19 June 2014

A preview of tomorrow’s post

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That’s the State Line outcrop south of the Guadalupe Mountains, along the Texas / New Mexico border. Know what you can find there? Tune in tomorrow to find out…

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17 June 2014

Weathering on Old Rag Mountain 2: Opferkessel

Yesterday, I pointed out an example of differential weathering on Old Rag Mountain, in Shenandoah National Park, in Virginia. Today, I’d like to shine the spotlight on another example of weathering to be seen along the trail there: little weathering pits that occur on the top of the granite outcrops. These are opferkessel.

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Some people call these “potholes,” a term I do not approve of in this context. To me, “pothole” implies focused abrasion (in a fluvial setting) – which is not how opferkessel form. Others opt to dub them “solution pans,” but I don’t love that one either, since the granite isn’t really “going into solution” at these sites so much as it is being subjected to increased levels of chemical (and, to a lesser extent, physical) weathering. The actual genesis of opferkessel is more prosaic and basic: the chaotic business of weathering a rock means some areas will be higher than others, and some will be lower. The lower spots tend to hang on to rainwater for longer, which means the minerals lining those low spots will be subjected to more chemical weathering (oxidation and hydrolysis mainly, in the context of granite). Consequently, those minerals “rot” faster than their neighbors in higher, drier positions. When the wet minerals weather away, the little low spot becomes bigger, and so can hold more water, and that can encourage more chemical weathering, and so on. It’s a self-amplifying process. Physical weathering may play a role, too – I can envision additional breakup of the granite due to the splashing action of impacting raindrops, the jostling of small waves, and even the grinding action of ice that forms in the opferkessel during winter months. Many of the opferkessel have living things in them, too, and the humic acids, hyphae, and even scrabbling of little legs may focus additional weathering power on the walls and floor of the opferkessel.

The actual summit (highest point) of the mountain (3,291 feet above sea level) has a very well-developed opferkessel, too:

IMG_2563(Note the quarter for scale on the “peninsula” at bottom center)

If you’re into weathering, Old Rag’s an excellent destination.

If you can’t get to Old Rag in person, here’s a GigaPan of an opferkessel there for you to explore:
link

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16 June 2014

Weathering on Old Rag Mountain 1: feeder dikes

Old Rag Mountain is a distinctive mountain in the eastern Blue Ridge of Virginia, contained in a little lobe of Shenandoah National Park. It’s a great hike on several levels: (1) it’s got no trees on the summit, so you can actually get a decent view from on top, (2) it’s got a great section of full-body rock scrambling on the Ridge Trail, and (3)  it’s long (9.2 miles round trip), which is why I brought a group of four of my Rockies students there last Friday for a training hike. Today and tomorrow, I’d like to show you some of the geology we saw there. Here’s one of the most striking geological features to be seen at Old Rag:

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The sharp-eyed among you will note that most of the rock in the picture is light-colored and coarse-grained – on both the left and right sides of the photograph. This is the Old Rag Granite, a Mesoproterozoic (~1.04 Ga) granite that intruded in the final phases of the Grenvillian Orogeny, during the assembly of the ancient supercontinent Rodinia.

Running through the middle of the photo, however, is something different: a rock that is dark in color, and fine-grained. This vertical seam is a feeder dike of the Catoctin Formation, a Neoproterozoic (~565 Ma) basaltic dike. Flowing through this ancient fracture was basaltic magma that fed surface eruptions of lava that flooded the landscape. Tectonically, this was during the breakup of Rodinia, and the opening of the Iapetus Ocean. The dike was metamorphosed to low-grade greenstone facies much later, during the Alleghanian Orogeny (~300 Ma), during the assembly of the more recent supercontinent Pangaea.

The mafic (iron and magnesium rich) minerals that make up the basalt are less chemically stable at Earth’s surface than the felsic minerals (potassium, sodium, and silica rich) that make up the granite. Consequently, they fall apart (weather) more quickly, and the dike is soon etched into an empty slot in the landscape. Many of these differentially-eroded Catoctin feeder dikes can be seen on the Old Rag hike, mainly on the Ridge Trail. The trail itself actually goes through three of them, which is a very cool perspective – to be walking within the actual slot that once hosted pulsing, gurgling flows of 1100° C magma, bled out during the death throes of Rodinia.

