Where on Google Earth? #215

With a helpful Twitter hint from Ron Schott, I won my second “Where on (Google) Earth?” challenge, the 214th edition of this popular geoblogospheric competition. As a result, I get to host the next one, Where on Google Earth? #215.

The aim of the game is to figure out where on Earth this satellite imagery comes from, and then post the coordinates (lat/long, UTM, whatever) and give a brief explanation of the geologic significance of the region. (I’ve got a full post ready to go that goes into more detail on the region; so you need only sketch out the flimsiest of details.)

Post your answer in the comments section once you’ve figured it out. The winner earns the right to host Where on Google Earth #216. If you don’t have a blog of your own, then I’ll be happy to host it here on your behalf. I invoke the Schott Rule, which says that you have to wait one hour for each past Wo(G)E that you’ve won before answering. Posting time is 9:00am on Saturday, October 16.

Here it is:

Please note that north is off to the upper right. You can enlarge the screenshot to full-size by clicking through twice. Good luck!

Friday fold: multilayer buckle folding demo

Check out this video I found online whilst uploading last week’s Friday fold:

This video was produced and published on YouTube by Markus Beckers, Michael Ketterman, Dennis Laux and Janos Urai.

It’s a nice demonstration of how multiple layers of material of different properties and different thicknesses can yield up different flavors of folds. In the movie, there are two materials present: white silicone and gray foam. The silicone layers are stronger (“more competent”) than the foam. But the two silicone layers are different thicknesses. It turns out that this ends up being a decisive factor in determining the way they fold.

We can explain this behavior using the Ramberg-Biot equation:

L = 2 π t (η / 6ηo)

where L is the wavelength of the fold (in other words, the distance from one antiform fold hinge to the next antiform fold hinge); t is the thickness of the folded layer; η is the viscosity (resistance to flow) of the silicone layer (or, in general, the more competent of the two layers); and ηo is the viscosity of the foam layers.

In other words, the (η / 6ηo) part of the equation reflects the viscosity contrast between the affected layers. In the video, this viscosity contrast is a constant, since we’re looking at two layers of the same stuff surrounded by the same matrix of other stuff. The only difference is the thickness of the two silicone layers.

So as far as our video up top is concerned, pay attention to the t value and the L value: the thicker the layer is, the larger the wavelength of the resulting fold. The thin layer has a lower t value, and so it ends up with a shorter wavelength: i.e., there are more folds packed into the same amount of vertical space as its stouter neighbor. The thick layer’s higher t value means it wıll have a proportıonately higher L value. It will have a longer wavelength, and fewer undulations will fit into the available vertical space.

Happy Friday, everyone! I’m heading back to DC tomorrow (from Turkey), so more regular posting wıll resume next week.

Words’ worth IV

Back on the first incarnation of this blog, I occasionally posted about words that bugged me. A few more have piled up since then, so here we go with the latest consideration of “words’ worth”…

First off, let’s consider the use of “outcrops” as a verb. This came up recently on this blog when commenter Tom Skaug pointed out that I was incorrectly using that term. He’s right of course, and has the dictionary citations to prove it. Technically, we should say that a particular rock unit “crops out” on a hillside. Mea culpa. I appreciate the correction. That being said, I know a lot of geologists who speak as sloppily as I write. Using “outcrop” as a verb is reasonably common slang in my circles.

Next, let’s consider some plural words. When reviewing an article recently, I saw the words “maximums” and “minimums” written out by a science writer. I suggested to the editor that these should be “minima and maxima” instead. The editor countered that real people (i.e., non-scientists) don’t speak that way, and that the accepted parlance among the general public is just to tack an “s” on the end of a word to make it plural. However, in Latin, the language that gives us these words, the plural would end with the addition of an “a.” When you look it up in a dictionary, both plural forms are listed. To add insult to injury, my computer’s automatic spell-checker function is putting the red zigzag under my correct Latin versions, and NOT underlining the “-s” versions. I’m beset on all sides!  Still, to me, “minimums” sounds clunky and clumsy, while “minima” is elegant and sleek, like a well-designed scientific instrument.

