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Radioactive Dating

Home earth Earth History Geologist Radioactive. Read about How do we know the Age of the Earth? Radiometric dating using the naturally-occurring radioactive elements is simple in concept even though technically complex. If we know the number of radioactive parent atoms present when a rock formed and the number present now, we can calculate the age of the rock using the decay constant. The number of parent atoms originally present is simply the number present now plus the number of daughter atoms formed by the decay, both of which are quantities that can be measured. Samples for dating are selected carefully to avoid those that are altered, contaminated, or disturbed by later heating or chemical events. In addition to the ages of Earth, Moon, and meteorites, radiometric dating has been used to determine ages of fossils, including early man, timing of glaciations, ages of mineral deposits, recurrence rates of earthquakes and volcanic eruptions, the history of reversals of Earth's magnetic field, and the age and duration of a wide variety of other geological events and processes.

The fossils, when combined with geologic mapping, allow the various exposed sections of the Pierre Shale to be pieced together in their proper relative positions to form a complete composite section Figure 1.

The Pierre Shale also contains volcanic ash that was erupted from volcanoes and then fell into the sea, where it was preserved as thin beds. There are three important things to note about these results.

First, each age is based on numerous measurements; laboratory errors, had there been any, would be readily apparent. Second, ages were measured on two very different minerals, sanidine and biotite, from several of the ash beds.

Third, the radiometric ages agree, within analytical error, with the relative positions of the dated ash beds as determined by the geologic mapping and the fossil assemblages; that is, the ages get older from top to bottom as they should.

Finally, the inferred age of the shocked quartz, as determined from the age of the melted feldspar in the Manson impact structure Meteorites, most of which are fragments of asteroids, are very interesting objects to study because they provide important evidence about the age, composition, and history of the early solar system. There are many types of meteorites. Some are from primitive asteroids whose material is little modified since they formed from the early solar nebula.

Others are from larger asteroids that got hot enough to melt and send lava flows to the surface. A few are even from the Moon and Mars. The most primitive type of meteorites are called chondrites, because they contain little spheres of olivine crystals known as chondrules. Because of their importance, meteorites have been extensively dated radiometrically; the vast majority appear to be 4.

Some meteorites, because of their mineralogy, can be dated by more than one radiometric dating technique, which provides scientists with a powerful check of the validity of the results. The results from three meteorites are shown in Table 1. Many more, plus a discussion of the different types of meteorites and their origins, can be found in Dalrymple There are 3 important things to know about the ages in Table 1.

The first is that each meteorite was dated by more than one laboratory Allende by 2 laboratories, Guarena by 2 laboratories, and St Severin by four laboratories.

This pretty much eliminates any significant laboratory biases or any major analytical mistakes. The second thing is that some of the results have been repeated using the same technique, which is another check against analytical errors. The third is that all three meteorites were dated by more than one method two methods each for Allende and Guarena, and four methods for St Severin. This is extremely powerful verification of the validity of both the theory and practice of radiometric dating.

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 isotopes , each 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. Is this a remarkable coincidence? Scientists have concluded that it is not; it is instead a consequence of the fact that radiometric dating actually works and works quite well. Creationists who wants to dispute the conclusion that primitive meteorites, and therefore the solar system, are about 4.

One of the most exciting and important scientific findings in decades was the discovery that a large asteroid, about 10 kilometers diameter, struck the earth at the end of the Cretaceous Period. The collision threw many tons of debris into the atmosphere and possibly led to the extinction of the dinosaurs and many other life forms. The fallout from this enormous impact, including shocked quartz and high concentrations of the element iridium, has been found in sedimentary rocks at more than locations worldwide at the precise stratigraphic location of the Cretaceous-Tertiary K-T boundary Alvarez and Asaro ; Alvarez We now know that the impact site is located on the Yucatan Peninsula.

Measuring the age of this impact event independently of the stratigraphic evidence is an obvious test for radiometric methods, and a number of scientists in laboratories around the world set to work.

In addition to shocked quartz grains and high concentrations of iridium, the K-T impact produced tektites, which are small glass spherules that form from rock that is instantaneously melted by a large impact. The K-T tektites were ejected into the atmosphere and deposited some distance away.

Tektites are easily recognizable and form in no other way, so the discovery of a sedimentary bed the Beloc Formation in Haiti that contained tektites and that, from fossil evidence, coincided with the K-T boundary provided an obvious candidate for dating.

How Does Radiocarbon Dating Work? - Instant Egghead #28

Scientists from the US Geological Survey were the first to obtain radiometric ages for the tektites and laboratories in Berkeley, Stanford, Canada, and France soon followed suit. The results from all of the laboratories were remarkably consistent with the measured ages ranging only from Similar tektites were also found in Mexico, and the Berkeley lab found that they were the same age as the Haiti tektites. The K-T boundary is recorded in numerous sedimentary beds around the world.

