Monday, January 17, 2011

The Brilliant-Colored Glass-Trees of the Petrified Forest: The Official Fossil of the State of Arizona

This is a close-up view of an ancient conifer in the Petrified Forest National Park that has been petrified. The process of permineralization has preserved the structure of the wood down to the microscopic level, but it also helps to prevent tissue compaction which would otherwise distort or crush the original structure. Structures such as tree rings, the root ball, branches and even the various tree tissues are easily identifiable in cross-sections.

About 225 million years ago, during the Late Triassic, northeastern Arizona was located near the equator. The entire region was located near the southwestern edge of Pangaea, a massive supercontinent that would soon break up and spawn the continents of the world we know today. Picture an enormous, flat basin with rivers and streams coursing through the lowlands. This is the time of deposition of the Chinle Formation, a mud and sand, river-floodplain deposit.

Growing along the waterways, tall coniferous forests thrived on the landscape along with cycads, lycopods, ginkgoes, ferns and giant horsetails, the dominant land plants (gymnosperms) in the age of the dinosaurs. Flowering plants (angiosperms) had not yet evolved. Most of the trees (or at least the ones that were fossilized) were a pine called Araucarioxylon arizonicum, the state fossil of Arizona. A diverse reptilian and mammalian assemblage also flourished in the ancient basin.

Eventually, large numbers of trees catastrophically fell victim, perhaps to floods originating in the now eroded highlands to the southeast or southwest. The fallen trees were carried downstream and buried along the banks of rivers and in stream channels. Rapid burial protected many of the trees from a fate of decomposition and fossilized them into the petrified logs we see today in the Petrified Forest.

For the petrification of the logs to happen, numerous events had to “come together.” This involves taphonomy, a fundamental branch of paleontology that aims to understand the processes behind the preservation of all fossils, ranging from common shells to dinosaur bones, and from the soft-bodied remains of trilobites to ancient fallen trees. It involves the study of the ecological, biochemical, geochemical and sedimentary processes that occurred in the environment before and after burial. Let’s continue to investigate the unique taphonomy of the Petrified Forest.

Distant volcanoes spewing ash into the atmosphere and carried by the wind to this corner of Arizona found its way into the river sediments. The ash contained silica (SiO2, commonly occurring as quartz), which was dissolved by the ground water and carried downward through the many rock layers overlying the buried logs. Quartz-crystals filled every hollow and pore space, even replacing the internal cellular structures and cell walls of the wood.

All that was left was to permanently color the logs, also supplied by the mineral-laden ground water. The structural arrangement of silicon and oxygen creates an open bonding structure that permits other ions such as various forms of iron to occupy interstitial positions in the molecule and bond to it, thus producing variations in color. Red and pink colors in the wood are produced by the presence of hematite, a form of oxidized iron - Fe2O3. The intensity of the color depends on the quantity of hematite present in the petrified wood. Iron dissolves in ground water when no oxygen is present, which becomes re-oxygenated as it moves though the tree trunks. This causes oxygen to bond with the iron. The iron then precipitates to produce a solid form of iron called hematite, which becomes incorporated into the cell walls of the wood. A similar process occurs when iron in tap water stains porcelain sinks. The soluble iron in ground water becomes oxidized into a solid form when it comes in contact with air, causing a reddish stain. The other colors seen in the wood, yellow, brown, orange, green, white, purple, blue, tan and black, are all oxides and sulfides of metallic elements such as manganese, carbon, cobalt and copper. 

The exposure of the logs was a consequence of the uplift of the entire Colorado Plateau. Fueled by the increased gradient from the uplift and a wetter climate, the forces of nature eventually eroded the rock overburden down to the Chinle. This left the petrified logs strewn haphazardly across the landscape.


  1. Why is the log looking so cleanly cut?

    1. The log has a very high silicon dioxide and other mineral content that essentially makes it brittle. Two hundred million years of landscape evolution and weathering once exhumed has essentially fractured the crystalline log along cleavage planes that are very sharp and angular.