copyright©photo of painted desert overlook courtesy of Cleet Carlton

Related Links

Petrified Forest National Park Information Page

National Park Service Petrified Forest Website

Buy Petrified Wood!

Petrified Wood Created in a Laboratory

How to Identify Petrified Wood

Visit the Petrified Forest

The author (left) and her sisters at Petrified Forest National Park, Arizona, 1992. Photograph courtsesy of Tom Wanner

Pseudomorphs

photo courtesy of http://en.wikipedia.org/wiki/Petrified_wood

Petrified wood is considered a pseudomorph. Pseudomorphs are minerals which change in their chemical composition, but retain their original shape. Though petrified wood starts as organic matter, the quartz crystals that it becomes retain the original structure of the wood, sometimes including it's rings (Scovil).

Quartz

quartz crystals from Mexico courtesy of Glenn and Heidi Reed

Properties of Quartz:

"Nor had those antagonistic forces been dormant, which are always at work wearing down the surface of the land: the great piles of strata had been intersected by many wide valleys and the trees, now changed into silex, were exposed projecting from the volcanic soil, now changed into rock, whence formerly, in a green and budding state, they had raised their lofty heads."

~Charles Darwin, after viewing a formation containing petrified wood in Argentina in 1835 (Daniels 9)

Petrified Wood

photo by Amy Wanner

Introduction

Petrified, or silicified, wood is a type of fossil that has been found all over the world in Argentina, Denmark, Mexico, China, Japan, New Zealand, Indonesia, and much of the southwestern United States. Petrified materials of more than 80 plant species have been found, the most common being walnut, sycamore, oak, magnolia, chestnut, redwood, maple, persimmon, and dogwood, with petrified palmwood being the state fossil of Louisiana (Daniels 11). It is often found at gem and mineral shows, and is sometimes used in jewelry, and sold popularly as bookends or clocks (National Park Service).

One of the largest deposits of petrified wood is found in the Petrified Forest National Park in Arizona. The land that is now much of Petrified Forest National Park was set aside in 1906 by president Theodore Roosevelt as the nation's second national monument, which Congress then made a national park in 1962 (Uhler).

Some of the largest specimenst found have been up to 10 meters long and 2 meters thick. It can weigh as much as 150-200 pounds per cubic foot (Holleran). It ranks a seven on the Moh's hardness scale, indicating that it cannot be scratched with a knife, but can scratch glass (Hochleitner 14).

copyright©photo courtesy of Cleet Carlton

Formation of Petrified Wood

Petrification, the process of becoming stone, happens when trees fall and die and are quickly covered with silt, clay, or mud, or fall in a swamp or the bottom of a lake. Some are covered with volcanic ash or lava. All of these things provide an anaerobic, or oxygen free, environment, which prevents normal decay of the plants. The petrification can happen when the material covering the trees is rich in silica. This is often due to it containing volcanic ash. As water seeps from the mud and silt it carries the silica and other chemicals with it. The chemicals then begin to enter the cells of the tree and replace the organic matter there with forms of quartz (Carlton, Scovil).

The petrification is conditional on several factors. It is affected by the tissue systems present and the state of growth of the plant. Then it is dependent on the the abundance and kind of mineralizing material, the temperature, and the duration and rapidity of the chemical activity. Other minerals, such as iron, can affect the amount of preservation of the plant tissues, cells, and other structural details (Daniels 11).

The time it takes for petrified wood to form varies. There are the most common pieces, found in Arizona, were formed in the Triassic period, 225 to 250 million years ago when this area was a more tropical environment. Due to plate movement and geologic changes this area has moved and gone through periods covered with water and then sediment. Now, after much wind and water erosion the sediments have been removed to reveal the giant cracked and scattered logs and fossils (Uhler, National Park Service). There are also younger examples of wood being transformed found around the world that are associated with mines. Timbers less than 2,000 years old have been found in copper and silver mines in the Mediterranean, Mexico, and Montana that appear on the surface to have turned into copper or silver. These exhibit the beginning of the process of petrification, and show that it can begin in a relatively short period of time. These timbers are not completely petrified and do not go through quite the same process as the silicified wood (Daniels 11)

Note the crystals formed on
the left side of the petrified wood
photo by Amy Wanner

