Geology 12
Presents
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.
• 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)
BibleBibleBible was interpreted as giving an date of 9:00 am, Tuesday, Oct 26, 4004 BC
Heb
rew
Writ
ing
Greek WritingGreek Writing
Old English to New Old English to New EnglishEnglish
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).
Radioactive core producing heat
Convection currents
Rates of Rates of SedimentationSedimentation
Salinity of Oceans
Salt is added to the oceans from rivers and removed through precipitation.
Absolute Time
• Is determining how long ago something happened (ma = millions of years)
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
Tree RingsTree RingsCount back in time to centre.
Range up to 14,000 yrs.
Gives indication of paleoclimate.
• 2. Varves: thin glacial sediments
1 year
Drop stone: fell out of melting iceberg.
Varves
• 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
Fission Track Dating
K-Feldspar Soda-ljme Glass Lexan
Apatite
Muscovite
• 4. Radiometric dating = unstable atomic nuclei decay at a known rate to stable ones.
Parent nucleus
“Unstable”
Daughter nucleus
“stable”
Decay Particle
• 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
Radiometric Dating
Please refer to note helper WS Please refer to note helper WS 8.45, side 28.45, side 2
Thank you.Thank you.
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%
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
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
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
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
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
• 2. High temperatures/pressures (metamorphism) can:– a) destroy fission tracks– b) use up parent/daughter isotopes distorting
the ratio– c) cause argon to escape
Preventing Error:
• 1. Make sure sample is frsh,unweathered, unheated, etc.
• 2. cross check with several methods of dating to see if dates agree.
General Rule
• Radiometric dating for igneous and meta’ rx’
• Index fossils and C14 dating for sed’ rx’
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