Age of the Earth

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Earth as seen from Apollo 17

Modern geologists and geophysicists consider the age of the Earth to be around 4.54 billion years (4.54×109 years).[1] This age has been determined by radiometric age dating of meteorite material[2] and is consistent with the ages of the oldest-known terrestrial and lunar samples.

Historically, estimates of the age were based on either creation myths in religious texts, or philosophical interpretations of geologic features, most notably by the Greek philosophers Theophrastus and Xenophanes.[citation needed] Biblical young earth creationists believe that the earth was formed as recently as 4004 BC,[citation needed] whereas Hindu beliefs have the universe enduring for billions of years before being destroyed and recreated in an endless cycle.[citation needed]

Following the scientific revolution and the development of radiometric age dating, measurements of lead in uranium-rich minerals showed that some were in excess of a billion years old.[3] The oldest such minerals analysed to date – small crystals of zircon from the Jack Hills of Western Australia – are at least 4.404 billion years old.[4] Comparing the mass and luminosity of the Sun to the multitudes of other stars, it appears that the solar system cannot be much older than those rocks. Ca-Al-rich inclusions (inclusions rich in calcium and aluminium) – the oldest known solid constituents within meteorites that are formed within the solar system – are 4.567 billion years old,[5] giving an age for the solar system and an upper limit for the age of the Earth. It is hypothesised that the accretion of the Earth began soon after the formation of the Ca-Al-rich inclusions and the meteorites. Because the exact accretion time of the Earth is not yet known, and the predictions from different accretion models range from a few millions up to about 100 million years, the exact age of the Earth is difficult to determine. It is also difficult to determine the exact age of the oldest rocks on Earth, exposed at the surface, as they are aggregates of minerals of possibly different ages. The Acasta Gneiss of Northern Canada may be the oldest known exposed crustal rock.[6]

Development of modern geologic concepts

Template:UnreferencedsectionStudies of strata, the layering of rock and earth, gave naturalists an appreciation that the Earth may have been through many changes during its existence. These layers often contained fossilized remains of unknown creatures, leading some to interpret a progression of organisms from layer to layer. Xenophanes interpreted fossil-bearing strata in much the same way during the 6th Century BC.

Nicolas Steno (17th Century) was one of the first Western naturalists to appreciate the connection between fossil remains and strata. His observations led him to formulate important stratigraphic concepts (i.e., the "law of superposition" and the "principle of original horizontality"). In the 1790s, the British naturalist William Smith hypothesized that if two layers of rock at widely differing locations contained similar fossils, then it was very plausible that the layers were the same age. William Smith's nephew and student, John Phillips, later calculated by such means that the Earth was about 96 million years old.

The naturalist Mikhail Lomonosov, regarded as the founder of Russian science, suggested in the mid-18th century that the Earth had been created separately from the rest of the universe, several hundred thousand years before. Lomonosov's ideas were mostly speculative, but in 1779, the French naturalist the Comte du Buffon tried to obtain a value for the age of the Earth using an experiment: He created a small globe that resembled the Earth in composition and then measured its rate of cooling. This led him to estimate that the Earth was about 75,000 years old.

Other naturalists used these hypotheses to construct a history of Earth, though their timelines were inexact as they did not know how long it took to lay down stratigraphic layers. In 1830, the geologist Charles Lyell, developing ideas found in Scottish natural philosopher James Hutton, popularized the concept that the features of the Earth were in perpetual change, eroding and reforming continuously, and the rate of this change was roughly constant. This was a challenge to the traditional view, which saw the history of the Earth as static, with changes brought about by intermittent catastrophes. Many naturalists were influenced by Lyell to become "uniformitarians" who believed that changes were constant and uniform.

Early calculations: physicists, geologists and biologists

File:Kelvin-1200-scale1000.jpg
William Thomson, aka Lord Kelvin.

In 1862, the physicist William Thomson (who later became Lord Kelvin) of Glasgow published calculations that fixed the age of the Earth at between 24 million and 400 million years.[7][8] He assumed that the Earth had been created as a completely molten ball of rock, and determined the amount of time it took for the ball to cool to its present temperature. His calculations did not account for the ongoing heat source in the form of radioactive decay, which was unknown at the time.

