Painted Desert-The Geology of Petrified Forest

Member Rating 5 out of 5 by Taylor252 on August 30, 2003

North of the Petrified Forest but included in the protection of the National Park is the area known as the Painted Desert. As the name implies, it is very dry (a desert) and the painting comes from iron and other minerals that colored the ancient layers that form this land. Most of the reddish and yellowish colors are caused by iron oxide minerals. (See pictures below for views of the banded hills and desert.) The scientific name for most of the land we saw is the Chinle Formation. In geological terms this formation isn't very old --180 to 200 million years. For example, it is the same age as the highest and newest layers of the Grand Canyon. It was laid down as sediment in a watery tropical environment that was this area in the Triassic period. That was also a time of many dinosaurs so as a result, there is an almost continuous supply of fossil bones discovered in this area.

As new as the Chinle Formation is, there is an even younger layer that can be seen in places. After the Chinle sediments were put down, a layer of lava from long gone, but formerly local volcanoes, created another layer called the Bidahochi Formation. It is aprox. 12 million years old and can be seen at Pilot Rock in the Painted Desert and a few other places. It is basically basalt.

You will occasionally see a butte in the park. How are they formed? It was a question that interested me. A butte is basically harder rock than that which is around it. The softer stone erodes away leaving the spire. Also some of them are old lava tubes from volcanoes. (This is essentially how the petrified logs were exposed again after such a long period of time. They are very hard rock surrounded by essentially sandstone which is very soft!) The rubble deposits of the actual volcanic cone have completely eroded away leaving the lava filled tube standing by itself. I didn't know this, but there are over 200 existing volcanic cones in northern Arizona and one, Sunset Crater, erupted only 1000 years ago. The area is considered to still be active.

Painted Desert

Petrified Forest National Park, Arizona

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