8 February 2017
I’ve always been fascinated by parasitism. Parasites are organisms that live on or in another organism (or organisms) in a way that detracts from the vitality of the host. Nothing in nature is redder in tooth and claw than the parasite. They represent a stark repudiation of the naive way many people think of evolution, with humans at the pinnacle, with a historical sense of purpose or progress, and with the morality that might imply. Parasites live in highly specialized ways, and they harm they leave in their wake is more personally palpable as a result. They exist, as we all do, to propagate their genes to the next generation, and the destruction they cause along that path can be macabre and fascinating. As Charles Darwin put it in a letter to Asa Gray,
I cannot persuade myself that a beneficent & omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars, or that a cat should play with mice.
The ichneumons Darwin refers to are wasps that sting caterpillars (or spiders, depending on the species) to paralyze them. They stash the living but still bodies in secret burrows or chambers, and lay an egg or a dozen on them. The wasp larvae hatch and eat the host alive, saving the vital organs for last so the meat is fresh. Imagine being paralyzed and eaten alive, feeling those hungry grubs chewing away at your flesh and being unable to do anything about it, until finally, when your bodily larder is emptied, you’ve outlived your usefulness to them, and they burst through your skin to live out their destiny. It’s a horror show, and it shows decisively that natural organisms don’t exist as exemplars of moral purpose. They don’t live to comfort us. They exist to reproduce, and if they have to go through you to do it, they will.
There are even nematodes that parasitize the parasitic wasps, and other parasitic wasps that protect their conquered caterpillar by hatching two varieties of offspring: (1) reproductively viable individuals, and (2) others that basically exist only as booby-traps to kill other species of parasitic wasps that might happen upon the caterpillar smorgasbord post-paralyzation. The body of a caterpillar is a brutal battleground. It’s fascinating the learn about.
Lucky for us that Carl Zimmer has written the book Parasite Rex (2000), a survey of all things parasitic. Like everything Zimmer touches, it’s awesome science and awesome writing, leavened with a bit of personal anecdote and humor. I really enjoyed it. Chapters examine the societal (mis)appropriation of parasites as a political analogy, the relationship of parasites and the immune system, how parasites can drive evolutionary change, and parasites’ social and economic effects on humanity. Along the way, we get insights into the detailed physiology and biochemistry of dozens of parasitic organisms and profiles of the scientists who study them. One surprising point the book makes in an excellent and convincing way is that parasites can have beneficial effects on a host organism, and even to the functioning of an ecosystem.
Did you know that Plasmodium vivax (the little protozoan that causes malaria) and intestinal worms together rob humanity of 80 million “life-years” annually? Did you know that parasites are the majority of species on Earth? Did you know that some parasites take control of their hosts’ sex lives, castrating some and performing sex-changes on others? Dig into the deep ecology of Wolbachia and its peers by reading Parasite Rex, and feel your jaw drop in horrified wonder.
7 February 2017
Today, let’s journey to Iceland, to a bit northwest of Reykjavík. This is a view from the top of the Grábrók cinder cone, across the valley to the east.
With very few exceptions, Iceland is a big pile of basalt, and that shows through in the walls of this valley, which display a stack of basaltic lava flows, intercalated in places with pyroclastic debris or volcaniclastic sediment.
One portion of the far wall of the valley shows some discontinuities in the layering:
I had a satisfying few minutes puzzling over multiple working hypotheses for this observation. Is there a fault along that little green gully? How best to explain the highly inclined basalt layer to the right of that? What to make of the grayish granular pocket in the middle? (I’d be curious to hear your reactions, if you’d care to record them in the comments below.)
The valley appears to have glacial influence, on the basis of U-shaped “hanging valleys” that feed into it, but the bottoms is relatively flat, which may reflect a heavy sedimentary “backfiling” of the valley post-glaciation, as with the Yosemite Valley in California. A river runs through it today, with several channels that merge and bifurcate:
Take a look at the whole scene in this GigaPan. If you don’t have Flash enabled on your computer, you’ll have to click through to explore it:
Link 0.55 Gpx GigaPan by Callan Bentley
See if you can find: (a) the “fault” site I discussed above, (b) a hanging valley, (c) a river channel, (d) columnar jointing, and (e) an outcrop showing at least five sequential basalt flows.
