Over at Apparent Dip, Thermochronic, Brian, and Chris Rowan have been suggesting some ideas for gifts to entice your favorite young person into geology.
I'll bet everyone can figure out who is responsible for each of these suggestions:
- I predict that a full Bouma sequence would be a big seller, but I can also envision a cute 'bottom-absent' turbidite, and a tough-looking, but lovable, mass transport complex.
- It's the cuddly magnets the kids go for.
- I've been working on a children's book in my head about the "Brave little biotite."
So now I need to wear two hats at once, and play "that weird mom whose kid talks about volcanoes on Mars."
My son is the proud owner of a stuffed T. Rex and a velociraptor puppet. (The velociraptor is kind of scary, and has been banned from the bed. T. Rex, however, does a lot of cuddling when he isn't RARRRRRing.)
I bought this book, Geology Rocks! 50 Hands-on Activities to Explore the Earth for one of my nephews. Activities include "A Quake-Proof Contest!", which involves building blocks and a washing machine, and playing plate tectonics with graham crackers.
His older brother got this for his birthday: Magic Tree House: Tsunamis and Other Disasters. (My sister actually suggested the series, and when we saw the tsunami book, well...)
And I swear I saw a plush trilobite for sale at GSA a couple years ago. I couldn't find it this year, though. The closest thing I've seen on the web is plush Martian life (complete with scientific disagreements about its meaning!), and a few non-extinct invertebrates. At GSA, I was left bringing home things like the official IRIS slinky, a dinosaur-in-an-egg from (I think) Mississippi State, and a spinning light thingy from the Planetary Geology division.
I think the best geologic gift for a little kid, though, would be a great big magnifying glass and a box full of interesting things to look at. (A piece of pumice, maybe, or polished piece of granite, or some sand.) And if you're around the kid, take him or her outside and go exploring. (Have you ever followed critter tracks in the snow, to try to figure out how many legs an animal had, or what it ate? Snow is good for talking about relative ages, too. And, yes, I do ask my four-year-old whether he thinks the rabbit hopped by before or after it stopped snowing. And layers of snow make very nice folds as they start to slide down the car's windshield, so there's even the potential to get into structural geology!)
But I'm open to other suggestions (or for clever ideas that just need the right marketing person).
(If you're shopping for adults, Andrew Alden at about.com has put together a list of geologic gift ideas. Some of them sound pretty good for older kids, too... I like the idea of a plate tectonic globe, or some geologic puzzles.)
Thursday, November 29, 2007
Over at Apparent Dip, Thermochronic, Brian, and Chris Rowan have been suggesting some ideas for gifts to entice your favorite young person into geology.
Monday, November 26, 2007
John Fleck asked for more info about my weather journal exercise. Here it is.
Every semester, I end Earth Systems Science by talking about weather and climate, and instead of giving quizzes, I make the students keep a weather journal.
Here's what I ask them to do:
1. (3 points) Keep a journal of the weather that occurs for the next three weeks. Every day, record:
➢ maximum and minimum temperatures (temperatures at Durango airport are available from the National Weather Service)
➢ clouds -- how much of the sky is covered (as a percent), what types of clouds are present
➢ precipitation -- does any occur? In what form (rain, drizzle, snow, hail, etc.)? Is it continuous in time or space, or does it occur as showers? How much falls?
➢ wind -- speed (qualitatively) and direction
➢ any other interesting phenomena
Detailed weather observations from the Durango airport are available at http://weather.noaa.gov/weather/current/KDRO.html . You may use the NOAA observations as a source, but I would also like you to include your own observations.
2. (2 points) Explain the weather patterns you observed. Check out weather discussions at the National Weather Service web site (http://www.nws.noaa.gov/) for information about national and global weather patterns.
