Post by: Dina Goldman, M.G., and Will Burgess

GPS Coordinates: 44.04621°N, 123.07380°W

Google Earth Map Site 5

Location and general description: 

Sat in Cascade Hall is this piece of petrified wood. Although it may look like wood, and is called wood, it is actually composed of minerals like quartz and agate. Over time, these minerals slowly replaced all the organic material of the wood, creating this inorganic representation of the ancient log. It’s part of a small exhibit, which provides a little context on its nature and history. It serves as a piece of geological artwork, and a learning experience for those passing by. The piece is a part of Oregon’s natural pre-history, and a rare representation of ancient organic life. 

Geological Observations:

The petrified wood has a mottled mixture of orange-brown, dark red, and yellow-green. Petrified wood is a fossil of a tree – even though it looks like a tree still, all of the tree material has been replaced with minerals. The wood has a pattern of concentric circles of light and dark stripes. The stripes vary in width. The outermost ring has a width of 2 cm. Some rings are discontinuous, and large sections of the wood contain little to no rings. The outer edge of the petrified wood is uneven, with lots of small bumps. The darkest spot on the wood is in the center.

Petrified Wood: What causes colors in petrified wood?

Contributed by: Dina Goldman 

Geological Question: When I saw this piece of petrified wood, the first thing that stuck out to me were the light green splotches (Figure 1) – not a normal color to see in a piece of wood! So, what causes colors in petrified wood? This question is related to geology because, while petrified wood looks like a tree, it is actually made of rock. Trees are not usually bright colors on the inside, so these colors must have come from the silicification process. Silicification is silica replacing natural tree fibers, turning the petrified wood into stone. This question is interesting because the non-tree colors give us clues about what happened in the silicification process. 

Description of a scientific article: To help me start answering this question, I read about the origins of petrified wood color in an article by Mustoe and Acosta (2016). This article is structured by going through each color and what could cause it in petrified wood. I chose this article because I can see it specifically analyzes red, green, and brown, which are colors in the petrified wood from campus. 

The intersection between peer-reviewed research and observations on campus: On campus, I observed that our petrified wood has a combination of orange-brown, dark red, and yellow-green. Red color comes from oxidized iron (Mustoe & Acosta, 2016). This means that the iron has combined with oxygen. This is the same as the red colored rust that you can see form on older metals with iron in them. The article did not come to a conclusion about the brownish color, but suggests that it might be left over in some way from the original organic material in the tree (Mustoe & Acosta, 2016). On the petrified wood from campus, I observed a yellowish green color, which isn’t discussed in the article. However, the article discusses some colors that are close: bright green is caused by chromium, dark green is caused by lots of iron, and light green is caused by a little bit of iron (Mustoe & Acosta, 2016). On campus, there is an information plaque that says that this tree was silicified by volcanic ash and silt. Volcanic ash is mainly silica, but can also contain iron. This is possibly where the iron that oxidized and caused these colors came from.

An answer to the question? This article mostly answered my question. I learned that iron is responsible for most colors in petrified wood, with the exception of very bright greens, which are caused by chromium (Mustoe & Acosta, 2016). However, the article’s research on brown colors was inconclusive and I don’t think that the green color in the petrified wood from campus matches any of the greens talked about in the article. I wish that the article used a different system of defining or describing colors, because it was hard to tell what they meant by “bright green,” “light green,” and “dark green.” Those descriptions are subjective and for that reason, I don’t feel very confident applying any of the conclusions drawn from them to my own sample. 

Something Additional I learned and future questions: I learned about a place called Petrified Forest National Park in Arizona, which has “rainbow wood,” where the wood has lots of bright pastel colors in one sample. It is caused by variations in iron concentration (Mustoe & Acosta, 2016). One future question this sparks for me is: How can we write about colors in a more consistent way in science?

Sources Cited: 

Mustoe, G., & Acosta, M. (2016). Origin of Petrified Wood Color. Geosciences, 6(2), 25. https://doi.org/10.3390/geosciences6020025

 

 

What can petrified wood tell us about the ancient climate of a certain location? 

Contributed by: Will Burgess 

Geologic Question: Being able to discern past events based on geology has always interested me. It’s the only way for us to get a real, physical grasp on what Earth’s past climates were like. It’s also our only indication for how the planet will behave in the future, although our current planet conditions are unprecedented and wildly unpredictable. Petrified wood is one of the few opportunities of ancient, organic preservation. Cell structures, growth patterns, and species type are all frozen in time and preserved for us within petrified wood. We can use them to discern ancient climates, ecologies, and how specific organisms responded to change. We can extract so much information just from the type of tree, let alone the morphological aspects (cell structure, ring shape) of the tree itself. So, what exactly can petrified wood tell us about ancient climates?

