A couple of other cool stuff, not directly related to the theme of the GSA Field Forum, that we saw at the site of the tafoni and the metates:

First, some petroglyphs. These are carved into the clay/oxide/biofilm layer known as "desert varnish," revealing the pink Bishop Tuff beneath:


And if you walk to the edge of the bluff, where the Owens River has chewed away at the edge of the volcanic tableland, you can see this:


That's the Sierras in the distance, the Owens River in the middle ground, Chalk Bluff Road, and then the south slope of the volcanic tableland. The Owens River is near its local base level here, and has produced some lovely meanders. You can see the current batch of meanders, plus older, cut-off loops here:


...And did I mention they have some fault geology there too?

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

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

That's a slab of the Shawangunk Formation conglomerate, from eastern Pennsylvania. I collected it a couple of years ago when I drove up to go fossil hunting at the Whaleback, but it wasn't until last year that I slabbed and polished it. (The slab measures 10 cm wide by 27 cm in length.) Then a couple of months to get around to scanning it, and finally a few months more before posting it. Sheesh.

It's a lovely quartz-rich clast-supported conglomerate, a ridge former in the Valley & Ridge province of the Appalachians. Like the Massanutten Formation, it's Silurian in age, and thought to be part of the "molasse" sequence shed off the Taconian mountain belt, first raised during the late Ordovician. It is interpreted as a relatively-high-energy fluvial system deposit; sediments laid down by rivers as the mountains next door were weathered and eroded.

Labels: appalachians, pennsylvania, quartz, sediment, silurian, valley and ridge, weathering

Today I took a group of students to Sideling Hill, a syncline in western Maryland. Here are a few photos from the trip. All photos by my iPhone, via Facebook (which is why the quality is lower than my usual standards):

The group all kitted out at the Sideling Hill Visitor's Center (which was closed due to budget cuts in Maryland):


Jared points out fast-weathering shale layers betwixt slower-weathering sandstone layers:


Diamictite outcrop on the far western side of Sideling Hill:


More diamictite... enigmatic sediments...

Labels: maryland, nova, primary structures, sediment, structure, teaching, valley and ridge, weathering

Here we have some nice little glacial striations exposed in the Grinnell Glacier cirque in Glacier National Park, Montana. These grooves were carved by pebbles and other clasts within the glacial ice as it flowed over this outcrop of the Mesoproterozoic Helena Formation (part of the Belt Supergroup). Perhaps some of the same pebbles you see in this photo were responsible for acting as carving tools -- though the 'hand' that wielded them, Grinnell Glacier itself, melted away from this point sometime since 1939.

Also of interest to me in this photo is the lingering stain of water around the joint set in the upper right. I'm fascinated at the interplay between physical and chemical weathering, and seeing stuff like this emphasizes how even a simple hairline fracture can help funnel water, with all its destructive effects, deeper into the heart of an outcrop. Weathering is focused on these areas, and in another century this outcrop may look quite different.

Labels: glacial landforms, glaciation, joints, montana, national parks, pleistocene, proterozoic, sediment, snow, weathering

Took all these images of joint surfaces this summer in Glacier National Park on my Rockies trip. Enjoy!

Appekunny Formation, with two concentric ribs:


Grinnell Formation, showing well-developed hackle fringe (rough area at bottom):


In the lovely fine-grained limestones of the Helena Formation:

Labels: montana, national parks, proterozoic, structure, weathering

Here's two pictures of quartz I took with my new toy, a Nikon camera/microscope/digital-picture-stitcher-togetherer. More to come, clearly. Click on each image to make it bigger.

Conchoidal fractures at the tip of a quartz crystal:

Blue quartz, a distinctive mineral that's found in Virginia's Blue Ridge province:

The blue color is apparently from inclusions of ilmenite and rutile...

Purty, huh? No sense of scale, though. Tough!

Labels: blue ridge, minerals, weathering

Almost a week after the field trip to Old Rag Mountain, and the Facebook-hosted pictures keep trickling in. Here's some shots by NOVA student Eileen Lodovichetti, and an ensuing discussion of feeder dikes and supercontinent breakup.

Here's a shot of the upper reaches of Old Rag, showing the characteristic spheroidal weathering of the Old Rag Granite and the relative lack of trees on top:

photo by Eileen Lodovichetti

...And here's a shot that Eileen took which shows the interior of one of the weathered-out feeder dikes we had to hike through on our way to the summit. You can actually see the classic geoprofessorial arm-waving caught in blurry motion!

photo by Eileen Lodovichetti

This is one of the coolest things about hiking Old Rag: after scrambling up on top of spheroidally-weathered granite domes, you drop into these tabular "hallways." The astute observer will note that the floor is made of a fine-grained, dark-green-colored rock, quite distinct from the light-colored, coarse-grained granite that makes up most of the mountain. These are dikes of metamorphosed basalt that intruded the granite during the breakup of the supercontinent Rodinia in the Neoproterozoic era of geologic time.

