17 January 2018
Here’s a puzzler to warm up your chilly brain this Wednesday morning:
Figure out the story told by this set of imprints in the snow. The branch of science called ichnology studies the traces organisms leave behind. There’s a neat little story here. If you’ve got a guess, then you can check your answer by watching the video that this screenshot came from. It was posted on Facebook by the town of Cobourg, Ontario, which caught it on a security camera. It’s pretty cool to watch the whole thing unfold. Enjoy!
13 January 2018
I was so impressed with After On that I went out an got the only other novel by Rob Reid, Year Zero. The plot set up is something rather ludicrous, but the novel works in spite of the silly premise. Here’s the idea: There are a lot of alien civilizations out there, and they are really advanced. Banded together into a Refined League, they have mastered almost all forms of art, but they are really deficient when it comes to music. They discover Earth’s music and it becomes an ecstatic addiction. They listen and listen, copying music files and lip-synching performances, and loving it. But then, 30 years in to the sonic orgy, somealien realizes they haven’t paid a cent for it, and are subject to legal penalties according to Earth’s copyright laws. Following legal precedent such as when a college student who shares 30 files has been slapped with a $675,000 fine, the aliens owe the citizens of Earth the entire wealth of the universe. So now some of them want to destroy the Earth to get out of the debt. And in steps our hero. Goofy, right? But it’s worth reading, because Reid is a really skilled writer with a terrific imagination. It’s super fun. And as an added bonus, the novel has at least one character in common with After On, as well as the social media app Phluttr, which was central to the subsequent novel. I like it when authors or auteurs manage to construct multiple independent stories that fit into the same fictional universe with elements like that, akin to Quentin Tarantino’s Red Apple cigarettes or the 1% overlapping novels The Golden Ocean and The Unknown Shore by Patrick O’Brian. Overall, I’d say that After On was a stronger book than Year Zero, but I enjoyed the alien music law fest too. It’s a nice escapist novel.
12 January 2018
In keeping with the Arizonarific theme of this week’s posts (thanks to my participation in the 2018 Structural Geology and Tectonics Forum), I thought I would wrap up my ‘geology of the Phoenix area‘ posts with a walk I took on my last day there. This was to what Google Maps calls “Hayden Butte,” but the locals call “A Mountain.” Not “a mountain,” but “the mountain called ‘A‘.” It has a big yellow “A” on its south side. It’s adjacent to the stadium where the ASU Sun Devils play, and on the other side is the vibrant Mill Street corridor, the beating heart of Tempe.
Here’s a view from the north, to emphasize the big-picture geology of the butte:
Most of the peak is made of lava flows with an andesitic composition. But these overlie Neogene sedimentary rocks. Both are interesting.
I began my visit there by summiting the butte. The trail is heavily used, which is great, but severely eroded, which is not so great. The views were nice – though it’s essentially an urban vista.
The rocks were fairly unextraordinary, porphyritic to aphanitic in texture, gray in color. They are apparently 18 million years old. I did spot a couple of small light-colored xenoliths, including this one:
I also spotted an interesting set of deflected fractures, resembling a kink fold:
Annotated copy, with the trace of fractures highlighted in white:
Given that these lavas flowed on Earth’s surface less than 20 million years ago, it seems unlikely that they have been subjected to compressional stresses at the brittle-ductile transition in order to kink. So I suspect this is just a visual match, a coincidence of form which isn’t really a kink band. That said, the fractures exist. Fractures form when stresses exceed the brittle strength of materials, so there could well be an interesting story to be extracted from these rocks.
I made a 3D model of this outcrop, which helps to convey the shape of these “kinked” fractures:
Then I walked clockwise around the butte, and found the sedimentary rocks. These were much finer than the coarse landslide breccias at Papago Park. Instead it was mudrocks and sandstones. One thing that was superlative about the site is some boffo soft sediment deformation: certain beds with high degrees of internal contortion.
The first glimpse was relatively symmetrical and open in its folding:
… But then things got more intense:
This is the signature of a density inversion – a deposition of wet sand on top of squishy mud. The sand then sags downward into the mud as big lobes, and to accomodate this relative motion, the low-viscosity mud squootches upward in flame-like projections.
