Understanding the mountains from the bottom up

By Cailey B. Condit Ph.D. student, Geological Sciences, University of Colorado – Boulder

 

Have you ever sat on a ski lift and looked out at the snowy peaks and wondered, “How did these mountains get here and how long have they been around?” Or spent a few minutes scrolling through NASA’s Instagram account and gasped at a beautiful picture of some mountain range from space and thought to yourself, “How does that form?” If you ever have had these kinds of thoughts, we have a lot in common. This is the kind of stuff I think about all the time.

Geology, tectonics, and time

Geology is the study of the solid earth, including the materials that make it up and the processes that shape it. I categorize myself as a tectonic geologist, meaning I try to understand the formation and deformation of rocks, using this information to unravel the geologic processes preserved within them to reconstruct the tectonics of the past earth. Tectonics is the study of the earth’s tectonic plates – a series of ridged plates consisting of the earth’s crust and upper mantle called the lithosphere that move across the flowing plastic asthenosphere ~50-200 km below. As these plates move slowly (typical rates are 3-10 mm/year – the rate of your fingernails growing) around the globe they can crash into one another, building giant mountain ranges like the Himalaya (Fig. 1a). These plates can also dive underneath each other (a process we call subduction) releasing water and volatiles and building chains of volcanoes like the Andes, the Cascades or the island of Japan (Fig. 1b). In some places these plates slide past each other, intermittently releasing energy in the form of earthquakes as they lock together and release in a cyclical manner (Fig. 1c, e.g. the San Andreas Fault in California). In other places like Iceland, the Mid Atlantic Ridge or the Pacific Rise, two plates spread apart, creating new material and growing in zones called spreading centers (Fig. 1d). Tectonic plates can interact in many other unique ways, but as a geologist, these are some of the major tectonic boundaries we study.

Figure1

Figure 1. Simplified cross sectional cartoons of common tectonic boundaries. A. Continental collision zone – location where two continental plates smash into one another. B. Subduction zone – location where an oceanic plate is diving underneath a continent. C. Transform boundary – location where two plates are sliding past each other. D. Spreading center – location where new material is being added to two oceanic plates moving apart.

A wonderful thing about geology is the unique aspect of time. In geology, all you have is time. When you’re studying something that is 4.54 billion years old, there’s a lot of time for things to happen, change, evolve, grow, and erode. The slow rates of geologic processes, accumulating over thousands, millions or billions of years can lead to stunning changes in landscapes and environments. Mountains can grow due to tectonic collisions, and erode away so that the only evidence we have of their existence is preserved in their deeply eroded roots. Geologists at heart are essentially historians, trying to reconstruct past events and environments; the major difference is that geologists look at time on a very different range of scales, from billions of years as continents collide or separate to seconds as a fault breaks.

The usefulness of analogs in geology

As a geologist, I look at a place like the Himalayan Mountains and wonder what processes are going on deep below those peaks that might lead to such beautiful and high mountains. Unfortunately, it’s impossible to directly observe the processes occurring below those magnificent mountains because they’re buried by ~40 km of rock, so I do the next best thing. I go to places where natural processes have beveled off the overlying mountains, exposing their roots and the rocks that have enjoyed ancient deep crustal processes. I study those rocks, now present at the earth’s surface, to understand the way portions of the lower crust responds, evolves and changes due to continental collisions. These processes are known as analog studies. Using a combination of field mapping techniques, chemical, isotope, and structural analyses and inverse and forward modeling I try to reconstruct the changes in rocks due to increased pressures and temperatures in the deepest parts of the crust. This is a field called metamorphic geology; it is the study of the formation of metamorphic rocks, rocks that have transformed due to deep burial from and changed in response to increased pressures, temperatures, and large tectonic forces. Along with this transformation, rocks plastically flow and fold, undergoing ductile deformation while deeply buried. Understanding the processes that occur in the lower crust is important for modeling the way large mountain belts are supported, grow, and evolve.

My field area is in southwestern Montana (Fig. 2a) in the Northern Madison Range directly north of Big Sky and about 40 minutes south of Bozeman. The rocks of the Northern Madison Range are ancient, many of them over three billions years old, and preserve evidence of a 1.8-1.7 billion year old collisional tectonic event similar to the continental collision that gave rise to the Himalaya. After almost two billion years, the mountains built during this event have long been eroded, bringing the deep rocks from the lower portions of the crust up to the surface, creating an ideal natural laboratory to study deep tectonic processes.

Figure2

Figure 2. A. Map showing location of the Northern Madison Range in southwestern Montana, USA. B. Example of a detailed geologic map created over several field seasons from the Northern Madison Range (adapted from Condit et al., 2015).

Fieldwork and research

I have spent several weeks each summer of my PhD in the field with the expressed purpose of creating geologic maps to elucidate broad tectonic patterns and collect samples for subsequent chemical, structural, and isotopic analyses. My field season starts in Boulder, Colorado where I pack up a large SUV with camping, backpacking, geologic mapping gear and a field assistant (thanks Kat, Phil, Craig, Diana, Kevin, and Josh, you’re all the best!) and take a long drive up to Bozeman, Montana. From here we load up with backcountry friendly meals and drive down to the Northern Madison Range. We head to a pre-planned trailhead, pack up our gear and head into the backcountry armed with 5-7 days of food and ample bear spray ready to do some science. After hiking in about 6-8 miles and gaining 4000 ft of elevation, we set up a basecamp, eat some delicious food, and get ready for the next day of mapping.

Geologic maps are one of the fundamental tools that all geologists use. These maps are essentially a cartoon, drawn on top of a topographic map, that outlines the location of different rocks-types, the location of samples, and where different geologic features like faults and folds occur (Fig 2b). Perhaps the most important aspect of creating a geologic map is that it forces you to make careful observations of the entire area you’re looking at and provides you with a basis for communicating your ideas about how that area formed to other geologists. Creating one of these maps usually consists of finding outcrops of rock, identifying them, describing them, and then taking attitudes (orientations) of relevant planar features (e.g. bedding plane, foliation, contact between units) to understand how these rocks extend across the mapping area (Fig. 3).

ConditFigure3

Figure 3. View to the south east of a ridge in the eastern portion of the Northern Madison Range. Geologists C. Condit, K. Kravitz and K. Mahan are returning to camp after a long and rainy day of mapping. Photo by O. Orlandini.

I spend much of my time in the field moving from outcrop to outcrop taking detailed notes on the rock type, minerals, structures, and orientations of deformation-related features at each spot. I often spend several hours at a single outcrop, sketching (Fig. 4), sampling, and reconstructing what I see within the rocks. After 4-6 days of hiking, mapping, note taking and sampling we load up our now empty bear bins with rocks (geologic field work is the one time where your pack gets heavier over the course of a backpacking trip) and hike out to the trailhead. A shower and a cheeseburger later and we get ready to do it all over again. A typical field season may consist of 2-4 sets of backcountry trips.

Figure4

Figure 4. Top: Annotated field notebook sketch of folded gneiss next to a field photograph of the same folds. Bottom: Field photograph of a metamorphic rock containing dime sized garnet next to annotated sketch of the same feature.

After my field season, I return to Boulder with several large bins of rocks and the real work begins. Some of the rocks are sent out for geochemical analyses to be used for forward modeling and characterization, while other parts are turned into petrographic thin sections – tiny slivers of rock (0.03 mm thick) that can be used on a petrographic microscope. Using this special transmitted light microscope I look at these thin sections, making observations about the relationship of minerals and structures. Through the integration of my field mapping, geochemical data, modeling, microscope observations, and isotopic radiometric dating, with previous research, I have been able to reconstruct a pattern of growth preserved within the ancient rocks in southwestern Montana that occurred 1.8-1.7 billion years ago (Condit et al., 2015). This growth occurred over a time period of ~80 million years and over a distance of ~100 km, and is similar to mountain belt growth patterns observed in other ancient collisional mountain belts and in the currently colliding Himalaya.

One of my favorite things about being a geologist is the excuse to get outside for my research. I decided to come to graduate school at CU in part because of the field-based project my advisor offered me. Although the vast majority of my research involves careful sample preparation, detailed lab work, and computer modeling, whenever I look at one of my samples I am returned to the beautiful setting it was collected in (Fig. 5). The rock acts as a touchstone that refreshes me and gives me the motivation to continue trying to unravel its secrets.

ConditFigure5

Figure 5. View to the north west of the headwall of Bear Basin in the Northern Madison Range. This same area is the central portion of the map in figure 2B.

If you’re interested in geology, feel free to follow my geologic themed Instagram: @geologyinaction

Reference:

Condit, C.B., Mahan, K.H., Ault, A.K., Flowers, R.M., 2015. Foreland-directed propagation of high-grade tectonism in the deep roots of a Paleoproterozoic collisional orogen, SW Montana, USA. Lithosphere 7, 603–610. doi:10.1130/L460.1

 

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2 thoughts on “Understanding the mountains from the bottom up

  1. I have a tectonics question: why are the plates the shapes they are, and moving in the direction they do? Put another way, why did the plate boundaries form where they are, and why are some boundaries spreading while others are compressing?

    I’ve never heard a good answer to this, so my best guess is the boundaries were lines of relative weakness in the crust that at some point in the earth’s cooling gave way to movement from thermal convection cycles created by the mantle’s temperature gradient and the earth’s rotation.

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    • You’re asking a great and fairly complicated question here!

      I’ll try to answer it as best I can in a couple parts.
      Plate boundaries form where they do for a variety of reasons, and generally geologist don’t all agree on exactly why they might form in a particular place. To first order, you’re correct, plate boundaries form in locations where there is some inherent (often inherited) weakness in the lithosphere (conductive crust and upper mantle). Often a spreading center or rift margin forms where there was once a collisional plate boundary that is reactivated.

      The actual mechanism that causes the plates to move is also not totally understood. Generally, convection in the asthenospheric mantle is thought to drive plate motion. Subduction of oceanic plates into the mantle can also create a force called “slab pull” that can impact spreading centers at the other end of the same plate and also potentially drive convection by adding more cool material to the mantle.

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