How accurate are radioactivity-based geological dates?

Introduction

A 2009 poll found that 39% of Americans agreed that “God created the universe, the earth, the sun, moon, stars, plants, animals and the first two people within the past 10,000 years” [Bishop2010].

Needless to say, such notions are completely in conflict with modern scientific research. Radiometric dating schemes used in geology and paleontology are based on known rates of radioactivity, a phenomenon that is rooted in fundamental laws of physics and follows simple mathematical formulas. These radiometric dating schemes have been refined and scrutinized for several decades. The latest high-tech equipment permits reliable results to be obtained even with microscopic samples.

Radiometric dating is self-checking, because, in most of these schemes, the data (after certain preliminary calculations) are fitted to a straight line (an “isochron”) by means of standard linear regression methods of statistics. The slope of the line determines the date, and the closeness of fit is a measure of the statistical reliability of the resulting date. Technical details on how these dates are calculated are given in Radiometric dating.

Reliability of radiometric dating

The overall reliability of radiometric dating was addressed in some detail in a recent book by Brent Dalrymple, a premier expert in the field. He wrote [Dalrymple2004, pg. 80-81]:

These methods provide valid age data in most instances, although there is a small percentage of instances in which even these generally reliable methods yield incorrect results. Such failures may be due to laboratory errors (mistakes happen), unrecognized geologic factors (nature sometimes fools us), or misapplication of the techniques (no one is perfect). …

The use of different dating methods on the same rock is an excellent way to check the accuracy of age results. If two or more radiometric clocks based on different elements and running at different rates give the same age, that’s powerful evidence that the ages are probably correct.

Along this line, Roger Wiens asks those who are skeptical of radiometric dating to consider the following (condensed from [Wiens2002]):

  1. There are well over forty different radiometric dating methods, and scores of other methods such as tree rings and ice cores.
  2. All of the different dating methods agree — they agree a great majority of the time over millions of years of time. Some [skeptics] make it sound like there is a lot of disagreement, but this is not the case. The disagreement in values needed to support the position of young-Earth proponents would require differences in age measured by orders of magnitude (e.g., factors of 10,000, 100,000, a million, or more). The differences actually found in the scientific literature are usually close to the margin of error, usually a few percent, not orders of magnitude!
  3. Vast amounts of data overwhelmingly favor an old Earth. Several hundred laboratories around the world are active in radiometric dating. Their results consistently agree with an old Earth. Over a thousand papers on radiometric dating were published in scientifically recognized journals in the last year, and hundreds of thousands of dates have been published in the last 50 years. Essentially all of these strongly favor an old Earth.
  4. Radioactive decay rates have been measured for over sixty years now for many of the decay clocks without any observed changes. And it has been close to a hundred years since the uranium-238 decay rate was first determined.
  5. Both long-range and short-range dating methods have been successfully verified by dating lavas of historically known ages over a range of several thousand years.
  6. The mathematics for determining the ages from the observations is relatively simple.

Applications of radiometric dating

Radiometric methods are used heavily in day-to-day research in paleontology and evolutionary biology, in order to test certain hypotheses.

One interesting and timely application of advanced radiometric dating techniques in paleontology is in attempts to disentangle a controversy as to whether or not all dinosaurs (and numerous other species) were extinguished by a giant meteorite impact at a spot just north of the present-day Yucatan Peninsula. In 2011, researchers at the University of Alberta in Canada used the uranium-lead method to date a fossilized dinosaur bone found in New Mexico to be approximately 64.8 million years old. At the time, the Cretaceous-Tertiary meteorite impact was thought to have occurred approximately 65.5 million years ago. This suggested that hadrosaurs such as the researcher’s specimen may have survived for roughly 700,000 years after the meteorite event [SD2011b]. But in February 2013, a team of researchers at U.C. Berkeley, using a state-of-the-art argon-argon scheme that permits a significantly more accurate date determination, found that the impact had occurred 66,038,000 years ago, while the mass extinction occurred 66,043,000 years ago. Given that these dates differ by no more than the statistical error bars of the measurements, they are essentially identical. Thus these new findings offer dramatic confirmation to the theory that the meteorite impact caused the extinction (although climate-related phenomena prior to that time may have exacerbated stress on these species) [Perkins2013; Choi2013].

Another ongoing debate is whether or not Neanderthals persisted in Europe until after humans arrived. In June 2012, researchers announced that some red handprints and dots in a cave in northwestern Spain are more than 40,000 years old, based on a uranium-thorium dating of the calcite covering of the specimens. These results raised the intriguing possibility that the artists who created these images were Neanderthal, since at the time Neanderthals were thought to have remained in the Iberian peninsula region until as recently as 35,000 to 40,000 years ago [Wilford2012]. However, in February 2013 researchers at the University of Oxford in the U.K, using a more sophisticated Carbon-14-based dating process, found that the latest Neanderthal sites are 10,000 years older than previously thought — i.e., they are 45,000 to 50,000 years old. Thus, for example, the handprints in Spain most likely are human, not Neanderthal [Callaway2013].

These two examples also underscore the futility in asserting that there is some sort of “conspiracy” or “groupthink” preventing the consideration of young-earth creationist views. Note that each of these studies have the potential to overthrow the beloved theories of numerous other researchers. If there are fundamental weaknesses in the general class of radiometric dating schemes (or in the particular schemes used in these two studies), why don’t the researchers whose results are potentially refuted come forward to publicly identify these weaknesses or flaws? The only believable answer is that there are no fundamental flaws in these schemes — they have withstood decades of rigorous examination within the scientific community and well deserve their reputation for reliability, although minor adjustments will be made from time to time as experimental techniques are further refined. For additional discussion, see Conspiracy.

Conclusion

Radiometric dating, like any other experimental discipline, is subject to a variety of errors, ranging from human errors to rare anomalies resulting from highly unusual natural circumstances. But while errors and anomalies can occur, the burden of proof is not on scientists to fully explain each and every error. Instead, the burden of proof is on skeptics of old-earth geology to explain why tens of thousands of other carefully measured ages are all internally and externally consistent. Indeed, there is no known physical phenomenon that can yield consistent results in many thousands of measurements, year after year, except one: that these specimens really are as old as the data shows them to be. As biologist Kenneth Miller has observed, “The consistency of [radiometric] data … is nothing short of stunning.” [Miller1999, pg. 76].

The reliability of radiometric dating is discussed in greater detail in this article: Reliability. Another related article discusses radiocarbon dating: Radiocarbon dating. A third article discusses whether a “time machine” is required to study the distant past: Time machine. A fourth article discusses the “uniformitarian” assumption and how it relates to evolution, radiometric dating and the age of the earth: Uniformitarian.

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