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13 June 2014

Friday fold: tortured turbidites at Devil’s Slide

Marek Cichanski (of De Anza College near the south end of San Francisco Bay) contributed this week’s Friday fold:

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Marek says:

The locality is a place near San Francisco called Devils Slide. It is a piece of the coastal highway built along a steep mountainside above the ocean. This unstable stretch of road was recently (finally!) replaced by some tunnels. And, amazingly, the old roadbed finally got converted to a nice paved hiking and biking trail! I never thought I’d live to see the day.

Most of the rocks here are Paleocene turbidites, IIRC. There is also Mesozoic granitic basement rock of the Salinian terrane at one end of the trail.

In the past, I couldn’t take my class here, because there was no safe place to park vehicles nor to stand by the side of the road. Now it’s all safe! I haven’t actually taken any students here yet, since the parking areas are small, and I didn’t even know the trail was finished until I was done leading my trip yesterday. But I sure hope I can take classes here, even if all we do is gawk at the cool geology and views!

This is also a great example of cool geology that people can visit when they go to AGU. A car would be pretty handy, although I think a bus line serves one of the trailheads – shades of `Streetcar to Subduction’. I think that for the more geologically-minded AGU attendees, it’s well worth skipping the least interesting half-day of talks to see this, if the weather isn’t rainy/windy. That blue-sky weather that seems to prevail so often during AGU week would be ideal, if one could get here before dark.

This image is one of a nice set that Marek shot at the trail. You should check them all out.

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12 June 2014

Slickensides in the Vulcan Quarry, Manassas, Virginia

While at the Vulcan Quarry in Manassas, Virginia, a few weeks ago, the principle interesting feature I saw on the individual blocks of rock we sorted through was slickensides. I saw dozens and dozens of examples, in both the hornfels and the diabase, but here are five nice examples to share:

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No sense of scale on this last one, because I didn’t feel safe edging up to the quarry wall (for fear of something falling off and popping me in the noggin), but this slickensided face is about 4 meters tall:

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This last one is the only in situ shot of the slicks – and I hope you’ll notice what EPCC geology faculty Rob Rohrbaugh noticed immediately: all the slicks that were in place on the quarry walls showed horizontal lineations, which (a) must post-date the rocks’ formation, making the faulting Triassic or later, and (b) implies horizontal (right lateral or left lateral) motion on the faults. Also, it’s worth noting that (c) there were bazillions of exposures of the subhorizontal slicks on the quarry walls – this was a pervasive rather than isolated phenomenon.

This kinematic interpretation is hard to square with a neat and simple “extension causes normal faulting” scenario for the opening of the Culpeper Basin. Clearly, there must have been some syn- or post-rifting wrench component to the basin’s extension.

…It would be a great project for an Honors student to look into!

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10 June 2014

Diabase intrusion at the Vulcan Manassas quarry

When I was flying back from Phase I of “Border to Beltway” in Texas this past March, I was delighted to photograph a bunch of local geology from the air, including this prominent diabase quarry in Manassas:

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I had never been to this particular quarry before, but as it turned out, it was one of our final destinations on Border to Beltway’s second phase, last month.

Here’s a closer view into the quarry, from its edge:

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There’s a big dike of diabase (mafic hypabyssal intrusive igneous rock) in the middle, cutting across layers of the Balls Bluff Siltstone, a Triassic sedimentary deposit that was baked to hornfels by the heat released by the intrusion:

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Up close, the diabase weathers out along joint surfaces, with its corners and edges reacting more rapidly (due to their greater surface area) with the water in the air:

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This produces some nice examples of spheroidal weathering:

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Here, Chris (our guide) and Josh (one of the B2B students) view a moderately-dipping contact between vertically-jointed diabase and shallowly-dipping Balls Bluff Siltstone (hornfels):

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We got to pick through a recent blast pile for souvenirs…

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…And being down in the pit gave us some additional perspectives on the contact between the two units:

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Later on, I’ll show you some other sights from within the quarry. Thanks so much to Vulcan and to Chris for admitting us and showing us around.