Okay, here’s another one. Occasionally, graffiti appear on the walls of the bathrooms here at the community college where I teach. When I spot a new scrawl, I write an e-mail to the cleaning staff alerting them to the vandalism. But what do I do when there’s just one little new jotting? Graffiti are plural; the correct singular of this Italian word is “graffito.” But that sounds vaguely ridiculous, right? “Dear Cleaning Staff, There is a new graffito in the men’s bathroom on the east side of the Shuler Building’s second floor.” I feel silly, and maybe a little pompous, if I use the correct singular form of this word. Anybody else have a word like that, where they know how to use it correctly, but they use it incorrectly on purpose for the ease of communication? (…Or possibly to avoid offending someone?)

Along similar lines, data are plural, while datum is singular. Most scientists are comfortable discussing a single datum, and are careful to only use “data” when there’s more than one chunk of information being discussed. But the general public doesn’t parse this distinction as finely. You’ll see “data” used to refer to what really is a lone datum.

Natural gas – I was thinking about this one while driving into work the other day, and the radio newspeople were talking about that big explosion a few weeks ago in San Bruno, California. It got me thinking about the term “natural gas.” What a dumb, non-descriptive term. I mean, do we ever refer to “natural liquid” or “natural solid?” Natural gas is annoyingly non-specific. I get it: it’s a cocktail of different gases, mostly methane, with a dash of ethane and maybe a few other volatile compounds too. If it were pure methane, we would call it “methane,” but it’s often not pure. It’s a mixture. So we can’t call it just “methane,” because that wouldn’t be accurate. The mixture occurs naturally, so we call it natural gas. We trade specificity for meaningless but accurate inclusiveness. Blech. The role of “natural gas” as a fossil fuel is ascendant; we’re going to be talking about it for some time to come. I think we need a better name for the stuff. Suggestions?

Deducing my first anticline

When I was done with my sophomore year at William & Mary, I embarked on a time-honored tradition among W&M geology majors: the Geology 310 Colorado Plateau field course. Jess alluded to this same course in her Magma Cum Laude contribution to this month’s Accretionary Wedge geology blog “carnival,” too.

My version of Geology 310 was led by the legendary Gerald Johnson (a.k.a. “Dr J”), a dynamic and enthusiastic educator who seemed particularly at home in the field. One day, he had us out in Utah (I think) somewhere, and pulled over to the side of the road so we could examine some tilted sandstone layers. We took a strike and dip reading, and plotted it on a map.

310A

Then we descended into a narrow valley, where Dr. J did some “geology at 60 miles per hour,” pointing out shale outcrops in a few places in the valley. Then we drove up the opposite side. We pulled over again. Same sandstone strata: we again took a strike and a dip on the beds. The data was then recorded on our maps with a strike and dip symbol, a broad, squat “T” shape, where the upper bar of the “T” is parallel to the strike of the bedding, and the vertical prong of the “T” is pointing in the dip direction.

310B

“Well,” Dr. J asked us, “What’s going on here?”

We were all silent, trying to puzzle it out. What’s the deal? What is he fishing for? Seconds ticked by, and no one had the right answer. We started to sweat… “Um, the sandstone beds are dipping to the west on the ridge west of the valley,” someone ventured, “and they are dipping to the east on the ridge east of the valley?”

“Yes, but what does that mean?” he replied. Silence…

Eventually, he relented, and spelled it out for us. Imagine this situation from the sides, he suggested, gesticulating the layers dipping off in opposite directions. “These are the same layers, so they were once laterally continuous…” He mimed a cross-sectional perspective:

310C

How could we connect these disparately oriented strata together?

310D

Bam! It hit me: I got the idea of an anticline at that point — the idea that a structure like an anticline could be so large that I couldn’t actually see it from my earthbound human-sized perspective, and I could only infer it from detailed measurements of the rock structures. It was a revelation to me: this valley and its surrounding ridges were part of a massive fold. The anticline must have breached in the middle, with the shale eroding away faster than the sandstone, producing a valley flanked by two ridges.

