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TITLE: Rocks From A Vanished Landscape


[Photo]
On every continent, cratons persist. They are made of low-density rocks that don't readily sink into the mantle. Because they've survived for so long, cratons are museums of plate tectonics. They contain clues to past environments and to some
of Earth's most valuable ore deposits.

 





[Photo: Komatiite]

Two-and-one-half billion years ago, Earth produced five times as much heat as today. Most komatiites formed prior to that time. Lavas rich in magnesium, they reflect unusually high eruption temperatures on the young Earth’s surface. The long, dark green crystals are amphibole that replaced pyroxene.


[Photo: Banded-iron]

Until about two billion years ago, Earth’s atmosphere contained one percent or less of its present oxygen levels. The layers of chert and hematite in this banded iron formation record seasonal changes in early Earth’s oxygen-poor ocean waters. The iron oxide precipitated when warm, oxygen-bearing water from the surface mixed with cold, iron-rich water beneath it.


[Photo: Platinum Ore]

A spectacular and unique ore deposit, the Merensky Reef is a 5.5-m (18-ft) thick layer in a gigantic, 2.1-billion-year-old igneous complex. It contains more than half of the world’s known platinum reserves.


[Photo: Conglomerate]

This rock contains tiny sedimentary particles of gold, uraninite, and pyrite. Such ores can no longer form. Higher levels of oxygen in today’s atmosphere would alter the uraninite and pyrite particles before they could be deposited.


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Smithsonian National Museum of Natural History Department of Mineral Sciences website Credits