Radiometric age dating methods in geology

How reliable is geologic dating?

radiometric age dating methods in geology

Radiometric dating is used to estimate the age of rocks and other objects Using Geological Layers & Radioactive Dating to Determine the. Radiometric dating calculates an age in years for geologic materials by method to determine the absolute age of geological processes. Radiometric dating or radioactive dating is a technique used to date materials such as rocks or By allowing the establishment of geological timescales, it provides a significant source of . The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature. Dating.

All of the dating schemes work from knowing the present abundances of the parent and daughter isotopes. There is little or no way to tell how much of the decay product, that is, the daughter isotope, was originally in the rock, leading to anomalously old ages. A good part of [Wiens' article] is devoted to explaining how one can tell how much of a given element or isotope was originally present. Usually it involves using more than one sample from a given rock. It is done by comparing the ratios of parent and daughter isotopes relative to a stable isotope for samples with different relative amounts of the parent isotope.

From this one can determine how much of the daughter isotope would be present if there had been no parent isotope. This is the same as the initial amount it would not change if there were no parent isotope to decay. Figures 4 and 5 [in Wiens' article], and the accompanying explanation, tell how this is done most of the time. There are only a few different dating methods. There are actually many more methods out there.

Well over forty different radiometric dating methods are in use, and a number of non-radiogenic methods not even mentioned here. A young-Earth research group reported that they sent a rock erupted in from Mount Saint Helens volcano to a dating lab and got back a potassium-argon age of several million years.

This shows we should not trust radiometric dating. There are indeed ways to "trick" radiometric dating if a single dating method is improperly used on a sample. Anyone can move the hands on a clock and get the wrong time.

Likewise, people actively looking for incorrect radiometric dates can in fact get them. Geologists have known for over forty years that the potassium-argon method cannot be used on rocks only twenty to thirty years old. Publicizing this incorrect age as a completely new finding was inappropriate. The reasons are discussed in the Potassium-Argon Dating section [of Wiens' article].

Be assured that multiple dating methods used together on igneous rocks are almost always correct unless the sample is too difficult to date due to factors such as metamorphism or a large fraction of xenoliths. Different dating techniques usually give conflicting results.

This is not true at all. The fact that dating techniques most often agree with each other is why scientists tend to trust them in the first place. Nearly every college and university library in the country has periodicals such as Science, Nature, and specific geology journals that give the results of dating studies. The public is usually welcome to and should! So the results are not hidden; people can go look at the results for themselves. Over a thousand research papers are published a year on radiometric dating, essentially all in agreement.

Besides the scientific periodicals that carry up-to-date research reports, [there are] textbooks, non-classroom books, and web resources. Anomalies As noted above, creationists make great hay out of "anomalies" in radiometric dating. It is true that some "anomalies" have been observed, although keep in mind that these have been identified by professional scientists in published literature, not by creationists or others outside of peer-reviewed scientific literature.

First of all, many of these claimed "anomalies" are completely irrelevant to the central issue of whether the Earth is many millions of years old. This is certainly true when errors are in the range of a few percent in specimens many millions of years old. This is also true of anomalies noted in carbon dates. Carbon dating cannot be used to date anything older than about 50, years, since the carbon half life is only years.

For additional discussion, see Radiocarbon dating. In any event, it is important to keep these anomalies in perspective. For example, out of literally tens of thousands of dates measured using the rubidium-strontium dating scheme see description of the Rb-Sr scheme in Agesonly about 30 cases have been noted where the individual data values initially appeared to lie nearly on a straight line as is requiredbut the result was later found to be significantly in error.

And each of these 30 cases is fairly well understood -- none of these is truly "mysterious" [ Wien ]. Anomalies and other objections that have been raised by creationists are dealt with in detail in Roger Wiens' article [ Wien ], Mark Isaak's book [ Isaakpg.

A detailed response to other claims of scientific evidence for a young Earth is given by Matthew Tiscareno [ Tiscareno ]. Radioactive isotopes and the age of the Earth Until recently, only a large scientific laboratories could afford mass spectrometers, which are the principal tool used to measure dates of rock samples.

But recently the prices of these devices have dropped to levels that even amateur meteorite hunters and others can afford. Samarium—neodymium dating This involves the alpha decay of Sm to Nd with a half-life of 1. Accuracy levels of within twenty million years in ages of two-and-a-half billion years are achievable.

