1. 21.3 – Absolute-Age Dating Objectives
describe how scientists date rocks and other objects using radioactive elements
Explain how scientists can use certain non-radioactive material to date geologic events
Main Idea: radioactive decay and certain kinds of sediments help scientists determine the numeric age of many rocks

2. Radioactive Isotopes
Measure the decay of the radioactive isotopes in igneous and metamorphic rocks
Radioactive Decay
Radioactive isotopes emit nuclear particles at a constant rate
Identity of an element is due to the number of protons is has
Radioactive decay is constant:
Regardless of pressure, temperature, or any other physical changes
Allows scientists to determine absolute age of object in which radioactive element occurs

3. Radiometric Dating
The ratio of parent isotope to daughter product indicates the amount of time that has passed since the object formed
When scientists date an object using radioactive isotopes, they are using a method called radiometric dating

4. Half-life and dating rocks
Half-life: length of time it takes for one-half of the original isotope to decay
After one half-life, 50% of the parent remains, resulting in a 1:1 ratio of parent-to –daughter product.
What would happen after 2 half-lives?

5. Dating Rocks
to date using radiometric dating, scientists examine the parent-daughter ratios of the radioactive isotopes in the minerals within the rocks
Scientists might use U-235, which has a half-life of 700 million yrs, to date a rock that is a few tens of millions of years old
If you wanted to date a rock that is hundreds of millions of years old, scientists might use U-238 due to its longer half-life
Might not work well if you use an element with a short half life to date ancient rocks…why?

6. Radiocarbon Dating Radioactive parent isotope Approximate Half Life
Potassium-40 (K-40)
1.3 billion years
Uranium-238 (U-238)
4.5 billion years
Uranium-235 (U-235)
700 million years
Carbon-14 (C-14)
5730 years
Scientists use C-14 to determine the age of organic materials, which contain abundant carbon, in a process known as radiocarbon dating

7. Radiocarbon Dating
Tissues of all living organisms contain small amounts of C-14 and large amount of C-12 (most abundant form of carbon)
Throughout life, C-14 decays but is continually replenished through respiration
Therefore, when organism dies, it no longer takes in C-14, so over time, the ratio of C-14 to C-12 decreases
Measuring the amount of C-14 can help determine how much time has passed since death

8. Other ways to determine absolute age
Tree rings
Many trees contain a record of time in the rings of their trunks (annual tree rings)
Each annual tree ring consists of a pair of early season and late season growth rings
Science of using tree rings to determine absolute age is called dendrochronology
helps confirm the results from radiocarbon dating
Matching tree rings from different wood samples

9. Ice Cores Analogous to tree rings
Like tree rings, they contain a record of past environmental conditions in annual layers of snow deposition
Summer ice tends to have more bubbles and larger crystals than winter ice