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10 November 2017

Friday fold: soft sediment deformation in California’s Castaic Formation

The Friday fold comes to us today from Bret Leslie of the Nuclear Regulatory Commission, a friend through the Geological Society of Washington. It shows soft sediment deformation in the Late Miocene Castaic Formation, a unit deposited in the Ridge Basin, a wrench basin that opened in a releasing bend along the San Andreas Fault:


Click to enlarge

Details of the site can be explored at this geocache description.

Thanks for sharing, Bret!

If you have a Friday fold you’d like to share, get in touch with me.

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3 November 2017

Friday fold: More kinks from Glacier National Park

This Friday, let’s return to Glacier National Park. Here are some folds in Helena Formation limestone:

Can’t see them? Fair enough – the point of maximum inflection appears to be hidden behind a snow-filled gully:

But in addition to that big fold, there are several kink bands in there, too. Let’s zoom in:

Here they are:

Zooming in further, on the right-most of these kink bands:

…And here, with the traces of bedding highlighted:

These strata are Mesoproterozoic. The folding is probably Cretaceous or Paleogene in age (related to the Sevier Orogeny).

Deep time, rocks that tell stories with multiple chapters: Good fodder for a Friday fold!

Enjoy your weekend.

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2 November 2017

Weathering in komatiite

This morning on Twitter I was reminded of komatiites, those “extinct” ultramafic lavas that were relatively common eruptions during the Archean. I’ve actually got a good number of komatiite photos to share from my time last year in South Africa’s Barberton Greenstone Belt.

But since time is short this morning, I’ll start with a short photo set of three images, all showing different aspects of how these ultramafic rocks weather in the relatively dry climate of the South African mountains:

Students: what weathering patterns do you notice in these images? I’ll come back to this post later and tell you what I see, but first, let’s hear your observations and interpretations.

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27 October 2017

Friday fold: Quantankerous veins

What does it mean for a vein to be “quantankerous?” Well, to start with, it’s quartz. Second, it has to be disagreeable or cantankerous. This vein, seen in meta-arkose of the Catoctin Formation near the summit of the Blue Ridge at Rockfish Gap (not Afton Mountain), is such a quantankerous individual:

You’ll notice its “S” shape, which might imply top-to-the-left kinematics. But just down the outcrop is this set of folds:

They beg to differ, having assumed a “Z” shape, which might imply top-to-the-right kinematics.

One whispers, “Top to the left!” The other hisses back, “No! It’s top to the right, you fool!”

Who’s truthful? Who’s lying? We can’t tell based on the testimony of these unreliable witnesses.

Of course, asymmetric folds really can’t be trusted as kinematic indicators, so we should leave these quarrelsome veins to themselves. Let them bicker about the kinematics of the western Blue Ridge for eternity, for all I care.

It’s Friday. We have better things to do.

Enjoy the weekend!

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24 October 2017

The View from the Cheap Seats, by Neil Gaiman

I’ve been reading a fair bit of Neil Gaiman over the past year or so: American Gods, Good Omens (with Terry Pratchett), and The Ocean at the End of the Lane. Each of those books is good in its own way, and each is fiction. I just finished a compilation of Gaiman’s nonfiction, and there is enough about it that I think is applicable to the audience of this blog to mention it here. The pieces included in the collection are mostly speeches, essays, and introductions to other people’s books. Some are celebrations of authors or artists I know well (Stephen King, H.P. Lovecraft, They Might Be Giants), while others are equally passionate descriptions of the work of people of whom I was previously ignorant (Harlan Ellison, Dave McKean, Diana Wynne Jones). That’s inspiring – it’s a revelation of new literary pathways available for me to stroll down in the years to come. There are numerous essays on key figures in the world of comic books, for instance, and high-level thoughts on science fiction, movies, libraries, reading, creativity, and genre writing. Though there is noticeable overlap across separate pieces in subject matter and even turn of phrase, I think this works to give you deeper insight into what Neil Gaiman’s perspective is on these various subjects. It imparts a sense of familiarity, in other words. Gaiman is a gifted writer, and when he sets out to be inspiring, such as when giving an awards speech, or a graduation speech, he can really churn it out. I closed the book feeling jazzed up to “make good art” and follow my geology outreach muse to the fullest extent possible. Here’s his “make good art” speech: If you haven’t seen it, you should. It will get you fired up, and it will give you a flavor for the character for this book.

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20 October 2017

Friday fold: Kink folds in Glacier National Park, part II

Over the summer, I treated you to a great big kink fold in the sedimentary rocks of Glacier National Park. Here’s another set:

Did you see both of them in that first picture? – one bigger down below, one smaller up above. Both kink bands dip to the left.

Let’s zoom in on the upper one:

There’s more where this came from – stay tuned for more…. and in the meantime, Happy Friday!

