical doctor in Leiden (1749)passionate about scientific inquiry“Theory of the Earth” -- processes are slow; take a long timeCharles Lyell ( )Scotsman who attended Oxford Universityfather was an avid naturalistrebelled against prevailing thought of “catastrophism”.“Principles of Geology” -- popularized Hutton’s viewsidea of “uniformitarianism” --same processes operating today occurred in the past….the present is the key to the past….
5 the key to the past relative time absolute time relative time vs. absolute timerelative timeorder of events or objects from first (oldest) to last (youngest)she is older than he is; she was born first and he was born lastabsolute timeage of events or objects expressed numericallyshe is twenty-one and he is nineteenstudy of timing of geologic events and processes is geochronology
6 relative time and relative order apply simple concepts to determine…• original horizontality• superposition• lateral continuity• cross-cutting relationships• inclusions• unconformities
7 relative age dating concepts original horizontalityall beds originally deposited in water formed in horizontal layerssediments will settleto bottomand blanketthe sea floor
8 relative age dating concepts superpositionyoungestwithin a sequence of undisturbedsedimentary or volcanic rocks,oldest rocks are at the bottomand youngest at the top….young upward…lateral continuityoldestoriginal sedimentary layers extendlaterally until they thin at edgescontinuecontinue
9 relative age dating concepts cross-cutting relationshipsa disrupted pattern is older thanthe cause of the disruptione.g. an intrusion is youngerthan the rocks it intrudes
10 relative age dating concepts inclusionsfragments of other rocks contained in a body of rockmust be older than thehost rocke.g.xenoliths in granite are olderthan granite and2) pieces of rock inconglomerate are olderthan conglomerate
11 relative age dating concepts unconformitiesa contact between sedimentary formations that represents a gapin the geologic record -- “gap” represented is variable (i.e. amount of time or the amount of missing section)different types of unconformitiesconformity• relatively continuous deposition• deposition of a sequence of parallel layers• contacts between formations do not represent significant amounts of time
13 relative age dating concepts different types of unconformitiesangular unconformity• contact separates overlying younger layers from tilted older layers• sequence of layersis not parallel• contacts between formationsmay represent significantamounts of timeangular unconformity
17 relative age dating concepts different types of unconformitiesdisconformity• contact separates beds (formations) that are parallel• sequence of layersis parallel• contacts betweenformationsmay represent significantamounts of time• missing time is difficult to recognize (may need otherinformation--paleosol?)
18 relative age dating concepts different types of unconformitiesnonconformity• strata deposited on older crystalline (metamorphic/igneous) rock• erosion surface on igneous/metamorphic rock covered bysedimentary rocks• large gap ingeologic recordnonconformity
33 relative age: correlation correlation -- determining time equivalency of rocks within a region, between continents, etc.how is this done?physical continuityphysically following a continuous exposure of a rock unit--most direct; easily done in some locations, not in otherse.g. within the Grand Canyonlithologic similarityassuming similar sequences of rocks formed at same time-- inaccurate if common rocks are involvede.g. the Grand Canyon and Zion National Parks
34 physical continuity -- Coconino Sandstone in Grand Canyon
35 lithologic similarity -- Coconino and Navajo Sandstones
36 lithologic similarity -- Coconino and Navajo Sandstones Navajo is much younger!