Geologists had trouble accepting such a short age for the Earth. Biologists could accept that the Earth might have a finite age, but even 100 million years seemed much too short to be plausible. Charles Darwin, who had studied Lyell's work, had proposed his theory of the evolution of organisms by natural selection, a process whose combination of random heritable variation and cumulative selection implies great expanses of time. Even 400 million years did not seem long enough.[citation needed]

In a lecture in 1869, Darwin's great advocate, Thomas H. Huxley, attacked Thomson's calculations, suggesting they appeared precise in themselves but were based on faulty assumptions. The German physicist Hermann von Helmholtz (in 1856) and the Canadian astronomer Simon Newcomb (in 1892) contributed their own calculations of 22 and 18 million years respectively to the debate: they independently calculated the amount of time it would take for the Sun to condense down to its current diameter and brightness from the nebula of gas and dust from which it was born.[8] Their values were consistent with Thomson's calculations. However, they assumed that the Sun was only glowing from the heat of its gravitational contraction. The process of solar nuclear fusion was not yet known to science.

Other scientists backed up Thomson's figures as well. Charles Darwin's son, the astronomer George H. Darwin of the University of Cambridge, proposed that the Earth and Moon had broken apart in their early days when they were both molten. He calculated the amount of time it would have taken for tidal friction to give the Earth its current 24-hour day. His value of 56 million years added additional evidence that Thomson was on the right track.[8]

In 1899 and 1900, John Joly of the University of Dublin calculated the rate at which the oceans should have accumulated salt from erosion processes, and determined that the oceans were about 80 to 100 million years old.[8]

Radioactive dating

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Overview

Rock minerals naturally contain certain elements and not others. By the process of radioactive decay of radioactive isotopes occurring in a rock, exotic elements can be introduced over time. By measuring the concentration of the stable end product of the decay, coupled with knowledge of the half life and initial concentration of the decaying element, the age of the rock can be calculated. Typical radioactive end products are argon from potassium-40 and lead from uranium and thorium decay. If the rock becomes molten, as happens in the Earth's mantle, such nonradioactive end products typically escape or are redistributed. Thus the age of the oldest terrestrial rock gives a minimum for the age of the Earth assuming that a rock cannot have been in existence for longer than the Earth itself.

Convective mantle and radioactivity

In 1892, Thomson had been made Lord Kelvin in appreciation of his many scientific accomplishments. Kelvin calculated the age of the Earth by using thermal gradients, and arrived at an estimate of 100 million years old.[9] He did not realize that Earth has a highly viscous fluid mantle, and this ruined his calculation. In 1895, John Perry produced an age of the Earth estimate of 2 to 3 billions years old using a model of a convective mantle and thin crust.[9] Kelvin stuck by his estimate of 100 million years, and later reduced the estimate to about 20 million years.

Radioactivity would introduce another factor in the calculation. In 1896, the French chemist A. Henri Becquerel discovered radioactivity. In 1898, two other French researchers, Marie and Pierre Curie, discovered the radioactive elements polonium and radium. In 1903 Pierre Curie and his associate Albert Laborde announced that radium produces enough heat to melt its own weight in ice in less than an hour.

Geologists quickly realized that the discovery of radioactivity upset the assumptions on which most calculations of the age of the Earth were based. These calculations assumed that the Earth and Sun had been created at some time in the past and had been steadily cooling since that time. Radioactivity provided a process that generated heat. George Darwin and Joly were the first to point this out, also in 1903.[10]

Invention of radiometric dating

Radioactivity, which had overthrown the old calculations, yielded a bonus by providing a basis for new calculations, in the form of radiometric dating.

File:Ernest Rutherford 1908.jpg
Ernest Rutherford in 1908.

Ernest Rutherford and Frederick Soddy had continued their work on radioactive materials and concluded that radioactivity was due to a spontaneous transmutation of atomic elements. In radioactive decay, an element breaks down into another, lighter element, releasing alpha, beta, or gamma radiation in the process. They also determined that a particular radioactive element decays into another element at a distinctive rate. This rate is given in terms of a "half-life", or the amount of time it takes half of a mass of that radioactive material to break down into its "decay product".

