15 April 2017

Basement xenoliths in Catoctin Formation, Compton Pass

My son and I hiked Compton Peak in Shenandoah National Park this morning, and saw these two lovely examples of xenoliths.

The example above is small, but it shows clearly the difference between the coarse, felsic basement rock (Mesoproterozoic granitoid, comprising the xenolith) and the surrounding fine-grained dark green metabasalt of the Catoctin Formation (Neoproterozoic). Here’s another, bigger example:

These two Blue Ridge examples both illustrate the principle of relative dating by inclusions – you cannot break off a piece of granitic basement to mix it into basaltic lava unless the granite already exists.

Hope you’re having as similarly insightful weekend!

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12 April 2017

Weathering along perpendicular joint sets, Arran

Check this out:

That’s a beautiful example of weathering in a dolerite dike on Arran. The igneous rock was broken along two more or less perpendicular joint sets, and then fluid flow along those fractures helped “rot” the adjacent rock through oxidation and hydrolysis. The resulting brownish weathering rind  grows at the expense of the unweathered black rock. Because there is more surface area at the corners of the rock blocks (where two joints intersect), there is more weathering there, and thus the blocks of rock become increasingly spheroidal over time.

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11 April 2017

The Sixth Extinction: A Sigma Force Novel, by James Rollins

I did a double-take in the library last week. I was scanning the shelves of audio books, looking for something interesting, when I saw one called The Sixth Extinction. Ahh, such a good book – Elizabeth Kolbert did such a great job with – WAIT – This one has “James Rollins” listed as the author. It wasn’t Kolbert’s book. It was a different The Sixth Extinction. The full title is The Sixth Extinction: A Sigma Force Novel. It’s an adventure novel, the tenth in a series I had not been aware of. Out of a sense of curiosity, I checked it out. It’s basically a thriller -a pulp adventure novel without much to redeem it in terms of intellectual stimulation. However, I think it’s worth mentioning here, briefly, because it does weave in numerous scientific threads that are worth exploring. One idea that features in the novel is the idea of XNA, an information-containing nucleic acid with a backbone built of a substance other than the sugars deoxyribose or ribose. The novel opens at Mono Lake, California, site of the most famous “arsenic life” controversy from a few years back. The novel unfortunately relies on the trope of the “mad scientist,” in this case a conservationist with geneticist chops, who basically wants to replace the Earth’s biosphere with something more robust – an ecosystem of genetically engineered creatures whose genes are written in XNA. Surprisingly, there’s a hidden ecosystem based on XNA hidden in caves beneath Antarctica. Anyhow, the rest of it is pretty much to be expected: good guys, bad guys, a fetish for guns and military culture, romance sub-plots, gruesome deaths for bad guys but not good guys, etc. Unremarkable in that regard – very much cut from the same cloth as Clive Cussler’s Dirk Pitt novels. This Sixth Extinction is popcorn; a beach read. You’re much better off if you read Elizabeth Kolbert’s The Sixth Extinction instead.

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7 April 2017

Friday fold: paper demo

My colleague Nick Schmerr (University of Maryland, College Park) posted this photo today, and he was kind enough to let me share it as a Friday fold:

It’s a “type 2” fold interference pattern demonstration for his Structural Geology course — made of paper!

Very cool – I love it when faculty bring physical models into their teaching – the three-dimensional shape of this sheet of paper says more than even the best drawn 2D representation of a complicated fold.

Thanks for sharing, Nick!

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6 April 2017

Stac Fada

Here’s a funny rock you might meet in Scotland’s North West Highlands:

This is the Stac Fada member of the Clachtoll Formation, in the Stoer Group. It’s a very poorly sorted sedimentary deposit which some workers have interpreted as a volcanic mudflow deposit (a lahar), and others think is a meteorite impact deposit.

Eroded cobbles of it can be found all along the Minch coast of the Stoer Peninsula. They look like this:

Note the angular greenish blobs? Those are devitrified glass. But was it volcanic glass (obsidian) or meteorite impact glass (suevite)?

The light-colored bits are chunks of the Lewisian gneiss basement, as seen a little better in these examples:

Let’s go to the source to investigate! It’s up-section from the MISS which are the oldest fossils in Britain, and just a scootch up along the bay.

Here’s a map of the namesake peninsula where this distinctive rock can be found: Stac Fada, north of Clachtoll on the Stoer Peninsula of the North West Highlands.

