RADIO-DATING AND THE CREATION "SCIENTISTS"
by Lenny Flank
(c) 1995
The most commonly-used methods of dating geological formations involve the process of radioactive decay. Certain atoms are unstable, and their nuclei sometimes break apart and change into another element through a process known as "radioactive decay". Some of these radioactive elements transform themselves by emitting a high-energy particle consisting of two protons and two neutrons, a process known as "alpha decay". Other radioactive elements decay when a neutron inside the nucleus breaks into a proton and an electron. The proton stays in the nucleus, and the electron is ejected at very high speed--a process known as "beta decay".
Probably the best-known of the radioactive elements is uranium, which is the heaviest element found in nature. The uranium nucleus comes in several versions. Each version is known as an "isotope". All isotopes of uranium have 92 protons in the nucleus (it is the number of protons which determines to which chemical element an atom belongs), but the number of neutrons can range from 141 to 146. Thus, the total number of particles in the nucleus (protons plus neutrons) in uranium can vary from 233 to 238. Each of these isotopes is identified by its "atomic number"--the total number of particles in its nucleus. Uranium, for instance, is found in three different isotopes, uranium-233 (abbreviated chemically as U-233), U-235 and U-238.
All of the isotopes of uranium are radioactive, and decay by emitting an alpha particle. Through a series of intermediate steps, the U-235 atom will decay to form an atom of the lead isotope 207 (abbreviated chemically as Pb-207). The Pb-207 atom does not undergo radioactive decay--it is "stable"--and thus over time all U-235 will tend to decay to form increasing amounts of Pb-207. Other chemical elements may have some isotopes that undergo radioactive decay, and other isotopes that do not decay--they are also "stable". The other radioactive elements will decay to form different stable "daughter elements".
Radio-dating is possible because of the fact that the decay of a radioactive element into its daughter element takes place at a constant rate, known as the "half-life", and the half-life of various radio-decay rates can be measured very precisely. U-235, for instance, has a half-life of 713 million years. If we start with a known quantity of U-235, say one pound, in 713 million years this material will consist of half U-235 and half Pb-207. In another 713 million years, half of the remaining uranium will decay, and the material will now consist of three-fourths lead and one-fourth uranium.
Conversely, if we calculate what the ratio of lead to uranium is in a given rock, we can calculate the length of time that has passed since the original uranium started decaying. For instance, if we determine that a rock consists of one-sixteenth U-235 and fifteen-sixteenths Pb-207, we know that a total of four half-lives have passed since the original uranium started decaying, and therefore the rock was formed approximately 2.8 billion years ago.
Since rocks are virtually never found in a pure elemental state, but consist of a number of different minerals mixed together when the rock was formed, it is entirely possible (and even likely) that some amount of lead was present along with the original uranium when the rock was formed, and geologists must therefore find a way to calculate how much of the lead in any given rock is "primordial", or present from the beginning, and how much is "radiogenic", or produced by radio-decay after the rock was formed. This is done using the fact that the isotope lead-204 is non-radiogenic, and is not produced by any process of radioactive decay. Any Pb-204 in a given rock, therefore, must be primordial. And since all of the isotopes of any given element are chemically identical, there is no way for any natural process to move Pb-204 into a mass of rock without at the same time moving all of the other isotopes as well. Thus, in a mass of primordial lead, the ratio of the 204 isotope to the others will remain the same, and this ratio will depend on the specific concentration of each of the other isotopes at the time the primordial lead was formed. This varies slightly from place to place, but the average rate is 15 parts lead-204 in every 1,000 parts of primordial lead. Thus, when radio-dating a rock using the uranium-lead method, we can estimate that for every 15 parts of lead-204 we find, 985 parts of lead-207 are primordial and are not the result of uranium decay. Whatever lead-207 is left after we subtract this amount must therefore be radiogenic, and by comparing this amount with the amount of uranium-235 left, we can calculate an age. (In practice, the actual calculation is much more complex since there are other lead isotopes which must be taken into account, but the description here is complete enough to illustrate how the process works.)
