Why is radioactive decay dating the most reliable method of dating the geologic past
Radiometric dating, often called radioactive dating, is a technique used to determine the age of materials such as rocks. It is based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates. It is the principal source of information about the absolute age of rocks and other geological features, including the age of the Earth itself, and it can be used to date a wide range of natural and man-made materials. The best-known radiometric dating techniques include radiocarbon dating, potassium-argon dating, and uranium-lead dating.
Dating Rocks and Fossils Using Geologic Methods
This overview was written to help absolute beginners get started. Radioactive dating is the procedure of calculating an age for an artifact by determining how much of the radioactive material has decayed and calculating how long that would take given the half-life how long it takes for half the material to decay of the material being tested. Several dozen methods exist, using different radioactive isotopes and decay products, with varied dating ranges and precision. The procedure, however, is difficult, and many tests have shown that it can be inaccurate, and it is at times not even considered reliable by mainstream scientists.
It is impossible to measure the age of something, except to time it as it actually occurs, so radioactive dating methods calculate the age, based on i measurements of quantities of specified materials, ii measurements of decay rates, and iii assumptions about the history of the sample. Suppose we have a tank partly filled with water , and a hole in the bottom through which the water is leaking out of the tank.
We wonder how long the hole has been there, that is, how old the hole is. We could measure a how much water the tank holds, b how much is still in the tank, and c the rate at which it is leaking out. We can calculate the age of the hole by subtracting b from a to find out how much water has left the tank, and then dividing this by c , the rate at which it is leaking out.
However, in doing so, we have, consciously or subconsciously , made a number of assumptions about other factors that could have affected the calculations. Our calculation of the age of the hole will not be accurate unless all these assumptions are true. Similar factors apply to radioactive dating methods. Unless these factors are known, the calculated dates will not be reliable. Sometimes a hypothesis must be made that may be plausible but has not been proven.
At other times an additional measurement can eliminate the need for one assumption, although no science can be done without assumptions at some level. For example, isochron methods do not assume any particular concentration of the daughter isotope in the original sample, but calculate that concentration based on other measurements. The reliability of that calculation will in turn depend on other conditions.
Different isotopes have different decay rates. Radioactive dating is usually considered most accurate if the age of the sample is not too much different than the half-life of the isotope used, although in favorable cases the age of the sample could be as much as time less than the half-life or 10 times more. The appropriateness of a particular isotope also depends on the mineralogy and history of the sample, so not all samples can be dated.
Carbon for example is considered accurate by its supporters on ages from 2 to 50 thousand years. Modern methods can detect essentially any Carbon, and therefore produce dates up to about , years. A sample is taken and prepared by removing any extraneous material, and removing any inclusions from the sample. The sample is then crushed and dissolved. The sample is then placed in a mass-spectrometer and a chart is produced showing the quantities of each element or isotope. That result is compared to decay curves to get a time interval.
That time interval is compared to calibration information and corrections made for known variations. This provides a calendar date with an error margin. This form of dating measures the decay of uranium within igneous zircon over a scale of tens of millions to billions of years. As uranium decays to two different isotopes of lead at different rates of decay, two clocks are inherently built into the system. If the two agree with each other, the confidence in the date will be high.
If they disagree, it may be because lead has been lost at some point in the history of the sample, for example, if there was an episode of heating above degrees C. The mineralogy of zircon makes it highly unlikely that either any uranium is lost from the crystal or that any lead was in the crystal to begin with. To deal with this possibility, an independent measurement is made from several points in the sample.
Usually, different amounts of lead will have been lost, for example more from the surface of the crystal than from the interior, so there will be a range of isotope ratios. If the differences are due to one episode of loss of lead, then these points will lie on a straight line, as is often observed. Given this confirmation of the confounding factor, the line of observations may be extended until it intersects the curve consisting of the possible values without loss of lead.
This value is taken as the true age of the sample. A variation of this method is also known as lead-lead dating. It does not determine the age of a single sample directly, but the time at which different samples with differing amount of uranium and lead were separated from a common pool. This method has particular significance because it is the only method that purports to give a value for the age of the Earth that is, the time at which the Earth and asteroids condensed out of the planetary nebula rather than only a lower limit on the age of the Earth that is, the age of the oldest surviving rocks on the Earth.
