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Radiometric Dating: How Does It Work?

Updated on January 28, 2013
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The discovery of radioactive decay in the early 20th century led to a number of advances in physics, medicine, and power generation. One area in which radioactive materials have been particularly useful is in geology.

Since radioactive elements have a measurable half-life - the period of time needed for an amount of the substance to decrease by half - they can be used to measure the ages of rocks containing them. Over the past century this technique, known as radiometric dating, has allowed geologists to precisely date rocks from the Earth, the Moon, Mars, and meteorites, allowing them to establish the age of the Earth and our Solar System. By dating the many rock strata containing ancient fossils, this technique has also allowed scientists to determine when different creatures lived on our planet and even how the continents were oriented at the time.

This article provides a brief introduction to the technique, using a specific example of a recently-dated formation to illustrate how radiometric dating is used to establish Earth's geologic history.

Common Dating Methods

(click column header to sort results)
Parent  
Daughter  
Half-Life  
U-235
Pb-207
700 million years
U-238
Pb-206
4.47 billion years
Rb-87
Sr-87/Sr-86
50 billion years
Sm-147
Nd-143
106 billion years
K-40
Ar-40
1.3 billion years
C-14
N-14
5,730 years
Microraptor gui fossil from the Yixian formation, Liaoning, China
Microraptor gui fossil from the Yixian formation, Liaoning, China | Source

Radiometric Dating Methods

Radioactive elements decay into daughter isotopes over time. By analyzing a rock sample in a mass spectrometer and measuring the ratio of mother to daughter isotopes in a rock, scientists can estimate how long ago the rock formed. Geologists use several different pairs of parent-daughter isotopes to establish the ages of rocks. When possible, multiple methods are used on the same rock or formation so that they will provide a check on each other. Although it is not possible to go back in time and measure the amount of parent isotope that was present in a rock, scientists can use the chemical properties of different elements to estimate the original ratio.

Uranium-Lead dating provides a good example of this. Zircon (zirconium silicate (ZrSiO4) is a common mineral in igneous rocks such as granite. When magma cools and crystallizes, zircon crystals formed will often contain trace amounts of uranium in place of zirconium. Lead, however, will be strongly rejected by the crystallizing zircon, as has been demonstrated in laboratory experiments.

As uranium decays over millions of years, it eventually turns into lead. Uranium-238 decays to lead-206 with a half-life of 4.47 billion years, and uranium-235 decays to lead-207 with a half-life of 704 million years. Measuring the proportion of lead to uranium thus provides a very accurate date when the rock solidified. Since there are two uranium isotopes and two lead isotopes with two different half lives, this method of dating also checks itself, providing two calculable ages for each sample.

Another method commonly used is Potassium-Argon dating. While potassium is a common element found in many rock minerals, argon is a noble gas that does not bind with other atoms. Potassium-40 decays to argon-40 with a half-life of 1.3 billion years. Since argon cannot form part of the lattice of minerals in igneous rocks and thus should not be present in them, argon-40 that is found in rocks forms there due to the decay of potassium-40. For rocks formed and preserved under ideal conditions the K-40/Ar-40 ratio will preserve a record of the rock's age.

However, not all rocks form and preserve under ideal conditions. Since argon is present in the atmosphere, tiny air bubbles left in a rock that cooled rapidly or formed from volcanic ash may preserve atmospheric argon, making the rock appear older than it is. On the other hand, extreme temperatures and pressures, such as during tectonic events, can fracture rocks and allow decay-product argon to escape, making the rock appear too young.

The argon-argon dating method, to be discussed in the next section, was created to correct for these errors. Instead of measuring the total amount of argon-40 and assuming that the rock started with zero argon content, this more complicated technique dates the rock using the ratio of different argon isotopes to each other.

Location of the Jiufotang Formation

A markerChaoyang, Liaoning, China -
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Illustration of the strata analyzed by He, et al. Argon-argon dating of volcanic ash deposits in Tuff L3002 and L3001 found them to be 120.3 million years old, give or take 700 thousand years.
Illustration of the strata analyzed by He, et al. Argon-argon dating of volcanic ash deposits in Tuff L3002 and L3001 found them to be 120.3 million years old, give or take 700 thousand years. | Source

An Example of Radiometric Dating

Northeastern China has become a fossil-hunting mecca in recent decades, with rich deposits yielding new species of fish, dinosaurs, amphibians, and early birds from the Tertiary Period. One of the richer deposits is the Jiufotang Formation near the city of Chaoyang, Liaoning Province, which has yielded fascinating fossils such as the four-winged dinosaur Microraptor gui and the pterosaur Saperonis .

