Okay, so maybe you recognize that. No? Take another look:



That's tafoni, peppering the Bishop Tuff on the volcanic tableland north of Bishop, California. I went there in September as part of a weeklong GSA Field Forum. Tafoni is a distinctive weathering pattern presumed to be caused by salt weathering, often in sandstones. This particular example wasn't in a particularly salty location, and the rock being weathered was the Bishop Tuff, a welded volcanic ash deposit. But it's clearly the tafoni pattern:



Here's some tafoni resources from the geoblogophere:
Through the Sandglass 1
Through the Sandglass 3
Tafoni from About.com 1
Tafoni from About.com 2
The Dynamic Earth 1
The Dynamic Earth 2
A previous mention here on NOVA Geoblog



And one more... ??

...Just kidding. This last one is a metate, a Native American grain-grinding depression. There were a couple of them at this location, too. Like the tafoni, it's a hole in the rock. Unlike the tafoni, it's man-made. Would you believe we didn't go there for the metates or the tafoni, but some normal faults instead? ...I'll have to share them in a future post.

Labels: anthropology, california, conferences, history, travel, weathering

It's been a while since I last posted about my time in Bishop, California, back in September, when I attended a GSA field forum on the structural and neotectonic evolution of the volcanic tableland.

For reference, here's a list of the previous posts about that trip:
...Faults of the volcanic tableland
...The Bishop Tuff
...The flipping fault

So, picking up where I left off, I thought it would be worth a post to mention the gorgeous drainage channels one sees etched into the top "Ig2" welded layer of the Bishop Tuff. These channels are interpreted as being Pleistocene in age, when the area was wetter than it is now.

Here is a photograph of the most spectacular of these channels, as viewed from the rim:

We visited this vantage on our second day in the field. A hiking path at the bottom of the dry channel imparts a sense of scale.

Here's a Google Map of the area from the perspective of a hawk:

Where the road comes most closely tangential to the canyon is the point where we stopped to take a look at it, and where the above photograph was captured.

Further upstream along the channel, we find it broken by normal faulting. Check out the view across this graben (a graben is a normal-fault-bounded valley, downdropped relative to the highlands next to it). There, you see the distinctive crescent-shaped profile of the drainage channel, but offset along several fault scarps:

There are three scarps on the far side of the graben, and an additional one that Peter is standing on, on this side of the graben. Just behind Peter, you can see a broken relay ramp, too. View is to the northwest; those are the Sierras in the distance.

Here is a Google Map of the area, showing the drainage channel crossing the graben. This conclusively shows that the channel must be older than the faulting which produced the graben.

This Google Map shares its southeastern corner with the northwestern corner of the first one I showed. You can see this for yourself by dragging either one in the appropriate direction. They both share the white-knuckled place where the road goes straight down the fault scarp, rather than sensibly down a relay ramp. That wasn't my favorite thing to drive.

Here's another drainage channel, similarly bone dry, that we visited in our fourth day in the field. Perspective is to the east: those are the White Mountains in the distance:


The Google Map shows a more interesting relationship this time. Instead of the faulting cross-cutting the channel's orientation, this channel approaches the graben to the southeast, curves around (deflecting from its original downhill course) and drops down the relay ramp to the northeast, into the graben (breaking up into multiple channels en route). There, it resumes its original downhill trajectory to the southeast:

This suggests that at least some of these faults were rupturing the "Ig2" layer at the same time that water was flowing over the surface (i.e. before the Owens Valley's climate dried out, post-Pleistocene). The stream's course and the faulting were coeval.

So what was the source of these streams? Did they originate on the volcanic tableland, or were they derived from the Sierra Nevada, prior to incision by the Owens River (which makes a deep canyon a mile or two west of here)? Fred Phillips, of New Mexico Tech, holds up a piece of evidence:

That is not a rounded cobble of the Bishop Tuff. That's a rounded cobble of granite. While the majority of cobbles in these channels are locally-derived chunks of the Bishop Tuff, there are also clasts which originated elsewhere, beyond the volcanic tableland itself. This suggests a source area with a granitic outcrop. One candidate location is Casa Diablo Mountain, north of the (south-sloping) volcanic tableland. Another possibility is the Sierras, to the west.

Another possibility entirely is that the source of the cobbles could be anywhere, and they were brought to the volcanic tableland not by streams but by paleoindians, who used them as grain-grinding stones in their metates.

Labels: california, conferences, faults, granite, pleistocene, rivers, travel, volcano, weathering

A nice example of plumose structure, enhanced by a pronounced joint set which cross-cuts the be-plumed surface. Hammer for scale. White Mountain front, California, September 2009.

Labels: california, joints, structure

Yesterday I went to see 2012, the new movie by Roland Emmerich. It was a LOT of fun. I love so-called "disaster porn" movies, the genre including Dante's Peak, Volcano, Cloverfield, Independence Day, the Day After Tomorrow, Deep Impact, the Core, etc. What these films have in common is that they gratuitously display major scenes of destruction as a way of luring audiences to the theater. Most have a plot stapled on too! And maybe some pretty celebrities!

The science is not all there, though. I'm sure you're shocked to hear that.

