21 September 2018
Here’s a lovely sight, contributed by reader Fred Atwood:
Those are quartz veins in one of my favorite local rock units, the Mather Gorge Formation. Fred reports,
This is at Madeira School in Great Falls between Black Pond and the Potomac.
The rocks around Great Falls, particularly those on the Billy Goat Trail’s “A” Loop, are exemplary in many regards. That’s why I am taking my Physical Geology students there next month. Of relevance to understanding Fred’s video is that there are two generations of quartz veins in these rocks: (1) a pre-late-Ordovician set, which were ductilely deformed during the Taconian Orogeny (local metamorphic ages ~464 Ma), and (2) a post-Taconian set, which are undeformed, cutting across their crumpled predecessors. The latter set of veins, whose traces appear as straight white lines on the rock outcrop surface, are gold-bearing, but that’s not the case for these “folded oldies.”
I guess we can console ourselves that they have something even more valuable than gold to share: beautiful deformation! sublime wiggles! classic contortions! uplifting undulations!
Thanks for sharing, Fred; Great outcrops!
19 September 2018
The talented science writer David Quammen has a new book out, and it’s excellent. The Tangled Tree explores endosymbiosis and horizontal gene transfer, two aspects of evolution that undercut the traditional ever-more-branching “tree of life” vision for the relatedness of living things. The lineage of organisms is not only divergent, but convergent too: populations diverge and sometimes merge, in whole or in part, complicating the traditional “ramose” structure of phylogenetic trees and evoking a more “reticulate” (net-like, or “tangled”) shape. The reality, revealed by the last 150 years of science (specifically molecular phylogenetics), is that especially for microbes, ‘mergers and acquisitions’ take place regularly, with huge implications for both public health and our understanding of evolution. Specifically, these insights allow us to re-conceive the source of genetic novelty that natural selection can then play with, as well as the actual (past-tense) history of life, including the line of descent that led from microbes to us. This is a history book, a history of the development of ideas, pushed along by interesting scientists who uncover interesting information about the history of organisms. It is a story of people, people who figure things out, who make new knowledge. It’s remarkable in that sense, very much focused on humans going through the process of science, rather than merely the empirical results of their work. These people collaborate and fight, synergize and snipe. I came away from reading it with a much better sense of who Carl Woese was, who Lynn Margulis was, who Ford Doolittle is. These people are essential in the story of how we came to understand who we are as a species, and Quammen is to be commended for his thoughtful, sympathetic rendering of their human essence. He’s a great writer: methodical, erudite, curious, measured. He consciously skips unnecessary information, and lets you know he’s doing it – this editorial sense of efficiency is much appreciated. At the same time, nothing feels rushed. Key notions, the ones that are really worth delving into, really get delved into! The book is written with a gentle redundancy, circling back just frequently enough to remind its readers of key ideas, figures, or data. There’s a lot to keep track of, and I appreciate this “reticulate” structure of the narrative. What’s more, Quammen has a Dan Brown-like talent for ending each chapter with a cliffhanger or teaser that leads directly into the introduction of the subsequent chapter. It’s delicious reading about a fascinating topic. Recommended.
14 September 2018
Today, I dug into my photo archives looking for folds that (I think) I haven’t shared before, and came across these two, from the beginning of the R40 road outside of Barberton, South Africa, on the road that leads up into Barberton Mountain Land, and toward the border with Swaziland.
The rocks here are sedimentary layers, and they show some relatively minor folding: Honestly, they’re pretty pristine for Archean aged deposits of mud, silt, and sand!
