Quartz veins on Pimmit Run

Last Sunday, I took a solo hike along Pimmit Run in Virginia, accessing the valley via Fort Marcy, a Civil War fortification off of the George Washington Memorial Parkway. As always, I did a bit of geologizing along the route. One theme that emerged from the day’s photos was quartz veins. These veins form when the host rock (in this case, the Sykesville Formation) cracked open in a brittle fashion, then silicon- and oxygen-bearing hydrothermal fluids flowed into that fracture. As the fluids cooled, the silicon and oxygen bonded together and precipitated quartz, sealing shut the fracture like a seam of glue.

Here’s one that I liked because it outcropped both above and below stream level:

In several places along Pimmit Run, I saw small zones of saprolitic bedrock, which is basically “rotten rock,” where the Sykesville Formation outcrops have been more pervasively chemically weathered. This one was so soft that I was able to dramatically plunge the blade of my Swiss Army knife into the rotted rock adjacent to an unweathered quartz vein:

Oblique view of the same outcrop:

As a structural geologist, quartz veins are interesting because they are extensional features whose orientation relates to the stress field these rocks experienced in the distant past. Once formed, however, they can also act as strain markers to show how subsequent deformations have affected these rocks. Here, for instance, is a folded quartz vein:

…and here’s a bonus tiger beetle:

Hol(e)y basalt, Batman!

Today, our theme is vesicles. Here are some images of vesicles in basaltic lava flows in the Owens Valley of California, the same spot where we saw the baked fanglomerate that I showcased a few days back.

In this photo (and the zoomed-in detail shot below), you can see a couple of things. One is the size difference of the vesicles as you go up in the flow. Bigger bubbles represent larger loci of low density, and hence will be more likely to rise in a fluid batch of lava. This is the inverse of the phenomenon that causes graded bedding (heaviest grains sinking first). The result is a “graded vesicular lava flow.”

Also visible are several cooling joints that intersect to form columns. At the lower part of these columns, you can see arrest lines perpendicular to the column. Each of these subhorizontal lines represents a single instance of fracture propagation as the column separated from the rest of the flow. In composite, they form a “crack panel” like others showcased here in the past.

Let’s take a closer look at these distinctive features:

…And here’s some big vesicles, big enough to host a Swiss Army knife for scale:

They aren’t as big as some I’ve shown here in the past, but they were the largest vesicles I saw on the Owens Valley Field Forum last September. One thing I find interesting about this batch of vesicles is how they deform one another. The big one in the upper right has several smaller ones above it that “wrap around” its left edge. I envision this as the small bubbles hanging out with ~neutral buoyancy (ascendancy power), when up from below comes this massive bubble. As it pushes up (with its greater buoyancy), they smear out to the side, out of the way.

Likewise with the pair of large vesicles at lower right: it looks like the big flat one was there first, with the smaller “egg-shaped” one rising up from below and impinging on its larger upstairs neighbor. If the lava has been less viscous, the two may have merged into one, as blobs in lava lamps may be seen to do: a minimizing of surface tension, a lowering of the surface-area-to-volume ratio. Why would the smaller impinge on the larger? As I’m envisioning it, there would be a viscosity gradient in the cooling flow, with cooler temperatures towards the top (and hence higher resistance to flow). Deeper in the lava, temperatures would remain warmer, and hence the lava would be less viscous. I’m thinking that the big flat bubble had essentially risen as far as it could, but its top side was cooler than its more ductile bottom side, and so the bottom side was less resistant to the nosy intrusions of upstart bubbles from below.

Do you see anything else worth discussing in these photos?

Crystal ghosts

The first time I went to the Billy Goat Trail (Potomac, Maryland) with geology as the goal (as opposed to mere recreation), it was 2002. The trip was led by a professor at the University of Maryland. I was a graduate T.A. then, and didn’t know anything about the local geology. I remember at the end of the trip, the professor sent us out to search for “kyanite ghosts” (pseudomorphs of sericite after kyanite, produced during retrograde metamorphism). We didn’t find them on that trip, but the evocative phrase “kyanite ghost” stuck in my head.

Several years later, after I had cultivated a deeper understanding of the story told by Billy Goat Trail rocks, I was poking around in the area near the trail’s “emergency exit,” and found something that fit the “kyanite ghost” bill. I took a photograph of it:

My next step was to confirm what I found with my mentor and local rock guru, the geologist E-an Zen. E-an had been training me to take over leading geology hikes as a volunteer for C&O Canal National Historical Park. I e-mailed him the photo above. E-an wrote back to congratulate me on finding and photographing the exact same outcrop that was used in Cliff Hopson’s 1964 book The Crystalline Rocks of Howard and Montgomery Counties to illustrate the pseudomorphs. Hopson used a pencil for scale, and I used a Swiss Army knife, but otherwise the photos are identically composed:

Image: Plate 20, Figure 2; Hopson (1964)

That’s pretty uncanny, eh? Two photos taken just over half a century apart, of the exact same square foot of clue-bearing rock.