Why do I do it that way? Well, mostly because that's what I brainstormed the first time I taught the class. But I wanted to create an assignment that would get the students to look at the world around them, and think about what they were learning in class, and connect them together. So when they wake up in the morning and see this:
... they have some idea why there is frost on the car windows but not on the house windows (as was the case at my house on Saturday). Or so, when the smoke from the Durango-Silverton train hangs down in the valley on a cold morning, they realize that they're seeing a temperature inversion.
And I didn't want to make the assignment too complicated - I didn't want them to need access to a barometer, or for them to get up in the middle of the night to measure the temperature.
So does it work? Well, it's hard to say. A few students make great, detailed observations. Other students do the bare minimum. And it seems like it's pretty hard to connect the local observations to bigger picture questions - I would need to have them look at the National Weather Service web site daily, or something.
I also hope that the weather journals will give me some concrete examples of things to discuss in class. I started class this morning (on humidity and the dew point and ways that clouds form) with a picture of frost on my car, and we talked about that (and about the experiences of students who have grown up in more humid climates with a lot of dew). We still didn't end up talking about some of the more interesting things I saw over the weekend, such as the fog that formed at dusk on Friday, after the sun had started sublimating the new snow. (I think - though I'm not certain - that I was seeing something like the crazy freezing fog that Albuquerque had last winter, when the snow sublimated and raised the relative humidity near the ground, and then the cold air near the ground condensed it into fog.)
This morning, however, I asked how many students had described frost in their weather journals. Only a couple students raised their hands. So it doesn't seem like most of them are paying very close attention. They've got a few more days, though, so maybe their observations will improve.
Thursday, November 22, 2007
If it hadn’t been for the wild turkey, we would never have found the breccia.
We were working our way toward a section of a fault zone where two entirely different senses of movement had been described. We had already taken an interesting but physically challenging route to the area, and as a result, we were hurrying to the spot where we expected to find the interesting rocks. And then we saw the turkey.
She – well, at least we thought it was a she – was moving rather slowly across an open area, dragging a wing. I had seen that kind of trick before, when I worked beyond treeline in Alaska, so I figured that she probably had chicks that she wanted us to avoid. So, polite geologists that we were, we decided to follow her.
She led us in a wide circle, back up the hill we had been coming down. No matter, we thought. We could always take a longer route once she was satisfied that her babies were safe.
And then we saw the outcrop. A great wall of rock, on the west side of the saddle. We went and looked at it... and there it was. The fault zone we had been chasing.
We followed it up the hill, found patches of deformed granite caught in the fault zone, and took samples. We haven’t analyzed them yet. Maybe they’re significant. Maybe not.
But we wouldn’t have seen them at all, if it hadn’t been for the wild turkey.
(Image source: Utah's Hogle Zoo. I forgot my camera that day. Oops.)
Happy Thanksgiving to those in the US, and happy Nov. 22 to everyone else.
Wednesday, November 21, 2007
Do you remember when you first learned to read a graph?
I don't. And that might be a problem for teaching.
I'm not talking about the various weird graphs that geologists use. (Stereonets, Ar release spectra, U-Pb concordia plots, ternary phase diagrams... ) I'm talking about a nice, straightforward, x-y plot. Two dimensions, commonplace variables like temperature and time, no logarithmic scales or ratios between variables or anything like that.
(Image source: Global Warming Art)
I started thinking about this while explaining a weather journal assignment to my Earth Systems Science class. I cover weather and climate at the end of the semester, in the slot that always gets shorted when I run out of time. But I figure that weather is something that the students ought to be able to observe, at least qualitatively, and I want to give them a reason to read the book and think about it, even when I don't get to all the material in class. So I have them keep a weather journal: every day they have to record temperatures, cloudiness, precipitation, wind direction, and anything they find interesting. The goal isn't to teach them how to professionally record weather observations - the goal is to get them to observe the world around them and relate what they see to the explanations from their book.
This year, I lost nearly an entire class because the students asked one good question: when, and where, and how, should they make their observations?
