Explain how decay rates of radioactive materials in rock layers are used to establish the timing of geologic events.
Earth and Space Science
In prior grades students learned about a few of the methods that have made it possible to uncover the history of our planet. In grades 9-11 students learn about the major changes in Earth systems over geologic time and some of the methods used to gather evidence of those changes. Methods include observation and measurement of sediment layers, using cores drilled from the sea bottom and from ancient glaciers, and the use of radioactive isotopes. Findings of Earth history include the existence of life as early as 3.5 billion years ago and major changes in the composition of Earth’s atmosphere.
Geologic time can be estimated by several methods (e.g., counting tree rings, observing rock sequences, using fossils to correlate sequences at various locations, and using the known decay rates of radioactive isotopes present in rocks to measure the time since the rock was formed).