Petrified wood (from the Latin root petro meaning 'rock' or 'stone'; literally 'wood turned into stone') is the name given to a special type of fossilized remains of terrestrial vegetation. Petrifaction is the result of a tree or tree-like plants having been replaced by stone via a mineralization process that often includes permineralization and replacement. The organic materials making up cell walls have been replicated with minerals (mostly silicain the form of opal, chalcedony, or quartz). In some instances, the original structure of the stem tissue may be partially retained. Unlike other plant fossils, which are typically impressions or compressions, petrified wood is a three-dimensional representation of the original organic material.
The petrifaction process occurs underground, when wood becomes buried in water-saturated sediment or volcanic ash. The presence of water reduces the availability of oxygen which inhibits aerobic decomposition by bacteria and fungi. Mineral-laden water flowing through the sediments may lead to permineralization, which occurs when minerals precipitate out of solution filling the interiors of cells and other empty spaces. During replacement, the plant's cell walls act as a template for mineralization. There needs to be a balance between the decay of cellulose and lignin and mineral templating for cellular detail to be preserved with fidelity. Most of the organic matter often decomposes, however some of the lignin may remain. Silica in the form of opal-A, can encrust and permeate wood relatively quickly in hot spring environments. However, petrified wood is most commonly associated with trees that were buried in fine grained sediments of deltas and floodplains or volcanic lahars and ash beds.
Permineralization is a process of fossilization in which mineral deposits form internal casts of organisms. Carried by water, these minerals fill the spaces within organic tissue. Because of the nature of the casts, permineralization is particularly useful in studies of the internal structures of organisms, usually of plants.
Process of permineralization, a type of fossilization, involves deposits of minerals within the cells of organisms. Water from the ground, lakes, or oceans seeps into the pores of organic tissue and forms a crystal cast with deposited minerals. Crystals begin to form in the porous cell walls. This process continues on the inner surface of the walls until the central cavity of the cell, the lumen, is completely filled. The cell walls themselves remain intact surrounding the crystals.
In silicification, the weathering of rocks releases silicate minerals and the silica makes its way into a body of still water. Eventually, the mineral-laden water permeates the pores and cells of some dead organism, where it becomes a gel. Over time, the gel will dehydrate, forming an opaline crystal structure that is an internal cast of the organism. This accounts for the detail found in permineralization. Silicification reveals information about what type of environment the organism was likely to have lived in. Most fossils that have been silicified are bacteria, algae, and other plant life. Silicification is the most common type of permineralization.
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.
Wood is composed mostly of holocellulose (cellulose and hemicellulose) and lignin. Together, these substances make up 95% of the dry composition of wood. Almost half of this is cellulose, which gives wood much of its strength. Cellulose is composed of long chains of polymerized glucose arranged into microfibrils that reinforce the cell walls in the wood. Hemicellulose, a branched polymer of various simple sugars, makes up the majority of the remaining composition of hardwood while lignin, which is a polymer of phenylpropanes, is more abundant in softwood. The hemicellulose and lignin encrust and reinforce the cellulose microfibrils.
Dead wood is normally rapidly decomposed by microorganisms, beginning with the holocellulose. The lignin is hydrophobic (water-repelling) and much slower to decay. The rate of decay is affected by temperature and moisture content, but exclusion of oxygen is the most important factor preserving wood tissue: Organisms that decompose lignin must have oxygen for their life processes. As a result, fossil wood older than Eocene (about 56 million years old or older) has lost almost all its holocellulose, and only lignin remains. In addition to microbial decomposition, wood buried in an alkaline environment is rapidly broken down by inorganic reactions with the alkali. Wood is preserved from decomposition by rapid entombment in mud, particularly mud formed from volcanic ash. The wood is then mineralized to transform it tostone. Non-mineralized wood has been recovered from Paleozoic formations, particularly Callixylon from Berea Sandstone, but this is very unusual. The petrified wood is later exposed by erosion of surrounding sediments. Non-mineralized fossil wood is rapidly destroyed when exposed by erosion, but petrified wood is quite durable.
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. Thus silicification begins within the cell walls, and the spaces within and between cells are filled with silica more gradually. Over time, almost all the original organic material is lost; only around 10% remains in the petrified wood. The remaining material is nearly pure silica, with only iron, aluminum, and alkali and alkaline earth elements present in more than trace amounts. Iron, calcium, aluminum are the most common, and one or more of these elements may make up more than 1% of the composition. Just what form the silica initially takes is still a topic of research. There is evidence of initial deposition as opal, which then recrystallizes to quartz over long time periods. On the other hand, there is some evidence that silica is deposited directly as quartz.
Wood can become silicified very rapidly in silica-rich hot springs. While wood petrified in this setting is only a minor part of the geologic record, hot spring deposits are important to paleontologists because such deposits sometimes preserve more delicate plant parts in exquisite detail. These Lagerstätte deposits include the Paleozoic Rhynie Chert and East Kirkton Limestone beds, which record early stages in the evolution of land plants. 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.
Wood can also be petrified by calcite, as occurs in concretions in coal beds. Wood petrified by calcite tends to retain more of its original organic material. Petrification begins with deposition of goethite in the cell walls, followed by deposition of calcite in the void spaces. Carbonized wood is resistant to silicification and is usually petrified by other minerals. Wood petrified by minerals other than silica minerals tends to accumulate heavy metals, such as uranium, selenium, and germanium, with uranium most common in wood high in lignin and germanium most common in wood preserved in coal beds. Boron, zinc, and phosphorus are anomalously low in fossil wood, suggesting they are leached away or scavenged by microorganisms.