Era (geology)

A Ärathem (English erathem ) is a unit that is used for chronostratigraphic subdivision of the Earth's history. The corresponding unit of time in geochronology is an era (English era ). It usually includes periods of several hundred million years.

Definition

The Ärathem is the second highest unit in the hierarchy of chronostratigraphic units of the earth's history. In Geochronology, this corresponds to a time period, the era is called. The terms Ärathem and era are often in the elderly and in the popular scientific literature, but only partially correctly used as synonyms. Here, the term era is much more prevalent. The boundaries of chronostratigraphic units are bounded by isochronous ( simultaneous ) stratigraphic surfaces that are defined by chronostratigraphic marker, not by absolute age. However, only the three most recent eras are chronostratigraphic defines the boundaries of the other eras are dated by absolute age. That that only the Äratheme which are older than the Paleozoic are also the same as the corresponding eras defined. The International Commission on Stratigraphy seeks, however, to define the pre- Paleozoic eras chronostratigraphic.

The duration of an era is not defined per se, but according to the historical context. Eras include with only one exception, always more than 100 million years, often several hundred million years. In the Proterozoic and Archean they were laid down by decision of the International Commission of Stratigraphy in almost regular intervals.

Outline of a Ärathems or an era

A Ärathem or an era can be divided into a non- specific number of systems ( chronostratigraphic ) or in cycles ( geochronological ). For example, the Paleozoic - Ärathem is divided into six systems, the Mesozoic - Ärathem only three systems. The Äratheme of Hadaikums are not further subdivided into systems. Several Äratheme or eras form a Äonothem or an aeon.

Classification of Earth's history in Äonothemen or eons and eras Ärathemen or

Currently, the Earth's history is divided into four Äonothemen or eons and divided ten Ärathemen or eras:

An annotated table of Äonothemen / eons Ärathemen / eras and their subdivisions can be found in Article Geologic time scale.