Here's Emmerich's scenario:
Planetary alignment --> solar flares --> neutrino storm --> neutrinos heat up Earth's interior --> crust gets detached and starts sliding around willy-nilly --> hilarity apocalypse ensues

Here's the problems with that:

1. Planetary alignments don't trigger solar flares.
2. Neutrinos (mostly) don't interact with matter in the Earth. (50 million neutrinos will pass through your body today without any issues that we're aware of.)

...but if you ignore those two basic fundamental problems, you can enjoy the death and destruction that result. It's a smorgasbord for the eyes.

One of the main characters is a geologist, which is cool. The scene of Yellowstone erupting was probably the neatest part for me, geologically. Even crazy man Woody Harrelson thinks so, and it's the last thing he ever sees. The magnitude 10-plus earthquake that hits southern California/Las Vegas is pretty awesome visually, but doesn't really square with geological realities of how earthquakes happen. Basically the way Emmerich runs the show, soCal breaks into a huge number of separate chunks which mainly move apart from one another, although occasionally they exhibit convergent motion (maybe 1% of the time). Mostly, it's huge chasms opening up, and people/cars/buildings/airplanes/trains/Russians dropping into them. And of course, "California slides into the sea" (that old trope which ain't actually what geologists predict for the Golden State*).

There's a plot, too, but who really cares about that? That's not why you go to see 2012. So I won't even bother. Go for the effects; revel in the destruction of the world, but try not to think about the death of billions. It's all about planetary chaos, adrenaline, eye-popping awesomeness. You know you like to watch.

Some reviews worth reading:

1. Rebecca Watson on Skepchick
2. Ian O'Neill on Discovery News

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* A better depiction can be seen in the AGI-produced Faces of Earth, episode 2, where Los Angeles (still a city much like the current one ~10 million years in the future), atop the block of continental crust west of the San Andreas Fault, slides past San Fransisco, briefly merging the two megalopoli into one.

Labels: california, movies, pseudoscience, science and society, sun

Returning now to some of the stuff I saw when I was out in Bishop, California for the GSA Field Forum I attended in September. One of the cool little spots we visited was "the flipping fault," a location on the Volcanic Tableland north of Bishop where an east-dipping fault scarp dies out and a west-dipping fault scarp starts up. Check it out:


Here, try one with annotations:


Here's a Google Map of the location, as seen from the perspective of a passing turkey vulture:

Notice how the road, Casa Diablo Road, goes right through the notch where the two meet. Complicating the picture a wee bit is a Pleistocene drainage channel which uses the same route between the two scarps (and diverges from the road in the lower-left).

Another view, further back and higher up:


And of course we must annotate that one too:


Recall that these are normal faults busting through the Bishop Tuff's upper welded layer, the "Ig2." In the annotations, I've sketched in the approximate position of the "hanging-wall cutoff" (lower boundary of each scarp) and the "foot-wall cutoff" (upper boundary of each scarp).

There are roughly equal numbers of east-dipping and west-dipping faults on the Volcanic Tableland. Originally, some creative structural geologists wanted to interpret this feature as an overall "propeller" shaped fracture: a so-called "flipping" fault (as in, it's one single fault that flips its dip direction in the middle). However, this was not the interpretation of our workshop leaders, who suggested that it was simply two faults that started independently and then propagated towards one another.

Taking a fresh look at these images now, almost a month after I visited the outcrop, I find that I agree with them. One thing that seems obvious to me now is how the east-dipping fault truncates on the face of the west-dipping fault scarp. My annotations reflect this interpretation. What do you think?

Labels: california, conferences, meetings, structure

As noted earlier, I had the good fortune to spend last week in Yosemite National Park, celebrating the wedding of my friends Jason and Lindsay, and in general poking around in one of the coolest places around. Below, a summary of the three-day trip:

Friday:
Lily and I flew to Modesto, California, and rented a car. It took about two hours to drive up to Evergreen Lodge, where we checked in and then headed out for a short hike in the Hetch Hetchy area. Hetch Hetchy was dubbed "Yosemite's sister valley" by John Muir in an attempt to keep it from being dammed. But the city of San Francisco had been destroyed in 1906 by earthquake-induced fire, and the call for a reliable water source was an important force in overpowering Muir's conservationist ideals. Ken Burns apparently explores this saga, the first instance of "development vs. conservation," in the second episode of his new National Parks series. (I saw the first episode, but haven't caught up on the rest of it yet.) The valley was dammed in the 1920's, creating the Hetch Hetchy Reservoir:


Here's the O'Shaughnessy Dam, named after the chief engineer of the project:


I didn't find it as spectacular as Yosemite, but it was sure a pretty place. Walking along the north side of the reservoir, I reaquanited myself with some fine Sierran granites and granodiorites. Here's a sweet little xenolith (or maybe an MME; how can you tell an MME from a mafic xenolith?):

Back to the Evergreen for the rehearsal dinner (Oktoberfest theme!) and then bed.

Saturday:
Up early, got some coffee, drove an hour to reach the Yosemite Valley. I liked how quiet things were compared to the throbbing pulse of summer. This view of El Capitan, for instance, is typically mobbed with tourists. This day, we had it to ourselves for five minutes or so, then shared it with one other car:


Time to stretch the legs! We decided to hike up to Vernal Falls. On our way up, the base of the falls was still in shadow, with low-angle morning sunlight dramatically illuminating the upper reaches of the falls:


Looking back down the valley we had climbed up... I like the dark shadow of the cliff merging with the dark shadows of the trees below:


But if we set the camera's F-stop a bit differently, we can see what's going on in all that shadow. There's the trail we climbed up, with fellow hikers for scale:


Up top, photographing the waterfall:


On our way back down, with more of the falls illuminated as the sun rises in the sky:


Looking north across the valley from where we parked our car, marvelling at the huge exfoliation joints there: rounding these exposed plutons into granite 'domes.'


