Questions:

Gabby: What can the garnets in our metamorphic rock tell us about how the rock formed and has changed since then?

Kennedy: How does the grain size and mineral composition in a rock reflect the location, intensity, and stages of stress during metamorphism?

Will: How did the garnet found in this metamorphic rock get in there?

Bridget: What causes the staurolite to have their specific cross shape in our metamorphic rock?

Post by: Gabby Studen, Bridget Donnelly, Will Klatt-Breed, & Kennedy Barksdale

GPS Coordinates: (+44.04602, -123.07402)

Google Earth (Site 3)

Location & general description: Found just in between the University of Oregon’s Pacific, Cascade, and Volcanology Halls, this metamorphic rock is embedded into the brick on the ground of the path winding between buildings. The waterfall art piece located nearby sets the scene for this unique rock sample. This rock is interesting because of its multi-colored layering and natural decoration of garnet. Garnets are a group of silicate minerals that share a common crystal structure but differ in composition.

Geological observations: When observing our rock, we can see that the layers within it vary in size, with the largest being 2 cm wide and the smallest being less than 1 mm (figure 1). Each layer appears to alternate between dark and light gray, indicating a possibility of metamorphic segregation, the physical and chemical movement of minerals into layers. This process may indicate that it is a gneiss, a metamorphic rock consisting of feldspar, quartz, and mica. The rock is embedded with many small crystals called garnet and staurolite, which are most common in metamorphic rocks. These crystals can be seen in shapes like spheres, triangles, and cylinders. When touched, the rock has a sandy texture and leaves traces of silt on your fingers. The surface glitters in the sun but the side has rough, jagged edges. Metamorphism is represented by a rock that has undergone transformation by heat and pressure.

 

What can the garnets in our metamorphic rock tell us about when and how the rock formed and how it has changed since then?

Contributed by: Gabby Studen

Geological Question: In analyzing our site of interest as a group, we had many mysteries to solve regarding our chosen rock. Not only were the patterns we saw fascinating, but we were also allured by the small crystals that seemed to appear throughout the rock. In asking about the rock seen in Figure one, we found that these small crystals are granite, commonly seen in metamorphic rocks. My question regarding this is: What can the garnets in our metamorphic rock tell us about when and how the rock formed and how it has changed since then? Metamorphic rocks are complicated, and understanding how they form can be very confusing. I think that this question is interesting, considering these garnets act as a sort of time capsule for us to uncover and learn details about the rock that we might not otherwise understand. It is also extremely helpful for those of us who are not super familiar with metamorphic rocks because it allows us to take a different approach to understanding how they form. I’m also biased in choosing both this site and the question because my birthstone does happen to be garnet. 🙂

Scientific article chosen: In trying to find an answer to my question, I read about the use of garnets within metamorphic rocks to date and further understand the processes that occurred to metamorphic rocks as they formed in an article by Baxter et al. (2017). This article explores the petrologic context (AKA the details about composition, origin, and structure of the rock) that the garnet minerals within rocks are able to provide to us through our analysis of them (Baxter et al., 2017). In describing these garnets as petrochronometers – mineral sources of information on the petrology and timeline of metamorphic rocks – and providing significant amounts of evidence to support their claims, the article is a great starting point for me to branch off when answering my question (Baxter et al., 2017). I think that this article will provide more than enough information for me to understand what the garnet is capable of telling us. 

Intersection between peer-reviewed research and observations on campus: The article explains that petrochronology as a science began with the discovery that the analysis of garnet’s chemical zonation (aka variation in the chemical composition dependent on the location within the garnet) gives us information about the rocks that the garnets reside within – similar to how tree rings tell us about the environment and age of a tree as it grows. In Figure B on page 470 within the article, they show the different zones within the garnet itself and what kind of composition exists there (Baxter et al., 2017). If we were to apply the same kind of measurement shown through the diagrams seen within the journal article to one of the garnets within our rock seen in Figure one, we would learn detailed information about the environment in which the rock and garnet formed. However, since we don’t have access to the time, skills, or technology to analyze our garnets’ compositions, we can learn from what we can simply see in observing our rock. The article explains that even though there are multiple growth paths, garnets can develop depending on which elements are present, one thing stands clear. As heat and pressure increase around and within the garnet, it grows. The article emphasizes how composition can change as the pressure within and size of the garnet shift, however, we can’t quite see that happening in our physical example (Baxter et al., 2017). In comparing our metamorphic rock’s garnets with others (like the ones we saw in class), it is clear that ours are on the smaller end of the scale. It is because of this that we can infer that our garnet might not have been subject to insane amounts of pressure and heat in the grand scheme of things. 

An answer to the question?: The article not only provided multiple answers to my question directly, but also broke down the terms that are used to label the processes I was asking about. The answer to my question is that garnet can tell us a whole lot about what the pressure, temperature, and location of the rock looked like as it was forming and how it has evolved since. The article explains that garnet petrochronology is the umbrella term to explain how we can measure and understand these details from simply analyzing the garnets within metamorphic rocks. Petro and chrono together refer to many different aspects of measuring rock composition, history, and more. These measured aspects include the rocks’ age through the decay of radioactive atoms within the garnets in the rocks/through variation of mineral composition of the garnets, timescales and temperatures experienced by the rock through the movement of atoms within the garnets, and more (Baxter et al., 2017). 

Something additional I learned and future questions: One thing that I learned from reading the article is that many people in the geological science community are unaware or not yet convinced of the power of garnets in providing knowledge about the history and life path of metamorphic rocks. The article explains that the idea of garnets as active participants in the practice of geochronology was introduced without the methods seen today to back that up, so many are hesitant to trust work done surrounding the topic (Baxter et al., 2017). I wonder what the garnet petrochronology studying scientific community can do to communicate their findings effectively to the broader geological/science community. I am also curious as to whether the situation of broader disbelief in these ideas has changed in the eight years that have passed since this article was written. Hopefully, these scientists can answer my question and successfully shed light on this amazing niche field of geology.

 

Sources Cited:

Baxter, E. F., Caddick, M. J., & Dragovic, B. (2017). Garnet: A rock-forming mineral petrochronometer. Reviews in Mineralogy and Geochemistry, 83(1), 469-533.

 

How does the grain size and mineral composition in a rock reflect the location, intensity, and stages of stress during metamorphism?

Contributed by: Kennedy Barksdale

Geological Question: Minerals have many identifying characteristics that lead to our understanding of rock properties (density, color, texture, and resistance). The feature we chose has a diverse array of visible minerals, a key reason for why it caught our attention. This leads me to wonder what other ways minerals can help us determine a rock’s geological history. How does the grain size and mineral composition in a rock reflect the location, intensity, and stages of stress during metamorphism? Metamorphism is represented by a rock that has undergone transformation by heat and pressure. This question relates to geology because it is asking how minerals in rocks are used to identify metamorphic processes and origin. Analyzing grain size and mineral composition helps us reconstruct the conditions under which the rock formed and the stresses it endured. This question intrigued me because geologists may be able to draw advanced conclusions that ease their research procedures, saving time and resources as well as broadening their understanding!

Scientific article chosen: To help me start answering my question I read an article by Ando et al. (2014) which discusses how mineral compositions can predict the metamorphic grade (intensity) rocks undergo, depicting the relative temperature and pressure of those conditions. Metamorphic grade has shown to be the best indicator of how deeply source rocks have been eroded, revealing different layers of Earth’s crust, and finding the depth from which the material originated. This article has a focus on garnet, a mineral present in our rock, and explains how its chemical composition varies with temperature and pressure conditions during growth, providing important information on the metamorphic evolution of source areas.

Intersection between peer-reviewed research and observations on campus: As temperatures rise, most minerals lose their vibrancy or are transformed into minerals with darker colors (Ando et al., 2014). In our metamorphic rock feature (figure 1), the garnets are a dim reddish-brown color, representing the high temperature and pressure environment that is required for their formation. Since the garnets are small and have lost a lot of their red coloring, turning shades of brown, gray, and dimmed red, we can infer that this rock was taken deep from within the earth’s crust or inside a mountain. Garnet and amphibole are among the most widespread heavy minerals in orogenic (mountain) sediments and are common in medium-grade river sediments sourced from the Himalayas and Alps. The appearance and abundance of minerals such as staurolite, kyanite, and prismatic sillimanite tells us how deep erosion has cut into a mountain alluding to where a particular sediment came from (Ando et al., 2014). This further proves that our rock came from deep within a mountain because it contains garnet, staurolite, and silicone dioxide. Garnets sometimes have rings around them that show the changes in heat and pressure throughout their formation. This is a helpful tool in determining the location, the length of the process, and the distance a rock traveled during its formation (Ando et al., 2014 and references therein). These layers can be very thin so I am unable to depict for certain whether or not the garnet in our rock has these layers or if they are just imperfections in the physical appearance of the minerals themselves. However, based on the diverse foliation (layers in a rock) I can assume that the garnet pieces also have some level of layering themselves. The foliation seen in the feature (figure 1) also indicates a high level of metamorphism and gives evidence that this is a gneiss rock, known for its distinct banding.

An answer to the question? My initial question was: How does the grain size and mineral composition in a rock reflect the location, intensity, and stages of stress during metamorphism? This article mostly answered my question because it examined how mineral compositions allow us to trace erosion patterns and exposed temperatures and pressures, though there is some trickiness in determining the conditions completely. Rock samples more abundant in TiO2 (titanium dioxide) than SiO2 (silicone dioxide) were most likely taken from lower crustal areas, with the opposite being true for upper crustal areas (Ando et al., 2014 and references therein). This explains that rocks with higher levels of TiO2 experienced higher levels of metamorphic grade. What makes determining these conditions difficult is that contact and regional metamorphic rocks have different chemical compositions in minerals. As tectonic uplift exposes rocks, different rock types produce sediments with distinct mineral compositions. The composition of garnets and amphiboles is not controlled only by the metamorphic grade of the proliths (parent rocks), but also the chemical composition and pressure conditions (Ando et al., 2014). This makes it more difficult to determine which factor directly led to the composition of minerals or if it was a combination of multiple. It would be helpful to analyze mineral alignment and placement, which displays stress and pressure levels. This article focused on mineral composition and didn’t go into detail about grain sizes, but with the information I have I can conclude that a metamorphic rock would have the same grain size as others that went through similar conditions.

Something additional I learned and future questions: Something I learned while reading this article was that due to their accessibility and extensive research history, the Alps provide an ideal setting for studying metamorphic isograds. An isograd is a line on a map connecting points of equal metamorphic grade in rocks (Ando et al., 2014). This interested me because my first thought would be how intimidating collecting samples from these mountains would be, but with this in mind, it makes sense that they would have diverse and rich sample material due to the massive amount of area they cover. This leads me to wonder, what makes the Alps accessible and what type of accessibility are they referring to? I am also curious as to what initiated all of the research projects performed on the Alps? And, can the information concluded be generalized to other mountain ranges?

Sources cited: Ando, S., Morton, A., & Garzanti, E. (2014). Metamorphic grade of source rocks revealed by chemical fingerprints of detrital amphibole and garnet.

 

What causes the Staurolite to have their specific cross shape in our metamorphic rock?

Contributed by: Bridget Donnelly 

Geological Question: When our group saw Figure 1, it was clear how visually interesting our rock was. Beyond just the condense nature of its metamorphic formation, there also appeared to be two different, smaller objects within the larger rock. When doing research as to what they are, we discovered the smaller colorful gemstones were garnet, and the elongated, more cylinder mineral was a staurolite. We hadn’t mentioned staurolite at all this year, so when looking into its specific qualities, its unique cross shape really interested me. This led me to my question: What causes the Staurolite to have their specific cross shape in our metamorphic rock? Within this question, staurolite refers to the silicate mineral that is seen in Figure 1 that rises off the face of the rock. The cross shape referred to in the question is less visible in Figure 1, as the cross side is likely facing inwards towards the rock. Finally, metamorphic rocks refer to a rock formed under heat and pressure, and seen in Figure 1. 

Scientific article chosen: To help me start answering this question, I read about the properties, and geochemistry of staurolite in the Casa Nova region of Brazil in an article by Gomes de Oliveria, et. al. (2024). I chose this article to get a better understanding of what exactly staurolite is, and because it goes into great specifics on methods and research with staurolite in relation to gems in Casa Nova. This article is mainly trying to understand the properties of the staurolite found in this region, and apply their findings towards the possible use of the staurolite found as gemstones. This article could help me answer my question by providing context to how staurolite is formed, alongside with any factors that could influence how its cross appears or varies. 

Intersection between peer-reviewed research and observations on campus: Gomes de Oliveira, et. al. (2024) describe the characteristics of staurolite, many of which overlap directly with the staurolite found in Figure 1. The features of staurolite are generally durable, dark colored, and with a cross at either 90 or 60 degrees, (Gomes de Oliveira, et. al., 2024). There’s a clear intersection between the durability noted in the article, and the durability seen in Figure 1. In our observations, we’ve learned that metamorphic rock is compressed with high pressure and temperature, making it very durable. However, the durability of the staurolite is seen even more, as it sticks out of the face of the rock in Figure 1. While the metamorphic rock around it has eroded down, the staurolite appears to be harder and more resistant to scratching and wearing down, proving its durability. Moreso, the dark color quality of staurolite noted in the research is seen as well, with Figure 1 showing dark grey staurolite. Interestingly, the 60 or 90 degree cross that is seen in the staurolite reflects the orientation of how the rock was cut. As mentioned above, the crosses in figure 1 are dipping into the rock so it’s not as easily observable. While the article goes on to give specific and tested information about the geochemistry of staurolite, as we don’t have the tools in class to take those types of observations, which allows less intersection. 

