What is Petrified Wood?
Petrified wood is a fossil. It forms when plant material is buried by sediment and protected from decay due to oxygen and organisms. Then, groundwater rich in dissolved solids flows through the sediment, replacing the original plant material with silica, calcite, pyrite, or another inorganic material such as opal. The result is a fossil of the original woody material that often exhibits preserved details of the bark, wood, and cellular structures.
Some specimens of petrified wood are such accurate preservations that people do not realize they are fossils until they pick them up and are shocked by their weight. These specimens with near-perfect preservation are unusual; however, specimens that exhibit clearly recognizable bark and woody structures are very common.
Petrified Wood by Many Other Names
A wide variety of names are commonly used for petrified wood. "Fossilized wood" is a general term for wood that has been petrified or preserved by other methods of fossilization. "Opalized wood" is petrified wood that has been replaced by opal, an amorphous form of silica. "Agatized wood" is wood that has been replaced by agate, a form of chalcedony or microcrystalline quartz. "Silicified wood" is wood that has been replaced by any form of silica, including opal and agate.
Other Petrified Wood Localities
Petrified wood is not rare. It is found in volcanic deposits and sedimentary rocks at many of locations worldwide. It is sometimes found where volcanic activity covered plant material with ash, mudflows or pyroclastic debris. It is found where wood in sedimentary deposits was replaced by minerals precipitated from groundwater. It is especially abundant around coal seams, although many of the wood specimens in these locations are casts and molds rather than petrifications. One almost unbelievable material from Western Australia is known as "peanut wood" because of its ovoid markings, but those markings are actually boreholes drilled by a clam!
In the United States, noteworthy locations where abundant fossilized wood can be seen include:
- Petrified Forest National Park near Holbrook, Arizona
- Petrified Palm Deposits in the Catahoula Formation of Louisiana, Texas and Mississippi
- Ginkgo Petrified Forest near Wanapum Reservoir, Washington
- The Petrified Forest near Calistoga, California
- Mississippi Petrified Forest near Flora, Mississippi
- The Gilboa Fossils near Gilboa, New York
- Florissant Fossil Beds near Florissant, Colorado
- Gallatin Petrified Forest near Yellowstone, Wyoming
- Escalante Petrified Forest State Park near Escalante, Utah
- Petrified Wood Park in Lemmon, South Dakota (a rock sculpture park - some made of local petrified wood)
Elements
Elements such as manganese, iron, and copper in the water/mud during the petrification process give petrified wood a variety of color ranges. Pure quartz crystals are colorless, but when contaminants are added to the process the crystals take on a yellow, red, or another tint.
Following is a list of contaminating elements and related color hues:
Petrified wood can preserve the original structure of the stem in all its detail, down to the microscopic level. Structures such as tree rings and the various tissues are often observed features.
Petrified wood is a fossil in which the organic remains have been replaced by minerals in the slow process of being replaced with stone. This petrification process generally results in a quartz chalcedony mineralization. Special rare conditions must be met in order for the fallen stem to be transformed into fossil wood or petrified wood. In general, the fallen plants get buried in an environment free of oxygen (anaerobic environment), which preserves the original plant structure and general appearance. The other conditions include a regular access to mineral rich water in contact with the tissues, replacing the organic plant structure with inorganic minerals. The end result is petrified wood, a plant, with its original basic structure in place, replaced by stone. Exotic minerals allow the red and green hues that can be seen in rarer specimens.
What Are Sedimentary Rocks?
Sedimentary rocks are formed by the accumulation of sediments. There are three basic types of sedimentary rocks.
Clastic sedimentary rocks such as breccia, conglomerate, sandstone, siltstone, and shale are formed from mechanical weathering debris.
Chemical sedimentary rocks, such as rock salt, iron ore, chert, flint, some dolomites, and some limestones, form when dissolved materials precipitate from solution.
Organic sedimentary rocks such as coal, some dolomites, and some limestones, form from the accumulation of plant or animal debris.
What are Igneous Rocks?
Igneous rocks are formed from the solidification of molten rock material. There are two basic types.
Intrusive igneous rocks crystallize below Earth's surface, and the slow cooling that occurs there allows large crystals to form. Examples of intrusive igneous rocks are diorite, gabbro, granite, pegmatite, and peridotite.
Extrusive igneous rocks erupt onto the surface, where they cool quickly to form small crystals. Some cool so quickly that they form an amorphous glass. These rocks include andesite, basalt, dacite, obsidian, pumice, rhyolite, scoria, and tuff.
Figure 1
Figure 2
My wife had mentioned that when she was a teenager in high school near here she used to walk through this park each day to and from school. She told me of a story about finding a whole petrified log near here. On a chilly afternoon we dropped the kids off with grandpa while she showed me around the area. I was astonished at how much petrified wood was everywhere! Most of the original log she had found has been chipped away, but much of it still remains. We hunted around and found many many samples! At the posted coordinates there are several examples of exposed petrified wood both on the cliff side and in the rocks around the base. I’ve included a couple photos of some specific examples to study.
Based on your studies and the information provided answer the following questions by sending me an email or via a message from the geocaching message system.
Question 1: look at the rock material surrounding the fossils, noting the texture and composition. Please describe briefly if this appears sedimentary or igneous and why.
Question 2: Looking at any of the exposed fossilized wood, what colors do you see and what elements might you guess there to be present.
Question 3: Find the fossil in Figure 1, about how long is this exposed piece? Estimate the diameter near the "base"?
Question 4: Find the fossil in Figure 2, Do you see any evidence of rings? Estimate the diameter of this tree when it was alive.
Thank you for taking the time to learn about these fossils. Be sure when you are out and about exploring to always take the time to observe the rocks and geology around you. You'll never know what you might find right in your own backyard!!
Here are some other pictures of samples in the area. I encourage you to explore the rocks and surrounding area to discover more!
Sources for this information can be found at https://geology.com/rocks , https://geology.com/stories/13/petrified-wood/ and some references from https://en.wikipedia.org/wiki/Petrified_wood.