One advantage of the uranium dating method is that rocks which contain U-235 also contain the isotope U-238, which decays to form lead-206 with a half-life of 4.47 billion years. This provides a method of cross-checking the dating results by comparing the date calculated from the U-235---Pb-207 series to that calculated from the U-238---Pb-206 series.
However, since in the uranium-lead process there is no way of precisely determining the original amount of primordial lead (the best we can do is use an estimate based on the average concentration of lead-204 found today), some error is introduced in this part of the calculation (most radio-dates using the uranium-lead techniques vary by a few percent plus or minus). Therefore the uranium-lead dating technique tends to give a wider range of dates than other methods, and it is generally considered to be the least precise of the radio-dating methods. As a result, it has largely been abandoned in favor of newer radio-decay methods. However, the oldest rocks so far discovered on earth have been uranium-dated to approximately 3.6 billion years old, plus or minus 0.5 billion years, while rocks from meteors and the surface of the moon, which are believed to have formed at the same time as the earth, have been dated to about 4.5 billion years. (The original surface of the earth has long since been destroyed through erosion.)
A much more precise method of radio-dating depends on the decay of the isotope potassium-40 (chemical abbreviation K-40) to form argon-40 (chemical abbreviation Ar-40) through beta decay, with a half life of 1.2 billion years. The precision of the potassium-argon method comes from the means it presents for determining the original amount of "daughter element" that was present in the primordial rock, thus eliminating the source of the error in uranium-lead dating. Many of the minerals containing potassium form precise crystalline internal structures, with a specific number of potassium atoms locked into a specific position. And since argon is a chemically-inert gas, there is little opportunity for any atoms of argon to become trapped within the crystals (the rocks selected for potassium-argon dating are almost always volcanic rocks which were liquid at the time they were formed, thus allowing any gaseous argon contamination to diffuse out of the liquid). Thus, each argon atom that is found should correspond to exactly one potassium atom which has undergone decay, and the amount of original potassium atoms can be known exactly because the mineral crystals will always contain a set number of potassium atoms per crystal. This makes it possible to determine the amount of radiogenic argon-40 very precisely, and thus greatly reduces the error in measuring the ratio of K-40 to Ar-40. And when the potassium-argon method is used on the oldest terrestrial rocks, we once again obtain the age of 3.8 billion years, plus or minus one or two percent. And meteors and moon rocks also date to about 4.5 billion years.
Another very precise method of radio-dating is called "isochron" dating, and depends on the beta decay of the isotope rubidium-87 (Rb-87) to strontium-87 (Sr-87), with a half-life of 4.8 billion years. The rubidium-strontium method takes advantage of the fact that three other nonradiogenic isotopes of strontium are usually found with strontium-87; these are Sr-84, Sr-86 and Sr-88. As with the isotopes of lead, all of the isotopes of strontium are chemically identical, and no means exists in nature to move one isotope without also moving the others, in the same ratios.
In any given mineral, no matter how much primordial strontium was originally present, the proportion of Sr-87 to Rb-87 will increase over time (as the rubidium decays to strontium), while the ratio of Sr-87 to each of the non-radiogenic isotopes (Sr-84, Sr-86 and Sr-88) will also increase. In other minerals present in the same rock, which may have different initial amounts of primordial strontium, the proportion of strontium to rubidium will differ (since they have different amounts of rubidium), but the ratio of the strontium isotopes to each other will be the same, since they are chemically identical and cannot be separated. Thus, in each mineral, over time, the ratio of Sr-87 to Sr-84 or Sr-88 will change, but this change will itself be proportional to the ratio of rubidium to strontium. When these ratios are plotted against each other, they will form a straight line. And the slope of this line will vary according to the change in the ratio of rubidium to strontium, i.e., according to the age of the rock.
These sloping lines are known as "isochrons", and they present a powerful method of radio-dating. In this method, there is no need to estimate the amount of daughter element that may have contaminated the sample, because if any strontium or rubidium has been removed or added from the original rock, this will produce a point that lies outside the isochron line, thus indicating that the sample has been contaminated. In any sample which produces ratios lying on a straight isochron line, it is a certainty that the sample is uncontaminated, and the calculated half-life age will be correct. Using this method, the oldest terrestrial rocks so far found have been dated to about 3.7 billion years, while moon rocks and meteors have been dated at around 4.2 billion years.