If any of the assumptions do not hold, then there is no reason to expect this to happen. In fact, when points are plotted for each of five meteorites that contain varying levels of uranium, a single data point for all meteorites that do not, and one data point for modern terrestrial sediments, all seven points lie on a single line. The slope of that line corresponds to an age of 4. Whereas uranium-lead dating relies on the properties of the minerals to expel lead atoms on formation, potassium-argon dating relies on the fact that noble gases, in this case argon, easily diffuse out of molten rock, but are trapped within the crystal if they are produced within the solidified rock.
The material being dated must contain potassium, but that is relatively common. The nearly ideal application for potassium-argon dating is lava that has been quickly cooled otherwise some of the radiogenic 40 Ar may escape. One of the biggest concerns is that the sample may be contaminated by atmospheric argon. Minor contamination can be taken into account by measuring the amount of 36 Ar in the sample. The atmosphere contains This method is applicable to the oldest rocks on Earth, and recent advances have pushed the range of precise dating down to a few thousand years, under ideal conditions.
For carbon radiocarbon, or 14 C dating, the sample is the remnant of a living organism which was in equilibrium with the atmosphere or ocean while it was alive, but when it died it ceased to absorb carbon and this isotope began to decay to nitrogen 14 N with a half-life of about 6, years. As the sample aged, more and more carbon converted to nitrogen Carbon dating is a little different than other methods.
With the other methods, the quantities of the parent and daughter products are added to determine the original amount of the parent material. With carbon dating, the daughter material, nitrogen, is gaseous and escapes from the sample, so the original quantity cannot be calculated that way. When the creature was alive, however, it was taking in both carbon and carbon in the form of carbon dioxide in a ratio which was based on the ratio of those two elements in the atmosphere today about 10 So by measuring the ratio of carbon to carbon and comparing it to the ratio that would have existed when the creature was alive, the age of the sample since the time it died can, in principle, be calculated.
Radiocarbon dating can be, and has been, calibrated by dating samples of known antiquity. In some cases, timber is used, and the age of the timber is found by counting its tree rings. Dating methods are often presented as accurate, objective, measures of the age of an item, but the public perception of their accuracy does not match the reality of the dating methods being inaccurate in many cases.
Scientists will refuse to accept a radiometric date that they consider to be wrong, and in those cases look for reasons to reject it. In effect, they will keep trying for an "acceptable" date until they find one that fits their ideas on how old their samples actually are. They were the remains of a woman, and were carbon dated to between 24, and 26, years old. Five years later more remains, this time of a man, were found metres warning. It has not been possible to use carbon dating on these remains, but the initial publication by their discoverer, Jim Bowler, and Alan Thorne estimated the age to be 28, to 32, years on the basis of geomorphological criteria and stratigraphic association with the previous find.
That study was criticized in the professional literature on a variety of grounds. In Bowler presented a review of the dating attempts, including OSL data from 25 new samples. In retrospect, some of the controversy arose because various effects like contamination of the bones with recent carbon added systematic error to the dating results, and some arose because the age of related objects like the layer of sand in which the body was buried could not be directly transfered to the find itself.
The intensity of the controversy was doubtless fueled by the significance the older date would have on the field of human origins, particularly in relation to the theory that had been proposed by Thorne. The extreme value propagated by Thorne was in the end about three of his error bars removed from the value later accepted. The KBS tuff east of Lake Turkana in northern Kenya is a layer of volcanic material between sedimentary layers that contain fossils, including hundreds of hominid specimins, which makes the dating of the layer important for evolution.
Dating this tuff is "complicated",  however, because, as concluded from conventional geological analysis, it is composed of particles that have been transported by water and redeposited, so older material was mixed with new material when the layer was deposited. The question became critical in , when Richard Leakey discovered a hominin skull below the KBS tuff. Depending on the date assigned to the KBS tuff, this skull would either fit into the framework of human evolution held by anthropologists at that time, or it would require dramatic modifications of those ideas.
Creationists would even see the possibility of throwing the dating methods themselves or the entire theory of evolution into question. The controversy was settled by when three separate studies showed that the tuff was actually 1. The first attempt to date the material was published by F. Fitch and J. Miller in The authors rejected this age on the grounds that fossils found beneath the layer and therefore older were only considered to be around two to five million years old, and attributed the discrepancy to the admixture of older material.
They recommended that new samples be collected from which suitable individual crystals could be separated. In , another lab, using K-Ar dating, reported dates of 1. Finally, an independent expert, Ian McDougall, was called in to settle the issue. His measurements, published in the first half of the s decade, found about 1. During the 's, several researchers attempted to resolve the controversy by using other methods. One of these was paleomagnetism.