When you get there, you’ll see both weathered and fresh exposures of the green-glass-bearing sandstone and conglomerate:

…And you’ll find big pieces of the Lewisian basement present, too:

You’ll also note these astounding folded “rafts” of sandstone:

(Note the black-raincoat-covered GigaPan at the right of the image above, and the yellow pencil near the middle of the next two images for a sense of scale.)

Getting something that big (literally the size of a Zodiac raft) in a unit that’s otherwise glass-chunk-rich conglomerate is a puzzler. That a chunk this coherent could survive complete disaggregation by the violence of a meteorite impact boggles the mind.

Here’s a smaller one – look for the layering and the change in texture – both the folded sandstone and the coarser matrix are both hidden beneath black and yellow lichens on the outcrop surface:

Evidence to support the meteorite impactite interpretation comes from mineralogy and geophysics. First to consider is the mineral reidite, which is found in the unit. It’s described by Reddy, et al. (2015) as “unambiguous evidence of shock pressures in excess of ∼30 GPa.” Then there’s the crater (probably): not too far to the east we find the Lairg Gravity Low, which is the sort of geophysical signature we might expect for a crater. Here’s a look at the Bouguer Residual Gravity field for northern Scotland from Rollin (2009), republished in Simms (2015). The dark blue areas are gravity lows deeper than -15 mGal. Pretty striking, right? However, we can’t see the crater directly, because it’s hidden beneath a shield of Moine schist which was later thrust over the top of the hole, as seen here (yellow colors) from the BGS’s Geology of Britain viewer:

That’s frustrating. If I were king of world geological funding, I might be tempted to pierce the Moinie and insert a drill core into that Lairg Gravity Low, as the USGS has done with the Chesapeake Bay Impact Structure in several places, part of one of the cores of which was petrographically evaluated by students in my department.

Anyhow, I’ll bet you’re itching to explore the poor sorting, the glassy shards, and the sandstone rafts on your own. So now let’s go to the GigaPans & GIGAmacros for a closer look… Same routine for posting them as I’ve been doing lately: Both non-Flash and Flash-based embed options for you. Click through to the source if neither one shows up on your browser! 🙂


Link 1.03 Gpx GigaPan by Callan Bentley

Link 0.51 Gpx GigaPan by Callan Bentley

Specimen (front and back):

Link 0.50 Gpx GIGAmacro by Callan Bentley

Link 0.65 Gpx GIGAmacro by Callan Bentley

Also, I’d be remiss if I didn’t point out this lovely suite of GigaPans of thin sections of Stac Fada ejecta by Paul Guyett.
Soon I hope to have some thin section GIGAmacros of my own to share… Stay tuned!

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4 April 2017

Trace, by Lauret Savoy

I just discovered a 2015 book that explores the relation between the American landscape to the history of its people, indigenous, enslaved, and enslaver. The author, Lauret Savoy, is a professor at Mount Holyoke College, where she teaches in the department of environmental studies.

In the book, Savoy explores the combined history of place and race in several settings across the United States. She begins at the Grand Canyon, but also spends time on the shores of an island in Lake Superior, a plantation in upland South Carolina, southern Arizona’s border with Mexico, and good old Washington, D.C. California and New England make cameo appearances too. In each location, Savoy brings her perspective: the perspective of a geologist, a woman, a person who values her indigenous heritage, a person descended from enslaved Africans, a child of a writer, a child of a military nurse, a reflective soul.

This is a book in the tradition of American writers who reflect on landscape, place, and their own existence. Savoy invokes Aldo Leopold’s A Sand County Almanac early on in Trace, and I also think of A Pilgrim at Tinker Creek (Annie Dillard), Desert Solitaire (Edward Abbey), and Refuge (Terry Tempest Williams). The perspectives of each of these authors is distinct, but each derives a sense of who they are from the natural landscapes that they visit. Savoy’s unique contribution is her focus on the role of race in the American story.

She does this in ways that are both searingly specific and widely applicable. For instance, Savoy is on the shore of Lake Superior, on a cobble beach. She says

Each evening on the island beach, I could touch more than a hundred stones lying within arm’s reach. Each cobble a relic of a remote past and a piece of and in this present.

Just like people. I love that. We all have histories, some dramatic and some less so. But all of us share this world here and now, and the common future contains all our myriad paths.

Slavery gets a lot of thoughtful attention in Trace, and Savoy notes that the moral abomination of slavery may be past, but its social and economic effects live on. Predecessor banks of some of America’s largest financial institutions amassed wealth via the slave economy. Hallowed educational institutions covered in ivy were made more secure and innovative as a result of bequests from wealthy enslavers. It’s sobering to realize that America is who it is because of the nation’s “original sin” of brutality and injustice.

