This was in last month's Scientific American: "Birth of an Ocean: the evolution of Ethiopia's Afar Depression." Great photos.

Labels: africa, oceans, plate tectonics, rift valleys, salt

I'm on the big island of Hawai'i for the Thanksgiving break; and I've really enjoyed trooping around and checking out the volcanic features. (Photos once I get back to DC...) The other night I saw Bela Fleck and the Flecktones perform in Waimea, and they were playing lots of Christmas tunes from their brilliant new album. The next day, hiking on Mauna Kea, the residual music mixed in my brain with the cool igneous geology I was seeing. The result? The Twelve Days of Volcanoes... Enjoy!

On the first day of Christmas my island sent to me:
a bunch of pahoehoe

On the second day of Christmas my island sent to me:
2 Pele's hairs
and a bunch of pahoehoe

On the third day of Christmas my island sent to me:
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

On the fourth day of Christmas my island sent to me:
4 falling blocks
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

On the fifth day of Christmas my island sent to me:
5 volcanoes
4 falling blocks
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

On the sixth day of Christmas my island sent to me:
6 basalts flowing
5 volcanoes
4 falling blocks
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

On the seventh day of Christmas my island sent to me:
7 tubes of lava
6 basalts flowing
5 volcanoes
4 falling blocks
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

On the eighth day of Christmas my island sent to me:
8 steam explosions
7 tubes of lava
6 basalts flowing
5 volcanoes
4 falling blocks
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

On the ninth day of Christmas my island sent to me:
9 green sand beaches
8 steam explosions
7 tubes of lava
6 basalts flowing
5 volcanoes
4 falling blocks
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

On the tenth day of Christmas my island sent to me:
10 billion vesicles
9 green sand beaches
8 steam explosions
7 tubes of lava
6 basalts flowing
5 volcanoes
4 falling blocks
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

On the eleventh day of Christmas my island sent to me:
11 craters glowing
10 billion vesicles
9 green sand beaches
8 steam explosions
7 tubes of lava
6 basalts flowing
5 volcanoes
4 falling blocks
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

On the twelfth day of Christmas my island sent to me:
12 voggy lungfuls
11 craters glowing
10 billion vesicles
9 green sand beaches
8 steam explosions
7 tubes of lava
6 basalts flowing
5 volcanoes
4 falling blocks
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

Labels: basalt, hawaii, humor, music, travel, volcano

Just got an e-mail from the author of this new blog about building stones... He's also the fellow who authored the recent coprolite article in Geotimes/EARTH magazine... Check it out: stories-in-stone.blogspot.com

Labels: blogs, geology

This was on NPR this morning. Made me laugh out loud.

Labels: humor, news, radio

Yesterday, I took a three Honors students and a colleague to Difficult Run, Virginia. This is a hiking trail that goes from Georgetown Pike, in the tony neighborhood of McLean, Virginia, down through a deep, steep river valley to the Potomac River.

As noted a couple days ago, the trail is right across the Potomac River from my beloved Billy Goat Trail. In a recap from that post, here's a map of the area... Feel free to switch it to "satellite" view.



Some discussion of the bedrock geology of Difficult Run can be found here, in an excellent field trip guide by Scott Southworth (USGS) and colleagues that's part of Excursions in Geology and History (Frank Pazzaglia, editor).

We began our trip by meeting up with Doug Dupin of the Palisades Museum of Prehistory, who joined us for our exploratory geohike. We walked a short distance down the trail and found a big (abandoned) quarry where it was rumored there was a good fault. This is one of these pieces of information that I heard somewhere, at some point. I couldn't find it in any literature, so maybe I heard it in discussion when I taught at George Mason University for a year between grad school and when I got my position at NOVA. Anyhow, I had never actually checked it out...

...So our first order of business was to review the criteria for identifying a fault: What would we look for? Fault breccia, fault gouge, slickensides, hydrous mineral veins, and of course, offset. However, here in the Virginia Piedmont, it's rare to have a good marker unit to compare on opposite sides of the fault: usually it's just schist on one side, schist on the other. In some places, you could add the presence of a fault scarp to that list, but being as how this was an old quarry, geomorphic features like that didn't seem likely. So our search focused on the search for fault breccia, fault gouge, veins of odd minerals, and slickensides.

