Geologic Time - A Visualization and Essay

Geologists have divided the whole of geologic history into units of varying length.  Together, they compose the geologic time scale of Earth history.  The major units of the time scale were delineated during the nineteenth century, principally by scientists in Western Europe and Great Britain.  Because radiometric dating was unavailable at that time, the entire time scale was created using methods of relative dating.  It was only in the twentieth century that radiometric methods permitted numerical dates to be added.

Here is an interactive diagram of the scale of geologic time.  Click on the right columns to zoom in, the left column to zoom out.  Data from the Geological Society of America

Structure of the Time Scale

The geologic time scale subdivides the 4.6 billion year history of Earth into many different units and provides a meaningful time frame within which the events of the geologic past are arranged.   Eons represent the greatest expanses of time.  The eon that began about 542 millions years ago is the Phanerozoic, a term derived from Greek words meaning "visible life."  It is an appropriate description because the rocks and deposits  of the Phanerozoic eon contain abundant fossils that document major evolutionary trends.

Another glance at the time scale reveals that eons are divided into eras.  The Phanerozoic eon consists of the Paleozoic era (paleo = ancient, zoe = life), the Mesozoic era (meso = middle), and the Cenozoic era (ceno = recent).  As the names imply, these eras are bounded by profound worldwide changes in life forms.

Each era of the Phanerozoic eon is further divided into time units known as periods.  The Paleozoic has seven, and teh Mesozoic and Cenozoic each have three.  Each of these periods is charactered by a somewhat less profound change in life-forms as compared with the eras.

Each of the periods is divided into still smaller units called epochs.  Seven epoch have been names for the periods of the Cenozoic.  The epochs of other periods usually are simply termed early, middle, and late.

Precambrian Time

Notice that the detail of the geologic time scale does not begin until about 542 million years ago, the date for the beginning of the Cambrian period.  The nearly 4 billion years prior to the Cambrian are divided into two eons, the Archean (archaios = ancient) and the Proterzoic (proteros = before, zoe = life).  It is also common for this vast expanse of time to simply be referred to as the Precambrian.  Although it represents about 88% of Earth history, the Precambrian is not divided into nearly as many smaller time units as the Phanerozoic.

Why is the huge expanse of Precambrian time not divided into eras, periods, and epochs?  The reason is that Precambrian history is not known in great enough detail.  The quantity of information that geologists have deciphered about Earth's past is somewhat analogous to the detail of human history.  The further back we go, the less that is known.  Certainly more data and information exist about the past 10 years than for the first decade of the twentieth century; the events of the nineteenth century have been documented much better than the events of the first century AD; and so on.  So it is with Earth history.   The more recent past has the freshest, least disturbed, and most observable record.  The further back in time. a geologist goes, the more fragmented the record and clues become.  There are other reasons to explain our lack of a detailed time scale for this vast segment of Earth history.

1. The first abundant fossil evidence does not appear in the geologist record until the beginning of the Cambrian period.  Prior to the Cambrian, simple lifeforms such as algae, bacteria, and worms predominated. All of these organisms lack hard parts, an important condition favoring preservation.  For this reason, there is only a meager Precambrian fossil record.  Many exposes of Precambrian rocks have been studied in some detail, but correlation is often difficult when fossils are lacking.

2. Because Precambrian rocks are very old, most have been subjected to a great many changes.  Much of the Precambrian rock record is composed of highly distorted metamorphic rocks.  This makes the interpretation of past environments difficult because many of the clues present in the original sedimentary rocks have been destroyed.

Radiometric dating has provided a partial solution to the troublesome task of dating and correlating Precambrian rocks.  But untangling the complex Precambrian record still remains a daunting task.

Terminology and the Geologic Time Scale

There are some terms that are associated with the geologic time sale but are not "officially" recognized as being part of it.  The best known, and most common, example is "Precambrian" -- the informal name for the eons that came before the current Phanerozoic eon.  Although the term Precambrian has no formal status on teh geologic time scale, it has been traditionally used as though it does.

Hadean is another informal term that is found on some versions of the geologic time scale and is used by many geologists.  It refers to the earliest interval (eon) of Earth history -- before the oldest-known rocks.  When the term was coined in 1972, the age of Earth's oldest rocks was about 3.8 billion years.  Today that number stands at slightly greater than 4 billion, and, of course, is subject to revision.  The name Hadean derives from Hades, Greek for underworld -- a reference to the "hellish" conditions that prevailed on Earth early in its history.

Effective communication in the geosciences requires that the geologic time scale consist of standardized divisions and dates.  So, who determines which names and dates on the geologic time scale are "official"?  The organization that is largely responsible for maintaining and updating this important document is the Interanl Comittee on Straigraphy (ICS), a committee of the International Union of Geological Sciences.  Advances in teh geosciences require tha tthe scale be periodically updated to included changes in unit names and boundary age estimates.

For exmaple, the geologic time scale shown in