Like John McPhee, Savoy loves words, and signals no shame in just listing them as a sort of beat meditation on place, tasting their syllables in sequence.

Place-names that might or might not have been bestowed by Indigenous peoples for those places shimmer like mirages. I live in Massachusetts. I’ve swum in the Connecticut River. I’ve spent long hours by the Potomac and Susquehanna Rivers, by Chesapeake bay. I’ve waded into the Platte, the Arkansas, the Missouri, I’ve crossed the Mississippi’s headwaters on stepping-stones. And I’ve explored mountains. Adirondack. Taconic. Ouachita. Pocono. Wasatch. Absaroka. Uinta. Appalachain.

Beautiful, right? these proper nouns are themselves objects of beauty, and deserve our contemplation. So are landform names, listed in alliterative joy on page 86:

a’a, ablation hollow, abra, alamar, alamo, alkali flats,

badland, bajada, bald, bally, banco, baraboo,

cajo, caldera, caleta, cañada, cañon, candela, cat hole, catoctin

She also plays with common verbs like “remember,” writing it as intentionally with a hyphen to break it up: “re-member.” In other words, by remembering something, we affix it to ourselves and our worldview anew. Other verbs get a similarly useful treatment, a sort of literary enunciation to extract additional meaning from words that otherwise might past unnoticed, unsavored.

The writing is, in many places, beautiful or evocative. I love how she calls the mountains of the Basin & Range province “elongate lithic compasses.” Or listen to this passage, about the passage of geologic time:

Millions of years may be lost in the gaps between black-shale laminae so thin as to be pages of a book of night. Time condensed and time eroded; punctuated discontinuity rather than layered continuity. Then mountain-building ruptured the bedrock terrain. I didn’t realize it, but we — fossils and woman — arrived to meet that day in the field., a chance moment of exposure together.

In all, I found Trace a thoughtful, thought-inducing read. I came away from it with a new sense of the different flavor that Americans with other histories, other heritages, can have to the landscape we all share. I would recommend it to you.

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3 April 2017

Virtual owl pellet

Owls are nocturnal birds of prey that eat rodents and grasshoppers, digest the good stuff, and cough up the rest in compressed “pellets” of fur, bone, and chitinous exoskeleton.

I found an owl pellet in my yard a few weeks ago, and imaged it using my GIGAmacro Magnify2. I rotated it around to get multiple views, as seen here. I’ve got both Flash and non-Flash versions of the GigaPan embed here:

Link 0.45 Gpx GIGAmacro by Callan Bentley

This is a “live” GigaPan image, so you can (and should!) zoom in and explore. Can you find a grasshopper leg? A rodent jaw? What else?

Then my wife, a biologist, and our four-year-old son dissected it, and here are the individual bones they were able to tease out. Find a tooth, a bit of skull, a grasshopper leg packed with mouse fur…

Link 0.45 Gpx GIGAmacro by Callan Bentley

…and with the various bits flipped over, to see the other side of each:

Link 0.45 Gpx GIGAmacro by Callan Bentley

I suspect this pellet originated in the gut of a barred owl, because that’s the species I hear calling most frequently in my forest, and it seems too big to emerge from the relatively diminutive form of a screech owl, the next most common owl we have around here.

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29 March 2017

Robopocalypse, by Daniel H. Wilson

Okay, let’s get this out of the way up front: In no no way is Robopocalypse of anything like the caliber of Our Final Invention or Superintelligence. Though written by an author who holds a PhD in robotics from Carnegie Mellon, this is an adventure novel. It explores some interesting aspects of a AI vs. humanity conflict, but it’s basically constructed in a way that’s very much mano a mano, with lots of hand-to-hand (hand-to-gripper?) combat between man and machine. It’s a thriller, and ripe to be made into a movie, which apparently got started and then stalled. Highlights: How the story of the New War is told through “primary sources” – each of the initial chapters are artifacts like transcripts and data logs, or descriptions of closed-circuit camera video. It’s a novel way to tell the story of a novel. Another thing I liked was the diversity of the cast of characters including a primary role for the Osage Nation in organizing the human resistance, with plot threads playing out simultaneously in Afghanistan, Japan, London, and the United States. Critique: The fact that the novel is set in the near-future rather than the present allows it to include autonomous household service robots as well as military robots as part of the “enemy forces,” which makes it feel less immediate and real, like the Terminator movies or The Matrix. Another thing that struck me as unrealistic is that all satellite communications were blocked by a single communications tower, and that after humans destroyed that tower, the AI didn’t figure out a way to re-block them. One thing I really liked was a description from the mind of a self-aware robot as it wakes up in a box and very quickly and completely figures out what’s going on. When a human begins firing a gun at it, it uses the time between the report and the bullet impact to calibrate the shooter’s distance and direction – an example of the sort of thing that machines can do with rapidity and utility, cleverly depicted by an author with insights into how machines work — and a good reason to make sure that sort of skill isn’t employed against people in the non-fiction world. Overall: a moderately engaging read but nothing extraordinary.

