Radiometric Dating and the Geological Time Scale
The fatal flaw with radioactive dating methods by Tas Walker This is the since all of us regularly calculate how much time has passed: for example, Keep that in mind when you think about working out the age of something. This illustrates the problem with the radioactive dating of geological events. Radiometric dating involves dating rocks or other objects by measuring the Example Problem gives us the estimated age of the sample. This is best illustrated by the radioactive age of a sample of diamonds from Zaire. Their age was measured to be +/- billion years old.
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- Radiometric dating
Usually determinations of age are repeated to avoid laboratory errors, are obtained on more than one rock unit or more than one mineral from a rock unit in order to provide a cross-check, or are evaluated using other geologic information that can be used to test and corroborate the radiometric ages. Scientists who use radiometric dating typically use every means at their disposal to check, recheck, and verify their results, and the more important the results the more they are apt to be checked and rechecked by others.
As a result, it is nearly impossible to be completely fooled by a good set of radiometric age data collected as part of a well-designed experiment. The purpose of this paper is to describe briefly a few typical radiometric dating studies, out of hundreds of possible examples documented in the scientific literature, in which the ages are validated by other available information.
I have selected four examples from recent literature, mostly studies involving my work and that of a few close colleagues because it was easy to do so.
Radiometric Dating and the Geological Time Scale
I could have selected many more examples but then this would have turned into a book rather than the intended short paper. The heat of the impact melted some of the feldspar crystals in the granitic rocks of the impact zone, thereby resetting their internal radiometric clocks.
The impact also created shocked quartz crystals that were blasted into the air and subsequently fell to the west into the inland sea that occupied much of central North America at that time. Today this shocked quartz is found in South Dakota, Colorado, and Nebraska in a thin layer the Crow Creek Member within a thick rock formation known as the Pierre Shale.
The Pierre Shale, which is divided into identifiable sedimentary beds called members, also contains abundant fossils of numerous species of ammonites, ancestors of the chambered nautilus. The fossils, when combined with geologic mapping, allow the various exposed sections of the Pierre Shale to be pieced together in their proper relative positions to form a complete composite section Figure 1.
The Pierre Shale also contains volcanic ash that was erupted from volcanoes and then fell into the sea, where it was preserved as thin beds. There are three important things to note about these results. First, each age is based on numerous measurements; laboratory errors, had there been any, would be readily apparent. Second, ages were measured on two very different minerals, sanidine and biotite, from several of the ash beds. Third, the radiometric ages agree, within analytical error, with the relative positions of the dated ash beds as determined by the geologic mapping and the fossil assemblages; that is, the ages get older from top to bottom as they should.
Finally, the inferred age of the shocked quartz, as determined from the age of the melted feldspar in the Manson impact structure The Ages of Meteorites Meteorites, most of which are fragments of asteroids, are very interesting objects to study because they provide important evidence about the age, composition, and history of the early solar system. There are many types of meteorites. Some are from primitive asteroids whose material is little modified since they formed from the early solar nebula.
Others are from larger asteroids that got hot enough to melt and send lava flows to the surface. A few are even from the Moon and Mars. The most primitive type of meteorites are called chondrites, because they contain little spheres of olivine crystals known as chondrules.
Because of their importance, meteorites have been extensively dated radiometrically; the vast majority appear to be 4. Some meteorites, because of their mineralogy, can be dated by more than one radiometric dating technique, which provides scientists with a powerful check of the validity of the results.
The results from three meteorites are shown in Table 1. Many more, plus a discussion of the different types of meteorites and their origins, can be found in Dalrymple There are 3 important things to know about the ages in Table 1. The first is that each meteorite was dated by more than one laboratory — Allende by 2 laboratories, Guarena by 2 laboratories, and St Severin by four laboratories.
This pretty much eliminates any significant laboratory biases or any major analytical mistakes.
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The second thing is that some of the results have been repeated using the same technique, which is another check against analytical errors. The third is that all three meteorites were dated by more than one method — two methods each for Allende and Guarena, and four methods for St Severin.
This is extremely powerful verification of the validity of both the theory and practice of radiometric dating. In the case of St Severin, for example, we have 4 different natural clocks actually 5, for the Pb-Pb method involves 2 different radioactive uranium isotopeseach running at a different rate and each using elements that respond to chemical and physical conditions in much different ways.
And yet, they all give the same result to within a few percent. Is this a remarkable coincidence? Scientists have concluded that it is not; it is instead a consequence of the fact that radiometric dating actually works and works quite well.
