Common Misconceptions Regarding Radiometric Dating Methods
"There are a number of misconceptions that seem especially prevalent among Christians. Most of these topics are covered in the above discussion, but they are reviewed briefly here for clarity.
1.
Radiometric dating is based on index fossils whose dates were assigned long before radioactivity was discovered.
This is not at all true, though it is implied by some young-Earth literature. Radiometric dating is based on the half-lives of the radioactive isotopes. These half-lives have been measured over the last 40-90 years. They are not calibrated by fossils.
2.
No one has measured the decay rates directly; we only know them from inference.
Decay rates have been directly measured over the last 40-100 years. In some cases a batch of the pure parent material is weighed and then set aside for a long time and then the resulting daughter material is weighed. In many cases it is easier to detect radioactive decays by the energy burst that each decay gives off. For this a batch of the pure parent material is carefully weighed and then put in front of a Geiger counter or gamma-ray detector. These instruments count the number of decays over a long time.
3
. If the half-lives are billions of years, it is impossible to determine them from measuring over just a few years or decades.
The example given in the section titled, "The Radiometric Clocks" shows that an accurate determination of the half-life is easily achieved by direct counting of decays over a decade or shorter. This is because a) all decay curves have exactly the same shape (Fig. 1), differing only in the half-life, and b) trillions of decays can be counted in one year even using only a fraction of a gram of material with a half-life of a billion years. Additionally, lavas of historically known ages have been correctly dated even using methods with long half-lives.
4.
The decay rates are poorly known, so the dates are inaccurate.
Most of the decay rates used for dating rocks are known to within two percent. Uncertainties are only slightly higher for rhenium (5%), lutetium (3%), and beryllium (3%), discussed in connection with Table 1. Such small uncertainties are no reason to dismiss radiometric dating. Whether a rock is 100 million years or 102 million years old does not make a great deal of difference.
5.
A small error in the half-lives leads to a very large error in the date.
Since exponents are used in the dating equations, it is possible for people to think this might be true, but it is not. If a half-life is off by 2%, it will only lead to a 2% error in the date.
6.
Decay rates can be affected by the physical surroundings.
This is not true in the context of dating rocks. Radioactive atoms used for dating have been subjected to extremes of heat, cold, pressure, vacuum, acceleration, and strong chemical reactions far beyond anything experienced by rocks, without any significant change. The only exceptions, which are not relevant to dating rocks, are discussed under the section, "Doubters Still Try", above.
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7.
A small change in the nuclear forces probably accelerated nuclear clocks during the first day of creation a few thousand years ago, causing the spuriously old radiometric dates of rocks.
Rocks are dated from the time of their formation. For it to have any bearing on the radiometric dates of rocks, such a change of nuclear forces must have occurred after the Earth (and the rocks) were formed. To make the kind of difference suggested by young-Earth proponents, the half-lives must be shortened from several billion years down to several thousand years--a factor of at least a million. But to shorten half-lives by factors of a million would cause large physical changes. As one small example, recall that the
Earth is heated substantially by radioactive decay. If that decay is speeded up by a factor of a million or so, the tremendous heat pulse would easily melt the whole
Earth, including the rocks in question! No radiometric ages would appear old if this happened.
8.
The decay rates might be slowing down over time, leading to incorrect old dates.
There are two ways we know this didn't happen: a) we have checked them out with "time machines", and b) it doesn't make sense mathematically. Both of these points are explained in the section titled, "Can We Really Believe the Dating Systems?"
9.
We should measure the "full-life" (the time at which all of the parent is gone) rather than the half-life (the time when half of it is gone).
Unlike sand in an hourglass, which drops at a constant rate independent of how much remains in the top half of the glass, the number of radioactive decays is proportional to the amount of parent remaining. Figure 1 shows how after 2 half-lives, 1/2 x 1/2 = 1/4 is left, and so on. After 10 half-lives there is 2-10 = 0.098% remaining. A half-life is more easy to define than some point at which almost all of the parent is gone. Scientists sometimes instead use the term "mean life", that is, the average life of a parent atom. The mean life is always 1/ln(2) = 1.44 times the half-life. For most of us half-life is easier to understand.
10.
To date a rock one must know the original amount of the parent element. But there is no way to measure how much parent element was originally there.
It is very easy to calculate the original parent abundance, but that information is not needed to date the rock. All of the dating schemes work from knowing the
present abundances of the parent and daughter isotopes. The original abundance N0, of the parent is simply N0 = N ekt, where N is the present abundance, t is time, and k is a constant related to the half life.
11
. There is little or no way to tell how much of the decay product, that is, the daughter isotope, was originally in the rock, leading to anomalously old ages.
A good part of this article is devoted to explaining how one can tell how much of a given element or isotope was originally present. Usually it involves using more than one sample from a given rock. It is done by comparing the ratios of parent and daughter isotopes relative to a stable isotope for samples with different relative amounts of the parent isotope. For example, in the rubidium-strontium method one compares rubidium-87/strontium-86 to strontium-87/strontium-86 for different minerals. From this one can determine how much of the daughter isotope would be present if there had been no parent isotope. This is the same as the initial amount (it would not change if there were no parent isotope to decay). Figures 4 and 5, and the accompanying explanation, tell how this is done most of the time. While this is not absolutely 100% foolproof, comparison of several dating methods will always show whether the given date is reliable.
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12
. There are only a few different dating methods.
This article has listed and discussed a number of different radiometric dating methods and has also briefly described a number of non-radiometric dating methods. There are actually many more methods out there. Well over forty different radiometric dating methods are in use, and a number of non-radiogenic methods not even mentioned here.
