Happy Tunguskaversary!

It's the Centennial of the mysterious Tunguska Explosion, which flattened something like 80 million trees in Siberia on 30 June, 2008 (Click the link to see Andrew's summary of the whole story). Most people agree now that this was caused by an asteroid or comet blowing up as it entered the atmosphere - but not until it was only a few km above the surface of the earth. "Impacts" such as this leave no crater, and no trace at all if over the ocean. Maybe there are a lot more dangerous impactors entering the atmosphere than we know about! the Sandia super computers who have calculated that the asteroid or comet which exploded was much smaller than previously thought. The drawback to this, of course, is that the frequency of smaller objects is greater (see this NEO size frequency diagram). Party on!


Mineral Party Tricks at the Orange River

To tell you the truth, I never thought much of fluorite. Seems too cutesy to be the favorite mineral of anybody serious. I mean, the formula is dead simple (CaF2) and the name is practically a giveaway for it anyway. It comes in all kinds of luminous colors, any color you want, and cleaves into darling little double-pyramids. Yah, it's pretty, and puppies are cute. Tell me something interesting.

Even in this abandoned mine - really just a half-dug-out vein about 1.5m thick - it's cute. Look at that bright purple and pale green. Fluorite does look great in those black light mineral displays - it fluoresces, it phosphoresces... and some specimens can also thermoluminesce...
My opinion on fluorite changed when my students showed me what you can do with it (but only at night, while camping, and in possession of a mad excess of messy, less pretty handfuls of fluorite chips).

When nobody's looking, toss a handful or a few bigger chunks into the campfire. Best if it's a little low to the coals and not too bright. A few minutes later, long enough for anyone who saw the toss to forget about it, small points of turquoise glow fade into view among the coals. They grow brighter and brighter and then POP! Like a little popcorn kernel the crystal explodes in a tiny shower of blue sparks. A whole handful is enough to emit quite a POP, too.


Geological Spouse

"Why do I always have to be 'For Scale'?"

Groot Winterhoek Reserve, Western Cape, South Africa, December 2007.


And what do you want to be when you grow up?

In the fall of 2000, I went to the GSA Meeting in Reno. I was 6 months out of college, 6 months into wage slavery under an egomaniacal boss in an environmental consulting firm. I knew I didn't want to stay where I was, but didn't really have a good idea of the transition from undergrad to graduate student, hadn't been a particularly good undergrad anyway. I remember a vague sense of disappointment from some professors when I put rugby ahead of school (time and time again). On the rugby team I was the leader, the decider, director of all things! In the classroom I was one of many, and not calling all the shots.

I didn't have any sense of the incremental pathway between a bad case of senioritis and professorship. I knew I wanted to chase my own ideas, regardless of the fact that I didn't have any! Without of view of that path, I never pictured myself walking it.

At the GSA meeting I met a new kind of character, one I had read about in John McPhee but didn't really see on the East Coast where I went to school. The old crusty cowboy kind, dirt in the beard, leather boots molded to his feet, plaid western shirt, plus or minus big lump of turquoise on bolo, belt buckle or pinky ring. I've always referred to them as GreyBeards, although some are neither grey nor bearded. This character is a bit of a composite, clearly. Here's my portrait of one, gruff, sh*t-talking, "good ol days" wildcatting, tall-tale telling, swaggering example (before his time, even): Don Foss in the field in 2001:

He's somewhere between Jules Verne and Deadliest Catch, if you get the picture. My composite character hovers somewhere between mining exploration and academia, with plenty of distrust for both sides. He is a desert rat. He has nemeses who look just like him. He might have an axe to grind about how things are done these days. He probably has a beard, or at least a mustache. He does not respond to attempts by Eager Young Geologists to charm him with their enthusiasm, but might be easily flattered if he is recognized by the same. He is 100% geologist, everywhere, all the time. He knows no other life.

In that last bit, I found a model, an identity. It explained, in so many ways, why even though I felt sooo far away from college (6 months, but a very transformative 6 months) I was having difficulty performing basic social functions, such as hiking and chatting. Or driving. Past road cuts. Or bidding a remediation job at a very well known archaeo-Olympic Tahoe ski resort without asking the age and mineralogy of the tills into which the UST were leaking. Geological details had moved from facts I had to cram for a test to welcome, necessary diversions to keep me entertained during increasingly repetitive jobs.

I iconocized a cast of males, many-post retirement, in part because of their pride in being a bit "dying breed", a bit TOO FIELD for even the GSA meeting. God forbid they ever meet the dark matter of AGU. But I digress. At the same meeting, I ran into peers I had known in college. Some of them were graduate students, presenting their first posters. I weighed myself against them, competitively. Sure, my grades were lower, but hadn't I helped her when she totally bottomed out on Norm Calculations? Hadn't I helped (another) her when we studied all night for the paleo exam? Yes, I had. I was just as good. So why did I feel so inferior?

