What is "erathem"

n. (context geology paleontology English) The total stratigraphic unit deposited during a certain corresponding span of time during an era in the geologic timescale.

colspan=3 style="text-align:center"|Eras mapped into Eons

colspan=3 style="text-align:center"|Eras in the Phanerozoic Eon

Geologic Era

Cenozoic

Mesozoic

Paleozoic

colspan=3 style="text-align:center"|Eras in the Proterozoic Eon
542.0 (+/- 1.0) Mya — 2500 Mya

Neoproterozoic

Mesoproterozoic

Paleoproterozoic

colspan=3 style="text-align:center"|Eras in the Archean Eon
2500 Mya — years > 3600 Mya
rocks older than 2.5 Billion years — rocks older than 3.6 Billion years

Neoarchean

Mesoarchean

Paleoarchean

Eoarchean

colspan=3|Note: Rocks older than ca. 2500 Mya old are rare due to tectonic activity recycling the earths crust.

__NOTOC__ In stratigraphy, paleontology, geology, and geobiology an erathem is the total stratigraphic unit deposited during a certain corresponding span of time during an era in the geologic timescale.

It can therefore be used as a chronostratigraphic unit of time which delineates a large span of years — less than an 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, Sulphur compounds, etc.— a so-called reducing atmosphere lacking much free oxygen which was bound up in compounds).

These simple organisms, Cyanobacteria ruled the still cooling earth for approximately a thousand million (over 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 a Cenozoic, a Mesozoic and a Paleozoic erathem or matching era name.

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.<ref name="ICS Chart"

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