Any thin sand layers within a larger body of mud get dragged along for the ride, acting as nice strain markers.
I saw one nice example of graded bedding:
More soft sediment deformation:
In spite of the hematite ± calcite on joint faces that transect the folded sandstone layers, I found the form of this fold to be quite exquisite:
Here, I annotated it for you:
…And here are 3D models of two examples of the soft sediment deforomation weathering out of the butte:
I think there’s more to be seen on this mountain, but I wish I’d brought another person with me. I encountered some sketchy characters in some of the gullies draining the peak, and I think it would have been wiser to have a bigger group than just one. I’d caution anyone else thinking of visiting to bring along some friends. Maybe a trash bag for collecting litter would also be a good idea!
Anyhow, this mountain is a real treasure to have so close to campus. I wish I could take my students on a short field trip to a place like this. I guess with this set of photos and 3D models, I can at least give them a taste of it.
11 January 2018
Yesterday I outlined the idea of metamorphic core complexes, as expressed in the South Mountains, south of Phoenix, Arizona. We examined the ductilely-deformed footwall rocks. But a bunch of rock slid off the top, too, breaking into domino-like chunks as it slid along the detachment fault. The local mountain called Camelback is mostly made of granite that originally derived from the South Mountains, but it’s on the other side of Phoenix now. It also features a distinctive unconformity, separating the granite from overlying oxidized Neogene landslide deposits – the “red beds” in the image below:
Today we will journey to Papago Park. It has many more examples of these red beds, which in spite of tilted bedding show horizontally-oriented holes of “mega-tafoni.”
One of these is known locally as “Hole in the Rock.” Here it is, with some people for scale:
Hole in the Rock is basically a small version of Camelback:
The granite here is the tip of a big block of hanging wall rock from the South Mountain detachment. It’s mostly submerged in a sea of modern sediment, but also bears a load of lithified breccia. This is the Neogene-aged Camel’s Head Formation. It’s pretty angular, and pretty poorly sorted:
The clasts comprising this sedimentary breccia imply a nearby source of lots of granite, and also some finely-banded rhyolite:
Therefore, there must have been a mountain nearby in the past to shed these clasts off – some local relief from which landslides and debris flows might issue. The mountain is now absent, either eroded to nothing or else buried under the Phoenix Basin’s sedimentary valley fill.
Zooming in on one of the boulders, you can see (in the lower right) a primary igneous contact between the granite and the rhyolite:
So we can actually say something about the structure of this long-departed mountain range by examining a pile of sedimentary clasts like these. Neat-o!
I should also point out that this red bed breccia weathers out beautifully as a series of small buttes looking south toward Tempe and southwest toward Phoenix:
It’s a neat place to visit, clamber around, and contemplate the annals of the former world.
10 January 2018
I spent last weekend at the 5th biennial Structural Geology and Tectonics Forum, hosted by Arizona State University in Tempe, Arizona. It was an excellent meeting, full of new insights for me, new connections, and opportunities to share stuff I’ve been working on.
The four day Forum began with a field trip to the Phoenix Mountains and the South Mountains. I’d like to share a few images from the trip here today.
We began in the Phoenix Mountains, at a public park called Dreamy Draw. Crossing under the highway, we were able to explore some more terrain to the north.
Our leader was Steve Reynolds of ASU:
He came equipped with a well-used sketchpad on which were drawn useful diagrams to explain the geology of the area:
Clearly, it’s not the first time he or co-leaders Julia Johnson, Ramón Arrowsmith, or Steve Semken have led people to see these rocks.
As you can see from the sketch, the structure of the Phoenix Mountains is a series of upright metasedimentary and metavolcanic rocks, folded into two isoclinal synclines separated by a fault.
