17 January 2015
The astronaut Sam Cristoforetti, currently aboard the International Space Station, tweeted this photo today:
It shows her view of Athens, Greece. Zooming in, you can see the dense urban sprawl of Athens filling a broad valley between several mountain ranges. Within this valley, several hills poke up like islands in a sea. How many do you count? Athens is one of many cities that claim to be built on or around seven hills, and I’d say the number is approximately seven. One of these hills is known as the Acropolis (Greek: Ακρόπολη).
You know the Acropolis – it’s that distinctive hill with the ruins of the Parthenon sitting atop it:
So what’s the geologic story here? Why is the Acropolis a hill?
Let’s turn to the geologic map of Athens by Gaïtanakis (1982), zooming in on the portion surrounding the Acropolis:
Dark green here is Cretaceous-aged Tourkovounia Formation. Light green is a mix of marl and sandstone known (counter-intutitively) as the Athens “Schist.” It is not a true schist in the sense that the term is used in modern geology – a medium-grade metamorphic rock with visible crystals (including perhaps porphyroblasts) but lacking gneissic banding. Instead, these rocks are very lightly metamorphosed sedimentary rocks. The Athens Schist is also Cretaceous in age, and perhaps parts of it are younger than that. Everything else on the map (the blues, tans, yellows, and circles) is Quarternary sediment of several varieties. I don’t care about that stuff; I’m focused on the bedrock – those two shades of green.
Now, what is the relationship between these two units? Is the contact conformable, unconformable, intrusive, structural, or what?
Consider these two cross-sections, the first Gaïtanakis’ map, and the second from a book by Higgins & Higgins (1976):
I’m really indebted to the article The geology of the Acropolis (Athens, Greece) by M. Regueiro, M. Stamatakis, & K. Laskaridis from a couple months ago for linking me in to these resources, which I would have had a much harder time finding on my own.
The limestone capping the Acropolis is the Cretaceous-aged (specifically Cenomanian-Turonian) Tourkovounia Formation. The layer beneath that, the Athens “Schist,” is Cretaceous too, but here’s the neat thing: it’s Maastrichtian to Eocene (?) in age. The age difference is estimated to be 30 million years. Consider this graphically, from the International Commission on Stratigraphy’s time scale:
In other words, the upper rock layer is older than the lower rock layer, a perversion of the principle of superposition. Note the “cataclastic limestone” (dark blue) in the Higgins & Higgins cross-section. Cataclasite is a fancy name for “fault breccia:” the crushed up rock that forms along a fault zone: this is a clue as to how superposition was subverted.
Consider this sunset shot of the hill from our hotel when we visited Athens in September, and the ensuing annotations:
The limestone capping the Acropolis is older than the marl + sandstone at the base of the hill (the “Schist”). It moved to its present position from an original position some distance away. It moved there along a break in the rock: a horizontal fault, often called a “thrust fault.” Hence, the hilltop is “allochthonous” (translation: “not from ’round these parts”). Meanwhile the base of the hill is more or less where it’s been ever since it was deposited, a condition graced with the appellation “autochthonous” (translation: “local yokel”).
Because the sandstone / marl of the Athens Schist is more susceptible to weathering in the arid climate of Greece than is the Tourkovounia limestone cap, the arrangement is expressed as a hill. The hill used to be larger, but is being nibbled away over time from the sides. It’s an erosional remnant of a much larger thrust sheet.
Here’s a Google Earth “3D buildings” view that I modified a bit in Photoshop to emphasize the topography of the hill, as well as our digs in Athens, and the take-home message of this blog post:
A klippe is an erosional remnant of a thrust sheet. Chief Mountain, Montana, is a classic example:
Of course, there are some differences between the two klippes…
(That reminds me of the Mountain Beltway banner… but older both above and below!)
Anyhow, here’s a zoomed-in look at the limestone cataclasite at the Acropolis along the fault zone:
Some big and small clasts in there. And a little guy for scale!
Tectonically, this faulting (presumed to be Eocene in age) is related to the compressional tectonics that are causing Africa and Europe to collide, closing the Mediterranean Sea (née Tethys Ocean) and building the Alps.
Here’s a look along strike of the trace of the fault zone, out across eastern Athens:
Here is the street west of our hotel, where the Athens “Schist” crops out:
What follows next are five field photos of this uninspiring unit:
One more detail: Sometime after thrust faulting, the contact (fault zone) between the Acropolis’ upper, older limestone and its lower, younger “Schist” was itself faulted. Let’s zoom into this little exposure of the contact, just below the cliff-forming limestone but above the tree line:
See how the contact there is jagged? That’s due to a number of low-offset block faults (~normal faults):
Obviously, this would have to be more recent than the thrust-faulting, since it cross-cuts the main thrust.
So, to summarize, the recipe for one Acropolis is:
- Deposit limestone in the late Cretaceous.
- 30 million years later, deposit sandstone and marl.
- In the Eocene, shove the limestone up and over the younger strata, shattering the bottom of the limestone in the process.
- Break up the fault with some small-offset normal faults.
- Differentially erode the landscape.
- Add people, who:
- build the Parthenon, and
- misname the sandstone and marl as “schist.”
Pretty cool story overall, eh?
“…Yep, dad. Pretty cool.”
Thanks to “Astro Sam” for posting the ISS photo this morning and inspiring me to finally write this post!
References utilized and cited in the preparation of this blog post:
Gaïtanakis, P. 1982. Geological Map of Greece 1:50.000. Athinai-Pireus sheet. Institute of Geology and Mineral Exploration, Greece.
Higgins, M. & Higgins, R. 1976. A Geological Companion to Greece and the Aegean. London, Gerald Duckwort & Co Ltd.
M. Regueiro, M. Stamatakis, & K. Laskaridis. 2014. “The geology of the Acropolis (Athens, Greece).” European Geologist, v. 38, November 2014.
16 January 2015
Zoltán Sylvester has contributed today’s Friday fold, an anticline in the Ross Sandstone of Ireland:
Click image to go to the source (full sized).
Thanks Zoltán! Happy Friday everyone.
13 January 2015
Check out the argillite boulder in the left midground of this GigaPan, which I’ve showed here before. It was taken at the Icefields Center parking area in Jasper National Park, Alberta:
There, you’ll find some lovely orange lichens, some iron oxide staining, some graffiti, and a fair number of sub-aligned glacial striations.
Also, at the top edge of the boulder, there’s a nice set of big hackles, running along the crest like the knobby backbone of a sea monster:
A wealth of lovely detail to be explored…
12 January 2015
Another pair of shots of the Ferrar mafic intrusives from Antarctica, courtesy of Lauren Michel…
Zooming in more…
There are some major disruptions to the strata – I wonder if the story is more complicated than my simple annotation suggests…
9 January 2015
Along Going-to-the-Sun Road in Glacier National Park, you can see this outcrop of Mesoproterozoic Grinnell Formation quartzite (former quartz sandstone) and red argillite (oxidized mud rock). These strata are part of the Belt Supergroup.
It proudly displays an asymmetric / overturned anticline.
The outer arc of the hinge of this anticline shows extensional fractures in the red argillite “coating”:
Those photos were taken by Tom Biggs (UVA) in 2013. Here’s a shot I took in 2010 of the same fold, showing its cross-sectional profile:
Note the primary sedimentary structures: the ripples (bedform) and their guts (cross-bedding), as well as evidence of fluctuating current strength (mudchip rip-up clasts “suspended” in quartzite):
I look forward to revisiting this fold in July 2015 as part of my Rockies field course (for which, by the way, applications are now open).
8 January 2015
Probably this comes from the Cambrian-aged Conococheague Formation, although the Beekmantown Formation (early Ordovician) is another possibility.
7 January 2015
My friend and colleague Lauren Michel, the King Family Fellow at the Perot Museum of Nature and Science in Dallas, Texas, sent me this image from her recent trip to Antarctica:
This is a beautiful example of a mafic igneous sill, probably of the rock known as “dolerite” (or diabase, to us Yanks). Lauren and I think it must be part of the Ferrar Large Igneous Province. (They are Jurassic in age, emplaced upon the breakup of Pangea.) The location is apparently in the Beacon Valley, near the Taylor Glacier. At first, the middle of the photo looks strongly like a fault. However, upon closer inspection, I’m not so sure.
Here, I have annotated it to show the apparent continuity of bedding:
Let’s take a closer look:
It seems like the thing that first looks like a fault is really a dike, and that the strata are more or less continuous across the field of view, with the exception of regions “A” and “B,” which don’t obviously match up, though the layers immediately below them do…
Well, is there anything else to see here? Sure there is!
There are various details of the igneous intrusion to be seen, like on the right side of the photo, where we can witness the fresh production of xenoliths along the roof of the magma chamber, where these “alien rocks” are being liberated through the process of stoping:
(I’ve previously highlighted stoping in Torres del Paine, Patagonia.)
And off to the left, we can see a subsidiary sill that’s opened off due to an offshoot dike from the main sill.
Great stuff – my students and I will see a similar sill/dike/sill relationship in the Purcell Sill, cropping out in the cirque above Grinnell Glacier in Glacier National Park, Montana, this coming summer.
Thanks for sharing, Lauren!
5 January 2015
My family and I went to the Strasburg Museum in Strasburg, Virginia, last fall, because (1) we’ve lived out here for two and a half years now without stopping in, and we felt “overdue” for checking it out, and (2) a big train is prominently featured out front, and my son is really keen on trains right now due to the “Thomas the Tank Engine” series of books. I don’t have much to report about the museum, but they do have a couple of fossils, and so I figured it was my duty as a geoblogger to document them and share them with the world.
Fossil trilobite pygidium (“tail plate”):
(About 1 inch or 2.5 cm across)
2 January 2015
Devonian metamorphic rocks (garnet-bearing gneiss) exposed on the western side of Cabbage Island, Maine:
And here it is in GigaPan form:
1 January 2015
A list of birds seen in my yard this year.
- Downy woodpecker
- Mourning dove
- Dark-eyed junco
- Tufted titmouse
- White-breasted nuthatch
- Black-capped chickadee
- Pileated woodpecker
- Red-bellied woodpecker
- Turkey vulture
- Hairy woodpecker
- Eastern phoebe
- Red-tailed hawk
- American crow
- American robin
- Bald eagle
- Brown creeper
- Barred owl
- Carolina wren
- Brown-headed cowbird
- Chipping sparrow
- Broad-winged hawk
- Black vulture
- Blue-gray gnatcatcher
- Ruby-throated hummingbird
- Yellow-throated vireo
- Sharp-shinned hawk
- Rose-breasted grosbeak
- Blue jay
- Indigo bunting
- Red-eyed vireo
- Barn swallow
- Baltimore oriole
- Yellow-rumped warbler
- Yellow warbler
- Hermit thrush
- Great crested flycatcher
- Scarlet tanager
- Common nighthawk
- Great blue heron
- Eastern wood-peewee
- Yellow-billed cuckoo
- Cedar waxwing
- Chimney swift
- Eastern bluebird
- Red-shouldered hawk
- Black and white warbler
- Yellow-bellied sapsucker
- Black-throated blue warbler
- Purple finch
- European starling
- Red-winged blackbird
Again, the total species count went up. This pleases me. It means I’m probably spending more time outside and learning to identify new birds.