... or Half Domes, as the case may be:


A view from further out, again with Half Dome the most striking landform:


Then, we headed back to clean up before the wedding. Great ceremony, amazing meal. Drinks, dancing, rhubarb jam, bluegrass, reminiscing with old friends and new. Ahhh.

Sunday:
Breakfast and coffee with the wedding party, then off to check out some big trees. We drove to the Tuolumne Grove of giant sequoias. It started snowing on the way there, but we didn't let that deter us. On the hike down from the parking area (where, by the way, they had closed the Tioga Road), we found this nice example of spheroidal weathering in an outcrop of granite:


But the real attraction was the enormous sequoia trees. Here's one:


And a dead one, with a car-sized hole cut through it:


I found these trees very impressive: they were just stunning in their grandeur and immense age. Snow continued to fall as we left. We had to get going to make our flight home. Somewhere on the way down the mountain, Garry Hayes and his wife passed us going up the mountain. Ships passing in the night -- sorry I missed you, Garry! We made a couple of roadside outcrop stops, then got back to Modesto and traded in the car for an airplane. Our "redeye" route back to DC took us through San Francisco and Los Angeles, and I ran into Thomas Friedman in the airport. Got back to BWI at 6am, and headed off to work...

Labels: california, glacial landforms, national parks, plants, travel, tv, water resources, xenoliths

My friends Jason and Lindsay had the good sense to get married in Yosemite this past weekend. So I enjoyed visiting the park, including some new places I hadn't been before, as well as the nuptial festivities. More later...

Labels: california, glacial landforms, national parks

There's been a lot of hubbub in the geoblogosphere over the past couple of days about caldera-forming eruptions. The trigger for all the discussion was a report about a really old (Permian) caldera-forming eruption in Italy. This invokes discussion of our modern worry-inducing "supervolcanos," like Yellowstone. There's also a lesser-known place in California, where the USGS maintains a volcano observatory there just like they do with Yellowstone: Long Valley Caldera. This caldera formed ~760,000 years ago, and deposited a lot of ash, collectively called the Bishop Tuff. The Long Valley Caldera's true area is about 300 km², although central sagging has generated ring faults that give it a topographic area of ~350 km². The Bishop Tuff is thickest right around the caldera, but ash from this eruption can be found as far away as Nebraska.

I got to see the Bishop Tuff firsthand the week before last when I spent a week in the Owens Valley as part of a Geological Society of America Field Forum. I was lucky to be introduced to the tuff by Wes Hildreth, a volcanologist at the USGS's Menlo Park office. Wes "wrote the book" on the Bishop Tuff, and shared an immense amount of information and perspective with the Field Forum participants. I am indebted to him for all the information I'm sharing here. Maggie Mangan just took over the reins of the Long Valley Observatory, and she also participated in the Field Forum. I also really benefitted from talking to her about the eruption. (Any errors that you may find here, of course, are my own.)

The Bishop Tuff is the most striking of many volcanic eruptions along this same system. It's the only one that has produced a caldera. It was preceded by dacite and basalt eruptions at 3.5 to 2.5 Ma, and then by rhyolite and obsidian during the appropriately-named Glass Mountain Interval, from 2.1 to 0.8 Ma. (The Glass Mountain Interval is pretty cool in its own right: at least 60 eruptive units, each high-silica rhyolite!) The focus of both of these was further to the northeast. That area is also home to some post-Bishop eruptions, the youngest of which is at Mono Lake (only 250 years ago). In 1989, a dike came within a few km of the surface, and degassed a CO₂ "burp" which killed trees near Mammoth Mountain, which lies on the caldera boundary.

The Bishop Tuff is compositionally similar from bottom to top: it's all rhyolitic pyroclastics, whether it's welded (fused together) or not. Some went north from Long Valley Caldera under the Mono Lake area, while the bulk of it went south towards Bishop, forming the Volcanic Tableland. it has a density of about 1.5 g/cm³.

In this photo, Kim Bishop (yes, that's really his last name) and Peter Lovely (yes, that's really his last name) check out the first of the ashfall deposits, dumped atop lake sediments in a cool outcrop on the southern margin of the Volcanic Tableland, north of Bishop and the Owens River:


A close-up of this contact:

The ashfall portion of the Bishop Tuff has 9 subunits, and you can see the first (F1) and the base of the second (F2) here, overlying the silty lake sediments.

Here's another outcrop, in the Owens River Gorge, where you can see the welded ashflow "caprock" up top, and down below, and outcrop that showcases nonwelded ashfall and ashflow deposits. I've put a box around the area that I'll zoom into in the next photo:


The ashfall deposits are finely stratified and well-sorted, with no reworking. Overlying them, the first of the ashflow (ignimbrite) units shows characteristic poor sorting: big blobs of pumice mixed in with the finer pyroclastics. Most of the ashflow is pinkish in color, but you can see here that the first of it is white, same as the ashfall:

Why pink in the ashflow portion? It's hot when it gets deposited, and heat retention promotes oxidation. The earliest ashflows were dumped atop ashfall (which gets deposited cold), and so likely lost much of its heat downward; hence less oxidation. The entire eruptive seqence is preserved in the Volcanic Tableland north of Bishop. Here, at the southern rim of the Tableland, we're getting the latest flows. The earlier flows didn't make it this far south.

