Ahh, unakite, how do I love thee? Let me count the ways.

Nay, let me calculate the ways. ...That's why we have algebra. Let x = unakite.




Snowy Lily boots for scale.

Previous X marking the spot post. Previous unakite post. You may also want to check out the recent unakite posts on Looking For Detachment, About.com, and Ron Schott's Geology Home Companion Blog.

Anyone able to tell me where these two unakite "X"s may be found? First one to give the correct answer in the comments wins a "GEOLOGY ROCKS" bumper sticker!

Labels: contest, granite, metamorphism

The other day, I mentioned the lineated granite gneiss we saw when hiking in Hickory Nut Gorge State Park on Thanksgiving Day (hey, maybe it's not quite up to the standards of last Thanksgiving's hike, but I'm cool with that). The next day, we headed to Chimney Rock to check out the scene there.

Here's a Google Map of the site (satellite view):


...and a zoomed-in view where you can see the tan ellipse of Chimney Rock itself:


Chimney Rock is located right at the Blue Ridge mountain front, where the mountainous terrain underlain by Grenvillian basement complex (Mesoproterozoic) gives way to the multiple metamorphosed oceanic terranes of the Piedmont province (like the metavolcanics mentioned earlier this week). Here's a view east across this physiographic (and geologic) boundary:

This boundary is called the Brevard Zone, a fault/mylonite zone of complicated structure. I don't know much about the Brevard Zone, but those Carolinian geologists are all over it. It's something I'd like to learn more about. If you have any particular expertise to contribute, please leave a comment telling us more (and giving us outcrop recommendations!).

Here's the star attraction of Chimney Rock Park:

You can see from the bridge and the flag that it has been developed, and much of the park exhibits "improvements" from the natural state.

Before climbing up to the Rock, we decided to hike out to the waterfall upstream. Chimney Rock projects from the wall of a deeply incised canyon carved by the Broad River. A tributary of the Broad flows over the lip of the canyon, providing a lovely waterfall. This scenic location was the spot where they filmed the final scene in the movie version of Last of the Mohicans:


Here's a view across the gorge (from underneath an overhang), looking towards the north:


At the site of the waterfall, I was intrigued to note that the rocky walls were exhibiting "onion skin" weathering (exfoliation jointing) that in my experience is more typical of granites (say, like those in the Sierra Nevada):


Here's a smaller version of the same phenomenon: a flake parts with its source rock, leaving the source rock more spheroidal than it was before. Oak leaves provide a sense of scale:


The rock exposed in Chimney Rock Park is a gneiss. I didn't see any here that was noticably lineated, but it had a pronounced horizontal foliation. The rock varies quite a lot in its texture and degree of deformation. Here's some photos:


Penny for scale:


Penny for scale:


In places, the metamorphic foliation has been deformed. Mainly this is evidenced in charismatic, high-contrast folds, but there is also some small-scale faulting visible, and some boudinage. Here are some images of the folds:


Isoclinal fold. Penny for scale:


Penny for scale (bottom):








Lily points above her head at some parasitic folding:


Here, Lily appears to hold up a big ellipse with an axial ratio of 6 or 7:

This is a section through an isoclinal fold, so that the fold axis is transected once on the left and once (on a differently oriented surface) on the right.

Finally we approached Chimney Rock itself, a looming monolith whose presence was indicating by a loudly flapping flagpole. When a gust of wind came along, the sudden clatter of the flag whipping in the wind was quite disconcerting. From below, it sounded a lot like a rockfall had initiated somewhere up above us. Note how the shape of Chimney Rock appears to be a compromise between the ~horizontal fissility of the gneiss and the spheroidal weathering associated with exfoliation:


Little wooden walkways and staircases are draped all over the face of the mountain, including a catwalk out to Chimney Rock itself:


Atop Chimney Rock, we found these little holes which were filled with water. I forget the name of these things - can someone remind me in the comments section below?

Essentially what's going on here is a self-perpetuating focusing of weathering. A small initial divot in the rock face allows water to accumulate. That water facilitates additional weathering through freeze-thaw action and chemical breakdown of the minerals in the gneiss. This weathering enlarges the size of the depression, which allows more water to accumulate, which triggers more weathering. It's a nice example of a positive feedback loop: a small initial perturbation auto-catalyzes itself into a much larger final effect. I've seen similar structures atop many mountaintops.

Labels: blue ridge, metamorphism, movies, north carolina, piedmont, structure, travel, weathering

Thanksgiving Day, Lily and I took a hike in Hickory Nut Gorge State Park, North Carolina, just south of the much-better-known Chimney Rock, which was closed for the holiday.



Just outside the park, on the public right-of-way, I collected this lovely chunk of granite gneiss which shows both foliation and lineation:


This is classic Blue Ridge province basement rock; it formed ~1.1 or 1.2 billion years ago during the episode of mountain building known as the Grenvillian Orogeny. We've got many of the same sorts of rocks (though slightly younger) up in Shenandoah National Park in Virginia. However, the thing that caught my eye about this one is the fact that it has such well-developed lineation. You're probably already familiar with foliation, the planar alignment of mineral grains in many metamorphic rocks. Lineation, a linear alignment of mineral grains, is somewhat less common, as it requires a different sort of stress field to form. In the scanned image above, you're looking straight at the plane of foliation. Within that plane of foliation is the pronounced lineation, which indicates that the maximum principle stress was directed perpendicular to foliation (plane of the screen), an intermediate principle stress was directed left to right, and the minimum principle stress was directed top to bottom, which is why the gneiss squootched out in that direction*. Hemmed in from the sides, smushed from the front and back, it had nowhere to go but "up." The strain ellipsoid here would be shaped something like a flatworm, or a baguette that had been run over by a steamroller.

* I'm assuming a monoclinic stress field.

Labels: blue ridge, granite, igneous, metamorphism, north carolina, structure

The Duke Quarry in 1928, three years after its purchase. Image: Duke University Archives.

Duke University is a private university with a strong reputation. It is located in Durham, North Carolina. The campus has a distinctive "Old World" look with its gothic architecture, and many movies and television series have filmed there. Duke gets all its exterior building stone from a quarry near Hillsborough in Orange County, NC, that they purchased 85 years ago. Here's a Duke Magazine feature on the quarry: "By the numbers." This seems like a pretty clever move to me: they end up owning the source of their own stone, and that allows them to "brand" the rock used as unique to Duke. The university has found brick to be six times as cost-effective for modern campus additions, though they're still finding ways to integrate "Duke Stone" in those buildings too, as this nice little video explains (RealPlayer video).

After checking out other metavolcanics in the North Carolina Piedmont, Rob took us to see the Duke Quarry. It's pretty cool: a big operation, with lots of interesting rock about. Here's a view from one of the active walls, looking out over the semi-vegetated quarry (with Rob in the middle distance):


Piles of the rock await Duke architects and stonemasons:


The rock itself is a meta-volcanic assemblage, metamorphosed to phyllite. Original volcanic clasts can be seen, stretched out parallel to the foliation:

So the deal here is that these are ~andesitic island arc volcanics, originally erupted out in the Iapetus Ocean somewhere. Later, they were accreted to the edge of the Laurentian continent (ancestral North America). There, they were squished and squeezed during Appalachian mountain-building during the Paleozoic. I don't know any ages for this particular unit: if anyone can supply either a crystalization age or a metamorphic age, I'd be interested to hear it. Please leave it in the comments section below, along with any published references I should read.

The color of the clasts and the matrix varies quite a bit from one area of the quarry to another.


In a couple of places, I was delighted to find bedding/foliation relationships exposed. Here, you can see bedding as ~horizontal layers in the photo (look for the grain size change):


And here it is again, a bit more explicitly (you're looking at the plane of foliation):


A lovely rock. I want to learn more about it...

Labels: igneous, metamorphism, north carolina, piedmont, volcano

For me, Thanksgiving meant an opportunity for geologizing. The little lady and I headed down to western North Carolina to spend the holiday with her family, stopping along the way to visit with our friend Rob Greenberg in Chapel Hill. Rob took us out on a morning of geologizing, visiting various metavolcanic rocks in the Piedmont province. It was a nice sampling of what North Carolina has to offer geological visitors.

Pyrophyllite-rich metarhyolite at Eno River State Park:


Crystal-lithic meta-tuff alongside the road:


Greenstone (meta-basalt) with amygdules rich in chlorite and epidote:


More information on each of these rock units here. Many thanks to Rob for taking the three hours to show us these outcrops (and one more, which I will discuss in a different post).

Labels: igneous, metamorphism, north carolina, piedmont, volcano

These days, I'm engaged in the lovely process of rediscovering the geologic record of Shenandoah National Park. This 'rediscovery' was prompted by the recent Virginia Geological Field Conference based at Big Meadows. While I wasn't able to attend in person (I was in Yosemite that weekend!), colleagues like Pete Berquist and John Weidner were there, as well as three of my Rockies students from last summer. They've all shared their perspectives on the conference with me, and John loaned me a copy of the field guide to the conference. This guide, authored by other colleagues like Chuck Bailey of William & Mary and Scott Southworth and Bill Burton of the USGS in Reston, makes for great reading. I'd link to it so you can read it too, but it's not online.

The guide led me to the revelation that there is a new geologic map of the park and the surrounding area that was published earlier this year by the survey. This map* is authored by Chuck, Scott, and Bill, along with their peers at the survey and other institutions. Why wasn't I informed? (Just kidding) It's a beautiful work of art and science. I'm having the NOVA duplicating services team print me out a copy this week.

The new insights offered by the map (and the VGFC field guide) include the fact that the oldest rocks in Shenandoah National Park are diverse and complicated. It used to be that geologists considered these rocks to be a granite gneiss called "the Pedlar Formation," which was intruded in places by younger granitoid plutons. Modern work in the park has revealed that it's more complicated than that. There are a dozen or more separate rock units comprising what the pros are now calling "the basement complex." These rocks are distinguishable based on texture, mineralogy, and age. (These newer, more precise ages are one of the key advances of recent work by John Aleinikoff of the USGS: the granitoids and their metamorphic successors have crystallization ages ranging from 1,183 Ma (+/-11 Ma) to 1,028 Ma (+/- 9 Ma).

I've updated my Shenandoah web page to reflect the new preferred terminology plus these new dates. More updates to come -- I've got many new tidbits of inspiration from reading the 100+ page write-up that accompanies the new map. The web page, like all of my web pages, is a work in progress. Nothing makes that clearer to me than a steaming helping of fresh science!

When I was out in the park last weekend, I found this new outcrop of the basement complex, which shows some of this intriguing diversity:


Annotated version:


The outcrop is on the hike over Bearfence Mountain, described (and mapped) in the new VGFC field guide. It's a granite gneiss, partially altered to unakite (the plagioclase and pyroxene in the graniotid reacted in the presence of water to generate epidote. A pronounced foliation is cut by no less than 3 separate sets of fractures, two of which are filled in with fibrous quartz, and another by something dark. The granitoid formed during the Mesoproterozoic Grenvillian Orogeny, and was deformed later in that same episode of mountain building. The fractures formed at some point after that: just when, I can't say. Vein sets 1 and 2 are infilled with apparently identical compositions, which would be consistent with them being contemporaries. Vein set 3 has something else lining its fractures. At first I thought it was just mildew, but Elli suggested some mineralogical possibilities. Vein set 3 does not show the same amount of dilation as the other two sets. Cross-cutting relationships show vein sets 1 and 2 cross-cutting vein set 3, which suggests I was too hasty in labelling them in my photo. "3" is the oldest; "1" and "2," despite their names, are younger. Maybe they're related to Neoproterozoic breakup of Rodinia, or Alleghanian mountain-building, or uplift? So many mysteries...

