Siderian

Proposed redefinition(s)2630 – 2420 Ma

Kranendonk et al., 2012
3000? – c. 2450 Ma

Shields et al., 2021
Proposed containerNeoarchean

Kranendonk et al., 2012
Neoarchean

Shields al., 2021
EtymologyName formalityFormalUsage informationCelestial bodyEarthRegional usageGlobal (ICS)Time scale(s) usedICS Time ScaleDefinitionChronological unitPeriodStratigraphic unitSystemTime span formalityFormalLower boundary definitionDefined chronometricallyLower GSSA ratified1990Upper boundary definitionDefined chronometricallyUpper GSSA ratified1990Atmospheric and climatic dataMean atmospheric O₂ contentc. 0.014007 vol %
(0% of modern)Mean atmospheric CO₂ contentc. 11000 ppm
(39 times pre-industrial)

The Siderian () is the first geologic period in the Paleoproterozoic Era and Proterozoic Eon. It lasted from 2500 to 2300 million years ago (Ma), spanning 200 million years, and is followed by the Rhyacian Period. Instead of being based on tectonic layers, these dates are defined chronometrically.

Most continental activity in this period revolved around the breakup of the supercontinent Kenorland. While this event mainly occurred in the North American craton, volcanic intrusions and dike swarms appeared in plates pertaining to Northwestern Europe, South Africa, and Australia in connection to Kenorland's rifting. In the ocean, emissions from hydrothermal vents contributed to the production and crystallization of minerals, along with varying concentrations of sulfur and iron. While this sedimentation circulated in the ocean, the amounts deposited on the ocean floor contributed to the development of banded iron formations, along with a diverse range of ores such as pyrite and magnetite.

Cyanobacteria continued to develop their molecular structure, with eukaryotes beginning to appear near the end of the period, and they contributed to the ocean's oxidation. Their presence eventually became the partial cause for the build-up of oxygen in Earth's atmosphere, becoming known as the Great Oxidation Event. This led to a decrease in methane and carbon dioxide, which were two major greenhouse gases at the time, dropping the overall global temperature below 0 °C. As a result of this, the Earth experienced three snowball events which are collectively known as the Huronian glaciation.