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The Absolute Geologic Time Scale has been developed by using a process called “Isotopic Dating”, in which the decay rates of certain radioactive materials are established and measured, then used as “clocks” to calculate the ages of various rocks.

The Geologic Time Scale is illustrated here to help you see the way geologists have broken down geologic time into sections, each with an interesting and fascinating name, and each containing a package of rocks deposited during a certain time period…over the world.

This lesson is based on an online booklet that provides an introduction to the study of earth's history, published by the USGS.Using careful analogies and written historical records, the authors help students understand the development of the geologic time scale, including how this depended on gathering evidence and making comparisons.That is because the PG knows that certain conditions existed on earth during each of the geologic time periods.Knowing that a certain rock was deposited in the “Pennsylvanian”, for example, may help the PG to interpret rocks in a certain area.Also, petroleum geologists are mainly interested in rocks from the Mesozoic and Paleozoic Eras.

This is because almost all of the oil and gas found so far is contained within these rocks.These eras are separated by catastrophic extinction boundaries, the P-T boundary between the Paleozoic and the Mesozoic and the K-Pg boundary between the Mesozoic and the Cenozoic.There is evidence that catastrophic meteorite impacts played a role in demarcating the differences between the eras.The scale shows how old certain rocks are in millions of years. You will see the Jurassic rocks were deposited about 138-205 million years ago.The Jurassic was the “age of the dinosaurs.” Here is a much more detailed time scale, courtesy of the GSA (Geological Society of America). The PG is not as interested in the age of rocks in years as she is in the relative sequence of their deposition, and the time period to which they belong.Students could complete the Geologic Time Activity, on the Exploring the Environment website, in which they compare geologic time to the length of a football field, as well as take an in-depth look at the geologic time periods.