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9 June 2014

The Life and Times of the Thunderbolt Kid, by Bill Bryson

brys Bill Bryson is a very fun writer. Like many people, I first dipped into his oeuvre when he published his book about hiking the Appalachian Trail. Though I bruised my ribs laughing as hard as I did, I went back and read many of his other books – about traveling in Australia, or the UK. The one that totally knocked my socks off, though, was his superb book about science, A Short History of Nearly Everything. If you haven’t read that one yet, it really should go to the top of your list. But this little book review isn’t about that book; it’s about a more recent offering from Bryson: a memoir of his childhood in the 1950s in Des Moines, Iowa, possibly the most middle-America location possible. I read it over the past week or so, and found it quick and enjoyable, and while it’s not as profound as A Short History of Nearly Everything nor as hilarious as A Walk in the Woods, it’s a solid memoir with fun, authentic rememberings of childhood and teenagerhood. Though I didn’t grow up in Iowa nor in the same decade as Bryson, some childhood experiences are ubiquitous among American children, and these rang with authenticity for me. The cultural reflections on that decade are the most arresting matter in the book (including some horrific documentation of racism), but most of his jottings don’t meet a very high intellectual bar, nor are they particularly serious. It’s an enjoyable book on that count – some small amount of “making you think,” but it’s dominated by “making you laugh.” An excellent choice for summer reading.

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6 June 2014

Friday fold: differential weathering of carbonate intraclasts in mudstone

Howard Allen is the documentarian of this week’s fold:

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Howard writes that this is:

Middle Cambrian Chancellor Formation rock with recessive weathering intraclasts(?). Hamilton Lake trail, Yoho National Park, British Columbia. My interpretation of this one is a little shaky–it was raining when I took the photo (in 1982) and I was hiking with a non-geologist friend, so I didn’t linger at the outcrop or record the precise location. I think this is a mudstone, probably siliceous, with carbonate intraclasts, or something similar (perhaps chunks of algal mat?); the whole thing has a rather supratidal look about it. The surface is roughly normal to bedding (a joint surface, I think), and the rock is clearly folded, as can be seen in the acute angle to the lower left of the lens cap. I’d like to get a better look at this in the future. Location is west of Emerald Lake, approximately 51.440, -116.555.

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5 June 2014

Palimpsest tales from a Silurian limestone

My favorite rocks are those that tell multiple stories – rocks that are “palimpsest” with subsequent “chapters” of their biography capable of being teased out, based on different features to be observed in the rock.

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What can we see in this small sample of the Silurian-aged Tonoloway limestone, from Corridor H, West Virginia?

To start with, it’s sedimentary, and stratified. There are multiple layers of fine-grained gray limestone:

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Limestone today is deposited in warm waters of tropical seas, like off the coast of Florida or the Bahamas.

The fine grain size suggests very calm, tranquil conditions of accumulation. Up toward the top of the sample, however, we see a change in the story – the water energy must have increased temporarily, because we see chunks of this fine-grained limestone “suspended” in a coarser-grained matrix. These “rip-up” clasts suggest a storm or similar high-energy event is “Chapter 2″ in the story of this sample.

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And then there is a deformational story which overprints the depositional tale. For instance, the left side of the sample has some lovely small scale faults:

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You can’t break a rock until it’s rock, so these structures suggest the sediment was no cohesively stuck together (lithified) into a rock by the time of their formation.

And then there are the joints, the more or less vertical fractures that cross-cut the bedding. Some of these brittle features show evidence of extension, as they are filled in with calcite to make white veins.

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One of the joints on the right side of the sample seems to have served as a conduit for weathering of the rock. It provides a crisp boundary between weathered and fresh rock:

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So the overall story this rock tells is: (1) limey mud was being deposited in calm conditions, probably in a shallow tropical sea, when suddenly (2) a storm (perhaps a hurricane?) hit, ripping up some of the earlier-deposited sediment and rolling the chunks around before dumping them anew, along with coarser chunks of lime from somewhere else, and then (3) it was lithified into limestone, whereupon (4) it was capable of being faulted, and having some of the layers climb on top of their neighbors under a regime of tectonic compression, and then (5) it broke and had fluids flow through those fractures, making (6) veins and then also facilitating (7) modern weathering.

Quite the saga imprinted on this little sample of rock! It’s amazing to consider how many stories are carried in the rocks that surround us every day.

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4 June 2014

Faults in the Tonoloway Formation, Corridor H

Here’s something fun:

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Those strata are Silurian-aged Tonoloway Formation carbonates. There are plenty of dessication cracks to be seen, as well as salt casts, among the layers exposed. But more eye-catching at this distance is the faulting that disrupts the high-contrast layers…

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Both (apparent) normal and reverse faults can be seen in this road cut. Exciting stuff!

We visited it two weeks ago on “Border to Beltway’s” penultimate field day.

Alan Pitts GigaPanned this outcrop two years ago, if you’d like to explore it on your own:

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