Here's one of my former Field Studies in Geology students, Mike Nelson, pointing out a similar dike along Skyline Drive, in the main part of the park:


Basically, the story goes like this: Around 1.2 to 1.0 Ga, continental fragments amalgamated into a supercontinent called Rodinia. In Virginia, this is recorded in the rocks of the Blue Ridge province, where the basement consists of granitoids (granites and related rocks) and metamorphosed granitoids (gneisses, mylonites). Among the youngest of these is the Old Rag Granite, which intruded the Pedlar Formation granite gness around 1.0 Ga.

Later, Rodinia broke apart, resulting in an extensional tectonic regime and mafic volcanism. Fractures opened up in the Old Rag Granite and funneled mafic magma towards the surface. Massive eruptions of basalt blanketed the landscape. The resulting layers of basaltic lava are known as the Catoctin Formation. At Old Rag Mountain, we can see some of the plumbing that led to these flood basalt eruptions: these are feeder dikes, because they "fed" the eruption above them.

Because the dikes (which were metamorphosed to greenstone during ~300 Ma Appalachian mountain-building) weather more rapidly than the Old Rag Granite, they are typically recessed into the landscape. That's what makes the "hallways" in the photograph above. Here's two more images, showing these weathered-out feeder dikes:



Check out how there's moderately-developed columnar jointing extending across the dike. These columns form perpendicular to the cooling front, and the dikes would have lost their heat out the sides. In horizontal lava flows, the heat is lost from the top and bottom surfaces, so you get vertical columns. Here, a vertical dike produces horizontally-oriented columns. Hikers appreciate these "steps" as they squeeze through the dikes on their way up the mountain.

Here's a map of part of Shenandoah National Park:


Please ignore the "hover" instructions at the lower right. I've reproduced the "hoverable" image below. Key: the orange is the Pedlar Formation. The pink is the Old Rag Granite, and the green is the Catoctin Formation. Feeder dikes of the Catoctin are shown as green lines.

Now, let's take away the map, and just preserve the orientation of the feeder dikes. This will tell us the overall tectonic stretching direction:
Various plate reconstructions show either Amazonia or the Congo craton offboard of Virginia at the time Rodinia broke apart and the Iapetus Ocean began seafloor spreading. I've illustrated it here as the Congo, but that might be wrong.

So: the hike up Old Rag is great exercise, and offers scenic views, but for those willing to consider the rocks and how they got there, it's an insightful view into the tectonic past.

Lastly, here's a lovely, well-developed weathering rind on the Catoctin meta-basalt (greenstone). When the dark green rock adjusts to the conditions at the Earth's surface, it breaks down, resulting in the tan/"buff" color on the outside. You're watching the rock "rot" from the outside surface, working its way inward:


More on the geology of Shenandoah National Park can be seen at this page on my website.

Labels: appalachians, basalt, blue ridge, field trips, metamorphism, mountains, national parks, nova, plate tectonics, shenandoah, structure, weathering

Last weekend, I took a group of students, mostly from NOVA but also 3 from GMU, up to hike Old Rag Mountain in Shenandoah National Park.

Here's a Google Map showing the terrain (and trails, which is a cool new addition to the already cool Google Maps):


The crew discusses debris flow deposits in the forest on the way up the mountain:

photograph by Charlie Corrick

The first spot where we get a nice view out over the valleys below:

photograph by Charlie Corrick

Spheroidal weathering in Catoctin Formation greenstone:

photograph by Jared Fortner

Spheroidal weathering in granite (the Old Rag Granite, 1.0 Ga):

photograph by Charlie Corrick


photograph by Charlie Corrick

Student Jared atop a spheroidally-weathered boulder of the Old Rag Granite:

photograph by me

Grain-size differences in the Old Rag Granite (balanced atop my leg):

photograph by me

Non-foliated Old Rag Granite (showing lovely "blue quartz"):

photograph by me

And the foliated version of the Old Rag Granite:

photograph by me

Labels: appalachians, basalt, blue ridge, field trips, geology, granite, igneous, mountains, national parks, nova, structure, weathering

Walking to my car the other day, I looked up at the embankment on my street, and noticed some geology there I hadn't seen before. Yesterday, with my camera, I climbed up the embankment (~15 feet) to investigate. Fortunately there were some trees to hold onto.

Sure enough, it was as I suspected: dikes of granite (subvertical in orientation) that, along with the schistose bedrock they cut across, had totally weathered to saprolite.

Keys for scale:


Originally, these dikes were emplaced during the late-Ordovician eastern-North American episode of mountain-building called the Taconian ("Taconic") Orogeny. Later, when they got exposed at the surface (or close to it) they began to "rot."

