Serpentine rheology and subduction zone earthquakes

There’s something about subduction zones. Maybe it’s the potential for magnitude 9+ earthquakes and major tsunamis... and for silent earthquakes, and slow slip events, and postseismic deformation. Maybe it’s the explosive volcanoes. Maybe it’s the accretion of stuff scraped off the downgoing plate. Maybe it’s my old fondness for minerals that look purple under a microscope. Maybe I’ve just been seduced by the name.

Anyway, although I’ve worked in rocks that have been down in subduction zones, I don’t have a very good idea about the nature of the faults that generate the immense earthquakes. I work on land, and on rocks that have made it to the earth’s surface; the subducting plate starts on the ocean floor, and goes down from there. So I don’t have a mental image of what’s involved – not like I have of, say, the Wasatch normal fault, or the Snake Range decollement.

An article in last week’s Science by Nadege Hilairet and co-authors, about experimental deformation of serpentine, gives me a better idea.

Minerals can deform in a number of different ways, depending on the temperature, the stress they experience, and the rate of deformation. At low temperatures, things deform elastically - they change shape in response to the stress until either the stress goes away, or they break. At higher temperatures, chemical bonds break and reform within the crystals, allowing the crystals to change shape without physically breaking. This can lead to some major differences in behavior when the rocks are stressed – at low temperatures, the release of elastic energy when rocks break and slip gives us earthquakes. At high temperatures, rocks behave more like Silly Putty – they’re elastic at short time scales (and can transmit seismic waves), but over longer times, they change shape at a rate that’s proportional to the stress they feel.

Different minerals deform by different mechanisms. Some change shape ridiculously easily – rock salt, for instance, flows so easily that it is used to equalize pressures in rock deformation experiments. Others, such as olivine, are strong even at quite high temperatures – that is one reason why the uppermost part of the mantle, the mantle lithosphere, is the strongest part of most tectonic plates.

And then there’s serpentine, the hydrous magnesium-iron-aluminum silicate that forms from adding water to mantle rock. It’s the mineral that dominates the state rock of California (and the rock found in the core across the San Andreas Fault). And it is, presumably, a mineral that forms in the mantle wedge above the subducting plate, as the downgoing plate loses its water. (Some subduction zones also have evidence that it really is there – seismic waves traveling through the overriding plate are slowed by a material that has the right seismic velocity and other elastic properties to be serpentinite.)

Serpentine is soft and slippery at surface conditions, so one might expect it to be weak in subduction zones, as well. But serpentine is brittle in low-pressure experiments (below 0.7 GPa, which would be equivalent to a depth in the neighborhood of 20 to 25 km) – near the surface, it breaks. The authors, however, managed to do deformation experiments at pressures of 1 to 4 GPa (equivalent to around 33 to 120 km – for a subduction zone beneath a continent, we’re talking about depths from the average base of continental crust to the base of the mantle lithosphere – though at a subduction zone, I’m not sure what is typically found at those depths).

They found that serpentine behaves differently at different pressures, presumably because different deformation mechanisms are at work. At both pressures, the behavior could be modeled as power-law creep. Power-law creep means that the strain rate is related to stress taken to some power n – that is, the higher the stress, the faster the strain rate, and the higher the value of n, the more sensitive strain rate is to increases in stress. Power-law creep is typical of deformation mechanisms involving movement of dislocations (defects in a crystal lattice). But at lower pressures, the stress exponent was higher – increasing stress really made deformation happen more quickly – which is typical of sliding along grain boundaries. At higher pressures, the behavior was closer to that of a viscous material (though still with viscosities that change with stress).

Ok, nice. So why, exactly, is this interesting? Well, for one thing, I usually think about the effect of temperature on deformation mechanisms, but I don’t usually see discussions of the role of pressure. Temperature’s important because intracrystalline deformation involves breaking chemical bonds, and higher temperatures generally make it easier to break and re-form bonds. In fact, I had always thought about the great depths of subduction-zone earthquakes as a function of the cold temperatures in the subducting slab. (The rock descends faster than heat can flow into it – among other things, that makes high pressure/low temperature metamorphic rocks possible.) So it’s interesting to see rheology controlled by pressure as well.

