Actually, the computation of the age would be affected if some of the daughter element originally present had been lost. For if extra daughter element were added, then we should arrive at too large a figure for the amount of the parent element that has decayed, and thus produce too high a value for the age of the rock. Geologists are not unaware of these assumptions, and they take great pains to construct ways of cross-checking them. Consider first the ways of computing D 0.
Argon is an inert gas, so that it does not occur in chemical compounds in original rocks. In some crystalline structures it can be trapped mechanically, but for other naturally occurring minerals it can be shown that this does not occur.
- Radiometric Dating and Creation Science.
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A second common method of radiometric dating involves the decay of uranium into lead. Here it is possible to use two decay processes, the decay of uranium into lead and the decay of uranium into lead Furthermore, the amount of lead originally present can be computed by considering another isotope of lead.
Radiometric dating
Hence, by measuring the amount of lead in a rock, geologists can estimate the amount of lead originally present. Given this value of D 0 it is then possible to use either decay process to calculate the age of the rock. If the results agree, they are said to be concordant, and geologists are usually confident that concordant ages are the true ages of the rocks under consideration. The second worry is that extra amounts of the daughter element may enter the system after the original formation of the rock, thus giving the impression that more of the parent element has undergone radioactive decay than has actually been the case.
In both the examples I have described, there are ways of checking that such intrusions have not occurred. Minerals can be tested for their capacity to absorb extra argon under experimental conditions designed to resemble their natural environment, and geologists can screen out, in this way, minerals that are liable to give erroneous results. In the second case, the existence of two separate decay processes provides a check on the assumption that the system has not been contaminated.
Creationists Blind Dates
If extra lead were to have been absorbed in the rock after the original formation, the new lead would have caused the calculated ages of the rock to diverge unless it contained the right proportion of lead to lead If the ratio of lead to lead in the newly introduced rock were greater than the ratio of lead to lead found in an uncontaminated system, the method of dating based on the decay of uranium to lead would give a relatively higher value than the method of dating based on the decay of uranium to lead Obviously just the opposite holds when the ratio of lead to lead is too small.
Hence someone who supposes that concordant ages are inflated must believe that the contaminating lead contained just the right proportion of the two isotopes. I want to emphasize that I have only dealt with two of the commonly used radiometric methods, and I have only outlined the most elementary of the checks that geologists use in applying them.
More details can be found in Eicher , chapter 6; and Faul From what I have said it might seem that the assignment of ages to rocks is still a bit uncertain. However, I hope that it will help to quell anxieties when I point out that a large number of independent methods have been applied to a vast array of different rocks. The result of this enormous array of tests is a consensus.
The ages assigned to various rock strata bearing distinctive types of fossils show extraordinary agreement. The many independent computations of the age of the earth during the last three decades almost invariably yield a figure between 4. Of course, there are occasional puzzling discrepancies. But geologists take these as signs that unanticipated factors have affected the system from which the result was obtained. They know that geological clocks, like other clocks, can go wrong.
Frequently, further investigation dissolves the anomaly by showing what the interfering factor has been. Let us now take up some of the Creationists' attempts to criticize radiometric dating. The main lines of attack are laid down by Morris.
He begins by identifying three assumptions of the use of radiometric techniques: The system must have been a closed system. The process rate must always have been the same " Morris a, We have already discussed statements akin to Morris's first and second assumptions. As will become clear shortly, the status of the third is a little different. Unsurprisingly, Morris believes that he can provide good reasons for doubting each of these assumptions in the case of every application of every method.
He claims that none of the assumptions is "provable, testable, or even reasonable" Morris a, Here are the reasons: There is no such thing in nature as a closed system. The concept of a closed system is an ideal concept, convenient for analysis but non-existent in the real world. The idea of a system remaining closed for millions of years becomes an absurdity. It i s impossible to ever know the initial components of a system formed in prehistoric times.