Hand for scale:


Here's a video showing how readily these dikes formerly known as granite deform by crumbling into pieces:


The main chemical weathering process that has happened here to make this possible is the hydrolysis of feldspar to produce kaolinite, a clay mineral. Large single crystals of potassium feldspar in the granite are now large amorphous masses of kaolinite, which has no strength when stressed.

Labels: dc, granite, igneous, metamorphism, ordovician, weathering

The day before Inauguration, I decided to celebrate George Bush's last day in charge of my country by taking a walk in the woods. Okay... that wasn't really my motivation. I was just procrastinating writing up my Structural Geology labs for the coming semester. Anyhow, for one reason or another, I took a stroll in the woods.

I brought my rootin' tootin' new camera with me, and took a few photos. I've got four things to show you: (1) some differential weathering, (2) some kink banding, (3) some cool effects in frozen soil, and (4) a critter.

(1) To start, check out this close-up photo of a stone bridge where the Klingle Valley merges with the Rock Creek Valley:


Several of the (local) stones used in the bridge are weathering at a faster rate than the mortar (cement) that holds them together. As a result of this differential erosion, the less-stable rocks are recessed into the face of the bridge:



Here's another one, where you can see that not all minerals are equally stable at Earth surface conditions. The large central quartz augen stands out in high relief as the micaceous & feldspathic schist around it weathers away.



Yet another: recessed about an inch into the bridge:



That's not all I saw. I also re-discovered the location of some kink bands along the Rock Creek Park bike path:



These kink bands are similar to the ones that Spring 2008 Honors student Victoria measured and analyzed in Broad Branch (also in Rock Creek Park), but these ones are in a different location, further south in the park.



What's worth noting about these kink bands is that they overprint the regional foliation of these schistose rocks. In order to do that, the force that generated the foliation must have been coming from one direction (call it east-west), causing the mineral grains to line up at right-angles to that stress. That allignment is what we call foliation. Later, a new generation of deformation came in from a different direction (call it north-south, approximately parallel to the foliation), kinking the pre-existing foliation. For more on kink bands in DC, see my "DC rocks" page.



(4) One of the disadvantages to hiking in Rock Creek Park in the winter is that it's pretty monochromatic. One of the advantages is that with all the leaves off the trees, it's a lot easier to see new stuff. It's great woodpecker-watching weather, for instance. I saw five woodpeckers of two species that day. Also, it makes it a lot easier to see where the trails are. I saw a new trail that I had never walked before, and so I decided to check it out. I'm glad I did. One thing that I saw that is pretty cool is this effect in frozen soil:



When water freezes, it expands in volume by about 9%, and that shows up here as the upper layer of wet soil froze, it expanded in all directions, pulling away uniformly from two large cobbles of quartzite. It almost makes it look like the quartzite cobbles shrunk in their "sockets," but really it's the "sockets" that got larger.

(4) Lastly, I was doubly glad to have taken the new trail because it was a "road less travelled" kind of deal. I was the only one there. As I trod along, suddenly I heard a scampering noise. It was a critter! It ran up a little gully and then paused as still as a stump, looking at me:



Can't see it? Try this zoomed-in shot:



It's a red fox! Vulpes vulpes, one of two wild canids we have in Rock Creek Park. Pretty good sighting -- only the fourth time I've seen one here (and I spend a lot of time in this park). And every one of those times was in the winter. Again, it's having those clean leaf-less views that allows hikers to see stuff like this.

Labels: critters, dc, ice, mammals, national parks, structure, weathering

It's the 50th anniversary of Chinua Achebe's Things Fall Apart, a reminder that things continue to fall apart. Like... rocks. ...and steel. Today, I'd like to share a "compare & contrast" of two kinds of fractures I saw on my Thanksgiving trip to Hawai'i. One is caused by a decrease in volume; the other is caused by an increase in volume.


Type 1: Columnar jointing (shrinkage fractures)









Columnar jointing results from the decrease in volume as hot lava crystallizes into cool rock. The overall shrinkage in the rock's volume is accomodated by fractures that (all else being equal) are oriented at 120-degree angles on the surface of the flow, and then propagate downward into the flow, perpendicular to the cooling front (isotherm of the critical fracturing temperature, which here is subparallel to the surface of the lava flow). Similar fractures form in drying mud, where the volume loss is due not to cooling but to the evaporation of water. Generally, these mud contraction fractures (a) don't go as deep, and (b) experience more volume loss, resulting in wider fractures. These are in the Mauna Lani resort area, on the western shore of the big island of Hawai'i.
_________________________________________________________________


Type 2: Rust blisters (expansion fractures)








Here, we see fractures forming not due to a loss of volume, but the opposite: an increase in volume! Here the metal (steel, presumably?) in the pole is oxidizing, and in completing that reaction, rust is forming. The layer of paint probably got nicked, water (probably saltwater?) got under it, and then the paint kept the water down there, facilitating the rusting reaction. As the rust formed, it swelled relative to the volume of the original metal. It expanded in the direction that offered the least resisting stress (out away from the surface of the pole). As the rust bumps grow, they impart a new stress on the metal/rust, and this causes fractures to form subparallel to the pole's surface. These are near Ka Lae ("South Point"), near the start of the hike to Green Sands Beach.

