Absolute Dating & The Age of the Earth

Transcription

1 Absolute Dating & The Age of the Earth How do we know how old rocks are? 3.96 Billion Year Old Gneiss

2 Geologic Time and Clocks Lifespan of a human Human civilization Modern humans Stone tools Age of oceanic crust Precambrian Explosion Oldest Rocks Age of the Earth Age of the Solar System Age of the Universe ~ 100 years ~10,000 years ~100,000 years ~1,000,000 years ~100,000,000 years ~540,000,000 years ~3.8 Billion years ~4.56 Billion years ~4.6 Billion years ~14 Billion years

3 Age of the Earth: Originally Based on Mythology Buddhist Tradition: Infinite Age (Cyclic) Han Chinese Tradition: 23 Million Year Cycle

4 Archbishop James Ussher (1654) ( ) 4004 BC October 23 9:00 AM

5 Most scientific attempts are based on principle that: Requires: 1. Natural Process 2. Occurs at a Constant Rate 3. Leaves a Geologic Record Age (Time) = Amount of Change Rate of Change

6 Georges Louis Leclerc Comte de Buffon (1779) Based on the cooling rate of iron, he calculated that the age of the earth was 75,000 years. For this he was condemned by the Catholic Church in France and his books were burned. Buffon also denied that Noah's flood ever occurred and observed that some animals retain parts that are vestigial and no longer useful, suggesting that they have evolved rather than having been spontaneously generated. [3] Despite this, Buffon insisted that he was not an atheist. [4] 75,000 yrs Cooling of Molten Ball

7 William Thomson, Lord Kelvin (1869) ( ) Million yrs Cooling of Molten Ball

8 Moreover, his theological beliefs led to speculation about the heat death of the universe. I believe the tendency in the material world is for motion to become diffused, and that as a whole the reverse of concentration is gradually going on I believe that no physical action can ever restore the heat emitted from the sun, and that this source is not inexhaustible; also that the motions of the earth and other planets are losing vis viva which is converted into heat; and that although some vis viva may be restored for instance to the earth by heat received from the sun, or by other means, that the loss cannot be precisely compensated and I think it probable that it is under compensated. [10]

9 John Joly (1899) Million yrs Saltiness of the Oceans ( )

10 Joly s calculations Present concentration of ocean salt = 32 gm/liter Present total volume of the oceans = 1.5 x liter \ Present total amount of salt in the oceans = 32 x (1.5 x ) = 5 x tonnes Present rate of salt entering the oceans = 5.4 x 10 8 tonnes/year \ The time for the salt to reach the present value = (5 x ) (5.4 x 10 8 ) = x 10 8 years This is approximately = 100 million years

11 John Phillips (late 1800 s) About Million yrs Accumulation of Sedimentary Rocks

12 George Darwin (late 1800 s) Charles's son About 56 Million yrs Evolution of the Moon Earth System He calculated the amount of time it would have taken for tidal friction to give the Earth its current 24-hour day. ( )

13 Marie and Pierre Curie The Discovery of Radioactivity (1896) Antoine Henri Becquerel

14 Becquerel s Mistake

15 Uraninite Uranium Ore

16 Bertram Boltwood Authur Holmes : Dated first rocks: 250 million to 1.3 billion years 1921: Earth about 4 billion years old!

17 Radioactive Decay Parent Isotope > Daughter Isotope + Decay Particle + Energy

18 Daughter Isotope Atomic Number = -2 Atomic Weight = -4 Alpha Decay Uranium > Thorium Alpha Particle + Energy

19 Beta Decay Daughter Isotope Atomic Number = +1 Atomic Weight = +0 Carbon-14 --> Nitrogen-14 + Beta Particle + Energy

20 Decay of Uranium 238 to Lead 206 Alpha Decay Beta Decay

21 Half Life Time it takes for half of the parent isotope to decay into daughter isotope Daughter Isotopes Parent Isotopes

22 Radioactive Isotopes Used for Absolute Dating parent daughter half life (years) 235 U 207 Pb 4.50 x U 206 Pb 0.71 x K 40 Ar 1.25 x Rb 87 Sr 47.0 x C 14 N 5,730

23 Dating & Radioactive Decay

24 Radioactive Decay N = N 0 e -kt Where N is the amount of the radioactive element in the rock now; N 0 is the amount originally in the rock, e ~ (natural logarithm); k is the decay constant of the radioactive element, and t is time. Half-life when N/N 0 = 0.5

25 Logarithms to various bases: red is to base e, green is to base 10, and purple is to base 1.7. Each tick on the axes is one unit. Logarithms of all bases pass through the point (1, 0), because any number raised to the power 0 is 1, and through the points (b, 1) for base b, because a number raised to the power 1 is itself. The curves approach the y-axis but do not reach it because of the singularity at x = 0.

26 Information Required for Radiometric Dating Initial Parent Isotope Content Half Life of Isotope Current Parent Isotope Concentration Closed System Remember: Age = Amount of Change Rate of Change

27 Mass Spectrometer

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29 Cooling of Igneous Rock When does a system become Closed? (i.e., What are you dating?) Metamorphism Death of Organic Material

30 Sedimentary Rocks: What are we dating?

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34 Geologic Time Scale P r e c a m b r i a n Eon P h a n e r o z o i c Proterozoic Archean Hadean Era Period Age (Myrs) Epoch C e n o z o i c M e s o z o i c P a l e o z o i c Geologic Time Scale Tertiary Quaternary Cretaceous Jurassic Triassic Permian Pennsylvanian Mississippian Devonian Silurian Ordivician Cambrian Neogene Paleocene Age of the Earth 4600 Myrs (4.6 Byrs) Source: Geological Society of America (1999) Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene

35 Back to the Age of the Earth Oldest Rocks on Earth (Acasta Gneiss, Northern Canada) - about 3.96 Billion Years Oldest Mineral Crystals on Earth (Zircon, Jack Hills Conglomerate, Western Australia) - about 4.4 Billion Years Age of the Earth Billion Years

36 Meteorites Carbonaceous Chondrite (Allende Meteorite) Iron Meteorite

37 Meteorite Ages Type Number Method Age (Gyr)) Chondrites (CM, CV, H, L, LL, E) 13 Sm-Nd / Carbonaceous chondrites 4 Rb-Sr / Chondrites (undisturbed H, LL, E) 38 Rb-Sr / Chondrites (H, L, LL, E) 50 Rb-Sr / H Chondrites (undisturbed) 17 Rb-Sr / H Chondrites 15 Rb-Sr / L Chondrites (relatively undisturbed) 6 Rb-Sr / L Chondrites 5 Rb-Sr / LL Chondrites (undisturbed) 13 Rb-Sr / LL Chondrites 10 Rb-Sr / E Chondrites (undisturbed) 8 Rb-Sr / E Chondrites 8 Rb-Sr / Eucrites (polymict) 23 Rb-Sr / Eucrites 11 Rb-Sr / Eucrites 13 Lu-Hf / Diogenites 5 Rb-Sr / Iron (plus iron from St. Severin) 8 Re-Os / After Dalrymple (1991, p. 291); duplicate studies on identical meteorite types omitted.

38 Other Forms of Absolute Dating Dedrochronology Fission Tracks