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18 October 2017

VGFC 2017: Limbs of arkose

This past weekend was the annual Virginia Geological Field Conference, hosted by Piedmont Virginia Community College, with a field trip led by Chuck Bailey, Megan Flansburg, Katie Lang, and Tom Biggs. We examined the geology of Albemarle County, Virginia, the county that surrounds Charlottesville. This is the Blue Ridge province; the county spans its width almost exactly. We began on the east limb of the Blue Ridge Anticlinorium, traversed its basement rocks (exposed in the middle of the big fold) and ended on the western limb, adjacent to the northern terminus of the Blue Ridge Parkway and the southern end of Shenandoah National Park.

If there was a theme for the day, it was “arkose.” There’s a lot of feldspar-rich sand that was deposited in central Virginia during the Neoproterozoic. Though the formations vary, the (meta-)arkosic flavor carries through the Lynchburg Group. A few examples:

Here’s a boulder contrasting the typical lichen-encrusted outer surface with a fresh face:

We visited the Mechum River Formation (worth a blog post in its own right since it appears to be glaciogenic), but I didn’t taken any new photos on Saturday.

There were also a few fine-grained units, like these lovely pelitic metasediments, now hosting substantial garnets:

At a highway entrance ramp I’ve traversed dozens of times without stopping, I got to examine pelitic layers within the arkose. Here you can see the contact as an arcuate feature dipping to the right in the photo:

…Zooming in:

Here’s that contact annotated, in case it’s a wee bit too subtle:

The pelite here has been metamorphosed to a phyllite, and bears small crenulated wrinkles:

Even the arkose at this site had a phyllitic matrix, as you can see from the tilting back and forth of this cut and polished sample:

At another site, we saw an unusual combination of finely interbedded arkosic sand and graphitic phyllite:

Whatever the depositional setting was for these sediments, there was a lot of organic carbon getting added to the mix. A “bayou” type setting with distributary channels was hypothesized as one explanation.

Cut samples and fresh rock from the site, the first showing the arkose mixed with graphite rich beds at the base:

More graphic, but still sand dominated:

Here’s one dominated by graphite with a lesser amount of intercalated sand:

Nothing but graphite:

At the final stop up on the Blue Ridge itself, we saw coarse arkosic sediments within the Catoctin Formation, which is otherwise dominated by greenstone (meta-basalt):

Contact between medium and coarse grained dark gray arkose:

Here’s the contact between the greenstone (below) and the arkose above:

It appears to be unconformable on the basis of the wiggly, undulating contact, but the facing indicators were insufficient to convince me that I could tell which way was paleo-up. So I’m not sure if this is the bottom of the arkose or the top (and then bedding was overturned, which is something reasonable to expect in the western Blue Ridge). Annotated:

I have a few more images from the trip to share, but they have a different theme, so I’ll save them for another post.

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16 October 2017

After On, by Rob Reid

It turns out that Rob Reid can write. This “novel of Silicon Valley” is a tour de force of writing. Reid shows off his chops at writing potboiler adventure stories, ironic Amazon reviews, and sparkling dialogue. It’s a story of Silicon Valley culture, of start-ups and venture capital and social navigation in the Bay Area, but it’s also a novel that explores artificial intelligence (AI) in a fun, engaging way. Much of the material covered in Life 3.0 or Our Final Invention is explored here in an approachable, character-driven fashion (James Barrat even gets a shout-out by name!). The decisive strategic advantage (see Bostrom’s Superintelligence for more details) that the world’s first AI gets is an interesting plot point to the story, as the world’s first AI is more interested in matchmaking (which she is astonishingly good at, considering she has access to all the world’s data). The book explores quantum computing, augmented reality, the casual erosion of privacy, self-commodification of services, bioterrorism, genetic engineering, the ethics of publishing sensitive data, generation of fake audio and video, government surveillance, and much more. The setting is today – and it very much feels like a novel of this moment in history. It won’t feel as fresh ten years down the line, because all the references and plot devices hinge on the tools in our pockets right now, the capabilities of our computers right now, and I guess also the things our computers cannot yet do – but will be able to pretty soon. So read it soon! The characters are brash, brilliant, and often stressed out. They act like us, having the conversations we’re having. The difference is that as they’re innovating and grappling with office politics, an AI emerges from the Facebook/Groupon/Google/Tinder/Apple-like company that they work for. The whole set-up is simultaneously a commentary on modern American techno-life and an adventure story. It was very enjoyable. I listened to the audio-book version, which was lively and exceptionally well produced. If you can’t stomach a nonfiction book about AI, you may find After On more palatable. I was very impressed with the book overall; I know I’m going to read more by Rob Reid in the future.

