An ocean turns to iron
The rock formations at Kryvyi Rih are a record of a planetary transformation. Known as Banded Iron Formations (BIFs), they consist of alternating layers of dark iron oxides and red, iron-poor chert. These layers record the Great Oxidation Event, a period that began around 2.4 billion years ago. Before this, Earth's oceans were filled with dissolved ferrous iron (Fe²⁺) and were almost entirely anoxic, or oxygen-free.
This began with the evolution of cyanobacteria, microorganisms that were the first to perform oxygenic photosynthesis. They released vast quantities of oxygen as a waste product. This new, free oxygen was toxic to most life at the time and immediately reacted with the dissolved iron in the seawater. The reaction converted the soluble ferrous iron into insoluble ferric iron (Fe³⁺) oxides, such as magnetite (Fe₃O₄) and hematite (Fe₂O₃). These iron oxides precipitated out of the water and settled on the seabed in fine layers.
The distinct banding seen in the rocks likely represents cyclical variations in oxygen production. A bloom of cyanobacteria would produce a pulse of oxygen, leading to the deposition of an iron-rich layer. When the bloom subsided, a layer of silica-rich sediment would settle, creating the lighter-colored bands. This process continued for millions of years, scrubbing the oceans of dissolved iron and allowing oxygen to eventually accumulate in the atmosphere. The main period of BIF deposition peaked around 2.5 billion years ago and largely ended by 1.85 billion years ago as the deep oceans became oxygenated.
The Kryvyi Rih Iron Basin
The formations here are part of the Kryvyi Rih Iron Ore Basin, a massive geological structure located within the Ukrainian Shield, a section of Earth's crust with rocks dating back over 3.5 billion years. The basin itself is a long, narrow strip, stretching over 100 kilometers in length but only 2 to 7 kilometers wide.
The iron-bearing rocks belong to the Paleoproterozoic Krivoy Rog Series, which is folded into a complex structure of anticlines and synclines. The main ore-bearing section can reach a thickness of up to 2,000 meters and contains seven to eight distinct seams of ferruginous quartzites and schists. The average iron content in the raw quartzites is between 30% and 40%, but in some of the richer ores, composed of martite and hematite-martite, the iron content can exceed 63%.
The scale of the deposit is immense, with industrial reserves estimated in the billions of tonnes. Mining in the region has been ongoing since the 1880s and has shaped the local landscape, with large open-pit mines and underground operations accessing ore bodies that descend to depths of over 1,000 meters.