| Eras mapped into Eons | |||
| Eras in the Phanerozoic Eon[1] | |||
| Geologic Era | Span of Years | Notes: | |
| present – 065.5 (+/- 0.3) Mya | many GSSP points | ||
| 065.5 (+/- 0.3) Mya – 251.0 (+/- 0.4) Mya | many GSSP points | ||
| 251.0 (+/- 0.4) Mya – 542.0 (+/- 1.0) Mya | mostly GSSP points | ||
| Eras in the Proterozoic Eon 542.0 (+/- 1.0) Mya – 2500 Mya | |||
| 542.0 (+/- 1.0) Mya – 1000 Mya | few GSSP points | ||
| 1000 Mya – 1600 Mya | all GSSA points | ||
| 1600 Mya – 2500 Mya | all GSSA points | ||
| Eras in the Archean Eon 2500 Mya – years > 3600 Mya rocks older than 2.5 Billion years – rocks older than 3.6 Billion years | |||
| 2500 Mya – 2800 Mya | (only GSSA points) | ||
| 2800 Mya – 3200 Mya | |||
| 3200 Mya – 3600 Mya | |||
| 3600 Mya – 4000 Mya | Earth's crust solidifies ca 3800 Mya[2] | ||
| Note: Rocks older than ca. 2500 Mya old are rare due to tectonic activity recycling the Earth's crust. | |||
It can therefore be used as a chronostratigraphic unit of time which delineates a large span of years – less than a geological eon, but greater than its successively smaller and more refined subdivisions (geologic periods, epochs, and geologic ages). By 3,500 million years ago (Mya) simple life had developed on earth (the oldest known microbial fossils in Australia are dated to this figure). The atmosphere was a mix of noxious and poisonous gases (methane, ammonia, sulfur compounds, etc. – a so-called reducing atmosphere[3] lacking much free oxygen which was bound up in compounds).
These simple organisms, cyanobacteria ruled the still cooling earth for approximately a billion years and gradually transformed the atmosphere to one containing free oxygen. These changes, along with tectonic activity left chemical trails (red bed formation, etc.) and other physical clues (magnetic orientation, layer formation factors) in the rock record, and it is these changes along with the later richer fossil record which specialists use to demarcate times early in planet earth's history in various disciplines.
Erathems are not often used in practice. While they are subdivisions of eonothems and are themselves subdivided into systems, dating experts prefer the finer resolution of smaller spans of time when evaluating strata.
Erathems have the same names as their corresponding eras. The Phanerozoic eonothem can thus be divided into Cenozoic, Mesozoic, and Paleozoic erathems. Similarly, the Proterozoic eonothem is divided youngest to oldest into the Neoproterozoic, Mesoproterozoic and Paleoproterozoic erathems, and the Archean eon and eonothem are divided similarly into the Neoarchean, Mesoarchean, Paleoarchean and the Eoarchean, for which a lower (oldest) limit is undefined.[4]
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