1.9-billion-year-old striped rocks that supply America's iron ore. Each layer recording ancient ocean chemistry as Earth's atmosphere slowly filled with oxygen.
James St. John, CC BY 2.0
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.
💡Fun Facts
At its peak in the 1940s, the Hull-Rust-Mahoning Mine alone supplied as much as one-quarter of all the iron ore mined in the United States.
The mineral Minnesotaite, an important component of the formation, was first identified here and is named after the state.
To expand the Hull-Rust-Mahoning Mine, the entire town of Hibbing was moved two miles south between 1918 and 1920, a process that cost $16 million and involved relocating 185 houses and 20 businesses.
Banded iron formations like this one are a finite resource that no longer forms on Earth, as today's oxygen-rich atmosphere prevents large quantities of iron from dissolving in the oceans.
The Hull-Rust-Mahoning Mine View is open daily from 9:00 AM to dusk, from mid-May through September. The Minnesota Discovery Center is open year-round with seasonal hours; typically open Tuesday-Saturday in summer.
Admission
Free admission to the Hull-Rust-Mahoning Mine View, though donations are accepted. The Minnesota Discovery Center has an admission fee that varies by age.
Accessibility
The new Hull-Rust-Mahoning Mine View, opened in 2019, features ADA-accessible paths and viewing areas. The Minnesota Discovery Center museum and trolley are also handicap accessible.