Some radioactive materials have short half-lives; some have long half-lives. Uranium, thorium, and radium have long half-lives, and so persist in the Earth's crust, but radioactive elements with short half-lives have generally disappeared. This suggested that it might be possible to measure the age of the Earth by determining the relative proportions of radioactive materials in geological samples. In reality, radioactive elements do not always decay into nonradioactive ("stable") elements directly, instead, decaying into other radioactive elements that have their own half-lives and so on, until they reach a stable element. Such "decay series", such as the uranium-radium and thorium series, were known within a few years of the discovery of radioactivity, and provided a basis for constructing techniques of radiometric dating.

The pioneers of radioactivity were Bertram B. Boltwood, a young chemist just out of Yale, and the energetic Rutherford. Boltwood had conducted studies of radioactive materials as a consultant, and when Rutherford lectured at Yale in 1904[11], Boltwood was inspired to describe the relationships between elements in various decay series. Late in 1904, Rutherford took the first step toward radiometric dating by suggesting that the alpha particles released by radioactive decay could be trapped in a rocky material as helium atoms. At the time, Rutherford was only guessing at the relationship between alpha particles and helium atoms, but he would prove the connection four years later.

Soddy and Sir William Ramsay, then at University College in London, had just determined the rate at which radium produces alpha particles, and Rutherford proposed that he could determine the age of a rock sample by measuring its concentration of helium. He dated a rock in his possession to an age of 40 million years by this technique. Rutherford wrote,

I came into the room, which was half dark, and presently spotted Lord Kelvin in the audience and realized that I was in trouble at the last part of my speech dealing with the age of the earth, where my views conflicted with his. To my relief, Kelvin fell fast asleep, but as I came to the important point, I saw the old bird sit up, open an eye, and cock a baleful glance at me! Then a sudden inspiration came, and I said, 'Lord Kelvin had limited the age of the earth, provided no new source was discovered. That prophetic utterance refers to what we are now considering tonight, radium!' Behold! the old boy beamed upon me.[12]

Rutherford assumed that the rate of decay of radium as determined by Ramsay and Soddy was accurate, and that helium did not escape from the sample over time. Rutherford's scheme was inaccurate, but it was a useful first step.

Boltwood focused on the end products of decay series. In 1905, he suggested that lead was the final stable product of the decay of radium. It was already known that radium was an intermediate product of the decay of uranium. Rutherford joined in, outlining a decay process in which radium emitted five alpha particles through various intermediate products to end up with lead, and speculated that the radium-lead decay chain could be used to date rock samples. Boltwood did the legwork, and by the end of 1905 had provided dates for 26 separate rock samples, ranging from 92 to 570 million years. He did not publish these results, which was fortunate because they were flawed by measurement errors and poor estimates of the half-life of radium. Boltwood refined his work and finally published the results in 1907.[3]

Boltwood's paper pointed out that samples taken from comparable layers of strata had similar lead-to-uranium ratios, and that samples from older layers had a higher proportion of lead, except where there was evidence that lead had leached out of the sample. However, his studies were flawed by the fact that the decay series of thorium was not understood, which led to incorrect results for samples that contained both uranium and thorium. However, his calculations were far more accurate than any that had been performed to that time. Refinements in the technique would later give ages for Boltwood's 26 samples of 250 million to 1.3 billion years.

Arthur Holmes establishes radiometric dating

Although Boltwood published his paper in a prominent geological journal, the geological community had little interest in radioactivity. Boltwood gave up work on radiometric dating and went on to investigate other decay series. Rutherford remained mildly curious about the issue of the age of the Earth but did little work on it.

Robert Strutt tinkered with Rutherford's helium method until 1910 and then ceased. However, Strutt's student Arthur Holmes became interested in radiometric dating and continued to work on it after everyone else had given up. Holmes focused on lead dating, because he regarded the helium method as unpromising. He performed measurements on rock samples and concluded in 1911 that the oldest (a sample from Ceylon) was about 1.6 billion years old.[13] These calculations were not particularly trustworthy. For example, he assumed that the samples had contained only uranium and no lead when they were formed.

More important, in 1913 research was published showing that elements generally exist in multiple variants with different masses, or "isotopes". In the 1930s, isotopes would be shown to have nuclei with differing numbers of the neutral particles known as "neutrons". In that same year, other research was published establishing the rules for radioactive decay, allowing more precise identification of decay series.

