Remnants of an Ancient Forest

Post by Alessi Kwan, Kaela Eckel, and Josh Flannery

 

 

 

 

 

 

 

 

 

 

 

GPS Coordinates: +44.04614,-123.07365
Google Earth, Site 4

Location and general description: This feature, shown in Figure 1 and Figure 2, is a piece of petrified wood located in Cascade Hall, which houses the Department of Earth Sciences. Petrified wood is wood that becomes fossilized over time by being buried. Mineral-rich groundwater eventually seeps into the wood, replacing its original cellular structure and turning it into an inorganic, rock-like material. This piece was donated to the University of Oregon in 2002 from the Haase Collection and is featured next to a display case that contains a description of the specimen’s origin.

Geological Observations: The petrified wood log is approximately 1 meter tall and 25 cm in diameter and has a variety of colors on its surface, including reds, browns, yellows, and greens. The surface is very smooth and seems to have been polished. The tree’s original bark pattern and some of the ring structures were preserved, so it appears like normal wood on the outside. The log is oval-shaped and appears to have been squished before it was petrified.

 

 

What Causes the Dramatic Colors of the Log?

Contributed by Josh Flannery

The petrified log in Cascade Hall has been around for a long time. Two-hundred and twenty million years, to be exact (according to the plaque that accompanies it). A lot has happened since it was a living tree. The dinosaurs died off (twice), plants learned how to make flowers, and some funny primates started walking around on two legs. But time has been kind to the log. Yes, it’s a little squished. You probably will be too in a couple million years. More importantly, it still looks like a log. Almost. Instead of having uniform brown or tan wood with brown bark like most living trees, the petrified log is a swirl of reds, oranges, yellows, and whites. It looks as if it’s been painted on purpose. Somewhere along its 220 million year journey, something drastic happened to the wood in the log that made it what it is today. I want to know so I can do it too. So I set out to answer the question: what causes the dramatic colors of the log?

 

Scientific Literature

I chose to incorporate the article Mustoe & Acosta (2016) because they provided the first peer-reviewed, in depth analysis of the coloration of petrified wood. Their goal was to establish a background in the field for future research, and to test previously untested knowledge claiming that iron was responsible for red and chromium for green, the two most common bright colors in petrified wood. I chose this paper because it was the most complete analysis of petrified wood colors and formation that I could find. In terms of determining what happened to our log, it was my only hope.

 

Intersection Between Peer-Reviewed Article & Campus Feature

After lengthy analysis including samples from the Triassic Chinle Formation in Arizona (the same place our log came from) and all around the world, Mustoe & Acosta (2016) found a single culprit responsible for the vast majority of coloration in silicified petrified wood (which is the kind ours is): iron. Iron is a unique element, they explain, because it can form so many different compounds with different colors. Mustoe & Acosta (2016), using advanced chromatography techniques, found iron to be responsible for many shades of red, orange, yellow, green, pink, purple, and brown. In fact, other elements such as chromium were found to only produce a very small proportion of the color observed (Mustoe & Acosta, 2016). 

Mustoe & Acosta (2016) explain that during the fossilization process, water rich in SiO2 (quartz) seeps into the wood and deposits silicon as well as other minerals, such as iron, which fill cavities in the wood. This process is called mineralization. These minerals replace organic material, leaving behind only rock. However, the original structure of the wood is largely preserved including the lattice of lignous (woody) cells that support the trunk of the tree. This structure often does not allow the trunk to be fossilized uniformly which can leave behind streaks and bands of different colors, as seen in Figure 2. 

As far as the origin of the minerals which mineralized the log, there are two possible explanations. The minerals can either come from the material directly encasing the log, or from mineral-rich groundwater which seeps into the substrate, but often both are responsible (Mustoe & Acosta 2016).

 

An Answer to the Question?

Well, mostly. The reason I say “mostly” is that in order to understand the exact composition of our log, we would need some really expensive equipment and someone who knows how to operate it (not me). And to understand the exact conditions which created the masterpiece of nature which we now have, someone would probably need to be there 220 million years ago. But that’s impossible, and we have the next best thing.

Given that chromium only produces a bright green color, and that iron is responsible for just about every other color of the rainbow, we can confidently assume that the vast majority of color in our log is produced by compounds containing iron. Mustoe & Acosta (2016) analyzed the composition of a piece of “rainbow wood” containing red, orange, yellow, pink, and beige from the Triassic Chinle Formation and concluded that all of the colors were entirely caused by the presence of iron compounds. If that isn’t as close to an answer as we can get, then I don’t know what is.

 

What I Learned and Questions I Still Have

I honestly had no clue that there are so many petrified logs out there. They have entire forests of petrified trees (like at Petrified Forest National Park in Arizona). I was really disappointed when I looked up a picture of the national park only to find that none of the trees are still standing. I was picturing more of a real forest.

I also learned a bit about fossilization, but not as much as I would’ve liked to. The whole idea of silicization and permineralization still kind of goes over my head. It just seems impossible to me how liquid water can turn something from a log into a rock.

Sources

Mustoe, G., & Acosta, M. (2016). Origin of petrified wood color. Geosciences, 6(2), 25.

