Radioactive dating - A Storehouse of Knowledge
Radiometric dating or radioactive dating is a technique used to date materials such as rocks or . Accurate radiometric dating generally requires that the parent has a long so steeply that the age of relatively young remains can be determined precisely to . This scheme has application over a wide range of geologic dates. Radioactive dating enables geologists to determine the age of rocks, plants, materials, etc. Easy to read, and nice looking site are on this site. Radiometric dating of rocks and minerals using naturally occurring, although there is a small percentage of cases in which even these generally reliable methods order to provide a cross-check, or are evaluated using other geologic information that The fossils, when combined with geologic mapping, allow the various.
Expressed in what look like precise calendar years, figures seem somehow better — both to layman and professional not versed in statistics — than complex stratigraphic or cultural correlations, and are more easily retained in one's memory. No matter how "useful" it is, though, the radiocarbon method is still not capable of yielding accurate and reliable results.
There are gross discrepancies, the chronology is uneven and relative, and the accepted dates are actually selected dates. If a C14 date supports our theories, we put it in the main text. If it does not entirely contradict them, we put it in a footnote. And if it is completely 'out of date', we just drop it. They send samples to the laboratories to be dated. Archaeologists never let me emphasize this never date their finds by carbon They only quote it if it agrees with their conclusions.
For ages less than a few thousand years, this can be done with historically dated material. Note, though, that this particular example is according to Egyptian chronologywhich is itself a matter of dispute. For dates older than this, and to fill in the calibration curve where no historically datable artifacts are available, the radiocarbon dates can only be compared with other dating methods.
The preferred standard, at least for dates up to several thousand years, is wood that has been dated by dendrochronologyi. Single trees can be dated by their rings back nearly 5, years, while matching the overlapping ring patterns of a series of trees is claimed to be able to extend the chronology back to 12, years before the present.
However, dendrochronology is not itself an exact dating method, and for older dates carbon dating is used to help determine dendrochronological dates, which is a circular exercise. Recently, by including a variety of other evidence such as varves and corals, a new standard for the radiocarbon calibration curve was agreed on, which extends to 50, years before the present.
To the extent that these "incremental" dating methods are valid, they remove the otherwise necessary assumption that the ratio of 14C to 12C in the atmosphere has not changed.
Carbon in deeply buried carbon Carbon makes up about 1 part in of the carbon in the atmosphere. Levels on the order of 1 part in can be precisely measured with state-of-the art mass spectrometer systems, although at that level it is difficult to rule out contamination during the processing and measurement.
In at the new Crinum Coal Mine in Central Queensland a piece of wood was found embedded in basalt, so the tree must have died shortly before or as a result of the lava flow. The radiocarbon age of the wood was determined to be around 45, years. Creationists hold the divergence of these dates to demonstrate the unreliability of one or both of the methods. Evolutionists consider contamination to be the likely cause of the finite radiocarbon age. Furthermore, the wood was collected by miners without an interest in or training to prevent contamination with modern carbon, so contamination during handling cannot be ruled out either.
Carbon that is buried deeper, such as diamonds   and deeply buried coal,  is less susceptible to in situ contamination. Potential sources of this carbon contemplated by secular geologists include contamination by bacteria after mining and production in situ from nitrogen by background radiation estimated to be capable of producing a radiocarbon age as young as 80, years. Creation geologists tend to believe that this residual carbon primarily reflects some combination of low concentration before the Flood and sequestration by burial during the Flood.
This scenario would require that carbon be exempt from the acceleration of decay rates believed to have occurred for other radioisotopes during the Flood.
Dating of historic lava flows In general, dates in the "correct ball park" are assumed to be correct and are published, but those in disagreement with other data are seldom published nor are discrepancies fully explained.
For classical potassium-argon dating, with a half-life of 1. Nevertheless, a check on the consistency of the method is possible by dating lava flows known to have occurred in the last few thousand years, or even within the last few decades.
If the method is reliable, then these calculations should return an age indistinguishable from zero. If they don't, then the magnitude, frequency, and causes of the false ages must be carefully examined, and the method improved, restricted, or abandoned, accordingly As outlined above, the potassium-argon method relies on the fact that diffusivity of argon is very high in molten rock and very low in solidified rock.
There are three situations, however, where this simple principle becomes more complicated. The sample may become contaminated with atmospheric argon in the field or during the analysis. This effect is relatively easy to correct because the isotopic composition of atmospheric argon is precisely known and stable. All that is needed is to measure the amount of 36Ar. The amount of 40Ar attributable to atmospheric contamination will be The sample may suffer some loss of radiogenic argon due to heating or weathering.
Weathering is the smaller problem because a trained geologist can recognize the signs of this and choose undisturbed samples. This will make the sample appear to have an age somewhere between the time of original solidification and the heating.
This should be kept in mind if there is any reason to suspect later heating, such as metamorphic changes,  but at any rate the loss of radiogenic argon is not able to produce an age greater than the age of solidification.
The sample may contain "excess argon" due to inclusions that are not melted with the lava. Magma, like other types of rocks, is usually a complex mixture of minerals in various states, and may contain solid particles ranging in size from microscopic crystals up to boulders. When it is transported to the surface as lava, any argon present can escape from the liquid matrix, but not necessarily from the solid inclusions.
If the potassium-argon age is included by analysis of the isotopes in the whole rock, then the argon measured will be the sum of the radiogenic argon in the matrix and the argon in the inclusions, both initial and radiogenic.
The calculated age will thus be greater than the time since the eruption. In Dalrymple  published a study of 26 distinct flows. Three cases showed an excess of 36Ar, yielding apparent negative ages up to 0. This study was followed in by work of Krummenacher  of another 19 flows.