Precisely dating these fossil finds had been difficult for many years, however, as there had not been much known about the rocks containing them. One piece of this information gap was filled by a study published in 2004 by a team led by Dr. Huaiyu He of the Institute of Geology and Geophysics, Chinese Academy of Sciences. Dr. He and her team conducted argon-argon and uranium-lead radiometric dating tests on a sample of tuff - rock formed from volcanic ash - in order to establish a minimum age for the fossil-bearing layers below.

The sample He's team used came from the lower of two layers of volcanic rock, named Tuff L3001. Once the sample, named Lx9, was taken to the laboratory at the Institute of Geology and Geophysics, it was crushed into small pieces. The purest crystals of K-feldspar, a potassium-bearing mineral, were then hand-selected, cleaned, and heated in a vacuum chamber to remove as much atmospheric gas as possible.

Sample Lx9 was then irradiated with neutrons in a nuclear reactor. This procedure causes potassium-39, a normally stable isotope, to decay into argon-39. Finally, the sample was placed in a mass spectrometer and heated with a laser to higher and higher temperatures, causing the crystal to break and the argon gas to be released. Based on the ratio of argon-40 to argon-39, the age of the sample was determined to be 120.3 million years old.

In order to confirm this date, zircons were selected from the same piece of rock and analyzed using uranium-lead dating. This method yielded a slightly older date - 124 million years, but with a four-million-year margin of error. The dating of this volcanic formation at 120 million years placed a minimum age of 120 million years for all of the fossils beneath it in the Jiufotang Formation. It also places a minimum age of 120 million years on fossils of similar types found elsewhere in the region.

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The Limitations of Radiometric Dating

Radiometric dating is not a perfect method of measurement. All dating techniques have an inherent margin of error, and even the most precise measurements are only estimates. And even though many dating techniques have self-correcting mechanisms, their results can be skewed by metamorphic processes or contamination that changed the chemistry of the rocks being studied.

Despite these limitations, careful sample selection and advances in technology have vastly improved the accuracy of radiometric dating over the past century. Radiometric dating techniques have given geologists, paleontologists, archaeologists and other scientists studying the past a very useful timeline of the history of our planet.

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    • scottcgruber profile image
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      scottcgruber 4 years ago from USA

      No, it's really not a flawed premise. Under normal circumstances, radioactive isotopes decay at a steady rate. The fact that we can alter atomic nuclei in particle accelerators and fission reactors doesn't put all of radiometric dating in doubt. The dinosaurs, as far as we know, did not have the bomb.

      Now, there is one known site of a natural fission reactor in Oklo, Gabon. About 1 billion years ago, this site had just the right mix of uranium and groundwater to sustain a fission reaction for about 100,000 years. The evidence of this is written into the rock as deposits of fission by-products. This particular rock strata would obviously not be a good place to use uranium-lead dating. However, since it is the only known example of its kind, it does not put all of radiometric dating or the known rates of radioactive decay in jeopardy.

    • profile image

      Disagrees 4 years ago

      If you hit K-39 and K-40 with fast neutrons, you get Ar-39 and Ar-40 very quickly. The idea of a fixed life cycle for isotopes has a totally flawed premise.

    • scottcgruber profile image
      Author

      scottcgruber 4 years ago from USA

      No, Zuma. Radioactive decay rates are not subjective. They are quite objective, determined by measuring the amount of a radioactive material that decays to its stable daughter isotope over time. You don't need to observe it for four billion years. A few minutes is plenty to establish the rate of decay.

      Here's an (admittedly flawed) analogy. Let's say a police officer pulls you over for driving 75 miles per hour on a highway with a 55 mile per hour speed limit. By your logic, she cannot possibly give you a ticket because she did not watch you for a full hour to see if you actually travelled 75 miles. Now, you could try to argue this in court, but I wouldn't recommend it.

      If you're intelligent, you've already seen the flaw in this analogy. A car can speed up and slow down as commanded by a driver. Radioactive materials do not work the same way.

