A slab of banded iron at the National Museum of Natural History in Washington, D.C.
From the placard:
Banded iron formations consist of alternating iron-rich and iron-poor bands. In this polished specimen from the Lake Superior region, the gray, metallic bands are nearly pure iron oxide (hematite). The red bands are jasper — silica chert colored by minute amounts of iron oxide.
These iron formations are found throughout the world in Precambrian deposits, but are rare in younger rocks. They are thought to represent a transitional stage in the development of our present oxygen-rich atmosphere. During the time when banded iron formations were deposited, vast amounts of iron dissolved in the oceans probably “soaked up” free oxygen by combining with it to form oxides. This “rust” then settled quietly to the bottom to create the iron deposits. Only when the supply of unoxidized iron was finally exhausted, which must have taken many hundreds of millions of years, did the amount of free oxygen begin to increase toward modern levels.
The characteristic banding is an enigma. It may correspond to “boom” and “bust” periods of oxygen-producing organisms, or, as others have suggested, to periodic upwellings of iron-rich water from deeper basins.