Ghosts under pressure

Callan has a great (and evocatively titled) post about "crystal ghosts" - metamorphic minerals that have been replaced by something else. Callan's examples are, like most of the ones that I've seen, "retrograde metamorphism" - replacement of one mineral as it cools. Textures like these form because we live on Earth's surface, and it's colder here than where metamorphism happens. In fact, we only see metamorphic minerals because it's hard for those retrograde reactions to happen - you've got to add water to get many of the minerals that form at lower temperatures.

But my favorite metamorphic ghost tells a different story:



This is a microscopic image, rather than a field photo - it's of a thin slice of rock with light passing through it. And it contains the same minerals that Callan discussed: kyanite, sillimanite, and andalusite. Three minerals with exactly the same formula, formed under different combinations of temperature and pressure.

Except they're all here in the same rock:



If all these minerals had formed at the same time, we would know the exact temperature and pressure at which they formed. But, as with most places where all three of these minerals are formed together, they probably didn't form at once. In this case, it looks like the kyanite replaced the andalusite. (In another sample from the same outcrop, kyanite fills the entire andalusite-like square.)

And that's really cool, partly because it's a texture that you don't see very often, and partly because it means that this rock got really hot first, and then was buried. (Andalusite forms at high temperature and low pressure; kyanite forms at higher pressure.)



And it fits with the story that my students and I had been working out before we found this rock: that a granitic magma worked its way up a fault zone while the fault was active, first heating the rock and then burying it.

Ghosts can tell fantastic stories, if you listen to them.

My research history, in time and P-T

I like Chris Rowan's idea of placing his research on a geologic time scale, so I stole his image and edited it:



Since I did some geochronology (actually, a major part of my PhD was explaining that it was impossible for my data to be meaningful), I felt like I should include the bad data as well as my preferred ages for events.

But the time scale doesn't have enough dimensions for my research, so here's version in pressure-temperature space.



All the green is actual data from Vermont. The blue arrow is supposed to represent my PhD work, except that the rocks were really lousy for finding any kind of data, and I didn't work on the low-pressure ones that told a better story. The red is Colorado, both my senior thesis and current work with senior thesis students. The yellow is one rock deformation experiment that a couple of my undergrad students in Vermont did one summer. And I figured if I was going to include the really low-pressure stuff, I ought to include the water quality stuff I've advised, but it doesn't really fit on the diagram.

Metamorphic rocks of "Middle Earth": boring, or magical?

I love movies with big landscapes, and the New Zealand landscapes of The Lord of the Rings are some of my favorites. So I just had to click on Brian's shared link to a geology.com news post, linking to a Discovery Channel article about 'Middle Earth' Mountains: Steep and Strong.

Executive summary: New Zealand has steep mountains but few landslides. What gives? (Or rather, what doesn't give?)

The answer is: metamorphic rocks. Or rather, the answer is the rapid uplift along the Alpine Fault, which has brought hot young rocks to the surface very rapidly. Many other places with metamorphic rocks (such as New England) have much gentler topography, because they aren't tectonically active. But New Zealand is blessed with the best of both worlds: an active plate boundary, and rapidly exhumed metamorphic rocks.*

Not all metamorphic rocks are strong - many break along cleavage or foliation planes, weakened by flaky sheet silicates (such as biotite, muscovite, and chlorite). But the weakest rocks, those with a lot of sheet silicates, come from metamorphism of aluminum-rich clays. If you metamorphose something that hasn't undergone much weathering - say, a sedimentary rock with a lot of volcanic fragments like a greywacke** - there will be fewer sheet silicates (especially muscovite).

So what are these strong rocks that can hold up spectacular mountains without failing in landslides?

A new survey of the mountain ranges that form the spine of New Zealand confirms the steepest are made almost entirely of tough but otherwise unexciting rocks called greywackes and schists. (Source: Discovery.com)

Ta-da! Greywackes (or maybe meta-greywackes?) and schists. And...

Hold on a minute. What was that modifying phrase?

...tough but otherwise unexciting...

Unexciting? Unexciting?

Those, my friends, are fighting words. (As bad as the gratuitous volcano slander happening on ScienceBlogs.)

And worse, I can't blame a biologist or the faceless media for the slander. It came from an avalanche researcher... a fellow geoscientist:

"They're pretty boring rocks," confirmed avalanche and landslide researcher Oliver Korup of the Swiss Federal Research Institutes in Davos, Switzerland. They are simply petrified deep sea sediments that have been pushed up to form the mountains, he said. "They don't even have fossils."

