How can radioactive dating help determine the absolute age of a rock (or rock layer)?

http://www2.lbl.gov/abc/wallchart/chapters/13/4.html

http://en.wikipedia.org/wiki/Radiometric_dating

thank you!

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Radioactive dating, also known as radiometric dating, is a method used to determine the absolute age of rocks or rock layers by measuring the abundance of certain radioactive isotopes and their decay products. Here's how it works:

1. Understanding radioactive decay: Certain elements exist in different forms, known as isotopes. Some isotopes are stable, while others are unstable, meaning they undergo radioactive decay over time. During decay, these unstable isotopes transform into a more stable form by releasing particles or energy.

2. Half-life: Each radioactive isotope has a specific half-life, which is the time it takes for half of the parent isotope to decay into the daughter isotope. The half-life can vary widely, from fractions of a second to billions of years.

3. Parent and daughter isotopes: When a rock forms, it may contain a certain amount of a radioactive parent isotope (e.g., uranium-238) and its corresponding daughter isotope (e.g., lead-206). By measuring the ratio of parent to daughter isotopes in a rock sample, scientists can determine how much decay has occurred and, consequently, how much time has passed since the rock formed.

4. Isotope dating methods: Several different isotopes and their decay products are used for dating rocks, depending on their half-lives and the age range being studied. For example, uranium-238 is commonly used for dating rocks that formed millions to billions of years ago, while carbon-14 is used for more recent events (up to about 50,000 years).

5. Measuring parent-daughter ratio: To determine the age of a rock sample, scientists measure the ratio of parent isotopes to daughter isotopes using various techniques. This can involve methods like spectrometry, mass spectrometry, or counting the radioactive emissions.

By comparing the measured ratio of parent to daughter isotopes with the known decay rate of the specific isotope, scientists can calculate the absolute age of the rock or rock layer. It's important to note that this method assumes the initial amount of parent isotope in the rock when it formed and that there has been no contamination or loss of isotopes since the rock's formation.

In summary, radioactive dating uses the decay of isotopes within rocks to determine their absolute age. By measuring the parent-daughter ratio and knowing the half-life of the radioactive isotope, scientists can calculate the time that has elapsed since the rock formed.