For another, the experiments show that, at depths in the range of approximately 30 to 100 km, serpentine is much weaker than most other likely minerals (pyroxene, plagioclase, and wet and dry olivine – the likely constitutents of oceanic crust and mantle). Is the likelihood of large earthquakes on subduction zones governed by the presence or absence of serpentine, rather than age of subducting crust or rate of plate movement? This would seem difficult to test, but if the seismic velocity and Poisson ratio of serpentinites are unusual, maybe tests are possible.

And what about those weird not-really-earthquake events that take place? Silent earthquakes, like the ones along the Cascadia subduction zone beneath Vancouver Island? Or afterslip – slow movement after a major earthquake, like occurred after the 2004 Boxing Day earthquake off Sumatra? (GPS data suggests that, during the month after the earthquake, slow slip equivalent to a M 8.7 earthquake occurred (Subarya et al., 2006).) The authors calculate that stresses in serpentine should relax on the same time scales described for slow earthquakes and afterslip, and imply that serpentine could explain some of the things that have been observed. There are some problems with that explanation, though. Modeling of afterslip following the Sumatra earthquake (Subarya et al., 2006), placed the slip in the same area as the initial earthquake, at depths shallower than 25 to 50 km, depending on the model they used to invert their ground deformation data. And Hilairet et al. cite studies saying that Sumatra has no evidence of serpentinization. So...the pieces don’t all fit together. Not perfectly.

This study, plus the recent study that found talc in the San Andreas Fault, are interesting because they tie together mineral behavior, rock mechanics, and seismology. The combination seems fruitful. I’ll be interested to see how the modelers who study triggered earthquakes work with these data.

References:

Hilairet, N., Reynard, B., Wang, Y., Daniel, I., Merkel, S., Nishiyama, N., and Petitgirard, S., 2007, High-pressure creep of serpentine, interseismic deformation, and initiation of subduction: Science, v. 318, p. 1910-1913.

Subarya, C., Chlieh, M., Prawirodirdjo, L., Avouac, J-P., Bock, Y., Sieh, K., Meltzner, A.J., Natawidjaja, D.H., and McCaffrey, R., 2006, Plate-boundary deformation associated with the great Sumatra-Andaman earthquake: Nature, v. 440, p. 46-51, doi:10.1038.

(Half of) 2007 in retrospect

I wasn't going to do the meme in which we post the first sentence of the first post of each month. I only started blogging in June, after all. But it does seem like a good way to reflect about the year, so here it is:

• June: NPR has had this series, off and on, in which listeners record interesting sounds and then explain them on the air. (The Sound of Mylonites)


• July: As students walked into my Earth Systems Science class on the first day of fall semester, 2005, they saw a satellite image I had just grabbed from NOAA’s website: Hurricane Katrina, aimed straight at New Orleans. (book rec: Storm World by Chris Mooney)


• August: Chris Rowan had a very thought-provoking post a couple days ago based on a suggestion by Propter Doc: that all scientific journal articles should include (possibly on the web, rather than in a print version) a lay statement written by the authors of the journal article. (summaries for non-scientists: what should they contain?)


• September: I grew up on the second-most-polluted lake in the state of Maine. (of lakes and rocks (or why I do what I do))


• October: I found the pink caldera on the China/North Korean border, so it's my turn to post something. (Where on (Google) Earth #56)


• November: I just got back from a two-day microprobe marathon - two days on what's essentially a scanning electron microscope that can do chemical analyses of very small crystals. (Teeny-tiny partial melt)


• December: My intro students turned in their papers for my new, kind-of-experimental, revised group project yesterday. (Teaching: a comment I like to hear)

I find the end of December an odd time for musing about endings and beginnings. The winter solstice makes me want to hibernate until the days get longer, and feels like a time for waiting, rather than a time for making changes. The equinoxes feel more like a time of change to me - the autumn equinox, in particular, feels like a time for new beginnings. (Maybe that's the result of the academic cycle, though, rather than the cycle of seasons.)

But today's the day that our year ends, so... patterns and resolutions:

• This blog has changed from me talking to myself about things I saw or things I read, to me talking to other people. It's nice to be part of a community, but I would like to go back to writing about odd observations more often.


• As the fall semester progressed, I talked about teaching more and more. Perhaps that's normal, given that I was teaching four courses and three labs.


• It's been a while since I've played Where on (Google) Earth.

• I can't decide which rules to follow for capitalizing post titles. GSA rules, in which only the first letter is capitalized? The rule of titles I learned as a kid, in which all nouns and verbs are capitalized? The Lazy Internet Rule, in which nothing is capitalized unless I want to draw attention to it? I can't figure out what looks best on the blog, and what looks best on the geoblogosphere widget that I haven't figured out how to install.
  