6 February 2017
Over the weekend, I launched a new initiative on Facebook to assist my friends in learning about scientific matters that they are interested in. I want to serve society in combating science illiteracy, and I think one way I can do that is to solicit questions or topics from those who want to learn more, and use those as a springboard for discussion. It’s time for me to let my social media network including you, dear reader, drive the content of the blog. I’ve set up a simple Google Form to allow anyone to submit questions anonymously.
Here are the first two questions, and my responses:
1) Why do geologists get so excited about folds?
This is a question that is entirely within my wheelhouse, since folded rocks are one of the most prominent features of this blog. I think I find them so fascinating because they are so counter-intuitive to basic human experience. I mean, here is a rock, a solid thing that would hurt us if it hit us on the head, and yet it clearly displays evidence that it has flowed as a soft, putty-like substance. To reconcile these two facts, we have to think beyond our normal experience. This is what science is so useful for, of course – systematic, empirical study allows us to escape our quotidian Ape Brain and come to terms with nature as it really is. Much of what science has revealed is counter-intuitive: Continents move. The Earth orbits the Sun. Our ancestors were fish. Quarks exist. The universe is vast.
When we allow the rocks to deform under higher temperatures than we can handle, at pressures much greater than we can survive, or for time spans much longer than our ephemeral lifetimes, the stiff, hard rocks can behave in astonishing ways. Rocks can flow. I think folds show this most clearly, though metaconglomerates and shear zones make the point in a different way. The realization that rocks can flow is fundamental: It’s a thought that forces us to change our assumptions about physical conditions, about the passing of time. It allows us to conceive of mantle convection. It allows us to imagine the formation of mountains as a dynamic process. It’s essential to understanding how the Earth works.
2) Why are some people born with tails, excessive amounts of body hair, and even what appears to be animal hands and feet?
The short answer to this one is “humans are animals.” We are creatures of flesh and blood, made of the same atoms and programmed by the same genetic code, as everything else on Earth. Ultimately, every living thing on this planet is a relative, though some are closer relations than others. Humans are apes, descended from tree-dwelling shrew-like critters, which were descended from hairy reptiles, which came from amphibians, which are basically fish that learned how to walk. We bear the marks of this extraordinary ancestry in our bodies. Our skeletons are built on the same basic body plan as every other mammal, but modified to allow us upright posture and big brains. It’s not a perfect arrangement: women’s hips are barely big enough to accommodate massive-headed babies to be born. We are plagued by lower back pain due to the S-shaped curve in our spine that animals like chimpanzees or hyenas lack. But in spite of the less-than-ideal tweaks that make use unique, the shared affinity is hard to dispute:
One of the most fascinating classes I took in college was Developmental Biology, the study of how a single fertilized egg grows into a whole organism, with many diversified cell types. Though developmental biology is not my professional specialty, I think I can make some general comments toward answering the question. In our mothers’ wombs, embryos grow according to tried-and-true developmental routines, mediated by biochemistry. The amount of hair is a function of how much the genes for hair growth are “expressed” (that is, copied and enacted by enzymes). People that have a lot of hair might (1) have genes that say “lots of hair” or (2) they might just have vigorous expression of the same genes that in someone else might result in less hair. So there are the instructions (genes) and then how frequently those instructions are read and enacted (regulation). We probably carry all sorts of genes that were useful to our non-human ancestors that are now “along for the ride” without being expressed as active blueprints for our development. Other inherited genes might be doubled-up or tripled, and that extra expression might result in very different effects in humans compared to non-human animals. So a gene for making hair follicles for instance, might be increased or decreased, to dramatic effect.
Another factor to consider is the presence of biochemical gradients. The length of toes, fingers and the backbone may be controlled by biochemical gradients that run from front to back (or top to bottom, or left to right). Let’s say there is some hypothetical chemical that is more concentrated toward one end of the growing embryo than the other end. When the concentration of this chemical (shown in purple in the sketch below) is high, the developing embryo starts making “head” stuff by turning on “head” genes, but if the concentration is low, the “head” genes are kept quiet, and instead, “tail” genes are turned on.
