Geologic Age Dating Explained - Kids Discover
Subject: Science. Subtopic: Geology. Topic: Relative, Absolute Dating and Geologic Time. Grade: 6. Instructions: Answer all questions to get your test result. Radiometric dating methods estimate the age of rocks using .. In a few cases, argon ages older than that of the Earth which violate local relative age .. Just to make the test unbiased, we will assign altitude limits to each. Subject: Science. Subtopic: Geology. Topic: Relative, Absolute Dating and Geologic Time. Grade: 8. Instructions: Answer all questions to get your test result.
This does not include dates from minerals that are thought to yield bad dates, or from igneous bodies with wide biostrategraphic ranges, where many dates are acceptable. He states that the number of dates within range are less than the number of anomalies, except for the Cenozoic and Cretaceous.
When one adds in the fact that many anomalies are unreported, which he gives evidence for, the true distribution is anyone's guess. There have been criticisms of John Woodmorappe's study, but no one has given any figures from the literature for the true percentage of anomalies, with a definition of an anomaly, or the degree of correlation between methods.
Steven Schimmrich's review of this study often concerns itself with John W's presentation of geologists explanation for anomalies, and not with the percentage of anomalies; the later is my main concern.
Here are a couple of more quotes about anomalies: The carbon age of the buried trees is only years, but some of the overlying volcanic material has a ,year potassium-argon age. Still another evidence for problems with radiometric dating was given in a recent talk I attended by a man who had been an evolutionist and taken a course in radiometric dating. The teacher gave 14 assumptions of radiometric dating and said something like "If creationists got a hold of these, they could cut radiometric dating to pieces.
Many sedimentary uranium ores are not. On another point, if we can detect minerals that were not molten with the lava, as has been claimed, then this is one more reason why there should be no anomalies, and radiometric dating should be a completely solved problem. But that does not appear to be the case, at least especially on the geologic column.
I'm not claiming that anomalous results are being hidden, just that the agreement of a mass of results, none of which has much claim to reliability, does not necessarily mean much. Picking out a few cases where radiometric dates appear to be well-behaved reminds me of evolutionary biologists focusing on a few cases where there may be transitional sequences. It does not answer the overall question.
And as I said above, I'm also interested to know how much of the fossil-bearing geologic column can be dated by isochrons, and how the dates so obtained compare to others. Concerning K-Ar anomalies, here is a quote from Woodmorappe's paper cited above, p. Gerling et al called attention to some chlorites yielding K-Ar dates of 7 to 15 b. It had been noted that some minerals which yield such dates as beryl, cordierite, etc.
They also pointed out that for the anomalies to be accounted for by excess argon, unreasonably high partial pressures of Ar during crystallization would have to be required. They concluded by suggesting some unknown nuclear process which no longer operates to have generated the Ar. Here is another quote from Woodmorappe about isochrons, since some people think that mixing scenarios or other age-altering scenarios are unlikely: If this condition does not hold, invalid ages and intercepts are obtained.
Models yield isochron ages that are too high, too low, or in the future, sometimes by orders of magnitude. The fact that the only "valid" K-Ar isochrons are those for which the concentration of non-radiogenic argon Ar36 is constant, seems very unusual. This suggests that what is occuring is some kind of a mixing phenomenon, and not an isochron reflecting a true age. The following quote is from http: We have analyzed several devitrified glasses of known age, and all have yielded ages that are too young.
Some gave virtually zero ages, although the geologic evidence suggested that devitrification took place shortly after the formation of a deposit. Back to top One of the main arguments in favor of radiometric dating is that so many dates agree with each other, that is, with the date expected for their geologic period.
But it's not evident how much support this gives to radiometric dating. If a rock dates too old, one can say that the clock did not get reset.
If it dates too young, one can invoke a later heating event. Neither date would necessarily be seen as anomalous. If lava intrudes upon geologic period X, then any date for the lava of X or later will not be seen as anomalous. And even if the date is one or two geologic periods earlier, it may well be close enough to be accepted as non-spurious.
If one does not know the geologic period of a rock by other means, then of course one is likely to date it to find out, and then of course the date agrees with the geologic period and this will not be seen as anomalous. So it is difficult to know what would be a reasonable test for whether radiometric dating is reliable or not. The percentage of published dates that are considered as anomalous has little bearing on the question.
Back to top The issue about igneous bodies may need additional clarification. If a lava flow lies above geologic period A and below B, then allowable ages are anything at least as large as A and no larger than B. This is called the biostratigraphic limit of the flow. Now, according to Woodmorappe's citations, many lava flows have no such limits at all, and most of them have large limits.