Numerous thin beds of volcanic ash occur within these coals just centimeters above the K-T boundary, and some of these ash beds contain minerals that can be dated radiometrically. Since both the ash beds and the tektites occur either at or very near the K-T boundary, as determined by diagnostic fossils, the tektites and the ash beds should be very nearly the same age, and they are Table 2.

There are several important things to note about these results. First, the Cretaceous and Tertiary periods were defined by geologists in the early s. The boundary between these periods the K-T boundary is marked by an abrupt change in fossils found in sedimentary rocks worldwide. Its exact location in the stratigraphic column at any locality has nothing to do with radiometric dating it is located by careful study of the fossils and the rocks that contain them, and nothing more.

Furthermore, the dating was done in 6 different laboratories and the materials were collected from 5 different locations in the Western Hemisphere. And yet the results are the same within analytical error.

In the early afternoon of August 24, 79 CE, Mt Vesuvius erupted violently, sending hot ash flows speeding down its flanks. These flows buried and destroyed Pompeii and other nearby Roman cities. We know the exact day of this eruption because Pliny the Younger carefully recorded the event.

They separated sanidine crystals from a sample of one of the ash flows. Incremental heating experiments on 12 samples of sanidine yielded 46 data points that resulted in an isochron age of 94 years. The actual age of the flow in was years. Is this just a coincidence? No it is the result of extremely careful analyses using a technique that works. This is not the only dating study to be done on an historic lava flow. Two extensive studies done more than 25 years ago involved analyzing the isotopic composition of argon in such flows to determine if the source of the argon was atmospheric, as must be assumed in K-Ar dating Dalrymple , 26 flows; Krummenacher , 19 flows.

Radiometric dating how does it work

Note, however, that even an error of 0. In this short paper I have briefly described 4 examples of radiometric dating studies where there is both internal and independent evidence that the results have yielded valid ages for significant geologic events.

It is these studies, and the many more like them documented in the scientific literature, that the creationists need to address before they can discredit radiometric dating. Their odds of success are near zero. Even if against all odds they should succeed, it still would not prove that the Earth is young. Only when young-earth creationists produce convincing quantitative, scientific evidence that the earth is young will they be worth listening to on this important scientific matter.

I thank Chris Stassen and 2 anonymous reviewers for their thoughtful comments, which led to important improvements in the manuscript. Excess argon within mineral concentrates from the new dacite lava dome at Mount St Helens volcano. While the lighter isotopes 12 C and 13 C are stable, the heaviest isotope 14 C radiocarbon is radioactive.

This means its nucleus is so large that it is unstable. Over time 14 C decays to nitrogen 14 N. Most 14 C is produced in the upper atmosphere where neutrons, which are produced by cosmic rays , react with 14 N atoms. This CO 2 is used in photosynthesis by plants, and from here is passed through the food chain see figure 1, below. Every plant and animal in this chain including us! When living things die, tissue is no longer being replaced and the radioactive decay of 14 C becomes apparent.

Around 55, years later, so much 14 C has decayed that what remains can no longer be measured. In 5, years half of the 14 C in a sample will decay see figure 1, below.

Therefore, if we know the 14 C: Unfortunately, neither are straightforward to determine. The amount of 14 C in the atmosphere, and therefore in plants and animals, has not always been constant.

Radiometric dating

For instance, the amount varies according to how many cosmic rays reach Earth. Luckily, we can measure these fluctuations in samples that are dated by other methods. Tree rings can be counted and their radiocarbon content measured. A huge amount of work is currently underway to extend and improve the calibration curve. In we could only calibrate radiocarbon dates until 26, years. Now the curve extends tentatively to 50, years. Radiocarbon dates are presented in two ways because of this complication.

The uncalibrated date is given with the unit BP radiocarbon years before The calibrated date is also presented, either in BC or AD or with the unit calBP calibrated before present - before The second difficulty arises from the extremely low abundance of 14 C. Many labs now use an Accelerator Mass Spectrometer AMS , a machine that can detect and measure the presence of different isotopes, to count the individual 14 C atoms in a sample. Australia has two machines dedicated to radiocarbon analysis, and they are out of reach for much of the developing world.

In addition, samples need to be thoroughly cleaned to remove carbon contamination from glues and soil before dating.

This is particularly important for very old samples. Because of this, radiocarbon chemists are continually developing new methods to more effectively clean materials. These new techniques can have a dramatic effect on chronologies. With the development of a new method of cleaning charcoal called ABOx-SC , Michael Bird helped to push back the date of arrival of the first humans in Australia by more than 10, years.

Moving away from techniques, the most exciting thing about radiocarbon is what it reveals about our past and the world we live in. Radiocarbon dating was the first method that allowed archaeologists to place what they found in chronological order without the need for written records or coins. In the 19th and early 20th century incredibly patient and careful archaeologists would link pottery and stone tools in different geographical areas by similarities in shape and patterning.

Then, by using the idea that the styles of objects evolve, becoming increasing elaborate over time, they could place them in order relative to each other - a technique called seriation. In this way large domed tombs known as tholos or beehive tombs in Greece were thought to predate similar structures in the Scottish Island of Maeshowe.


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