Petrified Forest National Forest

The Painted Desert and Petrified Forest are all part of one National Park. More of the petrified wood is in the more southern part of the park, though it is all over this area. The film we saw at the visitor's center indicated that there is petrified wood in every state in the US - I didn't know that. We had seen a stump in Yellowstone that was indicated as being a petrified tree, but I don't recall seeing it anywhere else.

Information from the Visitor Center:
Researchers believe that the life cycle of a petrified tree is something like this:
1) A living tree on the edge of a stream.
2) The tree dies and loses its leaves, branches, and bark.
3) The river undercuts the dead tree.
4) The tree topples into the water and is transported (5) a short distance downsream.
6) The log snags on a bank and is buried (7) under sand and mud in the stream channel.
8) Deep burial seals the tree away from bacteria and oxygen preventing decay. Eventually silica in the ground water infiltrates the tree replacing the organic material with quartz crystals. The log is "petrified".
9) Erosion of the surrounding rock layers re-exposes the petrified log.
10) Further erosion undercuts the log causing it to crack, break into segments, and eventually roll down the hill.

Trees to Stone: The colorful logs underwent a fascinating change. Buried beneath layers of silt, mud, sand, and volcanic ash, the logs were protected from decay. Mineral-laden ground water percolating down through layers of sediments carried silica, from the volcanic ash, and other trace minerals, saturating the absorbent dead wood.

Silica crystals (quartz) grew within the porous cell walls, continuing to form on the inner surface of the walls, and finally filling the central cavity (lumen) of the cells. If the process of petrification stopped, perhaps due to lack of water, the organic cell walls remained intact, thoroughly embedded with silica. This type of preservation is called permineralization. Permineralized petrified logs are tan and brown and tend to resemble modern wood. Touch the surface, however, and you will discover that they are indeed stone. (These were the type we saw at Blue Mesa - see Painted Desert entry.)

Most of the logs at Rainbow Forest (pictures below) are colorful agatized wood. If the petrification process continued, the remaining organic material degraded and was replaced by more silica crystals. Eventually, this results in the complete mineral replacement of the wood by silica. This creates a pseudomorph: a copy of wood.

The colors of the Rainbow Forest's brilliant petrified logs are due to trace minerals that soaked into the wood along with the silica. Iron minerals provide bright mustard, orange, rich reds, ochre, and black. Blue, purple, brown, and black, including graceful fern-like patterns, are caused by manganese minerals. Other minerals may be present in small amounts, differing from log to log. The colors are best seen in broken sections of petrified wood.

There weren't any prehistoric lumberjacks and the National Park Service doesn't break up the logs. The logs were broken naturally. Since petrified logs are composed of quartz, they are hard and brittle, breaking easily when subjected to stress. The hard logs are surrounded by softer sedimentary layers. During earth movement and as the sediments shifted and settled, stress on the rigid logs caused fractures. As erosion exposes the logs, weathering widens the cracks, particularly from ice wedging in the winger. Gravity helps the sections roll or fall away from their original orientation, however many of the logs in the area remain aligned.

What kind of trees were they? So far two main groups of trees have been identified. Conifers are cone bearing seed plants. Today conifers include pines, cypress, and cedars. There were also ginkgos, a bizarre group of trees related to cycads. Today there is only one living species of ginkgo, Ginkgo biloba.

 (end of information from Visitor Center)

A piece of petrified wood seen on the Blue Mesa Loop,

Inside the Visitor Center there were a couple of pieces of petrified wood that had been polished. Here is information about one of them.

This is the piece of wood described in the text in the picture above.

One of the pieces of petrified wood on the Rainbow Forest loop.

One of the logs on the Rainbow Forest loop - notice that it appears to be cut apart like my Dad used to chainsaw trees that fell on the farm!

Another view of the log that has broken apart

It was amazing to see these examples of petrified wood. One of the women working in the Ranger Station at the south end said that there is a LOT of petrified wood around outside the park. Indeed, we saw a LOT of it for sale (or even available for free in many places).