The Geologic Timescale Part of the Geology For Dummies Cheat
Sheet Geologists organize the 4.6 billion years of earth's
history into sections based on important changes seen in the
geologic record. The largest intervals are eons, with each eon
composed of many millions of years. Within the eons are eras,
which begin and end with dramatic changes in the types of plants
and animals living on earth. Within each era are multiple
periods, and within each period are multiple shorter epochs.
Because the intervals of the geologic timescale are based on
observed changes in fossilized remains on life on earth, the
span of time within each interval varies. While the divisions
may be confusing at first, learning about the events in earth's
history goes hand-in-hand with learning the geologic timescale,
shown below. (The numbers on the right side reflect the absolute
age, expressed in millions of years ago, of the earth at each
interval.) The geologic timescale is continually being revised
by new research and more accurat