How reliable are measurements of the age of the earth?

[This article has also been posted Here.]

A large fraction of the public still does not accept the most basic facts of modern geology, such as the notion that the earth is many millions of years old. For example, fully 45 percent of Americans insist that the earth was created at some time within the past 10,000 years [Gallup2004].

Much of this skepticism stems from the creationist movement, which has gone to great lengths to criticize the radiometric methods used to date rocks and fossils, such as Carbon-14, Rb-Sr and the K-Ar methods. Creationists cite “anomalies” that have been noted in the scientific literature as evidence that these methods are “unreliable.” As a single example, creationist Henry Morris [Morris2000, pg. 147] has highlighted the fact that K-Ar measurements of rocks from a 1801 lava flow near a volcano in Hualalai, Hawaii give apparent ages ranging from 160 million to 2.96 billion years, citing a 1968 study [Funkhouser1968].

So what are the facts here? How reliable are these measurements?

Indeed, geological dating methods, like the vast majority of scientific measurement techniques in many disciplines, are subject to anomalies. But such anomalies are hardly a secret in the field — they have been studied extensively in the literature, and most are well understood as due to various known phenomena — e.g., selective leaching of certain minerals from rocks, remelting of the specimen in the interim, or, in the case of Carbon-14 dates, historical variations in the rate of formation of Carbon-14. Modern dating procedures include steps to avoid such problems, and to cross-check results.

Along this line, because of its relatively short half-life, Carbon-14 measurements can only be used to date relatively recent items — i.e., less than about 50,000 years old. Thus any “anomalies” that Carbon-14 dating schemes may be subject to are completely irrelevant to the question of whether the earth is many millions of years old. With regards to the Hualalai lava mentioned above, this is unusual because it includes numerous xenoliths, typically consisting of olivine, an iron-magnesium silicate material, that are foreign to the lava, having been carried from deep within the mantle (as the authors of the 1968 study were careful to explain). These and other “anomalies” are analyzed in exhaustive detail in [Dalrymple2006].

Most recent radiometric measurements are very convincingly self-checked via “isochrons” — multiple data points from a single sample or rock layer are plotted, and the best straight-line fit is used to calculate the age. If the data points do not line on a very nearly straight line — if some lie far from the line — that indicates that the sample has been subjected to “anomalies” (for whatever reason), and the resulting dates are not reliable. If however they do all lie very nearly on a straight line (which nowadays happens most of the time), then this is compelling evidence that the data are reliable [Bailey2010a; Bailey2010b; Dalrymple2006; Stassen1998; Stassen2005].

New, high-tech equipment has eliminated many of the difficulties that afflicted earlier measurements. For example, the “SHRIMP” ion microprobe now in use in numerous laboratories around the world can reliably measure U-Pb and Pb-Pb ages from spots only 0.02 mm (i.e., 20 micrometers) in size within a zircon crystal [Dalrymple2004, pg. 60-62]. New dating schemes have been developed, such as the Argon-40-Argon-39 method, that are not vulnerable to some of the difficulties that potentially can afflict other schemes. Often it is possible to use completely independent dating methods on the same samples, thus providing some very strong cross-checking.

So what is the best explanation of the “anomalies”: (a) that every one of tens of thousands of careful, recent measurements, made using the latest high-tech equipment, self-checked with isochrons or the equivalent, are all simultaneously in error by nearly six orders of magnitude? or (b) that the handful of remaining anomalies that have not yet been fully explained are subject to some reasonable natural phenomenon that in time will be well understood? Obviously the second explanation is overwhelmingly more credible. Those who try to sweep enormous mounds of high-quality, consistent data under the rug are guilty of what is often referred to as the “forest fallacy” — i.e., someone who picks faults with the bark of a handful of trees, and then tries to claim that the forest doesn’t exist.

Overall, the picture painted by the large body of studies that have been published in the field is an extremely consistent one — when the same fossil layer is tested, anywhere in the world, the date is the same. As biologist Kenneth Miller notes, “The consistency of the data … is nothing short of stunning.” [Miller1999, pg. 76]. Of course none of this matters much for those who have no wish to be convinced by the geological record. But for the rest of us, there is no room for doubt.

  1. [Bailey2010a] David H. Bailey, “How are the dates of various geologic ages and fossil layers calculated?”, available at Online article.
  2. [Bailey2010b] David H. Bailey, “How reliable are geologic dates?”, available at Online article.
  3. [Dalrymple1991] G. Brent Dalrymple, The Age of the Earth, Stanford University Press, Stanford, CA, 1991.
  4. [Dalrymple2004] G. Brent Dalrymple, Ancient Earth, Ancient Skies: The Age of Earth and its Cosmic Surroundings, Stanford University Press, Stanford, CA, 2004.
  5. [Dalrymple2006] G. Brent Dalrymple, “Radiometric Dating,” available at Online article.
  6. [Funkhouser1968] J. G. Funkhouser and J. J. Naughton, “Radiogenic helium and argon in ultramafic inclusions from Hawaii,” Geophysical Research Journal, vol. 73 (1968), pg. 4601-4607.
  7. [Gallup2004] Gallup poll, available at Online article.
  8. [Miller1999] Kenneth R. Miller, Finding Darwin’s God: A Scientist’s Search for Common Ground Between God and Evolution, Cliff Street Books, New York, 1999.
  9. [Morris2000] Henry M. Morris, Scientific Creationism, Creation-Life Publishers, El Cajon, CA, 1985, reprinted 2000.
  10. [Stassen1998] Chris Stassen, “Isochron Dating,” available at Online article.
  11. [Stassen2005] Chris Stassen, “The Age of the Earth,” available at Online article.

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