14 February 2019
Darrel Cowan, professor of geology at the University of Washington, is a frequent Friday Fold contributor. He pitches in today with a real show stopper:
In 2010, I was on the south coast of Sicily with my Italian colleague, Prof. Stefano Lugli, from the University of Modena. We spotted this fold, looking SE, in what is called the “upper gypsum” unit of the Messinian evaporatic strata. Stefano wrote me that the location is at Giallonardo close to Siculiana.
I present the structure as a fault-propagation fold, and in the second image I add my interpretation: the position of the fault changes from parallel to layering to where it ramps up and cuts across the footwall strata into the hinge of the syncline.
Callan again here. That looks right to me; the annotation presented here is my own, with the fault drawn as a solid black line parallel to layering, and dotted in black where it ramps up across the strata. I’ve opted for that annotation scheme because while the rock in the fault zone appears crushed up (resulting the one zone of relatively poor exposure), it doesn’t appear like there’s much offset along this fault yet. In other words, I see this as an incipient fault-propagation fold. If deformation were to proceed, I’d really expect that sequence of massive-bedded gypsum to show more displacement on either side of the fault.
The other thing Darrel alluded to but I think deserves to be expanded on is that this package of “Messinian evaporatic strata” is extraordinary as a stratigraphic phenomenon in its own right, regardless of whether it’s been folded. About 6 million years ago, the Strait of Gibraltar closed, preventing the waters of the Atlantic Ocean from mixing with the Mediterranean Sea. The climate over the Mediterranean is arid, and as the evaporation:replenishment ratio approached infinity, the sea dried up. All the solutes swimming around as ions in that seawater became more and more concentrated as the water evaporated, and they eventually precipitated out as layers of salt and gypsum and limestone. It’s an extraordinary event which basically transformed this sublime body of water into a hellish sort of mega-Dead-Sea landscape for a time. Wikipedia has a nice introductory summary here.
We can infer then, that the deformation that Darrel and Stefano observed in Sicily must be younger than the Messinian “Salinity Crisis,” as you can’t deform something until it’s first formed, and that means it’s less than 6 million years old. This is a very young fold, popping up out the landscape. It may well be that movement along the fault will fold it further in the future, and that offset I want to see will be there someday after an earthquake or two.
11 February 2019
When Bugs Were Big, Plants Were Strange, and Tetrapods Stalked the Earth: A Cartoon Prehistory of Life Before Dinosaurs, by Hannah Bonner
It has been a while since I’ve reviewed any kids’ books here, but this one was so good that I just have to tell you about it. My son is now 6 and a half years old, and he’s interested in all sorts of natural history topics. Given that I’m a geologist, he’s probably more Earth-science-focused than the average kid, but my wife is a biologist, so he’s got plenty of interest in that too. He’s been very interested in bones and skeletons for a long time, and so when you put all of that together, you find a boy who’s going to be very predisposed to paleontology. He and I had an enjoyable day of middle-Paleozoic fossil hunting the other day, finding trilobites and other treasures. A subsequent visit to the library had us grabbing all sorts of books that might be relevant, and among the haul was When Bugs Were Big, Plants Were Strange, and Tetrapods Stalked the Earth: A Cartoon Prehistory of Life Before Dinosaurs (2003), by Hannah Bonner. Though the title is perhaps a bit long, the book is a perfect encapsulation of what we know about the Carboniferous and Permian, presented in elegant cartoons that really capture the organisms they describe. The prose is pitched at just the right level, and the jokes are witty and fun. Most importantly, the science appears to be entirely correct. This is so rare in my experience – many of the other “science for kids” books we read in my house have issues of emphasis or language that make it clear the authors aren’t practitioners of science themselves. But Hannah Bonner is the real deal – someone who’s clearly passionate about details (like taxonomy, anatomy, scientific nomenclature), respectful of the reader’s background enthusiasm and attention, cognizant of keeping her young readers aware of the big picture, and having fun the whole time. I’m really glad we stumbled onto this one, and you can bet your britches we’ll be seeking out others in this series, published by National Geographic.