I’m grateful to Dr. J for putting us through all stages of this exercise: collecting the incremental pieces of data, being forced to think about it in an attempt to come up with an interpretation, and then finally giving us the proper interpretation, once it had become obvious we weren’t going to get it on our own. This last bit is particularly important to me as an educator: sometimes it’s okay to spell it out for students, particularly if it’s their first time walking down a particular path. By revealing the “answer,” Dr. J guided my thinking from data to big picture structure to geomorphological interpretation in a way that I can only describe as “opening up a new pathway” in my mind. Once he showed the way to think about this sort of thing, it was suddenly very easy for me to visualize this sort of complicated four-dimensional story. Once the pathway was there, it was almost effortless to let my thoughts flow along that pathway. Weird how one’s perspective can change in a moment, and how that influences everything that comes after.

For me, this exercise and ensuing discussion constituted an important moment in developing my ability to think like a geologist. I don’t think my brain will ever be the same.

Friday fold: Siccar Point, Scotland

As with last week, I’m going to show you someone else’s fold today. This one should have strong resonance with most geologists, because it’s a fold in the tilted (and contorted) older strata exposed below the famous unconformity at Siccar Point, Scotland:

fold_siccar

I found this image on the British Geological Survey’s online repository of images, which are available for public use with attribution. I found out about the BGS photo repository via a post on StructuralGeology.org.

The photo was taken by T.S. Bain in 1979. Rock hammer (lower left) for scale.

The specific rock type here is shale, and their age is Silurian. Note the thinning of the limbs of the fold, and the relatively thick hinge area.

Happy Friday – may your workday rapidly thin (like the limbs of this “similar” fold), and your weekend be as thick as this fold hinge!

EARTH: the biography, by the BBC

Last week, I watched the BBC/National Geographic series “EARTH: The Biography,” hosted by Iain Stewart.

Stewart is a charismatic host, with a thick Scottish accent that cannot disguise his enthusiasm for geology. The five episodes focus on: volcanoes, ice, oceans, atmosphere, and “rare planet.” Overall, I thought the series did an good job covering some of the greatest stories in geology with an emphasis on presenting the latest ideas. Snowball Earth gets screen time, for instance, and the ocean-anoxia hypothesis for the end-Permian extinction, too. They also cover ocean acidification, a topic I feel deserves wider press.

The series is well-produced. Stewart zips all around the globe, and the editors seamlessly incorporate imagery from other BBC series (like Planet Earth) as supporting content where appropriate.

Here are some of the tidbits I gleaned from the show:

Two billion tonnes of the Andes are carried down the Amazon every year (in the form of sediment weathered and eroded off the Andes). Along similar lines, 40 million tonnes of dust from the Sahara Desert are dumped on the Amazon Basin every year. I wonder if the Sahara dust is included in their sediment volume estimates, or whether it is deducted since it’s not of Andean origin. Great statistics regardless.

They tell the story of Joesph Kittenger in the atmosphere episode. He did a skydiving jump from 90 miles up! After free-falling through almost the entire Earth’s atmosphere, this crazy dude lights up a cigarette! Those were the days.

Four million tonnes of the Sun’s mass are converted into energy every second. Whoa.

Humans now move more rock and soil than all natural processes combined. Ergo: Anthropocene.

The Mediterranean Sea loses three times as much water to evaporation than it gains from rivers and rain. Without the Straits of Gibraltar to let in Atlantic water, it will dry up (and it has dried up, multiple times in the past). In illustrating this, Stewart goes into a salt mine beneath Sicily and shows some BEAUTIFUL contorted salt laminae. Worth watching the whole series just for those gorgeous patterns. (here’s one shot)

The footage of Fayetteville Green Lake in New York is excellent — this is a deep lake with pronounced internal stratification of water and not much mixing — the deep parts of the lake have become anoxic and euxinic (enriched in H2S). They illustrate this by diving into it and the water turns PINK. It is presented, of course, as an analogy for one of the leading models for the end-Permian extinction: global ocean euxinia. It is astonishing to see pink water, and enticing to think about, but the show commits a major “fail” when they don’t tell what this substance is, or where it comes from. They describe the water as having “something deadly” in it, and then say it’s a “highly toxic poison,” or “a gas as deadly as cyanide,” but never do they (a) call it hydrogen sulfide, and (b) explain that it comes from certain kinds of bacteria that thrive in low-oxygen waters. Another complaint: they don’t say when the Permian-Triassic extinction occurred, just the same old saw about it being the “greatest” extinction in Earth history, and that it occurred “before the dinosaurs.” The word “Permian” is never used.