Here's a close-up of that basal ashflow, from the first outcrop. My field notebook is 18 cm "tall," for scale. Note the white color and all the large pumice clasts:

The iron to titanium ratio in these clasts suggests that they erupted at 770 to 800 degrees C. The temperature of the eruption increased as it progressed. This corresponds with an increase in the mafic content of the tephra over the course of the eruption. In the early layers, there's about 77.7% silica, but when you get towards the end of the eruption, you see that number drop to 74%, as well as a doubling of iron content, a quadrupling of the Ca content, and ten times as much magnesium as in the earliest strata.

Here's a close-up of some semi-welded material. This is float, so I don't know precisely where in the sequence it fits, but I would guess the "Ig1" layer, the lower of the two welded ashflows.


And another. One thing I noticed about a lot of the included pumice blobs is that their vesicles were all stretched out into cigar-shaped tubes (prolate), like an L-tectonite. Anyone have any idea what's up with that? I would expect oblate strain ellipsoids (pancake-shapes) here due to post-depositional compaction, but that's not what I noticed...


We made a trip to the lip of the Owens Gorge to look down on the upper ignimbrite (ashflow tuff) layers of the Bishop Tuff:


The first half of the eruptive sequence, dubbed "Ig1" (for "ignimbrite 1") is below the sharp line. In the upper half of the sequence, Ig2, you'll find rhyolite lithics that can be sourced to the earlier Glass Mountain Interval, as well as pyroxene-bearing pumice. You can see here some abortive cooling columns in Ig1:

Likely these don't extend very far down because as soon as they started forming, Ig1 was buried underneath piping hot Ig2 ashflow. This addition of heat disrupted the cooling front and truncated the fracturing process. Sorry I don't have a sense of scale in this photo: it's hard to do when you're photographing the opposite side of a deep gorge. I'd guess these columns are a meter or so across. In one spot, a little downstream (southeast) of here, you can actually see a little ashfall intercalated with these ashflows (it's the F9 ashfall subunit). This, Wes Hildreth told us, is most unusual and quite handy for interpreting the stratigraphy of the Bishop Tuff. The only other place he's seen such a thing is in the Valley of 10,000 Smokes in Katmai, Alaska.

Some close-ups of the Ig2 unit, which is classic "welded" tuff with nice pumice blobs and rhyolite lithics, as well as pyroxene-bearing pumice:


Rhyolite lithic clast in "Ig2" welded Bishop Tuff ashflow deposit:

That's likely from the earlier Glass Mountain Interval, through which the Bishop Tuff erupted.

The "Ig2" layer wasn't the last part of the Bishop Tuff eruptive sequence, but the stuff deposited on top of it was unwelded, and has since been eroded away. In order for a tuff to weld, it needs to be close to 600 degrees C when it stops (this temperature is for rhyolite: it's actually composition and H₂O dependent). But the welding process (essentially superhot glass fragments warp around one another and lock into place) has made for a resistant layer atop the modern Volcanic Tableland, and this layer preserves the weaker layers beneath, preventing them from being eroded (except, say, where a river incises downward through the caprock). It's a nice example of differential weathering. Cosmogenic ¹⁰Be measurements on the upper welded tuff suggest a modern weathering rate of 2 mm/1000 years.


Now here's the thing that I thought was most interesting about the Bishop Tuff: it's big, and it erupted quickly. There are about 200 km³ of ignimbrite (ashflow tuff), another 100 km³ of fallout (ashfall tuff) out to Utah and Nebraska, as well as 300 to 350 km³ of welded tuff that filled the downdropping caldera (2.5 km of subsidence). That's a lot of magma fluffed out and ejected onto the surface! But Wes calculated that this whole sequence, from the first puff of ash descending from above to the last of the sizzling nuee ardentes, lasted a mere 6 days: a single huge eruption! And, Wes added, "on the seventh day, it rested."

Further reading (I particularly recommend taking a look at Figure 5d!):
Hildreth, Wes, and Wilson, Colin, 2007. Compositional Zoning of the Bishop Tuff. Journal of Petrology 48 (5):951-999.

Labels: california, permian, pleistocene, volcano

I spent last week in the Owens Valley of California, attending a GSA field forum on the structure and neotectonics of the Owens Valley and the Volcanic Tableland north of Bishop. It was really cool, and I learned a lot. I'll be sharing images and ideas on the blog in days and weeks to come.

