Thin layers of iron-formation occur within the approximately 2.7 billion year old greenstone lava of northern Minnesota. The term is a contraction of “iron-bearing formation,” which is precisely what it is--a rock having in places as much as 30 percent iron. Iron-formation formed as iron-rich particles precipitated and settled to the sea floor during quiet periods in volcanic activity. The iron-formation we see today consists of thinly layered red, white, and black minerals. The red layers are jasper; the white--chert (mostly quartz); and the black are iron-bearing minerals--mostly magnetite (magnetic) and hematite (nonmagnetic). A much younger iron-formation (only 1.9 billion years old) occurs along the Mesabi Iron Range that extends from Grand Rapids to Babbitt. This iron-formation (above) formed by the same process, but its deposition also involved interplay among sea water, surface rain water, volcanic activity, and some of the world's oldest life forms (cyanobacteria). When upgraded in iron content by industrial processing, rocks of the Mesabi range yield an important ore called taconite (below).


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