Palaeos:		Geological Timescale
Timescale		Geological Time Tables

Geological Time

Introduction

Geologists and Paleontologists measure the age of the Earth and the history of life in ages of millions and even billions of years of "deep time". The entire history of humankind is but a blink of an eye next to the vastness of geological time. For this reason a special sort of "calendar" or "almanac" is required; one that measures not days, weeks, months or years, but millions and tens of millions of years. This is the Geological Time-Scale

The Geological Time-Scale is hierarchical, consisting of (from smallest to largest units) ages, epochs, periods, eras and eons. Each era, lasting many tens or hundreds of millions of years, is characterized by completely different conditions and unique ecosystems. For example, dinosaurs only lived during the Mesozoic era. Mammals have been predominant during the Cenozoic. And so on. We know that these rocks, and the fossils they contain, are tens or hundreds of millions of years old because they can be dated according to the amount of residual radioactivity.

Because it was derived from the stratigraphic charts of 18th and 19th century geologists, the Geological time-scale is usually represented as a vertical table to be read from the bottom up; the oldest eras and periods at the bottom, the youngest at the top. The reason for this strange convention is due to the table being a rough and rather symbolic representation of the layers of sedimentary rocks that make up the Earth's crust. The earlier layers were deposited first, the younger ones on top of them. More recently, other conventions are also used, such as horizontal (usually reading from left (oldest) to right (youngest), circular, or even, as shown in the above diagram) spiral. MAK, revised ATW080303, MAK110825

We follow the geological time scale as determined by the International Commission on Stratigraphy (ICS).  The ICS has not finished its job and gaps remain, particularly in the Early Paleozoic. Where gaps occur, we generally follow the Russian system for the Cambrian and the British system for the Silurian.  The choice of colours shown follows the semi-official color scheme of the Commision de la Carte Géologique du Monde.  On the whole, this scheme is uglier than its predecessors, particularly for the Precambrian; but the whole point of having international standards is that they supercede individual judgments.  For correlations we generally assume the Geowhen Database is authoritative unless we have good reason to believe otherwise (and we haven't found one yet).  The ICS does not recognize the Hadean, so we have adopted the suggestions Goldblatt et al 2010 (along with their new Chaotian eon), which replaces the earlier classification derived from the lunar stratigraphic column, and our own color scheme.  Each of the eons, eras, periods, and ages are, or will be, described in more detail on their respective pages.

Note that the divisions of the periods and eras are not to scale (The Riphean sub-era for example is longer than the entire Phanerozoic eon).  An asterisk (*) indicates an informal division, non-ICS name, or idiosyncratic use of an established name.  Various color charts can be obtained here.  The ICS time chart, using (more or less) this color scheme, is available here.  

* Not recognized by ICS

The above timescale is a work in progress and will doubtless be modified with future discoveries and suggestions. For example John Laurie, Daniel Mantle and Robert S Nicoll, Customising the geological timescale note that the 2004 version of the Geological Time Scale is mainly built around northern hemisphere datasets and, many of the biozones used in Australia were not included. There is also the problem of the length of particular ages, such as the Short Norian - Long Norian controversy (see posts, on Chinleana (and included links) and paper (pdf) by Paul Olsen; contra Spencer Lucas (The Triassic Timescale; currently we haven't updated this new paradigm in Palaeos although this is planned as part of the overall revision). MAK110920