Geon (geology)

The term geon (for geological eon) refers to large geologic time intervals.Geologists traditionally subdivide Earth history into a hierarchy of named intervals: eons, eras, periods, etc. (e.g. Jurassic Period of the Mesozoic Era). Likewise, historians subdivide the history ofman into intervals that are comparatively much shorter. In both geological and historical scales,the divisions of equal rank are characteristically of unequal duration, and the identification of a particular intervalis primarily based on its fossil, artifact, or cultural content (e.g., Carboniferous, Neolithic,
Dark Ages, Ming Dynasty). Both scales are calibrated against numerical ages obtainedseparately.

An alternative way of referring to the past is to use a scale with intervals of equal duration. Wespeak of a given decade, century, or millennium. For the enormously long geologic timeframe, it is advantageous to use corresponding large, equal time intervals encompassing theevents and processes that have shaped our planet. The development of mountain ranges,ocean basins, and continents takes tens to hundreds of millions of years, and large time units thusare convenient for discussing long-term trends. Astronomers use light years and parsecs to deal with huge distances, rather than kilometres.Geologists have geons to refer to large specified time intervals of Earth history. The geon scale is alsoapplicable to other planets with different histories, and to the universe itself.

Two usages of geon have been introduced in geology:

1) A geon is a unit “...taken to represent either the span of the average geologic period, or thethickness of the average stratigraphic equivalent, a matter of 60,000,000 years, and 50,000 feet [~15 km] of clastic depositions” (Woodward, 1929). Utilizing the currently accepted value of542 Ma, million years ago) for the beginning of the Cambrian Period, andusing 11 geologic periods in the Phanerozoic Eon, an updated value for Woodward’s geon wouldbe about 49.4 million years. Usage in this sense is not current.

2) A geon is a specified 100-million-year interval of geologic time, counted backward from thepresent. The geon scale can be likened to a ladder, each interval between rungs representing 100 million years. Geons are named for the leftmost part of the number representing age. For example, theEarth formed about 4550 million years ago, an event that is assigned to Geon 45 (interval below rung 45). Rocks formed at 1851 Ma or 1800 Ma both belong to Geon 18. The extinction of the dinosaurs at the end of the Cretaceous Period (065 Ma) belongs to Geon 0. (Hofmann, 1990).

References

Hofmann, H.J. 1990, Precambrian time units and nomenclature - the geon concept. "Geology", v.18, p. 340-341. [http://www.eps.mcgill.ca/~hofmann/]

Woodward, H.P. 1929, Standardization of geologic time-units. "Pan-American Geologist", v. 51,p. 15-22.