K40 can decay in two different ways: The ratio of calcium formed to argon formed is fixed and known. Therefore the amount of argon formed provides a direct measurement of the amount of potassium present in the specimen when it was originally formed. Because argon is an inert gas , it is not possible that it might have been in the mineral when it was first formed from molten magma.
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Any argon present in a mineral containing potassium must have been formed as the result of radioactive decay. F, the fraction of K40 remaining, is equal to the amount of potassium in the sample, divided by the sum of potassium in the sample plus the calculated amount of potassium required to produce the amount of argon found. The age can then be calculated from equation 1. In spite of the fact that it is a gas, the argon is trapped in the mineral and can't escape. Creationists claim that argon escape renders age determinations invalid.
However, any escaping argon gas would lead to a determined age younger, not older, than actual.
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The creationist "argon escape" theory does not support their young earth model. The argon age determination of the mineral can be confirmed by measuring the loss of potassium. In old rocks, there will be less potassium present than was required to form the mineral, because some of it has been transmuted to argon. The decrease in the amount of potassium required to form the original mineral has consistently confirmed the age as determined by the amount of argon formed.
See Carbon 14 Dating in this web site. The nuclide rubidium decays, with a half life of Strontium is a stable element; it does not undergo further radioactive decay. Do not confuse with the highly radioactive isotope, strontium Strontium occurs naturally as a mixture of several nuclides, including the stable isotope strontium If three different strontium-containing minerals form at the same time in the same magma, each strontium containing mineral will have the same ratios of the different strontium nuclides, since all strontium nuclides behave the same chemically.
Note that this does not mean that the ratios are the same everywhere on earth. It merely means that the ratios are the same in the particular magma from which the test sample was later taken. As strontium forms, its ratio to strontium will increase. Strontium is a stable element that does not undergo radioactive change. In addition, it is not formed as the result of a radioactive decay process. The amount of strontium in a given mineral sample will not change. Therefore the relative amounts of rubidium and strontium can be determined by expressing their ratios to strontium It turns out to be a straight line with a slope of The corresponding half lives for each plotted point are marked on the line and identified.
It can be readily seen from the plots that when this procedure is followed with different amounts of Rb87 in different minerals , if the plotted half life points are connected, a straight line going through the origin is produced. These lines are called "isochrons".
The steeper the slope of the isochron, the more half lives it represents. When the fraction of rubidium is plotted against the fraction of strontium for a number of different minerals from the same magma an isochron is obtained.
If the points lie on a straight line, this indicates that the data is consistent and probably accurate. An example of this can be found in Strahler, Fig If the strontium isotope was not present in the mineral at the time it was formed from the molten magma, then the geometry of the plotted isochron lines requires that they all intersect the origin, as shown in figure However, if strontium 87 was present in the mineral when it was first formed from molten magma, that amount will be shown by an intercept of the isochron lines on the y-axis, as shown in Fig Thus it is possible to correct for strontium initially present.
The age of the sample can be obtained by choosing the origin at the y intercept. Note that the amounts of rubidium 87 and strontium 87 are given as ratios to an inert isotope, strontium However, in calculating the ratio of Rb87 to Sr87, we can use a simple analytical geometry solution to the plotted data. Again referring to Fig. Since the half-life of Rb87 is When properly carried out, radioactive dating test procedures have shown consistent and close agreement among the various methods.
If the same result is obtained sample after sample, using different test procedures based on different decay sequences, and carried out by different laboratories, that is a pretty good indication that the age determinations are accurate. Of course, test procedures, like anything else, can be screwed up. Mistakes can be made at the time a procedure is first being developed. Creationists seize upon any isolated reports of improperly run tests and try to categorize them as representing general shortcomings of the test procedure.
This like saying if my watch isn't running, then all watches are useless for keeping time. What teachers are saying about Study.
Are you still watching? Your next lesson will play in 10 seconds. Add to Add to Add to. Want to watch this again later? Principles of Radiometric Dating. What is Carbon Dating? What is Relative Age? What is Relative Dating?
Absolute Time in Geology. Relative Dating with Fossils: Index Fossils as Indicators of Time. Methods of Geological Dating: Numerical and Relative Dating. Major Eons, Eras, Periods and Epochs. Prentice Hall Earth Science: Holt McDougal Earth Science: ScienceFusion Matter and Energy: Discover how scientists determine the age of fossils, rocks, and other geologic phenomena by using the known half-lives of isotopes within each specimen, a technique known as radioactive dating.
Radioactive Dating Ever wonder how scientists concluded the age of the earth to be about 4. Radioactivity Defined Elements occur naturally in the earth, and they can tell us a lot about its past. The Half-Life Isotopes decay at a constant rate known as the half-life. Try it risk-free No obligation, cancel anytime. Want to learn more? Select a subject to preview related courses: Radiocarbon Dating Since all living things contain carbon, carbon is a common radioisotope used primarily to date items that were once living.
Lesson Summary So, to sum this all up, radioactive dating is the process scientists use to conclude the ages of substances dating back several to many years ago by using the isotopes of elements and their half-lives. Unlock Your Education See for yourself why 30 million people use Study. Become a Member Already a member?
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What is Radioactive Dating? - Definition & Facts
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Radiometric Dating: Methods, Uses & the Significance of Half-Life
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