Description of Article: To achieve a satisfying grasp on what petrified wood can tell us, I looked at two articles. The first, an article by Elliot and Foster (2014), describes how the authors used the tree rings present in some petrified wood samples to discern ancient climate conditions. Using petrified wood samples, the article focuses on their implications for climate change during the Cenozoic time period, in an area of Southwestern Oregon. Their research is time period and location specific, and uses petrified wood samples to investigate past ancient climates, which pertains almost directly to my question. To get a little more specific, I looked at a second article by Ash and Creber (1992). This paper also uses petrified wood to investigate ancient climates, in the specific location of the Chinle Formation of Northeastern Arizona. I thought this would be a good choice as, interestingly, the sample this post is discussing comes from this exact formation. Our sample hails from the Chinle Formation during the Upper Triassic, the location and time period specifically covered in this paper. Ash and Creber’s (1992) observations are more closely linked to those of our own, and they are specifically tackling what the petrified wood from this area implies about the ancient climate. I believe that both of these articles will provide me with sufficient information on what petrified wood can tell us about the climate of a certain time period in a specific location.

Intersection between article and observations: We observed distinct tree rings on our petrified wood sample, which is something that the authors of both papers used to provide clues on ancient climates. Tree rings provide unique climate information as they are essentially physical records of tree growth, and we know how different climatic conditions impact tree growth. Tree ring distinction suggests an increase of seasonality, which is more common in temperate regions where temperatures and climate conditions fluctuate more throughout the year than in, say, a tropical climate (Elliot and Foster, 2014). This provides us with a great baseline of climatic knowledge just by observing tree rings. Elliot and Foster (2014) found an overall increase of distinct tree rings as their samples aged. This suggests an overall climatic shift from tropical (Early Eocene) to cool temperate (Early Miocene) in Southwestern Oregon. This implication was corroborated by previous climate models that indicated a similar shift. Ash and Creber (1992) also used tree rings to discern ancient climates, only in Arizona. As stated before, they did not observe distinguished tree rings in their samples (Ash and Creber, 1992). Similar to our observations, they mostly found discontinuous rings, which they believed to have been a result of unstable hormone transport, and not a result of seasonal changes in temperature (Ash and Creber 1992). By further examining the rings, they found no continuous bands of latewood, which is the wood that is grown during the latter part of a tree’s growing season. Continuous bands of latewood are characteristic of a temperate climate, so their absence indicates some other climate during this tree’s life (Ash and Creber 1992). Similar to our observations Ash and Creber (1992) also found large variations in ring width, which indicates that the tree grew for long periods of time without interruption. This implicates a consistent temperature over a period of multiple years (Ash and Creber 1992). This type of growth pattern is extremely similar to the tropical trees we see in areas like Hawaii and Malaysia. Many trees we see today that live in a monsoon system have poorly developed rings, very similar to those found in Ash and Creber’s (1992) study. 

Answer to Question: My initial question was, what can petrified wood tell us about the ancient climate in a certain location? Petrified wood can tell us a plethora of information regarding an ancient climate. And when taking a sample from a specific location, it can give us a very clear idea on what that area’s ancient climate was like. By examining the patterns of growth (represented by the tree rings), we can assume some very base ideas regarding an area’s climate. Clear, distinct rings indicate regular temperature fluctuations throughout a year (Elliot and Foster, 2014). This is indicative of a temperate climate, somewhat like what we experience here in Oregon. When growth patterns become less distinct, this suggests a constant state of temperature, indicative of a tropical climate (Ash and Creber 1992). Both papers addressed this topic and described the same patterns. To specifically address our piece of petrified wood, we can assume that when the tree was alive, during the late Triassic, it was experiencing a consistent tropical climate. This is due to the large variations of ring width, and severe ring discontinuity. Elliot and Foster (2014) looked at a period of climate change, and also described this trend of how ring distinction decreases as a climate becomes less temperate. This is corroborated by current climate models that place Arizona in a tropical climate during the Late Triassic. Both of these articles gave me the knowledge to be able to examine a piece of petrified wood, and be able to get a good grasp on what kind of environmental conditions that piece of wood experienced when it was alive. 

Future Questions and Learning: One thing I learned from my article is that trees in tropical areas are able to experience nearly year round growth, and this is shown in their growth rings. Also that these tropical trees give an interesting look into the hormones of the trees, as irregularities in growth patterns are often a result of a deficiency of hormones, and not linked to temperature. One question I have is, the Chinle Formation is a massive 280m thick formation consisting of thousands of petrified wood logs. So, was there a single event that covered all these trees? Why is this area so concentrated with petrified wood?

Sources Cited: