Morning, folks. I awake to a challenge from Chris at GoodSchist, to show where my local bedrock was at the time of Pangea's incipient breakup. (I think Chris chose the late Triassic, 220 Ma, since Ron Blakely's map of that time shows New Zealand clearly in the south.) It's an interesting time for the rock beneath Washington, DC. After have just experienced ~50 million years of crunching between North America and Africa, DC's tortured bedrock is now being stretched as Africa begins to pull away again. A series of rift valleys mark the stretching of the crust, shown clearly in the map as a series of NE-SW oriented lakes along the axis of the Appalachian orogen.

DC's future location is between two of those rift valley lakes: one to the east, one to the west. If I owned DC real estate during the Triassic, I'd be very interested in this process, because one of those rift valleys is going to become a new ocean basin, and one isn't. The one that isn't is destined to stop opening and fill in with dirt. It will be a failed rift valley, an aulacogen of sorts.

The question is: which one is the weakest link? If the one to the west breaks open, that will be the new Atlantic Ocean basin, and DC will stay hitched to Africa. If the one to the east breaks open, that will be the site of the Atlantic, and DC will stay hitched to North America.

As it turned out, the eastern rift was the one that connected up with other rifts to the northeast and southwest, and became the young Atlantic. The western rift, known as the Culpeper Basin, stopped stretching open, and got filled in with sediment. DC stayed attached to North America, and that's the way it is.

Labels: culpeper basin, dc, geology, mesozoic, piedmont, plate tectonics

Here's a photo of one of the cool things that my Honors students and I got to see on our recent trip up to Buffalo, New York (for the northeastern section meeting of GSA ):

That's an original, signed edition of the William Smith geologic map, brought to the meeting courtesy of the Buffalo Library. It is one of only two in the United States; the other is at the Library of Congress. The map found a home in Buffalo (of all places!) thanks to Chauncey Hamlin, the head of the Buffalo Museum of Science (then called the Buffalo Society of Natural Science) from 1920 to 1948. During his tenure, he assembled a collection of first editions of many seminal scientific works. First editions of Charles Lyell's Principles of Geology and Herbert Hoover's* translation of Agricola's De Re Metallica were also on display at the conference.

* Yes, that Herbert Hoover, at least according to Wikipedia.

Labels: geology, history, maps, meetings, nova

This morning, I popped a signed contract in the mail to Geotimes: they've asked me to draw a monthly cartoon for that geology-themed magazine. It will probably start in the August or September 2008 issue. Technical details still remaining to be settled include: what this cartoon will look like, and what it will discuss, and even what it will be called.

Geotimes managing editor Meg Sever and I have discussed a couple of possibilities: probably it will be vary in size and form: sometimes it will be a three panel strip, sometimes it will be a single panel (like The Far Side). The goal is less to be humorous (though that's always a bonus) and more to explain. In fact, Meg initially got the idea from an odd project I did for my senior "thesis" at William & Mary: The Cartoon Guide to Geology (1996). That was peppered liberally with bad jokes, but the primary goal wasn't to be funny -- it was to explain geology through a cartoon medium.

I bring this up now to seek the good advice of the geoblogosphere. Especially those of you who are Geotimes subscribers: what topics do you want me to cartoon about each month?

Also: what's a catchy title for a monthly geology cartoon? Any advice you have would be welcome!

Labels: art, geology, news

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

Snowball Earth. The Pleistocene Ice Age was the proving ground for our species. But an earlier episode of glaciation, dubbed Snowball Earth, stretches our conception of what the limits of climate change are: the ice reached from the Earth's poles to its equator! Scientists infer that the freezing event was ended due to volcano-induced global warming. The course examines the geologic, chemical, and biologic evidence for Snowball Earth. This course meets 8/4 to 8/10: three evenings (MWF, 6-9pm) and one Saturday field trip to local Snowball glacial deposits. GOL 299, section 071N: 2 credits. More details

Labels: appalachians, geology, nova, teaching

Last Friday, I had a fun local field trip, in search of ultramafic rocks included in the Piedmont's metamorphosed accretionary wedge complex. My companions on the trip were David and John, both of whom are retired gentlemen pursuing geology as a hobby. Because they're doing geology for fun, they are among the most dedicated and interested students I've met at NOVA. Friday's trip was something I've been meaning to do for a while, and both of them thought it sounded like an eye-opener, so they came along too.

Our goal was to find some new outcrops that we hadn't seen before. Of primary interest were several mafic and ultramafic bodies included in the larger metasedimentary complex of rocks that we know today as the Piedmont. As I've mentioned before, these Piedmont rocks are interpreted as being the rocks of an ancient (Neoproterozoic - Paleozoic) ocean basin. When the ocean basin closed during Appalachian mountain-building, the sediments of the ocean got squished and squeezed between North America and Africa. Mixed in with them were chunks of the ancient Iapetus Ocean crust, which would probably be recognizable as ophiolites if it weren't for that pesky regional metamorphism they endured as a result of the collision. Up and down the east coast, there are outcrops of these mafics and ultramafics along the presumed "suture" zone between ancestral North America and terranes (blocks of crust) that were once a volcanic island arc in the Iapetus Ocean. As with most geology field trips, we also found some other stuff worth noting, even though it wasn't our primary objective.