Potassium—argon dating This involves electron capture or positron decay of potassium to argon Potassium has a half-life of 1. Rubidium—strontium dating method[ edit ] Main article: Rubidium—strontium dating This is based on the beta decay of rubidium to strontiumwith a half-life of 50 billion years. This scheme is used to date old igneous and metamorphic rocksand has also been used to date lunar samples. Closure temperatures are so high that they are not a concern.

Rubidium-strontium dating is not as precise as the uranium-lead method, with errors of 30 to 50 million years for a 3-billion-year-old sample. Uranium—thorium dating method[ edit ] Main article: Uranium—thorium dating A relatively short-range dating technique is based on the decay of uranium into thorium, a substance with a half-life of about 80, years.

It is accompanied by a sister process, in which uranium decays into protactinium, which has a half-life of 32, years. While uranium is water-soluble, thorium and protactinium are not, and so they are selectively precipitated into ocean-floor sedimentsfrom which their ratios are measured. The scheme has a range of several hundred thousand years. A related method is ionium—thorium datingwhich measures the ratio of ionium thorium to thorium in ocean sediment.

Radiocarbon dating method[ edit ] Main article: Carbon is a radioactive isotope of carbon, with a half-life of 5, years, [25] [26] which is very short compared with the above isotopes and decays into nitrogen. Carbon, though, is continuously created through collisions of neutrons generated by cosmic rays with nitrogen in the upper atmosphere and thus remains at a near-constant level on Earth.

The carbon ends up as a trace component in atmospheric carbon dioxide CO2. A carbon-based life form acquires carbon during its lifetime. Plants acquire it through photosynthesisand animals acquire it from consumption of plants and other animals. When an organism dies, it ceases to take in new carbon, and the existing isotope decays with a characteristic half-life years.

The proportion of carbon left when the remains of the organism are examined provides an indication of the time elapsed since its death.

This makes carbon an ideal dating method to date the age of bones or the remains of an organism. The carbon dating limit lies around 58, to 62, years. However, local eruptions of volcanoes or other events that give off large amounts of carbon dioxide can reduce local concentrations of carbon and give inaccurate dates. The releases of carbon dioxide into the biosphere as a consequence of industrialization have also depressed the proportion of carbon by a few percent; conversely, the amount of carbon was increased by above-ground nuclear bomb tests that were conducted into the early s.

Also, an increase in the solar wind or the Earth's magnetic field above the current value would depress the amount of carbon created in the atmosphere.

Fission track dating method[ edit ] Main article: This involves inspection of a polished slice of a material to determine the density of "track" markings left in it by the spontaneous fission of uranium impurities.

The uranium content of the sample has to be known, but that can be determined by placing a plastic film over the polished slice of the material, and bombarding it with slow neutrons. This causes induced fission of U, as opposed to the spontaneous fission of U. The fission tracks produced by this process are recorded in the plastic film. The uranium content of the material can then be calculated from the number of tracks and the neutron flux. This scheme has application over a wide range of geologic dates.

For dates up to a few million years micastektites glass fragments from volcanic eruptionsand meteorites are best used. This is a reasonable scenario, since N is a non-radiogenic isotope not produced by decay such as leadand it can be assumed to have similar concentrations in many magmas.

Magma from the ocean floor has little U and little U and probably little lead byproducts lead and lead Magma from melted continental material probably has more of both U and U and lead and lead Thus we can get an isochron by mixing, that has the age of the younger-looking continental crust.

Radiometric dating

The age will not even depend on how much crust is incorporated, as long as it is non-zero. However, if the crust is enriched in lead or impoverished in uranium before the mixing, then the age of the isochron will be increased. If the reverse happens before mixing, the age of the isochron will be decreased. Any process that enriches or impoverishes part of the magma in lead or uranium before such a mixing will have a similar effect. So all of the scenarios given before can also yield spurious isochrons.

I hope that this discussion will dispel the idea that there is something magical about isochrons that prevents spurious dates from being obtained by enrichment or depletion of parent or daughter elements as one would expect by common sense reasoning. So all the mechanisms mentioned earlier are capable of producing isochrons with ages that are too old, or that decrease rapidly with time. The conclusion is the same, radiometric dating is in trouble. I now describe this mixing in more detail.

