Geologic Time Defined

The earth is estimated to be about 4.5 billion years old. Our knowledge of its history comes from a number of sources. The geologic time scale is constructed through scientific methods and calculations as well as from the interrelationships of geological features as observed in the field. The principle of uniformitarianism (“the present is the key to the past”) is helpful in that we can accurately measure the rates of geologic processes we see today and apply them to the geologic past. For example, we know layers of sediment build up on the ocean floor at the rate of about 1 millimeter per year. Thus, it would take over one million years of sedimentation to form a unit of shale 1,000 meters thick.

Geologists recognize two different kinds of time: relative time and absolute age. Relative time concerns the sequence of geologic events, and absolute age measurements concern the actual age of a rock or mineral.