It's always a bit dangerous when students ask a good question that I don't have a canned answer for. They get to see me think on my feet - not always a pretty sight. And this time, to try to explain my thinking (and perhaps inspired by reading graphical arguments on Tamino’s blog), I started drawing a graph.
It wasn't a very good graph - it was qualitative, illustrative of an idea rather than a compilation of data. But it showed (maybe) what I was trying to illustrate: that temperatures vary throughout the day, so perhaps they should describe their observations qualitatively, and then look at the National Weather Service official maximum and minimum daily temperatures for comparison from one day to another.
I don't know whether the graph made sense to the students or not - I don't have a good technique for quickly assessing whether students understand what I'm saying. I’ve recently learned that graphs don’t make immediate, intuitive sense to a lot of people, and I didn’t take the time to slowly explain what I was showing them. If the graph made sense, then the discussion might have been effective – I asked the class to tell me how the temperature should change throughout the day, and built the graph pretty slowly, so I hope they thought through the problem with me. But if they didn’t understand the graph... well, I might as well have been lecturing in Klingon.
So I’ve been thinking about what I could do to improve their graph comprehension (and to assess whether I’m doing it effectively). The weather journal assignment might be a good place to start – it should be relatively easy for them to graph the daily minimum and maximum temperatures over these three weeks. I wonder if they could use the graph to look at other things – maybe qualitatively annotate the graph with wind direction, wind speed, precipitation, types of clouds, so they could look for patterns? (It seems like that is one of the uses of graphs: they show patterns.)
But when I start thinking about what to make them graph, maybe I’m getting away from the point. What kinds of activities help them really understand what they’re doing?
- Does it help to graph points by hand, or is that simply tedious and frustrating?
- Does it help to graph data they’ve collected, or data related to things they’ve experienced, so the numbers are associated with what they measure?
- How much data is enough, and how much is too much? It seems as though there might be a balance between having enough data to see a pattern, and having so much data that the exercise becomes tedious. (If the students graph data from a spreadsheet, the tedium disappears... but the kinesthetic experience of plotting the dots, and the spatial sense that comes with it, might diminish.)
- If I ask questions about the graph, how do I avoid mistakenly teaching them that correlation = causation? Or is it ok to first see that correlation can help one to figure out causation, and then explain the fallacy?
- Are they going to be too burned out after Thanksgiving to handle an addition to their assignment?
(Side note: there’s a project in the works at SERC to create online modules to give math practice to students in introductory courses. Graph-reading is one concept that the developers may add. If you’ve got math concepts that you need geoscience students to understand, take the survey for the project.)
Monday, November 19, 2007
One of my senior thesis students is using some freeware written by an academic for her senior thesis research. Now, I love freeware, and I am in great debt to people like Rick Allmendinger and Frank Spear and Rod Holcombe and others who write software and then let the rest of us sponge off their hard work and programming expertise - for free.
But occasionally, relicts of operating systems past appear and confuse the heck out of undergrads used to web apps and commercial software packages.
(Image credit: Wikipedia)
Today's relict was PostScript. Now, I confess that I've never known much about PostScript, except that it was associated with printing, and that my maps in my dissertation always gave me a "PostScript error" when I tried to print them out. (That was back when we had to walk uphill both ways to the laser printer... *shakes cane at young whippersnappers who have just finished dissertation drafts, hooray!* ) Well, in the process of figuring out how to view the images (open them in Adobe Illustrator, and then edit them to our hearts' content), I ended up looking up the history of PostScript, so I could explain why a piece of software that I first encountered in 1990 would use that particular image format, rather than formats she had heard of.
And that led to stories about using a Mac that didn't have a hard drive.
And at that point, as my student looked at me in horror, I realized it.
I have become one of those people who talks about how it was in the "old days."
(And this was just a few weeks after a student looked at my eighteen-year-old solar-powered pocket calculator and teased me about it, and I threatened to give him a slide rule instead.)
