A planet holding its breath
Deep within the Amazon rainforest lies a geological formation that records Earth's first breath. The Carajás Iron Formation is a relic from the Archean Eon, a time before complex life. Around 2.7 billion years ago, microscopic cyanobacteria in the ancient oceans began performing photosynthesis, releasing oxygen as a waste product. This was new and toxic to the anaerobic world. Before this oxygen could build up in the atmosphere, it reacted with immense quantities of dissolved iron in the seawater.
The result was a slow, continuous rain of iron oxides—rust—that settled on the ocean floor. This process, repeated over millions of years, created the distinct layers of iron-rich minerals (hematite and magnetite) and silica-rich chert that characterize Banded Iron Formations (BIFs). The Carajás Formation is the largest and one of the best-preserved examples on the planet. Its layers, some just millimeters thick, are a direct mineralogical record of the Great Oxidation Event, a planetary-scale transformation that enabled all aerobic life. The entire formation is 300 to 400 meters thick, sandwiched between layers of ancient volcanic rock.
From ancient ocean floor to modern mine
Today, the Carajás Formation is the site of the world's largest open-pit iron ore mine, operated by the Brazilian company Vale. The deposit's total reserves are estimated at around 18 billion metric tons, with an exceptionally high average iron content of 65.4%. The main ore body, primarily soft hematite, is the product of intense weathering that leached away the silica, further concentrating the iron to depths of up to 300 meters. The scale of the operation is immense, involving multiple open pits and producing over 100 million tons of ore annually. An 890-kilometer railway line transports the ore from the mine to the Ponta da Madeira maritime terminal for export.
The iron-rich plateaus where the mine is located also support a unique and threatened ecosystem. Known locally as canga, this environment of ironstone rocky outcrops within the Amazon rainforest hosts hundreds of plant species, many of which are endemic and adapted to the metal-rich soils. This creates "islands" of savanna-like vegetation in a sea of dense forest, a landscape directly shaped by the deep-time geology beneath. The mining operation exists within the Carajás National Forest, presenting a continuous challenge of balancing resource extraction with the conservation of this rare biome.