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 – 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.

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.

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. It is more than four billion years old. Scientists figured out the Earth’s age by studying rocks. The rocks that you find near your house, in a park, or at school 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.

There is a special sort of calendar that shows geologic time. It’s called the geologic time scale. The geologic time scale shows 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 talk about when events happened long ago such as when a dinosaur lived, when a volcano erupted, or when an asteroid hit.