The creationists, of course, cannot accept an age for the earth of approximately 4.5 billion years, and therefore must find some way of demonstrating that the radio-date ages are incorrect. In his book Scientific Creationism, Henry Morris lists several reasons why he believes radio-dating is unreliable and should be discounted. But as we can see, none of these arguments have any validity:
Creationist Problems with Radio-dating:
"Uranium minerals always exist in open systems, not closed. . . Unless the system is known to have been a closed system through all the ages since its formation, its age readings are meaningless." (Morris, Scientific Creationism, 1974, pp 140-141)
As we have seen, determining the amount of "primordial" lead that was present before radio-decay started is indeed a limiting factor in the uranium-lead dating method. However, the use of lead-204 as a measuring stick allows us to make a reasonably precise estimate of the amount of original non-radiogenic lead, since the various isotopes are chemically identical and always are moved at the same proportion. It is indeed possible that enough lead may have entered or left the sample to throw off the accuracy of the date by a few percent, and this is one of the reasons why the uranium-lead method is no longer used. But even such an error would not be enough to allow for the possibility of a 6,000 year old earth. (Keep in mind that Morris is here arguing that enough lead entered the system to change the apparent date from 6,000 years to around 5 billion years, an error factor of almost a million percent.)
In any case, both the potassium-argon and the rubidium- strontium methods present precise methods of determining the amount of radiogenic daughter element, and the isochron method provides a precise method of checking for contamination.
"An even more important phenomenon by which these balances can be upset is that of 'free neutron capture', by which free neutrons in the mineral's environment may be captured by the lead in the system to change the isotopic value of the lead." (Morris, Scientific Creationism, 1974, p. 141)
While the capture of a neutron by an atomic nucleus is not impossible, it is a rare process which usually happens only in the presence of a large number of free neutrons (such as inside a nuclear reactor). There is no evidence that neutron capture can alter the ratios of geologic lead isotopes to any noticeable degree. Thus, capture of free neutrons might cause the observed ratio to vary by a few tenths of a percent, but certainly not near enough to produce such large errors as the creationists are postulating.
An additional problem is that neutron radiation is lethal to life forms, and a neutron flow large enough to produce the kinds of errors that the creationists are postulating would have killed all life and sterilized the planet long ago. The creationists have no explanation for how life managed to survive their postulated neutron capture.
"The uranium decay rates may well be variable". (Morris, Scientific Creationism, 1974, p. 142.
The entire method of radio-dating rests on the fact that the decay rates of radioactive isotopes, their "half-life", is constant. If the decay rate was faster or slower in the past than it is now, the entire method becomes unreliable.
It is somewhat ironic that Morris attempts to use the argument that "the decay rates might have changed over time" as a criticism of evolutionary theory, since, according to an earlier chapter of his book, it is the evolutionists who claim that the basic processes of nature have evolved over time, while the creationists assert that none of the basic laws of matter have ever changed:
"It seems obvious that the evolution model would predict that matter, energy and the laws are still evolving since they must have evolved in the past and there is no external agent to bring such evolution to a halt." (Morris, Scientific Creationism, 1974, p. 18)
"The creation model conversely supposes that the universe was simply called into existence by the omnipotence, in accord with the omniscience, of the Creator . . . The fact is, of course, all observations that have been made to date confirm the straightforward predictions of the creation model; namely, that the basic laws of nature are constant and invariable, and that the basic nature of matter and energy is likewise a constant. There is not as yet the slightest observational intimation that these entities are evolving at all." (Morris, Scientific Creationism, 1974, pp 17- 18)
Leaving aside for now the assertion that evolutionary theory predicts that the laws of nature "are still evolving" (an assertion which has not been made by ANY evolutionary scientist), we can see that Morris is plainly trying to have it both ways. The radio- decay rates of an element are determined by the strong and weak nuclear forces, which are in turn regulated by the laws of quantum mechanics, one of the most-verified of all scientific models. If radio-decay rates were different in the past than they are now, as Morris suggests above, then there must have been fundamental changes over time in quantum physics and in the structure of matter. Yet, according to Morris, such a fundamental evolution of natural laws has not taken place. On page 18 of his book, he asserts that the evolutionists must be wrong because the basic nature of matter (including, one presumes, quantum physics and the structure of matter) hasn't changed over time; yet on page 142 he is arguing that the evolutionists must be wrong because the basic nature of matter (i.e., quantum physics and the structure of matter) HAS changed over time. (Later, we will see the assertion that the speed of light, another basic property of the universe, must also, according to the creationists, have varied over time.) One wishes that Morris would at least be consistent in his balderdash.