Many geologists felt these new discoveries made radiometric dating so complicated as to be worthless. Holmes felt that they gave him tools to improve his techniques, and he plodded ahead with his research, publishing before and after the First World War. His work was generally ignored until the 1920s, though in 1917 Joseph Barrell, a professor of geology at Yale, redrew geological history as it was understood at the time to conform to Holmes's findings in radiometric dating. Barrell's research determined that the layers of strata had not all been laid down at the same rate, and so current rates of geological change could not be used to provide accurate timelines of the history of the Earth.

Holmes's persistence finally began to pay off in 1921, when the speakers at the yearly meeting of the British Association for the Advancement of Science came to a rough consensus that the Earth was a few billion years old, and that radiometric dating was credible. Holmes published The Age of the Earth, an Introduction to Geological Ideas in 1927 in which he presented a range of 1.6 to 3.0 billion years.[14] No great push to embrace radiometric dating followed, however, and the die-hards in the geological community stubbornly resisted. They had never cared for attempts by physicists to intrude in their domain, and had successfully ignored them so far. The growing weight of evidence finally tilted the balance in 1931, when the National Research Council of the US National Academy of Sciences finally decided to resolve the question of the age of the Earth by appointing a committee to investigate. Holmes, being one of the few people on Earth who was trained in radiometric dating techniques, was a committee member, and in fact wrote most of the final report.[14]

The report concluded that radioactive dating was the only reliable means of pinning down geological time scales. Questions of bias were deflected by the great and exacting detail of the report. It described the methods used, the care with which measurements were made, and their error bars and limitations.

Modern radiometric dating

Radiometric dating continues to be the predominant way scientists date geologic timescales. Techniques for radioactive dating have been tested and fine tuned for the past 50+ years. Forty or so different dating techniques are utilized to date a wide variety of materials, and dates for the same sample using these techniques are in very close agreement on the age of the material.

Possible contamination problems do exist, but they have been studied and dealt with by careful investigation; leading to sample preparation procedures being minimized to limit the chance of contamination. Hundreds to thousands of measurements are done daily with excellent precision and accurate results. Even so, research continues to refine and improve radiometric dating to this day.

Why meteorites were used

Today's accepted age of the Earth of 4.55 billion years was determined by C.C. Patterson using uranium-lead isotope dating (specifically lead-lead dating) on fragments of the Canyon Diablo meteorite and published in 1956.[2]

File:Paterson isochron animation.gif
Lead isotope isochron diagram showing data used by Patterson to determine the age of the Earth in 1956.

The quoted age of the Earth is derived, in part, from the Canyon Diablo meteorite for several important reasons and is built upon a modern understanding of cosmochemistry built up over decades of research.

Most geological samples from the Earth are unable to give a direct date of the formation of the Earth from the solar nebula because the Earth has undergone differentiation into the core, mantle, and crust, and this has then undergone a long history of mixing and unmixing of these sample reservoirs by plate tectonics, weathering and hydrothermal circulation.

All of these processes may adversely affect isotopic dating mechanisms because the sample cannot always be assumed to have remained as a closed system, by which it is meant that either the parent or daughter nuclide (a species of atom characterised by the number of neutrons and protons an atom contains) or an intermediate daughter nuclide may have been partially removed from the sample, which will skew the resulting isotopic date. To mitigate this effect it is usual to date several minerals in the same sample, to provide an isochron. Alternately, more than one dating system may be used on a sample to check the date.

Some meteorites are furthermore considered to represent the primitive material from which the accreting solar disk was formed. Some have behaved as closed systems (for some isotopic systems) soon after the solar disk and the planets formed. To date, these assumptions are supported by much scientific observation and repeated isotopic dates, and it is certainly a more robust hypothesis than that which assumes a terrestrial rock has retained its original composition.

Nevertheless, ancient Archaean lead ores of galena have been used to date the formation of the Earth as these represent the earliest formed lead-only minerals on the planet and record the earliest homogeneous lead-lead isotope systems on the planet. These have returned age dates of 4.54 billion years with a precision of as little as 1% margin for error.[15]

Why the Canyon Diablo meteorite was used

File:Canyon-diablo-meteorite.jpg
Fragment of the Canyon Diablo iron meteorite.

The Canyon Diablo meteorite was used because it is a very large representative of a particularly rare type of meteorite which contains sulfide minerals (particularly troilite, FeS), metallic nickel-iron alloys, plus silicate minerals.

