Home » Geologic Time

Geologic Time Scale


A Time Line for the Geological Sciences


Geologic time scale

Geologic time scale with a linear time axis. This time scale is available as a printable .pdf document. You can download this printable time scale and make copies for personal use.

Dividing Earth History into Time Intervals

Geologists have divided Earth's history into a series of time intervals. These time intervals are not equal in length like the hours in a day. Instead the time intervals are variable in length. This is because geologic time is divided using significant events in the history of the Earth.



Examples of Boundary "Events"

For example, the boundary between the Permian and Triassic is marked by a global extinction in which a large percentage of Earth's plant and animal species were eliminated. Another example is the boundary between the Precambrian and the Paleozoic, which is marked by the first appearance of animals with hard parts.


Eons

Eons are the largest intervals of geologic time and are hundreds of millions of years in duration. In the time scale above you can see the Phanerozoic Eon is the most recent eon and began more than 500 million years ago.

USGS geologic time scale

Detailed geologic time scale: The United States Geological Survey has published "Divisions of Geologic Time: Major Chronostratigraphic and Geochronologic Units." It is a much more detailed time scale than the simplified scale shown above. View a copy here.

Eras

Eons are divided into smaller time intervals known as eras. In the time scale above you can see that the Phanerozoic is divided into three eras: Cenozoic, Mesozoic and Paleozoic. Very significant events in Earth's history are used to determine the boundaries of the eras.



Periods

Eras are subdivided into periods. The events that bound the periods are widespread in their extent but are not as significant as those which bound the eras. In the time scale above you can see that the Paleozoic is subdivided into the Permian, Pennsylvanian, Mississippian, Devonian, Silurian, Ordovician and Cambrian periods.

Epochs

Finer subdivisions of time are possible, and the periods of the Cenozoic are frequently subdivided into epochs. Subdivision of periods into epochs can be done only for the most recent portion of the geologic time scale. This is because older rocks have been buried deeply, intensely deformed and severely modified by long-term earth processes. As a result, the history contained within these rocks cannot be as clearly interpreted.

Our geologic time scale was constructed to visually show the duration of each time unit. This was done by making a linear time line on the left side of the time columns. Thicker units such as the Proterozoic were longer in duration than thinner units such as the Cenozoic. We also have a printable version of the Geologic Time Scale as a .pdf document. You can print this timescale for personal use.

Author: , Ph.D.

More Geologic Time Scale Resources
Major Divisions of Geologic Time - US Geological Survey

Geologic Time Scale - Geological Society of America

Geology Wing - University of California Berkeley

Geologic Time Scale - University of California Berkeley




More home
  Tanzanite
  Mount Cleveland
  Salt Domes
  Kyanite
  What is a Maar?
  Rock Kits
  Range of Bear Species
  The East Africa Rift System

geology store

More From Geology.com:


geology news
Geology.com News contains news about geology and earth science from around the world.
Green Diamonds
Green Diamonds - one of rarest and most valuable diamond colors.
Dacite
Dacite - a light-colored extrusive igneous rock intermediate between rhyolite and andesite.
minerals
Minerals: Information about ore minerals, gem materials and rock-forming minerals.
Gemstones
Gemstones - Colorful images and articles about diamonds and colored stones.
Volcanoes
Volcanoes - Articles about volcanoes, volcanic hazards and eruptions past and present.
Zircon
Zircon is the primary ore of zirconium and a gemstone that is available in many colors.
Salt Domes
Salt Domes - Salt structures that are often associated with oil and natural gas accumulations.