A very common claim of young earth creationists in trying to reject the evidence for an old earth is to loudly proclaim that radiometric dating methods “makes assumptions” and that these “assumptions” are somehow fatally flawed or not supported by evidence. These claims generally land in three different categories: (1) radiometric dating assumes that initial conditions (concentrations of mother and daughter nuclei) are known, (2) radiometric dating assumes that rocks are closed systems and (3) radiometric dating assumes that decay rates are constant. Most young earth creationists reject all of these points. As a scientific skeptics, we ask ourselves: is this really the case? Let us critically examine each of these claims and see if they hold up against the science. While doing so, we will have to learn about how radiometric dating actually works.
There are many different kinds of radiometric dating and not all conclusions we will reach can be extrapolated to all methods used. Also, different radiometric dating techniques independently converges with each other and with other dating techniques such as dendrochronology, layers in sediment, growth rings on corals, rhythmic layering of ice in glaciers, magnetostratigraphy, fission tracks and many other methods. This serves as strong evidence for the reliability of radiometric dating methods.
1. How does radiometric dating work?
A lot of atoms are stable. Some are not. There exists different versions, or isotopes of many elements. These isotopes differ in the number of neutrons they have in their nuclei. Those isotopes that are not stable decay into daughter nuclei. Those that did the decaying are called parent nuclei. If you have a rock that contains radioactive isotopes, these will decay over time. As time goes on, the ratio of the parent to daughter nuclei will change and decrease (as more parent nuclei decay into daughter nuclei, the former decreases and the latter increases). Measuring this ratio gives us an idea of how long ago the rock formed.
But wait a second! Doesn’t this assume that the rocks are closed systems? Surely, if some daughter nuclei left the rock or parent nuclei entered the rock, the dates would come out all wrong! While this is technically true, there are several mini-industries dedicated developing methods and techniques to make sure that there is no contamination and check to see if the rocks where disturbed between forming and being tested by scientists. How is this done? Let’s find out!
2. Radiometric dating and testing for contamination and disturbances
On of the great things about many forms of radiometric dating is that they are self-checking. That is, you can see if the sample comes from rocks that have been disturbed (or contaminated) or not just by looking at the results. Now, creationists will claim that scientists are just somehow assuming that if samples show an age that does not fit their preconceptions, the sample must be contaminated or leaky. This is false. To see why, we need to look deeper into radiometric dating methods. A very important tool in radiometric dating is the so called isochron diagram and it holds the key to refuting the central creationist claims about radiometric dating.
One of the most beneficial things about it is that it can check itself for accuracy; the method tells you how well the rocks have been closed systems. An isochron diagram is obtained by looking at many minerals from the same rock or from rocks forming from the same parent mineral. Data is plotted on a simple two dimensional graph; the parent isotope on the x-axis and the daughter isotope on the y-axis. Both of these are divided or normalized by a stable isotope of the same elements as the daughter element. So on the x-axis, we have parent/(another stable isotope of the same element as the daughter) and on the y-axis we have daughter/(another stable isotope of the same element as the daughter).
If the samples have been undisturbed closed systems since formation, the data will fall on the same line (the isochron from which the diagram is named). The slope of this line is a function of the age of the rock. If the rock is older, the slope is higher. The reason scientists normalize with another stable isotope of the same element as the daughter is because most chemical or physical processes that occurs normally in nature does not differentiate between different isotopes of the same element when the difference in mass is as small as it is between isotopes of the same element that is used in radiometric dating. This means that the while different rocks contain different absolute amounts of the two isotopes, the ratio is same. At the time of formation for a rock, the isotopes for an element are homogenized and so the composition of a certain isotope is the same in all the minerals in the rock. But what happens when the rocks have been disturbed?
If a rock is heated during its lifetime, the system gets disturbed and some of the parent and/or daughter isotopes may move in or out of the rock. If so, the data will not fall on an isochron line, but will be all over the place. This tells scientists that the sample has been disturbed and cannot be dated with this particular method. So far from rejecting samples because they do not fit a preconceived notion of what the age should be, scientists reject samples because there is ample evidence that it has been disturbed: the data points do not lie on the isochron lines.
Scientists do not assume that rocks have been closed systems; it is a well-supported conclusion from experiments. But what about assuming that initial amounts are known?
3. Radiometric dating and initial conditions
A second property of isochron diagrams is that it actually gives the initial amount of daughter isotope as a result of the method. It is just the y-intercept of the isochron line. At this intersect, the ratio of parent/(another stable isotope of the same element as the daughter) is by definition 0 and so no amount of the daughter here is produced by decay of the parent in the rock. The initial conditions are just read off the graph; it is not just assumed.
4. Radiometric dating and decay rates
In a last ditch effort, young earth creationists exclaim that scientists just assume, without warrant, that decay rate are constant. However, this is not the case. Decay rates have been shown to be constant, despite very high pressure and temperature. Furthermore, by studying supernovas far away, scientist have determined that decay rates have been constant in the ancient past as well. Not only that, different radioactive isotopes decay differently and it is enormously improbable that a postulated difference in decay rates would affect all of them in the same way, yet as we have seen, different radiometric dating methods converge on the same date (within margins of error). Fourthly, decay rates can be predicted from first principles of physics. Any change would have to correspond to changes in basic physical constants. Any such change would affect different forms of decay differently, yet this has not been observed. As a final blow to the already nailed shut coffin of young earth creationism, had decay rates been high enough to be consistent with a young earth, the heat alone would have melt the earth.
Scientists do not assume that rocks have been closed systems, but they test for it. If all the data points fall on the isochron line, it has been a closed system; it it scatters, it has not and that rock is not used for dating with that method. Scientists also do not assumed that initial conditions are known; this is just read off the graph at the y-intercept. Finally, by studying supernovas, scientists know that decay rates have been constant in the past.
6. References and Further Reading
Dalrymple, G. B., (2004) Ancient Earth, Ancient Skies: The Age o the Earth and Its Cosmic Surroundings. Stanford: Stanford University Press.
Marshak, S., (2008). Earth: Portrait of a Planet. Third Edition. New York: W. W. Norton & Company.
Hedman, M. (2007). The Age of Everything. Chicago: University of Chicago Press.