Graduate Student, Laboratory of Steven Jacobsen
This rock formed on an ancient seafloor in Ishpeming, Michigan, not far from Northwestern University, about 2.4 billion years ago. Unlike today, the ancient oceans are thought to have contained abundant dissolved iron. When atmospheric oxygen became plentiful, this oxygen reacted with the dissolved iron to form solid iron oxides like magnetite (dark layers) in alternation with silica-rich chert (red layers). These alternating layers were formed during oxygen fluctuations that have yet to be completely explained. During the twentieth century, these rocks were mined to produce steel; today, we study them to learn about Earth’s paleoclimate.