Twenty-four people attended, plus the three conveners: David Ferrill, Alan Morris, and Nancye Dawers. Here's the team getting an orientation on Monday morning, looking west towards the Sierra Nevada. Note the time-honored geology field tradition of using magnets to hold posters and maps to the side of the van:


David discusses the tectonics and geomorphology of the "Eastern California Shear Zone," a transtensional zone between the Sierra Nevada and the typical Basin & Range. This area ranges tremendously in elevation: from Mount Whitney in the Sierras (14,494' elevation) to Badwater in Death Valley (-282'). The lurid colors on this elevation map show that:


A Landsat photo comes out at the next stop, looking northeast towards the Volcanic Tableland:


And yet another image, this one a beautiful side-scanning radar image of the Volcanic Tableland, which David and Alan (here assisted by Wes Hildreth) pulled out at a stop overlooking the Owens River Gorge (a canyon which dissects the Volcanic Tableland):


This image shows east-dipping normal faults as white stripes, and west-dipping normal faults as dark stripes:


This is the main reason we're all here: the young welded ashflow deposits of the Bishop Tuff (760 ka) record brittle strain as a result of the past 760,000 years of extensional and strike-slip tectonics. Due to the low rainfall and this excellent marker unit, you can really get a sense of how such systems operate. The faults are expressed topographically: a lovely marriage of structure and geomorphology.

Our first overview of the Volcanic Tableland, looking northeast from the Sierra Nevada over the fractured Bishop Tuff, towards the White Mountains in the distance:


Here's a Google Map of the Volcanic Tableland, showing the orange upper ignimbrite layer of the Bishop Tuff, and the north-south trending faults which rupture it. Green stripes are the Round Valley (southwest), Owens River (southern border, trending east-west), and Fish Slough (far east, trending north-south):


Here's another Google Marp, zooming in on some of the faults. Conveniently, Google opted for morning sunlight in this image, so it's "color-coded" the same way as the side-scanning radar image I showed you earlier: east-dipping fault scarps are light-colored, while west-dipping fault scarps are in shadow:

(Another very cool thing about this image is the northwest-southeast trending Pleistocene drainage channel -- more on that later!)

Many of the faults in the Volcanic Tableland are arranged in en echelon arrays, reflecting a broader zone of deformation:


In en echelon arrays of these normal faults, we find the individual fault segments are linked up with intermediary flexures of the the ignimbrite layer, called "relay ramps." This was a new term to me, but once I learned it, I saw them everywhere. Here's one atop the Volcanic Tableland:

(It's the shallowly-sloped bit in the middle, dipping towards us, bounded by two west-dipping fault scarps: the intensely-shadowed areas.)

Here, in Fish Slough, we see a couple of 'relict' relay ramps that have gotten cut off as the small fault segments linked up into a larger through-going fault. Pretty cool!


The group descends a relay ramp on their way back from a field excursion to the vehicles:


Annotated version of the photo above:


Relay ramps occur on many scales. This 'scaling' of fault systems (and deformation in general) was a theme at the field forum. Here's a Google Terrain Map of the Owens Valley area. Notice how, just west of Bishop, the Sierra Nevada front jumps to the west? That's a much larger relay ramp, the Coyote Warp Relay Ramp:


Looking west from the first stop at the Sierras, with the Coyote Warp Relay Ramp descending from upper left towards lower right:


Annotated version of the photo above:

That's a little taste to get you started. More geology from the Owens Valley in future posts...

Labels: california, faults, structure, volcano

Five more of the maps I scanned from my recently-entered-the-public-domain copy of Vernon Quinn's book A Picture Map Geography of the United States. As before, clicking on the image will take you to a bigger version of the map. Enjoy!

Labels: art, books, california, georgia, idaho, maps, utah, west virginia

You should check this out. Nice images. Two and a half minutes in length.

Thanks to Kevin Mattingly for alerting me to this tidbit.

Labels: california, energy, igneous

A couple of days weeks ago, I posted three photographs (reproduced below) and asked you to explain them. My sincere apologies that I haven't gotten the answers up sooner... it's been a crazy time. I've been swamped. And the explanations are not brief. Anyhow...

Here are my explanations -- and the winners for the contest!

A

This is a kink band that got reactivated fault. The tectonic stresses acting on these rocks changed over time, and with them deformation took different paths.

This kink band occured in an area of highly foliated metavolcanic rocks, which developed their transposed foliation (running left to right across the photo) due to transpression in the late Mesozoic. The orientation of the kink bands suggests that the second generation of deformation (the kinking) was caused by a maximum stress oriented at an angle of ~30 degrees to foliation. (see Figure 56, page 100 of my geology master's thesis). Some of the resulting deformation was taken up by (See Figure 52, page 95 of my geology master's thesis) kinking. If the second generation of deformation (kinking) were directed parallel to the foliation, we would expect to see conjugate pairs of kink bands, both at the same angle to foliation. But that ain't what we see... we see kinks in only one angular relationship to foliation. This tells us that the maximum stress (sigma-1 in part C of the diagram below) must have been coming in at an angle of about ~30 degrees to the foliation:


Later, those kink bands/faults were reactivated under a third generation of deformation, which then allowed those fault surfaces to open "void spaces" which instantly filled with whatever fluids were available. In this case, that appears to have been a quartz-saturated water, which filled in the void space with a deposit of milky quartz.

Winner? Kim came closest -- and also pointed out that this story is reinforced by looking around the area at similar exposures which show the same story. Kim, you win a bumper sticker!
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B

This is a strained metaconglomerate, and it provides a nice case-study in strain localization.

This photo speaks volumes to me, because my geology master's thesis was a "real life" check on the predictions of a forward numerical model. My advisor wanted to try and understand the development of lineation in shear zones (ductile faults) via computer modeling. So he came up with a cool model that made predictions about the orientation of lineation relative to foliation and relative to the shear zone's boundaries, and he sent me out into the real world to see if real shear zones played by those rules. And the two didn't match up perfectly.

One issue that may contribute to the lack of agreement between the Sierra Crest shear zone system and modeling predictions is that models distribute strain systematically across a shear zone, whereas it is instead localized in natural systems. The shear zone is itself a localization of strain, of course. The question is, 'how local?' In other words, at which scale(s) is strain being accommodated? Possible triggers for strain localization are many: rheological contrasts between lithologies (Nadin and Saleeby, 2004), variations in temperature or fluid flux (due perhaps to proximity to an intruding magma body) (McCaig, 1984; O'Hara, 1988; Tobisch et al., 1991), variations in stress (due perhaps to salients of wall rock which project into the shear zone or the presence of resistant blocks inside the shear zone), presence of fluids, and / or pre-existing structural heterogeneities. For whatever reason, certain areas within a shear zone may accommodate more strain than neighboring areas. Shear localization may occur on many scales.

Photo B above shows cm-scale localization of strain as small pebbles in a metaconglomerate wrap around a larger, central, less deformed clast. Pebbles immediately across strike from the large clast are more deformed than pebbles along strike from the large clast (i.e. those in the rigid clast's 'pressure shadow'). As a result, the orientations of the long axes of the surrounding pebbles (i.e. lineation) occur in a variety of orientations, a condition also seen in traces of the foliation. On a shear-zone-segment (km) scale, strain localization may be noted in the appearance of pods of relatively undeformed rock surrounded by well-foliated and lineated rock more typical of the shear zone. In the Gem Lake and Mono Pass segments of the Sierra Crest Shear Zone system, for instance, lozenge-shaped pods of clast-rich volcanic breccia (See thesis Figures 14, 15, and 21) were far less deformed than neighboring rock. The implication is that the deforming portions of the shear zone 'flowed' around these pods of more resistant material.

Winner? Growing Tedium came closest, though nobody wrote about the strain localization.
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C


I took this last photograph because it demonstrates well the relationship between bedding and foliation in these rocks. Bedding runs from the lower-left of the outcrop towards the upper-right. But within those beds, you'll notice that all the clasts are elongated vertically into elliptical shapes (ellipsoidal in three dimensions). That's because these rocks got squeezed from the sides when they were hot enough and under enough pressure to flow into new shapes. At this location, deformation played a light enough touch that we can still see relict bedding, but in most of the Sierra Crest Shear Zone, the rocks are much more pervasively deformed: they exhibit a transposition foliation, where no traces of their primary structures can be still be seen. So in some ways, Photo C is the opposite of Photo B: it's a zone of lesser deformation surrounded by a zone of greater deformation: a less-disturbed pocket of rock in an area defined by its disturbed rocks.

Here's how I interpreted this outcrop in my thesis:



Winner? Again, Growing Tedium came closest, by referencing the long axes of these clasts and the "bands" (bedding planes) which run through the outcrop at a 60-degree angle to the long axes. GT, please send me an e-mail with your mailing address, and I'll put your bumper sticker(s) in the mail to you ASAP.

Thanks to everyone for playing, and my sincere apologies for taking this long to get the answer up. (Is it apparent why it took me a while, now that you've read through this whole thing?) I've got a new, simpler contest planned for later in the week.
__________________________________________________________________

McCaig, A.M., 1984. "Fluid rock interaction in some shear zones from the Pyrenees." Journal of Metamorphic Geology 2, 129-141.

Nadin, E.S., and Saleeby, J.B., 2004. "Localization of shear along a compositional discontinuity: the Proto-Kern Canyon Fault, Sierra Nevada, California." GSA Annual Meeting Abstracts: Denver 2004.

O'Hara, K., 1988. "Fluid flow and volume loss during mylonitization: An origin for phyllonite in an overthrust setting, North Carolina, U.S.A." Tectonophysics 156, 21-36.

Tobisch, O.T., Barton, M.D., Vernon, R.H., and Paterson, S.R., 1991. "Fluid-enhanced deformation: Transformation of granitoids to banded mylonites, western Sierra Nevada, California, and southeastern Australia." Journal of Structural Geology 13, 1137-1156.

Labels: california, contest, structure

A reader of NOVA Geoblog forwarded me an announcement for a geology/education position at UNC-Chapel Hill, which led me to check out the rest of the UNC Geological Sciences website. (No, I'm not applying for the job -- quite happy where I am!)

I cited an important paper* by Allen Glazner in my geology master's thesis, which led me to poke around the author's website a bit. He has a nice collection of photos, including field work in the Sierra Nevada (and elsewhere).

One of my favorites is this awesome (and funny) shot of a shear zone. Check out the kinematics on that sucker! It's "textbook"!

Another is this mouthwatering fold.

There are also some great aerial shots featured. This series of the Deep Creek playa reminded me of a very cold night I spent camping in the Deep Springs Basin, then hiking out on the playa and finding a dead bat that had been mummified in the salt. Nice memories...

Anyhow, enjoy the whole series -- a pleasant way to while away fifteen minutes!

___________________________________________________________________

* The article I cited was a really interesting one:

Glazner AF, Bartley JM, Coleman DS, Gray W, Taylor RZ (2004) "Are plutons assembled over millions of years by amalgamation from small magma chambers?" GSA Today: Vol. 14, No. 4 pp. 4-11.