Postscript: You know, I still don't think anything with a GUI interface ought to count as "old technology" - after all, when I was little, I drew pictures on the punch cards that my dad brought home from work, and I remember a computer the size of a small building, complete with flashing lights and beeping sounds. *shakes cane some more*
Wednesday, November 14, 2007
We didn’t bring enough food into the mountains. We were helicoptered in for three weeks, but after one week, we discovered that we had eaten half the food. So by the beginning of the third week, we were on pretty short rations. When another research group arrived, on schedule, to share dinner with us, things could have gotten ugly. But they didn’t, because our visitors brought instant cheesecake, and we were camped on granite in Alaska.
And granites plus recent glaciation meant blueberries.
(Source: Maine State Berry)
I grew up in Maine, so that’s one wild food that I recognize – and eat, whenever possible. Blueberries are cultivated, even commercially grown, frozen, and sent to grocery stores around the country. But the best ones, in my not-so-humble opinion, grow wild on the barren granitic soil of coastal Maine.
Blueberries are difficult to cultivate, because they thrive in very acidic soil. And that’s why they are found on granite. Natural rainwater is somewhat acidic, due to dissolved carbon dioxide from the atmosphere. Natural soil water can have an even lower pH, due to the formation of organic acids during the decay of vegetation. The rock fragments that make up the rest of the soil can serve as chemical buffers as they weather – calcite (the active ingredient in Tums) dissolves and neutralizes acid, and other minerals consume hydronium ions as they break down by hydrolysis. But quartz doesn’t. It doesn’t dissolve very readily in surface waters, and when it does, it forms a very weak acid. It certainly is no good at neutralizing rainwater or soilwater (or the acid rain that we worried about in New England in the 80’s). And granite contains a lot of quartz. So granite is no good at neutralizing acids.
But it is very, very good at growing blueberries.
Coastal Maine, north and east of Acadia National Park, is intruded by granite after granite, formed during the middle period of Appalachian mountain-building. Several hundred millions of years later, it was scraped clean by continental glaciation, and since then it’s been doing its darnedest to develop something vaguely resembling soil. (If you haven’t guessed: yes, I have attempted to garden in New England. Yes, I have harvested many rocks. Can you guess why I am a geologist and not an agricultural scientist?)
It’s a lousy place to grow corn. Or broccoli. Or, well, most things. But the thin, acidic soil is exactly what the wild blueberries love.
In coastal Maine, the blueberries are harvested commercially. But I’ve worked around the margins of granites in Alaska and Vermont, and I can tell you: the blueberries grow on all of them. If you know the geology, and you look carefully, you can find them.
And then, when you run out of food, you can eat blueberry cheesecake.
(For The Accretionary Wedge #3, Rocks and Life, hosted at The Other 95%.)
Monday, November 12, 2007
I wish I could remember the times this past week when I've seen discussion of the sciences in general, and a list of what's included... and geology wasn't there. I'm hardly the first geo-blogger to notice and comment. But today I noticed a lack someplace else: The Open Laboratory, the science blogging anthology.
There were posts by geoscientists. There were climate and paleontology posts. There were oceanography posts. There were creation-science-debunking posts.
But geology? Nothing.
When I saw that, I nominated one post, but this is a crazy week for me, and I don't have time to find all the great posts I've read in the geoblogosphere in the past six months. And I'm a newcomer, too - I suspect there were a lot of great posts written before I started actively reading.
So help me out here, and let's keep ourselves from being so invisible.
Maybe if the blogosphere acknowledges that geology exists, schoolkids in California and Texas can start studying the earth sciences, too.
Well, I can dream, can't I?
edit: I remembered one of the places where I saw geology left out of a list: at Wikipedia, in its entry on "special sciences" (as philosophers apparently call them; I had never heard the term before).
Here's the relevant text:
The special sciences are those sciences other than physics that are sometimes thought to be reducible to physics, or to stand in some similar relation of dependence to physics as the "fundamental" science. The usual list includes chemistry, biology, neuroscience, and many others.