Variations in Petrified Wood

Quartz and varieties of quartz are the basic minerals that form petrified wood. This is also termed chalcedonized wood, with some of the variations being opalized or jasperized. The opal almost always occurs as a "common opal", and only indicates the wood retains more water. For precious opal, the type used in jewelry, to form, the groundwater conditions must be very stable in order that the spheres of silica settle out of the solution and arrange themselves into an orderly, crystal-like array. (Daniels 25)

The colorful bands found in most petrified wood, most commonly red, yellow, black, white, and sometimes blue. are caused by minerals that were in the mud and debris during the petrification process. The original quartz that is formed is colorless, but these minerals can cause it to form with certain colors (Desert USA). Acidic solutions containing iron and manganese can oxidize to create other colors. The table below shows which elements create which colors in petrified wood (Daniels 27).

copyright©photo courtesy of Cleet Carlton

Casts are another form of wood fossil. They contain none of the original plant structure, but are formed when a decayed piece of organic material is filled by another material. This other material can be mud or sand or can be largely composed of silicas, which can chalecedonize the space left by the original material. (Daniels 14)

Occasionally entire pine cones are found petrified. This is rare because coniferous trees only have cones during certain parts of the year. The cones are also usually preserved in an unopened state, tightly holding the seeds (Daniels 131).
limb cast from Arizona courtesy of Glenn and Heidi Reed

Other Petrified Wood Facts

- Every year visitors steal approximately 12 tons (over 24,000 pounds) of petrified wood from Petrified Forest National Park in Arizona. About 600 pounds of this is returned by guilty visitors, often many years later; however, it is still disappearing at rapid enough rates for the National Parks Conservation Association to have put the park on its Ten Most Endangered list in 2000. Even though petrified wood returned it still causes problems because it cannot be repositioned, and Park Rangers do not want to place it in the wrong spot because it could possibly affect scientific research carried out at the sites (Updike).

-Petrified wood has been used for thousands of years by indigenous peoples of the area. The Navajo believed the logs were the bones of the the great giant monster, Yietso, their ancestors had killed when arriving in what is now the southwest of the United States. (Updike)

- Natives of the Paiute tribe believed that the giant petrifications were spent arrow shafts and spears used by the Thunder God Shinauav and his enemies during a great battle. Silicified petrified materials were also used to make excellent arrowheads and spearheads and were also used as building materials, game balls, and tools. (Daniels 7)
arrowhead made of petrified wood courtesy of Bob Knowlton

Literature Cited

Carlton, Cleet. 1999. Golden Gate Photo-Petrified Forest Gallery. http://www.goldengatephoto.com/westus/petrify.html. Accessed 2005 March 15.

Crossley, John. The American Southwest. http://www.americansouthwest.net/arizona/petrified_forest/geology.html. Accessed 2005 March 15.

Daniels, Frank J. Petrified Wood: the world of fossilized wood, conifers, ferns and cycads, Western Colorado Publishing Company, Grand Junction, CO, 1998.

Desert USA. 2005. http://www.desertusa.com/mag00/jan/papr/rock.html, Accessed 2005 March 15.

Hochleitner, Rupert. Minerals: identifying, learning about, and collecting the most beautiful minerals and crystals. Barron's, 1994

Holleran, Patrick, Shannon Technologies. 2005. Park Vision. http://www.shannontech.com/ParkVision/PetForest/PetWood.html. Accessed 2005 March 15.

National Park Service, Park Geology. 1997. Geology of Petrified Forest National Park. http://www2.nature.nps.gov/geology/parks/pefo/ . Accessed 2005 April 11.

Scovil, Jeff. Minerals in Disguise. Earth, Vol. 6, Issue 5, p. 75, October 1, 1997. General Science Abstracts. Accessed 13 March 2005.

Uhler, John William. 2004. Petrified National Forest Information Page. http://www.petrified.forest.national-park.com/info.htm. 2005 March 15.

Updike, William A. Petrified Pilfering. National Parks, 0276-8186, May 1, 2000, Vol. 74, Issue 5/6. General Science Abstracts. Accessed 10 April 2005.

Photo Credits

Cleet Carlton, Golden Gate Photo

Bob Knowlton, Bob's Flint Shop

Glenn and Heidi Reed, Where on Earth?

Author:Amy Wanner junior biology major at Earlham College
Creation/revision date: September 21, 2005

Link to other Student Webpages for 2005 Earlham Physical Geology

Copyright © 2005 Earlham College. Revised September 21, 2005 . Send corrections or comments to wanneam@earlham.edu