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
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2012 State of Florida
