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Rare Earth

In Meteors by Brian Koberlein0 Comments

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About 1.85 billion years ago, in what would come to be known as Sudbury Canada, a 10 kilometer wide asteroid struck with such energy that it created an impact crater 250 kilometers wide. Today the chief industry of Sudbury is mining, all because of that ancient impact. In fact much of the mining industry is possible because of asteroid impacts in Earth’s early history.

When the Earth originally formed 4.5 billion years ago, it was molten enough that heaver elements such as iron, nickel, gold and then like settled toward the core, while lighter elements such as silicon and carbon settled near the surface. This is why the Earth has a nickel-iron core with a silicate crust. Of course a lot of the elements we desire are heavy metals, from iron and gold to rare earth elements like Neodymium. Unfortunately most of the Earth’s native heavy metals sank to the core in its early history. Most of them are completely inaccessible by modern mining.

But fortunately Earth was also bombarded by meteors in its early history, and this made mining practical in two ways. The first is that asteroids and meteors themselves contain vast quantities of heavy metals. By some estimates, a single mile-wide asteroid could contain twenty trillion dollars worth of precious metals. Since most of the asteroid bombardments occurred after Earth’s crust formed, these metals were deposited near Earth’s surface, making them easier to obtain. There is some evidence that most of accessible heavy metals are extraterrestrial in origin due to this process.

The second is due to large impacts such as Sudbury. With large impacts, part of the Earth’s crust are melted. As a result, deposited material then settles in layers as it re-cools. Heavier elements settle at the bottom of the crater, while lighter ones settle near the top. As a result, heavy metals are concentrated at the bottom layer of the crater, producing rich veins of ore. Impacts can also create other useful byproducts, such as impact diamonds and pockets of oil. Chicxulub crater (caused by the famous dinosaur extinction asteroid) near the Gulf of Mexico is a region with plentiful oil deposits, for example.

It’s worth noting that Earth’s geological history is complex, and there are other processes besides meteor impacts that can bring useful elements to Earth’s crust. But it is clear that one of those processes involves large impacts. If these impacts hadn’t occurred, it would be much for difficult for modern mining to sustain our technological way of life.

Paper: J. M. Brenan and W. F. McDonough. Core formation and metal–silicate fractionation of osmium and iridium from gold. Nature Geoscience 2, 798 – 801 (2009)

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