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13 October 2017

Friday fold: ptygmatic vein from Finland (NMNH)

Happy Friday! Here’s a sample from the Smithsonian Institution’s National Museum of Natural History in DC:

As the label says, we have a nice example of ptygmatic (“intestine like”) buckle folding here. It comes from Finland. The coarse equigranular crystals in the vein appear to be mainly potassium feldspar and quartz. The surrounding matrix has a pronounced foliation. Note the cuspate “flames” of matrix between the broad “lobes” of folded vein. These two materials were behaving quite differently at the time of folding. It’s a lovely sample.

And I hope you have a lovely weekend.

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11 October 2017

Pompeii vs. Herculaneum

What’s missing from this picture of Pompeii, outside of Naples in west central Italy?

I’ll give you a hint: here’s a photo of nearby Herculaneum:

I visited both sites this summer, and was struck by some of the differences.

First, what they have in common: Both towns were satellite Roman communities that were snuffed out in AD 79, when the volcano Vesuvius erupted violently. Those residents who didn’t escape during the early stages of the eruption were buried. Almost two thousand years later, their remains may be viewed in the context of the archaeological sites their towns have become.

The difference between the two photos above is that at Pompeii, excavations have removed almost all the pyroclastic material (ash and lapilli) that killed the people and preserved the town. At Herculaneum, much of it remains in place. While Pompeii pokes out into the air as you might expect a village to, Heculaneum feels like an excavation: it’s a hole in the ground. The walls of that hole are pyroclastics that the surrounding city have been constructed upon.

Today, let’s explore both sites together.

Some images from Pompeii:

There are plenty of vantages of Vesuvius, the great threat, off in the distance.

It’s a beautiful place – hauntingly beautiful if you think about what happened here.

The roads and floors:

Some art from Pompeii:

Building materials varied, but included thin bricks and blocks of travertine, here accompanied by vesicular basalt for artistic effect, and also a block of carved marble (modern) for the sign:

In some places, the basalt (quite common in the area because of the neighboring volcano) dominates:

Blocks of travertine  show the external molds of plants growing adjacent to the spring:

Full body casts of the dead at the Garden of the Refugees:

These striking forms resulted when human beings (likely slaves) were buried by a pyroclastic flow. The pyroclastics were already meters deep at the time of their demise, and their masters had likely fled the city. The slaves’ bodies rotted away over time (save for some bone material) and yet the surrounding envelope of ash had enough structural integrity to remain standing and preserve the shape of their bodies. Later infilling of this external mold with plaster of Paris created a cast of the original three-dimensional shape of their bodies.

The cast of a child who died almost two millennia ago:

Two millennia later, a child examines the lapilli that entombed this city:

This is the only pyroclastic material I observed in Pompeii; everything else must have been cleared away – and a tremendous amount of it, too. It seems to me that there is a missed opportunity for some geological education here. By seeking not to obscure the gorgeous archaeological features of Pompeii, the geological “murder weapon” has been removed from the scene.

Herculaneum, in contrast, sits in a hole in the pyroclastic deposits, surrounded by urban greater Naples:

Vesuvius can be seen looming in the distance here, too.

Here’s a room (now missing the wall closest to us, and the roof), where you can see dune-scale cross-bedding moving in from the former door (on the right) to fill the interior (on the left):

Zooming in on the cross-bedding:

Annotated to show the flow direction:

Herculaneum also preserves the bodies of the dead, but here they are skeletons.

These macabre remains are in a series of boat shelters at the former harbor of Herculaneum: the low archways along this wall:

It’s a sobering view.

One of the striking aspects of Herculaneum’s archeology is the preservation of carbonized wood at the site. Here is a bed that still persists, 2000 years after it was built: absolutely remarkable!

Some of this wood remains in architectural position, holding up walls and ceilings (though now reinforced with modern additions):

Along the walls of the excavation pit, you can see additional buildings, not yet revealed. The lapilli and fragments of the carbonized wood are crumbling into the open pit:

Some art at Herculaneum:

Differential weathering of blocks of yellow tuff, relative to the mortar that holds them in place:

Finally, here’s a stone* mill for making flour, one of two located in a bakery in Herculaneum:

*porphyritic vesicular basalt

Both sites are extraordinary windows into the past. They offer views of life during the heyday of the Roman Empire. Each has its strengths. Pompeii is enormous. We walked its streets for six hours in the blazing sun and hardly visited the same spot twice. Herculaneum (the excavated portion, anyhow) is much smaller – only a few blocks. Visitors have greater liberty to enter the buildings at Herculaneum, and it’s less mobbed by visitors. For these reasons, coupled with the visceral sense of being in the volcanic deposit, I think Herculaneum was the more powerful visitor experience. Ideally though, a stronger sense of how this violent day went down can be achieved by visiting both sites in tandem.

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