13 May 2017

Slump palimpsest, Corridor H

There’s a section of my favorite road, the lovely nowhere-to-nowhere Corridor H, that seems to be having some issues with slumping. I noted this in November of 2015, and I return to the topic today. Here’s a look at the slope, with old drainage “French drains” installed, and a fresh scarp transecting it just the same:

I see at least three small scarps there.

A short distance further to the east, there’s another example of a fresh (post-treatment) scarp:

This is the one I photographed previously.

Click to enlarge

 

Particularly striking, I think, is where this scarp cuts clean across one of the downslope-oriented drainage structures:

Clearly, this slope has slipped before, then highway engineers attempted to stabilize it, and then it slipped anew.

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11 May 2017

The Seven Hills of Rome: A Geological Tour of the Eternal City, by Grant Heiken, Renato Funiciello, and Donatella de Rita

I’m preparing for some time in Italy this summer, and picked up a couple of books to bring me up to speed geologically. The first is a geological guide for Rome. It’s structured around the archetypal “seven hills” of Rome, but the story is simpler in many regards than some other seven-hilled European capitals I could name. Rome’s geology appears to consist of four major units: older sedimentary rocks (which are Pliocene and Pleistocene, so still quite young in the grand scheme of things), deposits from the ancestral form of the Tiber River which incised and back-filled in alternation, depending on where sea level was, then volcanic deposits (mainly tuff) from nearby volcanic fields, and finally an Anthropocene layer of human debris. This last is unique in my experience – it’s actually a stratum of substantial thickness in some parts of the city, such that some older buildings are now in a “hole” as the city has been slowly built up (literally UP) around them. One hill is composed of almost nothing but olive oil amphorae! Imagine that: making a landscape feature from old bottles! Other interesting tidbits: much of the quarrying of tuff for building stone took place underground, as surface real estate was too valuable for other purposes. This however has resulted in occasional cave-ins of the old emptied-out mine workings below Rome’s surface. Also: the asymmetric damage to the Colosseum that gives it such a unique “look” results from it being built astride a geologic contact between Pleistocene sedimentary rocks and unconsolidated alluvial channel fill. When shaken by an earthquake, the loose sediment amplified the shaking, and thus the damage to the overlying structure: but only the half on top of the old river deposits. Additional portions of the text address travertine, marble, historical flooding on the Tiber River, volcanic and earthquake risk, and the aqueduct system that supplies the city with fresh water. A series of field trip itineraries are described in detail at the end of the book.

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9 May 2017

Ripples in Foreknobs

The Foreknobs Formation is a Devonian unit in the Valley & Ridge province of the Mid-Atlantic Region. It was deposited in relatively shallow near-shore conditions during the Acadian Orogeny.

On a field trip to Corridor H, a new highway transecting the West Virginian Valley & Ridge province on Monday, I stopped to document a couple of beds showing very nice ripple marks.

These ones are symmetrical, and thus likely represent oscillating waves:

Here, two wave sets interfere with one another, amplifying and cancelling out each other’s bedforms:

Here’s a GigaPan of the scene:


Link 0.44 Gpx GigaPan by Callan Bentley

And here’s a 3D model:

 

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5 May 2017

Friday fold: sea monster in stone

Digging into my photo archives for a suitably folded rock this fine Friday. Here’s what I came up with:

That has a very pleasing sinuosity to it, resembling drawings I made as a kid of sea monsters:

Those are folded limy mudrocks in Kootenay National Park, British Columbia, Canada. The prominent buckled layer is relatively pure limestone, with shalier strata above and below. I’ve featured folds from this outcrop previously here.

Happy Friday!

