What are banded iron formations and how did they form?

Banded iron formations (BIFs) are chemically precipitated sedimentary rocks consisting of alternating thin layers of iron-rich minerals—such as hematite or magnetite—and iron-poor minerals like chert. These formations are typically older than 1.7 billion years and are found on every continent.12

Their formation is closely linked to the evolution of Earth's atmosphere. During the Precambrian, Earth’s oceans contained high levels of dissolved iron. The emergence of photosynthetic bacteria that utilized water as a reducing agent released oxygen as a byproduct. This oxygen reacted with the dissolved iron in the seawater, causing it to oxidize and precipitate onto the ocean floor.134

This process acted as a massive "oxygen sink," consuming oxygen as quickly as it was produced for millions of years. Once the majority of the dissolved iron had been precipitated, oxygen began to accumulate in the oceans and eventually outgassed into the atmosphere, fundamentally altering Earth's geochemistry. While the exact mechanisms—including the potential roles of volcanic activity or specific microorganisms—remain a subject of scientific study, BIFs serve as a critical record of the transition to an oxygenated planet.431