For example, a flow lying on precambrian rock with nothing on top would have no limits on its dates. And such flows often have a large internal scatter of dates, but these dates are not considered as anomalies because of the unrestricted biostratigraphic limit. Other flows with wide biostratigraphic limits have weak restrictions on allowable dates. This is one reason why just reporting the percentage of anomalies has little meaning.
Thus these ages, though they generally have a considerable scatter, are not considered as anomalies. He cites another reference that most igneous bodies have wide biostrategraphic limits. Thus just by chance, many dates will be considered within the acceptable ranges. Again, the percentage of anomalies means nothing for the reliability of radiometric dating. Now, igneous bodies can be of two types, extrusive and intrusive. Extrusive bodies are lava that is deposited on the surface.
These cool quickly and have small crystals and form basalt. Intrusive bodies are deposited in the spaces between other rocks. These cool more slowly and have larger crystals, often forming granite. Both of these tend on the average to have wide biostrategraphic limits, meaning that a large spread of ages will be regarded as non-anomalous.
And if we recall that most radiometric dating is done of igneous bodies, one sees that the percentage of anomalies is meaningless. Thus we really need some evidence that the different methods agree with each other. To make the case even stronger, "Many discrepant results from intrusives are rationalized away immediately by accepting the dates but reinterpreting the biostrategraphic bracket," according to John Woodmorappe. This of course means that the result is no longer anomalous, because the geologic period has been modified to fit the date.
Finally, the fact that the great majority of dates are from one method means that the general but not universal agreement of K-Ar dating with itself is sufficient to explain the small percentange of anomalies if it is small. Back to top Now, the point about agreement is that whatever figure is given about how often ages agree with the expected age, is consistent with the fact that there is no agreement at all between K-Ar and other methods, since so many measurements are done using K-Ar dating.
And one of the strongest arguments for the validity of radiometric dating is that the methods agree. So when one combines all of the above figures, the statement that there are only 10 percent anomalies or 5 percent or whatever, does not have any meaning any more. This statement is made so often as evidence for the reliability of radiometric dating, that the simple evidence that it has no meaning, is astounding to me. I don't object to having some hard evidence that there are real agreements between different methods on the geologic column, if someone can provide it.
The precambrian rock is less interesting because it could have a radiometric age older than life, but this is less likely for the rest of the geologic column. It's not surprising that K-Ar dates often agree with the assumed dates of their geological periods, since the dates of the geological periods were largely inferred from K-Ar dating.
By the way, Ar-Ar dating and K-Ar dating are essentially the same method, so between the two of them we obtain a large fraction of the dates being used. Some information from an article by Robert H.
History of the Radioisotope based Geologic Time Scale Before the discovery of radioactivity in the late nineteenth century, a geological time scale had been developed on the basis of estimates for the rates of geological processes such as erosion and sedimentation, with the assumption that these rates had always been essentially uniform. On the basis of being unacceptably old, many geologists of the time rejected these early twentieth century determinations of rock age from the ratio of daughter to radioactive parent large.
Byincreased confidence in radioisotope dating techniques and the demands of evolution theory for vast amounts of time led to the establishment of an expanded geological time scale. The construction of this time scale was based on about radioisotope ages that were selected because of their agreement with the presumed fossil and geological sequences found in the rocks. Igneous rocks are particularly suited to K-Ar dating. The crucial determiners are therefore volcanic extrusive igneous rocks that are interbedded with sediments, and intrusive igneous rocks that penetrate sediments.
This verifies what I said about almost all of the dates used to define correct ages for geologic periods being K-Ar dates. Also, the uncertainty in the branching ratio of potassium decay might mean that there is a fudge factor in K-Ar ages of up to a third, and that the occasional agreements between K-Ar ages and other ages are open to question.
So the point is that there is now no reason to believe that radiometric dating is valid on the geologic column. Back to top Another issue is that sometimes the geologic periods of rocks are revised to agree with the ages computed. This also makes data about percentages of anomalies less meaningful. It sometimes seems that reasons can always be found for bad dates, especially on the geologic column.
If a rock gives a too old date, one says there is excess argon. If it gives a too young date, one says that it was heated recently, or cannot hold its argon. How do we know that maybe all the rocks have excess argon? It looks like geologists are taking the "majority view" of K-Ar dating, but there is no necessary reason why the majority of rocks should give the right date. The following quote is from the article by Robert H.
What is a Radioisotope Age? The relationship of a radioisotope age with real-time must be based on an interpretation. A discussion of rubidium-strontium ages in the Isotope Geoscience Section of the journal, Chemical Geology, specifically states that a radioisotope age determination "does not certainly define a valid age information for a geological system.
