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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