What exactly ARE eons and other time intervals shown on the geological time scale?

The geological time scale in Figure 5.4–1 shows time blocks identified in three different ways: “eons,” “eras,” and “periods.” You’ve also already heard us refer to a fourth type of time interval: an “epoch,” as in the human epoch (or Anthropocene). So you might wonder, what defines all these different terms? The answer depends on if you are speaking in ordinary language or if you are talking specifically about geological time.

Let’s start with ordinary language. In English, these words have the following basic definitions:

• A period of time can be any amount of time.
• An eon is a very long period of time.
• An era is period of time that is marked by notable events or characteristics. For example, we might say the “era of dinosaurs” to describe the time when dinosaurs roamed the Earth, or the “computer era” to describe the time since modern electronic computers were invented.
• The word epoch is often used as a synonym for era, For example, we could also talk about the “epoch of dinosaurs” or the “computer epoch.” Note that the word age is also often used in this same way, as in the “age of the dinosaurs” or the “computer age.”

In other words, aside from the fact that an eon usually means a very long time period, the other words — period, era, epoch, and age — can all be used interchangeably in ordinary language.

Let’s turn now to the geological definitions. Remember that different geological time periods are distinguished by changes in the rock or fossil record. Because some of these changes are more dramatic than others, scientists needed a way to describe different time periods that they would all understand. For this reason, an organization called the International Commission on Stratigraphy was put in charge of defining and naming time periods on the geological time scale. This organization has defined the following:

• Eons are defined as the longest and most significant divisions in the rock or fossil record.
• The eons can be divided into a series of eras.
• The eras can be divided into a series of periods.
• The periods can be further divided into a series of epochs.

Figure 5.4–1 shows only the eons, eras, and periods, mainly because adding the epochs would have made the figure too crowded. However, if you are interested, it is easy to look up the epochs. For example, the current “period” is called the quaternary, and it has traditionally been subdivided into two epochs: the Pleistocene, which spanned the time from the beginning of the quaternary about 2.6 million years ago until about 12,000 years ago, and the Holocene, which is the time since the end of the Pleistocene. Note that, if we agree that the Anthropocene has now begun, it would mean that the Holocene is over, having lasted only about 12,000 years.