I have some other criticisms, too…

The phrase “a blink of an eye, geologically” is used too often. Twice in the first episode alone!

They show an image of a comet moving like a badminton birdie, with the tail pointing back where the comet came from. This isn’t accurate — comet tails point away from the sun (dragged downstream by the solar wind).

At one point, when discussing the history of life on Earth, Stewart suggests that “life needs catastrophes.” I would argue that life has diversified due to catastrophes, but that catastrophes are not necessary for life to continue. In a non-catastrophic situation, life just perpetuates itself and may exhibit increasing specialization or genetic drift within the parameters available in its environment. But “needing” a catastrophe every now and again? Only if diversification of life is the goal — I take issue with this verb.

In another episode, Stewart is describing convection in the mantle, and says that “magma” is moving upwards. This is false: it is hot rock (a solid), less dense than neighboring relatively-cold rock. The “magma” idea for the Earth’s mantle is a popular misconception which Stewart is opting to elide rather than confront.

At another point, in praising the Moon, Stewart suggests that the planet Earth’s climate would have switching between freezing cold and boiling hot if it were not for the Moon’s influence. No explanation is given for this extraordinary claim. He may indeed have a chain of evidence and inference in mind when he says this, but without a robust explanation, this statement comes off as “because scientists say so”: an authoritative statement with no supporting detail which shows how science comes to a particular conclusion. Worse, he then cranks it up with the future fear factor — they go into great detail about how we have determined that the Moon is drifting further away from Earth over time, and then suggests ominously that Earth will then lose its climatic stability. So now we’ve got alarmism too, but again, no explanation of the supposed causative relationship is given.

Overall, it’s an enjoyable series, and I was pleased to have it to watch when I had the flu last week. Check it out, and let me know what you think.

Champlain thrust fault

champlain_01

Over the summer, I went up to Vermont to visit my friends the Clearys. Joe Cleary is a college friend and a talented luthier. He and his wife Tree and their children Jasper and Juniper have settled in Burlington, a lively town with a lot of cool stuff going on. Joe took time out one morning to show us a superb example of a thrust fault on the shore of Lake Champlain. It is on private property, but Joe got permission for us to hike there first. Our group that day consisted of Joe, Lily, and me, plus by a stroke of good luck, my pal Pete Berquist was in Burlington at the same time, with his friend Amy. The five us were Team Burlington for the day.

There are two rock units involved in the faulting at this location. Consider the first:

champlain_17

This is the Dunham Dolostone. It’s early Cambrian in age. It’s resistant to erosion, and stands up in cliffs above Lake Champlain. The distance from my ten little piggies down to the water is probably fifty feet. Below the Dunham Dolostone, you can find the Iberville shale. It is actually younger than the overlying dolostone. (We know this from unfaulted stratigraphy elsewhere in the region.) The Iberville shales are Middle Ordovician in age. They are relatively weak (‘incompetent’) rocks, and have been sheared out by the faulting. Here, Team Burlington demonstrates the sense of shear, by leaning over in the direction that foliation has rotated towards:

champlain_02

Looking in one direction along the base of the fault to show the differential weathering of the two units:

champlain_04

Flip it around 180°, and you see the same thing in the other direction:

champlain_06

Pete, Joe, and I crawled underneath the ominously overhanging dolostone to check out the detailed structure of the fault. Here’s Pete tickling the sheared out shales, looking for little sigmas…

champlain_05

The shales had nice veins of calcite running through them, and the high contrast of light and dark reveals some lovely folds, like this one:

champlain_03

Pete goes into professor mode, gesticulating and using the verb “shmoo” to describe the reaction of the shale to a gazillion tons of dolostone sliding over top of it:

champlain_07

Another nice fold (little tiny blue Swiss Army knife, 5.7 cm in length, for scale):

champlain_09

And another nice fold:

champlain_10

This fold is transitioning into a shear band:

champlain_16

Here’s my favorite part of the outcrop, a big fold with little parasitic folds all over it, showing opposite senses of shear on the opposite limbs of the big fold:

champlain_12

S-folds on the upper limb, Z-folds on the lower limb. Sweet, eh?