Super highly recommended. Top notch!
8 February 2019
Today’s featured Friday fold comes to us from “Effjot” – an environmental engineer with a focus on groundwater. In reply to my plea for fold imagery several weeks ago, he shared this example from Germany:
I note the well developed gently-inclined cleavage in the muddier layers, as well as the layer-perpendicular jointing in the blockier layers. Both limbs dip to the left, one steeply and one subhorizontally. The cleavage also dips that same direction, meaning that all else being equal, this steep limb (main focus of the photograph) is tectonically overturned.
Zoom in & see what I mean:
…Which is to say:
More detail in this blog post by the source author. (but it’s in German)
Also, he told me that, “This is the first fold I’ve come across as a child and realised rock could be bent. The first foundation for my interest in geology.”
What a great moment to have anchored in one’s memory – Realizing rock can bend – that’s huge! I remember a particular boulder of pinkish vein quartz that I learned as a child was called “rose quartz,” a term for a discrete phenomenon I could observe. Other phenomena, other terms would follow. For me, that boulder was my “geo-anchor,” the beginning of my Earth science awareness. This fold was Effjot’s.
Readers: Do you have a similar memory of the moment that geoscience became a subject of interest for you? If so, please share.
6 February 2019
We saw last week how glaciation carved out a valley in Scotland called Glen Roy. As the glacier ground into the landscape, it liberated tremendous numbers of sedimentary particles from the bedrock, which is composed of Dalradian metamorphic rocks (mainly porphyroblastic schist in my observation). Then once that now-U-shaped valley had been deglaciated, a new glacier dammed it, making a lake that rose and filled the declivity of Glen Roy, notching lakeshore terraces into the surrounding hill sides. As the valley filled with lake water, waves took the cobbles and gravel along the shores and turned them over and over, burnishing off the rough edges. Over years and years, this not only notched the valley walls at constant elevations, but it turned the sediment into accumulations of fairly well rounded gravels.
Here’s a piece of fresh schist, with prominent porphyroblasts of garnet and staurolite:
You’ll note that this second specimen has nice crenulations (small folds) in its foliation, and that its edges look a little less angular -rounded no doubt by the sloshing waves of the glacially-impounded lake that used to fill Glen Roy:
I collected only one specimen from the site, and this is it, both front and back, as two GIGAmacro images. You’ll note that it is an even more well-rounded cobble:
Link 0.8 Gpx GIGAmacro by Callan Bentley
Link 0.8 Gpx GIGAmacro by Callan Bentley
[If these embedded GigaPans show an error message, just refresh the page so they load properly.]
I particularly like the second view here because it shows a pronounced band of garnets crossing the cobble – a compositionally distinct layer within the metamorphic rock which is best explained by a compositional layer within its protolith. That is: this was a layer in the sedimentary rock that this schist used to be with the right elemental ingredients in it, so that when the rock encountered high temperatures and pressures, it reacted just so, and made a layer of garnets. You’re looking at an utterly recrystallized sedimentary bed in that zone of big red dots.
These metasedimentary rocks of the Dalradian Supergroup are a big player in northern Scotland. I also saw them at a bunch of other locations, as these blog posts show.
They’re everywhere! But this cobble was one of my favorites.