And in the next year:

• I want to do more blogging about peer-reviewed research. (Once a month seems like a good goal - it would make me set aside time to read articles, even during the busy time of the semester.) I love teaching, but it's easy to miss what's new in a discipline when I've got so many classes, and I don't want to become one of those greying professors whose classes forever reflect the ideas that were current when they were in graduate school.


• I'm also going to try to post some kind of photo every month, and talk about what I see. I've got a bunch of digital photos that I took last year, but never got around to blogging about them - perhaps they can fill the time until I can go outside. (Or I can post pictures of deformation in Silly Putty or chocolate chip cookies, for that matter.)


• And there are things that I started talking about last year - things that need finishing. I haven't looked at the pre- and post-tests I gave before showing An Inconvenient Truth, for instance, so I don't know whether the movie was useful at all.

Good idea: appreciating high school science teachers

Over at Apparent Dip, Thermochronic writes about appreciating his high school chemistry teacher, and suggests taking a moment to contact the teachers who influenced us and tell them so.

I graduated from high school 23 years ago, and a lot of the teachers who influenced me have retired. And the teachers who helped me most were a high school history teacher, and a sixth grade everything (including very good science) teacher.

Miss H. knows, I think, that she influenced me more than any of the other teachers in my high school. (At some point my college sent something to teachers that had influenced us. I don't remember the context, but I remember that Miss H. sent me a wonderful letter thanking me for mentioning her.) Miss H. taught history, but that wasn't all she taught. She started Ancient (European) History in the Paleozoic - despite, as I later learned, having found her introductory geology class incredibly boring. I suspect that there was some tension amongst the teachers about the unwillingness of either the biology or earth science teachers to teach things like evolution and the age of the earth. So a history teacher took up the slack. She also taught me to write research papers, and had us read historically relevant English literature (A Tale of Two Cities and Animal Farm). She pushed me to think, and to be goofily enthusiastic about intellectual pursuits. And that was as important for doing science as learning the content was.

Mrs. S. taught my sixth grade class - everything except social studies, if I remember correctly. She was the one who first told me to "tell them what you're going to tell them, then tell them, then tell them what you told them." (I told her that sounded stupid. I was punished for mouthing off, but I remembered the lesson in the end.) But what I really remember was a very creative physics/engineering lesson. We had to build something and - I don't remember whether we explained it to the class, or made a poster about it, or wrote a paper about it. But, anyway, my project involved transformers. (Not the toys - they, ummm, didn't exist yet then.) That was where I learned the relationship between electricity and magnetism. That project is the entire reason why I can trouble-shoot machines at all. I still think about it when I try to explain the earth's magnetic field, or when I read an explanation of how plasma is formed in our ICP, or when my car won't start. It was the first time a teacher had had me build something with wires and magnets and... well, it was really cool. I was rarely asked to create things with my hands (other than neat handwriting) - it was wonderful to be practical as well as theoretical.

Neither of them are teaching in the district any more. My mother might know how to contact them - so here's my appreciation, in public.

Carbonates and housecleaning

It's winter break. I'm home with the four-year-old, building a lot of things with Legos, playing snow leopards (we've been watching the Planet Earth series), and pretending to do some year-end cleaning.

There are a few household maintenance things that I... well, I should do them more often than I do. Turn the mattress on the bed, for instance. Change the air filter on the heating system. And change the wicking filter on the humidifier.

We got a medium-sized humidifier during the summer of 2002, when we were about ten miles from the edge of one of the many fires that were burning in the western US. Normally we cool our house in the summer by simply opening the windows at night. We live at 7000 feet, and it gets pretty cool here. But during the fire, the smoke settled into the valleys at night, and we kept the windows closed to keep it out. I had just finished my second year teaching here, and I had taught Earth Systems Science four times in a row, and I was all psyched to try cooling using evaporation. I wanted a swamp cooler, but I had no idea what I was looking for, so I came home with a humidifier and a fan. I'm not sure the humidifier did anything for the temperature, but it made the air in the house easier to breath. So now we usually turn it on at night, and fill it with water that we collect in old yogurt containers when we're getting ready to get into the shower. (Yes, I know a grey-water system might have been a better use, and might have allowed us to have a lawn that consists of more than twelve blades of buffalo grass. But I'm not a plumber. That's on the list of renovations I want to do.)

Anyway, whether or not the humidifier is useful, it only works when water can soak into its wick and then evaporate. That works for a while, but eventually, minerals precipitate on the top of the wick, and it needs to be replaced. So one of my missions yesterday was to find a new wick for the humidifier.

My first try was the locally-owned hardware store where I had bought the humidifier. They still carried the wicks, but they were out. So I tried Wal-mart. No luck; they only carried a handful of humidifiers. Same with Home Depot. In fact, the guy working there rolled his eyes and showed me the shelf full of DE-humidifiers that the main office kept sending them.

"They don't understand 25% humidity," he told me.

"Are they in Houston?" I asked.

"Atlanta," he replied.

So: three places in town that sold hardware, and no luck finding the wicks. I went home and looked at the crusty stuff on the wick, and thought cranky thoughts about the precipitation of calcite and other annoying non-silicate minerals. At least, I figured it was probably mostly calcite. Calcium is by far the most abundant cation dissolved in the nearest river, and even though my groundwater comes from a poorly sorted sandstone, I figured calcite was the most likely mineral. No acid to test it, though...

Wait a minute.

No, I don't keep a stash of hydrochloric acid at home. (Four-year-old in the house.) But I do, however, keep vinegar around. And vinegar's acidic.

In fact, I thought, maybe I could just clean the wick with vinegar. Dissolve off the precipitates, and put it back in the humidifier, good as new.

So I pulled out the wick and put it in the bathtub, and brought the bottle of vinegar upstairs. I poured a little vinegar on the precipitates, and...

Oh, yes. The satisfying FIZZZZZZZZZZZZZ of calcite in acid.

I poured more vinegar on it. It fizzed some more.

I plugged the tub, and kept pouring vinegar, trying to think of a way to make sure the vinegar had time to dissolve as much carbonate as possible.

Half an hour later, I had orange vinegar in my bathtub (not sure where the orange came from; I don't think there's much iron dissolved in my water), and an empty vinegar bottle.

The wick is still pretty crusty, and very soggy, and still needs to be replaced.

Maybe I'll have to try HCl next time.

Teaching: more on graphs and research in intro classes

While I'm on break between semesters, I'm working on revising and improving a new intro class group project that I introduced this past semester. (The lab sections are going to monitor a local stream, similar to what groups like Colorado Riverwatch do. We've got four to six lab sections, and each one will collect data from a different reach of the stream.) I'm especially trying to use this project to help students learn how to understand graphs better.

My experience this semester was mixed. That isn't surprising: a senior thesis student was still collecting baseline data, and a lot of the project was designed on the run. But I was still disappointed with the way the students interpreted their data. Several groups assumed that they could extrapolate data between data points when the data were collected at specific times two months apart. And I wasn't convinced that most of the students understood the numbers they had, or the units things were measured in, or the significance of the values.

So... it's another semester, and I'm revising the exercise. (It's rare that any new exercise works perfectly the first time around, anyway.) We're going to spread the exercise through several weeks (for fifteen minutes or so at the beginning of lab - we aren't going to cut other labs to make room for this one). Here's the basic schedule:

Week 1 - introduce project, use topo maps from field sites for the topographic maps lab

Week 2 - graphing-by-hand exercise, using monthly average discharges for the 2006 water year. This will be complicated because there is a reservoir between two of the sites, so they will graph data upstream and downstream of the reservoir. Hopefully, besides practicing their graphing skills, they will also start thinking about how water is stored and used in the western US.

Week 3 or 4 - use Excel to graph the fall 2007 data collected by the senior thesis student. (There will be a field trip on the other week. We live in the Southwest, and it can be clear and fairly warm, even in January. More field trips! Yay!)

Week 5 - choose groups (discharge, sediment, or one of two water chemistry groups) and write one page of background on the type of data being collected. (What is it, what controls whether it is high or low, why do we care about it.)

Week 6 - write one page (or less) about what they expect (given location of the field site, previously collected data, what they know about the weather at that time of year).

Week 8 - collect data in field.

Week 9 - ICP analysis for one of the water chemistry groups. (I still haven't figured out how to let the students see what the instrument does, without making them spend the entire day in the lab with me.)

