How Do Geologists Measure Geologic Time?

This time scale was constructed by putting in order rock layers that had similar features, such as rock types, environmental indicators, or fossils. Scientists examined clues within the rocks and determined the comparative ages of these layers. This process is called Relative Dating, and helps geologists determine the relative age of one event from another. For example, knowing nothing else about you, I can guess that you are younger than your mother; your actual age doesn’t matter if we can establish from independent evidence that someone, or some event, is older or younger than another.

As time progressed, scientists discovered and developed techniques to date certain rocks, including the age of the Earth itself. Geologists discovered the Earth was billions of years old (4.54 billion years old) and were able to establish a time frame for the Geologic Time Scale. This process is called Absolute Dating, which is the process of determining the exact amount of time that has passed since an object was formed or an event occurred.

Both absolute and relative dating have advantages and are still frequently used by geologists. Dating rocks using relative dating allows a geologist to reconstruct a series of events quickly, and can be used out in the field on a rocky outcrop. Relative dating also can be used on many different types of rocks, where absolute dating is restricted to certain minerals or materials. However, absolute dating is the only method that allows scientists to place an exact age to a rock.