Geon (geology)
Encyclopedia
The term geon 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 of
man 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 interval
is primarily based on its fossil, artifact, or cultural content (e.g., Carboniferous
, Neolithic
,
Dark Ages, Ming Dynasty
). Both scales are calibrated against numerical ages obtained
separately.
An alternative way of referring to the past is to use a scale with intervals of equal duration. We
speak of a given decade, century, or millennium. For the enormously long geologic time
frame, it is advantageous to use corresponding large, equal time intervals encompassing the
events 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 thus
are 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 also
applicable 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 the
thickness 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 of
542 Ma, million years ago) for the beginning of the Cambrian
Period, and
using 11 geologic periods in the Phanerozoic
Eon, an updated value for Woodward’s geon would
be 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 the
present. 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, the
Earth 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).
Geologists traditionally subdivide Earth history into a hierarchy of named intervals: eons, eras, periods, etc. (e.g. Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
Period of the Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
Era). Likewise, historians subdivide the history of
man 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 interval
is primarily based on its fossil, artifact, or cultural content (e.g., Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
, Neolithic
Neolithic
The Neolithic Age, Era, or Period, or New Stone Age, was a period in the development of human technology, beginning about 9500 BC in some parts of the Middle East, and later in other parts of the world. It is traditionally considered as the last part of the Stone Age...
,
Dark Ages, Ming Dynasty
Ming Dynasty
The Ming Dynasty, also Empire of the Great Ming, was the ruling dynasty of China from 1368 to 1644, following the collapse of the Mongol-led Yuan Dynasty. The Ming, "one of the greatest eras of orderly government and social stability in human history", was the last dynasty in China ruled by ethnic...
). Both scales are calibrated against numerical ages obtained
separately.
An alternative way of referring to the past is to use a scale with intervals of equal duration. We
speak of a given decade, century, or millennium. For the enormously long geologic time
frame, it is advantageous to use corresponding large, equal time intervals encompassing the
events 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 thus
are convenient for discussing long-term trends.
Astronomers use light years
Light Years
Light Years is the seventh studio album by Australian recording artist Kylie Minogue. It was released on 25 September 2000 by Parlophone and Mushroom Records. The album's style was indicative of her return to "mainstream pop dance tunes"....
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 also
applicable 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 the
thickness 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 of
542 Ma, million years ago) for the beginning of the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
Period, and
using 11 geologic periods in the Phanerozoic
Phanerozoic
The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...
Eon, an updated value for Woodward’s geon would
be 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 the
present. 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, the
Earth 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
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
Period (065 Ma) belongs to Geon 0. (Hofmann, 1990).