Imagine walking through a forest where the trees are not made of wood but stone—ancient, fossilized remnants of a world that existed millions of years ago. These petrified logs, once towering trees, have been transformed through a slow, fascinating process that replaced organic material with minerals, turning wood into rock. At places like Arizona’s Petrified Forest National Park, these stone-like structures stand as vibrant, colorful reminders of Earth’s prehistoric past.

How wood turns into fossils

Petrified wood forms through a process known as permineralization, which happens under very specific conditions. It all starts with the burial of organic plant material—trees and other types of wood—under layers of sediment. This sediment creates a protective environment, shielding the wood from decay by cutting off exposure to oxygen and organisms that would otherwise break it down.

This process requires a very specific set of conditions. First, the wood must be buried quickly, often by volcanic ash, mudflows, or sediment from rivers and lakes. This rapid burial is critical to prevent the wood from decomposing before the fossilization process can begin.

Once buried, groundwater plays a crucial role. The water, rich in minerals and dissolved solids, flows through the layers of sediment, seeping into the wood. Over time, these minerals replace the organic material bit by bit, molecule by molecule. The wood’s cellular structure, even down to the smallest detail like tree rings, can be preserved during this process.

What starts as a living organism becomes a rock-hard fossil, keeping its original shape and form while transforming into a completely different substance. Minerals like silica, calcite, pyrite, and opal are the most common substances that replace wood during this petrification process.

The final result is petrified wood that still looks like a tree trunk or branch but is now composed entirely of minerals. It can even retain intricate details, such as bark textures and internal structures that allow scientists to study these ancient plants.

Without the combination of rapid burial and mineral-rich groundwater, the wood would decay and decompose before it could turn to stone. However, when conditions are perfect, petrification can occur over thousands to millions of years.

The Petrified Forest National Park

Some of the most spectacular fossilized wood in the world is preserved at the Petrified Forest National Park in Arizona.

The park sits within the larger Painted Desert, a region known for its colorful rock formations. Its petrified logs are remnants of an ancient forest from the Late Triassic period, about 225 million years ago.

Back in that era, this region was a lush, subtropical area filled with towering trees, ferns, and other vegetation. Dinosaurs roamed the land, and rivers and lakes crisscrossed the landscape. The trees, primarily species of coniferous plants and other prehistoric flora, would fall into rivers or floodplains, where they were rapidly buried by sediment, volcanic ash, and mud. This swift burial preserved the wood from decay and set the stage for petrification.

In fact, during the Late Triassic period, much of the southwestern United States was located near the equator, covered by vast tropical forests and wetlands. The wood found in the park belonged to species of extinct conifers, cycads, and ginkgoes. These trees are ancestors of modern plants, but some, like cycads, still exist today in tropical environments.

Over time, tectonic forces and erosion exposed these fossilized trees, allowing visitors today to see what was once hidden beneath the surface. The petrified wood in the park often displays a dazzling array of colors, ranging from reds and oranges to greens and blues. These colors come from the different minerals that replaced the organic wood material. Iron oxides, for example, create red and orange hues, while manganese produces blue and purple shades.

Petrified Wood Around the World

While Arizona’s Petrified Forest National Park is one of the most famous sites for petrified wood, it’s far from the only one. Petrified forests have been discovered all over the world, from Argentina and Australia to China and Egypt.

One notable example is the petrified forest in Lesvos, Greece, which formed 20 million years ago when volcanic activity buried a subtropical forest. In Patagonia, Argentina, the Sarmiento Petrified Forest preserves massive petrified trees from the Eocene epoch, around 45 million years ago. The sizes and varieties of petrified trees in these regions provide valuable data on how ecosystems have evolved over millions of years.

Given its scientific and aesthetic value, petrified wood is protected in many areas around the world. In places like Petrified Forest National Park, it’s illegal to remove pieces of petrified wood. Unfortunately, illegal collecting still occurs, and it poses a threat to preserving these unique geological wonders.

When you walk through a petrified forest, you are essentially stepping back in time—millions of years in the past. These fossilized logs tell stories of ancient ecosystems and offer a tangible connection to the Earth’s deep history. As we learn more about the processes that transform organic life into stone, we gain a greater appreciation for the natural world and its ability to preserve life in extraordinary ways.