Our first stop (located thanks to Diecchio & Gottfried (2004) in USGS Circular 1264) was in Clifton, Virginia, where we went to see the unconformity between the Piedmont metamorphic rocks and the Triassic sedimentary rocks which overlie them in an ancient rift valley called the Culpeper Basin. Tragically, instead of a beautiful outcrop, we found freshly graded surfaces and several new McMansions. There was only a small strip of undeveloped land, about 20 feet wide and 50 feet long which had any rock left. But in that area, we found an outcrop of soapstone. Here, John scratches the soapstone (talc) with his fingernail. It's soft!

In this case, the soapstone is interpreted as being metamorphosed ultramafic rock. Close to it, we found this piece of conglomerate:

The conglomerate is the base of the sedimentary sequence in the Culpeper Basin: it's the Reston Member of the Manassas Sandstone Formation. Notice that it contains clasts of foliated metamorphic rocks -- these were derived from the older Piedmont rocks it unconformably overlies. The Piedmont rocks got metamorphosed during Appalachian mountain-building, and then when Pangea broke up, the Culpeper Basin (one of the Newark Supergroup basins) opened up and got filled in. The source for the infilling sediment was the neighboring area, not surprisingly including pieces of the Piedmont. Up-sequence, the conglomerate is overlain by the regular Manassas Sandstone, which is a rich brick red in color (classic Triassic red beds), and contains a wealth of primary sedimentary structures. I found this one piece, which unfortunately broke into chunks when I picked it up:

It displays ripple marks, raindrop impressions, and a few horizontal branching trace fossils. Anyhow, that was about it for the Clifton stop. We were bummed about the development destroying the outcrop. On to the next location, Indian Run, on the east side of Annandale. There, using the geologic map that accompanied Drake & Lyttle (1981), we walked along the creek bed looking for exposures of rock. We didn't have to go far before seeing some heavily-rusted green rocks:

The above photo is dominantly chlorite, but check this out:

Pyroxene-rich inclusions (xenoliths? olistoliths?) were observable in the heavily-weathered exposures. The outcrops here were saprolitic, meaning they were essentially "rotten rock." David was struck by how soft they were. He said "It feels like velvet!" We turned our attention to the more coherent specimens which were weathered out and deposited as cobbles in the streambed. I got a watermelon-sized specimen that's about 40% massive peridotite and 60% greenschist. (I showcased this leprechaun-colored specimen last night in Historical Geology lecture, when we were discussing the Taconian Orogeny.) We also found intriguing hints of mountain-building in clasts like this:

That's a couple of beautiful folds in gneissic metamorphic foliation. As above, the bright green minerals are chlorite. We also found some cobbles of sedimentary rocks mixed in with the locally-derived metamorphic rocks. For instance, here's a nice semispherical cobble of flint, likely derived from the flint-bearing limestones of the Shenandoah Valley:

How did this flint nodule travel ~50 miles from its source area to its current resting place in Indian Run? Likely, it was transported by an ancestral version of the Potomac River, which brought many westward-derived cobbles eastward during the Cretaceous. About 100 million years ago, this river deposited a layer of cobbles all over our local area, preserved today as the Potomac Formation. It unconformably overlies the Piedmont rocks, and can be found today as the basal layer of the Coastal Plain. It's even found as a layer topping our highest local hills. The exposures in Indian Run actually offered a nice view of the unconformity surface, with foliated metamorphic rocks below, and unlithified Cretaceous gravel deposits on top:

Just to close out this post, I'll show a few other cobbles found in the streams. Here's a gneiss containing big, beautiful porphyroblasts of garnet:

And here's a Skolithos-bearing boulder of the Antietam Formation (quartz sandstone / quartzite), which I originally posted a few days ago, but is so gorgeous it should be shown again if I'm talking about boulders.

Finally, as a preview of tomorrow's post, I'll show a boulder which hints at the complex relationship between the foliated metamorphic rocks (gneisses) of the Piedmont and felsic igneous rocks (granites) which were derived from the partial melting of the gneiss. In other words, this is a boulder of migmatite: rock that has experienced partial melting. We'll explore this in more depth with some in situ photographs tomorrow.

Labels: appalachians, culpeper basin, field trips, geology, nova, piedmont, virginia

Picking up with my series of posts introducing the work my Honors students are doing this semester: today we'll take a look at Spencer's project, which involves field work on a bedrock terrace (strath) of the Potomac River near Chain Bridge (which can be seen in the background of this photo). As before, ignore the datestamp in the lower-right of the photo. These pictures were taken last week, not in 2004.

This is in the westernmost corner of DC's "diamond" shape. The bridge leads across the river into Arlington, Virginia. As you can see, there's a lot of rock exposure here -- the sort of thing we go crazy over here in the east. As noted before, this is metagraywacke (sometimes metamorphosed to schist, sometimes to gneiss, sometimes just strongly foliated, and sometimes so lightly metamorphosed / deformed that it even preserves original sedimentary structures like graded bedding. The interesting thing about the Chain Bridge locality is that in amongst the metagraywacke are big chunks of other rock types. I'll refer to these as "clasts." Some geologists have interpreted them as sedimentary deposits; others as "olistoliths" (tectonically emplaced chunks in an accretionary wedge complex). Spencer is in charge of documenting the variety of these clasts, in hopes that it may tell us something about their ultimate source. Here's a big elongate clast of gneiss:

We had a good little field routine going: Victoria and Jason would go scout out clasts, and then mark their location with a chalk arrow. Then Spencer would document each clast's lithology and characteristics (e.g. foliation at an angle to regional foliation) and then photograph it. Once he'd photograph it, he "checked it off" with chalk. All of this chalk graffitti gets washed away with the next big rainstorm.

Some of the clasts are no longer in their original condition. The one below, for instance, bears a multitude of garnets, metamorphic minerals which reflect how the clast's original composition reacted to the higher temperatures and pressures of Appalachian mountain-building.

Another thing we saw a lot of in the Chain Bridge locality is erosional features related to the incision of the Potomac River into bedrock. Here's Jason showing off a pothole that drilled all the way through one outcrop:

Next time, we'll take a look at Jason's project.

Labels: dc, field trips, geology, nova, sediment, teaching

Here's a view in the creek bed of Spencer and Victoria looking for kink band outcrops. (Ignore the date stamp in the lower right: it is not accurate.)

A nice kink band. Width of photograph is ~25 cm.

Victoria takes the strike of the metagraywacke's foliation:

Here's a Z-fold in the foliation -- more of a kink "knot" than a kink band. The kinematic sense of motion in this photo is top-to-the-right (right-lateral):

Here, Jason and Spencer measure the orientation of a kink band:

A nice little outcrop of crenulation cleavage, showing porphyroblasts of chlorite (green/blue) and garnet (red/brown). The pencil is parallel to crenulation "wrinkles".

Next time, we'll take a look at the projects that Spencer and Jason are working on.

Labels: field trips, geology, nova, sediment, structure, teaching

Later this spring, I'm teaching a geology course for a "Natural History Field Studies" certificate program offered by the USDA Graduate School and the Audubon Society. A friend informed me today that the course listing is online. NOVA's a better deal for students (and professors, frankly!), but I'm hoping to tap into a new population of students this way. Our field trips will be to (1) Shenandoah National Park, (2) the Shenandoah Valley and Massanutten Mountain, and (3) the Billy Goat Trail. As such, these NFHS students will get a nice cross-section of Appalachian geology, as recorded in three separate physiographic provinces (Blue Ridge, Valley and Ridge, and Piedmont, respectively). Just thought I'd mention it.

Labels: geology, nova, teaching

Tuff Cookie is posting danger signs that geologists ignore, so I'll pitch in one of my own from last summer in Montana. This is in Glacier National Park, on the trail up to Grinnell Glacier. The trail was closed due to snowfields which crossed the trail in some spots. It was a little dicey crossing them, but there was no non-litigious reason to close the trail:

Labels: geologists, geology, montana

Labels: art, geology, humor

Picking up from yesterday's post about my hike along Windy Run in Arlington, Virginia...

Just downstream from the waterfall (and crossing the trail) is a recent rockslide. Between D.C. and Great Falls (12 miles upstream), the Potomac River flows through a canyon called the Potomac Gorge. It's hundreds of feet deep overall, and consists of a series of nested straths (bedrock "terraces"), each shaped roughly like (half) a canoe. (At the tip of each canoe is a waterfall leading up to the next strath). Where the vertical distance between straths is great, as it is at Windy Run, mass wasting events serve to break down the cliffs and reduce the crisp profile of the straths.


This rockslide happened in 2005, and the area of "raw" rock up at the top of the cliff reveals the source area for the rock debris below. I wish I had taken a photo of this three years ago when it was really fresh -- it would be an excellent place to do repeat photography to show how the talus pile and cliff face change over time. Upstream are several examples of older talus aprons that have been overgrown by plants and buried in soil. Already, you can see that a few Ailanthus trees (single, upright pole-looking things) have taken root in this fresh landscape.


Once you get down from the Windy Run trail to the Potomac Heritage trail, here's the view of the river, looking upstream. Virginia's on the left; D.C. on the right. A slight "shelf" can be seen on the Virginia side where a notch has been cut to host the George Washington Parkway.


As I hiked along, I found this dead mole. It's a big fat sucker, and it must be quite fresh: probably a casualty from the previous 24 hours. Lens cap is 5 cm in diameter.


More critter evidence: here's a couple of small tree trunks that were decapitated by a beaver. Again, this is recent -- note the fresh curls of wood shavings at the base of the trunk.


But enough with these living entities: let's look at some rocks. This is the metagraywacke rock that makes up most of the Piedmont in our area. This rock is metamorphosed to various degrees up and down the Potomac River, in some places all the way to gneiss and migmatite. In some places, it's schisty, but in others primary sedimentary structures are still preserved. Upstream by Great Falls, for instance, we find graded bedding in isolated less-metamorphosed, less-deformed areas. Down along this stretch of the river, it preserves a diversity of sedimentary clasts, as shown in this image:

Here, you're seeing the graywacke matrix mixed in with a bunch of dark chunks. Today, these dark chunks are mostly biotite, but that's metamorphic. Originally, they were probably mud clasts. Little pebbles of granite and vein quartz are mixed in too. It's worth noting that not only are they metamorphosed, but they're also stretched out in the same direction: foliated and lineated. Many are squashed into X>Y>Z ellipsoidal shapes (where the letters refer to the lengths of the different axes of the ellipsoid), like a mango seed. Lens cap is 5 cm in diameter.

Let's pause for a moment and bring people up to speed if you haven't previously spent any time thinking about Appalachian geology. These rocks are part of the Appalachian mountain belt, which runs from Newfoundland to Georgia (by one definition) or from Texas to Scandanavia (by a more inclusive definition). The Appalachian mountain belt consists of three provinces: from west to east: the Valley and Ridge, the Blue Ridge, and the Piedmont. Two of these are topographically mountainous today: the Valley and Ridge and the Blue Ridge, as their ridgey names imply. But the Piedmont certainly counts as part of the ensemble, and if you compare it to the other two, you'll find that it experienced the most metamorphism, the most deformation, and is intruded in many places with syn-orogenic granites (which neither of "the Ridges" can claim, at least not for Paleozoic orogenies). The Blue Ridge and the Valley and Ridge are deformed, yes, and even lightly metamorphosed, but the Piedmont is really where the action is: this is the center of the ancient Appalachian mountain range. These rocks experienced some serious continental convergence.

So what was the Piedmont before it was the Piedmont? An ocean basin. Before the Atlantic, before Pangea, there was an ocean basin off the "east" coast (it was really the south coast at that point, but no matter...). We call this dead ocean the Iapetus Ocean. The Iapetus was closed via subduction throughout the Paleozoic, and it closed for good when Africa rammed into North America, metamorphosing these rocks and raising the Appalachians. As subduction narrowed the Iapetus, sediments atop the oceanic crust were scraped off in a big jumbled pile called an accretionary wedge. (It is for this mixed-up melange that the infamous geo-blog carnival is named.) You want to see an accretionary wedge being scraped up today? Dive down to the Peru-Chile Trench, off the west coast of South America. You want to see a fresh one at the surface? Visit California's coast ranges, which are a Mesozoic accretionary wedge, raised above sea level. You want to see what an accretionary wedge looks like after it's been tectonically squeezed between two continents? Come to the Piedmont!

Our metamorphosed accretionary wedge consists of a bunch of the sediments that were deposited in the Iapetus Ocean, including what was originally graywacke (a mix of sand & mud). Occasionally, you find a sedimentary clast that's a bit more intriguing, like this one (white arrow):

What intrigues me about this little sedimentary cobble is the fact that it's foliated, which indicates metamorphism and differential pressure, but its foliation does not line up with Appalachian foliation. This cobble was foliated before it was deposited in the accretionary wedge. Therefore, it was derived from some area that had previously experienced mountain building & regional metamorphism (presumably a continent). That ancestral orogenic episode produced a source rock from which this cobble was derived. Then that cobble was deposited by sedimentary processes somewhere and (possibly later) incorporated into the accretionary wedge, which then was metamorphosed (& foliated) itself. Lens cap is 5 cm in diameter.

Here's another one, which shows its foliation a bit better:

When I see something like this, I start to wonder, where did this cobble come from? What was its sedimentary provenance? Is this a North American cobble that attained its foliation in the Grenville Orogeny (~1 Ga)? Is this an African cobble that got squeezed in some pre-Pangea Gondwanan orogeny? Is it derived from a nameless microcontinent that was formerly marooned in Iapetus oceanic crust (a la Madagascar) and is now accreted to some continent as an exotic terrane? Do the answers to these questions change how we think about the (1) closure of the Iapetus, (b) Appalachian Orogeny, (c) assembly of Pangea?

Elsewhere in the Potomac Gorge, there are other clasts in the accretionary wedge complex that encourage similar thoughts (for instance, you can check out the photos at the top of this page). Another question raised by these clasts is this: Does their position amidst such relatively fine grained sediments (the mud and sand of the graywacke) represent original deposition? Or is that simply tectonically-induced "shuffling" in the blender-like environment of the accretionary wedge? The rocks in an accretionary wedge are not stratigraphically coherent, but sometimes they have little areas that are. If these clasts are in their original depositional position relative to the graywacke matrix, what does that tell us? Are these landslide deposits? Or are these "Snowball Earth"-related glacial dropstones? Without the original sedimentary bedding (destroyed via orogenic metamorphosis & deformation), it's impossible to answer these questions, but it sure would be nice to know.

Lastly, I'll note that everything I've talked about so far (metagraywacke, mysterious clasts, quartz veins, granite intrusions, and regional foliation) are all cut by a series of joints, brittle fractures in the rock. These joints are arranged in a series of joint sets which intersect one another, resulting in the "blocky" nature to bedrock exposures in the Potomac Gorge (example). Here, along one Gorge-bounding cliff, I saw that the joints had begun to accomodate some sliding of the blocks of rock on either side. Technically, they aren't joints any longer, but faults, instead. Total offset is only a few inches, but it shows up well in a photo like this. Note the similar sense of motion on the more distant fault "scarp." A housekey (with pink ribbon attached!) is jammed into the closer fault to give a sense of scale.


All in all, an hour strolling along Windy Run provides some terrific opportunities for reflection on the checkered geologic past of the Piedmont and the Appalachians, and the continuing geomorphic evolution of the Potomac Gorge landscape. I enjoyed my little stroll. It was with reluctance that I turned around and headed back to the house to grade exams...

Labels: appalachians, faults, geology, mass wasting, plate tectonics, virginia

This past week, I stayed in Arlington, Virginia. My dad and stepmom were in London, and I was looking after my youngest siblings (both teenagers) by staying at dad's house and serving as the Responsible Adult. It's the same house I grew up in, and it has a lot of nice memories associated with it. At the end of the street, there's a little trail which leads off into the woods and downhill towards the Potomac River along a little creek called Windy Run. ("Windy" as in the weather, not as in sinuous, though it is that, too.) At the bottom, Windy Run launches off a waterfall and tumbles down into the Potomac Gorge. On Saturday morning, I decided to go take a hike down to Windy Run and reacquaint myself with the landscape and its rocks. Here's the view from the top of the waterfall looking across the river into D.C.


