SME
Grade 12 Science
Geologic time and radiometric dating
The main lesson, then each kind of problem it solves.
Geologic time and radiometric dating
Main lesson
Dating the oldest rocks with uranium-238
Use this for “A meteorite has as much lead-206 as uranium-238. How old is it?”
Bracketing a fossil layer between ash beds
Use this for “Which ash beds date a fossil layer, and how old can the fossils be?”
Any isotope: age from the daughter-to-parent ratio
Use this for “A rock holds 3 daughter atoms for every parent atom, and the half-life is 1.25 × 10⁹ years. How old is it?”
Dating volcanic ash with potassium-argon
Use this for “Volcanic ash holds 0.01 argon-40 atom for every potassium-40 atom. How old is the ash?”
A dike cutting the layers: cross-cutting
Use this for “A 105-million-year-old dike cuts the sandstone but not the shale above it. How old can the shale’s fossils be?”
Putting rock events in order
Use this for “A dike cuts three layers and stops at an eroded surface under a lava flow. Which happened first?”
The geologic time scale
Use this for “Order the spans of Earth’s history, from Precambrian time to the Cenozoic Era. How long was each?”