Radiometric dating is one method that is favoured by geochronologists to determine the ages of rocks. The method relies on the fact that certain isotopes (or kinds) of certain elements are radioactive. These isotopes, which are called radio-isotopes, are unstable and decay into other elements, which are often called ‘daughter elements’. For example, the isotope of the element uranium that is called uranium-238 decays into an isotope of lead that is called lead-206. Radiocarbon, an isotope of carbon that is called carbon-14, decays into nitrogen-14, while potassium-40 decays either into argon-40 or into calcium-40. Sometimes the decay process is simple and involves only one step, as in the case of radiocarbon; at other times, the process is more complex with several intermediate steps, as in the case of uranium-238, which decays into lead-206 via no fewer than fourteen steps.
The principle of radiometric dating is straightforward. When molten rock, such as a lava flow, cools and solidifies, any radio-isotope that is trapped in the rock will begin to decay into its daughter element(s). By measuring the amount of the parent and daughter element(s) in the rock, and by knowing the rate at which the parent element decays into the daughter element(s), the date at which the rock formed (i.e. when it solidified) can be determined.
There are, however, a number of assumptions that are made by geochronologists, and these have a significant effect on the accuracy of such age determinations. In radiometric dating, it is always assumed that the rate of decay of the parent element into its daughter element(s) is constant. Students studying science in schools, colleges and universities are taught that nothing can change the rate of decay of a radio-isotope. This, however, is not true. The scientific literature contains a number of reports from scientists who have discovered that the rates of decay of radioisotopes are not constant. For example, it has been discovered that the rate of decay of the radio-isotope beryllium-7 is pressure dependant.5 In other words, the rate of decay changes as the pressure changes. Furthermore, it has been shown that the amount of radiation (that is, the measure of the amount of the decay) emitted from molecular monolayers (that is, single layers of atoms) of carbon with radiocarbon added, is not the same as that calculated assuming a constant radiocarbon decay rate.6
There is also evidence that the decay rates of uranium into lead vary with time. The first indication comes from the study of pleochroic haloes. When a rock crystallizes, the crystals of the minerals in the rock often enclose minute grains of other minerals which contain radioactive uranium or thorium. When the uranium or the thorium disintegrates, the alpha particles that are emitted are slowed down by the crystals in which the grains of the uranium-or thorium-bearing minerals are embedded. When these alpha particles finally stop, crystal deformation occurs, and this is shown by a discoloration or a darkening of the crystals. When the crystal is examined under a microscope, these discolorations appear as dark rings called ‘pleochroic haloes’. The radius of a pleochroic halo in a particular mineral depends on the amount of energy of the alpha particle. This amount of energy depends on the probability of emission of the alpha particle which, in turn, depends on the rate of decay responsible for this alpha-particle emission. In other words, the size of the radius of the pleochroic halo in a particular crystal depends on the rate of the decay responsible for the alpha-particle emission. If it can be shown that the radii of pleochroic haloes corresponding to a definite decay in a particular mineral are of constant size, it can then be safely assumed that the rate of decay is constant. If, on the other hand, it can be shown that the radii vary, this is proof that the decay is not constant. Interestingly, it has been found that the radii of pleochroic haloes due to the radioactive decay of uranium and thorium do in fact vary in size in the same minerals.7 This proves that the radioactive decay rates of these two elements are not constant, but vary over time.
More evidence that the radioactive decay rate of uranium-238 has varied in the past has come from the amount of helium found in zircon crystals trapped in granite. These zircon crystals contain radioactive uranium-238 which, over time, decays to lead-206. During the process, eight helium atoms are produced; helium, being such a light gas, migrates quickly out of the zircon crystals and the granite. However, large amounts of helium are found in the zircon crystals, suggesting that in the past, the radioactive decay was much faster than it is today.8
The first assumption—that the rate of decay of a parent radio-isotope into its daughter is constant—is therefore invalid. The second assumption in radiometric dating concerns how much of the parent and daughter elements were present in the rock when the rock formed (or, more correctly, solidified). You may think that it is assumed that there was no daughter present, but this is not often the case. For example, the age of the earth is determined by measuring the amounts of uranium and lead on the earth and then assuming its composition (that is, the amounts of uranium and lead present) when it formed. Some people find this unbelievable, but it is clearly stated in geology textbooks.9 It cannot be overemphasized that no rock on the earth has ever been dated as being 4,600 million years old.
