What is Petrified Wood? Petrified wood is a fossil. It forms when plant material is buried by sediment and protected from decay by 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.
Some petrified logs contain a spectacular surprise. Cavities within them served as crystallization locations for quartz crystals such as the citrine (yellow, left) and amethyst (purple, right) shown here. Images by Petrified Forest National Park.
Petrified Forest National Park The most famous locality for observing petrified wood is Petrified Forest National Park near the community of Holbrook in northeastern Arizona. About 225 million years ago this area was a lowland with a tropical climate and covered by a dense forest. Rivers flooded by tropical rain storms washed mud and other sediments into the lowlands. Enormous coniferous trees up to 9 feet in diameter and 200 feet tall lived and died in these lowlands. Fallen trees and broken branches were often buried by the river sediments. Nearby volcanoeserupted numerous times. These eruptions blanketed the area involcanic ash with a high silica content.
Rapid burial allowed the plant debris to escape destruction by oxygen and insects. The soluble ash was dissolved by groundwater flowing through the sediments. The dissolved ash served as a source of silica that replaced the plant debris, creating petrified wood. Trace amounts of iron, manganese and other minerals were included in the silica and gave the petrified wood a variety of colors. These sediments, plant debris and volcanic ash became part of a rock unit known today as the Chinle Formation.
In the millions of years after the Chinle Formation was deposited the area was uplifted and the rocks deposited above the Chinle were been eroded away. The petrified wood is much harder and resistant to weathering than the mud rocks and ash deposits of the Chinle. Instead of eroding away the wood accumulated on the ground surface as the surrounding mud rocks and ash layers were eroded away. That is why areas of the Park are covered with a litter of petrified wood trunks, branches and fragments. Today, visitors to the park can observe the petrified wood and photograph it; however, collecting petrified wood in the park is prohibited.
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
Gilboa Fossil Forest 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 near Lemmon, South Dakota (a rock sculpture park - some made of local petrified wood)