The Siderian (//; Greek: sideros, meaning "iron") is the first geologic period in the Paleoproterozoic Era and lasted from 2500 Ma to 2300 Ma (million years ago, from Greek: Mega and Latin: Annum ). Instead of being based on stratigraphy, these dates are defined chronometrically.
Abundance of banded iron formations (BIFs) peaked early this period. BIFs were formed as anaerobic algae produced waste oxygen that combined with iron, forming magnetite (Fe3O4, an iron oxide). This process cleared iron from the oceans, presumably turning greenish seas clear. Eventually, without an oxygen sink in the oceans, the process created the oxygen-rich atmosphere of today. This event is known as the oxygen catastrophe, which according to some geologists triggered the Huronian glaciation.