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4 May 2017

Passing Strange, by Martha Sandweiss

Clarence King was a legend. He led one of the four great surveys of the American west (along with Wheeler, Powell, and Hayden) and eventually convinced Congress to establish One Survey To Rule Them All, an institution that ended up being called the United States Geological Survey. King was its first director, but he didn’t last too long in that position before resigning so he could pursue his own mining business ventures without any conflict of interest with his public position (hmmm). A biography of King might include his time in the California state survey, under Josiah Whitney. It would certainly include a re-telling of the famous Diamond Hoax that King helped uncover. And it would include tales of his convivial nature, an apparently legendary sort of extroversion. He was a gifted raconteur and conversationalist, and made friends of all he met. What might not get covered in as much detail is his extraordinary family life, a deceptive house of cards he carefully constructed and maintained, but which fell apart after his death. King pretended to be a black man (he “passed” for a light-skinned African American) and married a black woman, Ada Copeland, who had been born a slave just prior to the Civil War. King’s nom de mariage was “James Todd,” and so Ada became “Ada Todd,” assuming a last name she didn’t know was fiction. King traveled extensively for his work as a geologist, and basically lived out of hotels and clubs most of the time, except when he was home in New York, wherein he transformed into James Todd, home after a cross country journey as a railroad porter. He and Ada had a slew of children, some of whom survived and some of whom died young. When King contracted tuberculosis and died, he sent Ada a letter confessing his real name. He had influential friends provide modest, sustainable financial support to Ada and her children, who was now Ada King. Dependent on the money for the sake of her children, she kept relatively mum about the situation, but quietly pursued attempts to recover a trust fund that King had lied to her about. Eventually, three decades after Clarence King’s death, this culminated in a lawsuit that, while unsuccessful, brought the cross-racial marriage into the public awareness. Martha Sandweiss documents this surprising history in Passing Strange. I found the book to be compelling, well-paced, and astonishingly meticulously researched. It’s a fascinating look behind the curtain at one of the most famous American geologists, as well as an insightful examination of the state of race relations in the half century after the Civil War. Recommended.

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3 May 2017

Storm beaches have I loved

This morning, I riddled you this:

I got a few guesses on Twitter, none entered as comments on the blog, but a slew of hearty, enthusiastic conversation on Facebook. That’s where the people are, I guess. Many of them came up with great ideas to explain this odd scene of big angular rocks lying on top of what appears to be a “lawn” of grass…. Time to reveal what’s really going on!

That scene is on the Eshaness coast of mainland Shetland, just north of Dore Holm, and just south of the Eshaness lighthouse:

It’s a marine terrace with a thick mattress of turf developed atop it.

…And as any four year old will notice, there are rocks lying all over the grass there!

This is a striking fact, because the grassy marine terrace is somewhere around 20 meters above sea level. As you walk toward the cliff from the grass (don’t get too close!), you see the source for these boulders and cobbles: bedrock of Eshaness volcanics.

Some force is removing big chunks of that rock and relocating them 10-20 meters inland from the grass’s edge, a bit inland itself from the rocky precipice.

What force could that be? Well, if you’ve visited the Grind of the Navir, you might be predisposed to think about storm waves – immense, ungodly powerful storm waves, crashing into the cliffs and climbing up, reaching up 60 feet above their base, and smashing out rocks, tumbling them inland as the wave energy dissipates.

Hard to see a grassy plateau dotted with rocks as a “beach,” but I think that’s what this sedimentary deposit qualifies as: a storm beach. The blocks of rock here aren’t as big as those at the Grind, nor as concentrated, but I think it’s reasonable to infer they are more recently deposited than the grass they lay atop. And therefore I think the origin is roughly the same.

Here’s another example we saw a few days later, from the Yesnaby region of Orkney (perspective is looking to the south):

Click either one to enlarge

 

It may not be a coincidence that all three of these locations are on west-facing coasts of their islands. Things must get pretty hairy here come wintertime.

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Geopuzzle: what’s going on here?

Fancy taking a guess?

Answers tomorrow…

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2 May 2017

Leafing out to bring down CO2

It’s getting to be full-on spring here in the Fort Valley. Everything which was winter brown is now spring green.