Perhaps we should be proud that geology isn't on the list - maybe that means we rank above physics in the philosophers' hierarchy. But I suspect that geology isn't on the list because the philosophers don't even think about it existing, or maybe consider it a form of stamp-collecting.
Sunday, November 11, 2007
By this time, you've probably all heard about the fake paper "Carbon dioxide production by benthic bacteria: the death of manmade global warming theory?", published online in the fake Journal of Geoclimatic Studies. (It's been taken down, but the google cache is still here.) A number of people (including Rush Limbaugh) were taken in by it. But my question is: could it be used to help students learn to question confusing literature?
It's hard to get students to critically evaluate published ideas. It's even harder when their eyes cross when they read equations. So: would your students have recognized that this paper is bogus? Could it be used to keep them from assuming abstracts and press releases are settled science? (In this particular case, neither the journal nor the researchers exist, and a quick visit to the University of Arizona's website could confirm that. But... what red flags are in the paper itself?)
My favorite spoof line in the paper is one of the references:
Tibbold, WR and JD Rawsthorne (1998). Miocene, Pliocene and Plasticine fossil records for eukaryotic mass on the West African continental shelf. Journal of Submarine Research 18:5. 196-203.
The variables listed are also pretty funny:
"Where Q is raw mass, u is area, c is osmotic conductivity, Ψ is the vertical (neo-Falkian) benthic discontinuity, X is concretised diachronic invariance (P-series), F is trans-dimensional flow structure and jy is the non-rectilineal harmonic regressivity of the constant Δ."
"inter-annual variability of the asynchronistic (counterbifurcated) non-tardigrade log run"
"the polychromatic "coffeeground" Schumann factor for semi-particulate distribution"
But... you know, I regularly have students read papers that contain math they haven't studied. (Multivariable calculus and linear algebra and differential equations show up reasonably frequently, but our major only requires math through second semester calculus.) And I don't know if they would do more than skim the equations and the references.
If you showed this fake paper to students, what would you ask them to look out for?
Saturday, November 10, 2007
When I started in geology, I wanted to think big. Big collisions between continents, big rifts.
So it may be a bit odd to blog about things that are only a few millimeters across. (Well, I supposed it could be worse; I could be doing sub-atomic physics or something.) But I’ve been thinking small lately, because I got to spend time on an electron microprobe after years away, and I had to explain to a student why, exactly, we were spending hours trying to make sense of one crystal and the minerals surrounding it. Why not collect compositions scattered around the thin section? Why spend so much time trying to characterize one tiny part – what if we were missing something by not looking at the complete picture?
My answer was that sometimes there’s a huge story in a tiny texture. Especially when you’re dealing with metamorphic rocks.
I love metamorphic rocks, but I find them very difficult to explain to non-geologists. Ok, yes, there’s the name: metamorphic = changed shape. Changed by high temperatures and pressures. But there’s a world of chemistry hidden in that statement. The changes are chemical reactions that take place between solid minerals, minerals that no longer can coexist when the pressure is too high for their total volume, or the temperature is too high for their low entropy. Thermodynamics provides a theoretical reason why the minerals should tell a story; the difficult kinetics of the solid-solid reactions mean that it can be possible to tease out the early part of the history of the reactions.
And that means that, sometimes, you can see things like this:
This is a metamorphosed mud-rock that was heated by a nearby body of molten granite. The iron, magnesium, calcium, potassium, aluminum, and silicon are now organized into new minerals. Flakes of brown biotite and colorless muscovite. Garnet, too tiny to make jewelry in this case. Staurolite, honey-brown and cross-shaped in rocks, and pale yellow when cut thin like this. Andalusite, aluminum and silicon and oxygen, grown in long, squarish prisms at low pressures and high temperatures, most likely because it was heated by a magma at shallow (for us metamorphic types) depths............
