Palaeos: Palaeos Geological
Time Geological

Deep Time: The Geological Timescale


Although (as explained in the parent unit (more inclusive unit) of this topic) there are many possible timescales (which range from the very long (billions of years) to the very short) , Geological time, which deals in millions or tens of millions of years, is the main focus here at Palaeos. Therefore this page provides the starting point for an overview of Earth history. In exploring geological time, we explore the history and evolution of life in all its incredible diversity, the drift of continents, the rising and erosion of mountains, and long term climatic cycles of greenhouses and ice ages.

The Geological Time-scale

Scientists divide the Earth into a number of periods - the "Geological time-scale", according to the rock types and sort of fossils found in each one.  These divisions are pretty arbitrary, like all such man-made classifications, but they at least can serve as useful labels, so we can orientate ourselves in our journeys through deep time.   So the Paleozoic, the era of "ancient life" is characterized by fossils of invertebrates, primitive tetrapods, etc; the Mesozoic or era of "middle life", by fossils of dinosaurs etc, and the Cenozoic or era of "recent life" by mammals and modern plants and invertebrates. These eras are divided into periods, the system of which was established by the middle of the last century.  The periods are in turn divided into epochs, and the epochs are divided into ages called ages. (more on these subdivisions) We know these intervals lasted for millions of years, or even tens or hundreds of millions, because they can be dated them with a fair degree of accuracy according to the amount of radioactivity that occurs in the rocks.  We even have a rough idea of geological timescales on the moon and Mars.

The colours associated with each have no other significance, other than to brightly label jars of geological specimens, and brighten up the Palaeos timeline pages. By convention, the oldest units are at the bottom, the youngest at the top (representing the order of deposition of strata, as this diagram derives originally from 19th century geology) . To see our still very incomplete overview of the history of life on Earth, click on one of the following links or images. Or select a stage from the detailed colour-coded chart of the geological timescale.

EonEraPeriodbegin - end (Mya)
Phanerozoic Eon: PHCenozoic Era: CZNeogene N 23.0 - 
Paleogene E65.5 - 23.0
Mesozoic Era: MZCretaceous K146 - 65.5
Jurassic J200 - 146
Triassic T251 - 200 
Paleozoic Era: PZPermian P299 - 251
Carboniferous C359 - 299
Devonian D416 - 359
Silurian S444 - 416
Ordovician O488 - 444
Cambrian Є542 - 488
Precambrian*: PrcProterozoic PR2500 - 542
Archean AR3850 - 2500
Hadean* HAc. 4500 - 3850
Chaotian* CHc. 4600-4500

For the sake of brevity the eras and periods of the three Precambrian eons are not shown here. Asterisks show time units that are not officially recognised.

Geological Eras and Life on Earth

The geological time-scale is here used to define the major stages in the history of life on Earth. Here the four and a half billion year history of planet Earth is divided into six segments, although this is semi-informal classification, mixing eons and eras. A brief overview of each is shown below.


Teaching Documents about Stratigraphy and Historical Geology gets our best on the web award for its huge annotated listing of links and references (although I haven't checked to see if they all still work, many do, a few appear to be broken).

International Commission on Stratigraphy is the official site. A Phanerozoic Time Scale by F.M. Gradstein and J. Ogg, deals with problems and issues in creating an accurate Phanerozoic time-scale. They include a geological time scale chart for the Phanerozoic, that integrates currently available geochronological information (based on an article originally published in Episodes, Vol. 19, nos. 1&2, pp. 3-4, 1996). Gradstein, Ogg, & Smith (editors) A Geologic Time Scale 2004 remains the official and definitive coverage, although no doubt it will continue to be refined.

Moving on to other links, that up and coming resource of information on anything, Wikipedia, offers a page and detailed Geologic time scale chart, some of which I contributed to. National Geographic's Prehistoric Time Line provides a nice interactive timeline with very basic info but giving a good sense of events through time. History of the Earth pdf with timescale, paleogeographic maps, and life through time. The UCMP Web Geological Time Machine gives a comprehensive review of life on Earth by geological period; the Coccosteus site has an earlier version of our timescale. Geologic Ages of Earth History by Jeff Poling - a detailed and up to date chart showing every era, period, epoch and age of the geological time scale, together with time when began and during in millions of years.  The part of the chart marked in red shows the periods when dinosaurs were around. Niel Brandt's Evolutionary and Geological Timelines at Talk Origins give a brief listing of major events and an upside down geological time chart which, counter to convention but in keeping with common sense (e.g. reading from top to bottom) has the oldest at the top and the youngest at the bottom. MAK110920

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page uploaded M. Alan Kazlev 14 March 2002, last modified MAK110702
checked ATW030731, content by MAK110725, MAK110829, 110915, edited RFVS111108

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