In the limbs of these folds, the cleavage and bedding were close to parallel: both stood up vertically or close to it:
There were some quartz veins cutting across the bedding and cleavage, and also a few small kink folds, like these:
In some spots, there appears to have been a volcaniclastic component to the sediments: little white balls of pumice that got dropped into an otherwise muddy sedimentary package. When deformed, these pumice blobs got stretched out, forming a nice lineation that lies within the plane of foliation:
Here’s a second example of this LS tectonite:
We lunched enjoyably at Los Dos Molinos, a legendary Mexican restaurant on the south side of town.
Then it was up into the South Mountains, the largest municipal park in the United States at over 15,000 acres.
We went there to look at the footwall of the metamorphic core complex exposed there.
This was a site where the Phoenix Mountains’ granite and gneiss were uplifted, as the rocks on top slid off the top. There is a detachment fault (a low-angle normal fault), but the story is more intriguing than that, as the rocks beneath the fault show ductile deformational fabrics – low-angle mylonitic foliation. The rocks here flowed into a new shape while the rocks just above were breaking and sliding.
Here’s a look across a small canyon to see the strong mylonitic cleavage in its almost-horizontal orientation, controlling the weathering of the deformed granites near Dobbins Overlook:
Out in the distance, in the Phoenix Basin, are huge chunks of rock that slid off the top of the South Mountains, with just little nubbins poking up, iceberg-style above the sedimentary basin fill.
The granites in the footwall are equigranular at lower elevations (greater depths), but show an increasing penetrative fabric the higher in the mountain (and the closer to the detachment fault) you go. They are both foliated and lineated:
The mylonites are beautiful in outcrop: smeared-out granite is lovely stuff:
This outcrop is the source of the S-C mylonite depicted as the “textbook” example in the Davis, Reynolds, and Kluth structural geology textbook:
Note the different foliations in this next outcrop, ans mylonitic smearing accommodated rotation of blocks of granite within the metamorphic core complex’s footwall:
There are veins of pseudotachylyte here too: dark bands of fault glass that cut across the mylonitic fabrics, an indication that brittle conditions succeeded ductile conditions:
Pseudotachylyte is generated when frictional grinding of one rock on another during an earthquake instantaneously melts the rock along the fault. This melt freezes moments later, rendering a seam of glass: a “fossil earthquake!”
More veins of pseudotachylyte:
Ironically, conditions then reverted back to ductile – but only on these warm pseudotachylyte veins, which were rheologically weaker than their surroundings. Though solid, the glass seams were capable of flowing under lesser stresses than the rock around them, where the atoms were all bonded together in a network of strong minerals.
So: many of the pseudotachylytes have a mylonitic overprint!
The South Mountains is one of the first places where metamorphic core complexes were first identified by Steve Reynolds back in the late 1970s and early 1980s. It was exciting to visit, and I’d love to go back and explore a bit beyond Dobbins Overlook, where there was a pretty heavy visitation impact: a lot of people, and a lot of litter. The rocks make up for it, of course, but I’d love to roam around there more to get a better sense of the structure of a metamorphic core complex footwall.
Next up, we’ll visit some of the rocks from the hanging wall, which have been transported ~40 km away thanks to the detachment fault. Stay tuned…
5 January 2018
My friend Vali Memeti posted this photo of her husband Scott Paterson examining some folded rocks on the coast of Tasmania last week. It looks like Vali and Scott enjoyed a fun excursion to this “Even Further” Down Under location.
This is the Sulphur Creek geologic site, part of a geocache suite called “Created from Chaos,” though I don’t get too much additional information from their website. Presumably the rocks are sedimentary: shales and siltstones by the look of it(???). I’d love to learn more if anyone knows the geology of Sulphur Creek, Tasmania.
Scott was really helpful to me when I was a grad student, showing me around in the Sierra Nevada when I was working on documenting fabrics in the Sierra Crest Shear Zone System. I’m pleased to see him exploring awesome structural geology around the world!
Happy First Friday of the New Year, all. May 2018 be foldiriferic for you and yours!
3 January 2018
This prominent sea stack, on the west coast of the island of Hoy in the Orkney Islands, is “the Old Man of Hoy.” There are lots of Old Men in Scotland; some are pinnacles, and some are castles and some are sea stacks, and some of the sea stacks are Castles but not “old men“. Very confusing. I took these photos from a ferry boat deck, en route from Stromness in mainland Orkney to Scrabster on the Scottish mainland the summer before last.
