Petrified wood

Petrified wood is the fossilized remains of ancient forests saturated with mineral-filled water, which converted the woody tissues into stone. Scientists studying such preserved trees are interested in learning about prehistoric plants and their environments.

Petrifying Ancient Forests

The Latin word petros, meaning rock, is the source for the scientific term “petrification.” Petrified wood is actually stone remnants from prehistoric trees. During the Triassic Era, gymnosperms—seed-producing trees without flowers, such as gingkos and conifers—grew over much of the Earth's landmass. Volcanic eruptions triggered tremors, lightning, and heavy rains, which washed trees from higher elevations down to swampy valleys. As they were pushed downhill, the trees were stripped of their bark, branches, and roots from the force of the water's impact, and they also broke into pieces. Usually, trees soaking in deep, muddy water would decay, but silt rapidly and completely covered these trees, preventing exposure to oxygen and inhibiting aerobic decomposition. Volcanic ash in the floodwater consisted of inorganic compounds, such as magnesium carbonate and iron sulfide, and the trees also absorbed silicon dioxide (called silica) that had dissolved in groundwater. The minerals filled the spaces between cells in the tree trunks and branches. Molecules of these inorganic materials replaced molecules of organic tissues. During the subsequent millions of years, wood gradually became stone in the process of silicification. Assisted by extreme pressure and temperatures, the silica lodged in the wood was transformed into quartz. Plants that have undergone petrification are also referred to as being permineralized.

The trees remained preserved under the soil for millions of years until soil erosion and shifting plates exposed them. Manganese, lithium, copper, and iron created patterns of bright colors as wood fossilized. Some petrified wood displays varying rings of vivid colors, resembling agates. Other pieces are brown and look like driftwood. More significant than its beauty, petrified wood reveals information about the history of plants on the Earth. Unlike other fossils seen as an impression or compression, petrified wood is a three-dimensional representation of its organic material that preserves its external shape and internal structure. The preserved tree trunk sections also indicate the size of the Triassic Forest. Scientists have even seen chromosomes and stages of nuclear division in petrified cells. A termite nest was discovered in one petrified log, offering clues about that insect's communal evolution.

Identifying Petrified Wood

Geologists and paleobotanists analyze petrified wood samples to specify the type of ancient tree that became fossilized. A piece of petrified wood from a hardwood tree is carved into a small cube approximately 2.5 centimeters square. The cell structure of hardwood fossils is more diverse than that of softwood trees, causing its source to be more easily identified. Scientists choose pieces that exhibit an intact cell structure. Using a saw and grinding wheel as needed, they flatten the cube's surfaces and sand them to preserve the tree's natural rings and remove any marks made while shaping the cube. Researchers then examine the specimen with a microscope at ten times, forty times, and one hundred times magnification to scrutinize how the wood's bands and pores are arranged, both of which are crucial identifying characteristics. Softwood samples require greater magnification (four hundred times) of thin slivers, only one or two cells thick, to permit enough light to shine through when mounted on a slide.

Wood anatomists then describe the sample's cell structure, which they compare to records of previously identified petrified wood and existing trees. North Carolina State University and the International Association of Wood Anatomists created a computer program known as the General Unknown Entry and Search System (GUESS), which efficiently checks its databases for matching cell patterns. In addition to databases that contain information about existing hardwood and softwood trees, another database for fossil hardwoods has information on at least 1,356 types of trees of more than 1,200 species that have been identified. Codes direct researchers to scientific journal citations for detailed illustrations.

Early identification of petrified wood relied on seeking similarities with existing trees, even though the petrified sample may have represented an extinct tree type. Because most petrified wood lacks fruit or leaves to aid categorization, paleobotanists had to focus on unique aspects of cell structure to distinguish fossil trees. As petrified wood samples were described in journals, scientists developed models for comparison to determine the geological age and geographical source of fossil wood. Identification remained difficult, however, because the cell structures of some trees resembled those of other species more than trees in their own families.

