Geologists use radiometric dating to estimate how long ago rocks formed, and to infer the ages of fossils contained within those rocks. Radioactive elements decay The universe is full of naturally occurring radioactive elements. Radioactive atoms are inherently unstable; over time, radioactive "parent atoms" decay into stable "daughter atoms. When molten rock cools, forming what are called igneous rocks, radioactive atoms are trapped inside. Afterwards, they decay at a predictable rate. By measuring the quantity of unstable atoms left in a rock and comparing it to the quantity of stable daughter atoms in the rock, scientists can estimate the amount of time that has passed since that rock formed. Sedimentary rocks can be dated using radioactive carbon, but because carbon decays relatively quickly, this only works for rocks younger than about 50 thousand years.
Well, that exact thing has been done and the calculations turn out to be correct. We can measure the ratio of 14C to 12C, then use our equations to determine how old the log would have to be to have that ratio. Thus, we have radiometrically dated the log and therefore the earthquake that buried it.
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Depending upon how important it is to obtain a precisely correct date, corrections may or may not be made to the raw calculations. The amount of solar radiation varies with the year sunspot cycle. The amount of time it takes for 14C to percolate through the environment and food chain varies from one place to another, so the reservoir of 14C varies with time.
There may or may not be data available to make these calculations. You are commenting using your WordPress. You are commenting using your Google account. You are commenting using your Twitter account. You are commenting using your Facebook account. Notify me of new comments via email. Notify me of new posts via email. Skip to content Basics of Radiometric Dating December 9, This discussion is made in terms of 14C dating, but the same principles apply, regardless of which radioactive element is used.
We can work the other way, too. Share this: Tweet. Like this: Like Loading Leave a Reply Cancel reply Enter your comment here Fill in your details below or click an icon to log in:.
Email required Address never made public. Name required. When the number of electrons change, the shell structure changes too. So when an atom decays and changes into an atom of a different element, its shell structure changes and it behaves in a different way chemically.
How do these axioms translate into useful science? This section describes several common methods of radiometric dating. To start, let's look at the one which almost everyone has heard of: radiocarbon dating, AKA carbon dating or just carbon dating. Method 1: Carbon Dating. The element carbon occurs naturally in three nuclides: C12, C13, and C The vast majority of carbon atoms, about About one atom in billion is C The remainder are C Of the three, C12 and C13 are stable.
C14 is radioactive, with a half-life of years. C14 is also formed continuously from N14 nitrogen in the upper reaches of the atmosphere. And since carbon is an essential element in living organisms, C14 appears in all terrestrial landbound living organisms in the same proportions it appears in the atmosphere.
Plants and protists get C14 from the environment.
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Animals and fungi get C14 from the plant or animal tissue they eat for food. When an organism dies, it stops taking in C If we measure how much C14 there currently is, we can tell how much there was when the organism died, and therefore how much has decayed.
When we know how much has decayed, we know how old the sample is. Many archaeological sites have been dated by applying radiocarbon dating to samples of bone, wood, or cloth found there.
Radiocarbon dating depends on several assumptions. One is that the thing being dated is organic in origin. Radiocarbon dating does not work on anything inorganic, like rocks or fossils. Only things that once were alive and now are dead: bones, teeth, flesh, leaves, etc.
The second assumption is that the organism in question got its carbon from the atmosphere. A third is that the thing has remained closed to C14 since the organism from which it was created died.
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The fourth one is that we know what the concentration of atmospheric C14 was when the organism lived and died. That last one is more important than it sounds.
When Professor William Libby developed the C14 dating system inhe assumed that the amount of C14 in the atmosphere was a constant. A long series of studies of C14 content produced an equally long series of corrective factors that must be taken into account when using C14 dating.
So the dates derived from C14 decay had to be revised. One reference on radiometric dating lists an entire array of corrective factors for the change in atmospheric C14 over time. C14 dating serves as both an illustration of how useful radiometric dating can be, and of the pitfalls that can be found in untested assumptions. U and U are both nuclides of the element uranium. U is well known as the major fissionable nuclide of uranium.