File:Meteor.jpg
Barringer Crater, Arizona where the Canyon Diablo meteorite was found.

This is important because the presence of the three mineral phases allows investigation of isotopic dates using samples which provide a great separation in concentrations between parent and daughter nuclides. This is particularly true of uranium and lead. Lead is strongly chalcophilic and is found in the sulfide at a much greater concentration than in the silicate, versus uranium. Because of this segregation in the parent and daughter nuclides during the formation of the meteorite, this allowed a much more precise date of the formation of the solar disk and hence the planets than ever before.

The Canyon Diablo date has been backed up by hundreds of other dates, from both terrestrial samples and other meteorites. The meteorite samples, however, show a spread from 4.53 to 4.58 billion years ago. This is interpreted as the duration of formation of the solar nebula and its collapse into the solar disk to form the Sun and the planets. This 50 million year time span allows for accretion of the planets from the original solar dust and meteorites.

The moon, as another extraterrestrial body which has not undergone plate tectonics and which has no atmosphere, provides quite precise age dates from the samples returned from the Apollo missions. Rocks returned from the moon have been dated at a maximum of around 4.4 and 4.5 billion years old. Martian meteorites which have landed upon the Earth, have also been dated to around 4.5 billion years old by lead-lead dating.

Altogether the concordance of age dates of both the earliest terrestrial lead reservoirs and all other reservoirs within the solar system found to date are used to support the hypothesis that the Earth and the rest of the solar system formed at around 4.53 to 4.58 billion years ago.

Helioseismic verification

The radiometric date of meteorites can be verified with studies of the Sun. The Sun can be dated using "helioseismic" methods which strongly agree with the radiometric dates found for the oldest meteorites.[16]

See also

References

  1. "Age of the Earth". U.S. Geological Survey. 1997. Retrieved 2006-01-10.
  2. 2.0 2.1 Patterson, C., 1956, Age of meteorites and the earth: Geochimica et Cosmochimica Acta 10: 230-237.
  3. 3.0 3.1 Boltwood, B.B., 1907, On the ultimate disintegration products of the radio-active elements. Part II. The disintegration products of uranium: American Journal of Science 23: 77–88.
  4. 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 409: 175–178.
  5. Baker, J., Bizzarro, M, Wittig, N, Connelly, J, Haack, H, 2005, Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites. Nature 436: 1127–1131.
  6. Bowring, S.A. and Williams, I.S., 1999. Priscoan (4.00-4.03 Ga) orthogneisses from northwestern Canada: Contributions to Mineralogy and Petrology 134: 3-16.
  7. England P., Molnar P., Richter P., 2007. John Perry's neglected critique of Kelvin's age for the earth: A missed opportunity in geodynamics. GSA Today v.17 (1), 4–9. doi: 10.1130/GSAT01701A.1
  8. 8.0 8.1 8.2 8.3 Dalrymple, G. Brent, 1991, The Age of the Earth, Stanford University Press, pp 14–17, ISBN 0-8047-2331-1
  9. 9.0 9.1 England, Philip C.; Molnar, Peter; Richter, Frank M. (2007), "Kelvin, Perry and the Age of the Earth", American Scientist, 95 (4): pp. 342–349
  10. Joly, J. (1909). Radioactivity and Geology: An Account of the Influence of Radioactive Energy on Terrestrial History, London: Archibald Constable & Co., Ltd., p. 36. ISBN 1402135777.
  11. Rutherford, E. (1906). Radioactivity Transformations, Charles Scribner’s Sons, New York, 287 pp.; reprinted by Juniper Grove (2007) ISBN 978-1603550543.
  12. Rutherford: Being the Life and Letters of the Rt. Hon. Lord Rutherford, O.M. Cambridge: Cambridge University Press. 1939. Unknown parameter |kast= ignored (help)
  13. Dalrymple, G. Brent, 1991, The Age of the Earth, Stanford University Press, pp 74, ISBN 0-8047-2331-1
  14. 14.0 14.1 Dalrymple, G. Brent, 1991, The Age of the Earth, Stanford University Press, pp 77–78, ISBN 0-8047-2331-1
  15. Dalrymple, G. Brent, 1991, The Age of the Earth, Stanford University Press, pp 310–341, ISBN 0-8047-2331-1
  16. Bonanno, A., H. Schlattl and L. Paterno, (2006) The age of the sun and relativistic corrections in the EOS, Astronomy and Astrophysics [1]
  • Carlson R.W. & Tera F., 1998. Lead-Lead Constraints on the time scale of early planetary differentiation. Origin of Earth and Moon Conference, Lunar and Planetary Society. PDF abstract
  • Powell, James Lawrence, 2001, Mysteries of Terra Firma: the Age and Evolution of the Earth, Simon & Schuster, ISBN 0-684-87282-X
  • Terada, K & Sano Y., 2001. In-situ ion microprobe U-Pb dating of phosphates in H-chondrites. in Proceedings of the 11th Annual W.M. Goldschmidt Conference, Lunar and Planetary Society. PDF abstract
  • Valley, John W., William H. Peck, Elizabeth M. King (1999) Zircons Are Forever, The Outcrop for 1999, University of Wisconsin-Madison Wgeology.wisc.eduEvidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago Accessed Jan. 10, 2006
  • 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, v. 409, pp. 175–178.
  • Wyche, S., D. R. Nelson and A. Riganti (2004) 4350–3130 Ma detrital zircons in the Southern Cross Granite–Greenstone Terrane, Western Australia: implications for the early evolution of the Yilgarn Craton, Australian Journal of Earth Sciences Volume 51 Zircon ages from W. Australia – Abstract Accessed Jan. 10, 2006