So, we have here large, bladed crystals that formed as porphyroblasts of metamorphic minerals during prograde (↑P,↑T)  metamorphism, then those same porphyroblasts found themselves unstable as temperatures and pressures dropped (retrograde metamorphism; ↓P,↓T). Their elemental constituents found themselves in disequilibrium, re-reacted, and formed new minerals which occupied the same space and shape as the large, bladed porphyroblasts. Today, you’ll finded that these “large, bladed crystals” are really aggregates of sericite (super-fine-grained muscovite).

So the question is, what were the metamorphic porphyroblasts that formed at peak P/T (and were subsequently replaced)? I mentioned kyanite as one possibility, right? However, Hopson noted these ‘ghostly’ shapes as “sillimanite (?).” Kyanite and sillimanite have a lot in common, but they aren’t the same thing. Like their polymorph andalusite, both kyanite and sillimanite have the chemical formula Al₂SiO₅. Both also grow in long bladed crystals. Check out these examples to prove this to yourself: kyanite | sillimanite

But in spite of these similarities, there’s a big difference between kyanite and sillimanite: they are stable at different combinations of temperature and pressure. Consider this classic P/T diagram:

If the sericitized pseudomorphs on the Billy Goat Trail were once sillimanite, then it implies higher temperatures. If they were once kyanite instead, then the temperatures were potentially lower. These rocks have plenty of un-retrograded sillimanite, but George Fisher (1971) was the one to invoke kyanite as the peak-P/T-porphyroblasts. He uses petrologic evidence to make the case that they were once close to ky/and/sil triple point. He says:

…the pelitic rocks contain many stubby crystals of andalusite, partially altered to sillimanite, and now largely pseudomorphed by fine aggregates of sericite. Andalusite partially altered to sillimanite is common at this end [south] of the island*, while at the north end of the island only bladed crystals of kyanite altered to sillimanite have been found. It appears as if the rocks at this end of the island must have entered the sillimanite field from the andalusite field, while the rocks farther north entered the sillimanite field from the kyanite field. If so, the rocks in the center of the island must have passed close to the triple point in the system Al₂SiO₅., about 5000 bars pressure [0.5 Gpa], and 650° C. The presence of muscovite and quartz in the sillimanite-bearing rocks reinforce this conclusion…

I assume he’s basing those statements on detailed petrologic evidence, but I haven’t seen his thin sections myself.

Tangentially, we’ve only been discussing the metasedimentary rocks so far, but E-an Zen and Phillip Candela point out in a 1998 guide to the area (for the University of Maryland geology department’s 25th anniversary hike) that the amphibolite units (meta-igneous, presumably) also contain kyanite or sillimanite but have not melted, which suggests temperatures in the range of 540° to 680° C, and pressures between 4.2 and 7 kbar (0.7 GPa).

So which is it? Kyanite or sillimanite? I can’t claim to know the answer: perhaps someone with more metamorphic petrology experience than me can shed some light on which mineral they they think they see in these ghostly pseudomorphs.

When I was out on the Billy Goat Trail last Friday with my GMU Structural Geology students, we ended up in that same general area. I challenged them to find the pseudomorphs, and it wasn’t five minutes before several of the students found excellent (though small) outcrops. Not the one that Cliff Hopson and I found, but other ones! Here are some shots to show their discoveries:

I have two questions for you: (1) What’s your favorite example of retrograde metamorphism? and (2) Have you had a similar photographing-the-same-spot-someone-else-did-many-years-before-you experience?

______________________________________________

Bierman, Paul, Zen, E-an, Pavich, Milan, and Reusser, Luke (2004). The Incision History of a Passive Margin River, the Potomac Near Great Falls, in USGS Circular 1264: Geology of the National Capital Region. Field trip guidebook.

Fisher, George  W. (1971). The Piedmont crystalline rocks at Bear Island, Potomac River, Maryland. Maryland Geological Survey Guidebook No. 4, prepared for the 1971 annual meeting of the Geological Society of America, Field Trip No. 4.

Hopson, Clifford A. (1964). The Crystalline Rocks of Howard and Montgomery Counties. Maryland Geological Survey, Baltimore.

Zen, E-an, and Candela, Philip (1998). Department of Geology, University of Maryland: 25th anniversary geology hike to Great Falls, and the Chesapeake and Ohio Canal National Historical Park. Field trip guidebook: September 19, 1998.