More to come on this topic, surely, as I get re-introduced to my local national park.
__________________________________________
* Southworth, Scott, Aleinikoff, J.N., Bailey, C.M., Burton, W.C., Crider, E.A., Hackley, P.C., Smoot, J.P., and Tollo, R.P., 2009, Geologic map of the Shenandoah National Park region Virginia: U.S. Geological Survey Open-File Report 2009–1153, 96 p., 1 plate, scale 1:100,000.

Labels: blue ridge, conferences, granite, igneous, metamorphism, national parks, proterozoic, shenandoah, structure

On his popular science blog Pharyngula, PZ Meyers has a regular series of posts called "I get email," (example) wherein he discusses e-mails he gets. I get e-mail, too (as I'm sure, so do other science bloggers of all stripes). Here's one I got the other day from Brian, a recent graduate from one of my many almae matres (oh yeah, I took Latin). I post it here in case anyone else is wondering the same thing:

I have a simple question for you... I was out at the Pimmit Run-Potomac
confluence collecting rock samples with that awesome chlorite/pyrite/garnet
assemblage and I encountered a couple pieces of unakite float. I'm just
wondering about its provenance. Your blogs seem to indicate that unakite is
typically found in situ farther west in the Shenandoah which would be a pretty
long way to travel (and pretty cool too!) although I believe there is Antietam
around Mather Gorge so I guess it's not impossible; unless it was
anthropogenically relocated which would be much less cool. A little insight
would be greatly appreciated so I can wow my friends when describing what is now the piece de resistance in my fish tank.

So I wrote back with this (links are additions, since I'm blogging it):

Yes, you could certainly have found some Blue Ridge unakite as float in the Potomac Gorge. I've seen many other Blue Ridge Formations as float on the bedrock terraces of the Potomac: Catoctin Formation, Harpers, Weverton, Antietam (like you mentioned), and something that looks a hell of a lot like the Old Rag Granite. I've found well-rounded bituminous coal cobbles, too! I've found unakite further out, in the Coastal Plain, as well as blue quartz (which is unique to the Blue Ridge). So I think it's quite likely you could have found some unakite.

Anyone else have any questions? Like PZ, I could make this a regular series. The more local and the more geo-centric, the better.

Labels: blue ridge, coastal plain, i-get-mail, metamorphism, piedmont, rivers, sediment

On our hike to Grinnell Glacier this past July in Glacier National Park, I found lots of cool cobbles of float, mainly of the Mesoproterozoic metasedimentary rocks that make up the bulk of the park: the Belt Supergroup. One of these formations, the Helena Formation, is intruded by a diorite sill known as the Purcell Sill. It's a prominent rock unit showing up as a black stripe within the lighter-colored Helena Formation, exposed high on the glaciated walls throughout the park. Occasionally, you'll find pieces of it as float, and I noticed that the higher we climbed up, the more of it we saw. Here's one of my favorites among these pieces of the Purcell Sill:


This cobble shows beautiful slickenlines, small gouges into the rock as neighboring rock ground across its surface, along a fault. These physical gouges are decorated with a chemical accoutrement: the metamorphic* mineral epidote, which is a gorgeous grassy green.

Labels: faults, igneous, metamorphism, minerals, montana, national parks, proterozoic, structure

Following on from Sunday's post showcasing new outcrops seen recently along the Billy Goat Trail, here's a cool ptygmatically-folded quartz vein I saw:



Can't quite make it out? The boulder's kind of weathered, so let me highlight it for you:


...and a close-up of the left side, which is better exposed:


That's all I noticed that was new this time around... but next time I'm sure there will be something else. The Billy Goat Trail is the gift that keeps on giving...

Labels: maryland, metamorphism, piedmont, structure

Went for a hike on the good old Billy Goat Trail last Sunday and saw this beautiful outcrop. I love it how every time I walk that trail, I see something new and blog-worthy. Here you see the metagraywacke of the Mather Gorge Formation getting squished and squeezed under conditions of partial melting. Granitic magma (light-colored rock) is leaking out, while the foliated mafic residue (schist chips) are getting strung out and boudinaged under conditions of mountain-building. This granite yeilds late Ordovician isotopic ages (Taconian Orogeny, ~460 Ma).

Seeing an outcrop like this reminds me of making cheese: squeezing the liquid whey (felsic magma) out from the solid curds (higher-melting-temperature solid minerals like those comprising the 'schist chip' boudins). As orogenic forces squeeze from the sides, granite oozes out the top.

I love that there are outcrops where this process is caught in freeze-frame: not all the granite escaped from its migmatitic source rock here; instead the process stopped before it was complete, and through the luck of uplift and exposure by the probing erosion of the Potomac, we get a glimpse of a fundamental process in making the Earth look the way it does. A single outcrop shows rocks that were oceanic sediments, then became metamorphic schist, and now are were transitioning to igneous granite! That's pretty wild. We have caught the rock cycle red-handed.

Labels: analogies, granite, igneous, maryland, metamorphism, piedmont, primary structures

(Parts 1, 2, 3 & 4 of this series...)

Today we'll look at some of the structural geology photos I took in Hanging Canyon, Teton National Park, Wyoming. These are all rocks of the Archean-aged Wyoming Terrane (or "Wyoming Craton"), one of the most ancient pieces of crust that make up the quilt-like North American continent. They include both metamorphic and igneous rocks that have been suffered enjoyed being deformed by tectonic processes.

Labels: archean, igneous, metamorphism, national parks, nova, structure, travel, wyoming

This is another photo from Saturday's hike. Unakite is rumored to be the 'state rock' of Virginia, though it's not in the state code. Regardless of its official status, it sure is a distinctive sight: An epidotized granitoid, unakite is identified by the distinctive pairing of pistachio-green epidote and pink potassium feldspar. There's some grey/purple quartz there too. In the mid-Atlantic states, it's only found in the Blue Ridge geologic province. Here, on the trail below Dark Hollow Falls in Shenandoah National Park, my friends and I encountered this lovely boulder of unakite bearing a vein of milky quartz.

The original granitoid was Grenvillian in age, about 1.1 billion years old. Presumably the metamorphism took place during Alleghanian mountain-building, between 300-250 million years ago. Unakite has been quarried in Virginia for use as a building stone, and can be seen as tiles on the first terrace of the steps leading from the National Mall up to the southern doors of the Smithsonian's National Museum of Natural History in Washington, DC.

Labels: blue ridge, dc, igneous, metamorphism, minerals, museums, smithsonian, virginia

A quick sketch of a glacial boulder that I showed you two days ago...



Here's what caught my eye:



What else do you see here?

Labels: granite, metamorphism, new york, structure

On Saturday's Bedrock Geology of Washington, DC class, my students and I had the good fortune to stumble upon two geologists out doing field work: Tony Fleming, lead author of the geologic map of the Washington West quadrangle, and Steve Self, senior volcanologist with the Nuclear Regulatory Commission. They were out looking at the Sykesville Formation at Chain Bridge Flats, assessing a potential reinterpretation of the unit.

Fortunately, they were willing to take a little time and discuss their findings with the students. Here's a couple shots of Steve talking to the group:




I joined Steve and Tony in the field yesterday (Monday) too, looking at some outcrops on the other side of the river, and trying to make sense of them. Fun stuff! More on that at a later date...

Labels: dc, field trips, geologists, geology, igneous, metamorphism, nova, piedmont, sediment, volcano

So, it's a month until my Rockies class starts. I've been encouraging all the students to get in shape, because the high elevations, rough terrain and multimile distances we'll be hiking in Montana and Wyoming could really kick an east-coast flatlander's arse. So we've scheduled a few training hikes to help everyone physically prepare for the Rockies experience. Last weekend, we did a 5.5-mile circuit up the steep Little Devil's Stairs trail in Shenandoah National Park. I was joined by five Rockies students + one of their kids. Here's a map of the loop we did:



Here's a few photos of the hike, and the geology we encountered along the way:



John poses next to some jointed columns in the Catoctin Formation, a Neoproterozoic rift-related series of flood basalts (subsequently metamorphosed during Alleghenian mountain building).


End-on view of one of the columns:


Overhanging cliff showing columns weathering out along jointed surfaces:


Bob poses next to a cliff, helping me demonstrate how difficult it is to take a well-exposed photo in the jungle of the Virginia hardwood forest:


A wiggle in some columns:


Jared thought these columns were better than the first ones he saw, at Old Rag Mountain.


Here's me with a fifteen-foot-long section of columns, indicating that the flow from which this boulder was derived must have been at least fifteen feet thick, maybe more:




But it wasn't all columns. There was also a lot of column-less massive Catoctin Formation, and some nice inter-flow conglomerates which are interpreted as stream deposits that developed atop a cooled flow before the next flow erupted. These conglomerates imply a reasonable amount of time passed between successive eruptions of the Catoctin flood basalts. The lichens obscure the rock, but note for instance the fingernail-sized chunk of greenstone an inch above my hand:


More chunks in the conglomerate:


And more:


Jared guards the way forward:


The view from the top:

Labels: basalt, iapetus, igneous, metamorphism, nova, primary structures, proterozoic, sediment, shenandoah, structure, teaching

I've been meaning to go check out the Soapstone Valley for years, but finally got around to it on Memorial Day. The park is a valley that shoots off to the east from Rock Creek Park, with an eastern terminus at Connecticut Avenue:



I didn't have far to walk before I found my first cobble of soapstone. It felt soapy in my hand, and was easily scratched by my fingernail. (Fingernail = 2.5 on the Mohs scale of hardness; talc = 1) I found it interesting that the soapstone cobbles had less algae growing on them than the other cobbles in the stream... Hmm. Because they slough off their outer layers more easily? Or because there's something chemical going on that prevents algae growth?


Why does anyone care about soapstone? Well, people who care about prehistory are interested in soapstone because it was easily carved to make various artifacts. As a geologist, I'm more interested in it because it's a metamorphic rock that implies an ultramafic protolith. In other words, as the various rocks that would become DC's bedrock were squished and squeezed and heated during Taconian mountain-building, one of the ingredients in the mix may have been a peridotite. As the graywacke around it metamorphosed to metagraywacke, the putative peridotite metamorphosed into soapstone.

The stuff I found in Soapstone Valley is a talc schist with porphyroblasts or relict phenocrysts of something dark and chunky in it:


Here's a close-up. The big crystals were dark green, like augite, but they had a texture that looked more like hornblende. Not sure as to their identity. I'll put one under the microscope later to try and suss out the relationship between the cleavage planes.


They're definitely mafic though! Here's an example where the large crystals are rusted out:


So there was plenty of soapstone float, but no bedrock outcrops. At first, I was in the highly foliated metagraywacke schist of the Rock Creek Shear Zone...


...but as I headed upstream I found boulders of the Kensington Tonalite, implying exposures of the KT further up the valley...


... and sure enough, that's what I found. This is the Kensington Tonalite, a late Ordovician granitoid.


Where I first crossed the contact, I thought it looked a little odd, and then a later look at the geologic map of the Washington West quadrangle (Fleming, et al., 1995):

Fleming, et al., list it as a sheared biotite tonalite of the Georgetown Intrusive Suite, which I guess explains its appearance as distinct from the Kensington Tonalite.

When I got up to the eastern edge of the park, I saw the source of the stream:


The valley widens out here, almost as if the rock is weaker... And where concrete has been poured (to stabilize the slope??) the underlying rock is etched away: it's the super-soft soapstone...


Here's a boulder of soapstone (my fingernail scratches it to demonstrate that it's soft):


Here's the geologic map of the area. You can see Soapstone Valley cutting an east-west swath across the strike of the structures. ("ss" means "soapstone"...)

My annotations on Tony Fleming's map (reference below).

Reference:
Geologic map of the Washington west quadrangle, District of Columbia, Montgomery and Prince Georges Counties, Maryland, and Arlington and Fairfax Counties, Virginia. Anthony H. Fleming, Lucy McCartan, and Avery Ala Drake. U.S. Geological Survey (Reston, VA), 1995.
_________________________________________________________________

A quick tangent to note a milestone: this is my 700^th post on NOVA Geoblog. Thanks to everyone for reading. Looking forward to 700 more...

Labels: appalachians, dc, granite, igneous, metamorphism, minerals, ordovician, piedmont

Last Saturday was my Field Studies in Geology trip to Shenandoah National Park. Here's a few shots from the day's geologizing...

Garnets in the Pedlar Formation granite gneiss, oldest rock in the park at ~1.1 Ga.


Meta-basalt columns of the Catoctin Formation (photo by Mathina Calliope):


At the end of the trip, I have the students order a series of strips of paper with different geologic events in the park's long geologic history. They have to figure out the proper order based on what they learned that day:





Lastly, a group photo overlooking the Browntown Valley:

Labels: field trips, metamorphism, minerals, national parks, nova, shenandoah, teaching

After seeing the feldspar megacrysts in Maryland's Ellicott City Granodiorite two days ago, I wanted to share some even more impressive megacrysts, those found on the periphery of the Cathedral Peak Granodiorite pluton ['CPGD'] in California's Sierra Nevada mountains.

Here's a typical look at the CPGD close to its contact with metasedimentary & metavolcanic host rocks. It's chock-full of 3-7 cm crystals of potassium feldspar, set in a more typical-looking granodioritic matrix of sub-0.5 cm crystals:

This is a nice example of an intrusive porphyry. Not all porphyritic textures result from two phase cooling: The way the story usually goes is that the magma starting underground at a realtively slow rate, then the magma (solid crystals + remaining liquid) gets tapped and erupts, with the rest cooling at a faster rate on the surface. This one clearly shows a phaneritic (coarse-grained) texture throughout; it's just that some crystals grew bigger than others. I'm not an igneous petrologist, so I won't claim to understand why. Enlighten me if you know.

Here is a close-up of one feldspar crystal shows lines of mafic inclusions (earlier-crystallizing minerals like amphibole which were caught up in the advancing front of feldspar crystallization, and trapped in the larger feldspar crystal):

My mind wants to see this as a spiral pattern, like a snowball garnet, and hence to interpret this as a feldspar crystal rotating as it grew, but that's surely wishful thinking. Especially seeing as how there's no foliation to get wrapped up in the 'rotating' porphyroblast. But... I've never seen another igneous crystal that shows this same pattern. Anyone else? Trick of the light?

Now here's something really wild:

Recall that when I took these photographs in 2003, I was out in the Sierras looking at the Sierra Crest Shear Zone, a 1-2 kilometer wide zone of smooshed rocks adjacent to the eastern boundary of the Sierra Nevada Batholith. So mainly I was interested in these older "host rocks" which were metavolcanic and metasedimentary, but I was also interested in how they related to the batholith as a whole. In places, I could see clear evidence that the plutons of the batholith were sheared, too, and in other places they appeared to have intruded post-deformation. This photo shows that the Cathedral Peak Granodiorite came along after the bulk of the deformation had happened.

How do we know? (1) It's not especially foliated itself. (2) Here, magma oozed between the foliation layers in the metasedimentary rocks immediately adjacent to the pluton. These layers flexed to allow the magma to intrude; I think of curtains billowing underwater. Then, as the pluton inflated (or as regional deformation continued to squeeze these rocks; or both), a compressive stress was exerted on these mingled layers of foliated rocks and magma. The liquid magma squished out of the way, but the solid megacrysts were trapped, and the foliation flexed and wrapped around them.

Twisted food analogy: Say I make a peanut butter and raisin sandwich. (Seriously, they're good!) I have a piece of bread, and I smear it with a mix of creamy peanut butter and chunky raisins (the giant ones from Trader Joe's). I place another piece of bread on top. Then, because I value my geology more than my manners, I lean over like I'm going to perform CPR, and exert pressure perpendicular to the plane of the bread. The peanut butter, being ductile, squishes out the sides, while the raisins are trapped, and the bread deforms around them.

Such, such are the thoughts of the hungry field geologist...

Labels: analogies, california, granite, metamorphism, minerals, primary structures

The other day I mentioned the Setters Schist.

Here's a couple of cobbles of the same formation, but lower stratigraphically than the stuff we saw on the University of Maryland petrology trip. The basal Setters has beautiful metamorphic tourmalines lying willy-nilly within the plane of foliation:







According to Mindat.org, "the general formula for this group may be written:

• A = Ca, Na, K, or is vacant (large cations);
• D = Al, Fe²⁺, Fe³⁺, Li⁺¹, Mg²⁺, Mn²⁺ (intermediate to small cations - in valence balancing combinations when the A site is vacant);
• G = Al³⁺, Cr³⁺, Fe³⁺, V³⁺ (small cations);
• T = Si (and sometimes minor Al³⁺, B³⁺);
• Y = O and/or OH; and
• Z = F, O and/or OH."

Note the constant there: boron! ...A lot of boron! Three boron atoms per unit cell... These metamorphic rocks have a sedimentary protolith. Where did the pre-metamorphic sediments get all that boron from?

Any ideas?

Labels: geology, maryland, metamorphism, minerals, piedmont

After we had collectively collected a hundred pounds of samples from Mineral Hill, the final stop on the University of Maryland petrology trip was in scenic Ellicott City, Maryland, where we visited the Ellicott City Granodiorite (map to outcrops).

Like everything else on this trip, the ECGD is intimately tied in with the Taconian Orogeny (late Ordovician; caused by the collision of ancestral North America with a volcanic island arc in the Iapetus Ocean basin). However, unlike the Port Deposit Tonalite we looked at early in the trip, this one crystalized from magma at 435 +/- 15 Ma (U/Pb in zircon). It is not only much younger than the PDT, but it's also pretty young even for the Taconian Orogeny, which reached its peak around 460 Ma.

It's more potassic than the Port Deposit Tonalite, as these K-spar 'megacrysts' show:


This potassium feldspar 'megacryst' shows internal growth laminations, as small mafic bits got caught up in the growing feldspar crystal, which consumed and included them:

Not only does this help us see how the feldspar crystal's habit is a reflection of its internal structure, but it's also an example of the principle of relative dating by inclusions, expressed in a single mineral crystal! Pretty cool.

As with the PDT, xenoliths may be seen in the ECGD:


Parts of it are equigranular, and parts of it are highly foliated:


And of course my eye is always drawn to the structures, like these small faults offsetting dikes of granite which cross-cut the ECGD:




The real prize with the Ellicott City Granodiorite is to view first-hand the magmatic epidote it bears:


Most epidote is metamorphic. However, as Zen and Hammerstrom (1984) showed that epidote could also crystalize from a late-phase magma as the melt interacted with hornblende at high pressures (8 kbar; roughly 30 km depth). You'll note in the photo above the intimate association between the epidote and the hornblende. (I'm not super-confident on my titanite identification, by the way; this rock also bears similar-looking allanite. Please correct me if I'm clearly wrong.) E-an Zen has guest-posted to this blog before, and once upon a time he tasked me with searching for magmatic epidote near Haines, Alaska, in 2006. I didn't find any, but it did pique my interest. So it felt good to be able to finally see some of this rare beast. I was surprised to find it locally, considering the the original magmatic epidote paper referred mainly to west coast plutons from California to Alaska. I was also suprised because of the tremendous depth of crystallization it implied: 30 kilometers down? Wild! I collected a sample for the NOVA lab.

Thanks again to Rich Walker and Roberta Rudnick for graciously hosting me on this trip. I learned a lot, and I'm greatful for the opportunity to expand my local outcrop knowledge.

_________________________________________________________________

Reference:
E-an Zen and Jane M. Hammarstrom (1984). "Magmatic epidote and its petrologic significance." Geology, September 1984. Volume 12, no. 9, p. 515-518. DOI: 10.1130/0091-7613.

Labels: alaska, maryland, metamorphism, minerals, structure, xenoliths

Done with the Cockeysville Marble and fortified with chocolate malts from the Twin Kiss, we ventured on to "Mineral Hill," interpreted as a paleo-black-smoker site from the deep Iapetus. This is a zone of mafic and ultramafic rocks that have been metamorphosed and also mineralized with a suite of sulfide minerals, including pyrite, chalcopyrite, bornite, covellite, and carrollite (in fact, this is the type locality for carrollite). Presumably it was a SedEx-type deposit in the Iapetus Ocean basin. It is geographically associated with the Baltimore Mafic Complex, which is most readily interpreted as a dismembered slice of the Iapetus oceanic lithosphere (that is, an ophiolite). As the Iapetus closed during the Taconian Orogeny, it was accreted to North America and metamorphosed.

The petrology students start picking up pieces from the massive pile of tailings in search of treasures:


Talc shist (soapstone) with malachite:


More of the same:


I forget what this one was, but I loved the "spray" pattern of its bladed crystals:


Chrysotile asbestos:


Pyrite:




And lots and lots of magnetite! These are some of my refrigerator magnets stuck to it:


One more stop to go: the Ellicott City Granodiorite...

Labels: appalachians, maryland, metamorphism, minerals, ophiolites, ordovician, ore

We are now halfway through our documentation of the University of Maryland petrology field trip. As a reminder, we've already seen the Port Deposit Tonalite and the Setters Schist. Today, we meet the Cockeysville Marble.

The Cockeysville is famed in some quarters because of its role in the construction of the Washington Monument. The upper portion (most) of the monument is made of this rock, although it is a purer (higher CaCO3 content) marble than we see here at this outcrop near the Hunt Valley Shopping Mall. Really, this is more of a marble gneiss.

Rich and Roberta talk with the students about this new rock:


The Cockeysville Marble has a well-developed foliation at this outcrop. Impurities in the limestone protolith (probably clay) have metamorphosed into muscovite mica:


Whether these foliations reflect bedding is an open question in my mind. Here's a look at how the outcrop is weathering out. Lovely, just like a limestone in the way it's dissolving away:


It's reasonably coarse-grained:


Here's two close-ups of the stringers of muscovite mica:




Some structural geology was also apparent. Here for instance, is a fault/shear zone


...And here's a fold. It's an overturned fold; Note how the foliation dips at two different angles, though in the same direction:


Still can't see it? Okay, let me help:


The sharp-eyed among you probably noticed the boudinage on the left side of the fold. Here's a portrait of the most prominent boudin:

The most mica-rich domains acted relatively stiffly under deformation, while the calcite-rich domains flowed more easily.

I also found a surface decorated with slickenfibers (crystal fibers growing aligned in small spaces along the surface of a fault):


... and a close-up, so you can see that the opposite block of rock was moving from the bottom of the photo towards the top. Running your finger over this outcrop from bottom to top would feel relatively smooth, while running your finger over it from top to bottom would feel rough as your fingertip would catch on the little mineral "steps":


The Cockeysville is a lovely marble. But we needed lunch at the Twin Kiss drive-in. The day was advancing, and refueling became a serious issue. And then? On to Mineral Hill...

Labels: marble, maryland, metamorphism, structure

Yesterday, I showed you the Port Deposit Tonalite, stop #1 on the University of Maryland's annual ig/met pet trip. Today I'll share pictures of the next stop. We voyaged to the Hunt Valley Shopping Mall, where a lovely exposure of the Setters Schist can be found.

It's a lovely example of a classic-looking muscovite schist:


It is also chock-full of garnets! Millions and millions of them....

Some are small:


Some are medium:


Some are large:


Some are fresh:


Some are weathered:


Some are weathered-out:


There's also staurolite present:




Here's a nice big chunky staurolite:


In one localized zone, we also see some very big, rather lovely kyanite:




...Awesome! I love this suite of metamorphic minerals!

The Setters Schist is a highly metamorphosed pelitic rock (meaning that its protolith was clay-rich). It was presumably metamorphosed in the late-Ordovician-aged Taconian Orogeny, like everything else in the Mid-Atlantic Piedmont.

Next up, another member of the Glenarm Series, the Cockeysville Marble...

Labels: appalachians, maryland, metamorphism, minerals, ordovician, piedmont

Yesterday, I was fortunate enough to be able to tag along on the University of Maryland's petrology field trip, to five locations in Maryland showcasing a variety of igneous and metamorphic rocks. I'd like to thank Rich Walker and Roberta Rudnick for allowing me to come along on the excursion, and UMD graduate student Ryan Kerrigan for alerting me to the trip's interesting rocks in the first place. They have a crew of enthusiastic students, and some cool outcrops!

Our first stop was in northern Maryland's Cecil County. Along the banks of the Susquehanna River, just upstream from the I-95 bridge, is an abandoned quarry of the Port Deposit Tonalite.

Here's Rich and Roberta leading us into the quarry:


UMD students examine the semi-overgrown outcrops of the tonalite:


Tonalites are kind of like granites, except they have only very low amounts of potassium feldspar. This particular tonalite has a magmatic crystallization age of 515 Ma (U/Pb in zircon) and a metamorphic age of 490-480 Ma (Rb/Sr in biotite). Close-up of the rock's texture:


ADDITION: Kim notes in the comments that I didn't draw an explicit connection between the metamorphism and the metamorphic foliation that is so prominent in this photo. She's right: The wavy linear pattern you see in this photo is produced by minerals aligned by differential pressure. Squeeze the rock "top to bottom" and you produce a foliation that runs "left to right."

On the basis of isotopic evidence, the Port Deposit Tonalite is interpreted to have formed as an igneous pluton offshore of ancestral North America, underneath an island arc in the Iapetus Ocean. Later, subduction brought the island arc into contact with North America, triggering the Taconian phase of Appalachian mountain-building.

Here's a closer look at the texture and mineralogy. You can see some k-spar present here, though this was not a common mineral to see at the outcrop...


There were some nice xenoliths present, indicative of the host rock into which the PDT intruded:


Here's a quartz vein cutting through the tonalite. You'll notice that the vein is emplaced approximately perpendicular to foliation, suggesting the same maximum stress which imparted the foliation also extended the rock parallel to the foliation place, opening up fractures that when then fill with the most mobilizable minerals available (in this case, quartz):


If you look closely, you'll see that the fracture which opened up in the tonalite to allow this vein to be emplaced has a ragged edge (not a clean break):


Next up: the Setters Schist...

Labels: appalachians, cambrian, granite, igneous, maryland, metamorphism, ordovician, structure, teaching, xenoliths

After the Garth Run high-strain zone and a night hanging out by the campfire at Heavenly Acres with the William and Mary Structural Geology class, the second stop on our Structural Geology trip was in Shenandoah National Park, looking at the deformed meta-basalt columns on the Limberlost Trail. Longtime readers of the blog have seen these unique (in my experience) columns before, in a post from last May.

This is an outcrop of the Catoctin Formation, a series of (mainly) basaltic lava flows that erupted sometime older than 565 Ma (only the youngest, rhyolitic layers have been dated, and evidence suggested that significant amounts of time may have passed between the eruption of each stratum of basalt deeper down in the stratigraphic stack). As the lava cooled, it developed cooling fractures that formed perpendicular to the isotherms. These fractures likely initiated at the top and the bottom of the flow, and propagated towards the middle over time.

Later, during Alleghenian mountain-building (~300 Ma to ~250 Ma, roughly), the rocks were subjected to greenschist-facies metamorphism, and were deformed. The basalt's consituent minerals re-equilibrated and reacted to become other minerals, most notably chlorite and epidote (both of which are green).

Here's John and Joe checking out the columns:


Exquisite! Even arrest lines on the side of each column are preserved. In an undeformed basalt column, these arrest lines would be perpendicular to the column edge. Here, they have a pronounced angular relationship, indicating the shearing of the overall column:


Bobby measures the angular shear along the length of the column:




Goofball professor poses with column:


Jay plays the column like an electric guitar:


We found some nice plumose structure too:


Finally, we evaluated the concentric rings of minerals filling amygdules (vesicles that had been infilled with mineral deposits after lithification) in an attempt to determine whether they could be used as strain markers, or whether they may have attained their ellipsoidal shapes due to stretching of the bubbles in the originial lava (i.e. like this) and then been infilled with minerals:




...and then we were off to Field Study Area #3...

Labels: appalachians, basalt, field trips, igneous, metamorphism, national parks, primary structures, proterozoic, shenandoah, structure, teaching

A little follow-up to this morning's post of pictures from the Garth Run high-strain zone. A short distance away is the Quaker Run high-strain zone, which I visited in spring of 2003 with my geology graduate advisor, Dazhi Jiang, and two other structure students from the University of Maryland, College Park. Here's a beautiful sample of the mylonite I collected then:

Labels: appalachians, blue ridge, metamorphism, structure

I took my Structural Geology students on a three-day field trip this weekend to examine outcrops in the Blue Ridge and Valley & Ridge geologic provinces. Here's a few photos of the team at our first (of four) field study areas, the Garth Run high-strain zone:

Examining the structure and taking strikes and dips:






Fabric elements cross-cutting one another:


Mylonitic fabric:


Foliation wrapping around a feldspar porphyroclast:


This is kind of interesting: a big pancake (oblate ellipsoid) of blue quartz, with a potassium feldspar in the middle:


And if you zoom in close, you can see that the feldspar porphyroclast is broken in the middle (along the plane of cleavage) with non-blue quartz filling in the gaps:


I think this blue quartz likely formed in the pressure shadow of the resistant feldspar porphyroclast during flattening strain, and eventually that feldspar began to brittlely deform, extending in the direction of minimum principal stress.

Quite a bit of variation across strike:


The students found some nice euhedral garnets too, though this was a block of float from upstream, and not intimately associated with the high-strain zone itself:


More tomorrow, from Field Study Area #2...

Labels: blue ridge, field trips, igneous, metamorphism, structure, teaching, valley and ridge

Almost a week after the field trip to Old Rag Mountain, and the Facebook-hosted pictures keep trickling in. Here's some shots by NOVA student Eileen Lodovichetti, and an ensuing discussion of feeder dikes and supercontinent breakup.

Here's a shot of the upper reaches of Old Rag, showing the characteristic spheroidal weathering of the Old Rag Granite and the relative lack of trees on top:

photo by Eileen Lodovichetti

...And here's a shot that Eileen took which shows the interior of one of the weathered-out feeder dikes we had to hike through on our way to the summit. You can actually see the classic geoprofessorial arm-waving caught in blurry motion!

photo by Eileen Lodovichetti

This is one of the coolest things about hiking Old Rag: after scrambling up on top of spheroidally-weathered granite domes, you drop into these tabular "hallways." The astute observer will note that the floor is made of a fine-grained, dark-green-colored rock, quite distinct from the light-colored, coarse-grained granite that makes up most of the mountain. These are dikes of metamorphosed basalt that intruded the granite during the breakup of the supercontinent Rodinia in the Neoproterozoic era of geologic time.

Here's one of my former Field Studies in Geology students, Mike Nelson, pointing out a similar dike along Skyline Drive, in the main part of the park:


Basically, the story goes like this: Around 1.2 to 1.0 Ga, continental fragments amalgamated into a supercontinent called Rodinia. In Virginia, this is recorded in the rocks of the Blue Ridge province, where the basement consists of granitoids (granites and related rocks) and metamorphosed granitoids (gneisses, mylonites). Among the youngest of these is the Old Rag Granite, which intruded the Pedlar Formation granite gness around 1.0 Ga.

Later, Rodinia broke apart, resulting in an extensional tectonic regime and mafic volcanism. Fractures opened up in the Old Rag Granite and funneled mafic magma towards the surface. Massive eruptions of basalt blanketed the landscape. The resulting layers of basaltic lava are known as the Catoctin Formation. At Old Rag Mountain, we can see some of the plumbing that led to these flood basalt eruptions: these are feeder dikes, because they "fed" the eruption above them.

Because the dikes (which were metamorphosed to greenstone during ~300 Ma Appalachian mountain-building) weather more rapidly than the Old Rag Granite, they are typically recessed into the landscape. That's what makes the "hallways" in the photograph above. Here's two more images, showing these weathered-out feeder dikes:



Check out how there's moderately-developed columnar jointing extending across the dike. These columns form perpendicular to the cooling front, and the dikes would have lost their heat out the sides. In horizontal lava flows, the heat is lost from the top and bottom surfaces, so you get vertical columns. Here, a vertical dike produces horizontally-oriented columns. Hikers appreciate these "steps" as they squeeze through the dikes on their way up the mountain.

Here's a map of part of Shenandoah National Park:


Please ignore the "hover" instructions at the lower right. I've reproduced the "hoverable" image below. Key: the orange is the Pedlar Formation. The pink is the Old Rag Granite, and the green is the Catoctin Formation. Feeder dikes of the Catoctin are shown as green lines.

Now, let's take away the map, and just preserve the orientation of the feeder dikes. This will tell us the overall tectonic stretching direction:
Various plate reconstructions show either Amazonia or the Congo craton offboard of Virginia at the time Rodinia broke apart and the Iapetus Ocean began seafloor spreading. I've illustrated it here as the Congo, but that might be wrong.

So: the hike up Old Rag is great exercise, and offers scenic views, but for those willing to consider the rocks and how they got there, it's an insightful view into the tectonic past.

Lastly, here's a lovely, well-developed weathering rind on the Catoctin meta-basalt (greenstone). When the dark green rock adjusts to the conditions at the Earth's surface, it breaks down, resulting in the tan/"buff" color on the outside. You're watching the rock "rot" from the outside surface, working its way inward:


More on the geology of Shenandoah National Park can be seen at this page on my website.

Labels: appalachians, basalt, blue ridge, field trips, metamorphism, mountains, national parks, nova, plate tectonics, shenandoah, structure, weathering

The backlog of photos from my hikes several weeks ago still looms. I've showed you exotic cobbles, migmatites, graded beds, flood debris, and boudins, now for some folds...

As with the others, these are images from the Maryland Piedmont, along the Billy Goat Trail in C&O Canal National Historical Park.

Here's two repeats that fall, Venn-diagram-like, into the overlap area between the "graded beds" theme and the "folds" theme:




Now for some fresh, never-before-seen images:







(that's a fold cut twice oblique to its axis, resulting in an elliptical outcrop pattern).

Tiny folds:


Folds in one direction (top to bottom); boudinage in the perpendicular direction (left to right):


Found this one on the side of a cliff I probably should not have been scaling:


That's all for now... have a good Tuesday!

Labels: igneous, maryland, metamorphism, national parks, piedmont, primary structures, structure

I'm returning now to the slew of new images I shot a couple of weekends ago on the Billy Goat Trail (BGT). Previous posts from these back-to-back morning hikes here, here, here, and here.

Today's theme: boudinage, the stretching & breaking of more competent rock units, and the gaps in between the 'chunks' filled in with less competent (more 'flowy') rock units, or by magma or other fluids. It's a behavior that's neither purely brittle nor purely ductile, but somewhere in between.

Boudinage of granite in metagreywacke:


Ditto (although some of this looks closer to hydrothermal quartz than granite, but there is some K-spar present...):


Felsite boudins in amphibolitic gneiss:


Pretty cool here; you can see that fluid magma filled in the gaps between the boudins. When this boudinage happened, the surrounding amphibolite was too viscous to flow into the gap. Furthermore, the asymmetry of these granite-filled tension gashes indicates some shearing: Was it a sense of shear that was concurrent with the boudinage (top to the left)? That was my initial take, but Kim (in the comments) suggested an alternative, which I like more and more: initial boudinage, and then later shearing in the opposite direction (top to the right). See the discussion in the comments section for more insight...



Some of the weirdest rocks on the Billy Goat Trail are these ones near Trail Marker 2. They are coarsely layered by composition, but I'm not able to figure out quite what the heck is going on with them. Is it just a gneiss with compositional banding ~3 inches thick? Regardless, it shows boudinage, both in horizontal cross-section...



...and in vertical cross-section:


When a rock gets boudinaged in two directions, it records flattening strain perpendicular to the plane of foliation, and goes by the colorful moniker "chocolate table boudinage." (Think of a Hershey bar's grid-like segments. If you smashed your hand down on it, the square chunks would separate from another and move apart, perpendicular to the direction in which you're pressing on it.)


Here's a quartz vein (cross-cutting metagreywacke) that's been boudinaged:



Part of this vein is milky quartz (on the left: white & easy-to-see), but part is transparent quartz (looks kind of grey in outcrop; difficult to see against a grey host rock), so I've used the wonders of Photoshop to turn that portion white, too, in this modified image:



Here's a new boudin that I never had seen before, on a diversion trail off the main C&O Canal towpath due to a breach in the Canal after Tropical Storm Hanna last year:


Lastly, here's something new (to me) that I found on my hike. It's a gigantic boudin of amphibolite in the foliated felsic rock showing chocolate-tablet boudinage that I showed up above. Unadulterated photo:


...And with annotations:


This is a big, angular block of amphibolite (about 1.5 m across) that has the foliation of the "gneiss" wrapping around it. Along strike of the foliation, there are two big rusty square holes, where I interpret other big boudins of amphibolite have weathered out. (As I showed the other day, the granite stands up signficantly better to weathering than does the amphibolite.) I was somewhat astonished to recognize this as a big boudin: it has very crisp edges, and is huge in comparison to other boudins that I am familiar with. Neat-O! I'm going to take my structural geology students here in a couple of weeks and have them examine and interpret these structures.

Labels: geology, igneous, metamorphism, piedmont, structure

Picking up where I left off last week with cool new pictures of rocks from the Billy Goat Trail, today we examine igneous beasties...

As you may have picked up from previous posts on this blog [e.g. here], the rocks of the Piedmont province are essentially the mangled remains of an ancient ocean basin: deep sea sediments, oceanic crust, volcanic islands, even microcontinents -- and all were crushed between North America and Africa during the mountain-building that closed the Iapetus Ocean and formed the supercontinent Pangea. Along the Billy Goat Trail, Piedmont rocks are exposed that started off as deposits of mud and dirty sand, but then were metamorphosed during mountain-building. From the bottom of the ocean to the center of a mountain belt: that forces rocks to change. In some places, they heated up so much that they began to melt.

When rock partially melts, but then the melt crystallizes in places (i.e., it doesn't completely drain out of the source rock), we call it a migmatite. The Billy Goat Trail has some spectacular exposures of migmatite. Here's three shots from the downstream end of the trail:







If migmatitic rock rips open while it is in this partially-molten state, that generates cavitites that the fluid magma flows into and fills. Here, for instance, you can see a rip in the foliated migmatitic metagraywacke that is filled with granite.


Further away from the source rock, mobilized magma can fill in planar fractures that cut across older rocks of many varieties. These cracks are filled in with magma that cools into igneous rocks, and we call them dikes. Here is a new dike I discovered on my hike last week: a vertical dike of granite about one foot thick, cutting across non-migmatitic metagraywacke:


Here's a granite dike cutting amphibolite; weathered out in high relief:


Same dike, from a slightly different angle (I leaned over to the left), to show how it pokes up above the amphibolite like a little wall:


Metamorphosed (some epidote present) granite dike cutting amphibolite:


These fractures didn't open up wide enough to admit large volumes of fluid (either magma or hydrothermal solutions), but there was some fluid flow along them. How do we know? The rock immediately adjacent to each crack weathers out in high relief, suggesting a higher proportion of stable, tough minerals (like quartz). [We've seen this before.] The base rock here is fine grained amphibolite.


Contact between a small granite pluton (or a large dike?) and neighboring amphibolite:


Tension gash in amphibolite, filled in with a mix of potassium feldspar and quartz:


Xenoliths of foliated biotite-rich rocks which I interpret to be metagraywacke that has had all its felsic melt expressed from it, then ripped off by the growing granitic magma chamber (stoping) and dropped into the magma (relatively low temperature, so the biotite doesn't melt), and rotating around to new orientations which do not match the regional foliation orientation. I'm seeing these as shreds of the 'depleted' migmatitic source rock...


Closer-up of these xenoliths #1:


Closer-up of these xenoliths #2:


Another cool thing I saw on last weekend's hikes was pegmatite. Pegmatites are present where there is a particularly watery magma. Water, the universal solvent, helps act as a courier, ferrying atoms around to where growing crystals can access them and add to their bulk. As a result, pegmatites are characterized by really large crystals. These potassium feldspars are highlighted by lichens which grow at the interface between the feldspars and the surrounding milky quartz:


Those same black-colored lichens can also highlight the cleavage planes of the feldspars:


Another big-ass K-spar:


...and another:


I love this stuff. Hope you enjoy these igneous treats as I much as I enjoy sharing them.

Labels: igneous, maryland, metamorphism, piedmont

I mentioned seeing some cool stuff when I went hiking on the Billy Goat Trail last weekend.

One of the things that really caught my eye were multiple new exposures of graded bedding. These rocks began as deposits of sediment offshore from a volcanic island arc: they consist of turbidite deposits that were then squished and squeezed as that volcanic island arc collided with eastern North America during the closure of the Iapetus Ocean. As a result of this, they were metamorphosed and deformed. But in a few places, you can still see the relict graded beds that originated through the settling out of turbidity currents.

Here's some images:

I count four or five here:





A nice central fault zone displaced the central block downward:




This one is a little more subtle...


Here's one that's been turned upside down (by tectonics):


And there were also some folded examples:




A close-up of the hinge of this folded graded bed:


Pretty cool, eh? The only problem is these samples aren't on the Billy Goat Trail itself, which means I'll really never be able to show them to students except in photographs...

Labels: geology, maryland, metamorphism, national parks, piedmont, primary structures, sediment, structure

Labels: granite, metamorphism, piedmont, quartz, structure, virginia

Walking to my car the other day, I looked up at the embankment on my street, and noticed some geology there I hadn't seen before. Yesterday, with my camera, I climbed up the embankment (~15 feet) to investigate. Fortunately there were some trees to hold onto.

Sure enough, it was as I suspected: dikes of granite (subvertical in orientation) that, along with the schistose bedrock they cut across, had totally weathered to saprolite.

Keys for scale:


Originally, these dikes were emplaced during the late-Ordovician eastern-North American episode of mountain-building called the Taconian ("Taconic") Orogeny. Later, when they got exposed at the surface (or close to it) they began to "rot."

Hand for scale:


Here's a video showing how readily these dikes formerly known as granite deform by crumbling into pieces:


The main chemical weathering process that has happened here to make this possible is the hydrolysis of feldspar to produce kaolinite, a clay mineral. Large single crystals of potassium feldspar in the granite are now large amorphous masses of kaolinite, which has no strength when stressed.

Labels: dc, granite, igneous, metamorphism, ordovician, weathering

This weekend, my MSSE advisor John Graves was in town, and I took him out to a couple of field locations that I bring geology students to. We started off on Saturday afternoon on the Billy Goat Trail, where I went through the usual rigamarole, what with the Iapetus Ocean, Taconian Orogeny, migmatites, and what-not.

We also saw some cool fractures involving quartz, in two different situations, each instructive in its own way.

First, here at the base of the legendary "Traverse," is some metagraywacke that has fractured. Quartz-rich fluids flowed along these fractures, and the quartz they precipitated (presumably in interstitial spaces between grains?) made that particular zone on either side of the fracture more resistant to weathering than the non-quartz-infused metagraywacke. This "fortifying" effect falls off with increased distance from the fracture. Note that you can actually see the crack in each of these high-relief ridges; it's not a quartz vein per se, but a separate, related phenomenon. Penny for scale in both photos below -- one zoomed out, one zoomed in...





Second, check out these photos, of a spot near the downstream end of the Billy Goat Trail, where usually I don't have time to take students. The bedrock here is a migmatitic schist/gneiss. Here, you'll see ~vertical foliation cut by a ~horizontal quartz vein. Once again, a penny is for scale (this time held in place with some chewing gum, as the outcrop surface is vertical, striking at a right angle to foliation). These two structures are both representative of the same stress regime. With a dominant (tectonically-induced) stress directed ~horizontally, the various minerals in the original rock rotated (or grew) into new positions perpendicular to that stress (e.g., ~vertical). But that wasn't quite enough to accomodate the ~horizontal shortening. Some additional strain was accomodated by ~vertical extension through fracturing. That fracture was infilled with hydrothermal fluids that precipitated "milky" quartz, at almost a perfect right angle to the foliation:





John was suitably impressed, and we both appreciated the afternoon hike in EXCELLENT weather (55 degrees F; gorgeous!).