37 relative age: correlation how is this done?faunal succession (correlation by fossils)fossil species succeed one another through the layersin a predictable orderindex fossilshort-lived organism;points to narrow rangeof geologic timefossil assemblagegroup of fossilsassociatedtogether
43 absolute time natural clock is necessary -- radiometric dating (nuclear clock: decay of radioactive isotopes)-- dendrochrolonology-- astronomical methods
44 age of the Earth early methods: long debated • 1625: Archbishop Usher determined Earth was created in 4004 B.C.by counting generations in the Bible• Hindus regarded Earth as old: 2000 A.D. is 1.97 million yearsaccording to Hindu calendar• 1866: Lord Kelvin calculated age by assuming that Earth wasmolten and cooled to a solid; age between million years old.- did not know about radioactive decay (makes heat)- assumed all heat dissipated by conductionearly isotopic methods (radioactivity known in 1896)• 1905: first crude estimates yielded 2 billion year age• meteorites gave dates of 4.5 to 4.6 billion years old• modern uranium/lead methods yield values of 4.55 billion years
45 radioactive isotopes have nuclei that spontaneously decay -- emit or capture subatomic particlesparent: decaying radioactive isotopedaughter: decay daughterparentdaughterloss or gainloss or gain of neutron converts parent to daughter of same elementloss or gain of proton changes parent into entirely new daughter
46 electron capture (e- + p+ = n0) 3 primary ways of decayalpha decay (Z ≥ 58)particle has 2 neutrons and 2 protonsU238 Th23492 protons 90 protonsbeta decay (n0 = p+ + e-)breakdown of neutron into anelectron and a proton and lossof the electron to leave a proton(result is gain of one proton)K40 Ca4019 protons 20 protonselectron capture (e- + p+ = n0)capture of an electron by a protonand change of proton to neutron(result is loss of proton)K40 Ar4019 protons 18 protons
47 radiometric datinguses continuous decay to measure time since rock formedonly possible since late 1890’s -- radioactivity discovered in 1896as minerals crystallize in magma;they trap atoms of radioactive isotopes in their crystal structuresradioactive isotopes will decay immediately and continuouslyas time passes, rock contains less parent and more daughter
48 half-life amount of time it takes for half the atoms of the parent isotope to decaydifferent radioactive isotopes have different anddistinct half-livesif rock has 12 parents and 12 daughters--ratio of 1:1…original rock had 24 parents and one half-life has elapsed……after another half life, rock will have 6 parents and 18 daughters……ratio of 1:3---note that total number (24) remains the sameregardless of isotope, the ratio of parent to daughter atomsis predictable at each half-life
49 predictable ratios at each half-life exponential decay (half always remains)
51 (each has its own half-life) example: Uranium 238 decay to Lead 206 (stable)several steps(each has its own half-life)
52 most common dating systems • uranium-thorium-lead dating (previous example)U-238, U-235, Th-232each of these decays through a series of steps to PbU-238 to Pb-206 half-life = 4.5 byU-235 to Pb-207 half-life = 713 myTh-232 to Pb-208 half-life = 14.1 my• potassium-argon dating…argon is a gas--may escape(ages too young--daughter missing)K-40 to Ar-40 half-life = 1.3 by• rubidium-strontium datingRb-87 to Sr-87 half-life = 47 by
53 basic geochronological assumptions decay constants constant through geological time-- good reasons to believe this is correct from nuclear physics-- measurements of decay sequences in ancient supernovaeyield the same values as modern lab measurementssystem closed to adding or subtracting of parent/daughter-- isotopic system and type of mineral (rock) are important-- careful procedure is essential to correct analysisigneous rocks are most reliable for dating…metamorphism may cause loss of daughter products……sedimentary rocks will give ages of source rocks…
54 Instruments and Techniques Mass Spectrometry: measure different abundances of specific nuclides based solely on atomic mass.Basic technique requires ionization of the atomic species of interest and acceleration through a strong magnetic field to cause separation between closely similar masses (e.g. 87Sr and 86Sr).Count individual particles using electronic detectors.TIMS: thermal ionization mass spectrometrySIMS: secondary ionization mass spectrometry - bombard target with heavy ions or use a laserSample Preparation: TIMS requires doing chemical separation using chromatographic columns.
62 Independent Checks on Radiometric Ages Correlation of erosion with age on Hawaiian Island Chain: Dates increase in age to the NW as does erosion.Annual growth bands in Devonian corals: 400/yr yields date that is similar to radiometric date. Consistent with slowing of Earth rotation with time.Independent determination of Pacific plate motion yields age progression that is consistent with K/Ar dates of the island chains formed by “hotspots”.Agreement between magnetic “age” from deep marine sediments and radiometric ages of tuffs in East African Rift
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