Imagine standing in a lush semi-tropical forest with a 200 foot canopy of conifers and tropical flora. Slow moving streams and swamps populated with fish, clams, fallen logs and reptiles moved like blue ribbons that drained into an inland sea. A range of volcanic mountains called the Mogollon Highland filled the southern skyline, the source of the streams and rivers.
It is a scene that is hard to imagine 225 million years later, when the land we see today is an arid desert scattered with wood that has since turned to stone. Petrified wood is real wood that has turned into rock composed of quartz crystals.
One of the greatest concentrations of petrified wood in the world is found in the Petrified Forest National Park in northeast Arizona. Logs as long as 200 feet and 10 feet in diameter have been found in the park.
What turned the wood to stone?
Petrified wood has been preserved for millions of years by the process of petrification. This process turns the wood into quartz crystal, which is very brittle and shatters. Even though petrified wood is fragile, it is also harder than steel.
Petrified wood is known for its exquisite color and detail. Some pieces of petrified wood have retained the original cellular structure of the wood; the grain can easily be seen. Petrified wood can be found throughout the desert regions. It is easy to find and identify. It is used often in jewelry making and for other types of decorative artwork.
What is petrification?
The process of petrification starts with three raw ingredients: wood, water and mud. Petrification of the wood found in the Petrified Forest began during the Triassic Period when the primitive conifers fell to the ground and into the waterways, entering into their journey through time. The logs were swept and tumbled downstream with sediment and other debris. The streams traveled through a plain of lakes and swamps where the wood, sediment and debris were deposited along the way.
Four hundred feet of sediment was deposited in the plain by the rivers that originated from the Mogollon Highland volcanic mountain range. That layer of sediment is known today as the Chinle Formation. As the logs were deposited in the plain they were buried with mud and debris, beginning the petrification process.
The mud that covered the logs contained volcanic ash, a key ingredient in the petrification process. When the volcanic ash began to decompose it released chemicals into the water and mud. As the water seeped into the wood the chemicals from the volcanic ash reacted to the wood and formed into quartz crystals. As the crystals grew over time, the wood became encased in the crystals which, over millions of years, turned the wood into stone.
How did the tropical forest become a desert?
The petrified logs were buried in the sediment for millions of years, protected from the elements of decay. During this time the plain was covered by an ocean and another layer of sediment on top of the wood-rich Chinle Formation.
It wasn't until 60 million years ago that the ocean moved away and the erosion process began. More than 2,600 feet of sediment have eroded to expose the top 100 feet of the Chinle Formation.
What makes petrified wood colorful?
It is not wood that makes petrified wood colorful, but the chemistry of the petrifying groundwater. Minerals such as manganese, iron, and copper were in the water/mud during the petrification process. These minerals give petrified wood a variety of color ranges. Quartz crystals are colorless, but when iron is added to the process the crystals become stained with a yellow or red tint.
Following is a list of minerals and related color hues:
Copper - green/blue
Cobalt - green/blue
Chromium - green/blue
Manganese - pink
Carbon - black
Iron Oxides - red, brown, yellow
Manganese Oxides - black
Silica - white, grey