Suppose P p is the concentration of parent at a point p in a rock. The point p specifies x,y, and z co-ordinates. Let D p be the concentration of daughter at the point p. Let N p be the concentration of some non-radiogenic not generated by radioactive decay isotope of D at point p. Suppose this rock is obtained by mixing of two other rocks, A and B.

Suppose that A has a for the sake of argument, uniform concentration of P1 of parent, D1 of daughter, and N1 of non-radiogenic isotope of the daughter.

Thus P1, D1, and N1 are numbers between 0 and 1 whose sum adds to less than 1. Suppose B has concentrations P2, D2, and N2.

radiometric age dating methods in geology

Let r p be the fraction of A at any given point p in the mixture. So the usual methods for augmenting and depleting parent and daughter substances still work to influence the age of this isochron. More daughter product means an older age, and less daughter product relative to parent means a younger age. In fact, more is true. Any isochron whatever with a positive age and a constant concentration of N can be constructed by such a mixing.

It is only necessary to choose r p and P1, N1, and N2 so as to make P p and D p agree with the observed values, and there is enough freedom to do this. Anyway, to sum up, there are many processes that can produce a rock or magma A having a spurious parent-to-daughter ratio.

Then from mixing, one can produce an isochron having a spurious age. This shows that computed radiometric ages, even isochrons, do not have any necessary relation to true geologic ages. Mixing can produce isochrons giving false ages. But anyway, let's suppose we only consider isochrons for which mixing cannot be detected. How do their ages agree with the assumed ages of their geologic periods?

As far as I know, it's anyone's guess, but I'd appreciate more information on this. I believe that the same considerations apply to concordia and discordia, but am not as familiar with them. It's interesting that isochrons depend on chemical fractionation for their validity.

They assume that initially the magma was well mixed to assure an even concentration of lead isotopes, but that uranium or thorium were unevenly distributed initially. So this assumes at the start that chemical fractionation is operating. But these same chemical fractionation processes call radiometric dating into question. The relative concentrations of lead isotopes are measured in the vicinity of a rock. The amount of radiogenic lead is measured by seeing how the lead in the rock differs in isotope composition from the lead around the rock.

This is actually a good argument. But, is this test always done? How often is it done? And what does one mean by the vicinity of the rock? How big is a vicinity? One could say that some of the radiogenic lead has diffused into neighboring rocks, too. Some of the neighboring rocks may have uranium and thorium as well although this can be factored in in an isochron-type manner. Furthermore, I believe that mixing can also invalidate this test, since it is essentially an isochron.

Finally, if one only considers U-Pb and Th-Pb dates for which this test is done, and for which mixing cannot be detected. The above two-source mixing scenario is limited, because it can only produce isochrons having a fixed concentration of N p. To produce isochrons having a variable N pa mixing of three sources would suffice. This could produce an arbitrary isochron, so this mixing could not be detected. Also, it seems unrealistic to say that a geologist would discard any isochron with a constant value of N pas it seems to be a very natural condition at least for whole rock isochronsand not necessarily to indicate mixing.

I now show that the mixing of three sources can produce an isochron that could not be detected by the mixing test. First let me note that there is a lot more going on than just mixing. There can also be fractionation that might treat the parent and daughter products identically, and thus preserve the isochron, while changing the concentrations so as to cause the mixing test to fail. It is not even necessary for the fractionation to treat parent and daughter equally, as long as it has the same preference for one over the other in all minerals examined; this will also preserve the isochron.

Now, suppose we have an arbitrary isochron with concentrations of parent, daughter, and non-radiogenic isotope of the daughter as P pD pand N p at point p. Suppose that the rock is then diluted with another source which does not contain any of D, P, or N. Then these concentrations would be reduced by a factor of say r' p at point p, and so the new concentrations would be P p r' pD p r' pand N p r' p at point p.

Now, earlier I stated that an arbitrary isochron with a fixed concentration of N p could be obtained by mixing of two sources, both having a fixed concentration of N p.

With mixing from a third source as indicated above, we obtain an isochron with a variable concentration of N pand in fact an arbitrary isochron can be obtained in this manner. So we see that it is actually not much harder to get an isochron yielding a given age than it is to get a single rock yielding a given age.

This can happen by mixing scenarios as indicated above. Thus all of our scenarios for producing spurious parent-to-daughter ratios can be extended to yield spurious isochrons. The condition that one of the sources have no P, D, or N is fairly natural, I think, because of the various fractionations that can produce very different kinds of magma, and because of crustal materials of various kinds melting and entering the magma.

