Geologic Time Scale

Follow the links below, in Epochs, Periods and Eons, to brief lists of important geological events in the Pacific Northwest.

Notes:

1. Ma stands for millions of years ago, Ga stands for billions of years ago.

2. If you look at geologic time scales published over the years, you will see that some of the absolute ages have been adjusted. For example the end of the Cretaceous period and Mesozoic era, which was once listed as 65 Ma, is now listed as 66 Ma. This is an example of scientific progress.
   
   Most of the divisions between eras, periods and epoch are defined in terms of significant changes in the fossil record. However, in general, the absolute age of a fossil cannot be directly measured (except for radiocarbon ages on plants or animals that died less within the last 50,000 years). Instead, a layer of rock that can be dated with absolute methods (such as lava or volcanic ash) must be located in the same sequence of layers that include the sedimentary strata in which the fossils are found. The absolute age of the volcanic rock can then be measured. If the volcanic layer is superposed on top of the sediment with fossils, the fossils are older than the the volcanic material. If the fossils are in a sediment layer above the volcanic layer, the fossils are younger than the volcanic age. With an increasing number of measurements like this, the age of the fossiliferous sediment can be narrowed down more precisely.
   
   Some boundaries in the geologic time scale are not defined in terms of fossils. For example, the length of the Pleistocene epoch is defined in terms of evidence of cold climates and extensive continental ice sheets.
   
   Absolute ages in the geologic time scale may be changed for several reasons:
   • New layers of rock are discovered with key evidence of the end or beginning of a geologic time period. For example, discovery of more volcanic layers closer to sedimentary layers containing key fossils leads to a more precise age on the fossils.
   • Advancements in radiometric age-measuring techniques produce absolute ages that are more precise and accurate.
   • Definitions of which fossil types (or other environmental indicators) mark the end or beginning of a geological time period are revised.

   In recent years an international group of geologists has put together a revised geologic time scale in which the Tertiary period has been replaced by the Paleogene and Neogene periods.

   • In the revised geologic time scale the Paleogene period contains the Paleocene, Eocene, and Oligocene epochs, and the Neogene period contains the Miocene, Pliocene, and Pleistocene epochs.
   • The epochs of the Cenozoic era will retain the same names, and be in the same sequence, as before.
   • In 2009, geologists agreed to re-position the end of the Pliocene epoch/beginning of the Pleistocene epoch at 2.588 Ma, a significant change from the previous age of 1.86 Ma. It was determined that climate cooling and extensive glacial activity on several continents occurred from 2.588 to 1.86 Ma, so that time interval was removed from the Pliocene epoch and put into the Pleistocene epoch. The change was made because the Pleistocene epoch is defined by cold climates and continental-scale glaciations.

Despite the occasional adjustment of the absolute ages in the geologic time scale, most of the ages, including the age of the earth itself at nearly 4.6 Ga and the end of the Pleistocene epoch around 10,000 - 12,000 years ago, are not very different from what they were in geologic time scales published 50 years ago.

Source: International Union of Geological Sciences - International Commission on Stratigraphy: International Chronostratigraphic Chart (2013), http://www.stratigraphy.org/ICSchart/ChronostratChart2013-01.jpg

Holocene epoch

Pleistocene epoch

Pliocene epoch

Miocene epoch

Oligocene epoch

Eocene epoch

Paleocene epoch

Cretaceous period

Jurassic period

Triassic period

Permian period

Pennsylvanian period

Mississippian period

Devonian period

Silurian period

Ordovician period

Cambrian period

Proterozoic eon

Archean eon

Hadean eon