Geologic Time: Our Old Earth

We know that the Earth is very old – about 4.6 billion years old. This huge amount of time is called geologic time. Our understanding of Earth’s age comes from its rocks. The rocks that are exposed at Earth’s surface are all different ages. Some are quite young, recently made by volcanoes spitting lava or beach sand that was cemented together. Others are quite old, exposed as plate tectonic movements push ancient rocks to the surface or as younger rocks weather away. Scientists who study these rock layers have developed our understanding of geologic time based on their findings.

Basically, the 4.6 billion years of geologic time is expressed graphically in a timeline divided into broad sections of time called Eras and smaller sections of time called Periods. Having names for the different time periods helps people communicate about when events happened long ago such as when a dinosaur lived, when a volcano erupted, or when an asteroid hit.

Last modified August 20, 2004 by Lisa Gardiner.

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