Wait. That’s not andalusite.
Those dark line in the middle of the image... that’s the cleavage of kyanite, pale blue and beautiful in hand sample, harder to recognize in thin section. Kyanite, which has the same chemical composition as andalusite, but which grows at higher pressures, at greater depth.
That square in the middle – here, I’ll color it so you can see it –
- that square looks for all the world like andalusite. The outcrop is filled with those crystals, pulled apart, but still, in many places, andalusite. But not here.
That one crystal told me the story that I had gradually come to suspect in the years I worked in that field area. The rocks were heated, and deformed, and then buried a little more. (The deformation is evident in the curved lines of biotite around the andalusite. In non-technical terms, that rock’s been squashed after the square thing grew.)
I argued, once, that it had all happened while the granitic magma was intruding. I don’t think anyone has been back to argue otherwise since I stopped working there. But those textures – and other, more mysterious ones on the other side of the granite – those tiny textures are the best evidence I’ve got for a few kilometers of slip on a tricky, poorly exposed fault.
Tiny grains. Tiny textures. They would be easy to miss in any kind of random sampling scheme. But they’re there, and they tell a story.
(Edit: and the tiny thin section photos tell a story about how computer storage has gotten a lot cheaper over the past seven years. Those pictures are little because I took them in the early days of digital photomicrography. And I filled the $% hard drive of the computer attached to the camera, and had trouble printing the darn things, too.
Better take new pictures now that we've got a new scope.)
Thursday, November 8, 2007
I'm late to this meme, but I finally played the I rank number one on Google! meme (originated at The World's Fair).
And here are my results:
1. All of my faults are stress related
2. joy field work
3. structural geology wow
4. sound of mylonites
5. over-excited yeti
Didn't need to use quotes on any of them... which, I guess, implies that I use some unusual combinations of words. Though, really, it's kind of disappointing that "structural geology" and "wow" don't belong together all the time.
Wednesday, November 7, 2007
Here's the scenario:
Students are in a lab, modeling groundwater flow. They make an observation. They come up with an explanation for their observation. And then, they are asked to describe something, anything, they can do to test their model.
They say to make the same observations that they started with.
So. Repeating the experiment is reasonable to see if their observations were just a fluke, or whether they made some mistake. But they've got to do something different if they really want to test their hypothesis.
If I talk to them while they're working on this part of the lab, I tell them that they need to propose to do something different. But if I don't point out that there's a problem with confirming your model by going back to your original data... they don't see that there's a problem.
It strikes me that this is an important point about science, here. And it's a particularly tricky problem for geology, because we constantly look at things that have happened long ago. (That 1.4 billion-year-old granite whose contact aureole I'm studying? It's done intruding, it's cold, it's eroding. It's not going back to the mid-crust any time soon.) So we constantly have to think about what other things our models might predict. Is there any place we haven't looked, or any kind of data that we haven't collected, that could test our model?
If students don't understand that point, it's easy for them to conclude that geology is just a form of story-telling. Not that there's anything wrong with story-telling - I love a good story - but it's much, much more than that. And that, for me, is the fascinating part of it.
But how could I make that point to the students, better than I am doing it now?
Tuesday, November 6, 2007
I'm a structural geologist. And that means that when I see something like this:
... well, my first reaction is "oh, WOW!" But my second reaction is to squint a little, tilt my head to the side, and ask (usually silently, unless I'm teaching): "I wonder how that formed?"
Structural geology is a funny discipline. It has borrowed a lot of ideas from other fields (especially engineering and old-school Newtonian physics, but also from other subdisciplines in geology), and then built on them and redeveloped them for decades. And sometimes, when I teach the class, it feels like a bit of a hodge-podge of ideas. In particular, there are two somewhat different approaches to trying to understand a structure: kinematics and mechanics. Kinematics deals with the changes that a body of rock undergoes when it deforms – how does a rock of one shape change into a rock of another shape? Mechanics deals with the forces and stresses that cause a rock to change shape, and tries to predict the shapes of the final structures through the behavior of a certain material under certain stresses.