Week 10 - enter group data in Excel spreadsheet. (I've thought about using Google docs for this, but most of the students don't currently have gmail accounts. For this semester, I'm going to have the lab professor enter the data as students read it off.)

Week 12 - presentation and discussion of results. (Each group will present their original results, and then we'll discuss what they mean in the context of the other groups' results. I'm going to use Moodle to allow the students to share their powerpoints and graphs with one another.)

Week 14 - final papers on the project due.

I spent a good part of this week reorganizing the data in Excel, in hopes that I could make the entire graphing process simple enough that students could learn anything from it. Now, I know that Real Scientists don't use Excel. (Though, ummm, I do. Not for complex math, but for organizing data and doing simple calculations. It works pretty well for recalculating mineral compositions from microprobe data, and for doing unit conversions, and for calculating stream discharge.) And I also discovered that, ARGH, Excel 2007 has really been reorganized, and I had to relearn everything I had known about making graphs in Excel. (I'm still not sure how to remove the graphs from the data without losing everything... it must be possible to make the graphs static, not dynamic, but I haven't figured out how yet.)

But besides the general frustrations of working with newly updated software, I discovered a few other things that might be pedagogically interesting:

- Excel doesn't really care what units are associated with what data. It will happily (as a default) plot discharge, pH, turbidity, and ppm of Na dissolved in water... all on the same Y axis. It takes a bit of work to choose what to plot against what.

- The easiest X-Y plot to read (scatter connected by lines) helps a reader to keep track of which data points go together, but it suggested a continuity in the data that doesn't really exist.

- On the other hand, Excel recognizes dates, and spaces them with appropriate distances between them. That's nice, because the senior collected data in July, and then went off to work as a river guide for six weeks before coming back to really start work in September.

I'm asking students for examples of situations when they shouldn't plot data on the same graph. One has to do with units. But because I want them to see the changes in each variable through time (or along the length of the river), I also want them to think about whether the changes will be visible if both types of data are plotted on the same graph. (If one element is present at 0.1 ppm levels, and another is present at 100 ppm, then changes in the element with the low concentration are impossible to see.)

I'm also asking the students to look at the graphs and notice (qualitatively) which variables seem to rise and fall together, and which have opposite patterns, and which seem entirely unrelated.

And now, after a lot of cutting and pasting in Excel, I'm wondering: is this going to be enough? Too much? Will they learn anything from this? Will they be bored to tears, or will they see patterns that make them more curious?

Edit: I thought about another issue while I was waking up this morning.

Using Excel requires access to computers. We've got a department computer lab, but it only has four computers. We've got wireless access in the geology labs, but not all students have their own laptops. (Perhaps more do than I am aware. I'll have to ask them at the beginning of the semester.) There are computer classrooms on campus, but they are in high demand, and I don't want to drag the class across campus for a short exercise.

So. If they don't have access to their own laptops, which is more important: having all the students use Excel themselves, or having the instructor available while they work? I could have them work in groups in out computer lab (but then only one person touches the computer), or I could make this a homework assignment (and make them complete it without being able to call me over whenever they got stuck).

Maybe most of the students will have wireless-enable laptops, and we'll be able to work in the classroom. That would be ideal...

Seven things you didn't know about me

I was tagged by Chris R. for this meme. So... here it goes.

1. My high school biology teacher was a young earth creationist, and I have never had a biology class that covered evolution, though I used to read the last chapter of grade school textbooks - the one that talked about human evolution - on my own. My high school history teacher tried to make up for our inadequate science education by starting Ancient History with the Paleozoic.

2. I chose my undergrad college because the glossy propaganda included a picture of a snow sculpture of a lake monster.

3. I chose my graduate school because it does not snow there. Not in April, when I visited. Not in January. Not at all.

4. Within three months of arriving in grad school, I missed snow so much that I decided to do field work in northwestern Alaska.

5. All of my academic jobs have been in ski towns, even though I am a Nordic (not Alpine) skier.

6. After my first job interview, I was told that I was not enthusiastic enough about teaching, and that I should consider applying for jobs at research universities. So the next time I got an interview, I asked lots of detailed questions about the courses I would be teaching, and I went to the interview with a lot of ideas about the classes. (I got that job, and I have never applied for a job at a research university.)

7. At one point, I knew how to ask for beer in French, Spanish, German, Russian, Japanese, and Chinese. However, I prefer to drink red wine.

I'm not going to tag anyone, because I don't want to put anyone on the spot. But if you want to play, consider yourself tagged.

