The Siderian ( /saɪˈdɪəri.ən, sɪ-/) is the first geologic period in the Paleoproterozoic Era and Proterozoic Eon. It lasted from 2500 to 2300 million years ago (Ma), spanning a time of 200 million years, and is followed by the Rhyacian Period. Instead of being based on stratigraphy, these dates are defined chronometrically.
The name Siderian is derived from the Greek word sideros, meaning "iron", and refers to the banded iron formations formed during this period. The term was proposed by the Subcommission on Precambrian Stratigraphy as a subdivision of the Proterozoic Eon, and was ratified by the International Union of Geological Sciences in 1990. Since the Siderian is well-defined by the lower edge of iron-deposition layers and the initial appearance of glacial deposits, alternate names have been suggested to mark the upper half of the period stratigraphically. The term Oxygenian was suggested in 2012 due to the change in Earth’s atmosphere during this time, while the name Skourian was proposed in 2021 as a rock-based alternative. As of December 2024, the Siderian is the earliest internationally recognized period on the geological timescale.