4 February 2019
I just finished reading Gaia Vince’s 2015 volume called Adventures in the Anthropocene. The book chronicles the new version of Earth that humanity’s actions have enacted, exploring all sorts of relevant topics including biodiversity, energy use, urbanization, human population, ocean pollution, fish farming, deforestation, architecture, solar radiation management, etc. It’s quite comprehensive. The book I’m familiar with that comes close in scope and subject matter is Earth Odyssey by Mark Hertsgaard (1999). Both books document the authors’ travels around our planet, seeing environmental change first-hand, talking to people on the ground. They are works of reporting. It’s been more than a decade since I read Hertsgaard’s book, but I remember it being pretty depressing as he documents one horrific issue after another. The section on chemical pollution in China was particularly shocking, at least in my memory. The tone in my memory is one of “we’ve messed it all up.” A lot has changed in the past 20 years though, and it’s certainly justified to do a new “Earth odyssey” in the context of the “Anthropocene” perspective. We should be grateful to Gaia Vince for making that happen! In contrast to Hertsgaard’s book, Vince’s reporting takes a different perspective: her book accepts that some of this change is permanent and inevitable, and we’re going to be living with it no matter what. Vince accepts that the Anthropocene is our reality, and has a more utilitarian attitude to coping with it. But her book is not a downer; descriptions of problems are interwoven with the stories of those who are countering negative environmental problems with innovative solutions or sheer perspicacity. Artificial glaciers, for instance, or electrified reefs (to encourage calcite precipitation), or extracting fresh drinking water from fog with clever net-like structures. It is a book that is infused with hope. This is summed up nicely in the epilogue wherein Vince imagines her own son reading the book in the year 2100, and reflecting on the world that he lives in then, and how it contrasts with the world in which Vince wrote the book at the start of the century. There are some terrific innovations in this imagined future (fusion reactors), and there are some sacrifices (ocean acidification, the loss of coastal cities like Miami and New Orleans). It’s a clear-eyed book in that regard. Historical examples such as the ecological transformation of Ascension Island are cited as precedent. Predicting the exact details of the world of 2100 is difficult, as who knows what game-changers are beyond the scope of our imagination at this point. The eventual reality may not match what she sketches out here, but it’s probably close enough to a hopeful, grounded vision of the future to be useful in guiding us. I did find some small typographical errors of geological details, but these were relatively minor, and I don’t think they are fatal to the overall reportage or argument of the book. It’s a useful contribution with some compelling anecdotes and juicy statistics. For instance, I learned that a quarter of anthropogenic methane emissions come from anaerobic microbial activity in the sediments accumulated behind large dams. One sixth of India’s electrical consumption is dedicated to pumping water for irrigation. Per capita U.S. meat consumption has dropped by 10% in recent years. Actually, given that it was published in 2015 (and thus the actual who/what/when/where/why details are about the years preceding that), and the pace of change in our modern epoch is so rapid, it’s probably suitable to do a new version of this book again. As with the frequent updates from the IPCC, every five years is probably good for a new, engaging global survey of how humans and the planet interact.
1 February 2019
A Friday fold twofer: both from the sultanate of Oman, and both by Chuck Bailey of the College of William & Mary:
Chuck and W&M music professor Anne Rasmussen co-lead a field course in Oman called “Rock Music” – featuring explorations in their respective disciplines. These images are from the most recent iteration of that course, earlier this month:
Fortunately for the rest of us, Chuck Bailey blogs! We can read in more detail about Anne & Chuck’s peregrinations here:
Enjoy the armchair adventure, and happy Friday to you!
29 January 2019
To get our minds warmed up on this cold Tuesday, consider this scene:
That’s my son, frolicking in the Scottish landscape a few years ago. Here’s one of his companions, contemplating me:
What do you notice about the background of these two scenes, the valley called Glen Roy?
Let’s clear away the foreground distractions and take a good look:
The U-shape of the valley is a straightforward initial observation. Standing on the shoulders of giants, graduates of Physical Geology courses will be happy to tell you that indicates this valley has been glaciated. More subtle though are the horizontal lines on the hills bracketing Glen Roy.
Take a closer look, and compare the near hill to the far hill:
These lines are perfectly horizontal, and there are three of them. In the photo below, I’m standing at the elevation of the lowest of the three, as you can see the position of the line on the near hill lines up with the position of the line on the far hill:
The elevations of these lines are at 260 m, 325 m, and 350 m:
So what do they mean? What is the origin of these features?