Ok, then. Let's take a look at what they're calling boring:





(Images are screencaptures from The Fellowship of the Ring. There's obvious CGI on the second one, but the rocks, I suspect, are real.)

If these are boring rocks, I don't think I could handle the adrenaline rush from seeing exciting ones.

It's not just that metamorphic rocks are strong. It's that they remember. They've been through a lot - they still contain traces of their history as sedimentary rocks, sometimes in their textures, sometimes only in their chemical composition. They've been buried and heated and squashed, but they haven't succumbed to melting (or at least, not entirely). Their minerals tell the story of their burial and exhumation; their structures tell of the strains that they have endured. It's hard to tease out their stories, and much of what they experienced has been erased by more recent events. But still - these are rocks worth understanding. These are rocks with sisu.***

So no, it's not elfin magic that makes the mountains of New Zealand strong. It's the magic of things that seem boring and simple on the surface. More hobbit than elf, probably, in Tolkien's world. But wonderful, all the same.

*Chris Rowan did his dissertation on paleomagnetism in New Zealand, and although he is probably less likely to sing the praises of metamorphic rocks than I am (because metamorphic rocks are lousy candidates for paleomag), he knows the tectonics much better than I do.

**Why do many online definitions of "greywacke" describe it as a primarily Pale[a]ozoic rock? Is it because the definitions were written by British geologists, and greywacke is associated with mountain-building, and Britain was shaped by Paleozoic mountain-building? I bet the New Zealand greywackes aren't Paleozoic...

***Thanks to Joe Kopera for introducing me to the word sisu. A Finnish word that can be used equally for metamorphic rocks and some of my favorite literary characters - I love it.

Unpacking and the urge to classify

I spent the holiday weekend moving into a house in town. This may seem a bit weird, but I kind of like moving. It makes me go through drawer and bookshelf stratigraphy, and rediscover things from the Paleogene (well, ok, 2000) that I had forgotten I owned. In some cases, they were things that we didn't need any more (such as size 3T Halloween costumes, which went to a daycare yard sale). In other cases, they were potentially useful (such as unopened tubes of toothpaste). And then there were the interesting things - the set of pretty metamorphic index minerals (garnet, staurolite, kyanite, andalusite...) that I had packed away when my son was born, for instance.

Now I'm working on UNpacking. It's work, but it's also an opportunity, to make sure that we will be able to find all three bottles of ibuprofen when we need them. I needed some logical schemes to let me find medicine, books, CDs, dishes, food, etc.

About halfway through the bathroom drawers, I realized that I had developed a new classification scheme for my stuff.

Medicine and hygiene stuff is classified by how it is used. Cough, runny nose, sore throat, headache? It's all in the "respiratory ailments" drawer. Clothing malfunction? Needles, thread, safety pins, etc are together. The "skin" drawer has hand cream, foot powder, and diaper rash ointment (leftover, because Boudreaux's Butt Paste is useful for things other than diaper rash). There's a drawer for glasses cleaning and repair. There's a drawer for bleeding (with everything from Spiderman band-aids to massive pieces of gauze for injuries that I hope never to see).

My husband was a bit surprised by the practicality of the scheme - he half expected me to organize the medicine by chemical formula. (If I had taken organic chemistry, I just might have done that.) I suppose I could have organized things by shape (bottles vs boxes vs tubes) or by color or by expiration date. And that made me think about the ways that geological things are classified.

Take rocks, for instance. Our classification into igneous, metamorphic, and sedimentary is a good way to remember how rocks form, but it's horrible for students who are trying to learn how to tell apart nondescript dark-colored rocks. (Black limestone? Basalt? Hornfels? They can look very similar, despite being respectively sedimentary, igneous, and metamorphic.) And it isn't very useful for people who want to use rocks, either. Metamorphic rocks might make nice sculptures and polished bathroom tile, or they might be good building stone but difficult to polish, or they might be good for roofs or floors. (Or they might be good for figuring out the temperature and pressure of metamorphism, and for sparkling in the light, but not much good for anything else.)

Builders and stonemasons classify rocks differently than I do. Does it take a nice polish? Then it's marble (even if I would call it serpentinite or limestone). Is it hard and difficult to polish, with speckles of various colors? Then it's a granite, not a gabbro or a granodiorite or a gneiss. If it breaks into flat slabs, it's slate, whether I would call it slate, phyllite, mylonite, or thin-bedded sandstone. (And if it crumbles, it's shale. Even if it's really something volcanic.)

