Petrification as literally translated from the Greek is “to change to stone” In Greek mythology, the serpent haired Medusa was so ugly that one look turned the viewer into stone. The many boulders of petrified wood that occur around the area weren’t changed by Medusa’s glare. The often beautiful fossil wood often gets more than a casual glance from most collectors however. Tell me that you wouldn’t pick up a nice piece laying in the Powder River Valley if you could get it back home. (By the way, any rock you pick up, belongs in reality, to the land owner!) Fossil wood chunks are really desirable collectables. There are even entire forests of ancient trees that were preserved and are protected as national monuments. Fossil wood occurs in nearly every state of the union and in most countries of the world.
Petrified wood is commonly called wood opal. The “opal” part is to signify the small amount of water that is mixed in with the silica dioxide (SiO2) that the fossil is composed of. (Real Opal is SiO2 with some water between the atoms) This SiO2 siliceous material is the same basic stuff that quartz is made of but it is not crystalline like quartz is. I need to point out that other minerals can also replace the wood material. Coach Hayes (who has a geology background too!) presented me last year with a beautiful piece of petrified wood for my teaching collection from a secret site in southwestern Powder River County. Unusually this specimen has the petrifying minerals being iron oxide (hematite) and gypsum (selenite). It is not hard like the silica replaced wood but is very impressive none the less. The softer material would never survive water transport and as such is quite rare and will only come out of the ground in place (insitu). I understand a whole log of the material remains insitu. Thanks Coach!
One common process that petrifies wood might go something like this. An ancient forest became buried by sediments too quickly for the wood to decay in an oxygen environment. The wood was removed from oxygen and thus bacterial action for many hundreds of years to many thousands of years. During this time, the wood was exposed to minerals that were dissolved in (ideally) hot, acid ground water. This groundwater penetrated into the porous parts of the buried wood where the wood acted as a lattice work which was slowly replaced by the silica. The acid softened the wood fibers, changing the acidity, which caused the silica to come out of the solution and replace the wood fibers atom by atom. The wood was literally dissolved away by the acid water which left silica in its place. Technically, petrification indicates that the entire object was completely replaced by mineral material.
It is important to note that the process of Permineralization is slightly different as some of the original material remains with only the pores being filled with silica. Some petrified wood can be up to 15 percent original wood material. Voids in the wood are often filled with a geode like material. What ever the wood was replaced with originally may even be replaced again at a later date if the ground water chemistry changes. A sequence of events like wood to calcite to silica or any combination of the above is possible.
A myriad of color combinations is probable because of the various decay rates of the different parts of the tree rings and branches. This occurs along with the mineral content in fluids changing in composition/proportion over a broad time frame. Impurities (mineral salts) such as calcium carbonate (calcite), pyrite (fool’s gold), dolomite (magnesium rich calcite), iron and magnesium oxides all contribute to the various colors of red, yellow black and orange. Thus, different compositions over time of the impurities results in different colors. The quality of the replacement can be exquisite preserving even microscopic detail of the original flora.
There have been some recent news stories you might have seen where a lab in Washington State has created a process to artificially create petrified wood under controlled condition albeit on a very small scale. They might use the process to enable certain industrial uses but they won’t be cutting book ends for your fireplace mantle with them. Mother nature does it better in this case.
Keep your eyes on the ground and look at what you kick. One, it might hurt, or two, it might be petrified wood. I have even found beautiful petrified pieces as gravel in driveways.