15 October 2014
As mentioned last week, I took a solo field trip north of Perissa, Santorini, Greece, in search of subducted rocks.
The contact between the two main rock types (marble and schist) was prominent and visible from a great distance (see photos in previous post), but what was the nature of this contact? Did it represent conformable stratigraphy? Was it a fault?
Here’s a closer look at the contact:
In places, both units could be observed to show brittle deformation…
Close ups of brecciation in marble:
This suggested to me that the contact was faulted. It may also, broadly speaking, be stratigraphic, but the deformation I observed along the contact indicated a pronounced history of shearing.
Within the schist, for instance, there were pods of marble, as seen here:
I interpret these as boudins, or perhaps more accurately as asperities of the marble, dismembered by faulting and “tumbled” into the less competent schist below.
Three other examples of this follow, with smaller marble “podlets” entrained in the schist:
This looks to me like tectonic mixing of the two lithologies. Further down, below the contact itself, the schist was more “pure,” with fewer macroscopic marble inclusions. Some of the schist below the contact was grayish, some was greenish, and some was bluish. Here are a couple of exposures I saw early on, lower down the hillside:
Here is an example from the highest point on the hill that I hiked to (i.e. to the contact and no further):
That schist is blue!
Blueschist was what I was after on this excursion, given that it’s a high-pressure, low-temperature sort of metamorphic rock – the kind of thing that could really only form in a subduction zone.
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).
…And now I had spotted it! Here are a few samples:
As I learned several years ago in Turkey, the degree to which a rock “re-equilibrates” under high-pressure, low-temperature conditions is partly a function of those two physical variables, but also dependent on (a) the exact composition of the protolith and (b) the amount of water present and available to help facilitate chemical reactions (like the production of glaucophane). As a consequence, blueschist and blueschist-grade “greenschist” may co-exist in the same outcrop. Here’s an example of that; compare the hand-sample to the bedrock it rests upon:
I collected a couple of teaching specimens of both the blueschist, and the adjacent marble.
Some of the marble contained chert nodules that weathered out in positive relief:
Also, for the structurally-inclined, here’s a conjugate pair of kink bands (partly ruptured to become faults) disrupting foliation in some of the green schist:
Santorini’s geologic story doesn’t end with these subducted oceanic rocks, however. There is also a volcano there…
10 October 2014
You many recall the putative submarine mass transport deposit that Alan Pitts and I found on Corridor H, and that I’ve been back to several times with Dan Doctor of the USGS. Well, last month I was out there again, to GigaPan the site. While I was there, I took portraits of some of the folded and dismembered sandstone bodies (which I dubbed ploudins, as an amalgam between the word pillows –as in “ball and pillow structure”– and the word boudins).
Here are three of the best.
And to get a sense of the context in which these things formed, consider poking around this GigaPan of their source outcrop…
…and this GigaPan of the larger road cut in which it is found:
9 October 2014
One afternoon during my stay in Santorini last month, I went for a solo geology hike. I left our hotel in the beach town of Perissa, and walked north toward the prominent mountain where I had reason to suspect I would find marble and blueschist – the subducted remains of the Tethys Ocean basin. I headed towards a prominent cliff which had a karsty-looking hollow in which prominently sat a white church of some sort.
In the next shot, you can see the church at the upper right, but the focus shifts to the prominent geologic contact to the west (left) of it.
This is the contact, I inferred between marble (which weathers out more prominently in this exceptionally arid environment), and an underlying body of schist, which would be a valley-former:
For reasons I’ll reveal later, I think this contact may be a fault.
Here’s a view directly northward into the valley west of the church-cliff mountain.
There’s a new geologic unit in this shot. In the middle of the valley is a light-colored, stratified, easily weathered rock unit. From a distance, I inferred this to be volcanic ash from the eruption of Santorini’s volcano. Later, I confirmed this by collecting several large blobs of pumice from this unit.
So this is neat – in the most central hollow of this schisty valley, there’s a nice horizontally-layered ash deposit. That means the valley already existed at the time of the eruption, otherwise there would be no topographic basin to catch and hold this ash.
Here’s a Google Earth view of the valley, north of the beach town of Perissa:
Now for some annotations, to show the trace of the contact between the marble (M) and the schist (S), and the ash deposit (A) in the deepest part of the valley:
I also indicated where the towns are, and the location of that white church in the shadow of the most prominent cliff.
Looking across the valley to the west:
A final perspective, this looking obliquely northwest into the valley:
Note the little hollows (“caves”) cut into the lowermost outcrop of the ash.
Okay, so now, can we finally examine the schist itself, in search of the blue variety?
Patience, grasshopper…. Next week.
8 October 2014
My family and I watched this morning’s lunar eclipse from the deck. It was lovely. A few images…
Note the airplane track at the bottom of this one:
I love moments like this – nature acting both predictably and beautifully… and being able to share it with my family.
6 October 2014
Often, stylolites (pressure solution seams) are bedding parallel in susceptible sedimentary rocks. They are shaped like “beds of nails,” overall planar, but with pointy bits that poke up and down, perpendicular to that plane.
The stylolites form with an orientation that is overall perpendicular to the maximum principal stress direction, and the little “teeth” parallel to the maximum principal stress direction.
Often stylolites form in sedimentary rocks parallel to bedding (that is, horizontally) because the maximum stress is vertical (due to loading of overlying sedimentary layers).
But sometimes we find stylolites that cut across bedding at some angle, and these are inferred to have formed due to more horizontal (i.e., tectonic) stresses.
Here is an example (cross-sectional view, perspective parallel to the bedding plane) I found last month on Corridor H, West Virginia:
This is a chunk of Silurian-aged Tonoloway limestone. Closer in, you can see some additional detail of this structure:
Here’s a view of the bedding plane (“map view”), showing the trace of the stylolite cutting across bedding:
From a study of in situ examples of stylolites like these, one can infer the maximum principal stress direction (σ1) and thus potentially the direction of tectonic transport. They are subtle things, but in aggregate, a bunch of little stylolites can tell you about regional-scale tectonics.
These rocks were deposited during the Silurian, but obviously the stylolites would have formed well after that, sometime after the carbonate mud was lithified to limestone. Probably they are Alleghanian in age – due to the collision of ancestral Africa with ancestral North America during the late Paleozoic periods called the Pennsylvanian and Permian.
3 October 2014
Santorini’s a volcanic island. But the giant volcano built itself up over a nucleus of much older material, subducted sediments (now metasedimentary rock). This rock is typical of the Cyclades, the circular-shaped archipelago north of Santorini. As we flew there from Athens, I looked out the plane window in jet-lagged wonder at the scene of dry islands in a placid sea.
Here’s one that I saw:
I reckon what we see there can count as our Friday fold. Let’s trace out the bedding:
That’s the sort of fold you would not see from the ground – it is so big that it requires either an aerial perspective or a concerted mapping effort to detect it.
I love living in the age of Google Earth. After I got home, I was able to poke around on that fine program and identify the island I had photographed. It’s a peninsula on the southern part of the island of Antiparos. Here’s a Google Earth screenshot for comparison:
This sort of folding is precisely what we would expect in a metamorphic terrane. However, it’s not the sort of thing we would associate with a volcano. So, we’ll have to keep flying south for that…
29 September 2014
Over the weekend, I ran a 1-credit field course for NOVA, on the geology of Shenandoah National Park. I was about eight minutes early getting to the meet-up location, so that allowed me to check out a promising new outcrop of rock along the road (route 33, ~100 m west of Swift Run Gap). Here are two photos of it: charnockite (pyroxene-bearing granitoid or meta-granitoid), with weak foliation:
This is what my free-time geologizing has been reduced to: squeezed in to a few spare minutes here, a few spare minutes next week. No time for any more than that…
26 September 2014
Here’s Baxter, last Friday, in Athens, Greece. He’s checking out some folds in the marble that’s everywhere in that city:
This is a lovely example of passive folding, where all the rock layers being folded have about the same viscosity (low viscosity contrast between layers). No buckling, as a result…
Enjoy the weekend, hopefully passively!
23 September 2014
Once upon a time, there was an ocean. Its name was Tethys. The Tethys was born in the cozy embrace of Pangea.
But as Pangea broke up, Tethys felt the squeeze. Subduction began to close it.
As subduction recycled the Tethyan oceanic lithosphere, magma was produced and volcanoes erupted. Seafloor and seafloor sediments were dunked into the mantle, experiencing tremendous pressures but relatively low temperatures.
The Tethys shrank in size, hemmed in on all sides by colliding fragments of the former supercontinent. The modern Mediterranean is the “last gasp” of the Tethys. And it’s getting smaller and smaller over geologic time. The subduction continues. One spot where this is gloriously reflected is the island of Santorini.
South of Santorini is the Hellenic Trench, where Africa is being subducted beneath Eurasia. This subduction has been going on for a long time (since Pangea’s breakup in the Triassic), and the most amazing thing about Santorini is that it reflects two different aspects of that subduction story, one ancient, one modern.
You see, Santorini sits in the middle of a geographic Venn diagram: it’s where the modern Hellenic volcanic arc overlaps with the ancient subduction complex of the Cyclades Islands.
The bedrock of the Cyclades tells the earlier chapter of the story: seafloor and overlying sediments that traveled down a subduction zone into the mantle, only to rise again, transformed, to appear at Earth’s surface. The volcanic arc, of course, is a line of modern volcanoes, erupting lava and pyroclastics produced during more recent subduction. And Santorini has both.
Whereas most of the island is famous (and justifiably so) for its volcanic features, I was most interested in the green blotches on the map above – the spots where the Cycladic basement rock was exposed: schist and marble that had a tale of subduction to tell…
As fate would have it, my free trip to Greece with the Fugawies had me staying in Perissa, the stretch of rock in the southeastern part of the island, right in the middle of the two biggest exposures of the basement complex.
Next up: I’ll discuss what I found there…
22 September 2014
Last week, I did something unusual. I took a mid-semester vacation. My wife and I were in the rather extraordinary position of being gifted with a free trip to Greece, and though the timing was far from ideal for a pair of educators, we could hardly say no.
My wife’s mother became friends with a group of people back in the 1970s, when they were all young and backpacking around Europe. They met one afternoon in Athens near the American Express office, and went on to have a series of rollicking adventures together that bonded them for life. They called themselves the “Fugawies.”
Years later, some of the Fugawies have gone on to achieve considerable financial success, and when two of them were celebrating their wedding anniversary last winter, they decided that the treat they really wanted to enjoy was a reunion with their old friends, in the old places (plus some new ones). They decided to invite everyone for a ten-day trip to Greece, with airfare lodging, ground transport, and meals all on their nickel. Of course, the various Fugawies have grown older and had children, and in two cases, grandchildren. These descendants were considered to be part of the Fugawie fold, and were included in the extraordinarily generous offer.
So thanks to my wife’s mother’s friendship with these benefactors, my wife and son and I found ourselves in Santorini (Thera) and then Athens last week. I was able to see some pretty cool geology while I was there, and I intend to explore it over the course of the next week or two, as blog-composing time permits.
Let’s begin with this:
That’s a sea arch (~3 m tall) composed of pyroclastics from the eruption of Santorini, a massive affair that turned the once-conical island into a shattered hole in the crust. We’ll talk about the details of the eruption later, but for now, it’s probably enough to realize two things about this arch: (1) that the rock it’s made from formed through volcanic violence, piling up during a cataclysmic eruption, and (2) gradual erosion by the limpid waters of the Aegean Sea have carved it into this lovely shape.