How does radioactive dating work?

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Offline Kryptid

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How does radioactive dating work?
« on: 29/03/2007 03:37:00 »
I understand how radioactive dating works, but something about it concerns me. Let me illustrate. If we have a rock and assume that it was 100% carbon-14 at formation, and we now measure it to be 25% carbon-14 and 75% nitrogen-14 (I know nitrogen is a gas, but bear with me), then we can calculate that the rock has been around long enough to pass through 2 half-lives (2 x 5,730 years = 11,460 years). That makes the rock 11,460 years old.


This measurement seems to hinge on the fact that we <i>know</i> that the rock was originally 100% carbon-14. If, in fact, the rock was 50% carbon-14 and 50% nitrogen-14 at its formation, then it would actually be only 5,730 years old (only half the originally calculated age). It goes without saying that this is a significant deviance.

So then, how do scientists know what the original composition of rocks were?
« Last Edit: 06/04/2008 15:56:15 by chris »
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Re: How does radioactive dating work?
« Reply #1 on: 29/03/2007 05:00:33 »
Firstly, we don't use carbon dating for measuring the age of rocks; carbon dating would only be used for the remains of living matter (e.g. wood that was once trees, or animal matter).  Other radioactive isotopes might be used for various other objects.

When we date something based of radioactive decay, we rely on some mecahanism that sets the ratio of isotopes to some predetermined value.  In the case of living organisms, there is a constant ratio of Carbon-14 to Carbon-12 in the environement, and this is because carbon-14 is constantly created anew by cosmic radiation impacting on nitrogen-14 (I can imagine there might be variations to take into account as historic cosmic ratiation levels would vary).  Once a living thing dies, it cannot take up any fresh carbon-14, so the carbon-14 that is contained within its body will decay, and will not be renewed.  How must carbon-14 is left can then be compared to how much carbon-14 would have been in the environement when the thing was living, and absorbing carbon from its environment.
Radiocarbon dating

One of the most widely used and well-known absolute dating techniques is carbon-14 (or radiocarbon) dating, which is used to date organic remains. This is a radiometric technique since it measures radioactive decay. Carbon-14 is an unstable isotope of normal carbon, carbon-12. Cosmic radiation entering the earth’s atmosphere produces carbon-14, and plants take in carbon-14 as they absorb carbon dioxide. Carbon-14 moves up the food chain as animals eat plants and as predators eat other animals. With death, the absorption of carbon-14 stops. This unstable isotope starts to break down into nitrogen-14. It takes 5,730 years for half the carbon-14 to change to nitrogen; this is the half-life of carbon-14. After another 5,730 years only one-quarter of the original carbon-14 will remain. After yet another 5,730 years only one-eighth will be left. By measuring the proportion of carbon-14 in organic material, scientists can determine an organic artifact's date of death.


Because the half-life of carbon-14 is short, the older a specimen is, the greater the margin of error becomes. About 40,000 years ago is the oldest the technique is reliable. Radiocarbon is also less useful for historic sites or recent sites. The standard margin of error is plus or minus 50 years. Because of this, the technique usually cannot pinpoint the date of a site better than historic records and previous knowledge of the site.

A further issue is known as the "old wood" problem. It is possible, particularly in dry, desert climates, for organic materials such as dead trees to remain in their natural state for hundreds of years before people use them as firewood, after which they become part of the archaeological record. Dating when that particular tree died does not necessarily indicate when the fire burned. This is also true of the heartwood of a tree, which will appear younger than the outer rings of the same tree because it has had less time to incorporate carbon-14 into its makeup. For this reason, many archaeologists prefer to use samples from short-lived plants (such as weeds or crops) for radiocarbon dates. The development of accelerator mass spectrometry (AMS) dating, which allows a date to be derived from a very small sample, has been very useful in this regard.

Potassium-argon dating

Other radiometric dating techniques are available for earlier periods. One of the most widely used is potassium-argon dating (K-Ar dating). Potassium-40 is a radioactive isotope of potassium that breaks down into argon-40, a gas. The half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14. With this method, the older the specimen, the more reliable the dating. Furthermore, whereas carbon-14 dating can be done only on organic remains, K-Ar dating can be used only for inorganic substances: rocks and minerals. As potassium-40 in rocks gradually breaks down into argon-40, the gas is trapped in the rock until the rock is heated intensely (as with volcanic activity), at which point it may escape. When the rock cools, the breakdown of potassium into argon resumes. Dating is done by reheating the rock and measuring the escaping gas. The date received from this test is for the last time that the object was heated. Common dates tested are the firing of ceramics (archaeology), and the setting of rocks (geology).


Thermoluminesence testing also dates items to the last time they were heated. This technique is based on the principle that all objects absorb radiation from the environment. This process frees electrons within minerals that remain caught within the item. Heating an item to 350 degrees Celsius or higher releases the trapped electrons, producing light. This light can be measured to determine the last time the item was heated.


Radiation levels do not remain constant over time. Fluctuating levels can skew results - for example, if an item went through several high radiation eras, thermoluminesence will return an older date for the item. Many factors can spoil the sample before testing as well, exposing the sample to heat or direct light may cause some of the electrons to dissipate, causing the item to date younger. Because of these and other factors, Thermoluminesence is at the most about 15% accurate. It cannot be used to accurately date a site on its own. However, it can be used to authenticate an item as antiquity.