* The “island” in question is Bear Island, which is not really an island (except during times of highest flooding). It’s just the land between the C&O Canal and the Potomac River in the vicinity of the Billy Goat Trail.

The working life

It’s a rough life, working in the places I have to work… here are a few photos from yesterday’s field trip on the Billy Goat Trail with my NOVA Physical Geology students. Photos are courtesy Dr. Meg Coleman, who joined us for the hike.

A post-lunch lecture on river incision (note the two prominent bedrock terraces, a.k.a. “straths” in the background):

The crew climbing the dreaded “Traverse” section of the trail:

We had a nice hot day yesterday: almost 90° F! Tragically, the snack bar was closed when we got back to the visitor’s center, so we were denied our salutatory Italian ices. Back to the trail tomorrow, for the 4th of 5 trips this week…

Suess effect II: corals sing an isotopic song

Almost a year ago, on my old blog, I brought up the issue of the Suess effect. Go read that post if you don’t remember what the Suess effect is. If you want an executive summary, digest this: The burning of low-¹⁴C fossil fuels (because the fuels are old and the ¹⁴C has all decayed), lowers the total atmospheric ratio of ¹⁴C relative to other isotopes of carbon. The carbon in the atmosphere has become enriched in the lighter isotopes of carbon, ¹³C and ¹²C, as low-¹⁴C coal, oil, and natural gas get oxidized.

However, this same principle applies to ¹³C vs. ¹²C: plants preferentially take up ¹²C over ¹³C; lighter isotopic ratios are typical of photosynthesis-filtered carbon populations. So when isotopically light fossil fuels are burned, once again, the overall isotopic ratio of the atmosphere becomes “seasoned” with lightweight CO₂. The measure of the ratio of ¹³C to ¹²C is usually expressed as a value called δ¹³C. Higher δ¹³C = more ¹³C than ¹²C. Lower δ¹³C = more ¹²C than ¹³C. Here’s data from Mauna Loa (Hawaii) and the South Pole (Antarctica), showing variations in atmospheric δ¹³C over the past ~30 years:

Image: Ralph Keeling’s lab at Scripps

Here, you can see the annual seasonal cycle of the Earth’s plants “breathing” in and out (northern hemisphere photosynthesis dominates this signal), with higher δ¹³C values each boreal summer, as modern plants preferentially suck the ¹²C out of the air. The values decrease each winter as photosynthesis reaches its minimum, and decay pumps some of that light carbon (enriched in ¹²C) back into the air. And, of course, overall the trend is negative, showing the increasing abundance of fossil fuel carbon in our planet’s atmosphere.

A new paper by Peter Swart and colleagues in Geophsyical Research Letters takes a look at scleractinian corals, to see what they’re recording about the changing composition of our atmosphere. Corals are marine organisms, but the atmosphere and the oceans trade many constituents back and forth, including carbon dioxide.

The team collected corals from Florida and elsewhere in the Caribbean, and assessed their carbon isotopes over time. They compared these numbers with 27 other published studies of coral isotopic composition data. They found that 64% of the total corals studied (in the entire world ocean) show a statistically significant trend towards lower isotopic values (lower δ¹³C) over the period from 1900 to 1990. The average rate of decline is -0.01‰ per year. In the Atlantic Ocean, though, there was a higher rate of change in the isotope ratios, about -0.019‰ per year for the period of 1960 to 1990, which is identical to the rate of decrease of δ¹³C in the atmosphere over that same span of time. They note that the Atlantic has a higher “inventory” of anthropogenically-liberated CO₂ than the Pacific or Indian Oceans. (I’m not sure I understand why this should be the case; chime in if you can explain it to me.)

Please take a look at Figure 2 from the new paper:

Image: Swart, et al. (2010), Figure 2

In this graph, δ¹³C is plotted on the vertical axis, for both the atmosphere (data from Ralph Keeling) and from the sampled corals (and some published values sponges, too). Time (1800 to 2000) is plotted on the horizontal axis. Atlantic values are green. Pacific and Indian values are Barney-purple. The cited sclerosponges sampled (from the Atlantic) are a deep purple color. The atmospheric δ¹³C measurements are in light blue. You can see that over time, all of the values show a decrease in δ¹³C. The Atlantic signal stays truest to the atmospheric signal, while the Indian and Pacific signals show greater variability.

Take home point: whether we’re sampling the atmosphere directly or whether we are reading atmospheric isotopic changes filtered through the oceans via animal skeletons, we’re getting the same story: this study found that corals are reliable proxies for atmospheric changes. The burning of fossil fuels is liberating low-isotopic-weight carbon from subterranean reservoirs. This contributed carbon is fluffing up the overall isotopic weight of the carbon in our atmosphere. The atmosphere is communicating this change to us in an isotopic tune. Corals are harmonizing with the atmosphere, singing the same song at the same time.

