Petrified wood is a geological formation where a tree or tree-like plant transforms into stone. It is a distinct type of fossil, offering a unique window into ancient ecosystems. Unlike many other fossils that are impressions or molds, petrified wood retains the intricate details of its original organic structure.
How Petrified Wood Forms
Petrified wood forms when woody plant material, such as a fallen tree, is rapidly buried beneath sediment or volcanic ash. This quick burial creates an anoxic, or oxygen-deprived, environment that inhibits decomposition by bacteria and fungi. Water, often rich in dissolved minerals from surrounding sediments or volcanic ash, then permeates the buried wood. As this mineral-laden water flows through the wood’s cellular structure, minerals precipitate and fill the empty spaces within the wood’s cells.
Over extended periods, these minerals gradually replace the original organic material of the wood. The most common mineral involved is silica, often in forms like quartz, chalcedony, agate, or opal. Less commonly, other minerals such as calcite or pyrite can also be involved. This slow, molecule-by-molecule replacement maintains the wood’s original structure, including growth rings and cellular details, even as it turns into stone.
Understanding Its Fossil Type
Petrified wood is classified as a permineralization fossil, a process where minerals fill the porous spaces within the original organic material. In this type of fossilization, minerals, typically silica, crystallize within the cell walls and other voids, creating a stone replica of the wood. This mineral infilling strengthens the woody structure, preventing its collapse.
Beyond permineralization, petrified wood often involves the process of replacement. Here, the original organic compounds of the wood are dissolved and replaced by minerals, molecule by molecule. This replacement ensures that fine details, such as wood grain, tree rings, and even microscopic cellular structures, are preserved. This distinguishes petrified wood from other fossil types like molds or casts, where only the external shape or an impression is preserved without retaining the internal structure.
Identifying Petrified Wood
Petrified wood exhibits several characteristics that aid in its identification. Despite being made of stone, it retains the visual cues of its original woody form, including distinct wood grain patterns, tree rings, and sometimes bark textures. Its texture is stony, and it feels dense and heavy.
The coloration of petrified wood can be varied, influenced by trace mineral impurities present during petrification. For instance, iron oxides produce reds, browns, and yellows, while manganese may lead to pinks, oranges, purples, or blues. Green hues can result from copper, cobalt, or chromium. Petrified wood is found worldwide in sedimentary beds, often in areas with past volcanic activity, such as the Petrified Forest National Park in Arizona.