Three types of petrified wood are found in the Tertiary strata: nondescript silicified wood, petrified palm wood, and massive, silicified wood. Nondescript silicified wood has undergone silicification but still appears to be woody structurally. Difficult to identify because of its generic structure, nondescript silicified wood requires expert authentication for accurate labeling. Petrified palm wood has rod structures that reinforced the tissue strength before the wood grain became silicified and that look like spots or lines when the wood is cut. Popular among rock collectors, this type of petrified wood is also Arizona's state fossil. Massive, silicified wood is difficult to recognize because the tree's grain was destroyed during silicification, thus making identification reliant on awareness of the area in which the tree was located and comparison to other petrified wood in adjacent territory.

Petrified Wood in the United States

Arizona's Petrified Forest National Park is one of the best-known areas with petrified wood. This wood originated from a forest of giant conifer-like trees that grew from Texas to Utah. Located in northeastern Arizona, the park's 93,533 acres is home to one of the largest and most varied assortments of petrified wood in the world. The desert is dotted with stone log fragments. Although some visitors expect to see rock trees standing in clumps, similar to a natural forest, these petrified trees rest where they fell individually or in groups.

The park features four primary petrification zones. At Blue Mesa, petrified logs have emerged from an area of blue- and gray-colored clay. The Jasper Forest consists of a valley scattered with petrified wood. It was covered with logs at one time, but nineteenth-century prospectors collected the stones to sell. Nearby, a petrified tree trunk called Agate Bridge crosses a stream. The Rainbow Forest contains the largest amount of petrified wood in the national park and some of the largest logs, some of which are several meters long. The Agate House is a hut that sixteenth-century Indigenous Americans constructed completely from petrified wood. The Crystal Forest's petrified wood represents what was once a vast landscape of logs sparkling with quartz and purple amethyst crystals. By the twentieth century, prospectors and tourists had taken the most beautiful pieces; as a result, local residents sought government protection against further theft. In 1906, the petrified wood site was declared a national monument, finally achieving national park status in 1962.

The National Park Service tries to protect petrified wood by preventing the nearly 1 million annual visitors from seizing samples. Rangers patrol sites and ask tourists to report any thefts they witness. Despite these precautions, several tons of petrified wood disappear annually from the Arizona park. As a deterrent, local superstitions have been encouraged to warn people that illegally acquired fossils will bring them bad luck. Privately owned sites adjacent to the park offer collectors opportunities to search for petrified wood without any restrictions. Stores sell small pieces that have been artistically cut and polished at affordable prices, which people use to decorate their homes, gardens, and even aquariums. Petrified wood is both Arizona's state fossil and its state rock.

Because the ancient forest lived simultaneously with the dinosaurs, archaeologists look for dinosaur fossils near petrified wood in the Arizona park. Although collection and thefts have greatly reduced the number of petrified logs, authorities believe some areas of the park may shelter petrified wood buried as much as 100 meters beneath the surface.

Petrified wood has been discovered in many other regions of the United States, especially in areas in the West where volcanic activity occurred, such as Yellowstone National Park, and in areas where rivers and streams deposited large amounts of sand, such as Louisiana and Texas. Washington State is home to the Ginkgo Petrified Forest State Park, which contains petrified logs that began fossilizing during the Miocene epoch. This petrified wood is unique because it includes petrified ginkgo, an indigenous tree that no longer grows naturally there. An unusual type of petrified wood resembling pebbles and that originated in the Chehalis Valley is sometimes seen on the state's beaches.

The Calistoga, California, petrified forest is considered one of the best sources of Pliocene fossils similar to existing redwoods. Measuring more than 2 meters in diameter, the fossil logs reveal gray stone veins of quartz. Petrified wood in New Mexico's Bisti Badlands is not as colorful as neighboring Arizona's fossil wood. In Utah, petrified wood has been found near the Escalante River and the Coyote Buttes region near the Paria River. Petrified wood has also been located in the southern states of Mississippi and Alabama.

