ASBOG Practice Exam

Uranium-thorium dating is typically used for which of the following?

1. Igneous materials older than 1 million years

2. Carbonate materials younger than about 500,000 years

3. Metamorphic rocks

4. Organic samples older than 60,000 years

The correct answer is: Carbonate materials younger than about 500,000 years

Uranium-thorium dating is particularly effective for dating calcium carbonate materials, such as stalactites and stalagmites found in caves or corals, which typically form in environments that can be younger than 500,000 years. This method takes advantage of the radioactive decay of uranium into thorium, which occurs over a relatively short time scale compared to other radiometric dating techniques. Since thorium does not remain in solution and precipitates along with calcium carbonate, this provides a reliable chronometer for dating the formation of these deposits. In contrast, while igneous materials, metamorphic rocks, and organic samples do have dating methods available, they are less ideal candidates for uranium-thorium dating due to their respective characteristics and the timescales involved. For example, igneous rocks may take longer to crystallize, and organic samples may not be suitable if they are younger than the threshold of uranium-thorium dating or if they contain carbon that complicates dating through this method. Thus, the preference for carbonate materials of this age range makes choice B the most appropriate answer.