A GEOLOGIC TIME SCALE 2004

The geologic time scale is the framework for deciphering the history of the Earth. Since the time scale is the tool “par excellence” of the geological trade, insight in its construction, strengths, and limitations greatly enhances its function and its utility. All earth scientists should understand how the evolving time scales are constructed and calibrated, rather than merely using the numbers in them.

This calibration to linear time of the succession of events recorded in the rock record has three components:

1. the international stratigraphic divisions and their correlation in the global rock record,

2. the means of measuring linear time or elapsed durations from the rock record, and

3. the methods of effectively joining the two scales.

For convenience in international communication, the rock record of Earth's history is subdivided into a “chronostratigraphic” scale of standardized global stratigraphic units, such as “Jurassic,” “Eocene,” “Harpoceras falciferum ammonite zone,” or “polarity Chron C24r.” Unlike the continuous ticking clock of the “chronometric” scale (measured in years before present), the chronostratigraphic scale is based on relative time units, in which global reference points at boundary stratotypes define the limits of the main formalized units, such as “Devonian.”