In the potassium–argon dating method, it is often assumed that some of the argon-40 in the rock came from the atmosphere when it formed (solidified). This amount is then estimated. To do this, the amount of argon-36 in the rock sample is measured. It is then assumed that the ratio of argon-36 to argon-40 in the atmosphere today has remained unchanged in the past, even though there is no proof of this. From this ratio, the amount of argon-40 in the rock when it first formed is calculated. But such a calculation is based on two assumptions:
that some of the argon-40 in the rock came from the atmosphere when the rock solidified
that the ratio of argon-36 to argon-40 in the atmosphere has remained constant.
There is no proof whatsoever that these two assumptions are correct; more disconcerting still is the fact that there is no way of knowing whether they are correct or not.
With the rubidium–strontium dating method, it is impossible to distinguish between the strontium-87 in the rock that has been formed by the radioactive decay of rubidium-87, and the strontium-87 that was in the rock when the rock formed. The geochronologist estimates the amount of strontium-87 that was present when the rock formed by measuring the amount of strontium-87 in the calcium plagioclase crystals (certain calcium-containing crystals) in the same rock and then assuming that this was the concentration of strontium-87 in the potassium-bearing crystals in the rock when it formed. In practice, the geochronologist measures the total amount of rubidium-87, strontium-86 and strontium-87 in each mineral in the rock mass and then draws a graph on which the ratio strontium-87:strontium-86 is plotted against the ratio rubidium-87:strontium-86. This graph is called an ‘isochron diagram’; if all goes well (and often it does not), the date on which the rock solidified and crystallized can be calculated from the slope of the isochron that has been plotted. The intersection on the vertical axis is supposed to give the initial ratio of strontium-87:strontium-86. The problem is that there is no way of knowing if this ratio was a constant throughout the whole rock; if it was not, the isochron is meaningless.
The third assumption that is made in radiometric dating is that the system has remained ‘closed’ since the rock formed—that no parent or daughter element(s) was added to or taken out of the rock from the time of its solidification. It is simply not possible to know whether this assumption is justified or not. With the potassium–argon dating method, for example, potassium can easily leach out of the rock by rainwater percolating through the rock, because potassium salts are soluble in water. Furthermore, the argon produced by the decay of the potassium can easily diffuse through the rock, and the rate of diffusion will depend not only upon the type of rock, but also upon its depth in the earth’s crust, because pressure will affect the rate of diffusion. Both factors will cause a false age to be obtained for a rock where leaching or diffusion has occurred.
‘The proof of the pudding is in the eating’, as the saying goes. The proof of radiomet
ric dating methods is therefore in their accuracy. Usually, only one radiometric method of dating can be used to determine the age of a rock, so very often no check can be made on its accuracy—unless, of course, the age of the rock is known from historical sources. Potassium– argon dating has been considered to be one of the most reliable radiometric dating methods. However, doubts have been cast on its reliability by the discovery that old ages have been obtained by this method of dating for young rocks that formed as a result of volcanic eruptions. The fact that the rocks are young is known because they were observed forming. The data in Table 4 illustrates this point adequately10 and raises the following questions: If radiometric dating fails to give an accurate age of a rock whose true age we know from historical observation, how can it be trusted to give us the correct age for a rock whose true age we do not know from historical observation? If the methods do not work for rocks of known age, why should we trust them for rocks of unknown age?
But what about dating rocks when there is more than one method of dating that can be used? Surely it is reasonable to assume that different methods of dating should give the same age. Yes, this is a reasonable expectation, but instead, what are called ‘discordant ages’ are often found. We will look first at an example that was published in 1971. (Not many examples are found in the more recent evolutionary scientific literature. This is because evolutionists have been wary to publish such discordant ages, realizing that creationists will seize upon them and publish them in their literature in order to show the inaccuracies of radiometric dating methods.)