I. Theory of radiometric dating. What is radiometric dating? Simply stated, radiometric dating is a way of determining the age of a sample of material using the decay rates of radio-active nuclides to provide a 'clock.' It relies on three basic rules, plus a couple of critical assumptions. Radiometric dating. Geologists use radiometric dating to estimate how long ago rocks formed, and to infer the ages of fossils contained within those rocks. Radioactive elements decay The universe is full of naturally occurring radioactive elements. Radioactive atoms are inherently unstable; over time, radioactive "parent atoms" decay into.
It has a half-life of roughly million years. U is more stable, with a half-life of 4. Th is the most common nuclide of the element thorium, and has a half-life of All three of these nuclides are the starting points for what are called radioactive series.
A radioactive series is a sequence of nuclides that form one from another by radioactive decay. The series for U looks like this:. A indicates alpha decay; B indicates beta decay. We can calculate the half-lives of all of these elements. All the intermediate nuclides between U and Pb are highly unstable, with short half-lives.
Then any excess of Pb must be the result of the decay of U When we know how much excess Pb there is, and we know the current quantity of U, we can calculate how long the U in our sample has been decaying, and therefore how long ago the rock formed. Th and U also give rise to radioactive series - different series from that of U, containing different nuclides and ending in different nuclides of lead. Chemists can apply similar techniques to all three, resulting in three different dates for the same rock sample.
Uranium and thorium have similar chemical behavior, so all three of these nuclides frequently occur in the same ores. If all three dates agree within the margin of error, the date can be accepted as confirmed beyond a reasonable doubt. Since all three of these nuclides have substantially different half-lives, for all three to agree indicates the technique being used is sound. But even so, radioactive-series dating could be open to question.
The rock being dated must remain a closed system with respect to uranium, thorium, and their daughter nuclides for the method to work properly. Both the uranium and thorium series include nuclides of radon, an inert gas that can migrate through rock fairly easily even in the few days it lasts.
To have a radiometric dating method that is unquestionably accurate, we need a radioactive nuclide for which we can get absolutely reliable measurements of the original quantity and the current quantity. Is there any such nuclide to be found in nature? The answer is yes.
Which brings us to the third method of radiometric dating. Method 3: Potassium-Argon Dating. The element potassium has three nuclides, K39, K40, and K Only K40 is radioactive; the other two are stable. K40 is unusual among radioactive nuclides in that it can break down two different ways. It can emit a beta particle to become Ca40 calciumor it can absorb an electron to become Ar40 argon Argon is a very special element. Argon is a gas at Earth-normal temperatures, and in any state it exists only as single atoms.
By contrast, potassium and calcium are two of the most active elements in nature. They both form compounds readily and hold onto other atoms tenaciously.
Radiometric or Absolute Rock Dating
What does this mean? It means that before a mineral crystallizes, argon can escape from it easily. It also means that when an atom of argon forms from an atom of potassium inside the mineral, the argon is trapped in the mineral. So any Ar40 we find deep inside a rock sample must be there as a result of K40 decay.
That and some simple calculations produce a figure for how long the K40 has been decaying in our rock sample. What happens if our mineral sample has not remained a closed system?
Dec 09, Basics of Radiometric Dating December 9, This discussion is made in terms of 14C dating, but the same principles apply, regardless of which radioactive element is used. The usual isotope of carbon is 12C. The atmosphere contains carbon dioxide (CO2). At high altitudes, solar radiation changes a certain amount of 12C to 14C. This. Jun 17, Radiometric Dating PART 1: Back to Basics. PART 2: Problems with the Assumptions. PART 3: Making Sense of the Patterns. This three-part series will help you properly understand radiometric dating, the assumptions that lead to inaccurate dates, and the clues about what really happened in the past. Radiometric dating is a method used to determine the age of rocks and other materials based on the rate of radioactive decay. Learn about three common types of radioactive decay: alpha decay, beta.
What if argon has escaped from the mineral? What if argon has found its way into the mineral from some other source? If some of the radiogenic argon has escaped, then more K40 must have decayed than we think - enough to produce what we did find plus what escaped.
In other words, a mineral that has lost argon will be older than the result we get says it is. In the other direction, if excess argon has gotten into the mineral, it will be younger than the result we get says it is. An isochron dating method isochron dating is described in the next section can also be applied to potassium-argon dating under certain very specific circumstances.
When isochron dating can be used, the result is a much more accurate date. Method 4: Rubidium-Strontium Dating.