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Peter K. Swart, Lisa Greer, Brad E. Rosenheim, Chris S. Moses, Amanda J. Waite, A. Winter, Richard E. Dodge, & Kevin Helmle (2010). The 13C Suess effect in scleractinian corals mirror changes in the anthropogenic CO2 inventory of the surface oceans GEOPHYSICAL RESEARCH LETTERS, : 10.1029/2009GL041397.

Major hat tip to M.J. Viñas of AGU, who shared a copy of the article with me. Thanks!

Easter egg

Searching through my photo archives this morning for something suitably “Eastery”… something in pastel colors, perhaps? … a petrified lagomorph? … how about an egg, or something egg-shaped?

This is as close as I got:

This is in the Owens Valley of eastern California, showing a boulder of the Mesozoic Sierra Nevada Batholith bearing a faulted xenolith. I love outcrops like this, with a combination of primary structures (like the xenolith) and secondary structures (like the fault). And the fault surface appeared to have hosted some fluid flow, encouraging epidotization (hydrous metamorphism) along its surface. How appropriate, considering both the “cracked egg” implication of the round xenolith and the pastel tones of the green epidote.

I’ll annotate it up for you, because I know you love it when I do that:

Happy Easter, folks. Focus on the bunnies and candy, and not the zombies.

Diatom time

Today’s EPOD is of a diatom. Seems like as good an excuse as any to share a couple photos of a big brass diatom sculpture from the American Museum of Natural History in New York.

Finger for scale, but not really, since the sculpture isn’t to scale.

Here, ptyggie ptyggie ptyggie!

Yesterday, I took my GMU structural geology class to the Billy Goat Trail, my favorite local spot for intriguing geology. Unlike last year, we managed our time well enough that we got to clamber around on the rocks downstream of the amphibolite contact. Here’s Sarah, Lara, Kristen, and Alan, negotiating a steep section:

Justin, Joe, Nik, Aaron, Jeremy, and Danny find a chunky amphibolite boudin in metagraywacke. Notice how Jeremy is gesturing about the orientation of the metagraywacke foliation wrapping around the boudin.

The thing that we found that really made me happy were these ptygmatic folds. Most of my readers will doubtless already be familiar with ptygmatic folding, but in case you’re new to this, check out this photo (ballpoint pen for scale):

Ptygmatic folding is “intestine-like” in appearance. It results where there is a particularly high viscosity contrast (viscosity is resistance to flow) between the folded layer and the surrounding matrix. The higher viscosity material makes broad lobes, while the lower viscosity material may be found in the pointy cusps between those lobes. If ptygmatic folding is well developed, the limbs become parallel to one another (isoclinal), and the visual similarity to guts is disconcerting. Here’s a smaller version, a few feet away from the first one:

I’m headed back to the Billy Goat Trail today to discuss the trail’s geology with a crew from Sigma Xi‘s D.C. chapter. I wonder what we will discover today?

Baked fanglomerate

A quick post to share a few images of an outcrop I visited last September out in California’s Owens Valley. This is a spot where alluvial fans coming off the eastern Sierra Nevada were overrun by a basaltic lava flow (Jeff, Kim, Fred, and Kurt for scale):

The unofficial term for these conglomerates deposited by alluvial fans is “fanglomerate,” and it’s pretty cool to see the contact metamorphism at the top of the fanglomerate. There’s also some weakly-developed columnar jointing in the basalt. Here’s an annotated version, in case the contact wasn’t quite obvious enough:

Here’s a close up (Doug for scale), showing the orange zone of thermal metamorphism at the top of the fanglomerate as the lava flow above baked the hell out of it:

Groovy, eh? Where’s your favorite example of contact metamorphism?

Geological heroes: call for posts

Ed at Geology Happens recently hosted the twenty-third edition of the geoblog carnival The Accretionary Wedge.

I’ve volunteered to host the next edition, and I’ve chosen “heroes” as the theme.

I invite all participants (geobloggers and geoblog readers alike) to contribute stories of their heroes. It’s time to pay tribute to the extraordinary individuals who helped make your life, your science, and your planet better than they would otherwise have been.

The deadline for submission of posts will be Friday, April 23.

Once you’ve published your piece, leave a link to it in the comments below this post. On the weekend of April 24-25, I’ll aggregate all the submissions into a thoughtfully-composed masterpiece post and put it up for everyone to savor. Thanks in advance for your participation.

If you’re a geoblog reader, but not a geoblogger yet yourself, then I’ll be happy to publish your story here. We all have heroes worthy of sharing, right?

UPDATE (April 26, 2010): The Wedge has been accreted!