Rumeli Hisarı

Right after I got to Istanbul on this most recent trip, I took a taxi from my hotel down to the Bosphorus, to check out the Rumeli Hisarı, a fort complex built in 1452 by Sultan Mehmet the II in anticipation of the following year’s siege of Constantinople. It’s constructed at the narrowest point on the Bosphorus (660 m wide), with the aim of controlling boat traffic coming from the Black Sea. This narrow spot is today where they have the second of two bridges spanning the Bosphorus. It looks like this:

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It’s in Europe; that’s Asia on the far right of the photo. A few more shots of the fortress’s pattern of towers and interconnecting walls:

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Inside, I was pleased to note the variety of building stones. Here’s a nice porphyritic andesite which was a common constituent of the walls:
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And a folded limestone:

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Here are some yellowish blocks that are weathering away faster than the mortar which holds them in place. There is a Turkish 1-lira coin in front of the dark block near the center, to provide a sense of scale:

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Here’s a similar phenomenon playing out with some bricks used to make an archway, except here the mortar is the more rapidly weathering component:

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Check out this slab of brick… it’s got a curious adornment:

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Zoomed in to show this detail:

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Dog prints! Sometime a long time ago, maybe more than 500 years ago, a brick maker put out slabs of clay to dry, and some long-dead dog walked across it. The dog’s footprints are a kind of “historical trace fossil” that was then incorporated into this ancient structure.

Visiting the Rumeli Hisarı was a pleasant experience. I walked down along the Bosphorus next, peering into its surprisingly clear waters and counting jellyfish, then got a pide at a cafe. I caught another cab back to the hotel, and eventually fell asleep, a victim of jet lag…

rumelihisari03

Jointed Virgelle

One of the stops my Rockies students and I made this summer was a dinosaur paleontology tour through the Two Medicine Dinosaur Center in Bynum, Montana. The folks there are very accommodating, and at my request gave the class a bit of stratigraphic context for the dinosaur fossils. For instance, we visited the geologic formation which underlies the dinosaur-bearing Two Medicine Formation: it’s a beach sandstone called the Virgelle Formation. The Virgelle was deposited along the shore of the Western Interior Seaway, a Cretaceous-aged transgression of seawater onto the North American continent.

While our guide Corey discussed the primary structures that showed the unit to be “beachy” to my students, I got distracted by this outcrop:

virgelle_crackedField notebook for scale (long side 18cm).

So what’s so great about this? It struck me as a nice little demonstration of the relationship between stress directions and joint orientations. σ1 is our maximum principal stress direction (i.e., the direction of greatest stress), in this case caused by acceleration due to the force of gravity. σ2 is perpendicular to the screen of your computer (and the plane of the photograph): that is the intermediate principal stress direction. σ3 is our minimum principal stress direction (weakest stress), in this case pushing in from the sides (atmospheric pressure only, no overlying rock weight):

virgelle_cracked_2

By definition, σ1 is greater than σ3.

So we have a low-level confining stress paired up with the differential stress imparted by the heavy rock pushing down on the slab of sandstone beneath it. As long as that difference in stresses is greater than the strength of this weakly lithified Virgelle sandstone, then the rock will break, and the orientation of those breaks will be ~parallel to σ1, and ~perpendicular to the extension direction, σ3:

virgelle_cracked_3

You’ll also note that the bedding planes in the Virgelle sandstone are planes of weakness, accommodating the extension by allowing blocks of sandstone to slip sideways over what amount to small-scale “detachment faults” (low-angle, upper block sliding downward relative to lower block).

So does an understanding of these stress directions and the resulting structures’ orientation do us any good beyond this one lone slab of fractured sandstone?

Indeed it does. Keeping in mind that we are rotating our perspective from horizontal (“side view”) to vertical (“bird’s eye view”), consider the following map of central Asia:

baikal_ext_sigma

As the Indian subcontinent impacts the Eurasian continent, it moves towards the northeast. This results not only in the northwest-southeast-trending Himalayan mountain front at the site of impact, but also in extensional faulting further into the heart of the continent. Down-dropped blocks of crust in desert areas show up as northeast-southwest-striking rift valleys, but in wetter areas, those low-lying cracks fill with water, and show up to us as linear lakes.

baikal_ext_lakes

Lake Baikal in Russia is a famous example of this, but Mongolia’s Lake Hovsgol is a smaller version of the same thing. The lakes are oriented with their long axis ~parallel to the σ1 direction, as they have been opened up due to stretching in the σ3 direction.