Here, a sort of S-C fabric has developed, with foliation tipped over the the left, and then near-horizontal shear bands running along through it:

champlain_11

Here’s something weird. Perhaps a reader can explain it. Here’s a shot of some of the veins, with the same 5.7 cm knife for scale:

champlain_13

Now we’ve zoomed in, and you can see some detail in the vein:

champlain_18

What are those lines? Is that more “S-C” fabric? I mean, it can’t be cross-bedding in a vein… but I’m having trouble visualizing what process of shearing the vein could yield such a delicate, even distribution of dark material amid the vein fill. What the heck is going on here?

Okay, now that you’ve twisted your brain up thinking about that, you can relax with a structure whose meaning is obvious. Some artistic and romantic previous visitor (not a member of Team Burlington) had arranged pebbles weathered from the two rock units into a bimodal icon of love:

champlain_08

Displacement along the Champlain Thrust is estimated at 30–50 miles (48–80 km). These dolostones started off near the New Hampshire border, then crossed Vermont, almost but not quite making it into the Empire State! The Champlain Thrust is the westernmost thrust fault that has been associated with the Taconian Orogeny, a late Ordovician episode of mountain building associated with the docking of an island arc with ancestral North America. Looking up at the fault trace:

champlain_15

A final glance at the thrust outcrop, looking north and showing the fault’s gently-inclined easterly dip:

champlain_14

Joe, thanks for taking the time to bring us out there!

Friday fold: granite dikes, Barberton greenstone belt

FF5

Folded & boudinaged granite dikes in tonalitic gneiss, Barberton granite-greenstone belt, South Africa. From Passchier, CW, Myers, JS, and Kroner, A., (1990). FIELD GEOLOGY OF HIGH GRADE GNEISS TERRANES.

Very crudely annotated:
FF5_anno

This is a sweet example of how you can get different structures developing in different orientations relative to the principal stress directions. In this particular part of the Barberton Greenstone Belt, compression (orange arrows) operated from the top of the photo towards the bottom, and the rock stretched out from left to right (green arrows). Folds formed where granite dikes were compressed, but the same rock in a different orientation was boudinaged… Cool, eh?

So that’s your Friday fold! The boudinage is just a little bonus for you, because, hey, it’s Friday.

Drilling: what, why, and how

As mentioned, I spent a significant part of last weekend was spent on a paleomagnetic sampling project with collaborators from the University of Michigan. On Friday, this was our field area:

drilling06

That’s the south slopes of Old Rag Mountain, a popular Blue Ridge hiking destination because unlike many Virginia peaks, when you get to the top, you see some rocks instead of 100% trees:

drilling07

But we didn’t come here for the view. We came here for the dikes. Here’s the edge of one, with a pen for scale:

drilling04

These are dikes of basalt and meta-basalt of the Catoctin Formation which are presumed to be feeder dikes pumping mafic lava to the surface of Virginia around 570 million years ago, during the breakup of the supercontinent Rodinia and the opening of the Iapetan Ocean basin. The dikes cut across the Grenville-aged basement rocks, in this case the Old Rag Granite of about 1000 million years age. The Old Rag area is especially great because the dikes are less metamorphosed than they are in other parts of the Blue Ridge province, where the Catoctin has been cooked into greenstone. Here’s an annotated view of the previous photograph:

drilling04anno

As far as this project goes, we are interested in these dikes for the information that they (potentially) contain about the orientation of the Earth’s magnetic field in Virginia at the time of the supercontinent Rodinia’s breakup. By sampling these dikes and then analyzing the samples at their paleomagnetism lab back in Ann Arbor, Fatim and Matt hope to put some constraints on the question of paleo-Virginia’s latitude when these dikes cooled into solid rock.

As a reminder, you are not allowed to sample any rocks in any national park unless you have first applied for and been granted a research sampling permit by the National Park Service.

Close to the planet’s surface, the Earth’s magnetic field is shaped like a torus (or, in less technical terms, a doughnut, but one of those donuts with a pinched up midsection, and more of a dimple than a hole). It exits at the south magnetic pole, wraps north around the Earth, and plunges back into the inner core at the north magnetic pole:

magfield_normal

A magnetically-sensitive mineral forming in a modern rock would have an upward-oriented high-angle magnetism if it formed at high southerly latitudes, a moderate-angled upward orientation at moderate southerly latitudes, a horizontal, northward-pointing orientation at the magnetic equator, and then the reverse as you head towards the north pole: a moderate-angled downward orientation at moderate northerly latitudes, and a downward-oriented high-angle orientation if it formed at high northerly latitudes, just like the red arrows show in the above image.

