Interesting descriptions and explanations
of various rocks and minerals

page 3


How Does Wood Petrify?

petrified wood

Wood must first be covered with such agents as volcanic ash, volcanic mud-flows, sediments in lakes and swamps or material washed in by violent floods - by any means which would exclude oxygen and thus prevent decay.  A number of mineral substances (such as calcite, pyrite, marcasite) can cause petrification, but by far the most common is silica.  Solutions of silica dissolved in ground water infiltrate the buried wood and through a complex chemical process are precipitated and left in the individual plant cells.  Here the silica may take a variety of forms: it may be agate, jasper, chalcedony or opal.  The beautiful and varied colors of petrified wood are caused by the presence of other minerals that enter the wood in solution with the silica.  Iron oxide stains the wood orange, rust, red or yellow.  Manganese oxide produces blues, blacks or purple. All of this combined with millions of years turns the once thriving tree to rock.  Sometime the replacement of the cell structure of the tree is so complete that the growth rings, knots and other growth marks are still very obvious.

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