They’ve been dubbed “the parallel roads of Glen Roy,” but they aren’t really “roads.” One idea that might occur to your modern mind, following on from the glacial U-shape observation, is that they are glacial striations, but this doesn’t work because they don’t just go in one linear direction; instead they wrap around the landscape like contour lines on a topographic map, never deviating in their elevation. Glaciers don’t do that. Glaciers flow, and they flow downhill.
So what else is there?
Charles Darwin, that plucky young geologist freshly back from his voyage on the Beagle, took his ideas about crustal uplift and subsidence to Glen Roy in 1838. Darwin had been deeply impressed in South America by evidence of tectonic uplift: marine terraces at Coquimbo, Chile, in addition to his personal experience of an earthquake at Santiago. Of course, he had also been reading Lyell, and Lyell extracted a surprising story of recent subsidence and uplift from the Temple of Serapis in Alexandria, Egypt. He used a lithograph of three columns from the temple as the frontispiece for the first several editions of Principles of Geology:
Once you learn a compelling or useful idea, it’s easy to see it everywhere. Darwin applied these notions of vertical motions of the crust to Glen Roy. He thought Glen Roy was a former fjord that had been uplifted, and the “parallel roads” were ancient seashores. The next year (1839), he presented this idea formally to the scientific community.
Darwin was conscious of another hypothesis, pitched in the 1810s by John MacCulloch and Thomas Dick Lauder, who suggested these were not seashores, but lakeshores. The issue Darwin had with MacCulloch and Lauder’s idea was that he couldn’t come up with a good way to dam Glen Roy and impound the water. No dam, no lake. No lake, no way to make a lakeshore. And Darwin knew the crust moved up and down elsewhere, ergo that was what he considered the best explanation for Glen Roy.
Enter Louis Agassiz, who in 1840 published Studies on Glaciers, the expression of ideas he had been promulgating since about 1837. Now there was a ready answer for the source of the requisite dam: it was made of ice! Indeed MacCulloch and Lauder had been correct, and Darwin was wrong. Glen Roy had been dammed by a glacier (the Spean Glacier), and as that glacier advanced, it blocked the flow of fresh water coming down the valley. The impounded water (a lake) found its way out of the valley at a series of three different spillways, one at 260 m (a valley east of neighboring of Glen Spean), one at 325 m (a spillway connecting an arm of Glen Roy to Glen Spean), and finally one at 350 m (the head of the Glen Roy valley itself).
So this scene…
…now looked like this in the minds of time-traveling geologic thinkers:
(I did that with Photoshop!)
Here is a set of three panels from an informational sign at a Glen Roy overlook to show the sequence of events:
The parallel roads of Glen Roy will not last forever. These subtle etchings on the hillsides are being torn down by modern fluvial erosion. The tracks of gullies and debris flows now overprint the Pleistocene glacial geomorphology, and the sharp definition of the ancient lakeshores will degrade more and more through time:
It won’t be long (a few centuries, a few millennia) before this subtle landscape signal is lost to direct observation. So get there while you can!
This is a terrific place to contemplate geoscience controversies in general, and an excellent place to make the point that celebrated scientists such as Charles Darwin didn’t get everything right. Darwin proposed multiple hypotheses during his career. Some of those (such as evolution by natural selection) have stood the test of time and scientific scrutiny. Others (like his seashore explanation for the parallel roads of Glen Roy) have been shown to be inadequate, a poor match for the available evidence. In science, we value truth above all – more than the celebrated personalities who bring that truth to light. Good ideas survive to fight another day; flawed ideas get tossed in the dustbin. Darwin wasn’t a god; he was a scientist, a generator of ideas. Some of his ideas have been validated; others discredited. This is one of the latter.
If you want to explore Glen Roy on your own from the comfort of your laptop, here are four GigaPans I made of the valley and its “roads.” Each can be explored with your mouse/cursor:
Link GigaPan by Callan Bentley
Link GigaPan by Callan Bentley
Link GigaPan by Callan Bentley
Link GigaPan by Callan Bentley
[If you get error messages with any of these, refresh the page in your browser. That should fix it.]
