Click on the image to see it enlarged. |
Modern Geologic Time Scales
Time on the geologic time scale is not broken up into equal increments, but is rather chunked together in spans of varying length based on similarities in the fossil record and geologic events. Though the time scale may be drawn as a vertical timeline, a horizontal timeline, or even a spiral like the one you see here, it always conveys the same information: names that have been given to each chunk of time, and the major life forms and geologic events associated with each chunk of time. For example, on the spiral time scale shown above, you see some of these names (Devonian Period, Holocene Epoch, etc.). You also see pictures of the life forms associated with each division of time. If the time scale is drawn vertically, the oldest dates are on the bottom. If it is drawn horizontally, the oldest dates are at the left side. In the spiral diagram, the oldest dates are at the bottom.
To make the study of geologic time easier, scientists have divided time into categories: eon, era, period, epoch, and age. See how they are defined below.
Eon |
There are four eons on the geologic time scale; each one lasts for about one billion years. Each eon is divided into eras. |
Era |
There are twelve eras on the geologic time scale; each one lasts several hundred million years. Eras are divided into periods. |
Period |
There are many periods on the geologic time scale and each one varies in length of time, though most cover several million years or several hundred million years. Periods are often bounded by mass extinctions, or times when the fossil record indicates a time of widespread extinction of living organisms. For example, the extinction of the dinosaurs marks the end of a period known as the Cretaceous Period. Each period is further divided into epochs. |
Epoch |
Each epoch on the geologic time scale covers a span of tens of millions of years. |
Age |
An age is the smallest unit of time on the geologic time scale. Most cover several million years. You (and all of modern humans) live in the Atlantic Age of the Holocene Epoch of the Quaternary Period of the Cenozoic Era of the Phanerozoic Eon. |