Table 4: Old potassium–argon dates obtained for young rocks
Volcanic eruption When the rock formed Date obtained by
Potassium–argon dating
Mt. Etna basalt, Sicily 122 BC 170,000–330,000 years old
Mt. Etna basalt, Sicily AD 1972 210,000–490,000 years old
Mt. St. Helens, Washington State, USA AD 1986 Up to 2.8 million years old
Hualalai basalt, Hawaii AD 1800–1801 1.32–1.76 million years old
Mt. Ngauruhoe, New Zealand AD 1954 Up to 3.5 million years old
Kilauea Iki basalt, Hawaii AD 195 1.7–15.3 million years old
Although volcanic rocks cannot be dated by their fossils (because they do not contain any), the fossils in the adjacent sedimentary rocks are often used to date them. A report was published of a basalt rock in Nigeria being dated by the fossils in the adjacent sedimentary rocks as being of ‘Upper Tertiary Age’.11 This could be anything from two to twenty-six million years old. The uranium–lead radiometric dating method dated the same rock as 750 million years old. When geologists measured the amount of helium gas in the rocks (that is, a measure of the alpha-particle emission) rather than the lead content, the basalt was dated as fourteen million years old. To confuse the issue further, more differing results were obtained from the potassium–argon dating method (ninety-five million years) and from fission-track measurements (less than thirty million years). The results are summarized in Table 5.
Table 5: Ages obtained for a Nigerian basalt rock
Dating method Age obtained
Conventional Geology (based on fossils in adjacent rocks) 2–26 million years
Uranium–helium 14 million years
Fission tracks less than 30 million years
Potassium–argon 95 million years
Uranium–lead 750 million years
Here there was a complete lack of agreement in the ages obtained for the same rock using different methods of dating. Which age was correct? The straightforward answer is that no one knows, for the simple reason that no one saw the basalt solidifying. Such examples show the complete unreliability of radiometric dating methods.
In 1997 a group of creationist scientists started an eight-year research project to investigate the age of the earth. The group called themselves the ‘Radioisotopes and the Age of the Earth’ (RATE) group. Their objective was to gather data commonly ignored or censored by those working in the field of geochronology. One area of research they undertook was to collect a rock sample and then to determine its age using different radiometric dating methods. If the different radiometric dating methods are accurate, they should give the same age for the rock.
Table 6: Beartooth Mountains sample results
Radiometric dating method Date (millions of years) Type of sample
Potassium–argon 1,520 Quartz–plagioclase mineral
2,011 Whole rock
2,403 Biotite mineral
2,620 Hornblende mineral
Rubidium–strontium 2,515 5 minerals
2,790 Previously published result
based on 30 whole rock samples
Samarium–neodymium 2,886 4 minerals
Lead–lead 2,689 5 minerals
Table 7: Bass Rapids Sill sample results
Radiometric dating method Date
(millions of years) Type of sample
Potassium–argon 841.5 11 whole rock samples
665–1,053 Model ages from single whole rocks
Rubidium–strontium 1,007 Magnetite mineral grains from 7 rock samples
1,055 11 whole rock
1,060 7 minerals
1,070 Previously published age based on
5 whole rock samples
1,075 12 minerals
Lead–lead 1,250 11 whole rock
1,327 6 minerals
Samarium–neodymium 1,330 8 minerals
1,336 Magnetite mineral grains from 7 rock samples
1,379 6 minerals
Two of the sites from which they took rock samples have been dated by evolutionary geologists as being from the so-called pre-Cambrian period—that is, older than 543 million years. Rock samples were taken from the Beartooth Mountains of north-west Wyoming near Yellowstone National Park and from the Bass Rapids Sill in the central part of Arizona’s Grand Canyon, USA. All the samples (whole rock and separate minerals within the rock) were analysed using four different radiometric dating methods—potassium–argon, rubidium–strontium, samarium– neodymium and lead–lead. In order to avoid any criticism of bias, the analyses were contracted out to commercial laboratories located in Colorado and Massachusetts in the USA and in Ontario in Canada.
