Chinle Formation petrified wood

The petrified logs of Petrified Forest National Park, Arizona, come from the Late Triassic Chinle Formation, laid down by rivers more than 200 million years ago. Silica, much of it dissolved from volcanic ash, soaked into the buried wood and crystallised in its cells until the wood was almost solid quartz. Traces of iron, carbon and manganese give the rainbow colours, and cracks may hold clear quartz, amethyst, citrine or smoky quartz. Most logs are the conifer Araucarioxylon arizonicum. The densest wood lies in the Rainbow Forest bed of the Sonsela Member and the Black Forest Bed of the Petrified Forest Member. People made points, knives and scrapers from it for more than 13,000 years, and Ancestral Puebloans (about AD 950-1300) built houses of it.

Late Triassic Chinle Formation, mostly river and floodplain deposits. Members at the park, oldest to youngest: Mesa Redondo (about 225-227 million years), Blue Mesa (about 220-225), Sonsela (about 216), Petrified Forest (213 plus or minus 1.7) and Owl Rock (about 205). Logs of the Rainbow Forest sit in the Rainbow Forest bed of the Sonsela Member; the Black Forest Bed of the Petrified Forest Member also has abundant wood (NPS, Geologic Formations). The wood petrified as silica from volcanic ash crystallised inside the cells; uplift of the Colorado Plateau from about 60 million years ago cracked the logs (NPS, Petrified Wood).