15.12: Radiometric Dating
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Can wood give an age in another way?
In the section on tree ring dating, there was a photo of a ruin at Mesa Verde National Park. This photo is also from Mesa Verde. In archeological sites, wood can be dated by tree rings. It can also be dated by radiometric dating. Carbon-14 dating is very useful for ruins that contain wood. If the tree died around the time the ladder was created, then carbon-14 can tell the age of the ladder. If the ladder was built for the site, then that age will be the age of the archeological site.
The rate of decay of unstable isotopes can be used to estimate the absolute ages of fossils and rocks. This type of dating is called radiometric dating.
The best-known method of radiometric dating is carbon-14 dating. This method is also called radiocarbon dating. A living thing takes in carbon-14 (along with stable carbon-12). As the carbon-14 decays, it is replaced with more carbon-14. After the organism dies, it stops taking in carbon. That includes carbon-14. The carbon-14 that is in its body continues to decay. So the organism contains less and less carbon-14 as time goes on. We can estimate the amount of carbon-14 that has decayed by measuring the amount of carbon-14 to carbon-12. We know how fast carbon-14 decays. With this information, we can tell how long ago the organism died.
Carbon-14 has a relatively short half-life. It decays quickly compared to some other unstable isotopes. So carbon-14 dating is useful for specimens younger than 50,000 years old. That’s a blink of an eye in geologic time. But radiocarbon dating is very useful for more recent events. One important use of radiocarbon is early human sites. Carbon-14 dating is also limited to the remains of once-living things. To date rocks, scientists use other radioactive isotopes.
Other Radioactive Isotopes
The isotopes listed below (Table below) are used to date igneous rocks. These isotopes have much longer half-lives than carbon-14. Because they decay more slowly, they can be used to date much older specimens. Which of these isotopes could be used to date a rock that formed half a million years ago? How about half a billion years ago?
|Unstable Isotope||Decays to||At a Half-Life of (years)||Dates Rocks Aged (years old)|
|Potassium-40||Argon-40||1.3 billion||100 thousand – 1 billion|
|Uranium-235||Lead-207||700 million||1 million – 4.5 billion|
|Uranium-238||Lead-206||4.5 billion||1 million – 4.5 billion|
Limitations of Radiometric Dating
Radiometric dating is a very useful tool, but it does have limits:
- The material being dated must have measurable amounts of the parent and/or the daughter isotopes.
- Radiometric dating can be done on only some materials. It is not useful for determining the age of sedimentary rocks. For this, geologists date a nearby igneous rock. Then they use relative dating techniques to figure out the age of the sedimentary rock. They may not get it exactly, but there will be some idea.
- Radiocarbon is useful for relatively young, carbon-based materials. Other longer-lived isotopes are good for older rocks and minerals.
- Different isotope pairs are useful for certain materials of certain ages.
- Radiometric dating cannot be used if parent or daughter are not measurable.
- How would you determine which isotope pair to use to date a certain rock?
- How does radiocarbon dating work? What materials does it work best on?
- Which types of rocks are best for radiometric dating? Which are not good?
- If you dated a mineral from inside an igneous rock, what would that date indicate? How about from a sedimentary rock?
Use the resource below to answer the questions that follow.
- What do scientists hope to answer with radiometric dating?
- Is the whole rock, in this case graywacke, dated?
- How is the rock crushed? How is it powdered?
- Who explained radioactivity? What was his explanation?
- What is a mass spectrometer? How does it work?
- What is the spectrometer separating in this rock?
- How will scientists use this information to date the rock? What else do they need to know before they can get a date?
- Why is the dating of rocks important?