Obviously no one was present when such a system was first formed. Since creation is at least a viable possibility, it is clearly possible that some of the "daughter" component may have been initially created along with the "parent" component. Even apart from this possibility, there are numerous other ways by which daughter products could be incorporated into the systems when first formed. No process rate is unchangeable. Every process in nature operates at a rate which is influenced by a number of different factors.
If any of these factors change, the process rate changes. Rates are at best only statistical averages, not deterministic constants. These rejoinders make it apparent that Morris's formulations of the assumptions underlying radiometric dating are only akin to the assumptions examined above. When geologists calculate the ages of rocks, they do assume that the system under consideration has remained closed in one particular respect.
They suppose that none of the daughter element has been added or subtracted. However, this does not commit them to the idea that the system was completely closed, that it engaged in no exchange of matter or energy with the environment. Like his memorable argument about the evolving junkyard, Morris's first reply only demonstrates his lack of understanding of basic concepts of physics. The crucial question is whether we can ever be justified in believing that the system was never contaminated by extra amounts of the daughter element.
I have tried to explain how geologists can sometimes obtain good evidence for this conclusion. Similarly, the second point is misguided. Geologists do not have to suppose that the system originally contained none of the daughter element. What is important is that they be able to compute the amount of the daughter element originally present.
Clearly, it is required only that D 0 be known, not that it be zero.
Professor Timothy H. Heaton
It is perfectly possible to have excellent evidence for statements about events and situations that no human has observed. Geologists draw conclusions about the composition of original rocks by applying claims about the possibilities of incorporating elements into minerals, claims that can be tested in the laboratory. So, for example, the thesis that certain minerals would have contained no original argon rests on a perfectly testable and well-confirmed claim. While those minerals were in the molten state, prior to the solidification of the rock, argon would have diffused from them.
It is only after the molten rock has solidified that the argon formed through radioactive decay becomes trapped within it. Obviously, what is being applied in this case is our knowledge of the physical and chemical interactions of minerals and elements. Morris's third assumption, and his attempt to undermine it, raises a new issue. In deriving equation 4 , from which rock ages can be computed, I employed equation 1 , the equation of radioactive decay. I asserted that l , which measures the rate of decay, is a constant. Morris suggests that the assertion is unwarranted.
However, the claim that l is a constant does not descend out of thin air. It is the result of our knowledge of nuclear physics. Although the sciences sometimes teach us that the rate at which a process occurs can be affected by a number of factors, as when we learn that the rate at which water boils is affected by the pressure or that the rate at which mutations occur varies with X-ray irradiation, what we sometimes discover is that a process is impervious to outside influence.
Precious little affects the time of passage for a light ray between two points. Similarly, nuclear physics tells us that radioactive decay is well insulated against external interference. The reason is that the emission of particles from an atomic nucleus is under the control of forces that are enormously more effective at short distances than the forces at work in most physicochemical reactions.
Extensive attempts to modify these rates under a variety of physicochemical conditions have produced no effects. For example, his chief weapon in arguing for the possibility of variable decay rates is a vague proposal that the capture of free neutrons or the impact of neutrinos could affect decay constants Morris a, The latter idea is linked to a paragraph quoted from a "Scientific Speculation" column.
A third, very rare type of radioactive decay is called electron absorption. In electron absorption, a proton absorbs an electron to become a neutron.
In other words, electron absorption is the exact reverse of beta decay. So an atom of potassium K40 , atomic number 19 can absorb an electron to become an atom of argon Ar40 , atomic number The half-life of a radioactive nuclide is defined as the time it takes half of a sample of the element to decay. A mathematical formula can be used to calculate the half-life from the number of breakdowns per second in a sample of the nuclide. Some nuclides have very long half-lives, measured in billions or even trillions of years.
Others have extremely short half-lives, measured in tenths or hundredths of a second. The decay rate and therefore the half-life are fixed characteristics of a nuclide. Different nuclides of the same element can have substantially different half-lives. The half-life is a purely statistical measurement. A sample of U ten thousand years old will have precisely the same half-life as one ten billion years old.