A nearly 3-billion-year-old banded iron formation from Canada shows that the atmosphere and ocean once had no oxygen. Photosynthetic organisms were making oxygen, but it reacted with the iron dissolved in seawater to form iron oxide minerals on the ocean floor, creating banded iron formations.

The dark layers in this boulder are mainly composed of magnetite (Fe₃O₄) while the red layers are chalcedony, a form of silica (SiO₂) that is colored red by tiny iron oxide particles. Some geologists suggest that the layers formed annually with the changing seasons.