When I teach about rocks in my intro classes, I often mention the other names students might have heard for rocks. It's confusing to switch organizational schemes. (And not all students think about rocks in this way.) But maybe I could help some of the students by telling them about the various ways I could have organized my bathroom.

Rocks don't suffer deformation...

...they enjoy it.

Don't believe me? Then ask this muscovite, staurolite, and sillimanite, which have had the pleasure of growing in the space where an andalusite crystal has pulled apart:



Image with labels:



And a little explanation. Andalusite is one variety of aluminum silicate (Al2SiO5). There are three minerals with the same chemical composition: andalusite, sillimanite, and kyanite. They're stable at different temperature and pressures - andalusite is stable where it's hot at shallow depths (and is typically found in sediments that have been baked by the intrusion of magma); kyanite is found at higher pressures (that is, deeper in the crust); sillimanite is found at the highest temperatures (including in rocks that nearly melted). This rock tells a story: it was heated at shallow depths, and then was stretch while it was heated (and, I think, buried a little more).

(Elli, this is from Gallup Mills again. I'm working on probe data. The garnets have really cool zoning.)

Would it be unethical to publish this image?

I'm a field-based structural geologist/metamorphic petrologist. I try to sort out when rocks were squashed or heated or buried or cooled, and use that timing to make arguments about what happened in the middle or lower crust. And that means that my evidence is often visual: field photos of folded dike, or thin section photos of foliations, or backscatter electron images of minerals frozen in the midst of an ancient chemical reaction.

Back in the day, I used film. I'm not a great photographer, but I learned to develop black and white film to try to get decent images of thin sections. (Then I laid a piece of mylar film over the photomicrograph, sketched the outlines of the minerals with a pen, scanned the drawing, and re-did the entire thing on a computer in Canvas. And published the two images side-by-side.) I also would take many exposures of everything I photographed, because I could never tell what would show up well in the developed film. And backscatter images... those I took using big pieces of polaroid film, and it was hard to guess just what combination of contrast and brightness would work.

Now, I don't do any of that. I've got a little digital camera that I take into the field, a digital camera on our polarizing microscope, and digital images of every mineral I even consider probing. But I may be sloppier as a photographer. A photo might be a bit too dark, or too bright. I leave off the tungsten filter on the microscope, for instance, and then take a set of adjacent pictures, knowing that I can easily splice them together in Photoshop.

When I'm done, my photomicrograph looks like this:



Is it ethical to publish this? It's a doctored image, and an article in the Chronicle of Higher Ed says that means that it shouldn't be in a journal.

It's all automatic doctoring, though - I used Photoshop's automated photomerge command to splice the photos together, merged the layers, and used the auto color adjustment to fix the yellow color from the tungsten light. In the olden days, I would have taped photos together into a mosaic, or adjusted the microscope magnification to get the image I wanted. I would have used a tungsten-balanced film (after taking an entire roll of yellow images) to avoid the yellow color. I would have taken multiple exposures to make sure the image was not too bright and not too dark.

And the photo isn't raw data. You, the reader, can't do all the things that I did to make sure I was identifying the minerals correctly. You can't spin the stage and watch the biotite change color. You can't cross the polars and see that the andalusite is grey, and the high-relief mineral (kyanite??) is red. You can't put in the gypsum plate and spin the stage around (though I know you want to). You've got to trust me that the mineral I say is garnet is really black under crossed polarizers.

But you can see some of the things that I want you to see. See the little yellow staurolites? See those white things beside them? Those are pressure shadows... and the garnet doesn't have them. You can see why I think the kyanite has partially replaced the andalusite - and you can tell me whether you've ever seen a texture like that before.

Is it doctored data, an intellectual lie? Or is it an attempt to show you what I see when I look at the rock?

Night of the living superprobe?

I just got yet another Nigerian scam e-mail:

FROM THE DESK OF ZOMBO JEOL

And I thought... you know... I always wondered whether the microprobe was a zombie. All those late nights in the dark, eating my brain...

Good thing the lab I'm using now has a Cameca.

High-pressure metamorphism before 1.7 billion years ago?

The further back in time we look, the more difficult it is to figure out how plates have moved, or whether plate tectonics was even active. The ocean basins give us less than 200 million years of history, and in long-lived continental crust, younger events can wipe out the record of older events.