Everywhere I look, I see chlorophyll:

These eastern deciduous forests are busy gearing up for an astonishing feat: Together with their photosynthetic brethren across the northern hemisphere, they are getting ready to extract carbon dioxide (CO2) from the atmosphere and fuse it to water (H2O) pulled up through their roots to make glucose. In so doing, they disentangle atmospheric carbon from oxygen. They want the carbon, but they have no immediate need for the oxygen. There’s a surfeit of that stuff anyhow: they say, “Get rid of it!” Photosynthesis generates oxygen as a waste product, but it’s not the goal of the process. Inconsiderate plants dump it into the atmosphere without regard for how that reactive gas will impact other life on Earth (a bummer if you’re an anaerobic microbe, but it’s a major score for Animalia!).

The overall effect of this extraordinary ramping-up of photosynthetic activity can be seen in the concentration of the important atmospheric trace gas, CO2. Each boreal summer, its concentration drops by ~7 parts per million (ppm), drawn down by the greedy carbon-gulping of gazillions of green things: plants living on land and phytoplankton living in the water.

We can measure this effect. Examine the dashed red line in the graph below, from the CO2 monitoring station at Mauna Loa, a place far from huge concentrations of trees, as well as being remote from urban centers (that’s why they chose to stick the monitoring station there). Each year, the CO2 levels hit a high in the late boreal winter, and get drawn down to a minimum at the end of each boreal summer, when the trees are swollen with carbon extracted from the atmosphere, and seasonal photosynthesis draws to a close.

The black line in the graph shows what atmospheric CO2 levels would be if it weren’t for the seasonal cycle of photosynthesis turning on and turning off in the land-rich (and land-plant-rich) northern hemisphere. If there were no extra transfer of carbon from the Earth’s sedimentary rocks into the atmosphere, that black line wouldn’t have a slope; it would be horizontal on the graph. The further north you go, the further you are into the territory of the plants, and the greater the prominence of the seasonal signal. (if you click through to that graph, you’ll see that the range of seasonal variation is about 15 ppm at Point Barrow, Alaska, and about 1 ppm at the South Pole.)

As I gaze out on this sea of green, I marvel at the power of all these flappy green solar panels, these carbon sequestering units, each one responsible for yanking a handful of carbon atoms out of the atmospheric mix, sticking it down into the cellulose of the tree, where hopefully it will remain for some time. If not for this effect, there would be ~1% more CO2 in the atmosphere of the mid-latitudes, with the attendant heat-retention and ocean acidification properties for the planet. The summer trees in my yard save us from that!

Together with their green kin, they do what they can.

‘Tis the season for carbon sequestration here in North America. Gaze around you at the green gauze and contemplate the vital biogeochemistry at work.

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1 May 2017

Accretionary lapilli from Archean volcanic eruptions

When a volcano erupts ash into the atmosphere, static charges in the eruptive plume can sometimes produce spectacular displays of lightning. Those same electrostatic charges, coupled with the presence of water vapor, can encourage the ash to clump together in small concentrically-layered orbs called accretionary lapilli. The individual lapillus grows by adding layers of new ash on its exterior. A hailstone forms through a similar process, though made of ice. Both hailstones and lapilli have a concentrically-zoned structure.

Here is a suite of accretionary lapilli images from the Msauli Chert, exposed in the Barberton Greenstone Belt, South Africa, a kilometer or so from the Swaziland border.

Sometime around 3334±3 million years ago, a volcano erupted somewhere. The ash rose into the atmosphere, with dusty bits glomming onto to one another to make lapilli. Eventually, carried downwind, the lapilli grew big enough that they settled out, raining down into the sea, piling up in granular layers on the seafloor. Later deposition covered and protected the lapilli, preserving them from erosion and weathering. 3.3 billion years passed, during which time they were lithified and uplifted. Sometime recently, the R40 road was constructed through Barberton Mountain Land, connecting the South African town of Barberton to Bulembu, Swaziland. A roadcut there exposes these ancient primary volcanic structures to the inquisitive eyes of visiting geologists.

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28 April 2017

Friday fold: Two more from the Lewisian gneiss of Scotland

Happy Friday! Here are two more folds in gneisses of the Lewisian, in the North West Highlands of Scotland, near Tarbet.

Enjoy!

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