Radiometric dating multiple choice questions

Over time, uranium decays to lead. By measuring the amount of uranium and lead in a zircon and knowing the rate of decay, we can measure the age of the zircon. Lead is somewhat mobile, however, as is uranium, and so other methods have been devised that can date zircons even if some lead leaves the rock.

Fossils and Their Place in Time and Nature

The problem with this method is that zircons can include lead when they form, throwing off the date. They can also lose uranium. In addition, they can travel through lava without melting, so the date computed for a zircon may be measuring a much older event than the lava flow itself.

Even geologists recognize that ages given by zircons are often much too old, even for them. Furthermore, a batch of zircons from the same place will often yield widely different ages. So I guess we'll have to discard zircons as a reliable dating method. The next candidate dating method is fission track dating. Some minerals contain uranium which decays by fission. It splits in two, and the pieces fly apart through the mineral, creating fission tracks. These tracks can be made visible by etching with an acid solution, and then counted.

By knowing how much uranium there is in a rock and by counting the number of fission tracks, one can measure the age of the rock.

There are a number of problems with this method, and even geologists have had intense disagreements about its reliability. The ages often do not agree with what geologists expect. One problem is that certain constants involved in this method are not known or are hard to estimate, so they are calibrated based on the "known" ages of other rocks.


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If these other "known" ages are in error, then fission track dates are in error by the same amount. Another problem is that fission tracks fade at high temperatures. So if there are too few tracks, the geologist can always say that most of them faded away. To get a fission track date, one has to know something about the temperature history of a rock. Another problem is that uranium can be removed from a rock by water. If a sample loses 99 percent of its uranium, then the fission track date will be times too old. Another problem is knowing what is a fission track and what is just an imperfection in the rock.

Geologists themselves suggest that imperfections are at times mistaken for fission tracks, and admit that fission tracks are not always easy to recognize. Textbooks have beautiful, clean pictures of fission tracks, but I doubt that these illustrations correspond to reality. Along this line, it is interesting to note that for every fission of uranium , there are over a million decays by a process called alpha decay, in which a helium nucleus is ejected from the nucleus of uranium.

The alpha particle creates a long, thin trail of damage, and the former uranium nucleus recoils in the other direction, creating a short, wide track about one thousandth as long as a fission track. Not only this, but what's left of the uranium nucleus having lost the helium nucleus decays by thirteen more steps until it becomes lead, so there are over fourteen million other decays for every fission track. Over four million of these occur within a few days. All of these decays emit particles that damage the crystal structure. Some of these decays emit alpha particles, and some emit beta particles, which are energetic electrons.

In addition, many millions of gamma rays are emitted, which are high-energy electromagnetic radiation like X rays, and also damage the crystal structure. Perhaps the damage created by all this radiation can be increased by chemical action and be etched by acid to appear like fission tracks. Or if two alpha particle trails are close enough together, perhaps they can damage the crystal enough so that their combined trail will be etched away by acid like a fission track. Minerals are also subject to alteration by water, which may contain chemicals that react with the rock. Over long periods of time, all of these processes can damage the crystal structure, and it may be that when the mineral is etched with acid, track-like formations appear as a result.

Another problem is that fission tracks in some minerals, like zircons, can survive in lava, so the fission track date can be measuring an older event than the lava flow. Thus we cannot necessarily use this method to date the age of the fossils. I think fission track dating has more potential than the other methods, but in view of all of these problems, I think we'll have to discard fission track dating as a reliable method. There are still other methods, such as rubidium-strontium dating, which are based on the decay of a parent substance in this case rubidium to its daughter product strontium.

These methods all depend on knowing how much daughter product was initially present, which we cannot know. So we'll have to discard rubidium-strontium dating and similar methods as reliable dating methods There is also the so-called "isochron" method, which is a clever way to estimate the amount of daughter product present initially, so that one can then use rubidium-strontium dating and other methods to get reliable dates.

Unfortunately, isochrons can also be caused by mixing processes that have nothing to do with true dates. One study indicated that nearly all published isochrons have properties suggesting that they result from mixings, and thus are not giving true dates. Another study indicated that nearly all isochrons published have poor statistical quality. Geologists often make excuses to reject isochrons, anyway, when they don't like the dates. So I guess we'll have to discard the isochron method as a reliable dating method.