An answer to the question? While the article answered smaller questions within my main question, it did not fully answer my question. It gave me a better understanding of what staurolite is, including specifics on its qualities, but never explained directly how it got its cross shape. The article focused more narrowly on the author’s research in Brazil, and framed its discussion of the staurolite through the lens of gemology, meaning the study of gemstones. The article was most helpful in answering my question in the introduction of their study, which gave me better building blocks to move forward with researching how the cross shape is formed, because I now know how to identify and classify staurolite. 

Something additional I learned and future questions: One additional thing I learned from my article is that gemologists use tools such as refractometers to analyse and process gemstones. Specifically, I learned that a refractometer measures the degree a gem reflects light, (Gomes de Oliveira, et. al., 2024). Before reading this article, I knew almost nothing about the tools that geologists use to attain and process data, but have always been curious. Learning this made me wonder: How have geologist tools evolved over time, specifically looking back at the first “geology tools” from indigenous practices? I’m curious about the origin of geology, and how early studies of it have been able to classify and explore components of geology. 

Sources Cited:

Oliveira, I. G., Bezerra, I. P., Santos, L., Santos, B. R., Silva, L. S., Leal Neto, A. & Neri, T. F. O. 2024, ‘Gemology and Geochemistry of Staurolites from the Casa Nova Region, Brazil’, Anuário do Instituto de Geociências, 47:62755. https://doi.org/10.11137/1982-3908_2024_47_6275

 

How did the garnet found in this metamorphic rock get in there?

Contributed by: Will Klatt-Breed

Geological Question: When examining the rock shown in Figure 1, we all noticed that it contained smaller gems. We thought it was kind of weird, as we knew it was not a sedimentary rock, which has layers of sediments and usually other substances within them, such as fossils. Yet our rock was metamorphic, and we ran to class the next day to show what we found. From class, we learned that within this rock we just happened to walk by, there were small garnets embedded. WE’RE RICH! Which was my first thought, but soon realized they were much too small to be of any value, but it did get me thinking, how did the garnet found in this metamorphic rock get in there?

Description of scientific article: In my quest to find answers, a gift was bestowed upon me, a scientific article by Baxter et al. (2013). This article dives into the science of garnet formation, how garnets grow, and how geologists use them to study the world below our feet, such as the crust, mantle, and tectonic plates. The main point of the article is to show the importance of garnets and all the ways it is useful. I wanted an article that directly talked about more the gemstone as my question focuses on the garnets, than just metamorphic rocks themselves. 

Intersection between peer-reviewed research and observations on campus: This UO campus rock embedded with garnets was either regionally metamorphized where a large area experiences the same pressures and heat or was metamorphized in a subduction zone which is more likely as here in Oregon we have our very own subduction zone between the Juna de Fuca plate going under the North American plate forming the cascades (Baxter et al., 2013). According to Baxter et al. (2013), garnets in the crust grow at ultra-high temperature upwards of 2000 °C and ultra high pressures of about 25 GPa (1 GPa is about 10000 atm and on the surface of the earth we live at about 1 atm) during these kinds of conditions these gemstones can form from any protolith which is a unmetamorphized rock/ parent rock. Garnet can also preserve information about the pressure and temperature they experienced, so just imagine the story that is yet to be told about Figure 1. This means that the garnets we see are more than just pretty gemstones, but they’re records of deep Earth processes. This article is a catalog of the importance of garnets in the rock world.

An answer to the question? Yes, partially, the article does mention all of the different conditions needed in order for garnet to form in many rock species, as garnet is able to form in all three rock types. On some points leaves out the step-by-step guide for formation and how gemstones squeeze their way in, but it gives several different ways that garnet could have ended up encased within our rock. Garnet is one of the most common gemstones in our world. As stated before this very rock if from Oregon we can infer that this metamorphic process could have taken place in a subduction zone at great pressure and temperatures where a yet metamorphized rock rich in possibly Fe³⁺, Al, or Cr was then crushed so hard it formed small gemstones within itself (Baxter et al., 2013).

Something additional I learned and future questions: I learned a lot from this article, some things I never even thought I needed to know, the geological world this class has opened for me, and I have questions for it all. I learned that Garnets can be used as little time capsules. These gems can capture Oxygen, other minerals, and fluids from deep down in the mantle and crust, and when they come to the surface, geologists can then examine them and have a glimpse of what is occurring and happening below their feet (Baxter et al., 2013). Also, the fact that garnets can occur in so many different colors, from purple to green, and most commonly a deeper red. In further thinking, I wonder what some of the differences are in garnets that are formed within metamorphic rock compared to igneous or sedimentary rock? How does the composition change, or does it at all? Are they stronger or weaker?

Sources Cited: 

Baxter, E. F., Caddick, M. J., & Ague, J. J. (2013). Garnet: Common Mineral, Uncommonly Useful. Elements, 9(6), 415–419. https://doi.org/10.2113/gselements.9.6.415