DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Polished section cut across the trunk of a large petrified tree. Fossilised wood like this occurs in many places all over the world, and where it is abundant we usually have to do with ancient volcanism. Volcanic outbursts, with outpour of lava, ashes a.o. ejecta, make for the rapid collapse and burial of the trees, so that they cannot decompose normally. The covering volcanics that are often of siliceous composition, over geologic time bring about the conversion of the wood into chalcedony, a very hard and durable, siliceous mineral as seen in this example. Retrieved from the well-known Petrified Forest National Park, Arizona, USA. Actual diameter 95 cm.
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