Here's a view of the waterfall from the side. The big ice-rimed log at the base is about a foot and a half in diameter, to give a sense of scale:

On the way down the trail, there lies a big boulder of quartzite. This is my first rock. By that, I mean that this specific boulder is the first time I learned to put a name to a chunk of the Earth: my dad taught me that it was quartz, and I committed the name to memory. Today I would note that it's milky quartz, indicating hydrothermal deposition. (Tiny inclusions of water in the crystal lattice scatter incoming light and make it appear white.) Its upper surface is covered in black lichen. Pondering it anew on Saturday, I wondered if learning the name of this boulder in the late 1970s was the first step leading to me towards my ultimate career as a geologist. Lens cap is 5 cm in diameter.


My "first rock" lies at the base of a hill, below a linear trail of other quartz boulders. This array likely represents a subterranean vein of hydrothermal quartz, a common feature in the Virginia Piedmont.


For instance, here's a big vein of hydrothermal quartz (center) cutting across the metagraywacke host rocks at the top of the Windy Run waterfall. It's about a foot wide, and emplaced at a ~20 degree angle to the regional foliation (which strikes ~N25E). The quartz vein is oriented approximately vertically, just east of true north.


Here's some more vein quartz in the metagraywacke matrix. Foliation runs approximately left-right across this image. Note how there are large bodies of milky quartz arrayed semi-parallel to foliation: these are probably best interpreted as boudins: the results when a tabular vein of quartz was broken into chunks, and these chunks were smeared out along along the foliation during mountain-building. Boudinage (the process of producing boudins) is a somewhat brittle behavior (breaking) and somewhat ductile (smearing): under the proper combination of high temperature and directed pressure, quartz can act like pizza dough. It's capable of being molded, but also capable of separating into coherent pieces. We call these "boudins" because they resemble sausages strung out in a row ("boudin" is French for sausage). Here, only one boudin is shown, but click here for some other examples. The boudin is about 3 cm in thickness, to give a sense of scale.

There are also smaller quartz-imbued veins (white arrows, extended with dashed lines) in this rock, cutting across foliation at nearly right angles. Note how the "infusion" of quartz along these thin fractures makes them more resistant to weathering (they stand up in high relief, as seen in the lower left). This set of small quartz veins was likely emplaced at the same time the rock was being squeezed during mountain building, for reasons I explain in the next photograph.

So here's my stress interpretation of this rock. The big blue arrows represent the principal stress direction. To simplify, you could think of one blue arrow as representing Africa and the other as North America, pushing on these poor oceanic sediments caught in the middle. The yellow arrows represent extension. As the rock gets compressed in from "top" to "bottom," it gets squished outwards left to right. This deforms pre-existing quartz veins by rotating them into parallelism with foliation, and also potentially boudinaging them into chunks like the big one. The green ellipse demonstrates this overall process. One way to accommodate the rock's stretching in the yellow-arrow direction is by opening up small fractures (like the ones on the left) which get infilled with quartz.


On my walk, I saw a couple of exposures of hydrothermal quartz that strained the definition: that is, they weren't all quartz. Instead, parts of them (~5%) appeared to be granite pegmatite. In this shot, you can see several large crystals of potassium feldspar set in the quartz. Large flakes of muscovite were also semi-common. Lens cap is 5 cm in diameter.


Here's another shot of the same phenomenon seen elsewhere on the trail: large crystals of potassium feldspar and muscovite set in the "quartz vein." At what point do we stop calling these quartz veins and start calling them pegmatite dikes? Is a single crystal of non-quartz enough to change our perception of the fluid from hot mineral-rich water to wet magma? Like many things in geology, these features indicate that phenomena like dikes and veins are on a spectrum between end-members. In other words, there are shades of grey in how these things form (in addition to how we interpret them). By the way, the greenish hue is algae, not epidote. Lens cap is 5 cm in diameter.


Granite dikes (including pegmatitic ones) are reasonably common in the Virginia Piedmont. Here, as a Windy Run example, is a small granite dike I saw in a boulder on my Saturday walk. Lens cap is 5 cm in diameter.


Tomorrow, I'll explore a rockslide I saw on Windy Run, as well as the nature of the metagreywacke itself. Stay tuned, rockhounds...

Labels: geology, granite, structure, virginia

Yesterday's post was featured in the geoblogospheroidal carnival "The Accretionary Wedge." Check it out to see what the world's geologists are hmmmming about.

Labels: blogs, geology

The geoblogosphere spawns semi-monthly collections of blog posts on a particular theme, and this time around, Dr. Lemming is hosting with the theme of "things that make you go Hmmmm." The idea here is to write a blog post about something you don't understand in geology -- a mystery. Here's my contribution:

When I was in graduate school at the University of Maryland, I started hearing about a crazy notion that the entire planet had frozen over in the past. Apparently, multiple streams of evidence (chemical, isotopic, geologic, and magnetic) suggested that during the Neoproterozoic era of geologic time, the planet experienced a mega-Ice Age. There were even glacial deposits within a few degrees from the equator. If you've got glaciers operating within a few degrees of the equator, some scientists argued, then that means the Earth would have been entirely sheathed in ice. Its reflectivity ("albedo") would have been so high that most (~85%?) of incoming solar radiation would have been reflected back out into space, and that would have made the planet even colder, promoting more snow and ice. This positive feedback cycle would have reached a tipping point if the planet were covered in ice from the poles to approximately 30 degrees latitude: once it got that white, the "runaway albedo" feedback would have reached a tipping point, and wham, you've got a planet that looks like a great big snowball.