External links

Further reading

  • Baadsgaard, H.; Lerbekmo, J.F.; Wijbrans, J.R., 1993. Multimethod radiometric age for a bentonite near the top of the Baculites reesidei Zone of southwestern Saskatchewan (Campanian-Maastrichtian stage boundary?). Canadian Journal of Earth Sciences, v.30, p.769–775.
  • Baadsgaard, H. and Lerbekmo, J.F., 1988. A radiometric age for the Cretaceous-Tertiary boundary based on K-Ar, Rb-Sr, and U-Pb ages of bentonites from Alberta, Saskatchewan, and Montana. Canadian Journal of Earth Sciences, v.25, p.1088–1097.
  • Eberth, D.A. and Braman, D., 1990. Stratigraphy, sedimentology, and vertebrate paleontology of the Judith River Formation (Campanian) near Muddy Lake, west-central Saskatchewan. Bulletin of Canadian Petroleum Geology, v.38, no.4, p.387–406.
  • Goodwin, M.B. and Deino, A.L., 1989. The first radiometric ages from the Judith River Formation (Upper Cretaceous), Hill County, Montana. Canadian Journal of Earth Sciences, v.26, p.1384–1391.
  • Gradstein, F. M.; Agterberg, F.P.; Ogg, J.G.; Hardenbol, J.; van Veen, P.; Thierry, J. and Zehui Huang., 1995. A Triassic, Jurassic and Cretaceous time scale. IN: Bergren, W. A. ; Kent, D.V.; Aubry, M-P. and Hardenbol, J. (eds.), Geochronology, Time Scales, and Global Stratigraphic Correlation. Society of Economic Paleontologists and Mineralogists, Special Publication No. 54, p.95–126.
  • Harland, W.B., Cox, A.V.; Llewellyn, P.G.; Pickton, C.A.G.; Smith, A.G.; and Walters, R., 1982. A Geologic Time Scale: 1982 edition. Cambridge University Press: Cambridge, 131p.
  • Harland, W.B.; Armstrong, R.L.; Cox, A.V.; Craig, L.E.; Smith, A.G.; Smith, D.G., 1990. A Geologic Time Scale, 1989 edition. Cambridge University Press: Cambridge, p.1–263. ISBN 0-521-38765-5
  • Harper, C.W., Jr., 1980. Relative age inference in paleontology. Lethaia, v.13, p.239–248.
  • Lubenow, M.L., 1992. Bones of Contention: A Creationist Assessment of Human Fossils. Baker Book House: Grand Rapids.
  • Obradovich, J.D., 1993. A Cretaceous time scale. IN: Caldwell, W.G.E. and Kauffman, E.G. (eds.). Evolution of the Western Interior Basin. Geological Association of Canada, Special Paper 39, p.379–396.
  • Palmer, Allison R. (compiler), 1983. The Decade of North American Geology 1983 Geologic Time Scale. Geology, v.11, p.503–504. September 12, 2004.

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