How and where is petrified wood made?

ASK THE HORT AGENT

Question How and where is petrified wood made?

Answer Short answer, the organic matter and air space inside a tree are replaced over time with minerals. This is a very simple process, but the combinations of contributing factors can be complicated. These factors include mineral composition, temperature, time and water.

Typically when a tree dies, organic matter (which trees are made of) is broken down very quickly by aerobic decomposing organisms. Sometimes there are events which cause a tree to be covered up by soil and water. Land slides, volcanoes and meteorites are probably the most common of these events.

When a tree is covered deep in the soil, then it cannot get oxygen to feed the decomposing organisms. Hence the tree lays there without change. Over thousands and sometimes millions of years, minerals from the soil are absorbed into the pores of the wood.

Many different minerals or combination of minerals can fill the wood (called permineralization). Calcite, marcasite, or hematite may seep into the wood, but the most common mineral is silica or quartz. Other minerals such as iron or manganese add color when they slip in.

Since this process is gradual, these minerals crystallize in the wood and form perfect molds. Scientists can sometimes identify petrified wood down to the species of tree because the cellular structure of the wood has been replicated exactly.

Obviously, petrified wood is hard since the wood has been transformed into a rock. There are living trees which are also hard as rocks. Hardness of wood can be ranked using specific gravity measurements (specific gravity equals the density of a substance divided by the density of pure water). There is an entire category of hard woods called ironwoods. All of these trees have a specific gravity higher than one. This means they are really hard and will sink in water. The Snakewood tree (Piratinera guianensis) is nearly as hard as a copper penny.

Although not ironwoods, some of the hardest trees in the eastern United States include osage orange (Maclura pomifera), shagbark hickory (Carya ovata), persimmon (Diospyros virginiana) and hornbeam (Ostrya virginiana).

For more information about petrified wood and fossils, check out http://www.backyardnature.net/g/fossils.htm or http://www.fact-index.com/f/fo/fossil.html

Petrified wood can be found all over the United States. Large quantities can be found in Arizona and the Dakotas. Petrified wood is even Mississippi’s state stone. In Harnett County, North Carolina, petrified palm and tree-ferns (Tempskya) have even been found. Maybe with global warming we’ll be able to grow those tree-ferns again.

Special thanks to Rob Slonka and Oker Lee Thomas for taking me on a fantastic petrified wood expedition.

Gary L. Pierce

Horticulture Agent

Harnett County