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.
19 September 2014
Here’s an outcrop of Miette Group slate, seen at the intersection of the Icefields Parkway with the Trans-Canada Highway, just north of Lake Louise, Alberta:
There’s a lovely anticline just to the right of Zack, who obligingly provided a sense of scale. Also note how cleavage which is subparallel to bedding on the far left side of the outcrop, becomes perpendicular to bedding along the crest of the anticline…
12 September 2014
I took this image in 2005, when I was working up a geologic history of the C&O Canal National Historical Park. It’s a vein of quartz, gracefully folded within the Catoctin Formation. The exposure is along the railroad tracks at Point of Rocks, Maryland, easternmost extent of the Blue Ridge province on the north shore of the Potomac River. The Culpeper Basin begins about 100 meters to the east of this site.
Note that the Catoctin (a former basalt) has been metamorphosed to greenschist here, with a pronounced foliation. Here’s a Photoshop-highlighted and annotated view of the same fold:
It is typical when folding and foliation form at the same time due to the same consistent tectonic differential stress that the foliation should be parallel to the axial plane of the folds. We saw the same parallelism in completely different rocks in the Canadian Rockies a few weeks ago. You know what they call that? Uniformitarianism!
8 September 2014
Archaeotherium skull, on display at the Royal Tyrrell Museum in Drumheller, Alberta:
I love these beasts since I first encountered mention of them at Badlands National Park, and reading them dubbed “grizzly pigs” in the excellent book Cruising the Fossil Freeway really stuck with me – these were pigs filling a predatory ecological niche we don’t really see them in today.
5 September 2014
This one is in my folder of ‘structure’ images on my computer, but it’s not one of mine. I’m not sure where it came from. A TinEye search turns up nothing. Perhaps one of you can tell me?
Lovely subparallel kink bands… such gorgeous structure.
3 September 2014
Here are some rugose coral fossils (along with some cross-sectioned articulate brachiopod shells) to be seen in the Clearville member (~80 feet thick) of the Mahantango Formation, exposed on the north side of route 55, just west of the West Virginia / Virginia border.
These fossils are cool in their own right (what fossils aren’t?) but here they’re serving another purpose – they’re letting us know where we are in the stratigraphic stack. This is really useful when you’re out in the Valley & Ridge, and all the strata are folded and faulted and “reshuffled” in a way that makes it hard to keep track of where/when you are. They serve, in other words, as a marker horizon. I’m grateful to Dan Doctor (USGS) for cluing me into them.
1 September 2014
While in Waterton Lakes National Park, Alberta, my field class visited beautiful Red Rock Canyon to practice our field observation skills. This is a deep gorge cut into red and green argillite of the Grinnell Formation. Here is the view across the middle of Red Rock Canyon, at Tony, Kaatje, and Tom on the opposite side:
We gave the students 45 minutes to explore the place and make observations before we met up to knit those observations together into an interpretation, making sure we didn’t miss any salient details. Here’s a collection of shots showing Team Rockies at work:
So what did they see there besides a bunch of very red rock?
There’s some structure to be discerned as well… noticed readily thanks to the few green argillite layers amid all the vermillion.
Here’s a view from the bridge at the upstream end of the pedestrian walkway:
At first glance, that looks like a bunch of parallel faults showing a small amount of offset, but that’s just a trick of the perspective. Really, this is nothing more than a joint set, progressively weathered out to deeper and deeper levels. (So I guess those would be “fauxlts?”)
…But fear not; there are genuine faults to be seen here, too. Sean has noticed one obviously faulted outcrop:
Here’s another example of these apparently normal faults:
Note here the parallel joint set to the fault, highlighted with light green reduced chemistry, surrounded by so much oxidized rock:
These joints probably pre-existed when the stresses that triggered faulting went to work on these rocks. Select joints, ideally positioned, were then exploited as faults.
Here’s another example showing the parallelism of the dominant joint set and the normal faults:
The reduction of the walls of the joints raises a question in my mind: was the reduction of the “strata” primary or diagenetic? The color is pretty much the same between the two, and in places, such as just left of the fault above, you can see the reducing front wicking upward from the bedding parallel reduced zones into the joints and faults.