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