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I couldn't find one. So I set out to write one. This article was the result. Radiometric dating methods are the strongest direct evidence that geologists have for the age of the Earth. All these methods point to Earth being very, very old - several billions of years old.

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. The fourth one is that we know what the concentration of atmospheric C14 was when the organism lived and died. The story of radiocarbon dating shows science at its finest. Presented with a new method that gave answers different than existing methods, the scientists involved did not simply assume that either the old method or the new one was wrong.

They viewed the problem as a challenge, dug into it with all their energy, and didn't stop until they understood exactly why their C14 dates disagreed with traditional dates, what was wrong with their C14 procedures, and how to compensate for the problems in the future.

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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. However, after a few years a number of scientists got suspicious of this assumption, because dates obtained by the C14 method weren't tallying with dates obtained by other means. 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 isotopes of the element uranium.

U is well known as the major fissionable isotope of uranium. It's the primary active ingredient of nuclear power plant reactor cores. It has a half-life of roughly million years. U is more stable, with a half-life of 4. Th is the most common isotope of the element thorium, and has a half-life of All three of these isotopes are the starting points for what are called radioactive series.

A radioactive series is a sequence of isotopes that form one from another by radioactive decay. We can calculate the half-lives of all of these elements. All the intermediate isotopes between U and Pb are highly unstable, with short half-lives.

That means they don't stay around very long, so we can take it as given that these isotopes don't appear on Earth today except as the result of uranium decay.

We can find out the normal distribution of lead isotopes by looking at a lead ore that doesn't contain any uranium, but that formed under the same conditions and from the same source as our uranium-bearing sample. 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 isotopes and ending in different isotopes 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 isotopes 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 isotopes 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. It's always possible that migration of isotopes or chemical changes in the rock could yield incorrect results.

The rock being dated must remain a closed system with respect to uranium, thorium, and their daughter isotopes for the method to work properly. Both the uranium and thorium series include isotopes 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 isotope for which we can get absolutely reliable measurements of the original quantity and the current quantity.

Is there any such isotope to be found in nature? The answer is yes. Which brings us to the third method of radiometric dating. The element potassium has three isotopes, K39, K40, and K Only K40 is radioactive; the other two are stable.

K40 is unusual among radioactive isotopes 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.

It's one of the group of elements called "noble gases" or "inert gases". Argon is a gas at Earth-normal temperatures, and in any state it exists only as single atoms. It doesn't form chemical compounds with any other element, not even the most active ones.

It's a fairly large atom, so it would have trouble slipping into a dense crystal's molecular structure. 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.

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What does this mean? It means that potassium can get into minerals quite easily, but argon can't. 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.

We know K40's half-life, and we know the probability of K40 decaying to Ar40 instead of Ca That and some simple calculations produce a figure for how long the K40 has been decaying in our rock sample. However, again it's important to remember that we're dealing with assumptionsand we always have to keep in mind that our assumptions may be wrong. What happens if our mineral sample has not remained a closed system?

What if argon has escaped from the mineral?

existence of life is derived largely from radiometric dating. These long time periods are computed by measuring the ratio of daughter to parent substance in a rock and inferring an age based on this ratio. This age is computed under the assumption that the parent substance (say, uranium) gradually decays to the daughter substance (say, lead). Radiometric dating In , shortly after the discovery of radioactivity, the American chemist Bertram Boltwood suggested that lead is one of the disintegration products of uranium, in which case the older a uranium-bearing mineral the greater should be its proportional part of lead. 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. The rules are the same in all cases; the assumptions are different for each method.

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.

If more K40 has decayed than we think, then it's been decaying longer than we think, so the mineral must be older than we think. 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. Yet a fourth method, rubidium-strontium dating, is even better than potassium-argon dating for old rocks.

The isotope rubidium Rb87 decays to strontium Sr87 with a half-life of 47 billion years. Strontium occurs naturally as a mixture of several isotopes. If three minerals form at the same time in different regions of a magma chamber, they will have identical ratios of the different strontium isotopes. Remember, chemical processes can't differentiate between isotopes.

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 isotopes of strontium.

The decrease in the RbSr87 ratio is exactly matched by the gain of Sr87 in the strontium-isotope 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. The line's slope then translates directly into a figure for the age of the rock that contains the different minerals. 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.

Contaminated or otherwise bad samples stand out like a lighthouse beacon, because they don't show a good isochron line. There are numerous other radiometric dating methods: the samarium-neodymium, lutetium-hafnium, rhenium-osmium, and lead isochron methods just to name a few. However, I simply haven't time or room to deal with all of them. 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 isotope ratios. 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 isotope ratios are constant axiom 2? Regarding the first question: There are sound theoretical reasons for accept-ing the constancy of isotope half-lives, but the reasons are based in the remote and esoteric reaches of quantum mechanics, and I don't intend to get into that in this article.