      Radioactivity is a random, probabilistic process. It has been confirmed by experiment to be unaffected by temperature, pressure, chemical reaction, or other natural processes. The decay of any individual unstable nucleus is random.

      If you were to take one atom of U-238 and observe it for 4.47 billion years, the chance that it would decay to lead is 1/2. It might take a day to decay, or it might take 9 billion years. However, if you take several billion billion atoms of U-238 - the number of atoms in one kilogram of uranium - some will decay soon, most others will not. You should still get an average rate of about 12 million atoms decaying per second. If you repeat this exercise a few times, you'll have enough data to extrapolate the time needed for half the sample to decay.

      You seem to be suggesting that Marie Curie, Ernest Rutherford, Henri Bequerel, and others simply made up the decay rates of radioactive isotopes. That is a pretty serious accusation of scientific fraud. I hope you have some strong evidence to back it up, for your sake.

    • profile image

      zuma 4 years ago

      a)Using coolness of the earth to compute its age with the presumption that the earth would be in molten state:

      Using coolness to compute the age of the earth might not be reliable for the fact that its computation has presumed this earth could be in molten state or in other words, it could be in liquid form.

      However, the initial stage of earth could be either in solid state that would be fully or partially covered with or without water. The water might be either warm or cold and that I do not like. If the earth would be in solid state that would cover with or without water, it would not take much time for the earth to cool down. Thus, the computation of the age of the earth by means of its coolness would not be feasible since the earth might be in solid state cover with water.

      b)Benoit de Maillet (1656-1738), a French anthropologist and diplomat, measured declining sea level and arrived the conclusion that the earth would be 2 billion years.

      His computation would not be feasible since sea level could rise as shown in the website address:

      http://www.reuters.com/article/2012/07/01/us-clima...

      The rise of sea level has caused his computation of the age of the earth to be unreliable.

      c)Radioactive dating method has been used to test the same stratum of rock and yet the same results (within the margin of error) would produce. The reason to explain this is simple. Using the same isotope to test on the same stratum of rock would produce almost the same result due to the same rock would produce the same unstable atomic nucleus of ionizing particles and electromagnetic radiation in spite of its spontaneous emission.

      The following is the list of some isotopes that are used for dating:

      Parent daughter half-life

      Samarium-147 Neodymium-143 106 billion years

      Rubidium-87 Strontium-87 50 billion years

      Uranium-238 Lead-206 4.47 billion years

      Potassium-40 Argon-40 1.3 billion years

      Uranium-235 Lead-207 704 million years

      Uranium-234 Thorium-230 80,000 years

      Carbon-14 Nitrogen-14 5,730 years

      Question has to be raised. If all the materials as mentioned above would have been created ever since the beginning of this earth, how could the scientists compute the half life of decay rate for Lead-206 from Uranium-238 to be 4.47 billion years? The reason is simply that the half life of decay rate for, let’s say, Lead-206 from Uranium-238, should be 0 if they would have been created at the same time in the very beginning. As the decay could be 0 if these materials would have been created in the very beginning, how could the Scientists be sure of its reliability and to use it to compute the age of the earth to be billion years?

      Besides, even if one material could be the transformation from another, how do the Scientists compute the figure of half life decay rate? For instance, how could the Scientist get 4.47 billion years or not 4 thousand years or others for Uranium-238 to decay to Lead-206? This half year decay rate that has been established by Scientists has pushed the age of the earth and even fossils, i.e. dinosaurs, to billion years. Whenever they use this isotope to test a rock to guess its age, it would give them billion of years since the decay rate has already set by them in the first place to push up to billion years.

      Thus, radioactive dating method is rather subjective and not accurate since the half year decay rate is indeed questionable.

    • scottcgruber profile image
      Author

      scottcgruber 5 years ago from USA

      Interesting stuff, isn't it? I've always been curious about the process and decided to research it a bit. It turned out to be even cooler than I thought - they shoot rocks with lasers.

      Lasers!

    • melbel profile image

      Melanie Shebel 5 years ago from New Buffalo, Michigan

      Wow! I LOVE science-y hubs! I didn't know all of this stuff about radiometric dating. I'd like to watch a group of scientists (or actually participate) do this!