13
. "Radiation halos" in rocks prove that the Earth was young.
This refers to tiny halos of crystal damage surrounding spots where radioactive elements are concentrated in certain rocks. Halos thought to be from polonium, a short-lived element produced from the decay of uranium, have been found in some rocks. A plausible explanation for a halo from such a short-lived element is that these were not produced by an initial concentration of the radioactive element. Rather, as water seeped through cracks in the minerals, a chemical change caused newly-formed polonium to drop out of solution at a certain place and almost immediately decay there
. A halo would build up over a long period of time even though the center of the halo never contained more than a few atoms of polonium at one time. "Hydrothermal" effects can act in ways that at first seem strange, such as the well-known fact that gold--a chemically un-reactive metal with very low solubilities--is concentrated along quartz veins by the action of water over long periods of time
. Other researchers have found halos produced by an indirect radioactive decay effect called hole diffusion, which is an electrical effect in a crystal. These results suggest that the halos in question are not from short-lived isotopes after all.
At any rate, halos from uranium inclusions are far more common. Because of uranium's long half-lives, these halos take at least several hundred million years to form. Because of this, most people agree that halos provide compelling evidence for a very old
Earth.
14
. A young-Earth research group reported that they sent a rock erupted in 1980 from Mount Saint Helens volcano to a dating lab and got back a potassium-argon age of several million years. This shows we should not trust radiometric dating.
There are indeed ways to "trick" radiometric dating if a single dating method is improperly used on a sample. Anyone can move the hands on a clock and get the wrong time. Likewise, people actively looking for incorrect radiometric dates can in fact get them. Geologists have known for over forty years that the potassium-argon method cannot be used on rocks only twenty to thirty years old. Publicizing this incorrect age as a completely new finding was inappropriate. The reasons are discussed in the Potassium-Argon Dating section above. Be assured that multiple dating methods used together on igneous rocks are almost always correct unless the sample is too difficult to date due to factors such as metamorphism or a large fraction of xenoliths.
15.
Low abundances of helium in zircon grains show that these minerals are much younger than radiometric dating suggests.
Zircon grains are important for uranium-thorium-lead dating because they contain abundant uranium and thorium parent isotopes. Helium is also produced from the decay of uranium and thorium. However, as a gas of very small atomic size, helium tends to escape rather easily. Researchers have studied the rates of diffusion of helium from zircons, with the prediction from one study by a young-
Earth creationist suggesting that it should be quantitatively retained despite its atomic size. The assumptions of the temperature conditions of the rock over time are most likely unrealistic in this case.
16
. The fact that radiogenic helium and argon are still degassing from the Earth's interior prove that the Earth must be young.
The radioactive parent isotopes, uranium and potassium, have very long half-lives, as shown in Table 1. These parents still exist in abundance in the Earth's interior, and are still producing helium and argon. There is also a time lag between the production of the daughter products and their degassing. If the Earth were geologically very young, very little helium and argon would have been produced. One can compare the amount of argon in the atmosphere to what would be expected from decay of potassium over 4.6 billion years, and in fact it is consistent.
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17.
The waters of Noah's flood could have leached radioactive isotopes out of rocks, disturbing their ages.
This is actually suggested on one website! While water can affect the ability to date rock surfaces or other weathered areas, there is generally no trouble dating interior portions of most rocks from the bottom of lakes, rivers, and oceans. Additionally, if ages were disturbed by leaching, the leaching would affect different isotopes at vastly different rates. Ages determined by different methods would be in violent disagreement. If the flood were global in scope, why then would we have
any rocks for which a number of different methods all agree with each other? In fact, close agreement between methods for most samples is a hallmark of radiometric dating.
18.
We know the Earth is much younger because of non-radiogenic indicators such as the sedimentation rate of the oceans.
There are a number of parameters which, if extrapolated from the present without taking into account the changes in the
Earth over time, would seem to suggest a somewhat younger
Earth. These arguments can sound good on a very simple level, but do not hold water when all the factors are considered. Some examples of these categories are the decaying magnetic field (not mentioning the widespread evidence for magnetic reversals), the saltiness of the oceans (not counting sedimentation!), the sedimentation rate of the oceans (not counting Earthquakes and crustal movement, that is, plate tectonics), the relative paucity of meteorites on the Earth's surface (not counting weathering or plate tectonics), the thickness of dust on the moon (without taking into account brecciation over time), the Earth-Moon separation rate (not counting changes in tides and internal forces), etc. While these arguments do not stand up when the complete picture is considered, the case for a very old creation of the Earth fits well in all areas considered.
19.
Only atheists and liberals are involved in radiometric dating.
The fact is that there are a number of Bible-believing Christians who are involved in radiometric dating, and who can see its validity firsthand. A great number of other Christians are firmly convinced that radiometric dating shows evidence that God created the Earth billions, not thousands, of years ago.
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20.
Different dating techniques usually give conflicting results.
This is not true at all. The fact that dating techniques most often agree with each other is why scientists tend to trust them in the first place. Nearly every college and university library in the country has periodicals such as
Science,
Nature, and specific geology journals that give the results of dating studies. The public is usually welcome to (and should!) browse in these libraries. So the results are not hidden; people can go look at the results for themselves. Over a thousand research papers are published a year on radiometric dating, essentially all in agreement. Besides the scientific periodicals that carry up-to-date research reports, specific suggestions are given below for further reading, both for textbooks, non-classroom books, and web resources."
Radiometric Dating