The embryo stops growing one of these body parts when its chemical surroundings change. There are dozens of ways that could happen, but let’s just imagine what happens when something goes wrong, and the chemical gradient is altered in some way. Maybe it’s too strong, or maybe it fades off sooner than it should, or maybe it increases, then decreases, and then increases again. The development of the embryo will react to those changes, and toes might be too short as a result, or the pieces of the tailbone might tack on additional material, extending them to be longer than they would otherwise have been. Giraffes have the same number of vertebrae in their necks as humans do; they’re just a lot longer. Or perhaps the number of pieces of tailbone can be changed. Repeating segments is an easy recipe for evolutionary variation. Just ask the centipede, the boa constrictor, or the polycheate worm.
If there is some environmental change, it too could alter the gradient, or the reception of the growing embryo to the gradient. For instance, there is a medication called thalidomide that pregnant women used to take to combat morning sickness, but it changed developmental conditions so that many of their children failed to fully develop their arms and legs, being born with paddle-like appendages instead. The addition of something like thalidomide alters the developmental environment, and this is why it’s so essential for pregnant women to protect themselves from substances which would hurt the embryo’s development.
The tailbone (coccyx) is basically the end of the spine, and it shows the same pattern as the rest of the backbone: a series of segments analogous to vertebrae, but tapering off to a point. Depending on who you look at, you may find three segments to the tailbone, or four, or five, fused together to a greater or lesser degree. This variation is a reflection of either the genes of the individual, or else variation in the strength of some developmentally-relevant biochemical gradient as the embryo is growing. That variation is an observation which gives us an insight into the source of developmentally-induced variation in humans – the sort of conditions that might result in people having characteristics like those in the question. A non-human example is using the ancestry of the chicken to turn the bird “back” into a dinosaur. It just takes a few tweaks on what’s already there to create something that looks a lot more like a Compsognathus than a dinner.
The diversity we see in our human brothers and sisters is a reflection of our collective animal ancestry. It shouldn’t surprise us to see that some people have tails or extra hair, because we are descended from animals that had lots of hair and had tails. What would be truly weird is if something showed up that wasn’t in our evolutionary heritage: an unprecedented novelty like a set of wings growing out of our backs, or antennae sprouting from our foreheads. Don’t hold your breath for either of those to show up any time soon.
That’s all for this first installment of “Q&A.” If you have a topic you want me to explore, use the Google Form to submit your questions anonymously.
3 February 2017
Today’s set of Friday folds comes from just south of the Time Zone Bridge in the Canyon of the Salmon River, Idaho.
Can’t see them very well in that view? Very well, let’s switch from a perspective looking down on the plane of foliation to looking along the plane of foliation…
The rocks are mapped as the Squaw Creek Schist, a common unit near Riggins.
Basically, the edge of ancestral North America can be found in here. Folds exposed the Squaw Creek Schist are a result of the stresses of adding terranes onto North America’s westward margin. When Pangaea formed in the late Paleozoic, North America was moving relatively eastward, causing deformation and metamorphism along its eastern edge (the Appalachians). When Pangea broke up, North America started moving westward, opening up the Atlantic Ocean in its wake, as well as smashing into various terranes (chunks of crust) along its new ‘leading’ edge, the west. These terranes started off as oceanic crust, or island arcs, or microcontinents, out there in the wide Pacific Ocean (recently formerly known as Panthalassa). Multiple lines of evidence suggest that western Idaho was the “cowcatcher” on the front of the proverbial train. The Squaw Creek Schist could be thought of tectonic “roadkill” splattered onto the front of this continental Amtrak. As the continent thundered westward, it accumulated quite a bit of crustal debris all over its front bumper. Today we call that “Washington state,” and “Oregon.”
Here’s another nearby outcrop with a 3D exposure of a crenulated fold train:
These rocks were a stop on the excellent field trip I did looking at the accretionary tectonics of western Idaho, following the Rocky Mountain Section meeting of GSA last May.
2 February 2017
Today, let’s return to the Barberton Greenstone Belt in Barberton Mountain Land, South Africa. I visited there in late August of last year, and documented some of the extraordinary Archean rocks exposed in that place. The Barberton GeoTrail on the road called R40, which runs from Barberton to the Swaziland border is a great place to introduce yourself to some of the varieties of rocks that can be found here.
One lovely unit is this banded iron formation.
(There’s a pretty big difference in the vibrancy of the red depending on whether the sun was shining or not.)