25 January 2019
I’ve reached the bottom of my larder of fold photos, so today I’m perusing the many beautiful images on Marli Miller’s Geology Pics website. We have featured some of her photos previously, you may recall.
Folded felsic dike in mylonitic rock–indicates deformation continued after intrusion. Okanogan Metamorphic Core Complex, Washington:
Folded radiolarian (ribbon) chert, Oregon. These rocks belong to the Jurassic Otter Point Formation. Photo is about 15 meters across:
Folded schist in New Zealand’s southern Alps. Photo is about 0.5 m across:
This last one doesn’t have location information, but it’s a synform too perfect to neglect, so let’s close our gallery out with that image:
Happy Friday all!
And, I suppose it also needs to be said: Unhappy Day 35 of the U.S. federal government shutdown.
18 January 2019
This shows a section of high grade gneiss in the spillway of the Saluda Dam in Columbia, South Carolina.
Zooming in there, you can see a nice fold hinge on the left edge:
Although I don’t know the precise location of this outcrop, I did a bit of internet sleuthing… According to the interactive geologic map of South Carolina, these rocks appear to be gneisses in the Modoc fault zone: “A distinctive suite of mostly mylonitic rock with numerous mesoscopic to map-scale orthogneiss sheets oriented approximately parallel to the boundaries of the zone, upper amphibolite facies rocks.”
Thanks for sharing, Scott!
I hope everyone has a safe and rejuvenative weekend ahead. My thoughts this weekend are with all those geoscientists furloughed by the federal government over the budget impasse and are going on a month without work. They deserve better.
15 January 2019
As I mentioned yesterday, Galapagos land iguanas eat mainly the flesh and fruits of the Opuntia cacti that grow all over the dry lowlands of these islands.
As with most other cacti, the Galapagos Opuntia have spines to protect themselves from being munched by herbivores…
… Except where they don’t!
That’s right – In this video, you see my own defenseless hand, petting the cactus as if it were a cat!
Here’s a photo showing similar “cactus petting:”
Rather than being stiff and sharp and painful, these cacti have “spines” that behave more like hairs.
But doesn’t this leave the poor cactus defenseless against attack by vicious iguanas?
Yep! It sure does. …Good thing that doesn’t matter.
This is what happens when the cactus have a long time to grow and reproduce without being molested by vegetarian lizards.
On islands where land iguanas roam, cactus individuals are under strong selective pressure to have some sort of defense. Stiff spines are therefore essential for survival. Hungry, hungry lizards keep the pressure on the cacti to maintain strong defenses. On islands where the cactus grows and is never attacked by iguanas, however, that pressure is released. It’s no longer a “deal breaker” for survival if a mutation makes a cactus grow non-stiff spines. It doesn’t need spines any more, so it doesn’t matter. On iguanafied islands, spinelessness is a recipe for death. Dead iguanas don’t have babies. On noniguanified islands, spinelessness is either (a) fine and neither a good thing nor a bad thing or (b) maybe even a good thing, if making stiff spines is in some way energy or resource intensive — those resources are then freer to be reallocated elsewhere. All else being equal, the spineless, hairy cacti can survive, no problem! And that means they can reproduce, which means they can pass this trait on to their offspring.
Evolutionary prediction: if you were to release land iguanas on these “hairy cactus” islands, the iguanas would go to town on the defenseless cacti, strongly selecting against the “hairs-not-spines” trait. If there were a few individuals that still made stiff, sharp spines, they would be at a huge selective advantage. They would be selected as the survivors, and then would be eligible to go on and be the breeding stock that would be the foundation of the future cactus population on those newly-iguanified islands. The proportion of spines/hairs in the population would be predicted to increase.
It’s a neat trick to pet a cactus and not come away bleeding, but it’s also a nice prompt for a little lesson in how natural selection works.