What is the age of the Earth?

Submitted by Staci Loewy - California State University at Bakersfield and Carol Frost - University of Wyoming

Why is this question important?

"What is the age of the Earth" is as much a philosophical question as a scientific one. For centuries answers to this question have relied on key assumptions; these assumptions may have changed but still are necessary to answer the question.

What we know...

Early estimates for the age of the Earth:

James Ussher used both sacred and secular histories along with astronomical data and a variety of calendars to calculate the beginning of human history that he also took to represent the beginning of the physical world. His work, which includes exhaustive referencing, was published in 1658. In 1862 William Thomson (later made Lord Kelvin) published calculations of the age of the Earth based upon the assumption that the Earth was initially a molten sphere of rock. He determined the amount of time it took for the surface of this mass to cool to its present surface temperature (England 2006). His estimates ranged between 20 and 400 Ma. John Jolly took a different approach and assumed that at the start of Earth history the oceans were originally dilute. He then calculated the length of time it would take for the oceans to accumulate their present salt content by the erosion of the continents. His estimate was about 90 Ma (Jolly 1899). C.C. Patterson equated the age of meteorites with the age of the Earth. His Pb/Pb date on Canyon Diablo meteorite of 4.56 Ga published in 1956 is arguably one of the most important papers in geochronology and cosmochemistry.

Contemporary understanding of the age of the Earth:

The minimum age of the Earth is limited by the age of the oldest known object that formed on earth. The oldest zircon yields an age of 4.4 Ga (Wilde et al. 2001; Peck et al. 2001)

At the other end of the time spectrum the age of the solar system places a maximum age limit. The accretion of the terrestrial planets from the solar nebula's dust and gas can be envisioned as taking place in several stages (Halliday 2006):

  1. Settling of circumstellar dust to a planar disk (1000's of years)
  2. Growth of planetisimals to around 1 km in size (poorly known timescale)
  3. Runaway growth of planetary bodies of around 1000 km diameter (~100,000 yr)
  4. Formation of larger planets through late-stage collisions (200 My; Raymond et al. 2006)

The oldest dated objects in the solar system are calcium-aluminum-refractory inclusions (CAIs) in chondritic meteorites. These inclusions are enriched in major and trace elements that condense at high temperatures. They have relative abundances of stable isotopes that are essentially identical to those found in the terrestrial planets and meteorites. The absolute age of some CAIs is determined by U-Pb dating to 4.5672 +/- 0.0006 Ga (Amelin et al. 2002). Some work using both U-Pb and the extinct 26Al-26Mg decay schemes suggests that some CAIs may be older than 4.5695 +/- 0.0002 Ga (Baker et al. 2005).

What is the Earth's birthday?

The age of the Earth may be calculated by determining the amount of time necessary for the Earth to accrete from the oldest objects in the solar system. The concept of "mean life" of accretion "is the inverse of the time constant for exponentially decreasing growth and corresponds to the time taken for 63% of the planet to accrete" (Halliday 2004). This concept analogous to the half-life of radioactive isotopes is based on W and Pb isotopes and is model dependent. Earth's mean life is estimated to be approximately 11 Myr (Yin et al. 2002). If accretion began at about the time that the oldest CAI formed then the age of the Earth based upon mean life of accretion is approximately 4.559 Ga.

Others may consider the age of the birth of the Earth as the time of the final large impact that formed the Moon. After that time the mass and composition of the Earth has remained more or less constant. Estimates based upon the W isotopic composition of chondrite, Earth and lunar samples suggest that the earliest possible time for Moon formation is approximately 30 My after the formation of the solar system (Jacobsen 2005). Other estimates are between 40-50 My after the start of the solar system (Halliday 2003, 2006). According to this method the age of the Earth is approximately 4.540 to 4.520 Ga.

How to link this topic to the classroom

This essay provides a summary of the methods (with references) that have been employed to determine the age of the Earth, both historical and contemporary. It also provides a discussion of some of the issues involved in defining the Earth's "birthday". As the Earth is continuously evolving, it is difficult to determine what milestone we should use to indicate the time at which Earth formation was complete.

The information in this essay could be used in a lecture on the age of the Earth and/or incorporated into class discussions. For example:

  1. By what milestone should we define the Earth's Birthday? Why?
  2. The ages determined by the historical methods were much younger than those determined by contemporary methods. Where did the early workers go wrong?

The topic would be suitable for classes ranging from introductory geology to upper division classes.

References and other Resources

  • Amelin, Y., Krot, A. N., Hutcheon, I. D., Ulyanov, A. A. (2002) Lead isotopic ages of chondrules and Calcium-Aluminum-rich inclusions. Science, vol. 297, p. 1678-1683.
  • Baker, J., Bizzarro, M., Wittig, N., Connelly, J., and Haack, H. (2005) Early planetessimal melting from an age of 4.5662 Gyr for differentiated meteorites. Nature, vol. 436, p. 1127-1131.
  • England, P., Molnar, P., and Richter, F. (2007) John Perry's neglected critique of Kelvin's age for the Earth: A missed opportunity in geodynamics. GSA Today, Vol. 17, p 4-9.
  • Halliday A. N. (2003) The origin and earliest history of the Earth. In: Davis AM (ed) Meteorites Comets and Planets Treatise on Geochemistry 1, Elsevier-Pergamon, Oxford, p. 509-557.
  • Halliday A. N. (2004) Mixing volatile loss and compositional change during impact-driven accretion of the Earth. Nature, vol. 427, p. 505-509.
  • Halliday A. N. (2006) The origin of the Earth—What's New? Elements, vol. 2, no. 4.
  • Jacobsen, S.B. (2005) The Hf-W isotopic system and the origin of the Earth and Moon. Annual Reviews of Earth and Planetary Sciences, Vol. 33, p. 531-570.
  • Joly, J. (1899) An estimate of the geological age of the Earth. Scientific Transactions of the Royal Dublin Society, Vol. 7, p. 23-66.
  • Patterson CC (1956) Age of meteorites and the earth. Geochimica and Cosmochimica Acta, Vol. 10, p. 230-237.
  • Peck, William H. Valley, John W., Wilde, Simon A., Graham, Colin M. (2001) Oxygen isotope ratios and rare earth elements in 3.3 to 4.4 Ga zircons; ion microprobe evidence for high delta 18O continental crust and oceans in the early Archean. Geochimica et Cosmochimica Acta, Vol. 65, Issue 22, p. 4215-4229.
  • Raymond, Sean N., Quinn, Thomas, and Lunine, Jonathan I. (2006) High-resolution simulations of the final assembly of Earth-like planets I. Terrestrial accretion and dynamics. Icarus, Vol. 183, Issue 2, Pages 265-282.
  • Ussher James (1658) Annals of the World. Reprinted 2003: Pierce, Larry and Pierce, Marion (editors) Annals of the World: James Ussher's Classic Survey of World History Master Books, Green Forest.
  • 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 Gyr ago. Nature, Vol. 409, p.175-178.
  • Yin Q. Z. et al. (2002) A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites. Nature, vol. 418, p. 949-952.

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