Classification of Geological Time

Geological time has been divided up by different means into various divisions, getting progressively more narrowly defined.
There are a number of different ways of categorising geological time, depending on the basis of that division. There are two main classification systems, being those of chronostratigraphy, and that of geologic time. The latter is based on the actual time interval, the former on the rocks that were formed during that interval.
Thus, the rocks of the Devonian System were formed during the Devonian Period, where 'system' is the chronostratigraphical time unit, and 'period' the geologic time unit.

Other stratigraphic scales exist, which do not consider time. These include

• lithostratigraphy, which concerns the description of rock units based on their appearance and mineral composition, regardless of age or organisms contained within the rock units, and
• biostratigraphy, which concerns the use of fossil plants and organisms for dating and correlation.

The following discussion covers:

• Time units
• Boundaries
• History
• Present Framework

Time Units

The two main classification systems mentioned above are those of chronostratigraphy and of geologic time. The latter is based on the actual time interval, the former on the rocks that were formed during that interval.
The 2 systems are closely correlated, and sometimes the names are shared. The following table gives the correlations between the chronostratigraphic, and geologic, time units:

(To read more about eons, eras and periods, please go on to Periods.)

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Boundaries

All these divisions are of course 'man-made'; early geologists chose the names and boundaries dependent on what they saw in the rocks. Originally, the period boundaries were entirely relative, and based on the appearance (or disappearance) of certain classes or species of life. Thus, the end of a period may be marked by the extinction of a certain animal that thrived during the period but disappeared at its end, based on fossil evidence.
Often, but not exclusively, the end of a period was marked by a 'mass extinction event', when a large proportion of the life on the planet became extinct over a relatively short span of (geological) time - which may have been 100's of 1000's, or even millions, of years.

The best known mass extinction event is that of the Chicxulub impact, when a meteoric impact changed the world's climate and drove perhaps 85% of the fauna to extinction. This marks the end of both the Cretaceous Period, and the Mesozoic era.
However, the largest mass extinction was that of the Permo-Triassic, which marks both the end of the Permian Period, and the Palaeozoic era.

More recently, 'absolute' ages have been applied to the boundaries, based on radiometric dating of the rocks of the time. This has been most exact for the end of the Cretaceous, as the boundary is marked worldwide by an iridium layer which was the result of the impact. Elsewhere, it is less-well defined.
Care must always be taken when attempting to date different strata, and ages used should be either the relative ages, based (largely) on fossil correlation, or the absolute age, based on radiometric dating.
The 2 schemes ought not be used together, else recognised 'margins of error' may lead to confusion and error when overlaps of the different stages occur - it is not always easy to correlate the life on one continent with another, when it has evolved separately.

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History

The concept of a geological time-scale has been evolving since the middle of the 18^th century. The following describes its progression, to the present day.

The 18^th Century

A First Attempt, 1759

In 1759, geologists considered the world to be no older than 6000 years. This figure was reached by the Archbishop Ussher, who based his calculations on the creation of the world in 6 days, as described in Genesis.
Without putting ages to the rocks, geological time was divided based on the appearance of the rocks, and given Latin terms as follows:

• Primary - crystalline rocks
• Secondary - folded sedimentary rocks
• Tertiary - undeformed sedimentary rocks
• Quaternary - loose sediments

Note that 'Tertiary' and 'Quaternary' are still used to denote the sub-eras of the Cenozoic era, whereas the 'Primary' and 'Secondary' are no longer used in present-day geological time scales.

Progressing from this simple classification based on the appearance of rocks, further classification based on relative ages of fossils was derived.
Using these relative ages, the following eras, given names relating to Greek origins, were defined:

• Palaeozoic - old life
• Mesozoic - middle life
• Cenozoic - recent life

Within these eras were defined periods, which were named after places or unique characteristics.
These periods were further subdivided into epochs (See the page on Periods for more detail about these divisions).

However; all these classifications depended on the fossils content of the rocks, and their relative ages.
Really old rocks hold no fossils, so early geologists had no means of dating them, nor even arranging them into a relative age-sequence. All were thus called Precambrian, an informal term we still use today.

The 19th Century

In the 19^th century, more was known about the early rocks. The Precambrian rocks could then be divided into two eons:

• Archean - the beginning
• Proterozoic - earliest life

The younger rocks of the 3 above-named eras were collectively grouped into the Phanerozoic Eon, which meant 'visible life'. (This includes the Paleozoic, Mesozoic and Cenozoic.)

In 1878, the first International Geological Congress met, to form a global framework for future naming standards, as described below under 'Present Framework'

The 20th Century

In the 20^th century, radiometric dating allowed the various eons, eras, periods, and epochs to be dated.

Finally, 'real' or 'absolute' dates could be applied to rocks - and it turned out that the subdivisions are of very unequal duration. For instance, although a 'period' is a subdivision of an 'era', the Cretaceous Period at 79Ma is longer than the whole of the Cenozoic Era, which is a mere 65Ma.

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Present Framework

The International Geological Congresses have, since the late 19^th century, provided the geological community with an opportunity to create an organizational framework for meeting at regular intervals in a spirit of fraternal cooperation that transcends oceans, languages and wars.
The need for an International Geological Congress had been recognized for some time before its first meeting took place in Paris in 1878. Present were participants from the Great Britain (Thomas Huxley), USA, Canada, France, Russia, Sweden, Norway, Austria, Spain and Italy. (Germany refused to participate, because of the recent Franco-Prussian war (1870-1871) - even though she won).

The program of the Congress listed the following issues as topics for debate:

1. Standardization of geologic maps and reports with regard to nomenclature and symbols
2. Discussion of the boundaries and characteristics of certain rock systems
3. Representation and coordination of linear features (faults and veins)
4. Respective importance of fauna and flora in the delineation of rock systems
5. Importance of the mineralogy and texture of rocks in terms of their origin and age.

The most important result of the Congress was, undoubtedly, the adoption of resolutions establishing two International Commissions. One was charged with looking into the standardization of geological symbols, and the other, the standardization of geological nomenclature.

The International Union of Geological Sciences (IUGS), founded in 1961, is now the 'umbrella' organisation that encompasses the many different commissions that resulted from the first 2. That that concerns geological time is the International Commission on stratigraphy.

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To read more about eons, eras and periods, please go on to Periods.

For any comments, suggestions or contributions, please e-mail me at: portsdown@bbm.me.uk