Geology 12 Presents

Methods that didn’t work 1. Bible: 9:00 am Tuesday, Oct 26 th, 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)

Bible Bible was interpreted as giving an date of 9:00 am, Tuesday, Oct 26, 4004 BC

Hebrew Writing

Greek Writing

Old English to New English

Rate 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 Sedimentation

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 Rings Count 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 : 50 p : d

Radiometric Dating

Please refer to note helper WS 8.45, side 2 Please refer to note helper WS 8.45, side 2 Thank you. Thank you.

Element U T 1/2 = 3 years 12.5% 6.25% Years Half lives parent daughter 3.125%

Element V T 1/2 = 5 years 12.5% 6.25% How many years until: a) 50% gone: b) 75% gone: c) 87.5% gone:

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 Millions of years : : 75 1 : 3 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 T 1/2 Dating Range Daughter Rocks Dated or abundant in C ,000 N 14 Organic Matter carbonates, CO 2 U ma 80,000 – 2baPb 207 Uraninite, zircon K ba 100,000 -Ar 40 Muscovite,Kfs,Hbld 4.6 ba biotite, volc rx, U ba 10 ma – 4.6 baPb 206 Uraninite, zircon Th ba 0.5 ba – big bangPb 208 zircon Rb ba 1 ba – big bangSr 87 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. Ar 40

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 C 14 dating for sed’ rx’

Do Lab 8.5 Do WS 8.5 Do Unit 2 (Chp 8) Review WS