Petrified Wood

My Grandfather was not a geologist, but he had a great interest in the many strange "rocks" that he found. Some of the things he found were not rocks at all, but fossils and petrified wood. When he died, these curiousities were passed on to me.

When my husband and I were still dating, one of the first trips that we took together was a week-long trip full of geologic interest. We visited Grand Canyon National Park, Dinosaur National Monument, Canyon de Chelly National Monument, and Petrified Forest National Park. Because of my Grandfather's interest in petrified wood, and thus my Mother's interest, I have always been fascinated with how wood could become stone.

"Petrify" comes from the Greek word petra, meaning "stone." If you look at a piece of fresh wood, you'll see that it is composed of the hard, woody cells that are filled with air and water. If this piece of wood is submerged in water, or buried in an area with a lot of water seepage, small amounts of minerals that are suspended in the water will be deposited behind in the wood where air and water were in life.

Climate and environmental conditions will affect how well the wood is preserved. Once a piece of wood is broken from the tree, or dies from disease, it begins to decay. But decay proceeds at different paces depending on conditions. Oxygen, while necessary for animal life, is also necessary for many of the biochemical reactions that lead to decay. If oxygen is not available, decay proceeds at a much slower pace, and the wood can be preserved for a long period of time. Volcanic ash is also a great preserver because it coats the wood and keeps out oxygen.

An oxygen-less environment is not the only necessary ingredient for petrification to occur. A supply of mineral-rich water is also required. The two most common chemicals seen in petrification are silica (found in sand) and calcium carbonate (found in rocs like limestone and dolmite).

Petrification can occur in a variety of ways. Sometimes the mineral components are so fine that even the most delicate structures of the wood are preserved, so that the cellular structure can be seen. Sometimes the inner spaces become filled with minerals, but the cellular structure decays to leave only the minerals. Or the wood may decay only partially, and free crystals may develop within the spaces, creating something like a geode. You might find amethyst or other gem crystals if the petrification process is due to siliceous (from silica) petrification of calcite crystals in calcareous (from calcium carbonate) petrification.

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