The geologic time scale is a way of managing large amounts of time in a convenient chart. The scale is actually a measurement encompassing the entire history of the Earth—from its beginnings some 4.55 billion years ago to the present day. The largest divisions include eons, eras, and periods; the smaller time divisions include epochs, ages, and subages.

The actual divisions of geologic time are not arbitrary, or uniform. The larger divisions are based on major events that occurred sporadically over the Earth’s long history. For example, the end of the Permian Period, about 240 million years ago, was marked by a major catastrophe. Some scientists estimate that close to 90 percent of all species on the Earth died at that time, resulting in a major extinction event that may have been caused by huge volcanic eruptions or even a space object striking the Earth. The smaller divisions are usually based on specific local structures or fossils found within the rock. Most often they are named after local towns, people, and sundry other nearby associations.