Petrified wood is ancient wood that has been transformed completely into stone through a complex geological process. This material is classified as a fossil, representing the preserved remnant of terrestrial vegetation from a past geological age. The resulting stone retains the external shape and internal structure of the original tree, making it a unique and visually striking form of fossilized life.
Formal Classification of Fossil Wood
The question of a single “scientific name” for petrified wood is complicated because it is both a mineral material and a fossilized organism. The general paleontological term used to describe this material is Fossil Wood. In mineralogy, the most common classification is Silicified Wood or Agatized Wood, which describes the specific mineral composition of the stone.
Petrified wood does not have a single Linnaean species name like a living organism because it is an altered material, not a biological species. The closest scientific name is a paleobotanical genus and species designation, assigned only when the cellular structure is preserved well enough to identify the original tree. An example is Araucarioxylon arizonicum, the scientific name given to the extinct conifer that makes up the majority of the petrified logs in Arizona. This binomial nomenclature identifies the parent plant, not the stone itself.
Paleobotanists use this specific naming system to classify the original plant based on microscopic features. If the cellular structure is too degraded, the fossil may simply be labeled “Silicified Wood” with a geological age and location.
The Process of Permineralization
The transformation of wood into stone occurs through a specific process called permineralization, often referred to as petrifaction. This process begins when a tree trunk is rapidly buried under sediment, such as volcanic ash or mud, which quickly cuts off the supply of oxygen. The resulting anaerobic environment drastically slows the natural decay caused by bacteria and fungi, providing the time necessary for fossilization to occur.
Groundwater rich in dissolved minerals then flows through the surrounding sediment and permeates the buried wood. The most common mineral involved is silica, often dissolved from nearby volcanic ash. This mineral-laden solution seeps into every empty space within the wood, including the microscopic voids inside the cell walls and the larger spaces within the cell interiors.
As the water evaporates or conditions change, the dissolved silica precipitates out of the solution and crystallizes within the cellular structure. This mineral deposition fills the empty spaces, creating a rigid stone matrix. Over time, the original organic material, mainly cellulose and lignin, slowly decomposes and is simultaneously replaced by the depositing minerals. This fine-scale replacement allows the petrified wood to retain the intricate detail of the original wood grain and annual growth rings.
Mineral Composition and Coloration
Petrified wood is overwhelmingly composed of silica, which takes various forms including quartz, chalcedony, agate, and opal. Chalcedony, a microcrystalline variety of quartz, is particularly common in high-quality specimens. The hardness and durability of petrified wood, which measures between 6.0 and 7.0 on the Mohs scale, is due to this high content of silica.
The colors of petrified wood are not derived from the original tree but from trace elements present in the mineral-rich water during the permineralization process. These impurities become locked into the crystal structure of the replacement minerals. Iron oxides, such as hematite and limonite, are responsible for a wide spectrum of colors, including reds, browns, oranges, and yellows.
Other elements introduce different hues. For instance, copper and chromium often result in shades of blue and green. Manganese oxides can produce pink, purple, or black coloration. The specific concentration and oxidation state of these trace metals determine the final palette seen in many petrified wood specimens.