The Petrified National Forest in the Arizona desert is what’s left of a prehistoric, semi-tropical forest from the Late Triassic Period. After being buried under layers of sand for about 200 million years, the trees turned into quartz crystals, but they still look like wood.
The U.S. named the forest a national monument in 1906 to protect the petrified wood, but humans have known about it for more than 13,000 years, when wandering hunter-gatherers first stumbled across it.
Many of the petrified logs come from conifer trees called Araucarioxylon, which grew as high as 200 feet tall. These trees weren’t standing up in the ground when the process of petrification (becoming rock) began. In the Triassic Period, the trees grew along the waters of a large river system, and some fell into the rivers when they died, floating downstream and collecting in log jams.
The log jams were slowly buried in debris and mud, which contained silica from volcanic ash. The silica and other minerals seeped into the wood and crystallized, perfectly preserving the wood’s cellular structures. Over millions of years, the wood cells were replaced by minerals, and the trees literally turned to stone. The park’s “forests” are actually areas where the different log jams occurred: Crystal Forest, Rainbow Forest, Jasper Forest, and more.
Although quartz is very brittle and tends to shatter, it’s also harder than steel. So the petrified logs are much heavier than regular wood: up to 200 pounds per cubic foot. These stone logs come in a rainbow of colors, thanks to minerals and other elements they absorbed from the water, like copper and cobalt (green and blue), carbon (black), iron (red, brown, and yellow), and manganese (pink).
The forest is a popular tourist attraction, but it’s also a treasure trove for scientists, who study everything from the living plants and animals, to the climate and seismic activity, to things left behind by ancient people: artifacts, petroglyphs (rock carvings), parts of walls, remnants of roads, and more.