Photograph by Ralph Lee Hopkins
Chunks of petrified wood lay strewn in Arizona's Petrified Forest National Park. Most petrified trees here are of the species Araucarioxylon arizonicum, a towering conifer that grew in the humid, tropical lowlands of this region during the middle Triassic period, about 220 million years ago. Trees that fell here often landed in deep rivers, where they were quickly buried by sediment. Lack of oxygen inhibited decay, and heat and pressure over millennia turned the wood into solid quartz colored by impurities like iron, carbon, and manganese.