Petrified Wood
Petrified wood photo courtesy of Rick Von Glahn.
JB: This is Earth and Sky, with a question recorded this year at the Smithsonian Institution’s National Air and Space Museum.
Catie Liebeck: Hello, my name is Catie Liebeck. And my question is, how does wood get petrified?
DB: Catie, when groundwater passes through buried wood, it deposits minerals, usually silica, inside the tiny cell spaces. Over millions of years, these deposits replace almost all of the original organic material, turning the wood into stone.
JB: You need silica to petrify wood. Silica binds to cellulose, a major ingredient in wood cell walls. When it crystallizes, it preserves the microscopic structure of wood. So scientists can often identify ancient trees – even to species – from their petrified remains.
DB: Volcanic eruptions are thought to be key to the formation of petrified forests. In the Petrified Forest National Park in Arizona, a massive flood left trees buried in mud for millions of years.
JB: Volcanic eruptions to the west spewed ash that blanketed the park – released silica – and slowly caused the trees to petrify. In our time, scientists believe the forests buried by the 1980 eruption of Mount St. Helens may already be beginning the long process of petrification.
DB: Catie thanks for your question. And thanks today to the National Park Service and to the National Fish and Wildlife Foundation. We’re Block and Byrd for Earth and Sky.
The following people were interviewed for today’s program. Our thanks to:
Dr. Peter Heaney
Department of Geosciences
Pennsylvania State University
Dr. Phil Stoffer
Western Earth Surface Processes Team
U.S. Geological Survey
Dr. William Tidwell
Emeritus Professor
Brigham Young University
Relevant Web Links
Links: Petrified Forest National Park (U.S National Park Service)
Colors of Petrified Wood# (American Museum of Natural History)
Author’s Notes:
When minerals like manganese, or iron-rich hematite or pyrite are deposited along with silica, the result is a rainbow spectrum of colors in the petrified wood. Silica in the form of opal is usually whitish, and quartz is generally clear. The addition of hematite will give shades of red and pink, goethite will give yellows, browns and oranges, pyrite will give blacks, and manganese is responsible for blues and purples. It all depends which minerals are picked up from the soil and deposited by the circulating ground water.
Silica is not the only petrifying agent – calcite, hematite and pyrite can also petrify. But silica is by far the most common, in part because of its strong attraction to cellulose. For this reason, petrified wood is also called silicified wood.
When it first crystallizes, the silica is mixed with water in a deposit of soft, rough opal. This whitish opal, called potch opal, is a far cry from the iridescent opal one sees at a jeweler’s, although structurally, they are very similar. The iridescence of gem-quality opal is caused by the presence of water trapped in the silica. In the process of petrification, over the next 10-40 million years, the opal dehydrates and changes to chalcedony (kal-SED-nee) and finally to quartz. Petrified wood is mostly quartz, but traces of opal and chalcedony may be visible as well.
Additional Teacher Resources
DesertUSA: Petrified Wood
Petrified wood has been preserved for million 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. This site explores the natural history and forces that create petrified wood.
Wikipedia.com. The Free Encyclopedia: Petrified Wood
This site is a good resource for introducing students to the basic concepts of the petrification of wood. It provides a scientific explanation of petrified wood as well as its location in various parts of the world. Also provided are external links to more information.
U.S. National Park Service, Petrified Forest National Park, Arizona: Petrified Forest
This is the official website of the Petrified Forest National Park in northeastern Arizona. The site offers various pages concerning the processes of petrification, the history of the land, paleontology of the park, and an educational page for kids.
