When wood is not wood

The author standing amidst some petrified conifer logs (Araucarioxylon arizonicum, Araucariaceae). Photographed 03/06/2010 at Petrified Forest National Park, Holbrook Arizona.

At first glance, the preserved trees (largely Araucarioxylon arizonicum, Araucariaceae) at Petrified Forest National Park look like any other logs.  They have finely detailed bark, knots, wood grain, and even growth rings:

Petrified conifer log displaying wood texture and thick bark. Photographed 03/06/2010 at Petrified Forest National Park, Holbrook Arizona.

Petrified conifer log displaying wood texture and knot. Photographed 03/06/2010 at Petrified Forest National Park, Holbrook Arizona.

Petrified conifer log displaying growth rings and bark. Photographed 03/06/2010 at Petrified Forest National Park, Holbrook Arizona.

Closer examination reveals that the logs aren’t wood, they’re rock.  The organic components of the wood have been replaced by microcrystalline silica minerals, including quartz, amethyst, and citrine:

Petrified conifer log displaying quartz (white), amethyst (purple), and citrine (yellow) replacement. Photographed 03/06/2010 at Petrified Forest National Park, Holbrook Arizona.

So how did this happen?  Based on abundant local fossils and relative and radiometric age dating, we know the exposed sediments in northeast Arizona are from the late Triassic period, around 225-210 million years ago.  Fossils and sediment types indicate the area was a low, wet conifer forest/swamp at that time.

The sediments are from the Chinle Formation, consisting of alternating beds of mudstones, siltstones, and volcanic clay known as bentonite.  These mudstones and siltstones are formed in riparian (river) environments, and the bentonite indicates local volcanism.

Two models explain how the mineral replacement in these trees may have taken place.  The first suggests that periodic flooding may have knocked down trees into the river, where river sediments buried them.  Once buried, oxygen necessary for decomposition would have been unable to reach the trees.  Subsequent volcanism would have deposited layers of ash.  Water leeching from the surface would have carried the silica minerals from the ash down to the buried trees, where over a long period of time the minerals would have replaced the organic components of the wood.

The second model discards flooding and focuses on catastrophic volcanic eruptions.  A nearby eruption may have blown down the trees and then buried them in ash.  The rest of the story is as above:  water leeching through the ash would have carried silica minerals into the wood, replacing the organic components over time.

Regardless of the specifics, several factors had to be present:  burial to prevent aerobic decay, a source of silica (ash), and transport of the silica (water).

Fast forward some tens of millions of years, and uplift and erosion complete the story.  The Sevier and Laramide orogenies, mountain-building events between about 140 and 50 million years ago, lifted this area substantially.  Uplift would have enhanced the effects of erosion.  Sediments that had buried the Triassic trees and sediments have since been eroded away.

It’s worth the trip to explore Petrified Forest, not only for the preserved tree remains but also for the the beautiful Chinle Formation of the Painted Desert:

Chinle Formation of the Painted Desert/Petrified Forest, photographed 03/06/2010.

Chinle Formation of the Painted Desert/Petrified Forest, photographed 03/06/2010.

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About Jeremy Sell

Science and nature nerd.
This entry was posted in Botany, Geology, National Parks, Paleoecology, Paleontology and tagged , , , . Bookmark the permalink.

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