Yet a fourth method, rubidium-strontium dating, is even better than potassium-argon dating for old rocks. The nuclide rubidium Rb87 decays to strontium Sr87 with a half-life of 47 billion years. Strontium occurs naturally as a mixture of several nuclides. If three minerals form at the same time in different regions of a magma chamber, they will have identical ratios of the different strontium nuclides.
The total amount of strontium might be different in the different minerals, but the ratios will be the same. Now, suppose that one mineral has a lot of Rb87, another has very little, and the third has an in-between amount. That means that when the minerals crystallize there is a fixed ratio of RbSr As time goes on, atoms of Rb87 decay to Sr, resulting in a change in the RbSr87 ratio, and also in a change in the ratio of Sr87 to other nuclides of strontium.
The decrease in the RbSr87 ratio is exactly matched by the gain of Sr87 in the strontium-nuclide ratio. It has to be - the two sides of the equation must balance.
If we plot the change in the two ratios for these three minerals, the resulting graph comes out as a straight line with an ascending slope.
This line is called an isochron. When every one of four or five different minerals from the same igneous formation matches the isochron perfectly, it can safely be said that the isochron is correct beyond a reasonable doubt.
There are numerous other radiometric dating methods: the samarium-neodymium, lutetium-hafnium, rhenium-osmium, and lead isochron methods just to name a few. A full cite for this book is given in the bibliography.
Now, why is all this relevant to the creation-vs. Every method of radiometric dating ever used points to an ancient age for the Earth. For creationists to destroy the old-Earth theory, they must destroy the credibility of radiometric dating. They have two ways to do this. They can criticize the science that radiometric dating is based on, or they can claim sloppy technique and experimental error in the laboratory analyses of radioactivity levels and nuclide ratios.
Basics of radiometric dating
Option 1: Criticize the Theory. Is there any way to criticize the theory of radiometric dating? Well, look back at the axioms of radiometric dating methods. Are any of those open to question. Answer: yes, two of them are.
Or at least, they seem to be. Do we know, for a fact, that half-lives are constant axiom 1? Do we know for a fact that nuclide ratios are constant axiom 2? However, if all we had were theoretical reasons for believing axiom 1, we would be right to be suspicious of it. Do we have observational evidence? On several occasions, astronomers have been able to analyze the radiation produced by supernovas.
In a supernova, the vast amount of energy released creates every known nuclide via atomic fusion and fission. Some of these nuclides are radioactive.
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We can also detect the characteristic radiation signatures of radioactive decay in those nuclides. We can use that information to calculate the half-lives of those nuclides. In every case where this has been done, the measured radiation intensity and the calculated half-life of the nuclide from the supernova matches extremely well with measurements of that nuclide made here on Earth.
And when we look at a supernova in the Andromeda Galaxy, 2, years old, we see nuclides with the exact same half-lives as we see here on Earth. Not just one or two nuclides, but many. For these measurements to all be consistently wrong in exactly the same way, most scientists feel, is beyond the realm of possibility. What about nuclide ratios? Are they indeed constant? The chemical behavior of an element depends on its size and the number of electrons in its outer shell.
This is the foundation of the periodic table of the elements, a basic part of chemistry that has stood without challenge for a hundred and fifty years. The shell structure depends only on the number of electrons the nuclide has, which is the same as the number of protons in its nucleus. K39 is chemically identical to K40; the only way we can distinguish between them is to use a nonchemical technique like mass spectrometry. Water molecules containing oxygen are lighter and therefore evaporate faster than water molecules with oxygen However, as far as is known such fractionation occurs only with light nuclides: oxygen, hydrogen, carbon.
Sr86 atoms and Sr87 atoms behave identically when they bond with other atoms to form a mineral molecule. If there are ten Sr86 atoms for every Sr87 atom in the original magma melt, there will be ten Sr86 atoms for every Sr87 atom in the minerals that crystallize from that melt.
Option 2: Criticize the Techniques. The only other possible source of error is in laboratory technique. To translate theory into useful measurements, the lab procedures must be accurate. A contaminated rock sample is useless for dating. A sample that is taken from the surface, where atoms could get in and out easily, is also useless.
Samples must be taken by coring, from deep within a rock mass. To date a rock, chemists must break it down into its component elements using any of several methods, then analyze nuclide ratios using a mass spectrometer.
But we can try to minimize error. And when we do, the dates produced can be accepted as accurate.