Absolute Dating& The Age of the Earth

How do we know how old rocks are?

3.96 Billion Year Old Gneiss

Age of the Earth:Originally Based on Mythology

Han Chinese Tradition:23 Million Year Cycle

Buddhist Tradition:Infinite Age (Cyclic)

Archbishop James Ussher (1654)(1625-1656)

4004 BCOctober 239:00 AM

Most scientific attempts are based on principle that:

Age (Time) = Amount of Change Rate of Change

Requires:1. Natural Process2. Occurs at a Constant Rate3. Leaves a Geologic Record

Georges Louis Leclerc

Comte de Buffon (1779)

75,000 yrs

Cooling of Molten Ball

1707-1788

William Thomson, Lord Kelvin (1862)

(1824-1907)

Cooling of Molten Ball

20-400 Million yrs

John Joly (1899)

Saltiness of the Oceans

90 Million yrs

(1857-1933)

John Phillips (late 1800’s)

Accumulation of Sedimentary Rocks

About 100-500 Million yrs

George Darwin (late 1800’s)

Evolution of the Moon

About 100 Million yrs

(1845-1912)

The Discovery of Radioactivity (1896)

Antoine Henri Becquerel

Marie and Pierre Curie

Becquerel’s Mistake

Uraninite - Uranium Ore

Authur Holmes

1921: Earth about 4 billion years old!

Bertram Boltwood

1904-1907: Dated first rocks:250 million to 1.3 billion years

Radioactive Decay

Parent Isotope -->Daughter Isotope + Decay Particle + Energy

Alpha Decay

Uranium-238 --> Thorium-234 + Alpha Particle + Energy

Daughter IsotopeAtomic Number = -2Atomic Weight = -4

Beta Decay

Carbon-14 --> Nitrogen-14 + Beta Particle + Energy

Daughter IsotopeAtomic Number = +1Atomic Weight = +0

Decay of Uranium-238 to Lead 206

Alpha Decay

Beta Decay

Half LifeTime it takes for half of the parent

isotope to decay into daughter isotope

Daughter Isotopes

Parent Isotopes

Radioactive Isotopes Used for Absolute Dating

parentparent daughterdaughter half life (years)half life (years)235U 207Pb 4.50 x 109

238U 206Pb 0.71 x 109

40K 40Ar 1.25 x 109

87Rb 87Sr 47.0 x 109

14C 14N 5,730

Dating & Radioactive Decay

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

Mass Spectrometer

When does a system become “Closed”?(i.e., What are you

dating?)

Cooling of Igneous Rock

Metamorphism

Death of Organic Material

Sedimentary Rocks:What are we dating?

GeologicTimeScale

Era Age (Myrs) Epoch

0.01Holocene

1.8Pleistocene

5.3Pliocene

23.8Miocene

33.6Oligocene

54.8Eocene

65Paleocene

144

206

248

290

323

354

417

443

490

543

2500

3800

Precambrian

Phanerozoic

Eon

Proterozoic

Archean

Hadean

Period

Quaternary

Tertiary

Neogene

Paleocene

Mississippian

Cenozoic

Mesozoic

Paleozoic

Cretaceous

Jurassic

Age of the Earth 4600 Myrs (4.6 Byrs)Source: Geological Society of America (1999)

Geologic Time Scale

Devonian

Silurian

Ordivician

Cambrian

Triassic

Permian

Pennsylvanian

Back to the Age of the Earth

Oldest Rocks on Earth(Acasta Gneiss, Northern Canada)

- about 3.96 Billion Years

Age of the Earth - 4.56 Billion Years

Oldest Mineral Crystals on Earth(Zircon, Jack Hills Conglomerate,

Western Australia)- about 4.4 Billion Years

Meteorites

Iron Meteorite

Carbonaceous Chondrite(Allende Meteorite)

Type Number Method Age (Gyr))

Chondrites (CM, CV, H, L, LL, E) 13 Sm-Nd 4.21 +/- 0.76Carbonaceous chondrites 4 Rb-Sr 4.37 +/- 0.34Chondrites (undisturbed H, LL, E) 38 Rb-Sr 4.50 +/- 0.02Chondrites (H, L, LL, E) 50 Rb-Sr 4.43 +/- 0.04H Chondrites (undisturbed) 17 Rb-Sr 4.52 +/- 0.04H Chondrites 15 Rb-Sr 4.59 +/- 0.06L Chondrites (relatively undisturbed) 6 Rb-Sr 4.44 +/- 0.12L Chondrites 5 Rb-Sr 4.38 +/- 0.12LL Chondrites (undisturbed) 13 Rb-Sr 4.49 +/- 0.02LL Chondrites 10 Rb-Sr 4.46 +/- 0.06E Chondrites (undisturbed) 8 Rb-Sr 4.51 +/- 0.04E Chondrites 8 Rb-Sr 4.44 +/- 0.13Eucrites (polymict) 23 Rb-Sr 4.53 +/- 0.19Eucrites 11 Rb-Sr 4.44 +/- 0.30Eucrites 13 Lu-Hf 4.57 +/- 0.19Diogenites 5 Rb-Sr 4.45 +/- 0.18Iron (plus iron from St. Severin) 8 Re-Os 4.57 +/- 0.21------------------------------------------------------------------------After Dalrymple (1991, p. 291); duplicate studies on identical meteorite types omitted.

Meteorite Ages

Other Forms of Absolute Dating

Dedrochronology

Fission Tracks