My deskcrop has dizzy feldspars

I had a bit of a problem with this month's Accretionary Wedge (which is why I'm late posting). I don't actually have any rocks on my desk at the moment - I've just turned in grades, which means my desk is currently covered with textbooks, unread issues of GSA Today, unopened mail, scribbled notes from phone calls, a slinky, a pair of fault blocks, a pile of scrap paper that was meant to be used for a discussion about triple junctions, and possibly my missing coffee cup.

So I had to dig through the drawers of a rock cabinet, instead. (I rescued a lovely old wooden rock cabinet last time one of my colleagues was getting rid of stuff, and now my teaching collection is organized. Which is a good thing, because I can't find anything on my desk.)



I found this rock in an overgrown field in central Massachusetts. My field partner and I were following an old field trip guide, looking for the Pelham dome. I'm still not quite sure if we found it or not. But we did find this: a perfect example of a shear-sense indicator for class.

Metamorphic rocks usually do a rather imperfect job of telling about their deformational history. The minerals recrystallize as the rock changes shape, and it is possible that, several hundred million years later, it will be impossible to tell if the rock was squished or smeared. Squishing (otherwise known as pure shear) is what happens if you flatten bread dough by pushing down on it. Smearing (otherwise known as simple shear) involves deforming an object between two sides slipping past one another - imagine what would happen to the bread dough if it were caught within a fault zone (other than just getting really dirty, I mean, and possibly making an earthquake rupture less likely). A combination of the two is also possible - a rolling pin flattens dough by a combination of pure shear and simple shear.

If you want to know just how a rock changed shape, it helps to have a way to tell apart pure shear and simple shear. Generally, you need to find some kind of object that has tracked the movement - and this rock has one.

That feldspar has done a bit of spinning.



Objects that are caught in a shear zone should rotate. But they rotate at different rates depending on their shapes. Spherical objects will roll forever at the same rate - try rolling a marble between your hands. But flat objects will rotate fast when they're at a high angle to the shear zone, but will slow down and get stuck parallel to the shear zone boundaries. (Try rolling a poker chip between your hands.) In this rock, the white feldspar has grown a pair of tails (which also tell a story about deformation), and then the round center kept spinning while the tails stayed in place.

That kind of grain is called a "delta porphyroclast," because it looks a bit like the Greek letter delta, and because... well, I don't really know the etymology of "porphyroclast," to be honest. But it's a good word, however it got into geologic terminology.

Dizzy feldspars may not know where they've been, but they can tell you a bit about how they got there.

(The observant reader may note the red pen on top of the sample. That's not just there because my grading wasn't quite finished. The pen shows the direction of the stretching lineation - the direction that the rock was stretched during deformation. If you want to see shear sense indicators, you need to look at the side of the rock that's parallel to the lineation, and also perpendicular to the foliation, which in this rock is defined by the dark- and light-colored layers.)

So you're applying to graduate school...

Female Science Professor has posted a very useful discussion of e-mails to potential graduate advisors. If you are an undergrad applying to graduate school, you might want to check out her comments - it isn't often that you get insight into what grad school faculty want to hear from applicants! The comments also give a view of the range of opinions about pre-application e-mails.

(My experience... well, I had to walk uphill both ways in a snowstorm just to search a paper copy of the Directory of Geoscience Departments to find out which universities employed metamorphic petrologists. I suppose I could have sent pre-application letters via the Pony Express... *shakes cane* So, anyway, I'm glad to hear what faculty at research universities think of the now-traditional e-mails.)

Edit: Female Science Professor has added a new post discussing applications of MS vs PhD students. It's a useful read, as well. But keep in mind that she teaches at a major research university - there are a range of types of institutions out there, and for some of them, the terminal MS degree is the most advanced degree offered. If you are primarily interested in going into industry rather than academia, an MS degree might be the way to go - especially right now, when both the oil&gas and mining industries are hiring geologists. (They are also hiring people with PhDs, but PhDs take a lot longer to complete, and the job market can change a lot in five years.)

Visualizing heat flow around a pluton?

Here I am, not going to AGU. I'll do some end-of-semester blogging this weekend. But in the meantime, I'm looking for a new(ish) visualization tool.