Banded iron formation is a rock type that no longer forms on planet Earth. To get hematite and magnetite to precipitate onto the ocean floor, you first need to have iron dissolved in the seawater. This is hard to do if you have free oxygen in any quantity, the iron will react with the oxygen and precipitate out. So BIF is in fact a rock that records the local oxidation of iron, but that implies more or less global absence of oxygen, for the iron to be in solution in this quantity in the first place.
So BIF is evidence of a different ocean chemistry than what we have today, and that implies a different atmospheric chemistry than we have today. Like komatiite, it’s an “extinct rock” on our planet, evidence of a radically unfamiliar planet’s primordial operations.
The particular outcrop that’s “the BIF stop” on the Barberton GeoTrail also features some pronounced folding, which may represent soft sediment deformation. Here are few examples:
Here are two GIGAmacro looks at the front and back of a sample of BIF I collected in Barberton. You’ll need Flash to view these here; otherwise click through to see a non-Flash-based version of the image. Compare the layers of hematite to the layers of chert and jasper for yourself:
Link 0.52 Gpx GIGAmacro by Callan Bentley
Link 0.63 Gpx GIGAmacro by Callan Bentley
1 February 2017
Eshaness is in western Shetland, on the Northmavine Peninsula.
It’s a land of Devonian-aged mafic volcanic rocks, a cross-section through a stratovolcano the erupted hundreds of millions of years ago. The powerful forces of coastal erosion have chewed into these rocks, carving Shetland’s edge into a series of cliffs and sea stacks.
The rocks on display are mainly of two types: basalt and agglomerate (lahar deposits or pyroclastic ejecta making a rock sometimes dubbed volcanic breccia). One cliff, south of the lighthouse, shows basalt overlying volcanic breccia:
Zooming in on the top and bottom units:
Outcrops of the breccia nearer to the lighthouse:
And then there was this:
Perched atop the cliff top was an erratic: a boulder of granitic intrusion breccia, filled with angular fragments of older rocks of several flavors. This oddball was likely dropped here when Shetland was last glaciated. There’s nothing like it anywhere nearby.
The other side looks like this:
My son loved it! It seemed friendlier in its polished smooth texture than the craggy, poky agglomerate all around:
Here are three GigaPans of the site, if you’re interested in exploring some of the details. If you don’t have Flash enabled, click through to zoom in and explore them.
Link 0.58 Gpx GigaPan by Callan Bentley
Link 0.48 Gpx GigaPan by Callan Bentley
Link 0.63 Gpx GigaPan by Callan Bentley
31 January 2017
It has been a stressful few days for the United States of America.
Our President enacted a ban on people entering the country from several Muslim-majority countries, and it triggered confusion and protests at several international airports. A federal judge imposed a stay on the order, and yet Customs and Border Protection agents at Dulles Airport refused to abide, siding with the president instead. As Northern Virginia Representative Don Beyer pointed out, that’s a Constitutional crisis.
We have a constitutional crisis today. Four Members of Congress asked CBP officials to enforce a federal court order and were turned away.
— Rep. Don Beyer (@RepDonBeyer) January 29, 2017
While it’s to be expected that Democrats have been vocally opposed to the ban, very few Republicans have given voice to disapproval.
WHY is McMullin the rarity in talking like this?? This is how all our representative should be talking. This is what is happening. https://t.co/9XF25F9qsj
— emily nussbaum (@emilynussbaum) January 30, 2017
The tweet she links to is this one, a compilation of screenshots from Evan McMullin’s Twitter stream:
Evan McMullin Tweetstorm on Trump’s Executive Order: “This is a painfully common authoritarian move.” pic.twitter.com/SjeDpid2Oh
— Kyle Griffin (@kylegriffin1) January 30, 2017
I concur with Nussbaum’s opinion, and wonder why there aren’t more Republicans standing up for American mores in the face of an authoritarian within their own party. It seems to me as if they would have a super-strong incentive to stand up and be a leader in repudiating Trump, to be counted on the “right side” of history. Why aren’t they bucking the trend?
I’m reminded of the consensus on climate change. (Bear with me here on this analogy.)
Among scientists who work on global warming, most think that humans are causing it:
Image by John Cook
There’s a very strong incentive in a situation like that to be the lone brilliant genius who shows how wrong everyone else has been. For instance, Galileo is celebrated for overturning the prevailing model of how the solar system was arranged, and Wegener is lauded for being a visionary who brought us evidence for drifting continents before the greater society of scientists would accept such a crazy notion. If you can show decisively that the prevailing view is wrong, the world will hold you in very high regard.
No one has managed that task when it comes to climate change (probably because the consensus view is actually real). Ditto plate tectonics, a heliocentric solar system, the Big Bang, evolution, atomic theory, general and special relativity, etc. But imagine if they did. Imagine a scientist who found the data that disproved relativity or the Big Bang. Imagine the paleontologist that uncovered a fossil rabbit in the Precambrian! Such a person would earn fame in spades, commensurate with whichever trend they bucked, whichever consensus they disproved.
But where does that leave us with the Republican Congress? Their pie chart of who’s looking at their shoes and whistling vs. who’s speaking out in protest would resemble the pie charts for climate science. There’s an astonishing uniformity among GOP Congresspeople in their recalcitrance to address an issue of Constitutional importance. Though we’re talking policy leadership rather than empiricism, who will be bold enough to be their Galileo analogue, their Einstein, their Darwin? Scientists overturn the status quo with facts and logic; Republican politicians will instead have to use ethics and patriotism. Only a few are so far on record with a stance that places America in a higher priority than fealty to the GOP or the President. But I feel like each of them is holding the proverbial fossil rabbit in their hand. They are in a situation where they have a chance to distinguish themselves, and yet they are forgoing that opportunity.
There are a few notable exceptions: Evan McMullin has been a critic of Trump for a long time and willing to fight him on his chosen battleground, Twitter. So has conservative columnist David Frum (if you haven’t read his piece in the Atlantic that was released today, “How to Build an Autocracy,” I recommend it). John McCain and Lindsey Graham, long term senators and literal elder statesmen, have voiced opposition to the ban. Many voiced opposition to the notion of a “Muslim ban” when Trump was campaigning for the White House, including Mike Pence and Paul Ryan. But they’ve been very quiet since Friday afternoon. Some have described their actions as spineless, in creative ways. What’s holding everyone else back?
It looks like it’s about their chances of being reelected:
If their constituents support Trump, they feel that they must support Trump also, or else they will lose their job at the next election.
Simple enough equation, I guess, sad as it is to behold. “Ethics and Constitution be damned; it’s what the people of my district want.” But of course they would never be so bold as to actually articulate that stance on the record, where they might be held accountable for it.
It’s an important thing for an elected representative to stand up for the viewpoint of their constituents, and I respect that. But it’s shallow to pander to them, and it seems to me that in this case, the elected representatives have a responsibility to clearly outline where they stand on this issue, including whether they find it Constitutionally acceptable to impose a religious test on those who visit our nation. It would be great if we knew where they stood. It would be great if they were to make an effort at communicating with the public as to their views: they might convince some of them to their way of thinking. Moreover, we’ll be on solid footing in terms of whether history views them as (a) being complicit in advancing unconstitutional policy or (b) patriotic exemplars who value country more than party.
I’m grateful that the leaders of AGU and GSA released a joint statement today about the detrimental effects to geoscience of the ban. Professional societies have an obligation to stand up for their members and for their profession. One will hope that their peers on Capitol Hill will join them soon.
It is one of my most exquisite delights as a parent to read to my son. I love snuggling up with him on the couch in the evening and opening a book and diving in to share ideas together. In this blog post, I wanted to share three exceptional books that my family has chanced upon with you, in the hopes that you might read them with your kids, or give them to kids you know.
