NSES Content Standard D
Earth and Space Science: Origin and evolution of the earth system Grades 9-12, page 189 Geologic time can be estimated by observing rock sequences and using
fossils to correlate the sequences at 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.
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Benchmark 4D The Physical Setting: Structure
of Matter
Grades 9-12, page 80
The nucleus of radioactive isotopes is unstable and spontaneously decays,
emitting particles and/or wavelike radiation. It cannot be predicted exactly
when, if ever, an unstable nucleus will decay, but a large group of identical
nuclei decay at a predictable rate. This predictability of decay rate allows
radioactivity to be used for estimating the age of materials that contain
radioactive substances.