The geologic timescale. The column on the right is the enlarged top section of the column on the left. Click on image for full size (24K GIF) Modified from USGS

We know that the Earth is very old. Scientists currently understand that it is about 4.6 billion years old. (That's 4,600,000,000 years!) This huge amount of time is called geologic time. The evidence 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, made in the past few million years. Others are quite old – many millions or even billions of years old. These old rocks are usually quite deep within the Earth’s crust but are often exposed as plate tectonics pushes ancient rocks to the surface. Scientists who study rock layers developed a timeline of Earth history called the geologic time scale to describe the ages of various rock layers.

The geologic time scale expresses the 4.6 billion years of geologic time along a timeline that is divided into sections. Broad sections of time are called Eras and smaller sections of time are called Periods. Having names for the different sections of time helps people communicate about when events happened long ago such as when a dinosaur lived, when a volcano erupted, or when an asteroid hit.