And some types of evidence are quite fragile. For instance, when oceanic crust is subducted into the mantle, it moves more quickly than heat can flow into it, and is metamorphosed into distinctive rocks that form only at unusually low temperatures for their depths. These rocks, blueschists and eclogites, are convincing evidence for an old subduction zone... but they rarely survive their return trip to the surface. It doesn't make much heating to replace the unusual minerals of a blueschist with more common lower-pressure minerals.

In the 1980's, when I first learned about blueschists, there weren't any known that formed in Precambrian time. Did that mean that plate tectonics was a new phenomenon, or that the subduction-related rocks just hadn't survived more than 500 million years? In the years since then, older high-pressure rocks have been found, but they're still very, very rare. Precambrian geology is interpreted in terms of plate tectonics, for the most part, but the blueschists and eclogites aren't part of the evidence in most mountain belts.

That's been true of North America. My corner of North America is thought to have formed from collisions between lots of volcanic arcs, from around 1.8 to 1.6 billion years ago. The evidence comes mostly from metamorphosed volcanic rocks, and from Precambrian deformation. The arcs are here, but their subduction complexes are gone.

Or, at least, their subduction complexes are mostly gone.

At the Rocky Mountain/Cordilleran section Geological Society of America meeting that I was at last week, Nina Fitzgerald and Mark Colberg of Southern Utah University showed evidence of retrograded eclogites from southwestern Utah. The rocks weren't pristine, by any means, but the authors had good evidence that the rocks had been at much higher pressures. (Not extremely high pressures - the pyroxenes didn't contain as much sodium as they do in, say, the Franciscan eclogites from north of San Francisco. And they didn't find coesite or diamonds, like in the ultra-high-pressure rocks of Norway or China or the Alps.) But they were the best candidates for subduction-zone rocks I've seen described in the Precambrian of the American Southwest. And they're the oldest high-pressure rocks I've heard of (though I've been out of the HP loop for a while now).

And they were found by people at a school with no grad students, doing good field mapping and looking at thin sections.

Ref: Fitzgerald, N.E., and Colberg, M.R., 2008, Evidence for Paleoproterozoic high-pressure metamorphism and decompression melting in the Mojave-Yavapai suture zone, Beaver Dam Mountains, Utah: Geological Society of America Abstracts with Programs, v. 40, n. 1, p. 64.

Tiara rocks

Maria at Green Gabbro nominates the Bishop Tuff as the Tiara of the Sierra Nevada, because it's so sparkly. Now, welded ash-flow tuffs are very lovely rocks, especially with all those sparkly sanidine crystals. But princesses need company. So, I give you...


(Source: Vermont Geological Survey.)

The Gassetts Schist, Tiara of the Taconian Orogeny. (Or maybe Acadian. Argon gives exhumation ages, I believe. Though there's something creepy, like Brothers Grimm or something, about exhuming princesses.) Which would make the Green Mountains the Princess of New England, unless Maine, New Hampshire, and Massachusetts want to fight about it.

Thin section:


(Source: Union College.)

Muscovite/paragonite, biotite, garnet, staurolite, kyanite, quartz, chlorite... and wait, it's got too many minerals for an AFM diagram. That's ok. If you're a princess, you get additional components, so you can have extra phases, too. Because princesses totally can do thermodynamics.

Edit: If you prefer metals in your tiara, Silver Fox reminds us that there's big money to be made in mining these days. (Dr. Lemming would probably agree, if he weren't in the field at the moment.) So you can make your own tiara, or you can make a lot of money and buy one.

Love Letter to a Piece of Schist

Note: I was trying to think of something appropriately geological to write for Valentine's Day, and this is what came out. You have been warned.

Look. You were hot. Once. Maybe more than once. And when everything was in equilibrium, well, you were a thing of beauty. Your micas had that sparkle, and your andalusite was a porphyroblast. And when your garnets rolled, baby, there was nothing to do but watch your inclusion trail.

But let's face it. You were never particularly gneiss, and everything's cooled since then. Don't tell me that your textures have gotten complicated - you've retrograded. Your reactions are incomplete. You're retaining too much argon.

I changed you. And I suppose that's what I get for letting you take me for granite.

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.

A question of scale

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.)

