Petrified Wood

Petrified Wood is Arizona's official state fossil, designated in 1988. The most common species, Araucarioxylon arizonicum, is the fossil of a large conifer tree that grew in a Triassic tropical forest about 225 million years ago. Over millions of years, silica-rich groundwater replaced the wood cell by cell, turning organic matter into brilliantly colored stone.

Arizona's Petrified Forest National Park holds the largest and most colorful concentration of petrified logs in the world. Some logs measure over 170 feet long and preserve the original tree structure down to individual growth rings and bark texture.

Araucarioxylon arizonicum was a tall conifer, related to the modern araucaria family that includes the monkey puzzle tree and Norfolk Island pine. It could grow up to 200 feet tall with a straight, columnar trunk. During the Late Triassic, what is now northeastern Arizona sat near the equator in a tropical lowland crossed by rivers and flooded regularly by volcanic ash from eruptions to the west.

When the trees died or fell, fast burial in river sediment cut off oxygen and slowed decay. Silica dissolved from volcanic ash seeped into the wood through groundwater, replacing each cell over millions of years. The result is stone that preserves the original structure of the tree, including bark, annual growth rings, and sometimes even insect damage.

The colors come from minerals carried in that groundwater. Iron oxides produce reds, oranges, yellows, and pinks. Manganese oxides create blues, purples, and blacks. Carbon turns sections dark. Pure silica is white or gray. No two logs are identical, which is why the logs in Petrified Forest range from deep crimson to pale lavender within a few feet of each other.

By the time Arizona became a state in 1912, the Petrified Forest had already been a federally protected area for six years. President Theodore Roosevelt designated it as a national monument in 1906, specifically to stop the large-scale removal of logs that had been going on for decades. Ranchers had been grinding petrified wood for abrasive material, and collectors and tourists were hauling away entire logs.

Congress upgraded the monument to Petrified Forest National Park in 1962, expanding its boundaries to include the painted desert badlands to the north. By then, Araucarioxylon arizonicum was already the most studied and most famous fossil tree species in North America.

Arizona designated Petrified Wood as its state fossil in 1988. The choice was direct: no other state has a fossil as large or as visible as the logs scattered across the Chinle Formation in northeastern Arizona.

Petrified Forest National Park in Navajo and Apache counties is the center of Arizona's petrified wood record. The park stretches about 30 miles along the I-40 corridor and contains several distinct log concentration areas: Rainbow Forest, Crystal Forest, Jasper Forest, and Blue Mesa, each with its own mix of log sizes and mineral colors. Rainbow Forest, at the southern end of the park, holds some of the longest individual logs.

Petrified wood from the Chinle Formation is also found outside the park, across the broader Colorado Plateau of northeastern Arizona. The formation outcrops in Navajo and Apache counties well beyond the park boundary, and isolated logs appear across reservation lands and state lands in the region.