INTRODUCTION:
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This EarthCache will take you to the doorstep of a New Mexico Tech treasure, the New Mexico Bureau of Geology Mineral Museum. Coordinates will take you right to the parking area for the Museum. Admission to the museum is free, and it holds many outstanding geological samples. There is no need to enter the museum to accomplish this EarthCache, but both the Administration at New Mexico Tech and the Director of the Bureau of Geology hope you can take the time to visit. For those interested, more information about the museum can be found at:
https://geoinfo.nmt.edu/museum/
Directly in front of the museum at the listed coordinates is a spectacular collection of petrified wood from the Upper Triassic Chinle Formation in Holbrook, AZ. Holbrook is the nearest town to the Petrified Forest National Park. Petrified Forest National Park contains one of the largest and most colorful deposits of mineralized wood in the world, a good sampling of which is provided here for examination.
THE PETRIFIED FOREST:
The Petrified Forest is part of the Colorado Plateau physiographic province—a high-elevation region of basins, plateaus, and canyons, roughly centered on the Four Corners area of Colorado, New Mexico, Utah, and Arizona.[1] Two topological maps of the region can be seen side by side in the figure below.
The uplift that elevated the plateau is a relatively recent geologic event, occurring within the past 70 million years. This uplift created a lack of overburden in the Petrified Forest park which provides excellent exposures of Triassic deposits ranging from about 200 to 220 million years old. Petrified trees compose many of the exposed deposits.
During the time of the late Triassic, the four corners region was not high desert. In this era all the Earth’s continents were combined together creating the supercontinent Pangea seen below.[2]
The area where the petrified specimens exhibited at this EarthCache came from was located near the equator at the southwestern edge of Pangea. This was an area of rivers, alluvial planes, sand, and mud. The change in topography over time is seen below between Pennsylvanian and Triassic periods (~300 to 200 million years ago). The region which became the Petrified National Forest is marked by a red dot.
The lush forest occupying the area during the Triassic was home to many varieties of trees that have since become extinct. The specimens on display at the EarthCache are from the species known as Araucarioxylon Arizonicum. Detailed research on these trees estimated the largest specimen studied to have been approximately 10 feet in diameter and 200 feet tall.[3] The figure below provides an artist representation of the shape and structure of the trees.
PETRIFIED WOOD:
Over time, trees died or were knocked down by wind or the action of water. Rivers carried the trees downstream, breaking off branches and roots along the way. Many tree trunks came to rest on the banks of the rivers while others were buried in the stream channels. Most of the trees decomposed and disappeared, but some of the trees were petrified. Some logs were buried by sediment before they could decompose while volcanoes far to the west spewed tons of ash into the atmosphere. Winds carried ash into the area where it was incorporated into the thickening layers of sediment atop the trees.[4]
The exact mechanism of wood mineralization (being turned to stone, or petrified) is complex and the subject of various theories.[5] Well known is the fact that the wood must be left for a very long time in a condition where oxygen is not present in any significant amount and silica is available to permeate the woods fibers. Logs buried under layers of sediment in a body of water with ideal acidity meet both those key criteria.
To understand how trees become stone a quick review of their cellular structure is needed. Trees are basically composed of microscopic tubes which allow the transport of water and nutrients from the roots to the leaves. The figure below shows tubes that go from top to bottom (called Tracheids) and those that go from the inside to the outside (rays). The hollow in the interior of these tubes is referred to as Lumen. A picture taken under great magnification can be seen in the figure on the far right looking down some of the lumen in the tracheids. Note that tracheids have small holes (called pits) that allow liquids to flow between them. It is through this tubular highway system that the materials required to mineralize the tree flow.
With no oxygen or bacteria to consume the buried wood, water has time over many, many years to infiltrate the channels created by the lumen. The water caries dissolved silicate species that adhere to the sides of the channels and slowly build up mineral deposits. Over time the silicate species fill up the voids and undergo crystal changes. Many mechanisms exist for this to occur. In some cases, the cellular detail of the original wood is perfectly preserved, in others many details vanish. The figure below shows some microscopic images taken at various stages of mineralization (or Silicification), with frame A being early deposit of silica compounds on the cell walls (lumen interiors) all the way to frame C with complete filling of the lumen.[6]
COLORATION OF PETRIFIED WOOD:
A great deal of geology has to align perfectly to allow mineralization of wood. But this geology is inseparably intertwined with chemistry. Silicified wood can be mineralized with opal, chalcedony, or microcrystalline quartz. In pure form, these SiO2 polymorphs are colorless, but in nature they may occur in a wide range of colors.
