1. Absolute Age
Scientists can use several methods to determine absolute age:
Radioactive decay
Radiocarbon dating
Tree ring dating
Ice cores
Varves

2. Radioactive decay
The emission of radioactive particles and the resulting change into other isotopes over time is called radioactive decay.
This decay happens at a constant rate, so it can be used to determine the age of the minerals in a rock.
For sedimentary rocks, scientists date layers of igneous rock or volcanic ash above and below the sedimentary layers
This is the short version of radioactive decay.

3. Radiocarbon dating
This is used to determine the age of carbon material, or things that were alive.
The amount of carbon decreases over time which allows scientists to determine how long ago the object died.

4. Dendrochronology – Tree Ring dating
Every year, a tree produces two rings (a dark-colored and a light-colored ring).
Counting the rings allows scientists to know how old the tree was when it died. Used in combination with carbon-dating, this can help determine the age of a tree.

5. Ice Cores
Ice cores contain a record of past environmental conditions in annual layers of snow.
Geologists use ice-cores to study glacial cycles.

6. Varves
Bands of alternating light- and dark-colored sediments of sand, clay, and silt are called varves.
Using varved cores, scientists can date cycles of glacial sedimentation over periods as long as 120,000 years.

7. Geologic Time Scale p.591 Activity p. 167
Instead of drawing the scale, write the name of the period next to the questions for numbers 4 – 15 at the top.