Petrified wood

Formation

Petrified wood forms when woody stems are buried in wet sediments saturated with dissolved minerals; the lack of oxygen slows decay, allowing minerals to replace cell walls and fill voids. Wood consists mostly of holocellulose (cellulose and hemicellulose) and lignin, which together make up 95% of its dry composition. Cellulose, composed of polymerized glucose chains arranged into microfibrils, provides strength, while hemicellulose and lignin encrust and reinforce these microfibrils.

Dead wood is normally decomposed by microorganisms beginning with holocellulose; lignin is hydrophobic and decays more slowly. Exclusion of oxygen is the most important factor preserving wood tissue, as lignin-decomposing organisms require oxygen. Fossil wood older than Eocene (about 56 million years) has lost almost all holocellulose, leaving only lignin.

Wood buried in alkaline environments is rapidly broken down by inorganic reactions. Rapid entombment in mud, especially volcanic ash, preserves wood, which is then mineralized to stone. Non-mineralized fossil wood, such as Callixylon from Berea Sandstone, is rare and quickly destroyed when exposed by erosion.

Occurrences

Petrified wood can be found across the globe in sedimentary layers that date from around 390 million years ago in the Devonian period, when woody plants first emerged on land, all the way up to nearly modern times. Petrified forests generally come in two types: whole ecosystems that were buried by volcanic eruptions, where tree trunks often remain upright where they grew, and piles of driftwood that collected in river environments. At Yellowstone National Park, Amethyst Ridge contains evidence of 27 separate forest ecosystems that were buried one after another by eruptions, while Petrified Forest National Park is a classic example of driftwood gathered by rivers. Volcanic ash is especially good at preserving wood because it releases a lot of silica as it breaks down, so finding petrified wood in a sedimentary bed often points to the presence of weathered volcanic ash.

It can also form in arkosic sediments, which are rich in feldspar and other minerals that release silica during weathering. The warm, super-monsoon climates of the Carboniferous through Permian periods helped this process along, though preservation in volcanic ash is less dependent on climate and tends to keep a wider variety of species. Notable locations include sites in Egypt, Madagascar, Namibia, Sudan, China, India, Japan, Indonesia, Israel, Pakistan, and Saudi Arabia.

Did You Know?

• Petrified wood has limited use in jewelry but is mostly used for decorative pieces such as book ends, table tops, clock faces, or other ornamental objects.
• A number of Ancestral Puebloan structures near Petrified Forest National Park were constructed of petrified wood, including the Agate House Pueblo.
• Petrified wood is also used in New Age healing.