Tables 6 and 7 show the results for the radiometric age determinations for the rocks from the Beartooth Mountains and the Bass Rapids Sill, Grand Canyon, respectively.12 Evolutionary geologists believe that the rocks in the Beartooth Mountains are some of the oldest in the USA and that they are 2,790 million years old. Yet, as can be seen from Table 6, the radiometric dates obtained by the RATE group using four different radiometric dating methods show a scatter ranging from 1,520 to 2,886 million years, and the difference in the results obtained using potassium– argon dating amount to a staggering 1,100 million years! Evolutionary geologists believe that the age of the rocks in the Bass Rapids Sill in the Grand Canyon is 1,070 million years. Table 7 shows, however, that the radiometric dates for these rocks give ages ranging from 665 to 1,379 million years—a difference of 714 million years! Such data shows the complete inaccuracy and unreliability of radiometric dating.
We have seen that radiometric dating is based on the assumption that the decay rate of a radio-isotope is constant, even though there is experimental evidence that in one or two cases this is not so. It is also based on assumptions regarding the initial conditions which are not always known. Furthermore, it is based on the assumption that the system is closed, even though very often we simply do not know whether this is so or not. Because of the problems with these assumptions, we should expect radiometric dating to be unreliable. And this is what we discover. Potassium–argon dating, for example, is notoriously inaccurate and gives ages of millions of years for lavas that are known from written sources to be less than 200 years old. Different radiometric dating methods usually give different ages for the same rock, although occasionally concordant ages are obtained. These occasions when ages agree should not, however, be used as proof that radiometric dating is accurate a
nd that the earth is therefore 4,600 million years old. Yet, in spite of all this, many people have been led to believe that they can trust radiometric dating, especially radiocarbon dating.
Radiocarbon dating
Radiocarbon dating has obtained widespread use in archaeology and geology. Most people that I meet seem to think that radiocarbon dating can be used to date anything and that it has been used to prove that the earth is millions of years old. This method of dating, however, can only be used to estimate the age of materials of biological origin (e.g. bones, teeth, wood and so on), and even then, if it were accurate, it could only be used to date materials that are less than 50,000 years old.
This method of dating was developed in the mid-1940s at the University of Chicago by Professor Willard F. Libby, who was subsequently awarded the Nobel Prize in Chemistry in 1960 for this work. In this section we will look at the principles upon which radiocarbon dating is based, the assumptions inherent in this method of dating, and the extent to which the dates obtained by this method have been checked against accurate historically dated materials.
The basic theory behind radiocarbon dating is that, in the upper atmosphere of the earth, nitrogen is changed into a rare form of carbon, known as radiocarbon, due to its reaction with atomic particles called neutrons.13 These neutrons are produced as a result of the bombardment of the upper atmosphere by cosmic rays. The nitrogen–neutron reaction produces not only radiocarbon but also protons.
‘Ordinary’ carbon is carbon-12 that has six protons and six neutrons in its atomic nucleus. Radiocarbon is a different isotope (or kind) of carbon and is also known as carbon-14. Radiocarbon has the same number of protons in its atomic nucleus as carbon-12—that is, six—but it has two extra neutrons, making a total of eight. Unlike carbon-12, radiocarbon is radioactive and disintegrates back into nitrogen with the emission of an electron. This disintegration process is relatively slow. Radiocarbon is said to have a half-life of 5,730 years. This means that, starting with one gram of radiocarbon, after 5,730 years one half of it will have disintegrated into nitrogen and half a gram of radiocarbon will be left. After a further 5,730 years, half of this half-gram will have disintegrated and only a quarter of a gram of radiocarbon will be left. After another equally long period of time, only an eighth of a gram of radiocarbon will be left, and so on.
What About Origins? (CreationPoints) Page 10
