Unit 08 · Geologic Time & Earth History
The year closes with the deepest idea in geology: time itself. This unit covers relative dating — the stratigraphic principles from Unit 03 that order events without numbers — and absolute, radiometric dating that puts years on them through half-lives and decay curves; the geologic time scale of eons, eras, and periods; the mass extinctions that punctuate it; and the sweep of Earth history read as a single record. Beneath all of it sits James Hutton’s founding insight — deep time, with “no vestige of a beginning, no prospect of an end.” Mastery means you can order events, calculate an age, and place them on the scale Hutton first made thinkable.
| Criterion | Not yet | Approaching | Mastered |
|---|---|---|---|
| Relative dating principles | Cannot order rock layers or events. | Names superposition but misapplies cross-cutting or inclusions. | Orders events using superposition, cross-cutting relationships, and inclusions, and reads an unconformity as missing time. |
| Radiometric dating & half-life | Treats absolute age as a guess. | Defines half-life but cannot compute an age. | Computes an age from a parent-to-daughter ratio and a known half-life, and reads a decay curve. |
| The geologic time scale | Cannot place major events in order. | Names eras but not their sequence or defining events. | Places major events — the origin of life, mass extinctions, key evolutionary steps — within their eons, eras, and periods. |
| Deep time & Earth history | Thinks Earth’s past fits human timescales. | Repeats “deep time” but cannot reason at its scale. | Uses Hutton’s deep time to interpret slow processes and mass extinctions across the full sweep of Earth history. |
| Lab technique (radiometric-dating / half-life simulation) | Cannot model decay or read the result. | Runs a half-life simulation but misreads the ratio. | Runs a decay simulation, plots the curve, and converts a parent/daughter ratio into a defensible age. |
| Integration (cross-domain) | Treats the science as isolated facts; makes no cross-domain connection. | Names a link to history, reading, or writing but cannot defend why it matters. | Connects the unit to its anchor across History · Reading · Writing (plus chosen electives) and defends why the connection matters. |
“Half the parent isotope is gone, so one half-life has passed — that gives the age. Below the ash bed the layers follow superposition, and the tilted layer cut by the fault came first, so relative dating orders them even where I can’t date them. Hutton would call this deep time.”
“It’s really old rock. Half-life is how long something lasts. The bottom layer is older, I think, but I can’t put a number on it.”
You demonstrate this unit through a radiometric-dating simulation and a relative-dating sequence — computing an age from a half-life and ordering events by stratigraphic principles, explained aloud rather than on a multiple-choice test. A criterion counts as mastered only when your age calculation is defensible and you can place the events on the geologic time scale. Mastery is demonstrated, not awarded.
A 5-page clipboard packet — unit overview, key terms, the mastery rubric, anchor examples, and a score sheet you can print and grade against.