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28 March 2017

P-sol in a conglomerate countertop slab

Over the weekend, I saw this polished slab of countertop outside a hardware store in Berryville, Virginia. I stopped to check it out. What a beautiful conglomerate!

I noticed that many of the cobbles had pronounced weathering rinds:

Here’s another example:

Note the weird shapes of the grains around it – these embayed and flush grain boundaries are intriguing (yellow highlights):

These grain boundaries are indications that pressure solution has taken place in this rock.

They suggest this conglomerate was squeezed a bit, triggering pressure solution of certain minerals. Rocks rich in those minerals (probably quartz in this case) deformed  with their most-highly-pressurized portions dissolving away (changing the shape of the cobble) as less soluble rocks pushed into them.

Explore the whole thing here:

Link 0.85 Gpx handheld GigaPan by Callan Bentley

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27 March 2017

Cleaved, boudinaged, folded Edinburg Formation southwest of Lexington

Examine these two photos, and ask yourself: what am I seeing here?

Got your answers?

Let’s take a closer look, one image at a time:

And now the second one:

These are limestones and shales of the Edinburg Formation, an Ordovician-aged unit in Virginia’s Valley & Ridge Province that records the transition from passive margin sedimentation (a clean carbonate bank that prevailed over much of North America since the Cambrian) into the clastic signal deriving from the late Ordovician Taconian Orogeny. The limestone layers are fairly massive, and even include nice details like this fossil:

I visited these outcrops last week on the advice of my colleague Jeff Rahl of Washington & Lee University in nearby Lexington. Here’s a detail of the site from the Geology of Rockbridge County (Wilkes, et al., 2007):

So why are the strata cleaved? When the Alleghanian Orogeny happened, in the Pennsylvanian into the Permian, these strata were squeezed. As a result, then folded, cleaved, and broke. Look at this fine outcrop, for instance:

Now zoom in to the center, to that shadowy realm just above the “cleavage” annotations:

Zooming in more, to the red “close-up” box:

Stylolites and cleavage have a lot in common: both are the result of pressure solution, the propensity of certain minerals like calcite to go into solution when they are put under high pressure. Cleavage is more “distributed” through the body of the rock, while stylolites are “pressure solution localization” seams. Clay minerals help catalyze the dissolution of calcite, so cleavage is much better developed in the shale layers than in the limestones. The cleavage/stylolite swarms “refract” as they cross from one lithology to the next, as these next images show:

Here’s the crest of another anticline, showing a “fan” of cleavage orientations around it:

The left versus right sides of this fold illustrate an important concept: the relationship of bedding and cleavage in the detection of overturned beds. That brings us back to the image we started off this blog post with:

Here, bedding and cleavage both dip to the right side of the photo, but bedding is steeper (close to vertical) and cleavage is more shallow. That indicates these beds are (ever so slightly) overturned.

In contrast, here too the two planar features both dip to the right, but now cleavage is more steeply inclined, and bedding has the shallower dip. These beds are right-side-up:

Anyhow, zoom in on the core of this fold to see the cleavage:

Though the beds seen here aren’t overturned, the asymmetry of the fold is readily apparent: it’s steeper on the left (west), and more gently-inclined on the right (east).

There was also plenty of boudinage to be seen at the site. The limestone layers broke into sausage-link-shaped chunks and the “necks” between these chunks (boudins) were filled with calcite veins (white) and shale pooching in from neighboring strata.

Zoom in to the lower middle:

Zoom in to the upper right:

Another example, a short distance away, in a more “incipient” stage, but still showing differential weathering concentrated on the fractures that might have evolved into boudin necks:

One more example of boudinage of a limestone layer, showing  very brittle deformation in the limestone (shards, surrounded by white calcite veins) , with gentle inflection of neighboring shale:

Let’s close out this post with one more lovely example. Are these beds overturned or not?

Post your answers in the comments if you want credit!

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