Of course, the flow of the magnetic field occasionally reverses direction (emerging at the north magnetic pole instead, and flowing south), but the shape of the field doesn’t change:

magfield_reversed

So the angle of inclination of a fossil magnet should be the same regardless of whether it’s poking up or plunging down, relative to the surface of the Earth. In this way, paleomagnetism can reveal the approximate latitude (but not longitude) at which a rock formed.

But wait, is it really so simple? No, of course not. Check out the map below, showing the positions of the north geomagnetic pole over the past 2000 years, with numbers showing the position of the pole in a specific year CE. It moves! The circles around geomagnetic poles at 900, 1300, and 1700 CE are 95% confidence limits on those geomagnetic poles; the mean geomagnetic pole position over the past 2000 yr is shown by the square with stippled region of 95% confidence. These data were compiled by Merrill and McElhinny (1983) and plotted by Butler (1982).

secvar

So this map shows us that even though the magnetic pole does wander about a bit, 2000 years of data is enough to generate an average which is more or less coincident with the geographic pole. And therefore a statistically significant batch of data (spread over a 2000-year-or-greater spread of time) will also reflect that average pole position.

Meert, Van der Voo, and Payne (1994) made a first attempt at constraining the paleomagnetics of the Catoctin Formation. Four of their 32 sites were feeder dikes, sills, and host rock (Grenvillian basement complex). One of the things these authors did was that they performed a “contact test” on two of their dikes. A contact test is a way of using an igneous contact (as with a dike) to determine whether the whole region has been magnetically reset, perhaps by thermal activity accompanying contact metamorphism. Consider this situation:

contacttest1

You sample a dike and its surrounding host rock, at several distances away from the dike. You find that they all give you the same magnetic orientation. This suggests you have the magnetic signature of a later overprinting, not the original orientations of dike and host rock.

Now what if you found this, instead?

contacttest2

Here, your dike shows a distinct signature that is different from the host rock, and the host rock shows a uniform orientation except right next to the dike, where the heat of the intrusion has partially reset the (older) host rock’s magnetism. If I were to annotate this up (with color coding!), it would look something like this:

contacttest3

Passing the contact test is critical to tying the two rocks’ magnetic data to their age data. It’s only with a positive contact test that you can use this data to say anything about where Virginia (and thus ancestral North America, often dubbed “Laurentia”) was when the Catoctin dikes were intruded.

The contact test is something that our team wanted to repeat, with more dikes than just the two that were featured in the Meert, et al. (1994) paper. We also wanted to double-check their results, and verify, reject, or modify them as our data warranted.

The key to constraining the magnetic orientation of these rocks as precisely as possible is to constrain the current orientation of the samples as precisely as possible. We measured the strike and dip of the surface of each sample very carefully, before we extracted it from the bedrock. At Old Rag Mountain, we were not allowed to drill (Old Rag is a wilderness area with no motorized equipment allowed), so we were collecting oriented hand samples.

Here’s Fatim Hankard writing orientation data in her field notebook while Matt Domeier takes a strike and dip reading in the background, using his Brunton compass:

drilling02

Because these rocks are inherently magnetic (that’s why we’re sampling them, after all!), we have to control for the possibility that the rocks themselves might be throwing off our Brunton compass needles. A second compass is employed to control for any magnetic field coming off the rocks themselves. This is a solar compass. If you know exactly where you are (note Fatim’s GPS unit in the above photo), and when you are taking the measurement, you can use this solar compass to double-check the orientation you get from the Brunton compass.

