AP Notes on Geologic Time



Two time scales are used to measure the age of the earth.

1. Relative time scale – based upon the sequence of layering of rocks and the evolution of life.
2. Radiometric time scale – based upon natural radioactivity of chemical elements in rock.

Earth’s past has been organized into units according to events that took place in each period. Different spans of time on the time scale are separated by major geologic or paleontological events that took place (like mass extinctions). The boundary between the Cretaceous period and the Paleogene period is defined by the extinction of the dinosaurs and many marine species.

The largest defined unit of time is the eon.
Eons are divided into eras.
Eras are divided into periods
Periods are divided into epochs
Epochs are divided into stages
Eons -->eras, --> periods --> epochs --> stages

Key principles of the geologic time scale are:
1. Rock layers (strata) are laid down in succession with each strata representing a slice of time.
2. The principle of superposition – any given stratum is probably older than those above it and younger than those below it.

Factors that complicate the geologic time scale:
1. Sequence of strata are often eroded, distorted, tilted, or even inverted after deposition.
2. Strata laid down at the same time in different areas can have entirely different appearances.
3. Strata of any given area represent only part of earth’s history.

At this point - Proceed to the next topic: Geology - Plate Tectonics