If you’re anything like I was when I read my first structural geology textbook, your eyes just crossed and you have absolutely no idea what I just said. So let me try again, but with an example.
Take, for instance, the San Rafael Swell (the second photo). The rock layers used to be flat-lying sediments. They aren't flat any more. How did they get all tilted up like that?
The simplest way to explain monoclines (like the San Rafael Swell) is to imagine two wooden blocks underneath a couple thick blankets. Push one wooden block up, and the blankets are folded. Voila... monocline, above what’s known as a blind thrust fault.
Except that there are some space problems in my little sketch, and in my pile of blankets. My cat and my four-year-old’s stuffed blue puppy could hide in my model (and there’s something a bit scary about the image of a giant stuffed blue puppy hiding somewhere within the upper Paleozoic section of the San Rafael Swell... ). So the possible explanations are limited, first, by geometry: there’s a fixed volume of rock to work with, unless something odd happens.
There have been a variety of geometric and kinematic models for folded beds above fault tips. There’s John Suppe’s fault-propagation fold model, which is shown in most structural geology textbooks:
Suppe’s models fold the rocks above the fault, but don’t fold the rocks below the fault. They’re relatively easy to work with geometrically, though – the rock layers stay the same thickness, so it’s possible to draw them by hand.
There are more complicated kinematic models, such as the trishear models that many people, including Eric Erslev and Rick Allmendinger, work with:
Trishear models recognize that rocks deform both above and below the fault, and describe the deformation as occurring in a triangular zone with its apex at the fault tip. There are a number of parameters that can be varied, which makes it possible to fit the model very nicely to real faults and folds.
We talked about trishear models in my Advanced Structural Geology class – read Eric Erslev’s paper, and played with Rick Allmendinger’s software. But when we discussed them, the students had one big question: why do the rocks behave like that? They change shape in this triangular zone... but why?
Well, I have no idea, but I suspect that the answer would lie in rock mechanics. Which I’m not very good at. The basics, though, would involve thinking about what is required to break a rock, and what stresses bend a rock, and maybe would involve calculating the energy involved. And then all those equations get put into a computer model that predicts the changes in shape from the physics, and you get... well... I’m trying to find a good image, because I know that some very talented people have worked on blind faults from a rock mechanics perspective, but I’m having trouble finding an image that I understand, let alone one that I can explain.
There was a session on integrating kinematics and mechanics at the Geological Society of America meeting last week... but it didn’t answer my questions. There were mechanically modeled structures that looked reasonable, and others that raised more questions for me than they answered.
But my conclusion, at the end, was that there’s a lot more work to be done, to reconcile the physics of rock behavior with the geometry that results.
Which is probably good news for people who want to do research in structural geology...
Sunday, November 4, 2007
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.
Here's one of our images:
It captures the coldest melt that occurs in metamorphic rocks: the partial melting of muscovite. Muscovite is the glittery, flaky mineral used for things like making eye shadow sparkle. It's very common in metamorphosed shales, but it gradually gets used up in metamorphic reactions as temperature goes up, until finally it reacts with quartz to grow sillimanite (the tiny fibers in the image) and potassium feldspar. And it can also melt, a little.
And this one did.
There's a tiny bleb that looks like a pair of sunglasses in the middle of the grain. Maybe if I outline it you'll see it:
Most of the bleb is made up of quartz (qtz). But the tiny bright spot in the middle is potassium feldspar (kfs) - exactly what you should see if the muscovite (mu) melted a little bit, and then the melt crystallized without ever escaping.
1.4 billion years ago, this rock melted. A little. (There were also a few little dikes in the field, but because we were in a contact aureole, we weren't sure if they were in situ partial melts, or if they were part of the granite. Now I suspect they were in situ melts.)