Magnetite and Chert

March 13, 2012

Magnitite and chert

Photographer: Renee French
Summary Author: Renee French; Steve Jacobsen

These sinuous layers of magnetite (dark layers) in alternation with chert (red layers) were photographed on Michigan's Upper Peninsula (UP), near Ishpeming. The largest chert bands are about an inch or two (about 5 cm) wide. This rock formed on an ancient seafloor 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 and silica-rich chert. The alternating layers shown above formed during oxygen fluctuations that have yet to be completely explained. During the twentieth century, iron oxides were mined here to produce steel; today, we study them to learn about Earth’s paleoclimate.

This location is also home to an ancient continental rift that formed roughly 1.1 billion years ago and subsequently terminated for reasons unknown. The formation of the rift is debated. Current hypotheses point to a mantle plume or extensional stresses that would have been shut down by the Grenville orogeny. Earthscope seismic stations, part of the transportable array moving east across the country, are currently deployed over this rift and may provide data that will further our understanding of its origin and the reason for cessation. Photo taken on May 29, 2011.

Photo details: Camera Maker: Canon; Camera Model: Canon PowerShot SD1000; Lens: 5.8-17.4 mm; Focal Length: 5.8mm; Aperture: f/2.8; Exposure Time: 0.017 s (1/60); ISO equiv: 80; Exposure Bias: none; Metering Mode: Matrix; White Balance: Auto; Flash Fired: No (enforced); Orientation: Normal; Color Space: sRGB.