Petrification of wood can take place during periods of anywhere from a few thousand years to 250 million years. Usually, a tree falls and very soon is covered with mud or volcanic ash. The covering prevents the wood from oxidizing or rotting while the slow replacement of the wood takes place. Water, containing dissolved
minerals, mostly silica (silicon dioxide), seeps through the covering: and over time, the dissolved minerals replace the atoms of wood. The coloration of the petrified wood depends on the dissolved minerals. Most of the coloration is from metallic iron or various iron compounds. Other elements which contribute various colors include copper, cobalt, chromium, uranium, and manganese.
Petrified wood is found in several other countries besides the United States. Most in the U.S. is found in the southwest; the largest collection being in Arizona’s Petrified Forest National Park. While in most areas, the petrified logs are lying horizontally on the ground, cyclic volcanic activity in northwestern Wyoming has buried trees in their upright position with volcanic ash. These upright petrified trees may be viewed in the faces of eroded cliffs in the northeast section of Yellowstone National Park as well as one tree standing alone in its protective enclosure between Mammoth Hot Springs and Tower Junction.
All of the spans in this section are found in Petrified Forest National Park. They consist of petrified logs which have been undercut by erosion. Obviously, they are not natural arches in the usual sense, not having been eroded from sandstone or other types of rock. They are interesting, however, since their flat, unsupported spans have survived for decades or possibly hundreds of years. Also of interest is the great detail preserved in knots and the stubs of broken branches.
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