# Radioisotope dating rocks and fossils

### How Is Radioactive Dating Used to Date Fossils? | Sciencing

Absolute dating is used to determine a precise age of a rock or fossil through radiometric dating methods. This uses radioactive minerals that occur in rocks and. Radiometric dating. Geologists use radiometric dating to estimate how long ago rocks formed, and to infer the ages of fossils contained within those rocks. Jan 9, Radiometric dating is used to estimate the age of rocks and other objects Relative Dating with Fossils: Index Fossils as Indicators of Time.

Layers of rock are deposited sequentially. If a layer of rock containing the fossil is higher up in the sequence that another layer, you know that layer must be younger in age.

This can often be complicated by the fact that geological forces can cause faulting and tilting of rocks. Absolute Dating Absolute dating is used to determine a precise age of a rock or fossil through radiometric dating methods. This uses radioactive minerals that occur in rocks and fossils almost like a geological clock. So, often layers of volcanic rocks above and below the layers containing fossils can be dated to provide a date range for the fossil containing rocks.

**Radiometric or Absolute Rock Dating**

The atoms in some chemical elements have different forms, called isotopes. These isotopes break down at a constant rate over time through radioactive decay. By measuring the ratio of the amount of the original parent isotope to the amount of the daughter isotopes that it breaks down into an age can be determined.

We define the rate of this radioactive decay in half-lives.

If a radioactive isotope is said to have a half-life of 5, years that means after 5, years exactly half of it will have decayed from the parent isotope into the daughter isotopes. Then after another 5, years half of the remaining parent isotope will have decayed.

While people are most familiar with carbon dating, carbon dating is rarely applicable to fossils. Carbon, the radioactive isotope of carbon used in carbon dating has a half-life of years, so it decays too fast. It can only be used to date fossils younger than about 75, years. Potassium on the other hand has a half like of 1.

## Dating Fossils – How Are Fossils Dated?

This makes it ideal for dating much older rocks and fossils. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. Good discussion from the US Geological Survey: There are a couple catches, of course. Not all rocks have radioactive elements.

## Geologic Age Dating Explained

Sedimentary rocks in particular are notoriously radioactive-free zones. So to date those, geologists look for layers like volcanic ash that might be sandwiched between the sedimentary layers, and that tend to have radioactive elements. You might have noticed that many of the oldest age dates come from a mineral called zircon. Each radioactive isotope works best for particular applications.

### Radiometric dating - Wikipedia

The half-life of carbon 14, for example, is 5, years. On the other hand, the half-life of the isotope potassium 40 as it decays to argon is 1. Chart of a few different isotope half lifes: If a rock has been partially melted, or otherwise metamorphosed, that causes complications for radiometric absolute age dating as well. Good overview as relates to the Grand Canyon: Which are the youngest?

I also like this simple exercise, a spin-off from an activity described on the USGS site above. Take students on a neighborhood walk and see what you can observe about age dates around you.

For example, which is older, the bricks in a building or the building itself? Are there repairs or cracks in the sidewalk that came after the sidewalk was built?

### Geologic Age Dating Explained - Kids Discover

Have students work alone or in pairs to find an article or paper that uses radiometric age dating. What materials were dated? Which method was used e. Carbon 14, potassium-argon, etc What was the result what was the material? From the chart, which methods are best for older materials?