Any interpretation will reflect the interpreters presuppositions bias. Back to top Concerning the need for a double blind test, it would seem that there are many places where human judgment could influence the distribution of measured radiometric dates. It could increase the percentage of anomalies, if they were regarded as more interesting. It could decrease them, if they were regarded as flukes.
Human judgment could determine whether points were collinear enough to form an isochron. It could determine whether a point can justifiably be tossed out and the remaining points used as an isochron. It could determine whether one should accept simple parent-to-daughter K-Ar ratios or whether some treatment needs to be applied first to get better ages. It could influence whether a spectrum is considered as flat, whether a rock is considered to have undergone leaching or heating, whether a rock is porous or not, or whether a sample has been disturbed in some way.
Since one of the main reasons for accepting radiometric dates at least I keep hearing it is that they agree with each other, I think that geologists have an obligation to show that they do agree, specifically on the geologic column.
Geologic Age Dating Explained
Since we do not know whether or how much human judgment is influencing radiometric dating, a double blind study is most reasonable. And it should not be restricted to just one or two well-behaved places, but should be as comprehensive as possible. Back to top The following information was sent to me by e-mail: Radiometric dating is predicated on the assumption that throughout the earth's history radioactive decay rates of the various elements have remained constant.
Is this a warranted assumption? Has every radioactive nuclide proceeded on a rigid course of decay at a constant rate? This has been challenged by studies involving Carbon C At the temperature or pressure, collisions with stray cosmic rays or the emanations of other atoms may cause changes other than those of normal disintegration. It seems very possible that spontaneous disintegration of radioactive elements are related to the action of cosmic rays and the rate of disintegration varying from century to century according to the intensity of the rays.
The evidence for a strongly increasing change in the cosmic ray influx is most favorable especially in light of the decay of the earth's magnetic field. Most geochronologists maintain that pleochroic haloes give evidence that decay constants have not changed. Crystals of biotite, for example, and other minerals in igneous or metamorphic rocks commonly enclose minute specks of minerals containing uranium or thorium.
The a- alpha particles emitted at high velocity by the disintegrating nuclides interact, because of their charge, with electrons of surrounding atoms which slow them down until they finally come to rest in the host material at a distance from their source that depends on their initial kinetic energy and the density and composition of the host. Where they finally stop to produce lattice distortions and defects there generally occurs discoloring or darkening.
Each of the 8 a-particles emitted during the disintegration of U to Pb produces a dark ring in biotite. Each ring has its own characteristic radius in a given mineral in this case biotite. This radius measures the kinetic energy, hence the probability of emission of the corresponding a-particle and also the half-life of the parent nuclide according to the Geiger-Nuttall law.
The Geiger-Nuttall law is an empirical relation between half-life of the a-emitter and the range in air of the emitted a-particles. If the radii of these haloes from the same nuclide vary, this would imply that the decay rates have varied and would invalidate these series as being actual clocks. Are the radii in the rocks constant in size or are there variable sizes? Most of the early studies of pleochroic haloes were made by Joly and Henderson.
Joly concluded that the decay rates have varied on the basis of his finding a variation of the radii for rocks of alleged geological ages. This rather damaging result was explained away saying that enough evidence of correct radii for defferent geologic periods and sufficient variation in the same period have been obtained that one is forced to look for a different explanation of such variations as were observed by Joly.
Measurements were later made in an excellent collection of samples with haloes. It was found that the extent of the haloes around the inclusions varies over a wide range, even with the same nuclear material in the same matrix, but all sizes fall into definite groups. The measurements are, in microns, 5,7,10,17,20,23,27, and More recent studies have been made by Robert V.
Gentry also finds a variation in the haloes leading him to conclude that the decay constants have not been constant in time. Gentry points out an argument for an instantaneous creation of the earth.
He noted form his studies of haloes: For the Po half-life of 3 minutes only a matter of minutes could elapse between the formation of the Po and subsequent crystallization of the mica; otherwise the Po would have decayed, and no ring would be visible. The occurrence of these halo types is quite widespread, one or more types having been observed in the micas from Canada Pre-CambrianSweden, and Japan.
The Radiometric Dating Game
So, then, careful scientists have measured variations in halo radii and their measurements indicate a variation in decay rates. The radioactive series then would have no value as time clocks. The following quotation also suggests a cause for a change in the decay rate: Jueneman Industrial Research, Sept.
The remnant of that local big bang is a pulsar called Vela-X PSRwhich recent observations have positioned in the southern sky some 1, light years away, and which is considered to have given rise to the huge Gum Nebula Being so close, the anisotropic neutrino flux of the super-explosion must have had the peculiar characteristic of resetting all our atomic clocks.