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 isotope via atomic fusion and fission. Some of these isotopes are radioactive. We can detect the presence of the various isotopes by spectrographic analysis of the supernova's radiation.

We can also detect the characteristic radiation signatures of radioactive decay in those isotopes. We can use that information to calculate the half-lives of those isotopes. In every case where this has been done, the measured radiation intensity and the calculated half-life of the isotope from the supernova matches extremely well with measurements of that isotope made here on Earth.

Now, because light travels at a fixed rate a bit underkilometers per secon and because stars are so far away, when we look at a distant star we're seeing it as it was when that light left it and headed this way. When we look at a star in the Andromeda Galaxy, 2, light-years away, we're seeing that star as it was 2, years ago. And when we look at a supernova in the Andromeda Galaxy, 2, years old, we see isotopes with the exact same half-lives as we see here on Earth.

Not just one or two isotopes, 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 isotope ratios? Are they indeed constant? Well, let's think about it: Minerals form by recognized chemical processes that depend on the chemical activity of the elements involved.

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 isotope has, which is the same as the number of protons in its nucleus. So the shell structure doesn't change between different isotopes of the same element. K39 is chemically identical to K40; the only way we can distinguish between them is to use a nonchemical technique like mass spectrometry.

Note: It's true that some natural processes favor some isotopes over others. 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 isotopes: oxygen, hydrogen, carbon. The atoms used in radiometric dating techniques are mainly heavy atoms, so we can still use the axiom that mineral-forming processes can't distinguish between different isotopes.

So the processes that are involved in mineral formation can't distinguish between isotopes. 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. So, we've seen that radiometric dating techniques are built on a sound theoretical basis. 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 isotope ratios using a mass spectrometer.

If the lab technique is sloppy, the date produced isn't reliable. There's no way to eliminate the possibility of error. It can't be done. Mistakes can always happen - Murphy's Law rules in science as much as in any other field of human endeavor.

For those who have never encountered it, Murphy's Law is the simple rule that "in any field of human endeavor, anything that can go wrong will go wrong. But we can try to minimize error. And when we do, the dates produced can be accepted as accurate. When samples taken from different parts of a given igneous rock formation are dated by different people at different labs over many years, the possibility that all those measurements could be wrong is vanishingly small.

Some may well be wrong. If nine analyses agree, and a tenth produces radically different results, the odd-man-out is usually considered a result of some kind of error and discarded. If the researcher doing the work can find and document a specific problem in analysis that could produce precisely that observed wrong result, then it's virtually certain the odd-man-out is an error.

And some radiometric techniques have a much better success ratio than that. In any case, while it's true that there are numerous possible sources of error, there is no source of error that could account for the enormous difference between the year age demanded by young-Earth creationists and the 3.

It's completely illogical to think that these techniques could be wrong by that much. Creationist objections to radiometric dating techniques basically fall into three categories:. We've already seen that this doesn't hold up under examination.

The assumptions that are used in radiometric dating techniques are perfectly justified given current physics. Creationist geologist John Woodmorappe is the best known of the creationists who attempt this approach.

He's compiled a list of several hundred radiometric dates that are widely divergent from the values accepted by conventional geology. Based on this, he claims that radiometric dating methods don't produce consistent results, that geologists conceal radiometric dates which don't match what's expected, and that therefore the whole methodology of radiometric dating is worthless. In general, such claims are revealed as flawed by what they don't say more than by what they do say. In an article for the creationist journal Creation Science Research QuarterlyWoodmorappe listed odd aberrant dates, and claimed that there are many, many more.

What he did not say is that those were winnowed out of tens of thousands of radiometric dates which do give more reasonable results. But if we run dating tests on 10, samples and get aberrant results 3. If we test fifty samples from the same rock formation and we get 2 ages of 1 million years, 2 ages of million years, and 46 ages all clustered around million years, it's not a great leap of logic to conclude that the 4 aberrant results were in error, and the 46 clustered results are probably correct.

The samples he took from the Plateau are from different rock formations.

For any type of radiometric dating to work properly, all samples must come from the same formation. So it's not surprising that Austin's results make no sense. We're still not done, though. If all we had was the radiometric techniques that I've described, there would remain a possibility a miniscule one, but a real possibility nonetheless that the entire idea is grievously wrong, that some as-yet-undetected factor is throwing off all the hundreds and thousands of radiometric dates that have been produced based on rock samples from all around Earth and even beyond it.

But we have more than that. All these methods point to Earth being very, very old - several billions of years old. Young-Earth creationists - that is, creationists who believe that Earth is no more than 10, years old - are fond of attacking radiometric dating methods as being full of inaccuracies and riddled with sources of error.

When I first became interested in the creation-evolution debate, in lateI looked around for sources that clearly and simply explained what radiometric dating is and why young-Earth creationists are driven to discredit it.

I found several good sources, but none that seemed both complete enough to stand alone and simple enough for a non-geologist to understand them. Thus this essay, which is my attempt at producing such a source. Theory of Radiometric Dating.

