by Steven Newton, Public Information Project Director

The age of the Earth is 4.54 billion years (Ga or Gyr), a number which geologists have determined by several independent methods. Because of the recycling of crustal rocks by plate tectonics, no direct material from the earliest Earth still exists. However, geologists are able to analyze meteorites and lunar rocks returned by the Apollo missions.

Methods

Rocks in the 4-5 Ga range may be dated by measuring ratios of isotopes of uranium/lead, rubidium/strontium, potassium/argon, argon/argon, and neodymium/samarium. One way to judge the reliability of a radiometric measurement is to compare the results of different measurement techniques for the same sample.

Note that these methods do not include carbon dating. Creationists frequently describe carbon being used to date rocks; in fact, it cannot be used in that way, as rocks do not accumulate carbon in the manner that living material does. Also, the relatively brief half-life of ¹⁴C (5730 years) means that after ~50,000 years, so little ¹⁴C remains that the machine used to measure isotopes, a mass spectrometer, cannot reliably detect this amount within its normal background error.

Meteorites

Meteorites are pieces of the primordial solar system. They formed at the same time as Earth and the other planets. Meteorites test within a narrow time range between 4.48-4.56 Ga.

Lunar Materials

The Apollo missions returned 382 kg of lunar materials. Although much of this consisted of relatively young volcanic flows, older crustal highland rocks ranged from 4.3-4.5 Ga. In 2005, researchers used a refined technique involving tungsten isotopes and hafnium to find a lunar formation date of 4.527 Ga.

Timeline:

1. Minimum age of the solar system: 4.5695 ± 0.0002 Ga
   Based on radiometric dating using ²⁰⁷Pb/²⁰⁶Pb—²⁰⁴Pb/²⁰⁶Pb, (Baker et al., 2005)
2. Age of the Earth: 4.54 ± 1% Ga
   Numerous meteorite samples. (Patterson, 1956; Dalrymple, 1991)
3. Age of moon formation, 4.527 Ga. (Kleine, et al., 2005)
   Note that lunar rocks are slightly younger than the Earth, reflecting their origin as a piece of Earth’s mantle hurtled into orbit during a collision.
4. Oldest mineral yet discovered on Earth: 4.408 Ga ± 8 Ma
   zircon crystal in sediment, Jack Hills, Australia (Wilde et al., 2001)
5. Oldest rock yet identified on Earth: 4.031 Ga ± 3 Ma
   Acasta gneiss, Slave Craton, Canada (Bowring and Williams, 1999). As this went to press, a paper was published in Science (O'Neil et al., 26 Sep 08) reporting a rock dated at 4.28 Ga. If verified, this would replace the Acasta gneiss as the oldest rock yet found.

References

• Baker, J., et al., 2005. “Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites.” Nature, v. 436, 25 August 2005, p. 1127-1131.
• Bowring, S.A., and Williams, I.S., 1999. “Priscoan (4.00-4.03 Gyr) orthogneisses from northwestern Canada.” Contributions to Mineralogy and Petrology, v. 134, no. 1, January 1999, p. 3-16.
• Dalrymple, G. B., 1991. The Age of the Earth. Stanford University Press, 0804723311.
• Kleine, T., et al., 2005. “Hf-W Chronometry of Lunar Metals and the Age and Early Differentiations of the Moon.” Science, 9 December 2005, vol. 310, no. 5754, p. 1671-1674.
• O'Neil, J., Carlson, R.W., Francis, D., Sevenson, R.K., 2008. "Neodymium-142 Evidence for Hadean Mafic Crust." Science, 26 September 08, vol. 321, no. 5897, p. 1828-1831.
• Patterson, C., 1956. “Age of Meteorites and the Earth.” Geochimica et Cosmochimica Acta 10: p. 230-237.
• Wilde, S. A., Valley, J.W, Peck, W.H., and Graham, C.M., 2001. “Evidence from Detrital Zircons for the Existence of Continental Crust and Oceans on the Earth 4.4 Ga Ago.” Nature. 409: p. 175-178.

Resources:

Science Courseware has an excellent radiometric dating exercise called “Virtual Dating” that walks you through each step of the process of dating rocks. These exercises are especially helpful because of their explanation of corrections in the measurements.