Tavşanlı Zone field trip, part 1

Before the Tectonic Crossroads conference two weeks ago, I had the good fortune to participate in a Istanbul-to-Ankara geology field examining the Tavşanlı Zone, a tectonic suture zone where a portion of the Tethys Ocean basin closed. This paleo-convergent boundary is marked by a suite of interesting rocks, including blueschists, ophiolites, and eclogites. I’d like to share with you some of the things I saw along the trip.

This is one of the trip leaders, Aral Okay (pronounced “Oh-kai,” okay?), discussing the general geology of the area at our first stop. (The other trip leader was Donna Whitney.)

tav_1_01

I think in general, you can make out the east-west trend of the rock units on Aral’s map (where they aren’t obscured by alluvium). This reflects the approximate north-south convergence of the Tethys closure in Turkey. To visualize this, I’d like to call your attention to a paleogeographic interpretation of the Tethys Ocean from Ron Blakey, the talented mapmaker from Northern Arizona University:

tethys

See all those colliding east-west-oriented crustal fragments in the northwestern Tethys? Those are the pieces that will comprise future Turkey. As you can imagine, rocks caught up in these tectonic collisions got both deformed and metamorphosed. Some of them were even subducted to ~80 km depth, and then brought back up to the surface! At our first stop, we saw some blueschist-grade rocks that had a phyllitic texture. Here’s two of them:

tav_1_02

As usual, my eye was drawn towards the structures visible in these rocks. Here are a couple of nice little folds:

tav_1_03

tav_1_05

(The Turkish 1-lira coin is the same size as a U.S. quarter.)

I found this to be an interesting portion of the outcrop:

tav_1_04

That’s green phyllite on the left, and blue phyllite on the right. Allow me to annotate it for you:

tav_1_04anno

“Blueschist” and “greenschist” refer to two assemblages of minerals which supposedly represent different combinations of temperature and pressure. They are examples of metamorphic “facies,” as illustrated in this image:

facies

Image redrawn and modified by me from Figure 3 of Bousquet, et al. (2008), which is itself modified from Oberhänsli, et al. (2004), and also from University of British Columbia (1997), which is modified from Yardley (1988).

Theoretically, blueschists and greenschists should be forming at different combinations of pressure and temperature. Blueschist forms at high pressures, but relatively low temperatures. But here we have an outcrop of blueschist that is right adjacent to a greenschist (medium temperature and pressure), with no faulting in between. It was suggested to me by a blueschist expert that this was likely a reflection in differences in the initial composition of the protoliths. I found this explanation less than completely satisfying, but there was no time to discuss, for we were being called back to the bus, already gunning its engine and ready to roll down the road.

At our second stop, we found some metamorphic rocks that showed clear textural evidence of having had pyroclastic protoliths:

tav_1_06

There were lots of chunky bits in there.

tav_1_07

So it wasn’t just pelitic (muddy) rocks that were getting metamorphosed in this Tethyan suture zone, but volcanic rocks too!

More later… when we move on to stop #3

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?

Graph beauty: T vs. viscosity for lavas

I spent the day lazily reading the igneous petrology chapters of Petrology by Blatt, Tracy, and Owens (third edition, 2006). Last time I read it, I didn’t get all that much from the igneous section, but this time around that’s the thing that motivated me to delve into it again. I don’t remember enough about igneous petrology from my school days, and while I have a little breathing room this summer, it seemed to me that I could bone up on it a bit.

One thing that caught my eye this afternoon was Figure 4-15, on page 78. I have redrawn it for you here:

visctemp

I love beautiful graphs like this. It compares viscosity (resistance to flow, as measured in pascal-seconds; each pa-s is the same as 10 poise) to temperature (as measured in degrees Celsius). Five different compositions of lava are plotted: komatiite, basalt, andesite, dacite, and rhyolite.

First off, you’re no doubt struck by the inverse relationship between temperature and viscosity. The hotter the lava is, the less viscous it is (more runny; easier to flow).

Second, higher-silica-content lavas (rhyolites, dacites) are much more viscous than lower-silica content lavas (basalts, komatiites). Silica (and, to a lesser extent, alumina) form polymer-like chains. The more silica there is (up to 75 wt% in some rhyolites), the more of these sticky, web-like chains can form. This is why you can see lava dripping off the molten basalt in this video, but the molten granite clings to its source rock. Water actually interrupts the formation of these silica polymers, and thus lowers viscosity when it is present.

I’m also struck, looking at this graph, of the difference in temperatures plotted from left to right. This corresponds with observed lava eruption temperatures of different compositions. Low-silica lavas erupt at high temperatures, as they are chock full of high-crystallization-temperature mineral components. (They wouldn’t erupt at all at lower temperatures, because they would be solid.) High-silica lavas erupt at relatively low temperatures, as the components they contain will crystallize into minerals like quartz at relatively low temperatures.

Ironically, though we might think “high temperatures = more dangerous,” the opposite is true. Low-silica lavas tend to erupt effusively, with relatively little risk for human life. You can outrun a lava flow — with rare exceptions, even low viscosity lavas still only flow a few hundred meters per hour. The lower-temperature lavas, on the other hand, are the ones to worry about — because they’re viscous. This “stickiness” means they tend to clog up magmatic plumbing and allow greater pressures to build up. Couple this idea with the tendency of high-silica lavas to also be rich in dissolved gases, and you get a much more explosive style of eruption.

________________________________________________

Reference

Spera, F.J. (2000) “Physical properties of magma,” in Encyclopedia of Volcanoes, H. Siggurdsson, ed. San Diego CA: Academic Press. {Fig. 4}

Mount Taranaki

Searching around for the current Where on Google Earth, I found this astonishing place in western New Zealand:

wow

That’s Mount Taranaki, and evidently the vegetation change you see in the circular colored shape around the mountain must be due to a protected-area boundary. Check out the radial drainage pattern on that sucker!

Check it out yourself here.

Photos from Eyjafjallajökull

My friend Barry R., now residing in PostDocVille, Denmark, took a trip to Iceland last week to check out the eruption of Eyjafjallajökull. Unfortunately, by the time he got to the volcano, it was no longer spouting lava, but the scene is dramatic regardless.

You can sample some of his photos below, or see the whole album on Facebook.

Waterfall:

Glacial terminus and moraine:

Ash on ice (steam rising beyond the hills):

Where the volcano has melted the local ice:

Thanks to Barry for letting me share his volcano photos here. He’s the second University of Maryland alum to do so! It seems to be a trend…

Baked fanglomerate

A quick post to share a few images of an outcrop I visited last September out in California’s Owens Valley. This is a spot where alluvial fans coming off the eastern Sierra Nevada were overrun by a basaltic lava flow (Jeff, Kim, Fred, and Kurt for scale):

contact_meta_sub_lavaflow02

The unofficial term for these conglomerates deposited by alluvial fans is “fanglomerate,” and it’s pretty cool to see the contact metamorphism at the top of the fanglomerate. There’s also some weakly-developed columnar jointing in the basalt. Here’s an annotated version, in case the contact wasn’t quite obvious enough:

contact_meta_sub_lavaflow02_anno

Here’s a close up (Doug for scale), showing the orange zone of thermal metamorphism at the top of the fanglomerate as the lava flow above baked the hell out of it:

contact_meta_sub_lavaflow01

Groovy, eh? Where’s your favorite example of contact metamorphism?

Transect debrief 3: Rodinian rifting

The Grenville Orogeny, responsible for Virginia’s basement complex, was one mountain-building event among many that helped put together a Mesoproterozoic supercontinent called Rodinia. But Rodinia didn’t last: it broke apart during the Neoproterozoic to form the Iapetus Ocean basin. This rifting event is recorded in Virginia’s Blue Ridge province in the Swift Run Formation and the Catoctin lava flows.

It’s probably about time to start putting some of these rock units in stratigraphic context. Here’s my redrawing (and updating) of a cartoon Shenandoah National Park stratigraphic column based on an original by Tom Gathright (1976):
shenandoah_column

You’ll notice here that the Swift Run Formation is interbedded with the Catoctin Formation, a Neoproterozoic series of lava flows fed by fissure eruptions (kind of like what’s happening this week in Iceland).

Trickling downhill away from these fissure eruptions would have been flows of basaltic lava (tholeitic, indicating a mantle source chemistry). If you want a warmer modern analogue than Iceland, look to the Afar Triangle region of Ethiopia:

As with Neoproterozoic Virginia, the continental crust of modern Ethiopia is stretching, opening up topographic grabens which are being filled with clastic influx from the surrounding highlands and mafic lava which is formed from decompression melting in the underlying mantle, and funneled to the surface via feeder dikes. In places you will see streambed conglomerates interlayered with the mafic lava flows, and in places there are tuffs and rhyolites that are a (volumetrically-small) part of the package. Elsewhere there are lake sediments. The two bear a common geologic signature, despite being separated by thousands of miles and half a billion years. There’s that refrain again: Same as it ever was, same as it ever was.

Once on the surface, the lava cooled, and in some places, columnar jointing formed:

The cooling age on some of the rhyolitic upper units in the Catoctin Formation is 570-565 Ma (Rb/Sr on pyroxene by Badger and Sinha, 1988). Some mafic and felsic dikes (could be feeders) associated with the unit yield the same age via U/Pb (Aleinikoff and others, 1995).

At some point, ancestral North America (“Laurentia”) drifted away from the spreading center, and volcanism ceased. The crust cooled, subsided, and then a sequence of sedimentary rocks began to accumulate atop the cooled lava flows. This transgressive sequence of sediments (the Chilhowee Group) is the next thing up in the stack. More on that later.

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 = (ρr-ρm)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

Crucifix Site 1: Sediments

On the September 2009 GSA field forum in the Owens Valley, the final stop of our first day was to check out the so-called “Crucifix Site,” along Chalk Bluff Road (north of Bishop, California, at the southern margin of the Volcanic Tableland). It’s called the “Crucifix Site” because there is a metal cross erected there:chalk_bluff_stuff_13

This is the site of some pre-Bishop-Tuff volcaniclastic sediments. The place is interesting on several levels, including the sediments themselves, and the subsequent deformation they have experienced. Here’s a look at the outcrop:chalk_bluff_stuff_03

Some annotations help to call one’s attention to primary sedimentary structures and interpretations:chalk_bluff_stuff_03anno

Lots of the sediment itself was made of little beads of obsidian, usually surrounded with a “chalky” weathering rind:chalk_bluff_stuff_07

A cool little channel cross-section was visible, too:chalk_bluff_stuff_04

For some reason, this is what pops into my mind when I run into a well-exposed Cheshire channel cross-section:
chalk_bluff_stuff_04anno

Birds and wasps had tunneled into the softer layers, resulting in horizontal rows of holes. I tried to ignore these modern bioturbations so I could focus on the ancient tale in the rocks themselves. Some cool soft sediment deformation was visible, like these flame structures (upper part of the central gray layer):
chalk_bluff_stuff_10

Zoomed in on a pair of flame structures, and the down-sagged material between them:chalk_bluff_stuff_11

chalk_bluff_stuff_11anno

…And another set:chalk_bluff_stuff_12

chalk_bluff_stuff_12anno

This was odd: The lowermost layer (upon which my field notebook rests) is unperturbed, but the layers above it are all churned up in one small area (center), flanked by a couple of bird holes:
chalk_bluff_stuff_08

Zoomed-in on the area in question:chalk_bluff_stuff_09

Annotated, for your viewing pleasure; green is top of undisturbed layer; red shows boundaries of zone of disrupted sediment:chalk_bluff_stuff_09anno

I would be pleased to hear from anyone who knows more about sedimentology than me about the wavy bedding in the second (& third) photo, and this weird sediment disturbance in the lowermost photo. Also: with the flame structures, it looks like coarser material in the lower layer (gray) is the less viscous participant, while finer-grained (white) material is sinking downwards. Isn’t this the opposite of the way it usually works?

Pacaya Volcano, Guatemala

Today, some photographs from Guatemala. I know one of my geoblogopeers is down in Guatemala doing research, so I’ll be interested to hear her take on these photos. These photos all come to us courtesy of my friend Courtney, who is a librarian at M.I.T., and a fellow M.S.-graduate of the University of Maryland geology department. She shared these images with me about a year ago, and I intended to post them on NOVA Geoblog, but never got around to it. Well, the wait is over… it is time to feast your eyes on some lava! The full set of images is here.

Pacaya Volcano is about 50 miles south of Antigua. It is one of about 35 volcanoes in the country — and one of four active volcanoes. Courtney and her friends four-wheeled it part way up to save a couple hours of hiking time. The trip door to door from Antigua was seven hours or so. They drove to about 1850′ elevation (564 m) and hiked the rest. Pacaya is 2,552′ high (778 m), but they didn’t go all the way to the top, just high enough to get up close and personal with some lava.

Here they are hiking up across relatively fresh lava flows:

“Lava stairs” that the group used to hike up. Courtney says, “This was live lava about two weeks prior to this picture. When the guide told me how recent it was, I started to get a little panicked. I put my hand down on the ‘stairs’ and it was very warm to the touch. Yikes.”

The group approaches the incandescent lava river (seen here as a faintly orange band running from upper left to lower right):

Approaching the lava river itself:

Wow. I’m struck by the ‘natural levees’ that form on either side of the liquid flow. The overall morphology calls to mind the neural tube of an embryo…

Here’s an unsettling sight to see on the “trail.” Courtney reports very hot feet on this hike, so I’m really not sure whether this is safe or foolhardy.

In these next two images, watch a big chunk of solidified basalt (shaped like an anvil, dark in the first picture, rolled over to appear orange in the second picture) ride the current downstream, like a log floating down a river:

The hikers, evidently happy with their experience. You can see the lava river in the distance as an orange stripe on the side of the volcano:

I’d like to thank Courtney for sharing these photos with us. What do you think? Was this safe? Was it awesome regardless?

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