A planet's first breath
The Biwabik Iron Formation is a geological record of one of the most important changes in Earth's history: the rise of oxygen. This rock, part of the larger Animikie Group, formed approximately 1.9 billion years ago in a shallow sea that covered what is now northern Minnesota. Before this time, Earth's oceans and atmosphere had virtually no free oxygen. Iron dissolved readily in the seawater.
Then, photosynthetic microorganisms called cyanobacteria began producing oxygen as a waste product. This new, highly reactive gas immediately bonded with the dissolved iron, causing it to precipitate and settle on the seafloor as iron oxides like magnetite (Fe₃O₄) and hematite (Fe₂O₃). These iron-rich layers alternated with layers of silica-rich chert, creating the distinct banding visible in the rock today. Each pair of stripes represents a cycle of oxygen production and iron deposition, a record of ancient life terraforming the planet. This process continued for millions of years, creating a formation that reaches a thickness of 60 to 230 meters (about 200 to 750 feet). Only after the vast reservoirs of dissolved iron in the oceans were used up could oxygen begin to accumulate in the atmosphere, an event known as the Great Oxygenation Event.
From ancient sea to steel mills
The Biwabik Iron Formation is the center of the Mesabi Iron Range, a district stretching over 100 miles that is the main source of iron ore in the United States for more than a century. Since mining began in the 1890s, the Mesabi Range has produced more than 3.6 billion metric tons of iron ore.
Initially, miners targeted high-grade "natural" ores containing 50-70% iron. These deposits were so rich they could be scooped from massive open-pit mines and shipped directly to steel mills. The world’s largest open-pit iron mine, the Hull-Rust-Mahoning Mine near Hibbing, is more than eight miles long and 3.5 miles wide. By the 1950s, the high-grade ores were largely depleted. The industry's survival depended on a new process developed at the University of Minnesota to extract iron from the lower-grade rock, called taconite.
Taconite ore, which makes up the bulk of the formation, has an iron content of only 25-30%. The rock is blasted and hauled by 240-ton trucks to processing plants. There, it is crushed to a fine powder, and powerful magnets separate the iron-bearing magnetite particles from the waste rock. This iron concentrate is mixed with a clay binder, rolled into pellets about 10 millimeters in diameter, and fired at high temperatures. The final taconite pellets contain over 65% iron and are shipped via rail and ore boats on the Great Lakes to power the nation's steel industry.