The first is Older Than the Stars, by Karen C. Fox, illustrated by Nancy Davis. This book takes children on a tour through the history of the universe, from the Big Bang until now, connecting the discernible reality of their modern bodies with the ancient happenings in an energetic young universe. Atoms are the linking concept, and this is where the title comes from – the atoms that make up everything, including young readers, originated in the heart of ancient stars. Thus, the kids are “older than the stars,” since their carbon came from a pre-existing star. Concepts like the nebular hypothesis and supernovas are covered in simple, easy to understand terms like “gas in a giant puff” or “the blast intense enough.” You’ll note the rhyme between those two phrases. That’s part of the extended cumulative construction of the book – it builds the story up a concept at a time, connecting them in a cadence that ultimately leads to the young person reading the story. A third of it: “This is the blast intense enough / to hurl the atoms so strong and tough / that formed in the star of red-hot stuff / that burst from the gas in a giant puff / that spun from the blocks / that formed from the bits / that were born in the bang / when the world began.” Each page brings a new chapter to the story, and a new phrase added on to the extended sequence of events. Each concept is explained in a brief (~3 sentence) explanation at a higher cognitive level than the rhyme. The entirety is quite a beautiful whole. My kid loves reading it. For review or fodder for discussion, a timeline at the end recounts the temporal sequence of events at the proper scale.
Next, consider Grandmother Fish, by Jonathan Tweet, illustrated by Karen Lewis. As with Older Than the Stars, the point of this book is to ground the child’s perspective of their place in the world in the context of their evolutionary heritage. It has five major parts, each separated into a two-part sequence. It begins by meeting ‘grandmother fish,’ who lived “a long long long long long time ago.” Then you move on to grandmother reptile, grandmother mammal, grandmother ape, and grandmother human, with each new character warranting one fewer “long” in the description of when they lived. Two of each organism’s abilities are described, and then the parent reading the book asks their audience whether they can do those same things (for instance, the fish can wiggle and chomp). Then the child is shown an evolutionary tree, and asked to pick out the next organism in the line that ultimately leads to us (for instance, grandmother fish’s descendants include coelocanths and sharks, but you want to find the reptile among the five choices on offer). Ultimately, the book’s conclusion has us marvel at humanity’s many children, ourselves among them. It’s an inclusive picture showing a diverse crowd of humans, young and old, brown and tan, disabled and upright. We are all related, and our ancestors are shared. We can all wiggle and chomp and crawl and cuddle, etc. These abilities are a consequence of our evolutionary history, the abilities selected for in our distance ancestors. Pretty cool perspective! A vast two-page tree of life is included at the end, putting the species discussed in a greater context, allowing the child to trace with their finger the genetic survival of their own life all the way back to the origins of life on Earth. Pretty powerful, and attractively illustrated.
The final book I’d like to endorse in the genre of “great things to read with your kids to give them perspective on their existence,” is The Golden Rule, by Ilene Cooper, illustrated by Gabi Swiatowska. Unlike the previous two, this book isn’t about science, but instead about ethics and morals. It takes the form of a conversation between a boy and his grandfather, discussing ‘the golden rule,’ the idea that we should treat other people the way that we would like to be treated ourselves. The old man explains the concept to his grandson, and takes him on a quick tour of comparative religion, seeing how this essential, simple concept transcends the particular flavor of one’s religious faith, or the idea of faith at all. The boy is asked to consider how he would apply the golden rule in his own life, and whether he would like it to be applied to him. They consider how the world would be if more people lived by the golden rule – what would be the effects on governance and history? Ultimately, the grandfather encourages the boy to begin applying the golden rule in his own life. It’s lavishly illustrated with rich paintings and sketches of the boy’s imaginings as the concepts are discussed and he considers them. It’s multicultural and universal, and a great opportunity for parents to explore “how to act” with their children.
We are made of pieces of dead stars. Our ancestors were fish. We should treat each other the way we want to be treated. These are big concepts, and these books make them accessible to a child’s mind. I recommend them all.
27 January 2017
Martin Schmidt offers us this lovely guest Friday fold. He writes:
This is the Bradshaw Layered Amphibolite. This rock was part of the offshore island arc in the Cambrian, and on the geologic quad is mapped as part of the Baltimore Mafic Complex.
It’s a good looking unit!
The Piedmont province is full of fascinating rocks like this: rocks which started off as oceanic rocks of some flavor, that were then baked and masticated in the tectonic maw of the young, virile Appalachian mountain belt. While most of the modern “peaks” of the Appalachian Mountains lie west of Baltimore in the Blue Ridge and Valley and Ridge provinces, these rocks are the highest metamorphic grade, and often show the most deformation. Therefore, they represent the ancient ‘core’ of the mighty Appalachian Mountain Belt. These rocks suggest that mighty peaks once towered in Alpine or Himalayan fashion above the spot where today we find Baltimore, Washington, DC, and Richmond.
