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…

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Building stones of the Haghia Sophia

The Haghia Sophia (or “Ayasophia”) is an astounding building in old town Istanbul. It is an ancient cathedral turned mosque turned museum. Through all these incarnations, the Hagia Sophia has retained some features and had other ones added on: it is a palimpsest of architecture, symbology, and history. Walking through its soaring main chamber, or side passages and alcoves, visitors like me stand with necks bent and mouths agape. It is an unparalleled location for peeling back the layers of time.

Built in 532 CE by the Emperor Justinian, the cathedral rose on the same spot where two earlier churches had stood, the first of which was built in 360 CE. The name “Haghia Sophia” comes from the Greek for “holy wisdom.” For more than a thousand years, it served as the principal church of the Byzantine Empire. It was the world’s largest cathedral for thousands of years. The minarets were tacked on in 1453, after Constantinople fell to the Ottoman Empire:

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There’s a gazillion aspects of this building to discuss, but today I’d just like to share some images of the different building stones seen in and around the Haghia Sophia. To start with, here’s a “Verde Antique” (serpentenite breccia) sarcophagus outside the building:

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The floor stones in an interior hallway, worn smooth and shiny by millennia of human shuffling:
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And a bunch of shots of stones used in the interior walls …

Granite (verging on unakite?):

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Conglomerate:

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Rhyolite porphyry:

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Rhyolite porphyry with xenoliths (also used to construct a sarcophagus outside):

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Marble gneiss:

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Darker granitoid:

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There are also some structurally interesting rocks, like this red and white marble breccia that shows pressure solution. Notice the sutured boundaries of the white grains, and their pronounced long axes, 90° to that maximum pressure direction.
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Kind of reminds you of the Purgatory Conglomerate, right? (Me too.)

My favorite rock there is this lurid, gory red/white/black marble gneiss, as it displays ptygmatic folding (elsewhere it is also boudinaged):
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I wish I had more photos of this stuff. It’s great. It reminds me of guts!

Here it is in a typical display (pardon the blurriness of the photo): they “fillet” the rock and spread it open in the manner of a Rorschach blot. This produces an attractive symmetrical design, with minimal artistic effort:
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Another nice “butterfly” spread, this one of folded marble gneiss:
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Another one:
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Look close at this one. Note the little gray crosses in there? Let’s zoom in…

Here’s one closer-up:

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These are ancient Christian crosses, or rather, the holes where ancient Christian crosses were once mounted on the wall. When the Haghia Sophia was converted to a mosque in 1453, these Christian symbols were removed, and the holes cemented over to obliterate traces of the old religion. Here’s another one, where the cement has fallen away:

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Along similar lines, here’s some Arabic script carved into the railing of the second floor, marring a lovely marble breccia:
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Stuff like this just floors me. I mean, think about all the different people to lean on this railing over the past 1500 years. The Haghia Sophia’s history is so deep, with so many distinct overlapping layers. The mind reels…

A fantastic concentration of building stones may be found at the “Coronation” spot on the main floor of the building, where Byzantine kings were crowned:

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After several pleasant hours touring the Haghia Sophia, we got lunch at a great cafe nearby. Lily got lentil soup:

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…and I got an amazing pide, the Turkish style of “pizza”:

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Delicious rocks followed by delicious repast! Can’t complain…

The Blue Mosque

In Istanbul over the summer, Lily and I checked out the “Blue Mosque,” named for the predominant color of the mosaic tiles in its interior. It’s more formally know as “Sultan Ahmed Mosque,” named for the sultan who commissioned its construction in 1609. It is an elegant building:

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I loved the “pile of bubbles” effect of the multiple domes, and then the skyward piercing forms of the minarets.

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It also cuts an impressive silhouette at night:
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The mosque is open to the public, including tourists. To visit it, you are asked to remove your shoes. Women are asked to cover their hair. Here’s Lily in the appropriate garb:

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I was shocked to see how many tourists completely ignored this request, whether out of contempt for the fact that Islam treats men and women differently, or out of sheer cluelessness. I’m no fuddy-duddy, but it seems to me that when you’re visiting a house of worship, you should follow the requests of the host faith.

Once inside, we got a look at the tiles for which the place is named:

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There were some interesting uses of building stones. Consider this arch, made of alternating blocks of conglomerate and marble gneiss:

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The ceiling of the Blue Mosque soars high above, decorated with more tiles:

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An ugly addition to this elegant architecture is a rack of lights, loftily called a “chandelier,” suspended on long cables. I thought this modern tack-on was rather tacky, but I guess it makes prayer easier in the dark hours:

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The ceiling is held up by four enormous pillars; many architectural critics find these ungainly and obtrusive:

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After our visit, we went to get some traditional Turkish tea at a little place overlooking the Bosphorus. The tea is very sweet, but comes with extra sugar cubes regardless:

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Here’s the view of the Bosphorus, the straight separating European Istanbul from Asian Istanbul. You’re looking north in this photo, with Europe on the left, and Asia on the right:

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Duke Stone

I wrote last fall about my visit to the Duke Quarry, home of a charismatic metavolcanic rock used to face buildings on the campus of Duke University in Durham, North Carolina.

Here’s a sample of the “Duke Stone” that I brought back to NOVA, cut, polished, lacquered, and scanned. It’s quite lovely. You can click through (twice) for the biggest version:

Gorgeous, isn’t it?

DC fossil website is now live

Chris Barr’s informative website of the “Accidental Museum of Paleontology” that can be found in D.C. building stones is now live. You should go and check it out, and if you ever visit the city, you can use it as a guide for your tourism.

Falls of the James I: pluton emplacement

Last Friday, NOVA colleague Victor Zabielski and I traveled down to Richmond, Virginia, to meet up with Chuck Bailey of the College of William & Mary, and do a little field work on the rocks exposed by the James River.

Our destination was Belle Isle, a whaleback-shaped island where granite has been quarried for dimension stone for many years. The island has also served as a Confederate prison for captured Union soldiers during the U.S. Civil War, and later for various industries. Today, it is preserved as park land, utilized by a wide swath of Richmond’s populace for recreational activities, both licit and non.

Fortunately, a large area of the James’ river bed south of Belle Isle is kept relatively dry by a long low diversion dam upstream. As a result, there are some mighty fine horizontal outcrops of rock:

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The dam fed water into a hydroelectric power generation station, but that station has been abandoned for some time now:

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The power plant dam has yielded enough exposure that some bedrock mapping is possible for those with the curiosity and fortitude to attempt it. Here’s a simplified geologic map of the area, authored by Chuck and his student James McCulla:

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So you can see that most of the area is covered by sedimentary deposits of both modern and early Cenozoic vintage. Our goal, however, was the more interesting stuff beneath that. (All due respect to my sedimentological colleagues; the Coastal Plain just doesn’t get my juices flowing like ‘crystalline’ rocks do!)

So here’s what we came to see, the Petersburg Granite:

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This is an Alleghanian pluton, ~320 Ma, and quite large: it extends for tens of kilometers north and south (Petersburg, the namesake locality, is to the south). It disappears beneath the Coastal Plain to the east, and beneath the Richmond Basin (a Triassic rift valley) to the west.

You can see from the photo above that in some places the Petersburg Granite is massive and equigranular, and in other places it’s “foliated,” with long dark lines running through it. These lines are schlieren, curtainlike zones of differing mineral ratios: more mafics than felsics, for instance. The schlieren (German for “lines”) are usually interpreted as magmatic flow structures as higher-temperature-crystallizing mafic crystals raft together in a more felsic flow. At Belle Isle, the schlieren are steeply dipping and trend NNE.

In places, there were also pegmatite bodies that were concordant (~parallel) with this overall magmatic fabric. Here’s an example of that texture:

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And here’s a really big crystal of K-feldspar set amid finer-grained granitic groundmass. I guess you could call this a “megacryst”:

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Another thing we saw a lot of were dark-colored inclusions in the granite. These were dark due to lots and lots of biotite mica in them. Here’s an example; notice how the schlieren wrap around it:

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And another, with its long axis oriented parallel to the strike of the schlieren, suggesting alignment in the magma chamber before the granite set up:

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How should we interpret these mafic inclusions? Are they xenoliths; fragments of country rock that were broken off and included in the intruding granitic magma? Or do they represent a plutonic emplacement process — perhaps an earlier stage of crystallization, or an immiscible bolus of mafic magma floating like a lava lamp blob in the surrounding felsic melt? When they’re fine grained and lacking internal structures, as with the above examples, it’s really hard to make that call.

On the other hand, this one clearly shows fragmentation along the right edge, suggesting to me that it was a coherent xenolith at the time the enveloping granite set up into solid rock:
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That rules out the fluid-blob-within-another-fluid hypothesis, but is it country rock?

This one suggests that it is indeed country rock, as it is both foliated and kinked internally:
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Here’s a heart-shaped inclusion which also suggests that it is a genuine xenolith. As with the previous example, it displays internal foliation that has been folded:

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Victor ponders these xenoliths, as well as a dense clot of biotite (dark steak next to the yellow field notebook – not Chuck’s shadow, but parallel to it and closer to the photographer’s vantage point):

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The photo above also shows how the schlieren wrap around these xenoliths. Here’s an example where the schlieren “tails” leave the xenolith “higher up” on the left side than the right side, suggesting a sinistral (counterclockwise) sense of magma-flow kinematics:

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This one is a beauty. It’s almost perfectly circular in cross-section, though with little flanges coming off the upper left and lower right. However, the “tails” are both on the same side of the xenolith, so I don’t really feel like I’ve got a good bead on its kinematics:

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A few more shots of these xenoliths:

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This one is a cool one…

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… because when you zoom in on the edge, you can see it has some ptygmatic folding inside it. Like the foliation and the broader folding we observed earlier, this internal structure suggests that these are genuine xenoliths; fragments of pre-deformed country rock.

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Another xenolith, also showing this internal deformation of ptygmatically-folded granite dikes:

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…And this one shows internal boudinage:

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Chuck examines a small vertical surface to get a sense of what these xenoliths are doing in the third dimension:

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This next bit was a real treat for me. It’s no secret that I’m a huge fan of boudinage, that brittle-ductile phenomenon that separates a more competent rock type into sausage-like chunks while a less competent rock type flows into the void between those chunks. Here’s some schlieren that evidently became thick enough slabs of biotite that they were able to behave as semi-coherent sheets, subject to boudinage:

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…Not only that, but if you back out and follow these boudinaged schlieren along strike, you can see that they are folded, too! Check out these sweet isoclinally folded, boudinaged schlieren:

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Biotite-rich inclusions which I interpret as similar “scraps of schlieren” which became entrained in later magmatic flows:

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While everything I’ve talked about so far has been concordant with the dominant schlieren orientation (and thus reflective of main-stage magmatic flow in the Petersburg Granite), there are also some discordant features, like dikes, which cut across the regional fabric.

Here, for example, is an aplite dike:

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Aplite is very felsic and displays a “sugary” fine-grained texture. This aplite dike is quite a nice feature, traceable over a long distance across the outcrop. We followed it a ways to a spot that Chuck was particularly eager to show us: a spot where the aplite dike crosses an earlier pegmatite dike, and then both dikes are cut by a right-lateral fault and a fracture set which parallels the schlieren. Check it out in outcrop (note the positive relief on the aplite dike):

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And here’s a sketch of this outcrop (above photograph from the perspective of the lower right corner):

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What a fine spot to bring students and have them suss out the order of events! First came the massive granite, then the pegmatite dike, then the aplite dike, then sometime later under very different P/T conditions, the rock was fractured and we get fractures: some of which show an apparent right-lateral offset (faults; oriented ENE), and others where no offset is apparent (joints). This second set appears to be utilizing the schlieren as zones of weakness, as it is parallel to the schlieren (NNE) and often occurs along their biotite-rich traces.

Whether the faulting or the jointing came first is a question we’ll examine in the next episode

New metaconglomerate sample

Courtesy of my scanner, and my generous student Saadet M., whose family runs a building stone business here in northern Virginia… behold a lovely metaconglomerate:

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This is a really cool rock. I would rather have this than diabase for my countertop.

It doesn’t show up well in this scan, but the rock has a moderately well developed foliation in the matrix: I’d describe it as “phyllitic verging on schisty.”

The next two images are zoomed in on certain portions of the rock, showing what I think it really cool about this sample: some of the pebbles have started to fuse with one another, with the grain boundaries of one impinging on the grain boundaries of another. Kind of reminds me of a certain batch of fusilinids. Check it out – Here’s a small quartz pebble getting squished between a bigger quartz pebble and a pebble of something gray.
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Here’s another close-up, showing the pebble at left impinging on the pebble at right. Furthermore, this photo shows the “fused” texture that has developed in this metamorphic rock, as a fracture propagates from lower left to upper right, totally ignoring the “discontinuity” of the grain boundary:

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Pressure solution is dissolving the pebble at right, and the quartz that leaves is either leaving the rock entirely (perhaps filling a vein somewhere), or precipitating in low-pressure areas (“next” to pebbles, perhaps, as pressure shadows).

Very cool. Thanks again, Saadet!

Other geobloggers: have your students or colleagues or second cousins or co-coffee-shop-denizens brought you anything cool like this lately?