13 September 2018
Yesterday, I finished listening to the audiobook version of A New History of Life, by Peter Ward and Joe Kirschvink (2016). This book is only a couple of years old, and takes as its topic ‘the modern perspective’ on life’s long history on Earth, using the latest insights available. It aims to debunk old hypotheses that don’t stand up to new data, and to expand the purview of life’s reign on Earth beyond the Phanerozoic and flesh out its Precambrian details. It also argues for expanding life’s purview beyond Earth itself – both way back when with a planetary perspective on why they think life began on Mars, and also for the future, as they forecast the eventual demise of planet Earth to due the inevitable hostility that comes with solar evolution. Another major focus is on ‘greenhouse mass extinctions,’ the principal subject of another Ward book I consumed recently, Under a Green Sky. Ward and Kirschvink put true polar wander into Earth’s history along with the Snowball Earth glaciations, and they suggest that the first of these was unleashed by the CO2 drawdown associated with the O2 buildup of the Great Oxygenation Event – the first of ten (not five) mass extinctions in the record of life on our planet. This is a book that really emphasizes oxygen’s role in steering the fate of organisms, both positively and negatively. The authors invoke oxygen time and again as they discuss bird air sacs, dinosaur eggs, and big Carbonifeous dragonflies. Ward and Kirschvink manage to summarize and synthesize a tremendous amount of insights from the past few decades, including papers from their own careers and fields of interest, and others that are outside their professional wheelhouses, but that they gleefully delve into in their book. This is a jargon-heavy book, so it’s not going to be as useful to a novice as to someone like themselves – a professional interested to see what the current thinking is on everything that’s being presented in all those GSA meeting sessions that you don’t have time to get to. I found it an impressive work and I’m grateful to the authors for compiling it all in one place. That said, I don’t think I would use it as a Historical Geology textbook because of the high level of its writing – it assumes a fairly substantial incoming knowledge base for its readers, and pulls no punches when it comes to taxonomy or biogeochemistry. Another, minor, note: if you read it, go for the paper book version rather than the audiobook. I listened to the Audible version of it, and I lost several mm of enamel off my teeth from gnashing them every time the narrator mispronounced a word. The audio narration really could have used some “p(ear) review” before being published!
11 September 2018
After blogging about geovisualization, reader James Safranek alerted me to this new book about two of my favorite things: drawing and structural geology! I requested a review copy from the publisher, who kindly provided one. It’s great!
This is “a whole book” about drawing and geology and specifically structural geology. As such, it’s not going to be as pertinent to every reader as it was to me. But I found it a joy to read, with thoughtful advice about the drawing process (as opposed to the final outcome), our motivations for generating useful geological illustrations, and the specific process to follow when generating certain kinds of images.
To the first point, consider this quote, which is now part of my performance repertoire when I give my geo-visualization talk:
A special merit of drawing is that it requires us to look closely. The click of the camera cannot do this. While we are drawing, we must already geologically assess what we are drawing. Therefore, not just the drawing, but also the path to it, is relevant. Graphical representation — manual drawing — is nothing old-fashioned and superfluous, or just a nice pastime. In the digital age, it is urgently needed, because it teaches us to observe and reflect and it leads to concentration and mindfulness.
Another thing that resonated with me is Kruhl’s comments on “tools,” specifically paper and pen. He doesn’t like lined paper or graph paper, and I wholeheartedly agree. My field notebooks are blank. I even number the pages myself! And I prefer pen to pencil, and so does Kruhl. He lists the importance of consistent line width as one motivation (for as a pencil’s sharp tip is worn down, it creates wider lines), but I also hate the smudginess of ordinary pencils, but I’m also turned off by the dimness of the lines that “hard” pencils leave behind. They don’t smudge, but neither do they really write. I like pen, and Kruhl does too.
Kruhl addresses several kinds of image-making. In particular, I found his comments on drawing thin sections and drawing what he calls “stereograms” (regional structural summaries) to be illuminating. The examples he shows from his own work are nothing short of inspirational. It’s a fun read, and I’m going to have this one within easy reach on my bookshelf for the rest of my career. Recommended!
7 September 2018
Happy Friday to you all! I’ve previously showcased folded Conestoga Formation on the campus of Millersville University in Pennsylvania for the Friday fold. Today we’ll go a bit further afield, to the Burle Business Park on New Holland Ave in Lancaster. There are plenty of nice outcrops of Conestoga Formation limestone exposed there, and they have a few cool things to show us.
I visited this site on a field trip during June’s Eastern Section meeting of the National Association of Geoscience Teachers. Here’s Martin Schmidt, David Brink-Roby, and Don Duggan-Haas traipsing across one of the exposures:
Sedimentologically, the site is noteworthy because it includes some submarine landslide breccias, as seen here:
But for me, the best part was the plethora of sweet folds:
As a friendly reminder, I welcome submissions of “Guest Friday folds” for the blog. Just shoot me an email if you’ve got some cool folds to share!