This led Joe Kirschvink (of Cal Tech) to dub this episode of glaciation the "Snowball Earth," which is about as catchy a name as a scientific hypothesis is every likely to get. The idea was then heavily promoted by Paul Hoffman (of Harvard), who was seeing strange stratigraphic patterns during field work in Namibia. Among the evidence Hoffman eventually accumulated for the Snowball were the following: "dropstones" (boulders, presumably dropped by icebergs into fine-grained offshore marine deposits, squishing the layers beneath them); conformable stratigraphy of "tropical" carbonate topped by glacial tillites, topped by more "tropical" carbonate; carbon isotope anomalies in overlying "cap" carbonates indicating a massive inorganic dumping of precipitated CaCO3; delicate crystal fans (some meters tall) precipitated rapidly in the post-Snowball ocean; and the temporary reappearance of banded iron formations (BIFs), which had not been seen since the Paleoproterozoic (and indicated an anoxic ocean, such as one sealed beneath a layer of ice).

When Kirshvink pitched the initial hypothesis, he also proposed how the Snowball could have ended (in a deliciously short, non-peer-reviewed paper): he noted that just because the surface of the planet was frozen, that would have meant diddly to plate tectonics. Radiogenic heat from the Earth's interior would have continued to drive plate tectonic processes, and that meant subduction would have continued, beneath the icy rime. If subduction continued, that meant that volcanoes would have continued to erupt, and as Iceland and Antarctica show us today, volcanoes can erupt underneath glaciers. This is important because volcanic outgassing has a substantial percentage (~15%) of carbon dioxide (CO2), and CO2 absorbs reflected infrared radiation: it's a greenhouse gas.

But with the entire surface of the planet frozen, what would have happened to this degassed CO2? If the planet's surface is frozen solid, that means the hydrologic cycle would be shut down, and the usual means of removing CO2 from the atmosphere (e.g. photosynthesis & also deposition of carbonate sediments like limestones) would be non-functional. Any CO2 emitted by volcanoes would therefore likely linger in the atmosphere, building up in concentration over time. Eventually, Kirshvink suggested, it built up to levels that caused global warming which compensated for the ice albedo effect, and the absorption of all that radiation by the CO2 melted the Snowball.

As evidence for this audacious idea, Kirshvink pointed to the cap carbonates: all that limestone ("cap carbonate") deposited on top of the glacial units needed a lot of CO2 to be dissolved in seawater (and a lot of Ca+ too). The cap carbonates, it was suggested, represented the stratigraphic removal of all that built-up CO2 from the atmosphere. Once the levels of CO2 were drawn down to a non-hothouse level, the cycle could repeat itself. Modeling calculations suggest that it would take about 5 million years of CO2 buildup to melt the Snowball.

And this is what I don't get: if you've got an atmosphere full of CO2, I can see how that would melt the Snowball. But wouldn't it then acidify the ocean (with carbonic acid, like we're seeing today), making calcite dissolve, rather than be precipitated? If the ocean is undersaturated with respect to CaCO3, then that ocean should not host accumulations of limestone. How could the voluminous worldwide cap carbonates be deposited in an acidic ocean?

On the Snowball Earth website, a list of suggested reasons why Snowball Earth could not have happened are listed, along with Hoffman, et al.'s scientific rebuttals. But when they come to the question of acid oceans and the deposition of cap carbonates, you can almost see them shrug: "These are serious criticisms," they note. Hmmmmm.

Post-script: The idea is intriguing not merely scientifically, but also in terms of the way science gets done: by people, sometimes people with outsized personalities. Paul Hoffman promoted the idea with an "evangelical zeal" (according to Gabrielle Walker, who wrote a book about the whole idea and the scientists involved). Hoffman's relentless pushing of the idea ruffled a good many feathers. Some scientists fought back, motivated in part by these chafing interpersonal dynamics. There's nothing like a little scientific controversy, and this is what Walker's book focuses on, more than the details of Snowball science.

When I found that Jay Kaufman (of UMD-College Park) was interpreting a local diamictite(near Aldie, VA) as a Snowball Earth tillite (and the overlying marble layer as a cap carbonate), I thought "this could make a great class." Last spring, I applied for and received a grant from the Virginia Community College System to develop a 2-credit class for NOVA utilizing these local rocks as a gateway to understanding the Snowball Earth hypothesis. I offered the class for the first time last summer, and I'll be offering it again this summer in August. We were fortunate to get rock samples from Virginia's two putative Snowball deposits as well as a suite of samples on loan from Gene Domack of Hamilton College. These "Snowball Suite" samples include tillites and dropstones from Namibia, Greenland, Mauritania, and Canada, as well as international BIFs and cap carbonate samples. I have to tip my hat to Dr. Domack and his colleagues: making these samples available is a terrific service in support of geoscience education.