Deep in the heart of Permian-aged Pangaea, the Bradshaw Layered Amphibolite buckled and squirmed and recrystallized beneath massive mountains. Those mountains decreased in elevation to the west, and were drained by Mississippi-sized rivers that transported their sloughed-off sediment far to the west. The Petrified Forest in Arizona was buried in Appalachian-derived sediment, if you trust the testimony of its detrital zircons.
Then Pangaea broke up, and there was a much shorter path to the sea for the rivers draining the eroding mountains. Soon, eastward-draining rivers took over most of the Appalachian mountain belt (though there are still holdouts like the extremely inappropriately-named New River of North Carolina, Virginia, and West Virginia). The Atlantic-draining rivers incised deep into the crystalline core of the mountain belt, exposing the high-grade, high-strain rocks that formed deep in its guts. That’s what we look at when we gaze upon a scene like this.
Want to visit yourself? Martin offers directions:
These are located at -76.395071, 39.412562 on the Big Gunpowder Falls (River), and at an actual (!) Coastal Plain-Piedmont border because it’s the first rocks & rapids coming upriver on the Gunpowder from the Bay. A couple of bends above this point the river is nearly all potholed rocks and rapids, especially at low water. The red, blue, or white letters in several images are graffiti.
A friendly reminder to readers that I’m happy to display your guest Friday folds here.
26 January 2017
A distinctive landform in Shetland can be found offshore of Eshaness, a blocky sea arch islet called Dore Holm (“Door Island”):
That’s a lovely beast, you must admit. I made sure to feature a photo of it in my article about the geology of Shetland for EARTH magazine. You should read that, if you haven’t already.
I was delighted to view Dore Holm for myself last summer because the previous spring I had selected it as an example to use when discussing joints in the newly-revised 13th edition of Essentials of Geology by Lutgens, Tarbuck, and Tasa. I helped revise the chapter on Structural Geology and Mountain-Building (Ch. 11), and suggested Dore Holm as an aesthetically-elegant example of jointing. Here’s the figure from the book:
I’m pleased that my suggestions on the color coding of the annotations was followed, but here’s my version of the scene, using my own photo:
A few quick comments on the #dataviz aspects of this image: I like color coding labels to match the thing they are describing. I also really don’t like using arrows for labels. I prefer to retain arrows exclusively for indicating motion. The authors of the book made some different decisions than I did, and that’s their prerogative, but I like mine better.
So what about the geology of this situation? When relatively stiff sedimentary or volcanic beds (volcanic, in this case: Devonian in age) are stressed in the brittle regime (i.e. either cold and low confining pressure, or rapidly), they break. The breaking is accomplished with fractures that show little to no displacement, called joints. It takes energy to break a rock, and so the easiest way to break it is to make the shortest joint possible. For a stiff tabular bed, that means breaking it perpendicular to the plane of the bedding. These orthogonal joint sets are among the most common structures to be observed in stratified rocks. Let’s look at one of these beds from the side, and imagine three hypothetical crack orientations relative to the bed:
The one that’s oriented perpendicular to the bed has the shortest distance to travel, the least amount of rock to transect, the least number of bonds to break. It is thus the most energetically efficient orientation, the easiest job to accomplish.
Clever eyes among you will have noticed that there’s another joint set in the photo of Dore Holm, but it’s parallel to the plane of the 2D image (perpendicular to the line of the photographer’s perspective). If you take a view looking down on the site, say from Google Earth, you are looking down on the more-or-less horizontal volcanic bedding plane. You can immediately see that there’s another, mutually orthogonal joint set sculpting the island:
This second joint set is perpendicular to the first, and also perpendicular to bedding. These joints intersect in X or T patterns (I can’t tell which from here, but I’d venture to guess X), different from the Y-shaped intersections detailed yesterday.
The three mutually-perpendicular fracture sets or planes of weakness divide the rock up into a Qbert style pile of blocks that the ocean can then pick away at in order to sculpt the island. When the erosive power of wave action works in tandem with the geometry of systematically-broken rock, we get neat features like Dore Holm. It’s a scenic place, but it also offers an exemplary lesson in the deformation and weathering of rocks on our planet’s surface. The island’s shape is a reflection of the parsimonious nature of the expenditure of energy in natural deformation.