31 August 2018
Another guest contribution for the Friday fold. Today, my friend Maitland Sharpe chips in with this mountainside he saw in Italy, specifically the
Dolomites, near Rifugio Fontana, Longarone
…And here is the same image, annotated in red to show the trace of bedding:
The strata here appear to be carbonates. I’d have guessed limestone, but given the fact that this is the Dolomites, I guess the chances are good that they are dolostone layers instead. The folding style is somewhere on the spectrum between kink banding and chevrons. It’s loooooooovely!
Two close-ups follow, one annotated:
I’m not even going to attempt tracing out bedding in that last one – I fear it may drive me insane!
Thanks for sharing, Maitland!
Readers, please know that I value your contributions to the Friday fold series – I love seeing the folds you see on your travels.
A happy Friday to all!
27 August 2018
The subtitle of this wonderful book is “Watermen, Crabs, and the Chesapeake Bay.” It’s an excellent account of crab ecology in the Chesapeake Bay as it stood in the mid-1970s, and simultaneously a sympathetic portrait of the lives of the locals who capture those crabs for sale to the seafood market. The writing is thoughtful and calm, paced very similarly to John McPhee’s writing, rich in quotes from the watermen speaking their local vernacular. It’s no surprise to me that it won the Pulitzer Prize for general nonfiction in 1977 (two years after Annie Dillard’s Pilgrim at Tinker Creek and a year prior to Carl Sagan’s The Dragons of Eden). I found it utterly engrossing, and would recommend it to anyone with an interest in the Bay’s economy or culture, a yen to learn more about the seasonal rhythms of an extraordinary animal, or just a taste for a soft-shelled crab sandwich, one of life’s finest pleasures (in my humble opinion). The title, I should note, is a translation of the scientific name of the Chesapeake Bay blue crab, Callinectes sapidus, which I’ve always appreciated, given that the first two syllables are pronounced identically to my own first name. Recommended.
24 August 2018
It’s Friday, and I have another guest Friday fold to share:
This one is from my Denver friend Greg Willis, who tells me it’s
from near Arapaho Pass, near where we rain-hiked.
Ahhhh, yes – a singularly soggy hike up in the Colorado Rockies. I remember it well, and it looks like Greg had better weather on this jaunt!
Happy Friday to all!
22 August 2018
After a bit of a respite, it’s now time for a fresh edition of “you ask the questions” here on Mountain Beltway. Anyone can ask a question, serious or spurious, and I’ll do my best to answer it here. Use the handy Google Form to to submit your questions anonymously.
Here’s this week’s question:
7. What is foliation and what makes it so important to the structure of rock?
Foliation is a planar alignment of mineral grains in a rock. It’s dependent on those mineral grains not being spherical, but having some non-equant shape, like a flake or a line. If you get all those flakes (or all those lines) in a common orientation, it imparts a “fabric” to the rock, in the same way that the alignment of plant cells in a log give it a “grain.” Just as it’s easier to split a log along the grain than across it, it’s often easier to split a rock along the foliation than across it. In this case (easier to split in one direction), we’d say the rock has not just a foliation, but a cleavage. So a cleavage is a foliation that is also a plane of weakness.
Foliation can have several causes: it could be a primary feature, say due to flow in a molten rock mass, but more commonly it refers to a tectonic feature, imposed on a pre-existing rock by differential stress (stress that’s greater in one direction than in other directions). As such, foliation is a secondary structure, and example of strain, a deformational feature that develops in rocks that have been squeezed by geological processes. Because rocks in these settings are often recrystallized under high temperatures and pressures, rocks that are foliated are often also metamorphic rocks. But you can have the one without the other.
A common way to get foliation to develop is through pressure solution. Pressure solution is a phenomenon where certain minerals dissolve under high pressure, and re-precipitate under lower pressures. Calcite is particularly susceptible to this, but quartz does it too.
