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Precambian, Paleozoic, Mesozoic, and Cenozoic Eras Help

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By — McGraw-Hill Professional
Updated on Aug 30, 2011

Geochronological Units

If you visit a Museum of Natural Sciences or Natural History you won’t be looking at brightly colored paintings or finely crafted statues created by human artists. Instead, you will see thousands of fossils and preserved shells and bones of ancient marine life, plants, and animals created by Nature.

These exhibits provide a chronological history of the Earth by displaying rock, plant, and animal specimens. In addition, besides giving the location where each sample was discovered, most museums also date samples according to their place in geological time.

Geological time is measured and divided into various parts called geochronological units.

Although people sometimes use the word eon to mean a really long time, like “it has been eons since I visited with my cousin,” the term actually comes from the Greek word, eos , meaning “dawn.” Geochronological units start with the major divisions of time called eons . Eons are measured in millions of years. As time dating is refined, the boundaries of the eons may change slightly, but most geological dating is calculated with a margin of error ± 60 million years either way.

The three major eon divisions are the Archean , Proterozoic , and Phanerozoic .

The Archean (Greek for “ancient”) eon is commonly thought to include the oldest rocks known. It is often called the early Precambrian era which begins with the formation of the Earth, about four billion years ago until about 2500 million years ago. The Proterozoic eon is now thought of as being part of the late Precambrian era.

Figure 2-4 provides a United States Geological Survey geochronological timescale. Eons, eras, and periods are shown.

Geological Time Precambrian

 

Fig. 2-4. Compared to ancient eras, our modern time period has been very small.

Precambrian

The Archaen and Proterozoic eons are also known as the Precambrian eon. Rocks and fossils from the Precambrian time are calculated to be between 4 billion and 600 million years old, respectively. These super-ancient times, following the original formation and cooling of the Earth along with the formation of mountains, oceans, and much of the original development of life, are the subject of a lot of theory and speculation.

The Archaen (early Precambrian ) eon is the most ancient time period and considered by most scientists as the beginning of the time divisions. This was the time when diverse microbial life thrived in the primordial oceans. The atmosphere was still very much anaerobic (lacking in oxygen) from the belching of ancient volcanoes and the development of the original landmasses.

The Proterozoic (late Precambrian ) eon is the more recent of the two times. It was during this chapter in the Earth’s history that the earliest forms of single-celled plant and animal life, like protozoa, were thought to have developed.

Following these first ancient time divisions, the Phanerozoic eon was further divided into the Paleozoic , Mesozoic , and Cenozoic eras. Table 2-2 shows the time divisions of different eras and smaller subdivisions into periods and epochs .

Table 2-2 Geological time is divided into eras, periods, and epochs.

Eon

Era

Period

Epoch

Years (millions)

Phanerozoic

Cenozoic

Quaternary

Holocene

    0.1

 

 

 

Pleistocene

    2

 

 

Tertiary

Pliocene

    5

 

 

 

Miocene

  25

 

 

 

Oligocene

  37

 

 

 

Eocene

  58

 

 

 

Paleocene

  66

 

Mesozoic

Cretaceous

 

140

 

 

Jurassic

 

208

 

 

Triassic

 

245

 

Paleozoic

Permian

 

286

 

 

Carboniferous

 

320

 

 

Devonian

 

365

 

 

Silurian

 

440

 

 

Ordovician

 

500

 

 

Cambrian

 

545

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