Unfortunately the magnetism was also consistent with the younger date, so that the application of this method was not able to settle the controversy. Another method, which had previously been applied only to rocks thought to be much older than the KBS tuff, was fission-track dating. The first measurement, published in ,  gave an age of 2. Four years later Gleadow published a paper  in which he pointed out problems with the previous methods, proposed new methodology to overcome these problems, and applied if to a KBS tuff sample, with the result being 1.
Finally, it was possible to correlate the KBS tuff to the H2 tuff in the Shungura Formation by analyzing trace elements in the minerals. The H2 tuff is uncontroversially dated about 1. More examples of anomalous dates can be found on the research page. Carbon dating is arguably the most accurate radiometric dating method, given its ability to be correlated with historically-known dates, but it is not entirely reliable.
The assistant editor of the Anthropological Journal of Canada marveled that so much use is made of it:. In the light of what is known about the radiocarbon method and the way it is used, it is truly astonishing that many authors will cite agreeable determinations as "proof" for their beliefs. Radiocarbon dating has somehow avoided collapse onto its own battered foundation, and now lurches onward with feigned consistency. The implications of pervasive contamination and ancient variations in carbon levels are steadfastly ignored by those who base their argument upon the dates.
boundary in which two plates slide past each other without creating or .. of a naturally occurring radioactive isotope and its decay products, using known decay Why is radiometric dating the most reliable method of dating the geologic past?. When a radioactive isotope decays, it shoots off some subatomic particles, either in the form of alpha particles, beta particles, electrons or.
This overview was written to help absolute beginners get started. Radioactive dating is the procedure of calculating an age for an artifact by determining how much of the radioactive material has decayed and calculating how long that would take given the half-life how long it takes for half the material to decay of the material being tested. Several dozen methods exist, using different radioactive isotopes and decay products, with varied dating ranges and precision. The procedure, however, is difficult, and many tests have shown that it can be inaccurate, and it is at times not even considered reliable by mainstream scientists. It is impossible to measure the age of something, except to time it as it actually occurs, so radioactive dating methods calculate the age, based on i measurements of quantities of specified materials, ii measurements of decay rates, and iii assumptions about the history of the sample.
A technician of the U. Geological Survey uses a mass spectrometer to determine the proportions of neodymium isotopes contained in a sample of igneous rock.
It is an accurate way to date specific geologic events. This is an enormous branch of geochemistry called Geochronology. There are many radiometric clocks and when applied to appropriate materials, the dating can be very accurate.
RADIOMETRIC TIME SCALE
Radiometric dating , radioactive dating or radioisotope dating is a technique used to date materials such as rocks or carbon , in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. Together with stratigraphic principles , radiometric dating methods are used in geochronology to establish the geologic time scale. By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change. Radiometric dating is also used to date archaeological materials, including ancient artifacts. Different methods of radiometric dating vary in the timescale over which they are accurate and the materials to which they can be applied.
Chapter 9: Geologic Time
Smith is known as the Father of English Geology. Oxford Library. Our understanding of the shape and pattern of the history of life depends on the accuracy of fossils and dating methods. Some critics, particularly religious fundamentalists, argue that neither fossils nor dating can be trusted, and that their interpretations are better. Other critics, perhaps more familiar with the data, question certain aspects of the quality of the fossil record and of its dating. These skeptics do not provide scientific evidence for their views. Current understanding of the history of life is probably close to the truth because it is based on repeated and careful testing and consideration of data. The rejection of the validity of fossils and of dating by religious fundamentalists creates a problem for them:. Fossil sequences were recognized and established in their broad outlines long before Charles Darwin had even thought of evolution.
Comparisons between the observed abundance of certain naturally occurring radioactive isotopes and their decay products, using known decay rates, can be used to measure timescales ranging from before the birth of the Earth to the present. For example measuring the ratio of stable and radioactive isotopes in meteorites can give us information on their history and provenance.
Chapter 9: Geologic Time Concept 5 Quiz.
RADIOMETRIC TIME SCALE
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Why is radiometric dating the most reliable method of dating the geological past?
Despite seeming like a relatively stable place, the Earth's surface has changed dramatically over the past 4. Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free. These changes typically occur so slowly that they are barely detectable over the span of a human life, yet even at this instant, the Earth's surface is moving and changing. As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils. A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved. However, by itself a fossil has little meaning unless it is placed within some context. The age of the fossil must be determined so it can be compared to other fossil species from the same time period.
.Creation v. Evolution: How Carbon Dating Works