It posits that igneous pluton emplacement is really drawn out, for instance consider the case of the Half Dome Granodiorite, which took ~4 million years to crystallize:

Figure 5 from the paper. The caption reads: "Summary of geochronologic data for the Tuolumne Intrusive Suite, modified from Coleman et al. (2004). Ages are from concordant U-Pb zircon data. Bar height is equal to +/- 2-sigma error and bar color is keyed to rock unit color on inset map. Ages for units are arranged in sequence from outermost to innermost (Kse-Sentinel Granodiorite; Kga-Kkc-tonalite of Glen Aulin-Kuna Crest Granodiorite; Khd-Half Dome Granodiorite; Kcp-Cathedral Peak Granodiorite; Kjg-Johnson Granite porphyry). Horizontal scale is not linear distance, but places samples according to the fractional distance from outer to inner contact of individual units (see Coleman et al. [2004] for a complete discussion)."

I recommend reading the whole paper, especially if the details of pluton emplacement interest you.

Labels: california, field trips, granite, north carolina, plutons

Answers tomorrow... Only three entries so far... There's still room for another winner or two...

Labels: california, contest, structure

After seeing the feldspar megacrysts in Maryland's Ellicott City Granodiorite two days ago, I wanted to share some even more impressive megacrysts, those found on the periphery of the Cathedral Peak Granodiorite pluton ['CPGD'] in California's Sierra Nevada mountains.

Here's a typical look at the CPGD close to its contact with metasedimentary & metavolcanic host rocks. It's chock-full of 3-7 cm crystals of potassium feldspar, set in a more typical-looking granodioritic matrix of sub-0.5 cm crystals:

This is a nice example of an intrusive porphyry. Not all porphyritic textures result from two phase cooling: The way the story usually goes is that the magma starting underground at a realtively slow rate, then the magma (solid crystals + remaining liquid) gets tapped and erupts, with the rest cooling at a faster rate on the surface. This one clearly shows a phaneritic (coarse-grained) texture throughout; it's just that some crystals grew bigger than others. I'm not an igneous petrologist, so I won't claim to understand why. Enlighten me if you know.

Here is a close-up of one feldspar crystal shows lines of mafic inclusions (earlier-crystallizing minerals like amphibole which were caught up in the advancing front of feldspar crystallization, and trapped in the larger feldspar crystal):

My mind wants to see this as a spiral pattern, like a snowball garnet, and hence to interpret this as a feldspar crystal rotating as it grew, but that's surely wishful thinking. Especially seeing as how there's no foliation to get wrapped up in the 'rotating' porphyroblast. But... I've never seen another igneous crystal that shows this same pattern. Anyone else? Trick of the light?

Now here's something really wild:

Recall that when I took these photographs in 2003, I was out in the Sierras looking at the Sierra Crest Shear Zone, a 1-2 kilometer wide zone of smooshed rocks adjacent to the eastern boundary of the Sierra Nevada Batholith. So mainly I was interested in these older "host rocks" which were metavolcanic and metasedimentary, but I was also interested in how they related to the batholith as a whole. In places, I could see clear evidence that the plutons of the batholith were sheared, too, and in other places they appeared to have intruded post-deformation. This photo shows that the Cathedral Peak Granodiorite came along after the bulk of the deformation had happened.

How do we know? (1) It's not especially foliated itself. (2) Here, magma oozed between the foliation layers in the metasedimentary rocks immediately adjacent to the pluton. These layers flexed to allow the magma to intrude; I think of curtains billowing underwater. Then, as the pluton inflated (or as regional deformation continued to squeeze these rocks; or both), a compressive stress was exerted on these mingled layers of foliated rocks and magma. The liquid magma squished out of the way, but the solid megacrysts were trapped, and the foliation flexed and wrapped around them.

Twisted food analogy: Say I make a peanut butter and raisin sandwich. (Seriously, they're good!) I have a piece of bread, and I smear it with a mix of creamy peanut butter and chunky raisins (the giant ones from Trader Joe's). I place another piece of bread on top. Then, because I value my geology more than my manners, I lean over like I'm going to perform CPR, and exert pressure perpendicular to the plane of the bread. The peanut butter, being ductile, squishes out the sides, while the raisins are trapped, and the bread deforms around them.

Such, such are the thoughts of the hungry field geologist...

Labels: analogies, california, granite, metamorphism, minerals, primary structures

Labels: california, glacial landforms, mountains

Callan is a busy boy these days working on his science education master's. But... (mainly through discussions in my Structural Geology class at George Mason University) I've been reminded of some of the cool stuff I saw when I did my geology master's thesis in the high Sierra of California. Here's a couple of neat images from my field work that ought to convey some of the magic of doing structural geology in the "Range of Light."

The challenge I now put to you: explain what's going on in these images. I've labelled them "A," "B," and "C" for easy reference. Winners get a "GEOLOGY ROCKS" bumper sticker. One winner per photo -- whoever comes closest to describing the geology most completely & accurately.

A


B


C


Just a taste of the magic that a summer of field work imparts... :)

Answers in a couple of days...

Labels: california, contest, structure

Today I got the news that I've been accepted to two professional development opportunities that I'm really looking forward to:

1. SERC workshop: Strengthening Your Geoscience Program, June 2009, Williamsburg, Virginia
2. GSA Field Forum: Structure and Neotectonic Evolution of Northern Owens Valley and the Volcanic Tableland, California September 2009, Owens Valley, California

Yay!

