Absolute Time 2

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Jul 30, 2001

Now that we have an understanding of how radiometric dating works (see my previous article, Absolute Time 1) let's look at the types of rocks radiometric dating can be applied to, and some of the potential problems that radiometric dating can give.

Let's look at the chart of radioactive isotopes and their half-lives. Four of the isotopes, Potassium (K), Uranium (U), Thorium (Th), and Rubidium (Rb), are generally only found in rocks that have an igneous or metamorphic origin. The isotopes are used to date when the rock or mineral being tested became a closed system. A closed system is the point at which the rock or mineral is sealed so that no parent or daughter isotopes can enter or leave the mineral or rock. Once the system is closed, the parent isotopes begin to decay, slowly changing over time to the new daughter isotopes. Therefore, when we date the rocks and check the ratios of daughter to parent isotopes, we are actually dating the time when the rock or mineral became a closed system. For some rocks, like lava flows, that is generally the time when the rock solidified, but on some later igneous intrusions, the date may not yield the age of the intrusion, but the age when the mineral cooled below the temperature when closure occurred.

Carbon (C), is only associated with organic material. Radiocarbon dating is similar to other radiometric dating. All living organisms incorporate C-12 and C-14 in their tissues. The ratio of C-12 and C-14 is equal to the same ratios found in the atmosphere. When the organism dies, it stops building new tissue and the C-14 begins to decay into C-12. Geologists can then measure the amount of C-14 and C-12 to determine the age of the sample. Carbon-14's short half-life means that it is only reliable as a tool to date samples as far back as 70,000 years, after that there is generally not enough carbon-14 isotopes left in the sample to be dated. For a more detailed account of how carbon dating works, check out the information at C14dating.com.

From the above associations we can see that already the geologist is limited in the material that can be dated. Nearly all sedimentary rocks, which are the most numerous of rock types, are not able to be dated using any of the radiometric tools. This means that a large portion of the geological record is unavailable to be dated.