Caveat blog-reader: The kinematics and dynamics of central Asia are actually a lot more complicated than this simplistic picture I’ve painted. My main point in drawing the parallel between the two examples is that outcrop-scale structures can serve as analogues that can help us understand regional-scale processes.

Turkey update

Hey there folks,

Long tıme no blog. I am enjoyıng Turkey. We spent several cool days ın Istanbul, checkıng out the awesome buıldıngs and twısty streets and great food there. Took a boat tour up the Bosphorus, walked across the Golden Horn. The Haghıa Sophıa ıs amazıng… a Chrıstıan cathedral datıng to Emporer Justınıan, then retrofıtted to be a Muslım mosque after the fall of Constantınople ın 1453 to Sultan Mehmet II, and now a museum as decreed by the man who made Turkey a modern, semı-secular natıon, Attaturk. Pretty amazıng stuff, and lots of cool buıldıng stones employed ın ıts hıstory-soaked foundatıons. I also really enjoyed walkıng through the Tokapı Palace and eatıng fısh sandwıches on the docks of the Golden Horn. We checked out the Roman ruıns at Ephesus, and then journeyed to Pammukkale, a travertıne-deposıtıng hot sprıngs where I am wrıtıng thıs brıef blog post. Tonıght we take an overnıght bus to Cappadoccıa, a trıppy landscape of eroded tuff deposıts. More later (wıth pıctures! I’ve been takıng so many pıctures!) when I get the chance to download them from the camera and upload them to the blog.

Hope all ıs well; dıd I mıss anythıng whıle I’ve been unplugged?

-Callan

1453, by Roger Crowley

So,  I think I dropped a hint here that I was planning to travel to Turkey this summer. Lily and I will be there from the end of June until the middle of July. (And I’ll be going back in October for the Tectonic Crossroads conference.) In preparation for a trip like this, I enjoy doing some research and reading some books. There are a lot of books about Turkey, and I’ve got at least two more I want to get through before I go, but I wanted to tell you about one that I just finished.

It’s called 1453, and the author is Roger Crowley. The book is a nonfiction account of the battle for Constantinople in the titular year, just one among many attempts by Muslims to conquer this Christian enclave in their part of the world. Situated on a triangle of land between the Bosporus and the inlet called the “Golden Horn,” Constantinople has fended off many attacks over the years. In a position to control commerce between the Black Sea and the Mediterranean, the city was a fabled jewel: it inspired some men’s covetousness and other men’s wholehearted defense.

istanbulImage modified from the NASA image (public domain) here

The geography of Constantinople protected it from assault on two sides due to sea (Sea of Marmara side + Golden Horn side), and the third side (facing west) was protected by several layers of defense: a moat, an outer wall, and a taller inner wall. For a thousand years, they had defended their city from outside aggressors. In the year 1453, the Byzantine empire (a.k.a. the “Greeks”) was defending Constantinople from the imperial advance of the Ottoman Empire (a.k.a. the “Turks”). Constantine XI was leader of the Byzantines; Mehmet II was sultan of the Ottomans. After a two-month siege, the Ottomans triumphed, and the city was sacked; its inhabitants enslaved or slaughtered. The city itself was transformed: The Cathedral of St. Sophia was remade as a Muslim mosque, the Hagia Sophia. The name “Constantinople” was replaced with “Islambol,” which became “Istanbul” with time.

It was a battle between two “great” world religions, with “great” in this case meaning “big.” Both prayed to their respective ideas of God, beseeching the deity that their cause triumph over their enemies’. Every time something positive happened to advance their cause, they sang the praises of their chosen deity. Of course, when things didn’t go their way, they did not infer that their deity had forsaken them (or that no deities were involved), but that they only needed to pray harder.

Powered by their deep religious faiths, they cut one another’s heads off, chopped one another into pieces, impaled one another on pikes. In addition, the raiding Muslims speared babies in Constantinople and raped women. (If it were Christians were to have attacked a Muslim stronghold instead, it seems clear that the “morals” would have led to similar depravity and atrocity in the other direction. That didn’t happen in 1453, since the Muslim army was all men, and the residents of Constantinople were mixed men, women, and children. For example, the Fourth Crusade, a group of warrior Christians, sacked Constantinople in 1204: this Christian attack on other Christians was the only previous occasion when the city had fallen to invaders. )

I came away from this book:

  1. …thinking “wow” — all this shit played out more than 650 years ago, and Constantinople was already a thousand years old when that happened! History is deep in some parts of the world. Even “civilized” history…
  2. …thinking “wow” — the siege of Constantinople is an epic saga, far more compelling than Troy. The back and forth, the ingenuity, the personalities involved in the 1453 battle: they are epic. This book would make an amazing movie. Read it to find out how Mehmet gets his boats into the Golden Horn, or how Constantine protects the city walls from Mehmet’s cannonfire. How does the Pope react to the Ottoman threat? How to the Genoese merchants on the north shore of the Golden Horn attempt to remain neutral while secretly pulling for Constantine? How close did Mehmet come to giving up?  I won’t reveal this wild stuff here but it’s a roller coaster ride.
  3. …thinking that Roger Crowley is an exceptionally talented writer, if he can make me care about these people, this event, lost in the netherworld between 10,000 BCE and the present. This man can write. What else has he written? Bring them to me!
  4. …thinking that I am very motivated to visit some of the critical locales mentioned in this book, where the battle for Constantinople took historical turns one way and the other. I’ll be in Istanbul a month from now; Stay tuned.
  5. …thinking how astonishing it is that people continue to think that their religion is the right one, in spite of being surrounded by other people thinking the same dang thing about different deities. This tribal thinking (“my people are the chosen ones; your people are infidels”) leads to tremendous suffering and bloodshed: people who don’t think the same things you do are by definition no longer people, and may be treated as non-human. [begin rant] I am reminded of a quote from Steven Weinberg: “With or without religion, good people can behave well and bad people can do evil; but for good people to do evil—that takes religion.” For thousands of years, our species has fought itself with tribal self-righteousness. It’s always “us” and “them,” and religion is the most frequently-adopted tribal cloak. In their own minds, religion absolves its practitioners from their atrocities;  by supplanting reason, it leads to unspeakable acts and horrific history. I am impressed by those religious individuals who think critically about their faith’s offerings, and apply the theological precepts with a modicum of common sense and an independent sense of ethics. But many religious people disappoint me deeply, with a series of actions that wreck the world along with any notion of consistency or moral “high ground.”  After reading about the battle for Constantinople, or experiencing the 9/11 attacks, or following the daily news, I can’t help but think the world would be a better place with a purely natural sense of ethics, and supernatural moral frameworks banished to the dustbin of thought. “Hypothesis not supported.” [end of rant]

If you are at all into history, or at all into Turkey, read 1453. As a rule, I’m not into history, but I am very grateful that fellow Turkey traveler Greg Willis recommended this book to me. In return, I loaned him Orhan Pamuk’s Istanbul: Memories and the City. I look forward to offering my review of that tome in the weeks to come.

The coming flood

In January, a large landslide occurred in the Hunza Valley of Pakistan’s Karakoram Range, near the village of Attabad. Like the Madison River landslide in Montana (1959), or the Gros Ventre landslide in Wyoming (1925), a river was dammed by the slide debris, and the impounded waters began to rise.

At Gros Ventre, the landslide-dammed lake overtopped the debris and caused a catastrophic flood which killed 6 people in Kelly, Wyoming. At the Madison River, the U.S. Army Corps of Engineers feared another Kelly-style flood, with Ennis, Montana being the (larger) vulnerable town downstream. They carved a spillway through the debris which accommodated the flow the Madison River, though a “Quake Lake” still remains upstream of the dam.

Dave Petley has been covering this growing threat at Attabad since the initial landslide on his blog, Dave’s Landslide Blog. I think Dave’s coverage has been absolutely superb — it represents the best of what geoblogging can be. He has been soberly reporting the facts and offering his considered interpretations for more than four months. He has tracked the continuing mass wasting in the area, the Pakistani government’s attempts to dig a spillway, and the growing seepage through the dam (with attendant erosion). On an almost daily basis, he has been posting graphs showing the rising lake levels and decreasing “freeboard” (distance between the lake’s surface and the lowermost point on the dam — the spillway mouth).

Now, the day has arrived when the rising lake is projected to finally overtop the dam. Dave’s prognosis is not a positive one: the spillway appears to be inadequate in size to handle the flow of the river even at normal rates of discharge (and certainly not during floods). The material composing the dam appears to be easily erodible, which raises the likelihood that the overtopping waters will rapidly incise downward, widening the spillway gorge rapidly into a lake-draining chasm. A flood is not guaranteed, nor is it guaranteed that if there is a flood, that it will happen today — but the situation offers little hope for optimism. We might get lucky and avoid a catastrophe — but there seems to be ample reason for grave concern.

Dave Petley seems to have been a lone western voice raising awareness of this growing hazard, and I feel he should be strongly commended for it. Dave  is accompanied by coverage from the Pamir Times, and a daily lake level dataset being gathered by an on-the-ground volunteer team called “Focus.” One can only hope that their collective efforts have not been in vain. The people downriver of the slide will need to move to higher ground until the threat has abated. It seems unrealistic to expect Dave, the Focus team, and the Pamir Times don’t convince them via blogging. I would venture to say that the Pakistani government should have called a mandatory evacuation of the area several days ago. It is their responsibility to be sufficiently on top of things and protect their citizens.