I applied for grad school. I got one rejection, some serious wine-and-dining, and one tenuous, not yet funded option at a school I hadn't meant to apply to, in a subject which had been my weakest in college. STRUCTURE - A subject I had somehow understood to be the web in which all my other subjects were suspended but just couldn't get my head around at the time. But in a brief interview with a potential advisor (of which I did many, and so should you) I met somebody who presented the most interesting questions I had heard, in a fascinating field area, with deep global relevance for fundamental processes in HOW FAULTS WORK.

This potential advisor was not my iconic character in the sense I understood it - he was young (or so he looked!) and quiet. None of the bluster and pushy dictation of project (amazing project! big money and big papers!) I heard at other universities. But he asked me some pretty hard questions. Not about what i knew, but about what I wanted to know.

The project took me to new places, more meetings, new people. Realized that my iconic old men were probably rolling out the GB persona for GSA meetings, as much as I was rolling out my best "promising young student" persona. My icon morphed from a bearded old grump into something much wider and deeper. Best yet, I finally found the female side.

My first discovery was at the same Reno GSA all though I didn't recognize it at the time, I can still picture her so clearly. Long straight grey braid down her back, middle part. Corduroy skirt, birkenstocks, wool tights. Festooned with what must have been a brutally heavy thick beaded necklace of picture jasper, or some similarly non-precious ROCK. She was nearly hidden among a crowd of plaid dusty men in bolo ties. Bingo. The "Grey Braid". Many of these women are not "grey" in terms of being "old". I mean to say that they are venerable, they have a sense of history, and some are every bit as crotchety as the Grey Beards, whether old or young.

Once I picked up on this phenomenon I started watching for more women like her, and there they were. At a local society meeting I met an incredible retired USGS geologist with brightest red hair, wearing a sparkling gold ruffled blouse with a full slab of Green River Shale with a fish in it around her neck. My new fashion icons were generally younger than the original Grey Beards, maybe because there aren't as many women geologists in their post-retirement era at GSA, certainly there were fewer field geologists in their generation. I met a volcanology prof at a neighboring college - the first woman at the UofO volcanology field trip. She had a rattle snake skin on her office wall. She had dispatched this huge specimen by throwing her bowie knife through its neck while it menaced some not-so-supportive (male) fellow students. Hell yah! The Grey Braid mythology grew.

I met more and more women who struck me as heroes of geology. Of women in geology. My kind of geology. My kind of women. My reluctantly glamorous field camp mentor who did volunteer autopsies at the Nairobi zoo and carried her baby on her back to dinosaur digs. A USGS mapping geologist who carries a whip in the field and takes no prisoners. The great gun-toting field geologist women of Alaska, who move easily between tectonics and petrology, geochronology and geomorphology, present and past as if they can see the whole Great Land in four dimensions before them at any time and place. One while wearing a brightly colored vest she quilted herself.

These women all have striking stories behind them. Stories of finding their own way into doing the field geology they love, often through very circuitous paths, often making great sacrifices by leaving the straight-and-narrow career path. Each of them eventually created a niche for herself- maybe not perfect niche, but a balance between career and family and love that works.

I want to write a book about these women. Or rather, I want someone who can write to do it. I want it to be a big portrait book, like an art exhibit book, with beautiful photography of these women in all their non-precious jewelery, pyritized ammonite-wearing splendor. As a model I suggest Alison Owing's Hey Waitress! The USA from the other side of the Tray. I've been a fan of Alison's writing for some time, she writes these incredible anthologies of portraits which vary deftly from funny to profound, intertwining the stories to produce a portrait of a group of women without smoothing over the individual faces.

Why are the portraits so important, and why am I obsessed with Grey Braid fashion? Because it asserts so strongly the geologistness of these women, and the womanliness of these geologists. It represents a collective turning-of-the-back to mainstream ladies clothing, the clothes are functional, machine washable, there is almost a pioneer sensibility about the corduroy and wool. Layered on top of this functionality is a feminine flamboyance expressed in wacky color and irrepressibly geological, often uncomfortably heavy accessories. This look is every bit as vital and expressive as any form of fashion, and a good deal more individual than what my students are wearing these days.

In case you missed my birthday.


Charnockites.... what?

Hey geoblogosphere, help me out will ya?

I'm a brittle fault person by trade. Now I'm working on a project in the middle crust... slow, squishy and unfamiliar territory. Big mineral grains are nice though... I can identify them in hand sample! They fill a thin section! In most of my field areas, I was identifying minerals primarily through x-ray diffraction. So this is a nice change I suppose.

