DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Manganese nodules. Manganese nodules (polymetallic nodules) are sedimentary rock formations (concretions) found on seabeds. Nodules form when manganese and iron minerals in sea water are deposited on the surface of an object such as a tiny particle of animal skeleton. The minerals deposit over a long period, forming concentric layers of iron and manganese hydroxides around the core. Most nodules are between 5 and 10 centimetres in diameter, only growing around a centimetre in diameter every million years. Some ocean fields of nodules may one day be mined commercially.
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