Good luck and best wishes to the people of the Hunza Valley.

Lola and the maps

My cat Lola has a thing for big sheets of paper, particularly maps. Here she is this morning, “helping” me plan a summer trip to Turkey:

Is this dike a feeder?

ResearchBlogging.org

A new paper in the journal Geology examines an interesting question: how can you tell feeder dikes from non-feeder dikes?

The answer is, normally you can’t. Normally, there’s no way to tell for sure whether a given dike actually funneled magma to the paleo-surface, or whether it never reached the paleo-surface. The reason for this is that usually, the paleo-surface is gone by the time the dike is exposed at the modern surface to your scrutiny. In the new paper, a team of Japanese researchers examined the plumbing of Miyakejima Volcano, which collapsed during an eruption in the year 2000. The collapse opened up a view into the volcano’s “guts,” which showed the anatomical details of many dikes.

Here’s Figure 2B from the paper (reproduced, as with Figure 3 below, with permission of the publishers of Geology), showing the extraordinary exposures on this volcano. The authors report that they were able to trace an individual dike for more than 350 meters. In this example, you can follow a feeder dike up 150 m to find where it erupted at the paleosurface in a cinder cone!

So, given such an extraordinary exposure, how do you go about assessing the geometries of the dikes? The research team used photography as their tool. Hopefully it will be obvious that examining the dikes in person would be difficult and dangerous on a subvertical cliff many hundreds of meters tall — and on an unstable and crumbly volcano, to boot! So they took photos, and then did their measurements based on the photos. They claim a resolution of about 3 cm per pixel at a distance of about 1 km.

Filtering your data through a medium like photography is a good way to introduce error and bias to your study, and the authors took some steps to avoid that. They used good zoom lenses, aimed at the outcrop face from the safety of the opposite side of the caldera, and aimed them straight on to the dike outcrops (i.e., within 10° of the strike of the dikes, not necessarily orthogonal to the cliff face, since there is no guarantee the dike would intersect the cliff face at a right angle): so the apparent thickness was as close as reasonably possible to the true thickness. For each photo, they cut off a 20% margins on each side of the image (total cropped area: -40%), as a guard against the effects of lens distortion. Finally, they double-checked their accuracy by comparing in-person measurements of objects of known size on the caldera rim to their photo-measurements of those same objects.

They defined feeder dikes as those (as in the image above) which were observed to connect directly to the bottoms of spatter cones and diatremes.  They defined non-feeder dikes as those which terminated “either by tapering away inside layers or ending bluntly at layer contacts,” where the ‘layers’ being referred to are pyroclastics and lava flows within this stratovolcano. In total, they tallied up 165 dikes, 93% of which were “non-feeders.” Of these, they selected the 27 best-exposed (21 non-feeders and 6 feeders) for their analysis.

What did they find? To quote from their abstract:

A typical feeder thickness reaches a maximum of 2–4 m at the surface, decreases rapidly to ~1 m at a depth of 20–40 m, and then remains constant to the bottom of the exposure. By contrast, a typical non-feeder thickness reaches a maximum of 1.5–2 m at 15–45 m below the tip, and then decreases slowly with depth to 0.5–1 m at the bottom of the exposure.

Width vs. depth data from five representative non-feeder dikes are plotted in their Figure 3, top row, and three representative feeder dikes in the second row of Figure 3. Check it out:

Feeder dikes open up (get wider) at the surface, but the non-feeder dikes first get wider (gradually positive trend to these plots), and then abruptly pinch out up towards the tip (sudden leftward cant at the top of the plot). The authors ponder these dramatically different profiles, and offer an explanation.

They offer two equations which describe these dike profiles pretty accurately. If you’re not mathematically inclined, take a deep breath. We’ll translate in a few column-inches! The first equation is:

b = (2Po(1-v2)L)/E

where b is the thickness of the dike, Po is the magmatic overpressure (the pressure in excess of the normal stress on the dike at the point of measurement), v is Poisson’s ratio, a measure of how much volume is conserved during strain for the host rock. In other words, when a material is compressed in one direction, how much do the other directions pooch outward? Call it ‘poochiness.’  E is Young’s modulus, a measure of the elasticity of the host rock. L is the “dike-controlling dimension,” that is whichever of the dimensions of the dike (either the dip-dimension or the strike-dimension) is smaller. So, to translate this equation into “English” enough that even Rick Sanchez could understand it, equation #1 says, ” The thickness of a dike of a given height depends on how much pressure the magma opening and filling the dike is under, along with how ‘poochy’ and elastic the host rock is.”