Teeny-tiny partial melt

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.)

of lakes and rocks (or why I do what I do)

I grew up on the second-most-polluted lake in the state of Maine. It was nasty – it looked like pea soup, and it smelled like rotting vegetables. But it was also beautiful – blue reflections from the sky, green trees hiding the houses on the other side, eerie cries of invisible loons in the evening and morning. There was a big rock, just right for sitting on, out where the water was about four feet deep. On the beach, there were shiny flat pebbles just right for skipping, and little sand grains that stayed stuck to my feet, even when they were dry.

The lake’s problem stemmed from years of being over-fertilized – sewage from two upstream towns was poorly treated, houses around the lake had leaky septic tanks, and rumor has it that there used to be a potato chip factory that dumped peels into the lake. By the 1970’s, it was a prime example of a eutrophic lake, a lake that had gone scummy and bog-like before its time because of too many nutrients and too much algae.

My dad started working on the lake’s problems in the early 1970’s. He’s a mathematician, and he worked with a geology grad student to develop a computer model of phosphorus cycling in the lake. I didn’t experience the modeling (well, other than scribbling on used punch cards), but I did get to help with the water sampling a few times. Mostly I sat in the boat and watched them collect bottles of water and lower a secchi disk until they couldn’t see it any more. But it made an impression on me. I remember bringing a little bottle of water and zooplankton for show-and-tell in 2nd grade. And by the time I was in high school, the research had turned into an experiment in cleaning up the lake. There were some new sewage treatment plants built (eventually – one town finally built its new one just a couple years ago), but the big experiment was the new dam. The town excavated the lake’s outlet and, every fall, lowered the lake by more than 10 feet, in hopes of flushing the phosphorus downstream.

I was impressed (though I didn’t think through the implications of sending the problem down to the Kennebec River and the Atlantic Ocean). I was impressed enough to have megalomaniac 10-year-old dreams of becoming a scientist and figuring out how to make chemical reactions run backward and solve the world’s pollution problems. (I hadn’t studied the 2nd law of thermodynamics then, obviously.)

When I went to college, I wanted to study environmental chemistry. But there was a freshman seminar offered on environmental geology, and intro chemistry wasn’t offered until winter term, so I took geology to have some fun and kill off my writing requirement.

And I got hooked.

Field trips. We walked along the river and talked about currents and sediment and erosion and floods. We went down to a park and made geologic maps of flat-lying sedimentary rock by coloring along contour lines. (Did I happen to mention that I went to college in the Midwest?) The horizontal bedding, actually was a huge revelation. I had grown up in a world full of glacial erratics and widely spaced outcrops that had no obvious relationship with one another. The idea that rocks could be correlated from one outcrop to another astounded me. And beyond that, rocks were laid down flat. I can hear the people laughing already when I say this, but the priniciple of original horizontality was probably the single biggest revelation of my introductory geology class. (Well, that and plate tectonics – I had taken an earth science class in high school, but the textbook was outdated, and even in 1981 I got the impression that continental drift was some wacko idea that had been mostly discredited.)

Our final project for the class was to write about the geology and landscape of my hometown. That was the first time I ever looked at the geologic map of Maine. I didn’t understand it, and wrote about the location of my hometown on a drainage divide between two major river systems. But when I went home for winter break, I started noticing the rocks. In particular, I noticed one road cut on I-95, just outside Bangor. It was made of phyllite, and it glistened even when it was dry. And the layering was vertical.

If rocks started with horizontal layering, and the rocks around Bangor had vertical layering, then... something really cool had happened there, practically in my backyard. And I wanted to know more about it.

Chemistry was pretty much doomed as far as I was concerned. I held on to dreams of being an environmental geochemist for years – I monitored water quality as part of a campus job, I applied to grad schools with low-temperature geochemistry programs, and I worked for the USGS on a project dealing with mitigation of acid mine drainage. (And I kept taking chemistry classes, though physical chemistry.) But the rocks kept calling. Not just any rocks. Rocks that had been through a lot and had stories to tell. Rocks that had been buried, contorted, heated, transformed. Rocks that were once under the equivalent of the Himalayas (120 million years ago, or 380 million years ago, or 1.7 billion years ago). Down a subduction zone. Stretched and thinned in a metamorphic core complex. Baked in the aureole of a pluton. Metamorphic rocks are the survivors of geology. They have been through it all, and it has changed them, but they haven’t melted or broken apart.

And I like to know their stories.

Oh, and the rocks on the beach where I grew up? The sand grains and skipping stones were phyllites, and the big rock in four feet of water was a granite boulder. There were pebbles with andalusite in them, too, probably from the aureole of one of the granites or another. So I may not be studying lake water, but my rocks still remind me of home.