|225-MillionYear-Old Petrified Opal Tree Trunk Located in Arizona. Colorful crystal patterns are displayed in a cross-section of petrified wood in the Petrified Forest National Park, located in northeast Arizona. Photo by: Patrick Fuchs|
Over 200 million years ago, the logs washed into an ancient river system and were buried quick enough and deep enough by massive amounts of sediment and debris also carried in the water, that oxygen was cut off and decay slowed to a process that would now take centuries.
Petrified wood forms when woody stems of plants are buried in wet sediments saturated with dissolved minerals. The lack of oxygen slows decay of the wood, allowing minerals to replace cell walls and to fill void spaces in the wood.
Minerals, including silica dissolved from volcanic ash, absorbed into the porous wood over hundreds and thousands of years crystallized within the cellular structure, replacing the organic material as it broke down over time. Sometimes crushing or decay left cracks in the logs.
Most of the color in petrified wood comes from trace metals. Of these, iron is the most important, and it can produce a range of hues depending on its oxidation state. Chromium produces bright green petrified wood. Variations in color likely reflect different episodes of mineralization. In some cases, variations may come from chromatographic separation of trace metals.
Some 40 minerals have been identified in petrified wood, but silica minerals are by far the most important. Calcite and pyrite are much less common, and others are quite rare. Silica binds to the cellulose in cell walls via hydrogen bonding and forms a kind of template. Additional silica then replaces the cellulose as it decomposes, so that cell walls are often preserved in great detail.