1. Rock Dating Methods Radiometric Dating: using known decay rates (half-lives) of radioactive elements to determine the ages of rocks. Relative Dating: putting rocks in order, according to age

2. Radiometric Dating uses the known decay rates of radioactive atoms to determine the ages of rocks.

3. One “half-life” is the time it takes for half of a quantity of parent atoms to decay into daughter atoms.

5. Half Lives for Radioactive Elements Radioactive ParentStable DaughterHalf life Potassium 40Argon 40 1.25 billion yrs Rubidium 87Strontium 8748.8 billion yrs Thorium 232Lead 20814 billion years Uranium 235Lead 207704 million years Uranium 238Lead 2064.47 billion years Carbon 14Nitrogen 145730 years

6. When magma solidifies, parent and daughter atoms separate. Some of the crystals that form contain 100% parent atoms. This “re-sets” the radiometric dating clock. Time = 0 After 1 half-life (50% parents, 50% daughters

7. Relative Dating: Principles: Superposition: The youngest rock strata are… …on top Faunal Succession: Strata with similar fossils… … are of similar ages (true as long as the species only existed over a short time span) Crosscutting Relationships: If one feature cuts through another… …the one that does the cutting is younger. Original Horizontality: Rock strata are laid down… …horizontally. Any strata that are not horizontal have been perturbed. Relative Dating – Putting Rock Strata In Order

8. Letter “A” is the oldest item in this diagram. T is newest. Some letters represent occurrences, rather than rock samples (N represents erosion or glaciation, for example.) This order was determined by applying the principles on the previous page. Notice that H, R, and M are younger than everything that they “cut through.”

9. Radiometric dating is usually combined with relative dating to give us a more complete understanding of when different rock strata were formed. Based on the dating of volcanic ash, we may infer that rocks with this particular type of fossil are between 520 and 545 million years old