1. By stating that rock unit A is older than rock unit B, we are
comparing relative ages of the rock, not the rocks real
ages in years. This is RELATIVE AGE DATING
To determine the rocks actual age geologists can use
several techniques. This is ABSOLUTE AGE DATING
Radioisotopes (radiometric dating)
2. Index Fossils
3. The Geologic Time Scale

2. Radiometric Dating:
Certain radioactive chemical elements decay into other more stable elements.
Since the rate at which they do this is constant, we can calculate how old the object containing the element is by comparing the original amount to the amountof daughter product
IMPORTANT! Decay rate does NOT depend on
environmental conditions,such as TEMPERATURE

3. Radioactive parent isotopes and their stable daughter products
Potassium 40
Argon 40
Rubidium 87
Strontium 87
Thorium 232
Lead 208
Uranium 235
Lead 207
Uranium 238
Lead 206
Carbon 14
Nitrogen 14

4. The rate at which of any radioactive substance
decays into a more stable substance is measured
as the half-life for that substance
For example: Carbon14 has a half life of 5700 years.
What does this mean?
It means that if you start with any given amount of C14,
you will be left with 50% (half) of it after every 5700 years.

5. Half Life – The time it takes for ½ of any sample of a
radioactive substance to decay into a more stable
daughter product.
Radioactive Parent
Stable Daughter
Potassium 40
Argon 40
Rubidium 87
Strontium 87
Thorium 232
Lead 208
Uranium 235
Lead 207
Uranium 238
Lead 206
Carbon 14
Nitrogen 14
Half-life (yrs)
1.3 x 109
4.9 x 1010
4.5 x 109
5.7 x 103

6. How C14 works

7. Sample of C14
decay
11,400 yrs
5700 years
17,100 yrs

8. A half life graph always
looks like this
Half life periods

9. Half life periods