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Florida Geological Survey - Geologic Topics
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Florida Rocks and Minerals

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picture showing the Geologic scale

The Geologic Time Scale

Geologists use a geologic time scale to describe the 4.5 billion years that have passed since the earth formed. The geologic time scale is constructed with the present time at the top, and progressively older time divisions as you read down the scale. A hierarchy of time period categories is used to characterize geologic time. The broadest category is the Eon, followed by the smaller subdivisions Era, Period, and Epoch. For example, the broad Phanerozoic Eon covers the last 570 million years of earth history. The Phanerozoic Eon is divided into three Eras, the latest of which is the Cenozoic Era, depicted in the time scale on this page. All the rocks in Florida that lie within several-hundred feet of the land surface, as well as those exposed in rivers and mines, were deposited during the Cenozoic Era. The Cenozoic Era is further broken down into two Periods, the Tertiary and Quaternary, which are in turn divided into a number of Epochs. Geologists apply unique names to each Era, Period and Epoch. These are commonly based on the life forms which existed during each; for example Cenozoic means “new life”, because it marked the emergence and evolution of mammals, birds, insects and flowering plants into their modern forms. Since the development of radiometric age dating techniques, geologists have been able to place absolute dates on time periods shown on the geologic time scale. By comparing ratios of naturally-occurring radioactive elements and their decay products, such as Potassium and Argon, the age of rock deposited during a particular time period may be determined. Absolute ages for the Epoch boundaries in the Cenozoic Era are shown on this time scale. For more information on geologic time, see: http://pubs.usgs.gov/gip/geotime/contents.html A complete geologic time scale, compiled by the Geological Society of America, is available at: , http://www.geosociety.org/science/timescale/timescl.htm

Last updated: July 01, 2015

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