This is significant because it is known that neutrinos do interact with the nucleii of atoms, and it is also believed that much of the energy of supernovae is carried away by neutrinos. Back to top Isochrons are an attempt to avoid the need for an absence of daughter element initially in computing radiometric ages. The idea is that one has a parent element, X, a daughter element, Y, and another isotope, Z, of the daughter that is not generated by decay.
One would assume that initially, the concentration of Z and Y are proportional, since their chemical properties are very similar. Radioactive decay would generate a concentration of Y proportional to X.
A good general introduction to isochrons from an evolutionary perspective can be found at http: If the concentration of K varies in a rock, that it is unlikely for the concentration of added argon 40 to vary in a way that will yield an isochron. But if the concentration of K does not vary, then one can still get an isochron if the concentration of the non-radiogenic isotope Ar36 of the daughter product varies.
So let's call an isochron a "super-isochron" if the concentration of the parent element varies from one sample to another. Let's call it a "wimpy isochron" otherwise. The question is, what percentage of isochrons are super-isochrons, and how do their dates agree with the conventional dates for their geologic period?
I would think that it may be rare to have a super-isochron. If one is dealing with minerals that exclude parent or daughter, then one cannot get an isochron at all. If one is dealing with minerals that do not exclude parent and daughter elements, then most likely the parent element will be evenly distributed everywhere, and one will have a wimpy isochron that cannot detect added daughter product, and thus may give unreliable ages.
Whole rock isochrons may also tend to be wimpy, for the same reason. Even super isochrons can yield ages that are too old, due to mixings, however. False K-Ar isochrons can be produced if a lava flow starts out with a lot of excess Ar40 which becomes well mixed, along with potassium. Then while cooling or afterwards, a mixture of Ar36 and Ar40 can enter the rock, more in some places than others. Other isotopes of argon would work as well.
I believe that this will produce a good K-Ar isochron, but the age calculated will be meaningless. There is another way that false isochrons can be produced. For a wimpy isochron, say a K-Ar isochron, we can assume that initially there is a uniform concentration of K everywhere, and concentrations of Ar40 and Ar36 that form an isochron. Then a lot of Ar40 enters, uniformly, through cracks in the rock or heating.
This will retain the isochron property, but will make the isochron look too old. My reasoning was that if the lava is thoroughly mixed, then the concentration of parent material should be fairly constant.
If the concentration of parent substance is not constant, it could indicate that the lava is not thoroughly mixed. Or it could have other explanations. If the lava is not thoroughly mixed, it is possible to obtain an isochron from the mixing of two different sources, in which case the radiometric age is inherited from the sources, and does not necessarily yield the age of the flow.
Someone pointed out to me that many Rb-Sr isochrons are super isochrons. I find this information very interesting, and thank him for it. I'd be curious to know which strata they occur in, as my main interest is the geologic column of Cambrian and above.
My impression is that these are not on this part of the geologic column.
And how well do the dates correlate with others for the same formation? There are also mixing scenarios that can produce even super isochrons having invalid ages. And geologists admit in any event that isochrons can sometimes give false ages. Here is a mixing scenario for false isochrons. There are two sources of lava, A and B.
Suppose these mix together so that at point 0 we have only A, at point 1 we have only B, and in between we have varying concentrations.
Half way between there is a mixture of half A and half B, for example. Suppose X is a parent substance, Y is its daughter, and Z is a non-radiogenic isotope of the daughter. Suppose A has a little X and lots of Y and not much Z, all uniformly distributed, and B has some mixture of Y and Z, all uniformly distributed.
In a way this field, called geochronology, is some of the purest detective work earth scientists do. There are two basic approaches: Here is an easy-to understand analogy for your students: Absolute age dating is like saying you are 15 years old and your grandfather is 77 years old. To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones.
This is called the Rule of Superposition. This rule is common sense, but it serves as a powerful reference point. Geologists draw on it and other basic principles http: Relative age dating also means paying attention to crosscutting relationships.
Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type. Pretty obvious that the dike came after the rocks it cuts through, right? With absolute age dating, you get a real age in actual years. Based on the Rule of Superposition, certain organisms clearly lived before others, during certain geologic times. The narrower a range of time that an animal lived, the better it is as an index of a specific time. No bones about it, fossils are important age markers.
But the most accurate forms of absolute age dating are radiometric methods. This method works because some unstable radioactive isotopes of some elements decay at a known rate into daughter products. This rate of decay is called a half-life. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. Good discussion from the US Geological Survey: There are a couple catches, of course. Not all rocks have radioactive elements.
Sedimentary rocks in particular are notoriously radioactive-free zones.