Labels: climate change, CO2, geology, global warming, nova, snow, snowball earth, teaching

Check out this great German animation called "Das Rad" about the difference in geologic time and human time. It was nominated for an Academy Award ("Best Animated Short Film") in 2003. I think it's pretty clever.

Labels: geology, movies

We've had a cold week in the mid-Atlantic this week, and increasingly my thoughts turn to warmer conditions and the summer. Last year, this year, and next year, I'm scheduling time in Bozeman, Montana, to take classes at Montana State University. I'm working on a second master's degree in science education. It's a pretty cool program which mixes educational practice and "action research" with science elective courses, including plenty of geology offerings.

Today in the blog, I thought I would begin the process of share some images from my time out west last summer. I'll start with the Bridger Range, north of Bozeman. Here's a meadow where we parked the vans before hiking up into the hills on Dave Lageson's excellent Alpine Field Studies seminar:


Once we had huffed and puffed up about tree line, we started to see some pretty cool geology. Here for instance, you can see tilted, folded, faulted Mississippian-aged strata that have been carved into by a glacier. A few minutes after this photo was taken, the class walked straight down into this cirque and climbed up the other side: there's some serious gravity-fighting going on with a route like that. We had lunch on the other side at the top of that orange-colored chute in the upper left:


In the photo below, my hands bracket a tilted zone of paleo-karst in the Mississippian-aged Madison Limestone. With massive limestone above and below, this orangey zone speaks of a time when the limestone deposits of this area were exposed at the surface. Caves and sinkholes developed, as did an iron-rich paleo-soil. It probably looked a lot like modern-day Florida, without the strip malls and retirees. Later, the sea returned and deposited more limestone on top. The paleo-karst is obvious because it contains big blocks of limestone from cave-roof collapse, and is stained by hematite and limonite:


Fellow DC resident and geology educator Nez Nesbitt follows Dave Lageson (the instructor) south along the crest of the range. The drop to either side was substantial, including the headwall of a cirque to the left (east). The loose scree we were walking over added an additional challenge:

In all that scree on the slope we're walking over, there were some cool fossils, including this awesome crinoid calyx ("head" region) - front and back views:


Atop a peak, we paused for a break, and Dave unfurled his Tibetan prayer flags to flap in the wind. I was struck by how a simple little string of cloth imparted a really cool aesthetic to the mountain-top:


This is the trail leading down Sacagawea Cirque. There's some substantial switchbacking going on here:


Here's me atop the highest peak in the Bridger Range, Sacagawea Peak. The views are pretty good from up there:


The class spent the next day mapping glacial landforms in Sacagawea Cirque: it was fun, but I didn't take as many pictures then. When the mapping was over, I prowled through the lateral moraines for fossiliferous chunks of limestone, and found some awesome rugose corals and other treasures. These samples now reside in the NOVA Historical Geology teaching collection.

Labels: fossils, geology, glacial landforms, montana, msse, structure

In case you haven't yet heard the news, the school shooting that took place yesterday afternoon at Northern Illinois University's Dekalb campus was in a geology class. I don't know what class, but it was in a "large lecture hall" (CNN) and the instructor was apparently a graduate student (Washington Post). The shooter was apparently an ex-sociology graduate student (Post). I can't imagine how awful that must be. There have been plenty of previous school shootings (unfortunately), but hearing that it was in a geology class really clarified in my imagination the horror of such an event unfolding.

NIU's website with updates.
More from The Washington Post.
More from CNN.

Labels: geology, news

You may have seen Ron Blakely's excellent paleogeographic maps of the North American continent. Browsing around his site the other day, I found this nice sequential cartoon of the geologic steps it took to build up the rocks at the Grand Canyon.

My dad and my two brothers and I are going rafting down the Canyon this summer, and I'm looking forward to exploring the geology firsthand from the river level. My four previous trips to the Canyon have all started at the rim, then hiked down (sometimes to Plateau Point, sometimes to the river), and then back up in the same day. Staying at river level for over a week ought to be awesome.

Labels: art, geology, grand canyon

So here it is: the answer to the riddle of the cake. This image shows the t-shirt cake labelled with a few key geologic units to help make my explanation a bit more coherent.


The main problem here is that central package of strata that are tilted at an angle: the sandstone, limestone, and marl (plus the little brown layer in there that was too thin to label). If they're tilted at an angle, why aren't the layers underneath? The principles of superposition, lateral continuity, and original horizontality suggest that if these layers are tilted up at a crazy angle, then so should the underlying layers (i.e., basalt, siltstone, and shale #1). Instead, this drawing depicts what amounts to an upside-down angular unconformity bounding the tilted layers below, in addition to the regular, perfectly-acceptable angular unconformity bounding the tilted layers above. This is what I referred to earlier as a geologic "impossibility."

But Ron Schott, wily geologist that he is, pointed out another possibility: that this isn't necessarily an impossible situation, just an improbable one. As I suspect is usual, Ron is right. One way that you could get the t-shirt cake situation is with that "lower upside-down unconformity" surface being a low-angle thrust fault, like the Lewis Thrust beneath Glacier National Park in Montana. That way, a package of rocks including tilted layers gets slid laterally (sideways) along such a fault, bringing them to rest on top of some other flat-lying sedimentary layers. The upper unconformity could form either before or after faulting in this scenario.

