geology 12 presents methods that didn’t work 1. bible: 9:00 am tuesday, oct 26 th, 4004 bc =...
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Geology 12
Presents
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Methods that didn’t work• 1. Bible: 9:00 am Tuesday, Oct 26th, 4004
BC = beginning of Earth
• 2. Rate of cooling: Earth from a molten ball– a) George Buffon (1750): 75,000 yrs using
steel balls– b) Lord Kelvin (1800s): 20 – 40 ma
• Didn’t account for radioactive heating or convection currents in the mantle.
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• 3. Rate of sedimentation: James Walcott (1893): 75 ma– Didn’t know thickness of sediments; – didn’t account for erosion; – assumed constant sedimentation rate
• 4. Salinity of oceans: John Joly (1899): 100 ma– Assumed oceans were initially fresh water;– Didn’t know of chemical sedimentation
(precipitation)
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BibleBibleBible was interpreted as giving an date of 9:00 am, Tuesday, Oct 26, 4004 BC
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Heb
rew
Writ
ing
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Greek WritingGreek Writing
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Old English to New Old English to New EnglishEnglish
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Rate of CoolingRate of Cooling
Steel balls were used and extrapolated to the size of the earth.
Didn’t take into account radioactive heating (adds heat) or convection currents (removes heat).
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Radioactive core producing heat
Convection currents
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Rates of Rates of SedimentationSedimentation
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Salinity of Oceans
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Salt is added to the oceans from rivers and removed through precipitation.
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Absolute Time
• Is determining how long ago something happened (ma = millions of years)
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Absolute Dating Methods
• 1. Tree Rings: count back/inward from cambium to centre to get age of tree when it was cut– Also gives indication of paleoclimate– Ranges up to 14,000 years; mostly in arid
area
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Tree RingsTree RingsCount back in time to centre.
Range up to 14,000 yrs.
Gives indication of paleoclimate.
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• 2. Varves: thin glacial sediments
1 year
Drop stone: fell out of melting iceberg.
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Varves
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• 3. Fission Track Dating: minerals with or near uranium become “scratched” = etched by decay particles– Compare the number of etches to amount of
uranium = age– Range: 40,000 to 1.5 ma
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Fission Track Dating
K-Feldspar Soda-ljme Glass Lexan
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Apatite
Muscovite
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• 4. Radiometric dating = unstable atomic nuclei decay at a known rate to stable ones.
Parent nucleus
“Unstable”
Daughter nucleus
“stable”
Decay Particle
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• By measuring the remaining amount of parent nuclei to the amount of produced daughter nuclei, an age can be determined– i.e.: lots of parent : little daughter = young– i.e.: little parent : lots of daughter = old
• Half Life = time for half parent nuclei to decay
50 : 50p : d
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Radiometric Dating
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Please refer to note helper WS Please refer to note helper WS 8.45, side 28.45, side 2
Thank you.Thank you.
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Element U T1/2 = 3 years
12.5%6.25%
0 3 6 9 12 15Years
Half lives 10 2 3 4 5
parent
daughter
3.125%
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Element V T1/2 = 5 years
12.5%6.25%
How many years until:
a) 50% gone:
b) 75% gone:
c) 87.5% gone:
5 10 15 20
5
10
15
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Element W
1. What is the half life? Years
2. A rock sample contained a ratio of 3 stable daughter atoms for every 1 parent atom. The age of the sample is: ma.
2 4 6 8 10 12Millions of years
2.5
50 : 5025 : 751 : 3
1 : 1
5.0
12.5 : 87.5 1 : 76.25:93.75 1 : 15
3.1 : 96.9 1 : 31
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Element X
An igneous rock specimen contains 6.25% of its original uranium 235. How old the specimen in half lives?
12.5%6.25%
4
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Isotopes Used in Radiometric Dating
Isotope T1/2 Dating Range Daughter Rocks Dated or abundant in
C14 5730 100-70,000 N14 Organic Matter
carbonates, CO2
U235 704 ma 80,000 – 2ba Pb207 Uraninite, zircon
K40 1.3 ba 100,000 - Ar40 Muscovite,Kfs,Hbld
4.6 ba biotite, volc rx,
U238 4.5 ba 10 ma – 4.6 ba Pb206 Uraninite, zircon
Th 232 14 ba 0.5 ba – big bang Pb208 zircon
Rb87 48.8 ba 1 ba – big bang Sr87 same as K40
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Sources of Error
• 1. weathering can remove some of the parent or daughter isotopes distorting the ratio.– i.e. daughter elements removed ( )
makes it too young. Ar40
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• 2. High temperatures/pressures (metamorphism) can:– a) destroy fission tracks– b) use up parent/daughter isotopes distorting
the ratio– c) cause argon to escape
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Preventing Error:
• 1. Make sure sample is frsh,unweathered, unheated, etc.
• 2. cross check with several methods of dating to see if dates agree.
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General Rule
• Radiometric dating for igneous and meta’ rx’
• Index fossils and C14 dating for sed’ rx’
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Do Lab 8.5Do Lab 8.5Do WS 8.5Do WS 8.5Do Unit 2 (Chp 8) Review Do Unit 2 (Chp 8) Review WSWS