The sea stack and the wave-cut cliffs behind it are made of Devonian-aged Old Red Sandstone, as further to the north at Yesnaby, though you can see a textural and geomorphic change at the base, which is apparently a “plinth” of basalt. As at Dore Holm in Shetland, you can see the influence of prominent bedding-perpendicular joint sets in controlling the shape of the sea stack.
The west coast of Hoy is likely subject to the same monster storm waves that chew into the western rocks of Orkney and Shetland, producing features like storm beaches and the bonkers Grind of the Navir. I wish I’d had more time to visit the Old Man of Hoy in person, up a bit closer than the ferry would allow, but better to have seen it from a distance than to never have seen it at all…
Any readers have any idea what this strange rock is? It’s got wild patterns!
These samples were in the garden of a local rockhound in Lerwick; I have zero idea what the mineralogy is; I was just stunned by the “scale-like” pattern of gray and pink.
I’d be eager to hear your ideas what this might be. Thanks very much in advance!
1 January 2018
New year’s day is the time I tally up my accumulated bird species seen in my yard on the forested slope of Massanutten Mountain. This is my sixth such annual list. Here are the previous iterations:
Here we go, in chronological order of first appearance in our yard:
- Red-tailed hawk
- Red-bellied woodpecker
- Mourning dove
- Turkey vulture
- American crow
- White-breasted nuthatch
- Tufted titmouse
- Downy woodpecker
- Carolina wren
- Golden-crowned kinglet
- Sharp-shinned hawk
- Purple finch
- Barred owl
- Brown creeper
- Canada geese
- Eastern phoebe
- Eastern bluebird
- Blue jay
- Chipping sparrow
- Pileated woodpecker
- American robin
- Hairy woodpecker
- Pine warbler
- Fox sparrow
- Dark-eyed junco
- Brown-headed cowbird
- Screech owl
- Brown thrasher
- Eastern towhee
- Blue-gray gnatcatcher
- Ruby-throated hummingbird
- Yellow-throated warbler
- Red-eyed vireo
- Black-throated green warbler
- Red-breasted nuthatch
- Blue-headed vireo
- Great blue heron
- Broad-winged hawk
- Yellow-billed cuckoo
- Hermit thrush
- Scarlet tanager
- Great crested flycatcher
- Cedar waxwing
- Eastern wood-pewee
- Chimney swift
- Black vulture
- Northern (Yellow-shafted) flicker
- Rusty blackbird
- Yellow-bellied sapsucker
A few notes: The two in bold are “first time ever observed” here (by me) – and in both cases, that means not only my yard, but also the immediate area. I was very pleased to see the return of the red-breasted nuthatch, which has been absent for the past several years. I was bummed that no pine siskins showed up this year – they are one of the few treats of the winter birding season. One of my new year’s resolutions is to devote more time to birding, particularly in late April, when the happy combination of many migrants and relatively limited foliage makes for fruitful observing.
31 December 2017
My friend Betsy recommended I read this novel, and I’m glad she did. It’s of an unusual structure: a series of documents, arranged in more or less chronological order (with some variation for plot structure), written by 5 principal characters, two in particular. The two are a U.S. Army captain sent on an expedition to explore the Wolverine River Valley of Alaska, and his wife, who remains behind in Vancouver, Washington. The lovers are separated for almost a year while he makes his way through the wilderness. She chafes at societal expectations and delves into photography and ornithology. Meanwhile, he comes into contact with a very strange wild Alaska, where people and wild animals can transmute into one another, sometimes with nefarious intent, and sometimes just because that’s their nature. It’s an unsettling vision where some women are also geese, and the shaman can turn into a limping raven. There’s also a modern day exchange of letters, between a descendant of the explorer and a museum curator in the modern, no-longer-wilderness Wolverine Valley. This helps set up the context of the documents, and provides commentary on them, linking the “primary” source material with insights that come a century later. It’s well written and immersive, enjoyable to read over the holiday break. Nothing profound, but full of character and soulful intent.