Labels: basalt, hawaii, structure, travel, weathering

...So let me ask you something, especially you sedimentary geologists...

This is a sample of the Martinsburg Formation, a clastic unit shed off the Taconian Orogeny and into the adjacent basin. It's exposed in the modern-day Shenandoah Valley, where it overlies Ordovician carbonates, and is overlain by the Silurian Massanutten Sandstone (which is correlative to the Tuscarora Formation). It's essentially a graywacke, showing rhythmic bedding traditionally interpreted as turbidite deposits. I collected this sample in the Shenandoah Valley a year and a half ago, on a camping trip with my family.

Then I put it on the NOVA rock saw and sliced it in half. This chunk went to my dad's back yard, where I ground it down and polished it up. The result is a decent look at the internal structure of the unit (you can click on it for higher resolution):


Note the pretty uniform weathering rind wrapping around the whole thing, like crust on a loaf of bread.

UPDATE: Woe is me; I forgot to include a sense of scale. The sample measures about 10 cm (~4 inches) on a side.

Here's the thing that gets me... While this portion ('upper' 2/3 of the sample) shows a clear fining-'upwards' sequence....


...this portion of the sample (lower 1/3) appears to show a coarsening-'upward' sequence:


In other words, in this 'graded bed,' the coarsest grains appear about 1/3 to 1/2 of the way 'up,' from 'bottom' to 'top'... What gives? This isn't part of the traditional Bouma sequence, is it? How does a bed like this form?

I'd appreciate any enlightenment you can offer.

Labels: primary structures, sediment, valley and ridge, weathering

Geological travels in Northern Ireland, Part VI:

The word "joint" in geology refers to any fracture in a rock unit along which movement has NOT occurred. (If movement DOES occur along a fracture, that makes it a fault.)

The Giant's Causeway in County Antrim, Northern Ireland, shows jointing of a particular pattern: the intersection of the joints divide the rock into column-shaped pieces, shaped roughly like an un-sharpened pencil.

This is an image of two of the "Causeway basalt" layers exposed in a gorge east of the Giant's Causeway itself. Note their difference in size: slower cooling produces larger columns. Faster cooling produces smaller columns. Therefore the lower flow cooled off more rapidly than the upper flow.



Lava, when hot, takes up more volume than cold igneous rock. As it cools, the solidifying lava contracts. Because the whole volume of rock is contracting, evenly-spaced centers of contraction develop. Cracks open up to accomodate that contraction. This makes a honeycomb-style pattern, because 3 crack orientations is the minimum number necessary to allow contraction in every direction. These three orientations meet at an average angle of 120º.

The same phenomenon can be seen at Devils Tower, Wyoming.

The weird columnar jointing patterns at the Giants Causeway were used on the cover of Led Zeppelin's album Houses of the Holy (1973). While I was there, I thought about re-creating the album cover with geologists (clothed!) in the same positions as these kids, but I forgot to bring along the album as a reference. Tragic, isn't it?

The overall loss of volume of the (hot versus cold) rock can be estimated with a photograph like this. Divvie the photo into equal units of area, and then count up how many are solid rock and how many are empty air. About 1% shrinkage is seen here -- more than in other places I've seen columnar jointing.

Once formed, these joints allow water to penetrate into the lava flow. Water encourages both physical and chemical weathering of the basalt, enlarging the size of the fractures. Water, being the universal solvent, helps catalyze many chemical reactions. Basalt is a rock that is stable under certain conditions in the Earth's interior, but it is not stable at the Earth's surface, where conditions of temperature, pressure, and humidity encourage it to break down. These break-down chemical reactions start on the surface of the column and work their way inward, like a thousand mice nibbling on the exterior of a large block of cheese. Physical weathering takes place when the water freezes. When water becomes ice, it expands in volume by about 9%. This "wedges" open the cracks even more. Once widened, they can accomodate more liquid water, which can then freeze again, widening the cracks further.
The end result of these physical and chemical weathering processes is to break down the rock, from the outside in. Rotten rock sloughs off in sheets, exposing fresh rock from the interior for weathering to attack. This produces an overall "onion skin" effect. An original polygonal chunk of rock become spheroidal over time, as weathering reduces it in size and volume. Pound coin for scale.

Labels: antrim coast, basalt, geology, giants causeway, northern ireland, structure, weathering