1. You discover tetrapod fossils in layers 3, 4, 5, and 6
You discover tetrapod fossils in layers 3, 4, 5, and 6. Which fossil is the oldest? The youngest? How do you know?

2. Carbon 14 Normal Carbon Radioactive Carbon Highly unstable
Decays to N14

3. Carbon-14 decays to Nitrogen-14

4. The time it takes for half of a radioactive element to decay
Half-Life
The time it takes for half of a radioactive element to decay
The time it takes for a radioactive element to fall to half of its original value
50% carbon-14
50% nitrogen-14

5. Carbon-14
Decays to Nitrogen-14
Half-life years

6. Some Carbon-14 is always present in the atmosphere around us
Some Carbon-14 is always present in the atmosphere around us. This gets incorporated into our bodies. When we die, it stays there.

7. Carbon-14
Decays to Nitrogen-14
Half-life years
Remains in organisms after they die
Scientists can test fossils to see how much C- 14 and N-14 they contain to determine the fossils age
Only useful on fossils between ,000 years old (really only used up to 30,000)

8. Uranium Uranium 238 decays to Lead 206 Half life = 4.5 billion years
Half life = 710 million years
Combined, can be used to date from 10 million-4.6 billion years
Only used on rock!