So here's the scene. The protoliths are a TTG suite - tonalites, granodiorites - metamorphosed at amphibolite facies. Luckily for us, the biotite foliation is pre-syn folding... which means it is sometimes axial planar but often folded. Leucosomes abound; they are solid albite+/-quartz. There are some gorgeous folded meta-dikes of nearly straight amphibole - most likely primary basaltic dykes into the TTG suite - which I am planning to use as strain markers. Here's one in a quarry in the middle of Nampula, Dr. Micheque for scale:

Outcrop here is limited to isolated, steep-sided mounds of rock (inselbergs), it's a common geomorphology in the subequatorial Gondwanaland. There are many inselbergs around Nampula, with completely flat land in between. I have questions about that as well... but I'll save those for another post. Anyway, one of the inselbergs has a unique zone crosscutting it, a networked zone of kspar-qtz-mt veins with a smoky, greenish patchy alteration zone around them (Lee-anne Rudd, Hons 07 and trooper extraordinaire, for scale). What you see looks like wet patches on a jointed rock, but I assure you, at 40° in blazing sun, there is no water here, you are seeing the greenish alteration zones:
Here's a closer look at the thing. The edges of the blocks in the network are sheared, the biotite foliation is curved into the boundaries - implying some rotation of the blocks. (excuse the white chalky patches, which are hammer blows):

Previous descriptions by previous workers called this a charnockite zone. This led me to a big literature search on charnockite (which I never, I swear it, heard of as an undergraduate). I found basically that charnockite is a sparcely defined facies? or rock type? resulting from (rock containing hydrous mafic minerals) + (CO2) -> (rock containing orthopyroxene) + (k-rich, volatile-rich melt). Basically, the idea is that if rocks with a little bit of water in them are flushed with CO2 at high enough temp, the hydrous minerals will dehydrate and the water will cause partial melting of the less refractory minerals, in this case biotite. (More recent work has pointed out that the fluxing fluid need not be CO2, but it needs to be thirsty for water). This fit well with our field observations and we were happy. Until we got home with the rocks. And found no opx, anywhere. Worse yet, our samples from the general background look exactly the same in thin section as the samples from the green patchy alteration zones. No textural difference, no change in mineralogy, even some (scanty, preliminary) microprobe work didn't turn up any difference between the green greasy-looking patchy rock, and the ordinary biotite gneiss. Hmmm.

From a geochemical standpoint, WTF? How can something so obvious in outcrop disappear in thin section? Luckily we have a new secret weapon on board, a metamorphic petrologist who knows THERMOCALC, knows migmatites, would prefer pelites but will help us out anyway.

From a structural standpoint (my standpoint), looky here. We have a network of veins of (I don't know what), at roughly 90° angles. This network occurs in a narrow linear zone about 20m wide. It clearly accommodated the flow of some reactive fluid. And take a look at the meta-basaltic dikes in the blocks in this zone:

Clearly, and this is a new level of wishywash for me, this is a "locus of strain" here in this zone.

Now we come to the question: Falling back on my old friend the Mohr Circle, I would say that 90° fluid conduits are a sign of hydrofracture. Can that be true? Slow, ever so slow hydrofracture in which the opening of joints is accommodated by volumetric strain in the blocks between joints? More importantly, can I use these folded dikes as passive strain markers? And people, what are charnockites? Truly? Do they require CO2, or low aH2O magma, or what? And how do I find out?

* just so you know, that is not my compass. I'm a brunton girl to the grave.
** theres been a lot of chatter lately about appropriate use of the geoblogosphere. Allow me to push the limits here by asking for your opinions on current research.


Barbarians Attach Rowe

This is what it looks like when Rowes visit you at the airport. Yooo HOOOO!


Alaganik Slough

Alaganik Slough is only a short drive outside Cordova. We drove out one evening just before sunset with our friend Craig to check it out. Fishing just outside the Copper River Delta along the sand bars, you can only see that a vast flat area stretches inland with mountains on all sides. I was really looking forward to seeing the delta from inside.
There are lots of moose on the delta this time of year, and they are big ones!
The willows were flowering and attracting all kinds of pollinators,

...such as these surprisingly huge but herbivorous mosquitoes...

The Canada geese are here in full force:
The hooligan (aka euchalon, aka smelt) are running and got caught all over the banks of the slough at low tide. I don't know much about hooligans but these must be spawned out I suppose... they are known as candlefish because they are so oily that they can be dried and lit like a candle for stinky winter illumination. We watched some eagles taking them out of the river but they looked pretty bored. Obviously they are waiting for the big payday when the salmon run kicks into high gear.

The delta is unbelievably rich in color.


Rock of the Week #9

Here's a real beauty. In my low-grade, brittle line of work I don't often get to admire such prettiness in my sample collection. But I digress...
1. Name two of the many minerals present in this rock.
2. What is the name of this rock type?
3. How is this rock type formed?

Once again if you can answer 1. you can figure out 2, which should lead to 3. Both cut slab and weathered side are shown - sorry for the saw marks. The third photo is the most true to colour.

Solution for Rock of the Week #8:
This is a fine-grained, blueschist facies metabasalt from the Franciscan Complex, California, USA. It was collected at the Russian River, California where it was lying as a large knocker (exotic block) in a tectonic mélange. Mineralogy is glaucophane+garnet+epidote+albite+pyrite, consistent with an overall composition of hydrated basalt.

1. (1 pt) Rounded red mineral?
: 1 point given for garnet.

2. (1 pt) Fine-grained matrix?
: 1 point for glaucophane or sodic amphibole.

3. (1 pt) Rock name or metamorphic grade:
: 1 point for blueschist.