 

 

What can the chemical and physical characteristics of a petrified wood sample tell us about its history?

Contributed by Kaela Eckel

Geological question: I initially posed my question around stable isotope ratios after finding an interesting article on the subject. You can think of isotopes as forms of the same element that have differing weights, and a stable isotope is a form of that element that is not radioactive. After spending more time looking at the petrified wood sample in the University of Oregon’s Cascade Hall, part of the Chinle Formation in Arizona, I realized that physical characteristics likely also played a large role in uncovering a petrified sample’s past. After conveniently finding an article which touched on both the Chinle Formation and physical characteristics, namely tree rings, I modified my question to encompass both the chemical and physical aspects of petrified wood. Thus, what can the chemical and physical characteristics of a petrified wood sample tell us about its history?

Description of scientific articles: In order to better understand the chemical component of my question, I researched the relationship between the depositional environment (the environment in which the piece of wood was moved to from its origin) and the minerals found in a petrified sample. An article by Hassan (2019) looked into how stable isotope ratios of various molecules in a piece of petrified wood, this one from Egypt, can provide hints as to what the piece’s prehistoric environment and life looked like. Research highlighted that microscopic differences in the Egyptian wood allowed them to be categorized as Type A – comprised of finely grained quartz, goethite (which is an iron-rich mineral commonly found in soil), and moganite (chemically identical to quartz but physically different on a crystalline scale) – versus Type B – containing chalcedony (quartz + moganite), crypto-grained or sub-microscopic quartz, goethite, and gypsum (a soft, white mineral) (Hassan, 2019). The values of carbon and oxygen, which are isotopic compositions, found in the wood were influenced by various factors: CO2 from a living thing, the amount of time water stayed in the wood/environment, how salty the water was, and how quickly it evaporated (Hassan, 2019). All these findings contributed to the idea that the wood silicified, essentially became filled with silica-rich minerals and transformed into a rock, in a closed hydrological system where different carbon and meteoric water signatures were present; this environment was a lake-like basin (Hassan, 2019). A hydrologically closed system is a system of water where none can be added nor subtracted, hence why it’s considered “closed.” Meteoric water refers to water from the atmosphere; think rain or snow.

An additional article by Ash & Creber (1992) focused on the past climates the tree experienced while still living and how indicators are preserved when the wood becomes petrified. This article dealt specifically with samples from Arizona’s Petrified Forest National Park which is conveniently where the sample displayed in Figures 1 & 2 originates from. The research pair examined fossilized tree rings which are commonly used as indicators of climate as they expand and contract with wet and dry season lengths, and ultimately concluded that the area was favorable to the year-round growth of forests (Ash & Creber,1992). Looking on a broader scale, Ash & Creber (1992) recognized that trees throughout the region displayed similar ring patterns, suggesting a similar climate throughout the greater area rather than a series of local weather anomalies.

Intersection between articles and personal observations: Regarding climate, our sample’s well-preserved, visible rings (Figure 2) can be used to determine the region’s prehistoric climate during the tree’s lifetime. Ash & Creber (1992) compare the rings seen in the preserved trees of the Chinle Formation to those of modern tropical trees that experience year-long growth due to their warm, wet environment. Both lack the typical annual growth rings, a ring of light wood followed by a ring of dark, and instead exhibit random “interruptions” that could have been caused by brief climatic changes (Ash & Creber, 1992). As learned during a field day in class, the lighter rings correspond with spring and summer growth while dark rings represent tougher bark growth during autumn and winter months (Rahilly, 2025). Figure 2 exhibits these exact qualities: wide rings indicating uninterrupted growth occasionally broken up by thin rings that represent the “interruption” Ash & Creber (1992) observed. I unfortunately do not have the ability to run a stable isotope analysis on the Chinle Formation sample (Figures 1 & 2), however some comparisons could potentially be made to the Egyptian fossils studies by Hassan (2019) as both were silicified in water and contain similar minerals such as quartz.

The sediment beneath and above the wood also suggests things about when the silicification could have occurred. Beneath the wood were sediments from the Eocene epoch, while the sand and gravel that had covered the wood were traced back to the Oligocene epoch (Hassan, 2019). The Eocene precedes the Oligocene which, as we learned in lecture, follows the principle of superposition (Rahilly, 2025). It can then be hypothesized that the wood, which is between these layers, got there after the Eocene began but before the Oligocene ended. Thinking of our sample from Arizona, if the age of the wood was unknown, one could estimate when it began silicification based off of the age of the sediment below it and above it.

Answer to geological question: Can chemical and physical properties of a petrified wood sample tell us about the piece’s history? Absolutely. Stable isotope analysis can be used to show what isotopes, typically those of carbon and oxygen, are present in a sample. These values correlate with several identifying metrics that are used to trace the fossil’s history back to where and under what conditions it began to silicify. Though I personally could not run any tests on the sample I looked at, stable isotope analyses have been conducted on samples from the same formation. Physical properties, namely the size and hue of preserved tree rings, reveal much about the past climate(s) that the pre-petrified tree lived in. Our sample’s rings suggest a steady climate beneficial to tree growth. Possible abnormalities in data or appearance are always possible so the same findings may not be applicable to all samples.

