Petrified Forest National Park (AZ)

Petrified wood is a mysterious byproduct of geological processes. A petrified log looks just like wood on the outside but is usually a jumble of colorful quartz on the inside. How did this come to be?

Here too in Petrified Forest National Park is the Painted Desert, the name given to the “badlands” whose outrageous colors layer the eroded slopes in the northern part of the park. It’s hard to describe the impact that this riot of colors has on your traditional notions of landscapes. The Painted Desert actually extends beyond the park, reaching as far as the Grand Canyon and the northern part of Arizona, like a giant horseshoe. The intensity of the pastel colors changes depending on the time of day; the most breathtaking canvas is reserved for sunrise and sunset.

The biggest log specimens are found in the Giant Logs trail at the park’s southern end, including “Old Faithful,” 9½ feet in diameter. Old Faithful is a good example of permineralized wood (see Geology Notes). Here we came across a collared lizard who was happy to pose for all the photographers.

A few miles up the road is the Crystal Forest whose floor is still strewn with petrified fragments and sections of logs, even after years of specimen collecting by thoughtless tourists.

The eeriest trail in the park is Blue Mesa where the highly eroded hillsides of blue and gray bentonite clay gives the impression of an alien landscape. Here you can also see petrified wood in rubble piles where they fell from eroding hillsides.

Since taking specimens from the national park is illegal, you can purchase petrified wood at the many rock shops in the area. These stores presumably get their stock from private land. You can see them along Interstate 40. In Holbrook, where we stayed, Rainbow Rock Shop on Navajo Blvd has two dinosaur models made out of cement and reinforcing bars. There are piles of petrified wood all over the place, including some really nice, polished specimens displayed inside.

Petrified Forest was the last place we visited in the Southwest before returning home.

Geology notes: The geologic layer that is prominently exposed in the park is the Chinle formation. It is probably the easiest formation to identify in the Southwest because of its highly friable, typically sloping sides that result from its composition of siltstone, mudstone and claystone, and because of its characteristic layers of pastel colors. When exposed to wind and rain, it erodes rapidly to form sloping hills and narrow gullies. It is in this layer that petrified wood is typically found. Because Chinle erodes rapidly, new petrified wood is exposed all the time.

Over 200 million years ago, ancient conifers were buried by sediment and volcanic ash. This phenomenon must have been sudden because entire trees were buried and subsequently petrified. This graveyard is devoid of oxygen to hasten the decay of the wood. Then quickly, petrification begins. In some cases, the minerals in the water filled up the very cells of the plant, thereby preserving the very structure of the wood. This process is called permineralization. In most cases, however, large-scale replacement of organic matter took place where very little, if none of the plant’s original structure remained.