- The Rhyacian Period ( /raɪˈeɪsi.ən/; Ancient Greek: ῥύαξ, romanized: rhýax, meaning “stream of lava”) is the second geologic period in the Paleoproterozoic Era and lasted from 2300 Mya to 2050 Mya (million years ago). Instead of being based on stratigraphy, these dates are defined chronometrically.
