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The banded iron formation theory.

The banded iron formation mystery

Banded iron formations may be the most beautiful, economically important and enigmatic rocks ever created on this plant. They are made up of alternating layers of iron ore and silica; some layers only a fraction of a millimiter across extend uninterrupted for kilometers. The worldwide heyday of banded iron formations was about 2 billion years ago. After that, their numbers fell dramatically.

The number of hypotheses on the origin of these formations has grown over the decades, but scientists have yet to settle on any one. In most of the theories, the oceans were saturated with vast amounts of dissolved iron. Periodically, perhaps because of seasonal "blooming" of oxygen-producing organisms or the upwellings of the iron-rich water, the iron was oxidized and precipitated out in layers.

Now Robert M. Garrels at the University of South Florida in St. Petersburg has added a new wrinkle to the proposals. The marine ideas, he says, can't explain why the iron layers don't contain calcium carbonates, an important component of today's oceans. So he proposes that the formations were made when freshwater streams brought the iron into bays after the rains. The thin layers could have formed when the waters evaporated each dry season, he says.

Rigel L. Lustwerk at Pennsylvania State University in University Park has another idea. She notes that as best as scientists can tell, the major episodes of iron deposition coincided with glaciation. In addition, recent studies suggest that the oceans were much warmer at that time than they are now. Lustwerk suggests that the iron formations were deposited when iron-and silica-rich warm ocean waters flowed toward the glaciated polar regions. The suddenly cooled waters could no longer hold the minerals, so they were precipitated out. The alternating layers formed, she proposes, because the two minerals have different settling rates.
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Author:Weisburd, Stefi
Publication:Science News
Date:Dec 6, 1986
Words:305
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