Frozen in Time, Petrified Forest National Park, Arizona, United States
Photograph by Nick Zantop
Frozen in time since the Late Triassic, a lone petrified log rests in the sand of Petrfied Forest National Park in northeastern Arizona.
Petrified Forest National Park is famed for its fossils, particularly those of fallen trees that grew about 225 million years ago in the Late Triassic period of the Mesozoic era. During the Late Triassic, the area that is now the park was near the equator on the southwestern edge of the massive supercontinent Pangea, and the climate was humid and sub-tropical. What is now northeastern Arizona was then a low plain flanked by mountains to the south and southwest and a sea to the west. Rivers flowing across the low plain from the highlands brought with them sediment and organic matter including large trees. While most organic matter decays quickly, some was swiftly buried and remained intact and later became fossilized.
Petrified wood is the result of a tree completely transitioning to stone through the process of permineralization. All of the organic materials have been replaced by minerals. The petrification process occurs underground, when the wood becomes buried beneath sediment and is initially preserved due to a lack of oxygen which inhibits aerobic decomposition. Mineral-laden water flowing through the sediment deposits minerals in the cells of the wood and as the trees lignin and cellulose decay a stone mold forms in its place.