1. Dating (but not the kind you’re thinking of….
Geologic Time
Dating (but not the kind you’re thinking of….

2. How Do We Know How Old ?? So many different ways to date things.
Principle of Superposition
Relative Dating w/ Index Fossils
Radiometric Dating
Radiocarbon Dating

3. Principle of Superposition
States that in undisturbed layers of rock, the oldest is always on the bottom and the youngest is on top
Strata are layers of sedimentary rock

4. Relative Dating
Used by determining the age of a layer of rock – older are below and younger are on top
Some fossils appear for only a short period of time and if dated, can tell the age of the rock they are found in
Fossils that are used to determine the relative age of a rock are called index fossils

5. Index Fossils

6. Radiometric Dating A somewhat complicated process
When a rock is cooled and hardened from magma or lava, it may contain the element uranium
Uranium changes into lead over time
After 4.5 billion years, half of the uranium will have turned to lead

7. Radiometric Dating
Therefore, scientists can tell how old a rock is by how much uranium and how much lead there is in a rock
After 4.5 billion years (one half-life) there will be 50% uranium and 50% lead
After 9 billion years there will be 75% lead and only 25% uranium

9. Radiocarbon Dating
Scientists also use carbon-14, a rare form of carbon, to date events in the recent past
Carbon-14 has a half life of 5730 years
Carbon-14 dating is only good for things that are up to 50,000 years old
Carbon-14 decays and turns into nitrogen gas

10. Carbon-14 is found in all living things that take it out of the environment to build cells and tissues
Therefore, carbon-14 dating is only useful on items such as fossils, bones and wood (that are less than 50,000 years old)