Crystal lattice defects produce transparent gray “smoky” quartz, and opaque “milky” quartz and may be caused by an abundance of microscopic fluid inclusions. Optical effects produce the iridescent “fire” in precious opal, where the orderly arrangement of silica microspheres causes reflected visible light to be divided into spectral colors. The exact source of colors in petrified wood has received little serious research, but a recent article has shed light on the origin of the colors in the specimens displayed at the EarthCache location.[7]
To fully appreciate the origin of mineral colors requires a deeper dive into chemistry than a single EarthCache would permit. Basically, there are certain elements which lend themselves to producing color. These are well known in the world of solution chemistry. When these elements lose electrons, species are created which can absorb light energy and produce a literal rainbow of colors. The most common elements and the colors they typically produce in solutions is seem in the figure below.
Chemists call these transition metal elements, and they reside in a very particular area of the periodic table. For those with a burning desire to learn some chemistry, these elements (number 22-29) and their place within the periodic table are displayed below. Being a chemist, I could not resist creating this graphic. Back to the petrified wood . . .
As in chemical solutions, these same elements mixed in mineral compounds also produce color. So while most silicate species are colorless, mixed with these elements they can take on many colors. In petrified wood the water permeating the trees not only contains the silicates to create the mineralization of the wood, it also contains many dissolved minerals that carry a wide range of elements.
Researchers looked at a Holbrook specimen very much like the ones in the outdoor exhibit highlighted by this EarthCache. They analyzed trace minerals across multiple areas of coloration in this “rainbow wood”. The sample they studied is shown below.[7] The numbered boxes represent different colored areas analyzed.
In this specimen, the darkness of the color was mainly attributed to Iron concentration. The darker the color, the more iron was present. When purple and dark red regions were analyzed, it was found those areas also had some Manganese present. The graph below shows the amount of iron present in all six areas examined in the last picture. Iron (chemical symbol Fe) concentration is shown in parts per million (PPM) associated with the color of the wood. Manganese is abbreviated Mn for its chemical symbol.
Samples where green coloration was present was found to be caused by one of two things. Dark green coloration was caused by iron containing salts. Light green color was created by the presence of Chromium (chemical symbol Cr). The graph below shows these analyses.
Finally, the geology and chemistry of the rocks in the area is not divorced from that of the petrified wood. The same source of coloration exists for both sand/dirt and mineralized trees. This is evident in pictures of both the logs from the Petrified National Forest and the dunes in the Painted Desert seen below.
QUESTIONS:
1. The samples on display at this EarthCache came from the Colorado Plateau. Is where you are standing currently part of that region? The first picture should help you decide.
2. Observe the petrified samples distributed around the outdoor exhibit. Did the mineralization totally eradicate the expected characteristics of the stumps, or is there visual evidence they were once trees?
3. Explain what you observed that led to your answer to Question 2.
4. Examine the specimens on display to for the presence of Chromium. Do you see any evidence of this material being present?
5. Based on your observations, do you believe the largest mineralized specimen (nearest the parking lot) has more of less Iron overall than the smaller specimens further in the back? Explain your logic.
Note: While a photo of you at the site is not a logging requirement, one will go a long way in proving a visit if answers provided are questionable for any reason.
REFERENCES:
1. http://npshistory.com/publications/pefo/nrr-2010-218.pdf
Keller Lynn, K., “Petrified Forest National Park Geologic Resources Inventory Report Natural Resource Report” NPS/NRPC/GRD/NRR—2010/218, National Park Service, Fort Collins, Colorado.
2. https://www.britannica.com/science/Triassic-Period
3. http://npshistory.com/publications/pefo/nrr-2010-218.pdf
Sidney R. Ash & Geoffrey T . Creber, “THE LATE TRIASSIC ARAUCARIOXYLON ARIZONICUM TREES OF THE PETRIFIED FOREST NATIONAL PARK, ARIZONA, USA”, Palaeontology, Vol. 43, Part 1, 2000, pp. 15–28.
4. http://npshistory.com/brochures/pefo/trees-to-stone-2011.pdf
Petrified Forest: Trees to Stone, National Park Service U.S. Department of the Interior, January 2011
5. http://petrifiedwoodmuseum.org/pdf/permineralization.pdf
Mike Viney, Petrified Wood: The Silicification of Wood by Permineralization, The Virtual Petrified Wood Museum.
6. https://www.mdpi.com/2076-3263/5/4/286/htm
George E. Mustoe, “Late Tertiary Petrified Wood from Nevada, USA: Evidence of Multiple Silicification Pathways”, Geosciences 2015, 5, pp. 286-309.
7. https://www.mdpi.com/2076-3263/6/2/25/htm
George Mustoe & Marisa Acosta, “Origin of Petrified Wood Color”, Geosciences 2016, 6, pp. 25-49.