More recently Snelling   reported the potassium-argon dates obtained for 13 samples from a single lava flow from the 20th century AD.
Radiometric Dating Does Work!
Five of these showed, as one would expect, an age indistinguishable for zero, but seven showed calculated ages between 0. In summary, there are several factors that can lead to the method providing an incorrect age. Argon-argon method In the 40 years since the seminal studies of Dalrymple and Krummenacher, a number of very important improvements have been made to the potassium-argon method, most notably extending it to the argon-argon method, which allows much more precise measurement of the isotope ratios, easier exclusion of contaminating crystals, and the measurement of an "isochron", which in turn provides a measurement of the excess argon in the sample, the possibility to detect - and in some cases correct for - argon loss, and a consistency check of the data.
When the argon-argon method is applied to the matrix of recent lava flows, avoiding any inclusions, and considering only samples which yield a clean isochron, the million-year anomalies of the K-Ar method are claimed to be no longer found.
A good example of the accuracy of this method when the conditions are ideal is the dating of the famous eruption of Vesuvius in 79 A. Consistency between methods and between repeated measurements For methods, such as U-Pb, that are not able to date samples as young as 50, years, the only recourse—other than Astronomical dating discussed above—is to compare the calculated dates with dates calculated from other radiometric methods. In the best case, a chain of comparisons back to historical dates can be established, but in any case a general check on consistency is also very valuable.
Another way to check the consistency of a dating method is to verify that the dates provided by multiple tests of a single object, or tests on a number of objects that should be the same age, yield consistent results.
One difficulty is that many samples rocks have a complex history. Since different methods have different "closure temperatures", an episode of heating may reset one radiometric clock but not another. Some events will change the rock in such a way, for example through partial loss of the daughter isotope, that an isochron is no longer produced. These effects limit the applicability of radiometric dating and complicate its interpretation without necessarily calling the fundamental principle into question.
Therefore, to test the fundamental principle by looking at the consistency between different methods, it is important to choose samples that are unlikely to have been modified since their formation.
The ideal objects in this respect are meteorites.
What Is Radioactive Dating, and How Does It Work?
These have been in space since their formation near the beginning of the solar system, and have not been subjected to the thermal, mechanical, and chemical processes on Earth that might compromise the assumptions of the method.
Dozens of meteorites have been dated, many of them several times, many of them with multiple techniques. The majority of the ages fall between 4. Commenting on the work done on one of these meteorites, Dalrymple writes  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.
What does radioactive dating enable geologists to determine?
Another good test of consistency under ideal conditions is the Cretaceous-Tertiary K-T boundary, which exists world-wide and was identified by geologists in the early s, long before radiometric dating was invented. Dalrymple summarizes  radiometric age measurements made on 3 different minerals and on glass, by 3 distinctly different dating methods, each involving different elements with different half-lives.
The dating was done in 6 different laboratories and the materials were collected from 5 different locations in the Western Hemisphere. The calculated ages are the same within analytical error 0. Although radiometric dating has been shown to be consistent under ideal conditions, it is harder to answer the question of how good it is under more typical conditions.
It is widely recognized that there are many factors which can lead to a date which is inconsistent with other information about the sample. Some of these can be corrected or at least recognized using various advanced techniques, but others will never be discovered. There are, however, exceptions.
The known effects are either very small or occur under extreme conditions, so that they, by themselves, can not significantly alter dates calculated with radiometric methods. However, the fact that rates can vary opens the possibility of other, as yet unknown, ways in which rates could change. Electron capture and internal conversion These decay processes can be affected in a few light atoms because they contain electrons in orbitals that interact with the nucleus as well as with the electrons of adjacent atoms.
Under conditions found in very hot stars, heavy nuclei can be stripped of most or all of their electrons, enabling a decay mode known as bound-state beta decay. This effect was first observed in No one has measured the decay rates directly; we only know them from inference.
Radiometric dating - Wikipedia
Decay rates have been directly measured over the last years. In some cases a batch of the pure parent material is weighed and then set aside for a long time and then the resulting daughter material is weighed. In many cases it is easier to detect radioactive decays by the energy burst that each decay gives off. For this a batch of the pure parent material is carefully weighed and then put in front of a Geiger counter or gamma-ray detector.
These instruments count the number of decays over a long time. If the half-lives are billions of years, it is impossible to determine them from measuring over just a few years or decades. The example given in the section [in Wiens' article] titled, "The Radiometric Clocks" shows that an accurate determination of the half-life is easily achieved by direct counting of decays over a decade or shorter.
Additionally, lavas of historically known ages have been correctly dated even using methods with long half-lives. The decay rates are poorly known, so the dates are inaccurate. Most of the decay rates used for dating rocks are known to within two percent. Such small uncertainties are no reason to dismiss radiometric dating. Whether a rock is million years or million years old does not make a great deal of difference.
To date a rock one must know the original amount of the parent element. But there is no way to measure how much parent element was originally there. It is very easy to calculate the original parent abundance, but that information is not needed to date the rock. 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. Uranium—lead dating method[ edit ] Main article: Uranium—lead dating A concordia diagram as used in uranium—lead datingwith data from the Pfunze BeltZimbabwe. This scheme has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two-and-a-half billion years.
Zircon has a very high closure temperature, is resistant to mechanical weathering and is very chemically inert. Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event. This can be seen in the concordia diagram, where the samples plot along an errorchron straight line which intersects the concordia curve at the age of the sample.
Samarium—neodymium dating method[ edit ] Main article: 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,   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: