Welcome to the My Science Life feature of Dr. Scott E. Johnson, Professor of Tectonics and Structural Geology, and Director of the School of Earth and Climate Sciences at the University of Maine.
My area of expertise would generally be classified as Structural Geology, but that title encompasses many sub-areas. My work is primarily focused on how the solid Earth responds to different types of loading (applied forces) over a range of spatial and temporal scales. An example of long-term, large-scale loading is the interaction of tectonic plates, such as the Pacific and North American plates. In California, these plates slide by one another and the boundary between them is called the San Andreas Fault.
An example of short-term, small-scale loading is the energy released around a propagating earthquake rupture. This energy can cause extensive grain-scale cracking in rocks surrounding big earthquake faults like the San Andreas. Finally, another type of loading is thermal, because when minerals change temperature they either expand (heating) or contract (cooling). Thermal loading leads to grain-scale forces large enough to crack rocks. In general, loading leads to deformation, and one of the main aims of my work is to examine the mechanisms by which deformation occurs given specific loading conditions. In that sense, my area of expertise could also be called Earth rheology, where rheology is the study of how materials respond to applied loads.
What is your job like on a daily basis (typical day)?
During the 9-month academic year, I try to get up around 4 AM and do research for 2-3 hours (after telling myself that only a crazy person would voluntarily get up at 4 AM!). This typically involves reading, writing, and running numerical models designed to explore deformation of rocks. I am most effective at research in the morning when my head is clear and I am not constantly distracted.
Have coffee with my wonderful wife Bethany, who needs a little more sleep than I do.
We both head to the gym for 1.5 hours. Health and fitness are lifelong priorities for us.
Go to the office for much of the rest of the day. My routine there is akin to managed chaos. There are classes to teach, graduate students to mentor, and because I am Director of the school, there is always a load of administrative work to do. I teach an upper-level course called Structural Geology, and an introductory-level course called Dynamic Earth. I also teach graduate-level courses when needed. My administrative work includes things like raising private funds, strategic planning, course scheduling for the coming semester, approving expenditures from the school’s annual budget, signing personnel action forms and myriad other forms, instructing the Peer Committee on annual reviews of faculty performance, running monthly faculty meetings, and a host of other duties. The administrative work is sometimes burdensome, but it gives me a deep sense of satisfaction knowing that in shouldering these responsibilities I am freeing my faculty colleagues to focus on their research and teaching.
Head home and, among other things, play guitar for an hour or two. I like different types of music, but these days I play mainly classic rock and metal. Bethany is a drummer, which makes playing extra fun and rewarding. Bethany and I also love to cook, so we typically spend an hour in the kitchen every night making something healthy that is also actually tasty!
I’ll typically spend an hour before bed doing more research and clearing out the email (and sneaking in a few more guitar licks!).
Tell us about your research
I have a wide range of interests in the general fields of tectonics, structural geology, and rheology. I have worked on magmatic systems, tectonic evolution of magmatic arcs and mountain belts, fracture-controlled migration of oil and gas, dynamic fragmentation in the earthquake cycle, and the coupling between deformation and chemical reaction in deformed metamorphic rocks.
Over the course of my career, I have mapped 100s of square kilometers of amazing geology in Australia, New Zealand, Nepal, and Mexico, and have traveled to many other parts of the world to attend conferences and field trips. Apart from being in the field, I also love rock microstructures, so I spend a lot of time examining them using a variety of tools including optical and scanning electron microscopes, and numerical tools like the one shown in Figure 1.
Most recently I have been working on deeply eroded earthquake faults to better understand the earthquake cycle and the rheology of earthquake faults at depth. We know a lot about earthquake faults in the upper few kilometers of Earth’s crust because we can access these crustal levels through surface observations, drill holes, and high-resolution seismology experiments. But if we want to know, for example, what the San Andreas Fault might look like at 10-15 kilometers depth, we must find a similar ancient fault that has been exposed by millions of years of erosion. One such fault is the Norumbega Fault System in Maine, which is one of the best ancient analogs for the San Andreas Fault currently exposed at Earth’s surface. My research group has been working in the Norumbega system for nearly a decade. The rocks we work with were deforming at temperatures of 400-500 degrees Celsius when the fault was seismically active (~300 million years ago), which corresponds to depths of 10-15 kilometers. I enjoy multidisciplinary projects and bringing together people from different fields to tackle difficult problems in the Earth sciences. Working with my engineering colleagues has been particularly rewarding because we have been able to make progress in areas that were too mathematically and computationally challenging for me to tackle on my own.
In the example shown, the electron backscatter diffraction technique was used via a scanning electron microscope to map the minerals and their crystallographic preferred orientations from a 20-micrometer thick polished rock section. The data are used as input for the numerical toolbox, which reconstructs the microstructure and applies the appropriate elastic stiffness matrices to each mineral grain.
In Figure 2, the “rock” is placed under a compressive hydrostatic load of 180 Megapascals (equivalent to a depth of ~7 kilometers in the continental crust) and then allowed to cool by 100 degrees Celsius. When rocks cool down, the individual mineral contract by different amounts in different directions. In other words, they possess anisotropic thermal expansion properties. In this example, we plot the maximum principal stress (in Megapascals), where red (positive) is tension and blue (negative) is compression. This anisotropic contraction leads to very large grain-scale tensile stresses, which would cause microcracking. This is very important in engineering but also in Earth sciences because such microcracking leads to changes in the rock rheology, and generates transient permeability that facilitates fluid flow. This has important application in geothermal fields and the kinetics of metamorphic chemical reactions, for example.
Vel, S.S., Cook, A.C., Johnson, S.E., Gerbi, C., 2016. Computational Homogenization and Micromechanical Analysis of Textured Polycrystalline Materials. Computer Methods in Applied Mechanics and Engineering, 310, 749-779, https://linkinghub.elsevier.com/retrieve/pii/S0045782516308192
What are some of the biggest challenges in your field?
Because planets are quite complex, we are still very much in the early stages of our understanding of Earth. There are many fascinating and societally important challenges to address. In the broad fields of tectonics and structural geology, there are several topics that stand out as grand challenges. The Tectonics and Structural Geology community in the USA is in the process of completing the final draft of a vision statement for the National Science Foundation that outlines five grand challenge for future research. These are: (1) understanding the evolution of Earth and other planets through time, (2) understanding the mechanisms by which Earth deforms, (3) understanding the physical and chemical behavior of earthquake faults like the San Andreas Fault in California; (4) understanding the dynamic interactions between processes occurring at Earth’s surface and those occurring in Earth’s interior, and (5) meeting societal needs while advancing these research objectives.
Taking number 2 above, for example, members of our community have long understood the importance of formalizing a conceptual framework and quantitative description of how the solid Earth deforms. Deformation directly impacts society through processes such as earthquakes, volcanic eruptions, changing landscapes, and the formation of natural resources. All aspects of this deformation are important regardless of whether it involves mountain-building caused by the interactions of tectonic plates over millions of years, surface rebound following the melting of ice sheets over thousands of years, the rise of magma to feed volcanoes over days to weeks, or the near-instantaneous movements on faults during earthquakes.
What advice do you have to those pursuing a career in your field?
Find something in the Earth sciences that excites you and then dedicate your professional life to pursuing it. Work hard, but play hard too, and pay attention to your health and hobbies. To be happy within your career, you must love what you do. For me, becoming a geologist was all about being outdoors, and becoming an academic was all about having the freedom to choose what I work on for research, and therefore what I teach to students. You never really know where you will end up in a career or in life, but if you find something that genuinely excites you, and you go after it with passion, chances are good that it will take you someplace rewarding.
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