Petrified Wood Worldwide

Scientists have also found petrified forests in other parts of the world. A petrified forest on the Greek island of Lesvos was named a protected site by presidential decree. A museum was established to encourage scientific research about this Greek forest's origins. Scientists have determined that the petrified wood began fossilizing during the Late Oligocene to Lower-Middle Miocene epochs. Unlike other sites of petrified forests, the Lesvos fossil trunks are erect and still have intact roots penetrating into fossilized soil and fossilized branches, leaves, cones, and seeds.

Scientists realize that these trees were fossilized where they grew, offering insight into the environment and climate of ancient Lesvos. The ancient trees of Lesvos were well preserved during the petrification process, and such details as rings indicating age and growth patterns are visible. Paleobotanists have determined that both gymnosperms and angiosperms (flowering trees) grew in the ancient forests, along with pteridophytes, such as ferns. Many of these ancient species no longer grow in the Mediterranean. Instead, they are found in Asian and American tropical and subtropical regions, indicating that Lesvos's petrified forest presents information about how the earth evolved as the continents moved apart.

Other sites of fossilized wood also reveal details about the planet's development. Argentina's Petrified Forest on the Central Steppes was created after the formation of the Andes Mountains. Larger than American wood fossils, Argentinean petrified wood includes pieces 27 meters long and 3 meters in diameter. The petrified forest in Namibia on the African continent contains giant tree trunks that are as long as 30 meters. Petrified wood has also been discovered in Australia, India, England, the Republic of Türkiye (Turkey), and Switzerland.

Petrified Wood's Legacy

Paleobotanists research petrified wood to determine how the Earth and the plants that have grown on its surface have changed since ancient geological periods. They study how plants are related and descended from similar ancestors. In addition to compiling lists and descriptions of thousands of modern tree species, scientists have noted existing trees represented by petrified wood and cataloged types of ancient trees identified solely by fossil samples. Paleobotanists realize that many more tree species have existed during the Earth's existence than are documented and that many species, existing and extinct, await discovery. They note that pieces of fossil wood are often the oldest known specimens of a tree species and might be the predecessors of living trees.

Petrified wood helps researchers comprehend how trees have evolved to adapt to environmental and climatic conditions. For example, researchers have hypothesized that the cell structure of tree xylem has not changed as much as fruit and leaves. Fossil wood also sometimes reveals the reason for the demise of extinct trees. The study of petrified wood has altered how scientists perceive both ancient geological ecosystems and modern environments by offering a perspective on which plants lived on the Earth and survived, adapted, or died according to changes in the atmosphere and crust.

Discoveries of petrified wood continued to be made in the twenty-first century. In 2023, a couple in Wyoming discovered a 60-million-year-old petrified forest when they uncovered a 35-foot-long section of a prehistoric tree, believed to be a metasequoia, along with other large, petrified trees and leaves resembling modern-day magnolias. A 19.5-meter-long petrified tree was found intact, with its branches and root system, on the Greek island of Lesbos in 2021. In 2024, geologists discovered 350-million-year-old tree fossils of the Sanfordiacaulis tree in southeastern Canada. Scientific research into petrified wood continues to give scientists a better understanding of the Earth’s past, including ancient ecosystems, climates, and species. Their study remains integral to the fields of education and tourism. 

Principal Terms

Miocene: geological epoch of the Tertiary period in the Cenozoic Era, beginning about 26 million years ago

Oligocene: geological epoch about 38 million years ago in the Tertiary period of the Cenozoic Era

Pliocene: geological epoch in the Tertiary period of the Cenozoic Era that began about 12 million years ago

Pteridophyte: plants, such as ferns, which have leaves, stems, and roots but no seeds or flowers

silicification: a process in which molecules of silica that are dissolved in groundwater replace the organic material in fallen tree trunks and branches

strata: layer of earth where petrified fossils are located according to the time period in which the organism lived

Triassic: period of time about 225 to 195 million years ago at the beginning of the Mesozoic era, when dinosaurs lived

xylem: plant cells through which water and nutrients move

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