The problem is that now there is nothing left! It's also interesting that geologists frequently admit that these different methods usually don't agree with one another. So the next time you see a museum exhhibit with a sign saying that some fossil is so many tens or hundreds of millions of years old, I hope you'll take that with a large grain of salt.

If there is real evidence that these fossils are hundreds of millions of years old, then I want to know about it. These processes correspond to changing the setting of the clock hands. Not infrequently such resetting of the radiometric clocks is assumed in order to explain disagreements between different measurements of rock ages. Some more quotes from the same source: In the lead-uranium systems both uranium and lead can migrate easily in some rocks, and lead volatilizes and escapes as a vapor at relatively low temperatures.

It has been suggested that free neutrons could transform Pb first to Pb and then to Pb, thus tending to reset the clocks and throw thorium-lead and uranium-lead clocks completely off, even to the point of wiping out geological time. Furthermore, there is still disagreement of 15 percent between the two preferred values for the U decay constant. Potassium volatilizes easily, is easily leached by water, and can migrate through the rocks under certain conditions.

Furthermore, the value of the decay constant is still disputed, although the scientific community seems to be approaching agreement. Historically, the decay constants used for the various radiometric dating systems have been adjusted to obtain agreement between the results obtained. Argon, the daughter substance, makes up about one percent of the atmosphere, which is therefore a possible source of contamination. However, since it is possible for argon to be formed in the rocks by cosmic radiation, the correction may also be in error.

Argon from the environment may be trapped in magma by pressure and rapid cooling to give very high erroneous age results. Rubidium parent atoms can be leached out of the rock by water or volatilized by heat. All of these special problems as well as others can produce contradictory and erroneous results for the various radiometric dating systems. So we have a number of mechanisms that can introduce errors in radiometric dates. Heating can cause argon to leave a rock and make it look younger.

In general, if lava was heated after the initial flow, it can yield an age that is too young.

Multiple choice

If the minerals in the lava did not melt with the lava, one can obtain an age that is too old. Leaching can also occur; this involves water circulating in rock that can cause parent and daughter elements to enter or leave the rock and change the radiometric age. Thus it is easy to rationalize any date that is obtained.

If a date is too old, one can say that the mineral did not melt with the lava. Maybe it got included from surrounding rock as the lava flowed upward.

Carbon-14 Radioactive Dating Worked Example - Doc Physics

If the date is too young, one can say that there was a later heating event. One can also hypothesize that leaching occurred. But then it is claimed that we can detect leaching and heating. But how can we know that this claim is true, without knowing the history of rocks and knowing whether they have in fact experienced later heating or leaching? The problems are compounded because many of the parent and daughter substances are mobile, to some extent. I believe that all parent substances are water soluble, and many of the daughter products as well.

A few sources have said that Sr is mobile in rock to some extent. This could cause trouble for Rb-Sr dating. In fact, some sources say that Sr and Ar have similar mobilities in rock, and Ar is very mobile.

Clark Larsen Answers Students FAQs

Especially the gaseous radioactive decay byproducts such as argon, radon, and helium are mobile in rock. So if a rock has tiny cracks permitting gas to enter or escape or permitting the flow of water, the radiometric ages could be changed substantially even without the rock ever melting or mixing. Now, there is probably not much argon in a rock to start with.


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  • So the loss of a tiny amount of argon can have significant effects over long time periods. A loss of argon would make the rock look younger. In a similar way, argon could enter the rock from the air or from surrounding rocks and make it look older. And this can also happen by water flowing through the rock through tiny cracks, dissolving parent and daughter elements.

    It would be difficult to measure the tiny changes in concentration that would suffice to make large changes in the radiometric ages over long time periods. I also question the assertion that argon, for example, is excluded from certain minerals when they crystallize and never enters later on. Geologists often say that ages that are too old are due to excess argon. So it must be possible for that excess argon to get in, even though the crystal is supposed to exclude it. Here is one such reference, although this is to a mineral that does not exclude argon: In a few cases, argon ages older than that of the Earth which violate local relative age patterns have even been determined for the mineral biotite.