Wood is a pretty hardy material, but if the conditions are just right, and sufficient time passes, it can turn into something even stronger: stone.
Petrified wood is, quite literally, wood that's been 'stonified' -- as petra means rock or stone in Greek. Think of it as a wood that has become mineralized. In petrified wood, the biological fibers and structures that normally emerge in trees are replaced by minerals such as opal or quartz. This makes petrified wood a unique (and spectacular) type of fossil, but as we'll see, it needs very specific conditions to form.
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What is petrified wood
It's not that uncommon for fossils to form. Fossils typically show up in sedimentary rocks, because the processes that generate igneous and metamorphic rocks aren't really suitable for forming and preserving fossils. Most fossils are formed around the hard parts (like the skeletons) of animals. But the skeleton parts are, themselves, minerals -- whereas a tree isn't.
The trunk is the part that has the greatest chance of being petrified. The trunk contains several different parts, but 95% of the dry composition of the wood consists of holocellulose and lignin. Holocellulose typically decays slowly, but lignin is much slower to decay. Also, microbes can only decay lignin if there is oxygen present; you can probably see where this is going already.
In order for petrified wood to form, the wood must be preserved in an environment without oxygen. In practice, wood is preserved from decomposition when it is rapidly entombed in mud or other water-saturated sediments, or volcanic ash.
Many key processes in geology are triggered by water, and this is no exception. Petrified wood is formed through a process called permineralization, and water is the key agent here too.
Permineralization happens when materials from plants or bones are impregnated by mineral water and are gradually replaced by minerals. This typically happens when water fills the pores of the tissue and brings minerals with it. The most common mineral for this is quartz, but more exotic minerals can play a role too.
Underground, where this process happens, water isn't two parts hydrogen and one part oxygen -- it has a lot of minerals dissolved in it. This mineralized water gradually replaces the original tissue; bit by bit, molecule by molecule, biological tissue turns to mineral. This gradual replacement also explains why so many of the original details of the original structure are preserved.
This is also why the process is particularly interesting to researchers (as we'll see shortly) because so much of the original detail is preserved. Permineralization can also preserve soft tissue, in addition to hard tissue, which makes it all the more interesting. The decay of the cellulose and lignin and the mineral filling of pores needs to be in equilibrium for cellular detail to be preserved with fidelity.
However, the end result of the process (the petrified wood) is a mineralized fossil. Unlike other plant fossils (which are often just 2D impressions of the original structure), petrified wood is a 3D structure. It's also more resilient than most fossils, which makes it likely to endure a long time even if it is exposed to the elements, which is not the case for most fossils.
So technically speaking, petrified wood is rather permineralized wood -- because there are other types of fossilized wood as well.
Not all fossil wood is petrified
When thinking of fossilized wood, you may be tempted to think of coal -- but contrary to popular belief, coal isn't really fossilized wood.
Coal forms from plant matter, and the vast majority of the world's coal was formed all at once because of an evolutionary curiosity: basically, large trees had evolved, but the bacteria and fungi that could rot wood had not. Basically, wood evolved before wood-eaters. Large stacks of wood riddled the land, and they were buried and preserved during a period called the Carboniferous, which literally means 'coal-bearing'. There are younger deposits of coal, but far fewer, as they need to be preserved in much more special conditions (basically in environments without oxygen).
Another type of non-petrified, fossilized wood exists -- fossil tree stumps. You may have seen a tree stump in the water sometime. The majority of these tree stumps are not fossilized, but if the conditions are just right, a mold of the stump can be formed, preserving its original shape.
While this is another pretty spectacular form of wood fossilization, it also shouldn't be confused for petrified wood. Petrified wood preserves elements of the tree structure, such as the woody tissue or growth rings, whereas the fossilized tree stumps only preserve the shape, without anything else.
How old is petrified wood
The oldest petrified wood is about as old as woody plants themselves, dating from a period called the Devonian, some 390 million years ago when woody plants first appeared on land. Scientists have found petrified wood dating from the Devonian all the way to the near-present. However, some periods (and locations) are more favorable to the formation of petrified wood.
For instance, warm megamonsoon climates (such as the Carboniferous-Permian period, from around 360 million years ago to 300 million years ago) seem to yield a greater number and diversity of petrified wood species, possibly because they produce more driftwood, as is the case with Petrified Forest National Park. However, what really turbocharges the formation of petrified wood is volcanic eruptions.
Volcanic eruptions, particularly the types of eruptions that produce a lot of ash, generate exactly the type of tomb that favors permineralizations. In some areas, the results can be spectacular -- the Yellowstone Petrified Forest, for instance, shows 27 successive forest ecosystems buried by subsequent eruptions.
Petrified wood science
For starters, the environments that petrify wood are also great at conserving other forms of fossilization -- which is why petrified wood forests are prime environments for finding other fossils. In 2020, for instance, geologists studying a vast expanse of petrified wood in eastern Arizona discovered a new type of reptile. The reptile, called Skybalonyx skapter, is 220 million years old.
Fossils from other fossilized forests have helped researchers better understand the biodiversity and climate of these areas. Researchers can look for signs of abundant rainfall or drought that are reflected in the petrified wood, and they can even find clues relating to potential pests or wildfires.
In fact, researchers even want to use petrified wood to study Doggerland, the now-submerged area of land that connected Britain to continental Europe. There are also things about the evolution of trees that can be inferred from the evolution of trees themselves, if the preservation is just right.
It can take millions of years for petrified wood to form, but scientists are also getting better at replicating this process in a lab, much faster than that -- which could help us understand this amazing type of fossil even more.
While some people use petrified wood as gems or for New Age healing voodoo, it's much more than this. Our world is full of wonders, and petrified wood is definitely one of them.