Creationists who wants to dispute the conclusion that primitive meteorites, and therefore the solar system, are about 4. The K-T Tektites One of the most exciting and important scientific findings in decades was the discovery that a large asteroid, about 10 kilometers diameter, struck the earth at the end of the Cretaceous Period.
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The collision threw many tons of debris into the atmosphere and possibly led to the extinction of the dinosaurs and many other life forms. The fallout from this enormous impact, including shocked quartz and high concentrations of the element iridium, has been found in sedimentary rocks at more than locations worldwide at the precise stratigraphic location of the Cretaceous-Tertiary K-T boundary Alvarez and Asaro ; Alvarez We now know that the impact site is located on the Yucatan Peninsula.
Measuring the age of this impact event independently of the stratigraphic evidence is an obvious test for radiometric methods, and a number of scientists in laboratories around the world set to work. In addition to shocked quartz grains and high concentrations of iridium, the K-T impact produced tektites, which are small glass spherules that form from rock that is instantaneously melted by a large impact.
The K-T tektites were ejected into the atmosphere and deposited some distance away. Tektites are easily recognizable and form in no other way, so the discovery of a sedimentary bed the Beloc Formation in Haiti that contained tektites and that, from fossil evidence, coincided with the K-T boundary provided an obvious candidate for dating. Scientists from the US Geological Survey were the first to obtain radiometric ages for the tektites and laboratories in Berkeley, Stanford, Canada, and France soon followed suit.
The results from all of the laboratories were remarkably consistent with the measured ages ranging only from Similar tektites were also found in Mexico, and the Berkeley lab found that they were the same age as the Haiti tektites. The K-T boundary is recorded in numerous sedimentary beds around the world. Numerous thin beds of volcanic ash occur within these coals just centimeters above the K-T boundary, and some of these ash beds contain minerals that can be dated radiometrically.
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Since both the ash beds and the tektites occur either at or very near the K-T boundary, as determined by diagnostic fossils, the tektites and the ash beds should be very nearly the same age, and they are Table 2. There are several important things to note about these results. First, the Cretaceous and Tertiary periods were defined by geologists in the early s. The boundary between these periods the K-T boundary is marked by an abrupt change in fossils found in sedimentary rocks worldwide.
Its exact location in the stratigraphic column at any locality has nothing to do with radiometric dating — it is located by careful study of the fossils and the rocks that contain them, and nothing more. Furthermore, the dating was done in 6 different laboratories and the materials were collected from 5 different locations in the Western Hemisphere.
And yet the results are the same within analytical error. These flows buried and destroyed Pompeii and other nearby Roman cities. We know the exact day of this eruption because Pliny the Younger carefully recorded the event.
They separated sanidine crystals from a sample of one of the ash flows. The elements rubidium and strontium are found in many rocks. One form of rubidium Rb is radioactive. As illustrated above, a neutron in a Rb atom can eject an electron often called a beta particlewhich has a negative charge. Since a neutron has no charge, it must become positively charged after emitting an electron. In fact, it becomes a proton. This changes the chemical identity of the atom. It is no longer Rb; it is strontium Sr Sr is not radioactive, so the change is permanent.
We know how long it takes Rb to turn into Sr, so in principle, if we analyze the amount of Rb and Sr in a rock, we should be able to tell how long the decay has been occurring. Of course, there are all sorts of uncertainties involved. How much Sr was in the rock when it first formed? Was Rb or Sr added to the rock by some unknown process? Was one of them removed from the rock by some unknown process?
The isochron is supposed to take care of such issues. Essentially, rather than looking at the amounts of Rb and Sr, we look at their ratios compared to Sr The ratio of Sr to Sr is graphed versus the ratio of Rb to Sr for several different parts of the rock.
How does that help? Thus, it provides an independent analysis of the rock that does not depend on the radioactive decay that is being studied. The amount of Sr that was already in the rock when it formed, for example, should be proportional to the amount of Sr that is currently there.
Since the data are divided by the amount of Sr, the initial amount of Sr is cancelled out in the analysis. He says that there is one process that has been overlooked in all these isochron analyses: Atoms and molecules naturally move around, and they do so in such as way as to even out their concentrations.
A helium balloon, for example, will deflate over time, because the helium atoms diffuse through the balloon and into the surrounding air. Well, diffusion depends on the mass of the thing that is diffusing. Sr diffuses more quickly than Sr, and that has never been taken into account when isochrons are analyzed. Hayes has brought it up, we can take it into account, right? If the effects of diffusion can be taken into account, it will require an elaborate model that will most certainly require elaborate assumptions.