In fact, considering all of the processes going on in magma, it would seem that such mixing processes and pseudo-isochrons would be guaranteed to occur. Even if one of the sources has only tiny amounts of P, D, and N, it would still produce a reasonably good isochron as indicated above, and this isochron could not be detected by the mixing test.

I now give a more natural three-source mixing scenario that can produce an arbitrary isochron, which could not be detected by a mixing test. P2 and P3 are small, since some rocks will have little parent substance. Suppose also that N2 and N3 differ significantly.

Such mixings can produce arbitrary isochrons, so these cannot be detected by any mixing test. Also, if P1 is reduced by fractionation prior to mixing, this will make the age larger. If P1 is increased, it will make the age smaller. If P1 is not changed, the age will at least have geological significance.

But it could be measuring the apparent age of the ocean floor or crustal material rather than the time of the lava flow. I believe that the above shows the 3 source mixing to be natural and likely. We now show in more detail that we can get an arbitrary isochron by a mixing of three sources.

Thus such mixings cannot be detected by a mixing test. Assume D3, P3, and N3 in source 3, all zero. One can get this mixing to work with smaller concentrations, too. All the rest of the mixing comes from source 3. Thus we produce the desired isochron. So this is a valid mixing, and we are done. We can get more realistic mixings of three sources with the same result by choosing the sources to be linear combinations of sources 1, 2, and 3 above, with more natural concentrations of D, P, and N.

The rest of the mixing comes from source 3. This mixing is more realistic because P1, N1, D2, and N2 are not so large. I did see in one reference the statement that some parent-to-daughter ratio yielded more accurate dates than isochrons. To me, this suggests the possibility that geologists themselves recognize the problems with isochrons, and are looking for a better method. The impression I have is that geologists are continually looking for new methods, hoping to find something that will avoid problems with existing methods.

But then problems also arise with the new methods, and so the search goes on. Furthermore, here is a brief excerpt from a recent article which also indicates that isochrons often have severe problems. If all of these isochrons indicated mixing, one would think that this would have been mentioned: The geological literature is filled with references to Rb-Sr isochron ages that are questionable, and even impossible. He comes closest to recognizing the fact that the Sr concentration is a third or confounding variable in the isochron simple linear regression.

Snelling discusses numerous false ages in the U-Pb system where isochrons are also used. However, the U-Th-Pb method uses a different procedure that I have not examined and for which I have no data.

Many of the above authors attempt to explain these "fictitious" ages by resorting to the mixing of several sources of magma containing different amounts of Rb, Sr, and Sr immediately before the formation hardens. AkridgeArmstrongArndtsBrown, Helmick and Baumann all discuss this factor in detail. Anyway, if isochrons producing meaningless ages can be produced by mixing, and this mixing cannot be detected if three or maybe even two, with fractionation sources are involved, and if mixing frequently occurs, and if simple parent-to-daughter dating also has severe problems, as mentioned earlier, then I would conclude that the reliability of radiometric dating is open to serious question.

The many acknowledged anomalies in radiometric dating only add weight to this argument. I would also mention that there are some parent-to-daughter ratios and some isochrons that yield ages in the thousands of years for the geologic column, as one would expect if it is in fact very young. One might question why we do not have more isochrons with negative slopes if so many isochrons were caused by mixing.

This depends on the nature of the samples that mix. It is not necessarily true that one will get the same number of negative as positive slopes. If I have a rock X with lots of uranium and lead daughter isotope, and rock Y with less of both relative to non-radiogenic leadthen one will get an isochron with a positive slope.

If rock X has lots of uranium and little daughter product, and rock Y has little uranium and lots of lead daughter product relative to non-radiogenic leadthen one will get a negative slope.

This last case may be very rare because of the relative concentrations of uranium and lead in crustal material and subducted oceanic plates. Another interesting fact is that isochrons can be inherited from magma into minerals.

Earlier, I indicated how crystals can have defects or imperfections in which small amounts of magma can be trapped. This can result in dates being inherited from magma into minerals. This can also result in isochrons being inherited in the same way. So the isochron can be measuring an older age than the time at which the magma solidified. This can happen also if the magma is not thoroughly mixed when it erupts. If this happens, the isochron can be measuring an age older than the date of the eruption.

This is how geologists explain away the old isochron at the top of the Grand Canyon. From my reading, isochrons are generally not done, as they are expensive. Isochrons require more measurements than single parent-to-daughter ratios, so most dates are based on parent-to-daughter ratios.

Types of radiometric dating methods

So all of the scenarios given apply to this large class of dates. Another thing to keep in mind is that it is not always possible to do an isochron. Often one does not get a straight line for the values. This is taken to imply re-melting after the initial solidification, or some other disturbing event. Anyway, this also reduces the number of data points obtained from isochrons. Anyway, suppose we throw out all isochrons for which mixing seems to be a possibility.

Due to some published anomalies, I don't think we know that they have any clear relationship to the assumed dates. It is also interesting that the points for isochrons are sometimes selected so as to obtain the isochron property, according to John Woodmorappe's paper.

Do the various methods correlate with one another? We have been trying to give mechanisms that explain how the different dating methods can give dates that agree with one another, if the geologic column is young. But if there is a variation, such effects could help to explain it. It's not only a matter of incorporation in minerals either, as one sometimes does whole rock isochrons and I suppose parent-daughter ratios of whole rock, which would reflect the composition of the magma and not the incorporation into minerals.

We all seem to have this image in our mind of the various dating methods agreeing with each other and also with the accepted age of their geologic periods. So we are investing a lot of time and energy to explain how this marvelous agreement of the various methods can arise in a creationist framework.

The really funny thing to me is that it is very possible that we are trying to explain a phantom of our imagination. The real radiomatric dating methods are often very badly behaved, and often disagree with one another as well as with the assumed ages of their geological periods.

It would really be nice if geologists would just do a double blind study sometime to find out what the distributions of the ages are. In practice, geologists carefully select what rocks they will date, and have many explanations for discordant dates, so it's not clear how such a study could be done, but it might be a good project for creationists. There is also evidence that many anomalies are never reported. Concerning the geologic time scale, Brown writes: Maybe only 15 in all.

It is possible that the reason is that uranium-lead dates so rarely agree with the correct dates. So there may not be anything to explain.

radiometric age dating methods in geology

For example, it's not clear to me that we need to worry about isochrons or whether U and U dates etc. I'd like to know how often this happens, in any case, especially on the geologic column of Cambrian and above. People should read John Woodmorappe's articles on radiometric dating to see some of the anomalies. One might say that if there were problems, then geologists wouldn't use these methods. I think we need something more solid than that. The correlation was not very good.

I assume he would have mentioned if any others had been done. What we really need is the raw data on how these dates correlate, especially on the geologic column of Cambrian and above. We need to see the data to know if there is really any need to explain anything away. Many anomalies never get published, according to John Woodmorappe's references; other quotes indicate that the various methods typically disagree with each other. A few years ago I took a course in the "Evolution of Desert Environments".

We were standing on the Simi Volcanic flow, about 80 miles south of the south end of Death Valley. The instructor was a well known geologist and evolutionist from Cal. He told us that the upper end of the flow was dated atyears, the middle of the flow was dated at 50, years, and the toe of the flow was dated at 20, years.

He then noted that the whole flow probably occured and solidified the surface at least within weeks. He then said, based on his observation of the rates of evolution of desert environments he thought the flow was less than 10, years of age.

He then said "radiometric dating is the cornerstone of modern historical geology and we get this kind of variation?

He was also not happy with the published dates on the flows in the Nevada Atomic Bomb Test site where one of the volcanic flows showed a reversal of isotope ratios and gave a value of 20, years in the future! These data were, in fact, published in Science magazine in about November of Please note, these were not MY ideas but the statements of a convinced, tenured, evolutionary geologist who apparently really wanted to beleive in the credibility of radiometric dating.

I am just reporting what HE said! Thus, there apparently ARE some problems in that kind of radiometric dating. Jon Covey cited some references about this, and it will take a lot of work to understand what is going on from a creationist viewpoint. But this is another factor that could be causing trouble for radiometric dating. If there is a proof that this could not be so, then I have missed it. I would not want to use a scale that might be right and might be wrong.

And I'm curious to see how discordia relate to the possibility of fractionation -- I did look into them at one time. But this point is sufficiently complicated that I can't see the implications right away. In general, when an area is so complicated that I can just barely understand it, then there may be problems with the area that are more complicated still. But my inclination is to think that the same kinds of mixing processes that produce isochrons can also produce discordia.