What is petrified wood and how does wood turn into stone?

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A cut and polished cross section of petrified wood.

Credit: Jeff Scoville

A section of fossilized conifer from McElprang Wash, Arizona, USA. The green color is due to silica and chromium replacing organic wood. It dates back to the Triassic Period.

Fossilization is a valuable process for learning about the past, but it also creates tripping hazards.

According to Angela Pillar, collections manager at Rice University’s Northwest Museum of Rocks and Minerals, Mary Murphy was excavating for fossil wood when she stepped out of her camper and tripped over a pine cone. A typical pine cone isn’t heavy enough to tip over someone, but this was no ordinary pine cone. The specimen was millions of years old and had been transformed into stone by minerals, water, and time.

Murphy and her husband, Dennis, who loans most of the museum’s petrified wood collection, were avid collectors of petrified wood. But “she never [the pinecone]”As long as you don’t trip over it,” Pyrrha replied, “it won’t look anything special. It’ll just blend in with the other pine cones on the ground.”

I’m sure we’ll find some amazing fossil wood in a few million years.

Angela Piller Collections Manager, Rice Northwest Museum of Rocks and Minerals

Petrified wood and its petrified pine cones often look like normal wood until you pick them up and realize they are heavier and harder than you would expect.

One of the museum’s exhibits features a fireplace with petrified wood logs, but Piller says the fire chief had to make sure it wasn’t actually usable: “Not only is it not usable, it’s not even wood,” she says with a laugh.

How does wood become fossilized?

Petrified wood is classified as a fossil, and some samples date back hundreds of millions of years. Essentially, this material is the biological components of the wood replaced with minerals, mostly silica, but also fluorite, pyrite, and calcite.

“Typically, a tree falls in a forest, and then it gets eaten by insects, it rots, there’s bacterial activity, and all of these normal things happen, so for fossil wood to form, you have to stop those things from happening, or you have to make them happen very slowly,” Piller says.Two conditions are needed for fossil wood to form: an oxygen-free environment that stops decomposition, and exposure to mineral-saturated water.

One common way these conditions occur simultaneously is a volcanic eruption. For example, imagine a tree falls into a lake, sinks to the bottom, and then a volcano erupts nearby. Normally, when a volcano erupts and sends a lahar (a flow of organic matter and volcanic sediment, such as lava and water) flowing through a forest, the trees and plants simply burn away. But if there is a lake in the path of the lava flow, the lahar will cover the water and then cool and create a cap over the lake, trapping the wood of the fallen tree in an area with very little oxygen. This causes two things to happen. First, the wood will decay much slower than wood exposed to air, because the organisms that contribute to decay will be starved of oxygen. Second, minerals carried by the water will eventually make their way through the lava cap and slowly replace the fibrous wood. This replacement happens at a cellular level, preserving fine details like the texture of the bark and the tree rings in the trunk.

But a volcanic eruption isn’t necessary: ​​Something like a landslide that covers logs in mud can also help petrify the wood. A similar process can also occur with other components of wood, such as pine cones and walnuts. The museum’s fossilized hazelnuts are just a few millimeters in diameter, less than half the size of a grain of rice.

But you’re unlikely to find petrified wood just lying on the ground. Like any fossil, it needs to be dug up and freed from oxygen-blocking sediments. Petrified wood is susceptible to erosion when exposed to the elements, and like regular rock, pieces can be broken down by water or animals. If Murphy had tripped over a fossilized pine cone, she’d probably have been excavating at the site and had probably dug it up without realizing it, but if she’d tripped over something while hiking, it’s probably just any old rock.

Identifying the type of fossilized wood requires knowledge of both geology and biology, and often a microscope to view the cell structure. But we can use information about what tends to form where to infer where the sample came from. For example, fossilized wood from Arizona is often colored red, due to iron working its way into the wood along with silica.

Do-it-yourself fossilization

If you want to make your own petrified wood, all you need is an argon furnace. In 2004, Youngsun Shin and colleagues at the Pacific Northwest National Laboratory Duplicated They studied the petrification process in a lab using pine and poplar wood, and published their findings in the journal Nature. Advanced Materials(2005, DOI: 10.1002/adma.200400371The researchers observed a coating of silicon dioxide on the wood’s cellular structure after soaking the wood samples in a hydrochloric acid solution for two days, followed by a solution containing silica for two days. After air-drying the samples, the team heated them to 1,450 °C in an argon furnace to complete the petrification process. Because silica can replicate wood’s existing cellular structure, wood may serve as a convenient template for creating complex ceramic materials at extremely small scales, the researchers say in their paper.

Fossilized wood chips.

Credit: Jeff Scoville

A cross section of a fossilized sycamore tree from Hampton Butte, Oregon, USA, dating to the Miocene epoch. Organic wood has been replaced by silica, and the red, yellow, and green colors are likely the result of different iron redox states during fossilization.

But if you don’t have access to a lab-grade furnace and want to obtain petrified wood anyway, be sure to check local regulations regarding rock hunting, the colloquial term for collecting rocks and petrified wood as a hobby. While small amounts are typically permitted on U.S. public lands managed by the Bureau of Land Management, rock hunting is generally not permitted in national parks or on private land.

Like other fossils, fossilized trees give scientists clues about what an area was like millions of years ago. The process of fossilization not only preserves the tree itself, but also traces of ecosystem activity: one candle tree in the museum’s collection even shows the direction of water flowing through it, just like a photograph. But while the fossilized wood we can study took hundreds of millions of years to form, that doesn’t mean all the samples are from the distant past. More recent events could create the conditions for even more fossilized wood to form in the future.

“Given that Mount St. Helens erupted and released a massive amount of ash, this would be a great way to bury something and preserve petrified wood,” Pillar said. “I’m sure we’ll find some amazing petrified wood in a few million years.”



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