How Is Petrified Wood Formed?
Wood that is petrified has been changed by nature into stone. 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.
Here is an example of how petrified wood is formed: Millions of years ago, a tree fell into a flooded stream and was quickly buried in mud or sand. Water, with minerals dissolved in it, seeped into the buried log and slowly filled the decaying cells of wood with mineral matter as hard as rock. After many centuries, the wood itself was all gone, and what remained was a perfect copy of the wood – in stone. The colors in the stone are caused by the various minerals in petrified wood.
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 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 volcanoes erupted numerous times. These eruptions blanketed the area in volcanic ash with 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.
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 replace by any form of silica, including opal and agate.
Petrified wood is often used in lapidary work. It is cut into shapes for making jewelry, sawn into blocks to make bookends, sawn into thick slabs to make table tops, and sawn into thin slabs for clock faces, cut into cabochons, used to make tumbled stones and many other crafts. Small pieces of petrified wood can be placed in a rock tumbler to make tumbled stones.