Here’s Matt’s solar compass butted up against one of our Old Rag samples. Note the shadow being cast by the compass’s nomen, and also note the “arrow with a prong” strike and dip symbol that we wrote directly onto the face of the sample with a Sharpie:

drilling01

Next, take a look at a photo of a sample once extracted. We label it redundantly, not only in terms of the orientation lines, but also in terms of the sample’s identity. That way, we’re less like to find a bunch of scratched-up but un-identified and un-orientable rock samples once the van gets back to Michigan:

drilling03

While poking around, I found this interesting feature at the edge of one of the dikes. I’m hoping one of my more petrologically-inclined readers may be able to offer me some kind of interpretation of this pattern:

drilling05

What I noticed is that in the first few mm of the dike, right up against the contact with the host rock, there are no white lathes of plagioclase feldspar. These relatively large feldspar crystals are phenocrysts, big chunky crystals that grow in the magma when it’s cooling relatively slowly underground, but then entrained in the flow as it moves upwards into the dikes, whereupon the surrounding liquid chills rapidly to make fine-grained basalt. So there are no phenocrysts right at the edge of the dike, then there are a bunch, all aligned with one another (but with no preferred sense of imbrication, so far as I can tell), and then there are more phenocrysts in the bulk of the dike, but they are (a) less concentrated, and (b) lack any preferred orientation. Let me annotate it for you, then go back and take another look at the unannotated version, so you can see what I’m referring to:

drilling05anno

Okay, petrologists, I want to hear from you: How should I interpret this?

Back to the paleomag… On Saturday, we went to another location to sample. This one was much more convenient because (a) it was right on the side of the road, and (b) it wasn’t a wilderness area, so drilling was allowed. This was at the lovely selection of Catoctin dikes downhill (north) from the Little Devils Staircase overlook, on Skyline Drive in Shenandoah National Park. Here’s a charismatic dike with Matt acting as a sense of scale:

drilling08

Annotated:

drilling08ANNO

We unpacked the gasoline-powered diamond-grit-tipped drills and hooked them up to the water pump. We put on ear- and eye-protection, and got to work:

drilling09

One the sample has been drilled out, you’re left with an empty hole. The white liquid is the cooling water with suspended dust from the abraded rock. This hole is about 3 cm in diameter:

drilling11

The core (2.5 cm diameter) that came out of that hole:

drilling10

In our field area, a core this size of the dike rock takes about ten minutes to extract. Basement rock (host rock) takes longer, as it’s made of harder minerals.

One worry is that the core will snap loose while you are drilling it out. If this happens, it may start rotating in the hole, and you will lose all sense of how it was originally oriented, which means you’ve just wasted a lot of time for no gain in data. To protect against this possibility, we used a technique of scoring a second circle with the drill bit, partially overlapping our actual core like a Venn diagram:

drilling12

This way, if the core snaps off, you can line up its arc with the rest of the circle inscribed on the outcrop next to the hole. Whew! Core saved!

Fatim extracting another core:

drilling13

After the core is drilled out (but still in its hole), Fatim oriented it. Notice the new array here – it’s a stand with slots into the drill-hole, then has a Brunton compass atop it with a solar compass atop that:

drilling14

As you can see with this example, the solar compass is just about to become useless as the afternoon shadows advance! Next up, record all the orientation information (trend and plunge of the cylinder’s axis), and then score the core with a line:

drilling15

Fatim and Matt sampled for two more days after I had to leave them due to other obligations, like teaching. They are headed back to Michigan today. Soon, hopefully, we’ll see whether our sampling campaign yields any meaningful results… Stay tuned!

As a final note, I would like to point out that this collaboration was born when Fatim read my blog post on feeder dikes and then proposed that we combine her paleomag skillz with my dike-location knowledge. It’s not the first time that my blogging has yielded a great opportunity, but it seems to be a shining example of how virtual connections online can lead to tangible work in the real world. The blog-curious should take note.

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References cited:

R.F. Butler. PALEOMAGNETISM: Magnetic Domains to Geologic Terranes. Originally published by Blackwell in 1984, 248 pp. Updated online 2004. Retrieved September 15, 2010, from http://www.pmc.ucsc.edu/~njarboe/pmagresource/ButlerPaleomagnetismBook.pdf.

J. G. Meert, R. Van der Voo, and T.W. Payne. “Paleomagnetism of the Catoctin volcanic province: A new Vendian-Cambrian apparent polar wander path for North America,” March 10, 1994. Journal of Geophysical Research 99, No. B3, pp. 4625-4641.

R. T. Merrill and M. W. McElhinny, The Earth’s Magnetic Field, Academic Press, London, 401 pp., 1983.

Friday fold: Kinky metagraywacke from DC

Fourth edition of the “Friday fold;” second one via video. Happy Friday!

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