The second equation is:

Po = (ρrρm)gh + ρc + σd

where ρr is the density of the host rock, ρm is the density of the magma, and g is the acceleration due to gravity. The variable h is the dip dimension (height) of the dike (measured upward from the source magma chamber), ρc is the excess magmatic pressure in the source chamber before rupture (dike injection), and  σd is the difference between the maximum and minimum principal stresses. Let’s translate this one, too: “The pressure exerted by the magma filling a growing dike depends on the difference between the density of the magma and the host rock it’s intruding into, as well as the force exerted on the magma by gravity.  Another important factor is whether there are significant tectonic stresses impinging on the dike as it forms.”

So where does that leave us in interpreting Figure 3, showing those different profiles for feeder dikes versus non-feeder dikes? Equation #2 says that the magmatic overpressure in a dike (Po) will increase as the dike propagates upwards (gets taller, in other words: h goes up). And equation #1 says, if the magmatic overpressure increases, then the dike will get thicker. That’s why the non-feeder dikes get thicker and thicker in a nice gradual way as you trace them upwards.

An additional factor is related to the density. You can lower the density of a magma if you allow the gases in it to expand under lower pressure regimes (i.e., at shallower depths). The basaltic lava from this volcano has been previously measured to have about 2% water by weight. As this water exsolves from the magma at shallow depths (lower pressures), it will make bubbles that expand, and lower the density of the magma. However, at shallower depths, the rock surrounding the dike is under less pressure too, so they both decrease their densities in tandem.

Deviations from the expected dike geometries can be observed in some of the field measurements. For instance, in the lower-right-hand corner of Figure 3, dike “110-01” flares out to a wider thickness right as it crosses a stratum of “poorly consolidated scoriaceous tuff” within the volcano. The authors suggest that this rock type has a lower Young’s modulus. Because it’s poorly consolidated, it’s less elastic. A lower E value in equation #1 results in a larger b value, the thickness of the dike. Cool!

Now, the feeder dikes have a constant thickness all the way up. To the authors of the paper, this suggests that in the course of the eruption, these dikes reached a stress equilibrium with the surrounding host rock. Magma, being fluid, flowed away from highly-pressurized zones, and the dike thickness “evened out.” And why do the feeder dikes abruptly get wider at the top? The authors postulate a couple of possible reasons: First to consider is the elastic free-surface effect, which is essentially saying that as a dike approaches the surface of the Earth, half of the surrounding rock elasticity is lost (replaced by air), and so that control “hemming in” the dike is lost, and the dike expands. Second, erosion is probably an important factor, as the flowing magma churns away at the wall rock, breaking it down thermally as well as dynamically. In other words, some of the rock that used to be there at the edge of the fissure has been abraded or melted away as a consequence of all that lava flowing out of the dike and away over the surface.

Take home message? To quote the authors, “Feeders propagate and grow as non-feeders before they reach the surface. Therefore, the geometric difference between these types of dike… is primarily a reflection of the feeders reaching the surface.”

I’m interested in feeder dikes because Neoproterozoic feeder dikes of the Catoctin Formation are a significant piece of the geologic story of Virginia’s Shenandoah National Park:

…But these are interpreted as feeder dikes. To my knowledge, no one has claimed any particular outcrop in the Blue Ridge province as a spot where you can actually see the dike flare out and transition into a Neoproterozoic spatter cone. I picked up the Geshi, et al. paper in the first place because I wanted to know whether there was some measurable aspect of the Shenandoah dikes’ geometries that could tell me if indeed they were feeder dikes. The problem is that the exposure in Virginia (especially vertically) isn’t quite as good as the exposure on the inside of Miyakejima’s caldera. We’re lucky if we get 10 meters of vertical exposure, and there’s no suggestion from the Miyakejima data that that 10 m is sufficient to “profile” the dike sufficiently precisely to say whether it’s got a feeder geometry or not, especially if you don’t know where in the dike’s profile that 10 m vertical segment lies. So maybe all we Virginians can do is just interpret: we’ve got a bunch of Neoproterozoic dikes cutting basement rock, and atop the basement rock a bunch of Neoproterozoic lava flows, therefore some of those dikes are likely to be feeders.

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Geshi, N., Kusumoto, S., & Gudmundsson, A. (2010). Geometric difference between non-feeder and feeder dikes Geology, 38 (3), 195-198 DOI: 10.1130/G30350.1