The Rhyacian (pron.: // or |reai-ashian| Greek: Ρυαξ (rhyax), meaning "stream of lava") is one of the first geologic periods in the Proterozoic that lasted from about 2,350 Mya to around 2,055 Mya (million years ago).
The first known eukaryotes began to evolve in the Rhyacian period. The multicellular Francevillian Group Fossils, at 2.1-Gyr-old are from the Rhyacian period.
- The Huronian global glaciation began at the start of the Rhyacian and lasted until 2,200 Mya.
- The Bushveld Complex and other similar intrusions formed during this period.