Banded Rocks Tell About Past

Banded rocks from the midwest are giving us an idea about our past. They are known as Banded Iron Formations, or BIFs. They are thought to have formed about 1.7-3.8 billion years ago deep under the ocean. The bands are chert and minerals like hematite and magnitite. The current theory is that hydrothermal fluids mixed with seawater and created these elements, which fell down in layers on the seafloor. They dominated the Early Proterozoic era, but were no longer created as of 1.7 billion years ago. Their demise would have starved bacteria that fed off of the iron in the minerals, and maybe even allowed for different kinds of organisms to form.

Link: http://www.sciencedaily.com/releases/2009/10/091011184428.htm