Contact metamorphism happens when magma intrudes rocks and heats them up. It's possible to calculate how hot rocks should get at various distances from a body of magma, and to predict the mineral assemblages that one should find, and how quickly the rocks should cool off. It involves partial differential equations, but they've been used since the early 1900's. There's an analytical solution for the one-dimensional simplification*, and there has been plenty of modeling of other geometries and the effect of fluid flow. I've used some models myself in past research.

But I don't have a good, interactive demo that students can use to figure out what the equations mean.

I'm not looking for a Matlab script or anything complicated for research. I'm looking for something that a student could play with to see if a pattern of metamorphic temperature data is consistent with a simple, one-dimensional model of the cooling of a tabular intrusion.

I've used a nice program written by Simon Peacock in the late 80's/early 90's, but it used an old Mac system, and an old Fortran compiler.

Anybody seen anything like this?

*Reference: Carslaw, H.S., and Jaeger, J.C., 1959, Conduction of Heat in Solids: Oxford University Press, 510 p. Umm, the equation is not 510 pages long, though if it were, that would be all the more reason to find some kind of program to help students visualize the results.

Teaching: thinking globally, observing locally

Yesterday morning, the National Weather Service issued a blizzard warning for my local mountains. It was the perfect set-up for a winter storm: a storm off the coast of Baja California, a jet stream that looped way down to Baja, and then up across central Arizona and straight for our mountains. I was on my next-to-last day discussing weather in my intro class, and it made for a perfect discussion. We could talk about winds, about low and high pressure, about the role of the mountains in controlling the weather in Durango and Denver.

The students were attentive and engaged, in the way that’s only possible when the professor is discussing the possibility of a powder day with a class full of skiers.

And then it fizzled.

It’s been raining. Raining hard. The temperature has stayed in the mid-30’s (F), even through last night. If it had been just a few degrees colder, there would be more than a foot of snow on the ground. But even though it has cooled a bit this afternoon, and there are now some big, fluffy flakes in the air, the snow isn’t sticking. I can see where the snow line probably is: there’s a layer of low clouds around 8000 feet, and that’s where the NWS is currently predicting the snow accumulation stops.

I only live at the edge of the mountains, and the ski area reportedly has two feet of fresh snow, and expects at least another foot. So the students won’t be disappointed.

But the lack of snow down here is important. Snow stays on the ground for months, and a lot of the water soaks into the ground. Rain runs off. And the cold and the snow help the pinyons and the Ponderosas fight off the bark beetles that have been killing the trees and making the summer fire hazards that much worse.

One storm doesn’t mean much. But changes in the snow level are one of the predictions of climate scientists like Jonathan Overpeck (who spoke here a couple years ago). And on Monday I’m talking about climate. So: do I use this storm as an example?

In some ways, that would be a very dangerous thing to do. Many people confuse climate with weather, and assume that if a winter is snowy, or a summer is cool, or a hurricane season is less active than expected, then global warming has turned out to be wrong. But weather and climate are not the same thing: weather is local and short-term; climate is regional or global, and deals with longer-term trends.

On the other hand, global climate can seem distant and irrelevant. Even with CNN and the internet and Google Earth exercises, it’s easier to care about things in one’s own backyard. One of the successes of An Inconvenient Truth, in my opinion, was the way it took a global scientific issue and made it personal, made it worth caring about.

So. Should I use the local weather as an example, and risk students walking out of class convinced that a single storm is what confirms or denies the existence of global warming? Or should I stick with discussing evidence that has been peer-reviewed by specialists, and remember that I’m professionally a structural geologist, not a meteorologist, and I am probably missing some crucial explanation for why this storm appears to be fizzling out, at least at my elevation?

(As I write, of course, the snow has begun to stick. Maybe we’ll get something out of this one after all.)

Teaching: a comment I like to hear

My intro students turned in their papers for my new, kind-of-experimental, revised group project yesterday. One of the students, a senior who had been avoiding science classes for five years, talked to me about the project a number of times, trying to figure out what it all meant.

It was great brainstorming ideas with a motivated student - that's one of my favorite parts of teaching. But the best part of our conversations was one comment she made. She hadn't taken a science class since her junior year in high school, she said... and now she regretted it. Not because she suddenly wanted to be a scientist, but because she felt like she had missed out on something important.

"Why do we take just one science class to fulfill a requirement?"

I'm going to suggest that she take more, just for the fun of it, if she has time before graduating. And if not, there are books, and natural history museums, and public talks, and hikes with guidebooks, and...