Banded Rocks Across Upper Midwest States Reveal Early Earth Conditions

MADISON, Wis., Oct. 12 (UPI) — U.S. scientists say banded rocks found across upper Midwest states and elsewhere offer clues about the Earth’s environment of more than 2 billion years ago.

University of Wisconsin scientists say such rocks, called banded iron formations or BIFs, formed between 3.8 billion and 1.7 billion years ago at what was then the bottom of the ocean. The stripes represent alternating layers of silica-rich chert — a variety of silica — and iron-rich minerals like hematite and magnetite.

Although BIFs are a rich source of information about the geochemical conditions that existed on Earth when the rocks were made, interpreting the clues has produced controversy for decades, Professor Huifang Xu said.

He said previous hypotheses about band formation involved seasonal fluctuations, temperature shifts or periodic blooms of micro-organisms — but all of those theories left many open questions.

Now Xu and colleagues from Indiana University and the Sandia National Laboratory have developed a BIF formation model offering a more complete picture of the environment when the rocks developed, including interactions of the rocks with water and air.

The new theory of how the bands developed and what they reveal about the early ocean floor’s composition, seawater and atmosphere appears in the early online edition of the journal in Nature Geoscience.

Copyright 2009 by United Press International

Categorized | Nature & Ecosystems
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