The Ectasian (from Greek ectasis, "extension") is the second geologic period in the Mesoproterozoic era and lasted from 1400 Ma ago to 1200 Ma (million years ago). Instead of being based on stratigraphy, these dates are defined chronometrically.
Name derives from continued expansion of platform covers during this period.
Evidence of a eukaryotic red algae, Bangiomorpha pubescens, has been identified from ca. 1200 Ma old rocks in the Hunting Formation (Somerset Island, Canada). This is the oldest known sexually reproducing organism and therefore the earliest known complex multicellular organism.
1,265 to 1,269 million year old Bear River dikes are interpreted to represent the western extension of the Mackenzie dike swarm.