Common Methods of Radiometric Dating. Possible Sources of Error. Creationist Objections to Radiometric Dating. Independent Checks on Radiometric Dating.

Summary and Sources. 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. The rules are the same in all cases; the assumptions are different for each method. To explain those rules, I'll need to talk about some basic atomic physics.

There are 90 naturally occurring chemical elements. Elements are identified by their atomic numberthe number of protons in the atom's nucleus.

All atoms except the simplest, hydrogen- 1, have nuclei made up of protons and neutrons. Hydrogen-1's nucleus consists of only a single proton. Protons and neutrons together are called nucleonsmeaning particles that can appear in the atomic nucleus.

A nuclide of an element, also called an isotope of an element, is an atom of that element that has a specific number of nucleons.

Since all atoms of the same element have the same number of protons, different nuclides of an element differ in the number of neutrons they contain. For example, hydrogen-1 and hydrogen-2 are both nuclides of the element hydrogen, but hydrogen-1's nucleus contains only a proton, while hydrogen-2's nucleus contains a proton and a neutron.

Uranium contains 92 protons and neutrons, while uranium contains 92 protons and neutrons. Many nuclides are stable - they will always remain as they are unless some external force changes them. Some, however, are unstable - given time, they will spontaneously undergo one of the several kinds of radioactive decay, changing in the process into another element.

There are two common kinds of radioactive decay, alpha decay and beta decay. In alpha decay, the radioactive atom emits an alpha particle. An alpha particle contains two protons and two neutrons. After emission, it quickly picks up two electrons to balance the two protons, and becomes an electrically neutral helium-4 He4 atom. When a nuclide emits an alpha particle, its atomic number drops by 2, and its mass number number of nucleons drops by 4.

Thus, an atom of U uranium, atomic number 92 emits an alpha particle and becomes an atom of Th thorium, atomic number A beta particle is an electron.

When an atom emits a beta particle, a neutron inside the nucleus is transformed to a proton. The mass number doesn't change, but the atomic number goes up by 1. Thus, an atom of carbon C14atomic number 6, emits a beta particle and becomes an atom of nitrogen N14atomic number 7. A third, very rare type of radioactive decay is called electron absorption.

In electron absorption, a proton absorbs an electron to become a neutron. In other words, electron absorption is the exact reverse of beta decay. So an atom of potassium K40atomic number 19 can absorb an electron to become an atom of argon Ar40atomic number The half-life of a radioactive nuclide is defined as the time it takes half of a sample of the element to decay.

A mathematical formula can be used to calculate the half-life from the number of breakdowns per second in a sample of the nuclide. Some nuclides have very long half-lives, measured in billions or even trillions of years. Others have extremely short half-lives, measured in tenths or hundredths of a second. The decay rate and therefore the half-life are fixed characteristics of a nuclide.

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Different nuclides of the same element can have substantially different half-lives. The half-life is a purely statistical measurement. A sample of U ten thousand years old will have precisely the same half-life as one ten billion years old.

Obviously, the major question here is "how much of the nuclide was originally present in our sample? Such cases are useless for radiometric dating. We must know the original quantity of the parent nuclide in order to date our sample radiometrically. Fortunately, there are cases where we can do that. This is the second axiom of radiometric dating.

The third and final axiom is that when an atom undergoes radioactive decay, its internal structure and also its chemical behavior change. Losing or gaining atomic number puts the atom in a different row of the periodic table, and elements in different rows behave in different ways. It may not form the same kinds of compounds.

Why not? 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. 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.

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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.

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. 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.

Radiometric dating of granitic intrusions associated with the Caledonian orogeny yields ages between about million and million years. The igneous activity that produced such intrusions constituted the final stages of subduction and obduction (that is, overthrusting of the edge of one lithospheric plate. Simply stated, radiometric dating is a set of methods for determining the age of a sample of material using the decay rates of radioactive isotopes to provide a 'clock.' It relies on three basic rules, plus a couple of critical assumptions. The rules are the same in . Jan 23, Radiometric dating measures the decay of radioactive atoms to determine the age of a rock sample. It is founded on ufatgirlnmotion.comovable assumptions such as 1) there has been no contamination and 2) the decay rate has remained constant.

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.

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? 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.

# Radiometric dating plate tectonics

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.

Jan 01, One clue is the relative ages of the rocks on the Hawaiian Islands and Emperor Seamount chains. Using the standard radiometric dating technique that measures the radioactive decay of potassium to argon (the potassium-argon method), we learn that the . IIRC, radiometric dating showed that the further you get from Hawaii, the older the igneous rocks on these islands/submerged peaks, and the age is consistent with the measured movement of the tectonic plate. Describe methods radiometric dating to figure out since the theory that the results of plate tectonics was the geologic time scale reaching into deep time. Edu - 20 september was invented by dating. Screenshot of rocks revealed a beautiful theory of radiometric dating the earth history of plate tectonics is now well established.