Friday, February 20, 2009

To Turn Wood to Stone, . . .

Take thousands of trees toppled by floods, volcanoes, earthquakes, or other natural disasters. Cover with massive quantities of mineral-laden ground water. Add silt, mud, sand, and volcanic ash in massive amounts. Cover (to cut off oxygen) and set aside for several millenia. The process is complete when the minerals and silica from the volcanic ash have percolated down through the layers of sediment and saturated the absorbent dead wood.

If the wood is exposed to the air "early," the petrified logs are tan and brown and tend to resemble wood, although they are actually stone.

If the process continues, more and more of the wood is replaced by silica crystals. Erosion over many millenia exposes the petrified logs, and since the logs are composed of quartz, they are hard and brittle, breaking up when subjected to stress.

We were fortunate to be part of a tour of the Petrified Forest (east of Holbrook, AZ) led by Ranger Rita Garcia. This was an off-trail, hands-on experience with a chance to find pieces of wood, quartz, and skeletons.

The ground was covered with fossils and petrified material, so it was surprisingly easy to see a variety of each category.

My eye was drawn to the larger quartz pieces, but the bones were white, so they became easy to identify.

The only difficult part of this tour was comprehending how the wood became transformed into the colorful manifestations of iron, carbon, manganese, and sometimes cobalt and chromium.

At one point, we were asked to run our fingers through the gray material shown in this photo. One of our group identified this as a clay. It is bentonite clay and is used in cat litter because of its absorbency. And this same silica compound is used as an abrasive in toothpaste. Yup.

We were near the end of our tour when Ranger Rita asked, "Now that we're back on the trail, what difference do you notice?" Here is a section of the trail she asked us to assess. After a few seconds, we noticed that about 10 feet on either side of the trail (a portion of which is shown on the left in the photo) were completely devoid of any fossils, wood, skeletons, small stones . . . .

It seems that tons of petrified wood are removed from the Park each year by visitors taking "one little piece" at a time. This is especially sad because 90 percent of the petrified wood in Arizona is outside tha Park, and some of it is on public land. People could take pieces of wood from that land at no cost.

Along the trail, there were only large sections of logs. They were certainly interesting but nowhere near the beauty and variety that the off-trail tour provided. Thank you Ranger Rita.

As we left the Park, we saw this roadrunner which seemed to be posing for a photograph. All the time we were in Albuquerque, we missed taking a photo of this fleet-footed state bird of New Mexico.

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