Something additional I learned & future questions: Hassan (2019) notes that stable isotope analyses have only been reported in two cases, one of which being the Arizonan forest that our sample comes from. I am thus curious if there is sufficient data from this site due to how well the wood was preserved or if it is merely a question of abundance, access, and adequate technology. Knowing, too, from the research and findings of Ash & Creber (1992) that trees from throughout the region had nearly identical growth rings, I would be interested in seeing samples from other trees to compare. Also knowing that the American southwest’s climate is much different now than the research pair suggest, I wonder what role climate change could play in future petrified specimens from the same geographical area.

Sources:

Ash, S. R., & Creber, G. T. (1992). Palaeoclimatic interpretation of the wood structures of the trees in the Chinle Formation (upper triassic), Petrified Forest National Park, Arizona, USA. Palaeogeography, Palaeoclimatology, Palaeoecology, 96(3–4), 299–317. https://doi.org/10.1016/0031-0182(92)90107-g

Hassan, K. M. (2019). Stable isotope ratios of carbonate and organic carbon from silicified tree trunks, petrified forest, New Cairo, Egypt – possible interpretations of palaeoenvironment. Geochemistry International, 57(5), 564–574. https://doi.org/10.1134/s0016702919050045

Rahilly, K. (2025). “Personal communication”. HC 241: Geology of Campus.

 

What processes lead to the fossilization and preservation of wood as petrified wood?

Contributed by: Alessi Kwan

Geological Question: When my team first suggested focusing on petrified wood for this post, I didn’t really know what it actually was. I had heard the name before, but I wasn’t sure what a piece of wood had to do with geology. After the first visit to our site, I was amazed at how the petrified log looked on the outside, exactly like a regular piece of wood, but when I felt it, it felt like any other rock. This led me to wonder: what processes lead to the fossilization and preservation of wood as petrified wood? This question is interesting because not many people know the specific processes by which wood becomes preserved as rock, and it will reveal the rare conditions that occur to form this unique feature. 

Description of scientific article: The article I chose by Mustoe (2017) discusses the mineralization processes of wood. This refers to the way wood is converted into an inorganic mineral form. The two processes are permineralization, which involves minerals filling the voids in wood and preserving organic tissues, and replacement, where organic tissues are destroyed and replaced by minerals. Mustoe (2017) challenges the idea that these two phenomena happen independently of each other and instead suggests they are overlapping processes. The degree to which wood structure is preserved depends on how quickly minerals deposit compared to how fast organic tissues decay (Mustoe, 2017). Mustoe (2017) will help me answer my question by examining these processes and how they both contribute to the petrification of wood. 

Intersection between peer-reviewed research and observations on campus: The petrified wood log shown in Figures 1 and 2 closely reflects the geological processes described in Mustoe (2017). Both the display description and the article highlight that fossilized wood forms through a combination of permineralization and replacement. The sign explains that the log was washed downstream and into a logjam, where ash from distant volcanoes mixed with silt and debris to bury the log before it decomposed. After the volcanic ash buried the log, groundwater carrying quartz, rich in the mineral silica, slowly infiltrated the cavities in the wood, filling voids and replacing the cell structures. Mustoe (2017) found that silica starts depositing in the cell walls through a process called organic templating, where silica binds to the molecular structure of wood before filling larger spaces. This suggests that our feature was primarily formed through replacement, but permineralization also occurred when groundwater carried minerals into the cavities within the wood. This aligns with Mustoe’s (2017) findings on the overlap between the two processes. 

An answer to the question? Mustoe (2017) does generally answer my original question by explaining the two processes that lead to the fossilization and petrification of wood, but it also disproves the belief that these are separate occurrences. Mustoe (2017) suggests that a better, more encompassing term for the formation of petrified wood is “mineralization,” a more general term that describes how wood becomes fossilized through a combination of mineral infiltration and chemical replacement, driven by environmental conditions such as pH, groundwater chemistry, and the rate of decay. This term captures the full fossilization process without forcing a false distinction. My question is very broad, however, so more information about other minerals involved and the actual chemical reactions happening at the cellular level could provide a more comprehensive answer. Overall, though, I now have a better understanding that the petrification of wood is not one simple process, but a complex and gradual process with many factors.

Something additional I learned and future questions: One additional thing I learned from Mustoe (2017) is how scientists use scanning electron microscopy (SEM) and hydrofluoric acid treatments to determine whether permineralization or replacement was the dominant process in a sample of petrified wood. SEM images can show whether minerals filled the wood’s cells or completely replaced them, while hydrofluoric acid can dissolve silica to reveal if any original wood tissue remains. Learning this, a new question I have is: which process was more dominant in the petrified log at our site? In order to preserve the specimen here, using hydrofluoric acid would not be the best idea, but I wonder if it would be possible to use SEM to figure out the exact fossilization history. 

Sources Cited:

Mustoe, G. E. (2017). Wood Petrifaction: A New View of Permineralization and Replacement. Geosciences, 7(4), 119. https://doi.org/10.3390/geosciences7040119