Petrified forest wood is a remarkable remnant of the geological history of the Earth. The crystal-like appearance of this ancient wood attracts thousands of visitors each year to Petrified Forest National Park in Arizona. The landscape in this park is dotted with logs that appear to be cleanly cut with a saw, but the chemical composition of the wood is solely responsible for its shape. The color of the log only deepens the mystery.
How Did Petrified Wood Form?
The petrified logs in Petrified Forest National Park are over two hundred million years old. The Black Forest Bed has been dated to about 211 million years old and the Blue Mesa region is about 218 million years old. During this prehistoric period, it is believed that large trees were carried along ancient rivers and buried underneath large amounts of sediments. The process was so quick that the trees were buried without the presence of oxygen.
In this encapsulated setting, the wood decayed extremely slowly. Various minerals would seep into the wood, and over thousands of years, would crystallize within the cells. All the organic material within the wood decomposed and cracks in the wood were filled with crystals.
A Petrified Forest
Most of the trees that were washed away and buried two hundred million years ago in the present day Petrified Forest National Park were coniferous trees, tree ferns, and ginko trees. About a dozen types of petrified wood have been identified. Most are classified as Araucarioxylon arizonicum, while a few have been named Woodworthia and Schilderia. These ancient trees may be related to some species currently growing in Chile and Australia, namely the Bunya Pine Tree and the Norfolk Island Pine.
What is Petrified Wood Made Of?
Petrified forest wood is mostly composed of quartz. Quartz is made of silicon dioxide, also known as silica. It is believed that the silica originated in volcanic ash and settled over the buried trees. Over the years, the wood absorbed the mineral and crystallized. Impurities in the quartz, including iron, carbon, manganese are also found in petrified wood.
Color, Weight, and Hardness of Petrified Wood
The color of petrified logs varies depending on the mineral composition. Usually, there are striations of purple, yellow, black, and rarely green. These colors are due to the trace minerals present in the quartz. Petrified logs are very heavy. A cubic foot of petrified wood weighs between 160 and 200 pounds. Petrified logs are hard. On the Mohs Hardness scale, where 1 is talc and 10 is diamond, petrified logs are between 7 and 8. Although petrified forest wood is hard, it is also brittle. The crystal structure of quartz features cleavages along angles. Any stress along these angles will cleave the wood. This gives the appearance that the logs were cleanly cut.
More Facts About Petrified WoodSome petrified wood still contains organic material and may appear like actual wood. This type of wood has undergone a fossilization process call premineralization. Basically, one part of the wood hasn’t been crystallized with quartz. Petrified wood is used as a reference for deducing the climate of the Triassic Period. Petrified wood is used as a gemstone in jewelry and as an ornamental stone in some types of clocks and furniture.
Where is the Petrified Forest?
Petrified Forest National Park
Petrified Forest Road,off Interstate 40
Chambers, AZ 86028
(928) 524-6228
References:
“Petrified Wood.” National Park Service.
“Petrified Forest National Park FAQ.” National Park Service.