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Iron Ore Deposits
Eventually, photosynthesis by the earliest forms of plant life (a form of life capable of feeding itself instead of feeding off of others) began to produce significant amounts of oxygen. One important thing to know about oxygen is that it likes to react with things. It is one of the most reactive of all the elements in nature. This means that it will readily attack and attach itself to other elements. Iron, in particular, is readily attacked by oxygen.Other forms of heterotrophic early life (life forms which eat things outside themselves) had been producing waste products such as iron in the form of pyrite (a rock which is sometimes called "fools gold" because it resembles gold) which built up in the early ocean. As oxygen began to be produced, a curious phenomena occured. Large amounts of iron which had accumulated in the early ocean were attacked by the accumulating oxygen. When oxygen (O2) reacts with iron (Fe) containing substances such as FeS2 (pyrite), iron ores are produced. Over a period of a billion years, huge deposits of iron ores were laid at the bottom of the sea. This activity took place between 3.5 and 2.5 billion years ago. Iron ores mined today in the United States, Australia, and South Africa, are part of the huge deposits laid down at that time. Once the oceans were swept clean of iron, then the oxygen could begin to accumulate in the atmosphere, and respiration
by sophisticated life forms could begin in earnest. It took a billion years for this process to complete. When it was finished, it closed the period in the history of the Earth which we call the Archean.
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