Absolute dating and geologic time

Objective: Explain radioactive decay and half-life, distinguish their meanings, and apply them to a described case.

Radioactive decay: Unstable isotopes transform at statistically predictable rates into other nuclei. Half-life: A half-life is the time required for half of a large population of parent atoms to decay. Radioactive decay is predictable for large populations of atoms, not as a schedule for one atom. After each half-life, half of the remaining parent population persists. Dating requires a suitable isotope system and interpretation of initial conditions and later changes. Numerical ages can constrain a sequence of events, but the dated event must be identified. Sedimentary deposition is not automatically dated by the crystallization age of an included mineral grain. Application: A parent isotope decreases as daughter products accumulate. This most directly illustrates radioactive decay. A closed sample retains one quarter after two half-lives. This most directly illustrates half-life.

Objective: Explain closed system and dating-event distinction, distinguish their meanings, and apply them to a described case.

Closed system: A closed dating system has not significantly gained or lost relevant parent or daughter material since the dated event. Dating-event distinction: A numerical age dates a specific geological event rather than automatically every event affecting the rock. Radioactive decay is predictable for large populations of atoms, not as a schedule for one atom. After each half-life, half of the remaining parent population persists. Dating requires a suitable isotope system and interpretation of initial conditions and later changes. Numerical ages can constrain a sequence of events, but the dated event must be identified. Sedimentary deposition is not automatically dated by the crystallization age of an included mineral grain. Application: A mineral retains its isotope products after crystallization. This most directly illustrates closed system. An igneous mineral age estimates crystallization rather than later erosion. This most directly illustrates dating-event distinction.

Objective: Explain isotope suitability and geologic time scale, distinguish their meanings, and apply them to a described case.

Isotope suitability: An isotope system must have a useful half-life and suitable sample material for the age range. Geologic time scale: The geologic time scale organizes Earth's history using geological and biological evidence and numerical constraints. Radioactive decay is predictable for large populations of atoms, not as a schedule for one atom. After each half-life, half of the remaining parent population persists. Dating requires a suitable isotope system and interpretation of initial conditions and later changes. Numerical ages can constrain a sequence of events, but the dated event must be identified. Sedimentary deposition is not automatically dated by the crystallization age of an included mineral grain. Application: Radiocarbon is not selected to date an ancient granite formation. This most directly illustrates isotope suitability. A timeline places eras within billions of years of Earth history. This most directly illustrates geologic time scale.