Labels: metamorphism, msse, piedmont, quartz, structure

Last Friday was the first day of Structural Geology at George Mason University. Though I'm a full-timer at NOVA, GMU talked me into teaching Structure this semester, too. I've done this once before -- my first job out of graduate school, in fact. Then (in 2005), it was very stressful for me, and I'm not sure that I did a very good job. Now, though, I'm much more confident as an instructor, and I feel like I've got a better grasp of some of the essential ideas and techniques: both structural and pedagogical.

For the first day of class, I took a page from Kim of All My Faults Are Stress Related, who recently described a simple but effective "first day of structure" exercise in a post. Inspired by this idea of nurturing structural curiosity right from the start, I gathered up a collection of 36 samples of deformed rocks (plus a few non-deformed ones as "decoys") and laid them out on tables in our classroom:

Most of them were samples from my personal collection, which resides in my office at NOVA, but there were NOVA teaching lab samples too, and I added a few more interesting ones I found at Mason, like this ptygmatic fold in a granite dike:

The instructions to the students were twofold: First, visit each sample and describe it as fully as possible, noting in particular its "structural significance" (which I declined to define more explicitly). Then, once everyone had done that, get together as a whole class and organize these samples into groups based on common features. How many groups? Which features? They had to decide.

I took as my mantra a quote my friend Bridget (a writing instructor at NOVA) found:

"Teaching should be as experimental as writing." -Donald Murray

So I was conducting an educational experiment...

Starting the class in this way felt unfamiliar to me -- everyone "knows" that the first thing you're supposed to do is distribute the syllabus and spell out the gameplan for the semester. Or perhaps start with an introductory lecture. So it was kind of eerie and uncomfortable for me to sit still and quiet off on the side while a roomful of eager students (that I had only just met) went to work.

I sat back and made observations. One student was miming squeezing and stretching rocks with his hands -- "replaying" the stresses that he interpreted must have acted on the rocks to leave behind such structures. (Kim has another post up, just today, about the role of gesturing while teaching and learning geology.) I was pleased when (umprompted by me) they started using supplies like hand lenses, rulers, percentage charts, and hydrochloric acid to quantify the samples' characteristics.

Another student picked up a metaconglomerate with stretched pebbles whose boundaries were somewhat indistinct. His pen moved over the surface of the sample, visually tracing out the place where one stretched pebble stopped, and the next began.

Later, a student set aside a chunk of slate with plumose structure on its surface. With raised eyebrows, he said, "I can't say much about that!" A few minutes later, the sound of stippling resounded in the room as one student sketched a grainy sample.

Periods of quiet work were interrupted periodically with joking commentary. The students in this class (mostly guys) appear to have really bonded with one another during previous geology classes. They are all seniors, with the exception of one geography graduate student. It's good to see that they are comfortable with one another.

During the groupwork portion of the exercise, when the students were organizing the samples into clusters based on shared characteristics, I continued my silent observations. "Let's organize them by stress direction," one student said. "But not fault direction?" asked another. "How about directionality, regardless of what it's direction of," came the reply.

They ended up choosing these titles for their groups: "Slickensides," "Bends and folds," "Smashed together," "Tension," and "Undeformed." It was cool to watch this process play out. I had put out one sample of tension gashes in a limestone (extensional fractures infilled with calcite). The sample was one of the few that I had labelled. That went into the "Tension" group, of course. But what about that other sample with the quartz veins? Was that the same kind of thing? It's a different mineral...

The most classic exchange went like this:

Student 1: "I'm confused."
Student 2: "It [the organizational system] made sense at first."
Student 1: "...Like a lot of organizational systems in geology!"
(laughter)

Finally, once consensus has been achieved, we all walked around to the various piles of rock and I talked in a general sense about the structural importance of each one. The students appeared to be pretty engaged with this discussion: after all, they had invested some serious time in trying to figure these samples out; now they wanted to know what they really meant. My discourse on the samples stretched to about an hour. All told, the whole lab, grouping, and ensuing discussion lasted about three and a half hours. I felt really good about the exercise as a way of generating structural thinking during our first few moments (and hours) of class. I preferred this way of starting class to the traditional approach.

Satisfied that we were off to a good start, I passed out the syllabus.

Labels: igneous, metamorphism, sediment, structure, teaching

As I mentioned yesterday, the Virginia Department of Geology and Mineral Resources has an excellent rock garden outside their office in Charlottesville, displaying a diverse suite of large rock samples from across the state's five physiographic provinces.

Here's Rick Diecchio (George Mason University) providing a sense of scale for the rock garden:


Here's a few of the samples that caught my eye, with my shoe providing a sense of scale (size 12, specifically) in each image...

Aquia Formation sandstone with Turitella fossils (Paleocene); King George County:


Balls Bluff Siltstone with mudcracks (Triassic); Culpeper County:


Conococheague Formation collapse breccia (Cambrian); Augusta County:


Cranberry Gneiss (?) showing well-developed lineation (Mesoproterozoic); Grayson County:


Kyanite quartzite (probably Ordovician metamorphic age); Prince Edward County:


Fossil Sigillaria tree trunk from the Wise Formation (Pennsylvanian); Wise County:


Unakite, the state rock of Virginia according to some (Mesoproterozoic); Rockbridge County:


Here's a link to the PDF (1.82 MB) with all the details about all the rocks in the garden, an impressive achievement just like the symposium.

Labels: conferences, fossils, igneous, meetings, metamorphism, primary structures, sediment, virginia

Dave at the Geology News blog is hosting this month's Accretionary Wedge on the topic of "favorite places to do field work."

My favorite place to do field work is in California's "range of light," the Sierra Nevada.

I did my geology master's field work in the eastern Sierra, along the Sierra Crest Shear Zone, a major high-strain zone which parallels the eastern edge of the Sierra Nevada Batholith through older meta-sedimentary and meta-volcanic host rocks.

In 2003, I spent the summer out there, starting with my first field area at lovely Gem Lake:

An angular unconformity can be seen in this image as the tilted (close to vertical) metasedimentary and metavolcanic rocks (orange and gray) are overlain by dark colored "Tertiary" basalt flows. A big talus slope of basalt chunks makes a black triangular shape that heads downhill toward the lake. In the distance, where the land rises appreciably, the granites (and granodiorites) of the batholilth begin.

We camped on this peninsula sticking out into Gem Lake:


Dazhi Jiang (Then of UMD-College Park; now at the University of Western Ontario) and USC's Geoff Pignotta examine strained metavolcanics near Gem Lake:


Here's me with the Ritter Range in the background:


Glacial striations sculpting my strained metavolcanics:



Field gear:

Labels: california, field trips, glacial landforms, granite, igneous, metamorphism, structure, travel

Yesterday, I took a three Honors students and a colleague to Difficult Run, Virginia. This is a hiking trail that goes from Georgetown Pike, in the tony neighborhood of McLean, Virginia, down through a deep, steep river valley to the Potomac River.

As noted a couple days ago, the trail is right across the Potomac River from my beloved Billy Goat Trail. In a recap from that post, here's a map of the area... Feel free to switch it to "satellite" view.



Some discussion of the bedrock geology of Difficult Run can be found here, in an excellent field trip guide by Scott Southworth (USGS) and colleagues that's part of Excursions in Geology and History (Frank Pazzaglia, editor).

We began our trip by meeting up with Doug Dupin of the Palisades Museum of Prehistory, who joined us for our exploratory geohike. We walked a short distance down the trail and found a big (abandoned) quarry where it was rumored there was a good fault. This is one of these pieces of information that I heard somewhere, at some point. I couldn't find it in any literature, so maybe I heard it in discussion when I taught at George Mason University for a year between grad school and when I got my position at NOVA. Anyhow, I had never actually checked it out...

...So our first order of business was to review the criteria for identifying a fault: What would we look for? Fault breccia, fault gouge, slickensides, hydrous mineral veins, and of course, offset. However, here in the Virginia Piedmont, it's rare to have a good marker unit to compare on opposite sides of the fault: usually it's just schist on one side, schist on the other. In some places, you could add the presence of a fault scarp to that list, but being as how this was an old quarry, geomorphic features like that didn't seem likely. So our search focused on the search for fault breccia, fault gouge, veins of odd minerals, and slickensides.

A few minutes in, we found some slickensides on this boulder of float:

This is a boulder of migmatitic phyllonite, with a wavy texture due to mylonitic flow at depth. (The picture doesn't show this very well at all, though you can see faint undulations 'cascading' from the top of the photo towards the bottom. It's much clearer in cross-section.) Anyhow, the 'slicks' are a faint upper-left to lower-right lineation seen on this surface, one or two degrees off from the orientation of the ballpoint pen. The surface you're looking at here was a fault plane at some point in its history. Ballpoint pen for scale.

We did eventually locate the fault, uphill from this boulder. It was characterized by a zone of fault gouge (pulverized rock), three inches wide to a foot wide in places, and highly oxidized (presumably by oxygen-rich meteoric waters percolating along this fractured surface)... but there were no good marker units to judge the total offset.

Here's a different section through a similar rock (though I wouldn't apply the "phyllonite" textural description to this one). Instead of looking at the plane of foliation here, we're looking at a surface which is perpendicular to the foliation plane(s)....

Here in this image, you can see two cleavages... One which runs roughly upper-left to lower-right through the photo, defined by gneissic banding including bands of granite (light-colored; late Ordovician in age... Taconian Orogeny). A second cleavage runs roughly left-to-right through this photo. This second cleavage overprints the first. The overall interpretation is that the first cleavage developed due to lower-left-to-upper-right compression, forming the foliation defined by alternating bands of different compositions of minerals in an upper-left to lower-right direction. The second cleavage formed due to compressive stress sub-parallel to the pre-existing foliation, deforming it into a series of tight folds. The limbs of these folds line up parallel to one another, defining the second-generation, overprinting cleavage. Can anyone else add to this interpretation? Dime for scale.

Along Difficult Run itself, the outcrops were all relatively recently scoured (in 1972 by Hurricane Agnes), so there are some good exposures. As I noted earlier this week, the area shows some nice exposures of granite pegmatites (keys, and the edge of the Pazzaglia volume, for scale):


On our field trip yesterday, we took at closer look at these beautiful pegmatites, and the associated amphibolite bodies. Take a look at this close-up... Dime for scale.

What's going on here? You've got a beautiful (euhedral/subhedral) example of an orthoclase feldspar ("potassium feldspar") crystal amid a bunch of quartz. But look closer at the feldspar crystal... this sucker has been fractured in many places, and it's shot through with very small veins of quartz. Somehow, as this pegmatite dike was cooling, the earlier-crystallizing feldspar was broken and intruded by the presumably-still-fluid silica-rich magma. Anybody able to expand on this interpretation and shed some light on how this all played out? Or contradict it and give a different story to explain this relationship?

In the neighboring amphibolite, we checked out these cool ridges of resistant rock which are centered on thin fractures. Here, you see a couple of intersecting joint sets, each of which was the "plumbing system" for silica-rich hydrothermal fluids (my interpretation). These silica-rich hydrothermal fluids impregnated the surrounding amphibolite with quartz, which made the immediately-adjacent areas more silica-rich, and hence more resistant to weathering and erosion: Hence, now that they've made it to the surface, they're weathering out in high-relief. Dime for scale.