Another improbable situation: the marl, limestone, and sandstone are the oldest layers, and they were tilted at angle, eroded, and younger layers were deposited on top: shale #1, siltstone, basalt. Then everything got folded in a really big overturned fold (like a nappe), putting them upside-down in this location. Then erosion attacked that, from the top down, and etched away the older rocks, leaving the younger sedimentary strata upside-down. Then deposition resumed with the shale #2, producing the upper angular unconformity. In this scenario, both angular unconformities are real, but superposition is pretty much thrown out the window.

OK -- new contest: can you come up with any other geologically coherent possibilities to produce a central series of tilted strata bounded above and below by horizontal strata in the manner shown? Same prize.

Many thanks to students Will and Hannah, who asked me questions about this one all day today.

Labels: geology, humor

Contest du jour: Tell me why this t-shirt design (recently re-issued at Threadless.com) is wrong. There's a major error in that sketch somewhere that makes it a geological impossibility.



Hint: it has nothing to do with the fork and plate. That's just a joke, not a misconception. First one to answer correctly wins a GEOLOGY ROCKS bumper sticker!

Labels: geology, humor

When she sees a geologic map of the eastern U.S., my cat Lola attempts to impress me by lining herself up with the trend of Appalachian structure. While noble in intent, she's not especially accurate. In the photo below, you can tell that she's off by about 20 degrees. Based on this, I conclude that cats have no natural instinct for structural geology. She can't use a Brunton compass, either.

Labels: geology, humor, lola, maryland

The Accretionary Wedge is a every-once-in-a-while compendium of geology blog posts on a particular theme. This episode is about geological misconceptions, mostly, but also a bit about pie. Yes, pie. Check it out here.

Labels: blogs, geology

The Maine Geological Survey maintains a terrific website with lots of information about the state's umpteen gazillion geological locales.

I feel like you could run a virtual field trip to Maine with the wealth of quality information and and images they have on this site. It's all well illustrated with lots of photos of structures and geologic contacts.

Learn more about the granite dikes at Pemaquid Point Lighthouse.

Or learn about where to find pillow basalts.

Or check out the giant purple crystals at Mount Apatite.

Or check out the distinctive dark feldspars of Maine's only "shonkinite".

It's all there, plus much, much more! Enjoy.

Labels: geology, maine, websites

Here are two examples (on opposite sides of the world) of places where a dark-colored host rock has been intensely fractured (maybe even "shattered") and then felsic magma squirted into and filled those fractures, solidifying into granite. In the first example, differential weathering has etched away the less stable dark-colored minerals of the host rock, exposing the more-stable granite dikes in high relief. I like the high contrast between host rock and intrusion, and the visual similarity between these two far-flung locations experiencing the same geologic process. That's uniformitarianism for you.

Lake Manapouri, near Te Anau town, southern South Island, New Zealand.

Photo by Andrew Birch.

Georgetown Intrusive Suite, exposed on Rock Creek Parkway, Washington, D.C.

Photo by Callan Bentley.

Labels: geology, granite

I've recently discovered some new geology blogs, and I thought I'd share them:
One of my favorite posts was Good Schist's "Accretionary Wedge #4" which invites other bloggers to participate on a certain theme. The theme for #4 was "deskcrops" -- the rocks that geologists keep on their desks. Anyone who has visited my office knows it's full of rock samples, so I got a big smile on my face checking out the rocks retained by blogging geological colleagues elsewhere in the world. Check them out for yourself here.

Labels: blogs, geology

The Northeastern section meeting of the Geological Society of America will be held March 27-29 in Buffalo, New York. I plan on attending it, and I can provide transportation for NOVA students up to the meeting and back. I think it would be awesome to get a whole van-full of people (Honors students and interested others) to take a little roadtrip up there and go to the meeting. The cost for students to attend the 3-day meeting is $45 if you are already a GSA member, or $55 if not. That's a great deal. Other costs: lodging and food. (I'm planning on staying in the hotel where the conference is being held, but there are undoubtedly cheaper options nearby...) I've reserved a NOVA van for us to drive up in, so transportation costs would potentially be zero.

I plan to go up one day "early" for the March 26 field trip to look at the rocks of the field trip: "Silurian - Early Devonian Sequence Stratigraphy, Events, and Paleoenvironments of the Niagara Peninsula Area of New York and Adjacent Ontario, Canada." (Full-day trip, Wed., 26 March. Cost is an additional $30 for students. Lunch provided. Participants must have passports to cross into Canada.)

More information on the meeting is here: http://www.geosociety.org/sectdiv/northe/08mtg/. If you are a NOVA student, let me know as soon as possible whether you want to go up for this meeting/trip. I think it'll be a lot of fun.

Labels: geology, meetings

Over the winter break, I finished reading the textbook Petrology by Harvey Blatt, Robert Tracy, and Brent Owens. "Petrology" is the branch of geology that is the study of rocks. (A lot of people coming into an introductory geology class for the first time think that geology is the study of rocks, but of course it is not. Geology is the study of the planet Earth.) This is a sure sign of me being a geology geek, but I really enjoyed it. The igenous section was least enlightening, but I really enjoyed the sedimentary section and the metamorphic section. I learned a bunch of new things about limestones, bentonites, the Barrovian sequence of metamorphism, and other fun stuff. It's in the Annandale branch of the NOVA library. If you've had at least one semester of geology and want to learn more about a specific rock type, I recommend it.

Labels: books, geology

Geological Travels in Northern Ireland, Part VII:

Ground moraine being used (quite appropriately) as a golf course, east of Port Rush.