Here’s an example of evidence for this phenomenon: “bearded” porphyroclasts in metamorphosed turbidites exposed on the north shore of the Isle of Arran, Scotland, just west of Lochranza:
Now an annotated version , to show the shape of the stiff, strong grain (porphyroclast) that resists dissolution, and the “beard” or “fringe” of precipitated quartz that grows in the “pressure shadow” adjacent to the grain:
All that quartz in the fringe adjacent to the porphyroclast had to come from somewhere. And where it came from is likely the direction 90° to the long axis of the fringe. It dissolves in the highest-stress direction (up and down in these photos) and re-precipitates in the least-stress direction (left and right in these photos). Here’s another example, also from Lochranza:
This one shows an additional feature, a dilational crack that has partially split the porphyroclast and opened up a little vein (also in the low-pressure zone). I’ve highlighted it with a blue arrow:
A rock body full of grains like these, shortened in one direction and enhanced in another, attains a fabric. It gets foliated. These Scottish rocks I’ve shown so far are interesting because the bedding (defined by grain size) and the foliation (defined by alignment of the mineral grains) are parallel:
That implies the maximum stress direction was perpendicular to bedding, which is to say, vertical (all else being equal).
In many circumstances, this is not the case. Next, take a look at an outcrop showing ~horizontal bedding and ~vertical foliation (Chancellor Group limy slates in Yoho National Park, British Columbia). Here, they are perpendicular to one another:
All else being equal, this might be the default expectation for metasedimentary rocks: the bedding is initially horizontal, and so is the tectonic shortening direction (thanks to plate tectonics), so we would expect a more or less vertical foliation.
Here is an outcrop showing bedding and foliation in metasedimentary rocks (Frog Lakes Conglomerate in the high Sierras of California), showing a non-parallel, non-perpendicular relationship:
Here is bedding and cleavage expressed in limestone and shale/slate of the Edinburg Formation, near Lexington, Virginia:
There’s clearly a lithological control on foliation’s development, right? It’s only weakly developed in the pure limestone, but where there’s a bit of clay in the rock layers, foliation/cleavage is pronounced and obvious. [Link to a GigaPan showing a similar outcrop]
Again there is an angular relationship between the two planar features. But this one illustrates nicely how bedding and cleavage both dip to the right, however bedding is steeper (closer to vertical) and cleavage is shallower (more moderately dipping), the opposite of the California example. This tells us that these beds have been tectonically inverted (flipped over).
Where seams of pressure solution are localized with big gaps between them, we dub them stylolites, and to believe the testimony of marker units such as calcite veins, they are apparently capable of dissolving away huge volumes of squeezed rock:
Where the pressure dissolution is more evenly distributed through the body of rock, and in particular where it changes the average grain shape within the rock or aligns non-soluble platy minerals (such as clays or micas), you get a proper foliation (± cleavage), like this example from the Swift Run Formation in the Virginia Blue Ridge:
And here is an example from a slab of building stone in Baltimore, Maryland:
Now for some interactive media: embedded GigaPans and GIGAmacros and also 3D models. We’ll start with the Chancellor Slate (shown earlier in this post), because it’s such a beautiful example of bedding and cleavage relationships:
Here’s an example of mylonite, a highly foliated rock formed along hot fault zones:
Here’s an example of cleavage refracting across sedimentary layers with varying amounts of clay vs. silt (Neoproterozoic Konnarock Formation of Virginia):
Here is an example of gneiss (Archean basement of Montana), which shows a much coarser flavor of foliation: it’s a 3D model, so you can spin it around and examine how the foliation looks from different angles.
Here is another 3D model, an example of an “L-S tectonite” (phyllitic metavolcanics from the Phoenix Mountains of Arizona), which shows a well-developed lineation on the plane of the foliation:
As for the second part of the question, about why foliation is important, I’d say that the single biggest reason is that it is one of the principal signatures of tectonic shortening (in one direction); implying tectonic elongation in another direction. Mountain belts form when rocks are shortened horizontally and thickened vertically. Though mountains are ephemeral features on the face of our planet (because erosion reduces their topographic expression over geologic time), the rocks at their roots will show foliation, and that tells us about the ancient episodes of tectonic squeezing they have enjoyed.
If you have other questions about science, the Earth system, or anything else, please ask them.