The geological time scale

Historical records only go back a few thousand years, and are inadequate to treat most geological processes.The geological time scale is based on the the geological rock record, which includes erosion, mountain building and other geological events. Over hundreds to thousands of millions of years, continents, oceans and mountain ranges have moved vast distances both vertically and horizontally. For example, areas that were once deep oceans hundreds of millions of years ago are now mountainous desert regions.

How is geological time measured?

The earliest geological time scales simply used the order of rocks laid down in a sedimentary rock sequence (stratum) with the oldest at the bottom. However, a more powerful tool was the fossilised remains of ancient animals and plants within the rock strata. After Charles Darwin's publication Origin of Species (Darwin himself was also a geologist) in 1859, geologists realised that particular fossils were restricted to particular layers of rock. This built up the first generalised geological time scale.

Once formations and stratigraphic sequences were mapped around the world, sequences could be matched from the faunal successions. These sequences apply from the beginning of the Cambrian period, which contains the first evidence of macro-fossils. Fossil assemblages 'fingerprint' formations, even though some species may range through several different formations. This feature allowed William Smith (an engineer and surveyor who worked in the coal mines of England in the late 1700s) to order the fossils he started to collect in south-eastern England in 1793. He noted that different formations contained different fossils and he could map one formation from another by the differences in the fossils. As he mapped across southern England, he drew up a stratigraphic succession of rocks although they appeared in different places at different levels.

By matching similar fossils in different regions throughout the world, correlations were built up over many years. Only when radioactive isotopes were developed in the early 1900s did stratigraphic correlations become less important as igneous and metamorphic rocks could be dated for the first time.

Geological divisions

Divisions in the geological time scales still use fossil evidence and mark major changes in the dominance of particular life forms. For example, the Devonian Period is known as the 'Age of Fishes', as fish began to flourish at this stage. However, the end of the Devonian was marked by the predominance of a different life form, plants, which in turn denotes the beginning of the Carboniferous Period. The different periods can be further subdivided (e.g. Early Cambrian, Middle Cambrian and Late Cambrian).

Terms

• Faunal succession: is the time arrangement of fossils in the geological record.
• Formations: are stratigraphic successions containing rocks of related geological age that formed within the same geological setting.
• Ga: is an abbreviation used for billions (thousand million) of years ago.
• Geochronology: is the study of the age of geological materials.
• Ma: is an abbreviation used for millions of years ago.
• Palaeobiology: is the study of the evolution of life during geologic time.
• Palaeobotany: is the study of ancient plants.
• Palaeontology: is the study of ancient lifeforms.
• Stratigraphic succession: is a sequence of layered sedimentary rocks.

 References

• Dickin, A.P., 2000. Radiogenic isotope geology. Cambridge University Press, 490p.
• York, D., and Farquhar, R.M., 1972. The Earth's age and geochronology. Pergamon Press Ltd, 178p.

Ian Graham
Last Updated: 3 December 2009

Tags geoscience, Earth, dating, geological, time scale, era, period, epoch, carbon, radioactive,