A bit further downstream, Doug showed us a 'cave' (central dark area, just to the right of the waterfall) between the bedrock and a big slab of sloughed-off migmatitic metagraywacke:

We each edged into the 'cave' to the end, where Doug has shown that a distinctly-rectangularly shaped hole admits a direct beam of sunlight during the fall and spring equinoxes. From the inside, it's a striking arrangement, enough to make you wonder whether it's anthropogenic. However, from the outside I was unconvinced that the hole's position was anything other than natural. Doug's initial intepretation of the site was strongly influenced by the fact that there are some unambiguous petroglyphs a short distance away from here, and based on this proximity, I think it's acceptable to infer that Native Americans may have visited this cave. However, I interpreted the opening to be completely natural, with no need to invoke anthropogenic modification in any way.

We hiked on along a ridge overlooking Mather Gorge, sighting a fox and an accipiter (Coopers? Sharp-shinned?) and a few vultures, and returned to the parking lot as the sun dipped low in the sky. On the way back to campus, Honors students Ana and Hope fed us Swiss cookies and cheese & crackers. Altogether, it was a pretty great way to spend a November afternoon...

Labels: birdies, faults, field trips, granite, igneous, metamorphism, ordovician, piedmont, rivers, virginia

This week, I'm taking some of my Honors students to Difficult Run, Virginia.

It's right across the Potomac River from my beloved Billy Goat Trail. Here's a map of the area:



Some discussion of the bedrock geology of Difficult Run can be found here, in an excellent field trip guide by Scott Southworth (USGS) and colleagues that's part of Excursions in Geology and History (Frank Pazzaglia, editor).

Here's a look at Difficult Run, looking upstream from below one of the several waterfalls there:



These outcrops were all relatively recently scoured (in 1972 by Hurricane Agnes), so there are some good exposures. We're going to look for a fault reported to be there, as well as the incision geomorphology of Difficult Run itself, and some nice exposures of granite pegmatites (keys for scale):





This field trip is less a guided tour, and more of an exploration, so I hope when we get back, I'll have some photos of new and interesting things to share.

Labels: faults, granite, igneous, metamorphism, ordovician, piedmont, rivers, virginia

The Virginia Community College System (VCCS) organizes conferences occasionally where faculty in different disciplines can get together. This weekend was the "peer conference" for the natural and physical sciences. It was held at the lovely mountain resort called Wintergreen, in central Virginia's Blue Ridge Mountains.

Here's a map of the area:

That's the Shenandoah Valley on the left (part of the Valley & Ridge province), the Blue Ridge in the middle (running from NE to SW), and the Piedmont province on the far right. Wintergreen is a bit SW of Charlottesville.

The conference was fruitful and interesting. I enjoyed getting to meet a bunch of the other VCCS geology faculty and discussing what we want to do in the future in terms of supporting one another and professional development. I gave a talk about new technologies in geology instruction, which included information about the geoblogosphere and other sundry web resources I use. My colleague Erik Burtis at NOVA-Woodbridge led us on a cool "field trip" to Glacial Lake Missoula, via Google Earth.

I spent a lot of time talking with Pete Berquist, from Thomas Nelson Community College, discussing next summer's Regional Field Geology of the Northern Rocky Mountains course. We laid out a series of goals for the students, and created a tentative itinerary. Pete and I took a great hike at the end of the first day, poking around in the rocks and watching the sun set over those gorgeous mountains. Friday evening, there was a cool astronomy session, where Ed Murphy from UVA showed us the Ring Nebula, the Andromeda Galaxy, and assorted other stuff in outer space. He had a great laser pointer that extended a green laser line up about 80 feet into the sky... Very useful for pointing things out. Low light levels in the forested mountains meant excellent stargazing. Saturday morning, Bill Warren of Lord Fairfax Community College gave a good talk about the global energy crisis, and potential solutions. I picked up a few good resources there that I'll use next semester in teaching Environmental Geology. And then when the conference concluded, there was a geology "hike" out to look over the landscape. By driving us to a couple of different overlooks, Doug Coleman of the Wintergreen Nature Foundation showed us spots where we were able to look east into the Piedmont, and west into the Valley & Ridge. Pretty cool, though we didn't look too closely at the actual rocks exposed there. Fortunately, I have an inclination to do that on my own... as you'll see below:

Catoctin Formation greenstone (meta-basalt), showing chlorite-rich portions (left) and epidote-rich portions (right). Quarter for scale.


More Catoctin, the volcanic breccia layer. Lots o' epidote. Quarter for scale.


Is this a quartz vein or a granite dike?
At first glance, it appears to be your standard hydrothermal quartz vein full of milky quartz, but then you'll notice that it's not just quartz. There are also two crystals of orthoclase feldspar in there. (The dark shapes are just empty holes & shadow, not mafic minerals.) I pointed this phenomenon out before, but I'll state it again: I think that hydrothermal quartz veins and granite dikes are not separate phenomena, but points along a spectrum of composition. Quarter for scale.

Looking southeast towards the Piedmont:


Looking northwest towards the Valley & Ridge:

Labels: appalachians, blue ridge, conferences, igneous, metamorphism, piedmont, valley and ridge

Last week was field trip week for me. I led trips to the Billy Goat Trail on Tuesday and Thursday, and to Washington, DC, on Saturday.

On the Physical Geology field trip to the Billy Goat Trail, we saw rocks like amphibolite, metagraywacke, and migmatite:







Hope and Ana checking out the migmatite:


The group poses with the migmatite, to show how close anatexis is to their hearts...


Jane examines lamprophyre in a weathered-out dike:


Noting the characteristics of metagraywacke:




Traversing 'Pothole Alley'... Joel looks chilly...


Our lunch spot... Alex pretends to dive into the Potomac River...


Traversing 'The Traverse':


On the Historical Geology field trip to DC on Saturday, we were amused to find a jack-o-lantern that had facial hair resembling mine...



But that's not all! We also saw some geology. While you can get a more complete picture at my "DC Rocks" webpage, I'll post a few new photos of new outcrops here...

Here's a nice slab of granite (very angular) set in metagraywacke matrix (metamorphosed accretionary wedge complex)...


Here's two members of the Georgetown Intrusive Suite, showing the (earlier) gabbro stoping xenoliths into the (later) granite:


I love field trips. I love seeing my students light up at being outside, at getting a handle on the stuff we talk about all semester in class. I think field trips are super duper important.

Labels: dc, field trips, granite, igneous, maryland, metamorphism, nova, piedmont, teaching, xenoliths

Yesterday, I pulled up the Venetian blinds in my office window at NOVA, and this is what I saw:


Naturally, I had to take a photograph. It's puuurty.

While I had the camera out, I figured I'd shoot a few photos of the rest of my office, since it's full of all sorts of interesting clutter. Rather than explaining what all the doodads are in these photos, I figured it would be more fun to just post them and see if you can identify them all:















Have fun!

Labels: fossils, igneous, maps, metamorphism, minerals, nova, plants, sediment

Several years ago, (former) NOVA student Theresa R. put together a nice little webpage with rock and mineral photos. My favorite part is a "gray rock quiz" at the end. Check it out and see how well you do!

Labels: igneous, metamorphism, minerals, nova, sediment, websites

This weekend, I spent three days on an extended field trip down to southwestern Virginia with NOVA adjunct geology instructor Chris Khourey and four of my Honors students. We left Annandale on Friday morning, and made our first stop at Willis Mountain, Virginia, site of one of the most productive kyanite mines in the world.

Here's a Google Map of the mountain:


The Kyanite Mining Corporation was very gracious in hosting us. I'd particularly like to thank Mike Morris, who took two hours out of his day to show us the site and the mining operation.

Why mine kyanite? It's used as a refractory mineral: that is, one that won't melt under high temperatures. A lot of their kyanite is heated in kilns to produce a second mineral, mullite. The mullite is even more stable than kyanite in high temperature refractory situations. (It won't melt until it hits over 1800 degrees C!) Additionally, they cleverly saw up big blocks into dimensional stone for countertops and the like.

The kyanite mined at Willis Mountain is in a quartzite which also includes a fair amount of pyrite and hematite. We heard about the different procedures used to extract the non-kyanite minerals so that their end product is relatively pure and of constant quality.

Here's Mike showing the overall anticlinal shape of the deposit:

It's a plunging anticline, as you can probably make out from the Google Map terrain view up top.

Some of the dimensional stone, which I think is pretty spectacular:


Close up of the kyanite (light blue, on left) in the dimensional stone.


Nearby Baker Mountain also hosts kyanite deposits, which show a deeper blue color (Mike wasn't sure why, but suggested that chromium may be responsible):


Inside a huge storage building where the mullite (white powder at our feet) is stored:


Atop Willis Mountain itself, showing the weathered kyanite quartzite exposed there:


Honors students ask questions of Mike:


Mike and Chris standing near some fresh boulders of kyanite quartzite:


It wasn't all metamorphism and mining... I also noticed these nice raindrop impressions in a drying mud puddle:


After lunch atop the mountain, we hopped back in the van and hightailed it for southwestern Virginia, on our way to the Virginia Geological Field Conference. More on that tomorrow.

Thanks again to Mike and the good folks at the Kyanite Mining Corporation for hosting our visit!

Labels: conferences, economics, field trips, metamorphism, minerals, nova, piedmont, teaching, virginia

When I look back on my four years of undergraduate geology education, the one thing that strikes me as the most important thing I learned is the age of the Earth. It sent my mind reeling to recognize what a huge old planet I was on, and how ephemeral was my own species' time on it. I was a blip, a temporary arrangement of carbon, hydrogen, oxygen, and a handful of other elements that would last a while, and then disassociate. Material and energy passed into me, and out. This kinetic chemical phenomenon known as me would soon pass, and the Earth would keep turning. The human species would reach its zenith, then collapse (or evolve into something else), and the Earth would keep turning. The continents would rift and crash and the map of the Earth would soon be obselete, and the Earth would keep on turning. Climates change, meteors hit, "rivers shift, oceans fall, and mountains drift" (REM, 1985), and still the planet keeps on spinning, keeps on orbiting, keeps on keeping on.

The day I really realized the age of the Earth wasn't the day I heard "4.6 billion" in lecture. It was the day I sat there studying and grasped it internally -- it clicked that it was immensely, unimaginably old. My temporary human mind was a short-time-scale phenomenon, and it was impossible for this small cerebral system to get a grip on the true scale of the planet's age. While I would never really know (comprehend/appreciate) the age of my planet, I tapped into something fundamental that day. Looking back on it now, I'm reminded of John Playfair's words when his pal James Hutton took him to Siccar Point for the first time: "The mind seemed to grow giddy by looking so far into the abyss of time" (1805).

When I made that cognitive leap (by essentially realizing it was impossible for me to fully make the cognitive leap), I got stuck on geology. I connected to the study in a way I hadn't done before. Suddenly I was subject to a dizzying temporal vertigo, as if a layer of flooring had crumbled away leaving me gazing into a bottomless pit. The realization gave a whole new perspective on things, and it was exhilarating. It felt like one of the conversations when you're getting to know someone, and realizing that they are both intriguing and yet never completely knowable. It draws you in, connects you. Without getting too gushy, it's kind of like falling in love. I've been a geologist ever since.

