How Does Petrified Wood Form?

Colorful crystal patterns are displayed in a cross-section of petrified wood in the Petrified Forest National Park, located in northeast Arizona. Arterra/Getty Images

Glen Rose, Texas is a place where the prehistoric and the modern are visibly intertwined. One hundred and fifteen million years ago, the area was a lush, marshy expanse teeming with dinosaurs. Trees were also plentiful — and many of them were gradually turned into a type of fossil known as petrified wood.

You don't have to be a science buff to appreciate this material. Indeed, as the people of Glen Rose learned, the fossils work nicely as a brick substitute. In the 1920s and 1930s, buildings made out of petrified wood popped up all over the city. Dozens of these establishments — including a petrified speakeasy — are still standing there today.

Resourceful citizens in Glen Rose also turned the wood into a major export. After all, there's always been a demand for it. Artists in 16th-century Florence used petrified wood to make fancy mosaics. Hundreds of years later, sculptors are still carving the material into tabletops, necklace beads and countless other products. Nowadays, a good-looking slab of petrified wood can sell for hundreds or even thousands of dollars.

This is to say nothing of the wood's scientific value — which is considerable. When we look at petrified wood, we're looking at a long-dead tree that was turned to stone and yet still retained its basic structure.

Burial and Preservation

How is such a transformation possible? Well, it can only take place under the right set of circumstances.

When an organism dies, it usually decomposes. Such is the circle of life. The process that most people call "rotting" is a type of decay which sets in as microorganisms break down organic matter. Usually, a dead, fallen tree will be subjected to this process.

Once in a while, though, a newly-deceased tree (or some other kind of woody plant) gets rapidly buried by mud, silt or volcanic ash. This blanketing material then shields the dead tree from oxygen. Because oxygen is the main driving force behind the decaying process, the smothered plant will begin to decompose far more slowly than it normally would.

Meanwhile, mineral-laden water or mud seeps into the dead tree's pores and other openings. As our plant's internal structure gradually breaks down, its organic material (wood fibers) gets replaced by silica and other minerals. Over a period of a few million years, those minerals will crystalize. The end result is a rock that appropriates the shape and structure of our original tree.

Dazzling Chemistry

The level of detail we find in some specimens is downright astonishing. Petrified logs with well-preserved knots, branches, and leaves have been found. Fossil-hunters also come across the occasional log with root structures attached to its base. Moreover, in certain petrified log segments, it's possible to count the growth rings.

There's also the matter of coloration. Cross-sections of petrified wood often showcase a glistening rainbow of colors, which is why the fossils are so beloved by artists. The different hues are produced by different minerals. For example, some petrified logs have a red or pink tint to them. Internal hematite is responsible for this hue. Now if there's a greenish color in your favorite hunk of wood, that means a mineral called "native iron" is inside the fossil. And shades of black are associated with "fool's gold" — also known as pyrite.

Find a Forest Near You

Caches of petrified wood can be found all over the world, from New Zealand to Greece to Argentina. Within the United States, there's an especially-famous motherload in eastern Arizona. Petrified Forest National Park is about an hour's drive away from Winslow. Two hundred and twenty-five million years ago, it was the site of a dense forest loaded with conifers and about a dozen other tree species. Log jams were often created when deceased trees fell into the prehistoric rivers that ran across the landscape. Scores of these plants were then buried rapidly in sediment and silica-rich volcanic ash.

Up in the Pacific Northwest, there's a much younger petrified forest with a more dramatic origin story. Around 15 million years ago, volcanic eruptions sent molten lava flowing across central Washington. Near the present-day city of Vantage, elms, sycamores, conifers, and ginkgoes would periodically die and then sink to the bottom of local lakes. Once that lava touched the lake water, it hardened into pillow basalt (an igneous rock, which is formed by the cooling of molten magma or lava). This encased the logs, shielding them from oxygen and allowing the petrification process to occur.

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