Any other geobloggers or geoblog-readers going to be at either?

Labels: california, conferences, virginia

Started by Lockwood, perpetuated by Silver Fox... [UPDATE: Geology Happens, Geotripper, Hypocentre & Phreatic Ramblings have chimed in, too. The latter even posted about a huge paleofalls...] As per the geoblogospheric standard, the idea is to bold the ones you've been to.

#10 Lower Calf Creek Falls, Escalante National Monument, Utah
#9 Lower Falls of the Yellowstone River, Yellowstone National Park, Wyoming
#8 Upper Whitewater Falls, in southwestern North Carolina
#7 Snoqualmie Falls, between Snoqualmie and Fall City, Washington
#6 Havasu Falls, Supai Village, Havasupai Indian Reservation, Grand Canyon, Arizona
#5 Shoshone Falls, Twin Falls, Idaho
#4 Multnomah Falls, Columbia River Gorge, Oregon
#3 Bridalveil Falls, Yosemite National Park, California
#2 McWay Falls in Julia Pfeiffer Burns State Park, Big Sur, California
#1 Niagara Falls, Niagara, New York

Bonus Waterfall #1 [via Lockwood]: Salt Creek Falls, Oregon
Bonus Waterfall #2 [via Silver Fox]: Palouse Falls, eastern Washington
Bonus Waterfall #3 [from me]: Deer Creek Falls, Grand Canyon, Arizona (photo above)

For the record, I kind of don't get the appeal of waterfalls. I mean, they're cool and all, but they don't strike as particularly complex (and therefore, not particularly interesting)... I mean: gravity, right? ...It pulls water downhill... What's the big deal? (I had a conversation this summer along these lines at Waterfall #9 on this list, with a similarly-minded fluvial curmudgeon.)

...But people love them - When I poll my Physical Geology students at the end of the semester about what their favorite part of our Billy Goat Trail geology field trip, only a third or so invoke the migmatite, a third or so cite the physical challenge of climbing "The Traverse," and a third or so claim that viewing Great Falls was their favorite part. To each their own, I reckon: I'm glad they got something meaningful out of the trip... but I can't claim to understand it.

In my twisted worldview, Deer Creek Falls is interesting not merely because it's scenic (and a great place to go swimming), but because the waterfall issues from the Great Unconformity, and thus has geologic significance: It satisfies the intellect as well as aesthetic sensibilities.

Labels: arizona, california, grand canyon, new york, wyoming

Dave at the Geology News blog is hosting this month's Accretionary Wedge on the topic of "favorite places to do field work."

My favorite place to do field work is in California's "range of light," the Sierra Nevada.

I did my geology master's field work in the eastern Sierra, along the Sierra Crest Shear Zone, a major high-strain zone which parallels the eastern edge of the Sierra Nevada Batholith through older meta-sedimentary and meta-volcanic host rocks.

In 2003, I spent the summer out there, starting with my first field area at lovely Gem Lake:

An angular unconformity can be seen in this image as the tilted (close to vertical) metasedimentary and metavolcanic rocks (orange and gray) are overlain by dark colored "Tertiary" basalt flows. A big talus slope of basalt chunks makes a black triangular shape that heads downhill toward the lake. In the distance, where the land rises appreciably, the granites (and granodiorites) of the batholilth begin.

We camped on this peninsula sticking out into Gem Lake:


Dazhi Jiang (Then of UMD-College Park; now at the University of Western Ontario) and USC's Geoff Pignotta examine strained metavolcanics near Gem Lake:


Here's me with the Ritter Range in the background:


Glacial striations sculpting my strained metavolcanics:



Field gear:

Labels: california, field trips, glacial landforms, granite, igneous, metamorphism, structure, travel

Would you like a little geoblogosphere with your coffee this morning?

There's some great stuff out there today...

Andrew Alden (Geology.About.com) showcases the Fransiscan melange on a trip to Shell Beach.

Watch Perito Moreno glacier do some AWESOME calving at En Morrenas (Spanish-language geoblog). Watch the whole thing for perspective (3 minutes), but the really spectacular collapse occurs at ~2 minutes into the video. Watch the splash and watch the huge chunks of ice go zinging off into the surrounding air. Wild!

Dave Petley (Dave's Landslide Blog) reviews the dangers of a collapse of a volcanic flank in the Canary Islands, and what it means for Atlantic Ocean tsunami risk.

And for the geobloggers in the house, Chris proposes getting together in January at a science blogging conference in North Carolina. I think this could be cool. I just signed up.

Time for another cup of coffee... Good morning!

Labels: blogs, california, geology, glaciation, landslide, meetings, north carolina, south america

An interesting piece on NPR discusses how joshua trees (Yucca brevifolia) will react to climate change. It revealed a fact I had not previously recognized: that during the Pleistocene, joshua trees habitat expanded thanks to the digestive efforts of the Shasta ground sloth (Nothrotheriops shastensis). Sloth dung deposits are full of j-tree nuts, and since the sloths expired 13,000 years ago, the trees haven't been able to move as far or as fast. Half of their current habitat in California and Nevada may be too hot and too dry within the next 50 to 100 years. The graphic above is from NPR, which produced the story as part of their "Climate Connections" series.

Labels: california, fossils, global warming, national parks