In any case, there is no evidence whatsoever to indicate that radio decay rates might have differed in the past from their present values, and there are several reasons from quantum mechanics why one would not expect them to significantly vary at all. The strong and weak nuclear forces which govern radio-decay are very powerful, but operate at only very short distances (less than the diameter of an atomic nucleus). They are not affected by temperature, pressure, magnetism, or any other known physical phenomenon. Even under the most extreme environmental conditions which can be produced in the lab, the decay rates of radioactive elements have not been observed to vary by more than four percent--Morris's hypothesis requires that these rates must have varied by up to one million percent. Obviously, Morris's assertion that radio-decay rates may have varied greatly in the past is completely without foundation.
"The daughter elements were probably present from the beginning". (Morris, Scientific Creationism, 1974, p. 143)
As we have seen, estimating the amount of non-radiogenic or "primordial" daughter element which was present before decay began is a problem (though not an extremely large problem) with the uranium-lead method. In the K-Ar and Rb-Sr methods, however, there are methods of precisely determining the ratios of radio-element and radiogenic daughter element, as well as ways to determine if the sample has been contaminated.
"Modern rocks formed in 1801 near Hualalei, Hawaii, were found to give potassium-argon ages ranging from 160 million years to 3 billion years." (Morris, Scientific Creationism, 1974, p. 147) "It would seem that the only remaining virtue of potassium ages is that they often yeild ages of millions and billions of years, and are therefore generally compatible with the evolutionary model." (Morris, Scientific Creationism, 1974, p. 148)
The implication here is that the potassium-argon method (and by extension all other radio-dating methods) are wildly inaccurate and give widely divergent dates, and that therefore evolutionary scientists simply save the ages they like and toss out those they don't like.
In citing the Hawaii data, however, Morris typically neglects to mention the whole story. Some of these tests were done on "pillow basalts" which form during underwater volcanic eruptions.
It was suspected by geologists that dissolved argon gas from the surrounding sea water might enter the newly emerged lava, and would not be able to escape quickly enough to dissipate from the rock before it cooled, and that therefore argon might become trapped inside the potassium crystals. To test this, geologists selected an area of basalt that was known to have formed during an eruption in 1801, and used the K-Ar method to date the outer surface. The average date obtained was 22 million years, thus demonstrating that such rocks were indeed contaminated and were not suitable for radio-dating. As geologist G. Brent Dalrymple reported, "The purpose of these studies was to determine, in a controlled experiment with samples of known age, the suitability of submarine pillow basalts for dating, because it was suspected that such samples might be unreliable . . . The results clearly indicated that these rocks were unsuitable for dating, and so they are not generally used for this purpose." (cited in Strahler, 1987, p. 206)
The remaining tests were done on each of the islands in the Hawaiian chain. And, since the Hawaiian Islands were formed several hundred million years apart by volcanic eruptions and are not all the same age (the large island of Hawaii is the youngest, and the islands become progressively older as one travels west along the chain), it should not be surprising that the radio-dates given for each island will differ from the others.
There are occasionally anamolous radio-dates which are produced in various laboratories, but these amount to only a few percent of the total. In every case, reasons for the discrepancy have been found--either geological contamination or errors in the testing process. None of the creationists has ever explained why, if all of the radio-dating methods are so unreliable, hundreds of different samples tested by hundreds of different laboratories all over the world, using a variety of different radio-dating methods, have all agreed on the same date for the age of the earth--approximately 4.5 billion years.