As I learned more, both in school and on later peregrinations around the world, I found that geology was a great traveling companion. No matter where I went, geology was there with me, showing me new things, giving me insightful perspective. I was looking at the world through geology-colored glasses, and finding that it had a lot to show me. The world made more sense on an elemental level. Hills made sense; rivers made sense; mountains made sense. While I couldn't claim to fully understand any of these phenomena, I could claim a connection to them now that wasn't there before. They were no longer random in my mind; they had a place in the overall system, and it took geology to make me realize it.

So this perspective has stuck with me, and it's what inspired me to pitch "geology as a connector" as this month's Accretionary Wedge theme. (Newbies: the Wedge is a semi-monthly geoblogosphere carnival wherein different geobloggers contribute posts organized around a central theme.) I was curious about what I would get, and I didn't want to restrict my peers' submissions by specifying what kind of connections should be written about.

Sure enough, different people interpreted connection differently. Tromping around in the mountains doing geologic mapping yields more than insights into local structure and stratigraphy, as BrianR of Clastic Detritus discusses how his field work has connected him to the messy reality that is nature.

Jess at Magma Cum Laude is starting her first semester as a graduate T.A., and is going to employ a teaching technique that connected her to the pervasive nature of geology: everything that the Earth puts out for the purpose of assembling Oreo cookies. Something as simple as an Oreo can be the vehicle through which students realize the manifold ways they depend on the Earth every day.

Where are the boundaries between sciences? Is geology a subset of environmental science, or physics? Or both? How do we define the different parts of Nature that we study? Using a Venn diagram, Hypocentre at Hypo-theses explores the connections between geology and other sciences, particularly in the environmental realm.

Similarly, Mel uses a diagram to explore connections in her post at Ripples in Sand. How does geology connect to paleontology? Join Mel in looking at the taphonomic bridge. (And wish her congratulations on her wedding while you're at it!)

Joining the crowd in her first Accretionary Wedge post, A Life Long Scholar (at The Musings of a Life-Long Scholar) makes a connection between the very small and the very large. In trying to answer questions about massive tectonic plates, sometimes geologists must turn to little bundles of mass a few micrometers across. Check out her post to see how garnets can reveal the secret histories of the continents.

And then there are the personal connections. In Looking for Detachment, Silver Fox was the first one to submit a post on the "connection" theme with her description of how different members of the mining and exploration community connect to one another over time and space (Nevada, of course). How do Charles Manson, Kevin Bacon, and exploration geologists all fit together? Read her post to find out.

MJC Rocks of the Geotripper blog has contributed a real treat: an exploration of the connection of geologists teaching geologists through time. It turns out that his academic lineage goes all the way back to Agassiz and Cuvier! A pretty impressive consideration which will surely inspire the rest of us to investigate our own geologic pedigrees.

Finally, over at Harmonic Tremors, Julian shares a story of how his knowledge of geology led him to make a personal connection with one of his cinematic idols, director Brad Bird. If you've seen the Incredibles, you're familiar with Bird's high quality entertainment. When Julian heard that Bird was working on a movie called 1906 about the great San Francisco Earthquake, he wrote a letter to clear up some inconsistencies in the book upon which the movie is based. The talented director took the time to write back to Julian, thanking him for the "seismic tutorial."

Enjoy the various and sundry posts -- follow these digital connections to other geologists in other parts of the world, and feel connected to the larger community of earth scientists. Thanks to everyone who contributed. If I've missed anyone or if anyone wants to submit a late post, give me a shout or post a link in the comments.
________________________
References:
Playfair, John (1805). Transactions of the Royal Society of Edinburgh, vol. V, pt. III.
REM, (1985). "Feeling Gravity's Pull," Fables Of The Reconstruction, IRS records.

Labels: blogs, earthquakes, field trips, geology, metamorphism, minerals, mining, movies, plate tectonics, teaching

Yesterday morning, I took a jaunt with a local amateur geologist, Owen P., to go look at some outcrops in streambeds in and adjacent to Silver Spring, Maryland.

Owen wanted me to look at these surfaces, our local unconformity between foliated metamorphic rocks of the Piedmont below, and unconsolidated sediments of the basal Coastal Plain above (cell phone for scale):
The lower rocks are metagraywacke schist of the Sykesville/Laurel Formation (different aspects of the same thing, as far as I am concerned, and not worthy of two different formation names). They were metamorphosed during the Taconian ("Taconic") Orogeny, ~460 million years ago. These rocks were then eroded, and new sediments deposited on top of that eroded surface -- this is an unconformity like the ones I posted about over the past couple of days out in Wyoming and Arizona.

My host thought the layer above the unconformity might be tsunami deposits associated with the Chesapeake Bay bolide impact at 35.5 million years ago. However, that's not what I saw. Instead, the high proportion of angular quartz, and the fact that it was clast-supported rather than matrix supported, suggested to me that the upper layer was a gravel deposit from this very stream. It was good for me to see such a collection of angular clasts atop the unconformity -- on hilltops in DC, I'm used to seeing the Potomac Formation in this position. It's a Cretaceous-aged river deposit, with a real mix of sand, clay, and well-rounded (mainly quartzite) cobbles.

Another look (with cell phone for scale):


After I explained why I didn't buy the tsunamite hypothesis, but encouraged him to keep looking, Owen took me to another cool location, on Northwest Branch (a creek) just outside the Beltway at Burnt Mills Park. Here's a location map:


There, we found an outcrop of migmatitic metagraywacke very reminiscent of the one I visited on Four Mile Run in Arlington, VA, in March of this year. Cutting down, Northwest Branch has exposed a complex of clearly metasedimentary, clearly granitic, and not-so-clearly transitional migmatitic rocks. It's pretty cool, and not only because some of the potholes went all the way through the rock, making wormhole tunnels that a geologist can (and will) crawl through...


I found a couple of cool igneous contacts. Here's a dike of granite cutting through metagraywacke. I like this outcrop because it shows that these things are in fact filled-in cracks, and cracks have a propagating edge, a tip. Most granite dike exposures don't show this fracture edge, but this one does. In spite of the graffiti, it's a good look at that process caught in the act.


And here's a nice example of cross-cutting relationships. Host metagraywacke (notice the pebble-sized clasts of various lithologies in the upper left) is cut by two granite dikes: first a finer-grained, darker-colored one, and then by a coarser-grained, lighter-colored one. Beauty!


Thanks to Owen for showing me these outcrops -- I appreciate the interest and the invitation!

Labels: granite, maryland, metamorphism, migmatite, primary structures, sediment

This weekend, I took a backpacking trip in Shenandoah National Park. Thought I would share a few photos today: scenery first, geology second...

Here's the view looking east from Skyline Drive:


The temperature difference due to elevation was striking. It was still early spring up on the top of the mountains, on Skyline Drive:


...But down below, it was green and lush (and sodden with pollen!):


I camped out for two nights near Corbin Cabin, and did a day-hike around Thorofare Mountain on Saturday, visiting this waterfall at lunchtime:


The geology of Shenandoah National Park is interesting: it records the assembly of the early supercontinent Rodinia at about a billion years ago, and then the breakup of Rodinia about 600 million years ago. The first event recorded is the generation of granite gneisses and granites due to the Grenville Orogeny. The oldest unit in the park is the 1.1 Ga Pedlar Formation, a granite gneiss. There's a slightly younger granite which intrudes it called the Old Rag Granite (~1.0 Ga), but I didn't see any outcrops (or float blocks) of it, so I'll not mention it further. There's a thin, patchy sedimentary cover called the Swift Run Formation deposited directly atop the granite gneiss and granite, providing a nonconformity surface. Atop that is a series of volumnious tholeiitic basalt flows: these mafic extrusions record the breakup of Rodinia and the opening of a new ocean basin: the Iapetus. In many places in the park, you can see "feeder dikes" of the Catoctin cutting through the older plutonic and metaplutonic rocks (see image below). There are also some sedimentary rocks layered atop the Catoctin (the Chilhowee Group), recording the transgression of the Sauk Sea on the North American platform. But I didn't encounter any good outcrops (or float blocks) of them on this trip, so I'll stick to the tectonic story: the Pedlar Formation shows us Rodinia getting put together, and the Catoctin Formation shows us Rodinia breaking apart. Later metamorphism due to Appalachian mountain-building resulted in changes in both of these rocks (development of "blue quartz" in the Pedlar, and the Catoctin metamorphosed to greenstone).

Here's a massive dike (possibly a "feeder dike" feeding surface lava flows) of the Catoctin basalt cutting through the Pedlar Formation granite gneiss, just north of the Marys Rock Tunnel. Note the columnar jointing extending perpendicular to the walls of the dike:


Having covered all that, I now propose to spend the rest of this blog post showing you the variety of cobbles and boulders in my campsite. I camped at the little wedge of land above the confluence of two streams. One stream's catchment basin was Catoctin, and the other drained outcrops of Pedlar. As a result, the "float" in my camp was all either Pedlar Formation or Catoctin Formation. I'll just run through them one after another so you get a sense of the range of variety in each formation.

You'll notice that the Pedlar is sometimes coarse, sometimes fine, sometimes well foliated, sometimes not so much. You'll also notice that the Catoctin varies a lot in terms of its extrusive texture: sometimes aphanitic (fine-grained), sometimes amygdular (formerly vesicular), sometimes it even runs to volcanic breccia. All of these original lithologies have been metamorphosed to various degrees in the Catoctin, which here can be seen by comparing the amount of green in the rock. This green comes from two metamorphic minerals: chlorite and epidote. Enjoy!

Pedlar Formation:



















Catoctin Formation:

Labels: appalachians, basalt, blue ridge, granite, iapetus, metamorphism, national parks, primary structures, proterozoic, shenandoah, virginia

Later this month, I'm leading a tour for "Walkingtown, DC" a twice-annual event sponsored by Cultural Tourism DC, a nonprofit organization. My tour is called "History Before History: the geologic saga of Washington, DC." I'll be leading the tour on both Saturday, April 26, and Sunday, April 27, from 1-4pm. If you're in the area, consider coming along. We'll be discussing the deposition of sediments in the Iapetus Ocean, generation of an accretionary wedge, the Taconian Orogeny, the Rock Creek Shear Zone, emplacement of the Georgetown Intrusive suite, and finally the erosion of the young Appalachian mountains and the deposition of dinosaur-fossil-bearing river gravels atop the unconformity: the Potomac Group. As a bonus, we'll even visit a thrust fault which ruptures the unconformity at the intersection of Adams Mill Road and Clydesdale Place, NW. It's a nice little jaunt through prehistory. However, this hike was extremely popular last year: we had ~300 people show up! So I've asked Cultural Tourism DC to institute a reservation system this time around: I'm limiting participation to 30 people per day. Act now to reserve your place by calling or e-mailing Cultural Tourism DC.

Here's two pictures of the mad crowds last spring. I get the heebie-jeebies just thinking about it:

Labels: action, dc, dinosaurs, geology, granite, metamorphism, sediment, unconformities