Home > Encyclopedia of Science > Geologic Time

• History of the concept of geologic time
• Relative dating
• Absolute dating
• Words to Know
• Geologic Time Scale
• Determining the age of Earth

Geologic time describes the immense span of time—billions of years—revealed in the complex rock surface of Earth. Geologists have devised a geologic time scale that divides Earth's history into units of time. These units are eras, periods, and epochs. A unit is defined in terms of the fossils or rock types found in it that makes it different from the other units. Eras, the four largest time blocks in the scale, are named to indicate the fossils they contain: Precambrian (before ancient life), Paleozoic (ancient life), Mesozoic (middle life),...