Petrified Wood
By Larry Larason
Logjam of petrified tree trunks in the badlands, Petrified Forest National Park Wilderness.
The Painted Desert, Petrified Forest National Park in nearby Arizona contains some of the world’s most colorful petrified wood. And it’s plentiful in the region; vendors gather it to sell to tourists. It may be iconic of the Four Corners, but we don’t have a monopoly on petrified wood; it’s found around the world. One notable petrified forest is located just outside Cairo, Egypt, near the pyramids. Another, in Argentina, contains trees more than 10 feet in diameter and up to 100 feet long.
In the Four Corners you can find petrified wood most commonly in the Triassic Chinle sediments that were laid down by a river system flowing from Texas and Oklahoma to the Pacific Ocean. But it also occurs in places like the Cretaceous beds of the Bisti Badlands. You can probably find it in every state west of the Mississippi River. I’m not sure about east of the Mississippi, but there is a rather famous site at Gilboa, New York where 375-million-year-old, Devonian-Period trees are the oldest known examples of petrified wood.
For petrifaction to occur, wood must be buried to keep it from decomposing.
And there must be silicon and water. There is some silicon almost everywhere since it composes 28 percent of the Earth’s crust. But it needs to be dissolved in water to make it available for petrification. Petrified wood is most often found where there is volcanic ash in the soil, specifically high-silica ash, such as rhyolite. The small particles of ash allow silica to be more easily dissolved. Volcanoes were common during the Mesozoic Era [Triassic, Jurassic, and Cretaceous Periods] in the Southwest, so many beds of rock that formed during that time are likely to contain a portion of ash. In particular, beds of the Chinle Formation and those of the Cretaceous sea and shoreline deposits both contain ash from volcanoes to the west and south of our region. Most of the ash has been chemically altered to clay, making these deposits sticky when wet. But the ash was fresh when logs were being petrified.
Polished slice of petrified wood (Photo by Michael-Gäbler).
Petrifaction occurs when organic [carbon based] compounds are replaced by another compound. It may involve several chemicals including calcite [calcium carbonate] and pyrite [iron sulfide]. But by far the most common is silica [silicon dioxide]. Silicon is the second most common element at Earth’s surface after oxygen. Carbon is the fifteenth most common. Silicon and carbon are next to each other in one column of the periodic table of the elements. They have been described as “kissing cousins” because of their similarities, and there has been much speculation about silicon as an alternate basis for life. Although the two elements are chemically similar their differences are more important in making carbon the basis of life on Earth. Both of them latch onto oxygen readily, but when carbon is oxidized it becomes carbon dioxide, a gas, which is easily exhaled. Silicon, however, becomes silica, sand, a solid, which, no matter how fine the particles, would be much more difficult to eliminate.
After it is buried, the wood is attacked by bacteria, which create acid conditions that cause more rapid precipitation of the silica. The wood becomes somewhat squishy until enough petrifaction has occurred to strengthen it. The process of changing wood to stone is fairly rapid, probably requiring no more than a century. I read that some sawed planks left by pioneers have been found petrified in western Nebraska, but, although this was in a reputable publication, I haven’t been able to verify it.
There are a couple of myths that should be dispelled. Petrified logs very seldom have any bark on them. For this reason paleontologists used to think that the logs in the Petrified Forest National Park must have grown somewhere else and been washed in by flash floods to where they were buried and petrified. But then some geologist had an A-ha! moment when he looked at the logs in the wood pile he kept for his fireplace, and noticed that after a year or so the bark sloughed off. Not only does it slough off easily, it also decays more rapidly than the wood itself. Now it is believed that the petrified trees grew near where they are found. Also, some stumps in growth positions are found in parts of the park.
Another myth: petrifaction preserves the cellular structure of the wood. That’s not always the case. The kind of petrified wood we make bookends out of is also called “agatized wood.” Just looking at it, even without a microscope, shows that, while it has the shape of the log, little or no tissue structure is preserved. The gray, tan, or yellowish pieces that actually look like wood are the kind you would examine with a microscope. In fact, in almost every case some of the original wood is preserved by being encapsulated in the petrifying silica. So, the less colorful specimens are better for paleontological study. I saw a petrified log in the Bisti once that was a uniform gray. It looked very much like a piece of firewood left too long in the sun and weather. I’m sure that under a microscope it would have looked just like fresh wood.
We have used wood since we first became human. The uses of wood are myriad. We can’t build with petrified wood, but we can marvel at once living tissue being transformed into a stony, but faithful, replica.
