Chron (geology): short geological time interval

Characteristics and contexts

Usage of the word is pragmatic rather than strictly hierarchical. In magnetostratigraphy a "polarity chron" denotes an interval of predominantly normal or reversed geomagnetic polarity preserved in sediments and lavas. In biostratigraphy a related term, "biochron," describes the time range over which particular fossil taxa or assemblages were characteristic. The length of a chron therefore depends on the process being recorded and may range from thousands to, in some cases, more than a million years; in many studies the intended meaning is an interval substantially shorter than major subdivisions.

History and practical use

The concept of naming short intervals of time grew with improved stratigraphic techniques and radiometric dating. Researchers use chrons to correlate layers across regions, to align sedimentary sequences with global events, and to subdivide long records for higher-resolution study. Chrons are especially valuable when a clear, synchronous marker—such as a reversal of Earth’s magnetic field—is present in multiple locations.

Examples and importance

• Polarity chrons: intervals of consistent geomagnetic polarity that help synchronize far-flung sedimentary and volcanic records.
• Biochrons: intervals defined by the first and last appearances of particular fossils, used in regional correlation.
• Chronozones and correlations: chrons provide the fine-scale temporal framework that complements larger time units in stratigraphy and geochronology.

Distinctions and notes

It is important to distinguish a chron (a span of time) from the physical rock unit that records it (a chronostratigraphic unit, often called a chronozone). Terminology and precision vary between specialties, so authors will usually specify whether they mean a polarity chron, a biochron, or another kind of short time interval. For further general context on stratigraphic and temporal terms see Earth science resources.