9-12

F4: Describe ways that scientists measure long periods of time, and determine the age of very old objects.

Organizing Questions
Elaboration	Geologic time can be estimated by observing rock sequences and using fossils to correlate the sequences of various locations. Current methods include using the known decay rates of radioactive isotopes present in rocks to measure the time since the rock was formed. NSES D3b p. 189
Specific Ideas	Radioactive isotopes are unstable and undergo spontaneous nulcear reactions … The decay of any one nucleus cannot be predicted, but a large group of identical nuclei decay at a predictable rate. This predictability can be used to estimate the age of materials that contain radioactive isotopes. NSES B1d p. 178
Developmental & Instructional Implications
Examples

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