A few minutes in, we found some slickensides on this boulder of float:

This is a boulder of migmatitic phyllonite, with a wavy texture due to mylonitic flow at depth. (The picture doesn't show this very well at all, though you can see faint undulations 'cascading' from the top of the photo towards the bottom. It's much clearer in cross-section.) Anyhow, the 'slicks' are a faint upper-left to lower-right lineation seen on this surface, one or two degrees off from the orientation of the ballpoint pen. The surface you're looking at here was a fault plane at some point in its history. Ballpoint pen for scale.

We did eventually locate the fault, uphill from this boulder. It was characterized by a zone of fault gouge (pulverized rock), three inches wide to a foot wide in places, and highly oxidized (presumably by oxygen-rich meteoric waters percolating along this fractured surface)... but there were no good marker units to judge the total offset.

Here's a different section through a similar rock (though I wouldn't apply the "phyllonite" textural description to this one). Instead of looking at the plane of foliation here, we're looking at a surface which is perpendicular to the foliation plane(s)....

Here in this image, you can see two cleavages... One which runs roughly upper-left to lower-right through the photo, defined by gneissic banding including bands of granite (light-colored; late Ordovician in age... Taconian Orogeny). A second cleavage runs roughly left-to-right through this photo. This second cleavage overprints the first. The overall interpretation is that the first cleavage developed due to lower-left-to-upper-right compression, forming the foliation defined by alternating bands of different compositions of minerals in an upper-left to lower-right direction. The second cleavage formed due to compressive stress sub-parallel to the pre-existing foliation, deforming it into a series of tight folds. The limbs of these folds line up parallel to one another, defining the second-generation, overprinting cleavage. Can anyone else add to this interpretation? Dime for scale.

Along Difficult Run itself, the outcrops were all relatively recently scoured (in 1972 by Hurricane Agnes), so there are some good exposures. As I noted earlier this week, the area shows some nice exposures of granite pegmatites (keys, and the edge of the Pazzaglia volume, for scale):


On our field trip yesterday, we took at closer look at these beautiful pegmatites, and the associated amphibolite bodies. Take a look at this close-up... Dime for scale.

What's going on here? You've got a beautiful (euhedral/subhedral) example of an orthoclase feldspar ("potassium feldspar") crystal amid a bunch of quartz. But look closer at the feldspar crystal... this sucker has been fractured in many places, and it's shot through with very small veins of quartz. Somehow, as this pegmatite dike was cooling, the earlier-crystallizing feldspar was broken and intruded by the presumably-still-fluid silica-rich magma. Anybody able to expand on this interpretation and shed some light on how this all played out? Or contradict it and give a different story to explain this relationship?

In the neighboring amphibolite, we checked out these cool ridges of resistant rock which are centered on thin fractures. Here, you see a couple of intersecting joint sets, each of which was the "plumbing system" for silica-rich hydrothermal fluids (my interpretation). These silica-rich hydrothermal fluids impregnated the surrounding amphibolite with quartz, which made the immediately-adjacent areas more silica-rich, and hence more resistant to weathering and erosion: Hence, now that they've made it to the surface, they're weathering out in high-relief. Dime for scale.


A bit further downstream, Doug showed us a 'cave' (central dark area, just to the right of the waterfall) between the bedrock and a big slab of sloughed-off migmatitic metagraywacke:

We each edged into the 'cave' to the end, where Doug has shown that a distinctly-rectangularly shaped hole admits a direct beam of sunlight during the fall and spring equinoxes. From the inside, it's a striking arrangement, enough to make you wonder whether it's anthropogenic. However, from the outside I was unconvinced that the hole's position was anything other than natural. Doug's initial intepretation of the site was strongly influenced by the fact that there are some unambiguous petroglyphs a short distance away from here, and based on this proximity, I think it's acceptable to infer that Native Americans may have visited this cave. However, I interpreted the opening to be completely natural, with no need to invoke anthropogenic modification in any way.

We hiked on along a ridge overlooking Mather Gorge, sighting a fox and an accipiter (Coopers? Sharp-shinned?) and a few vultures, and returned to the parking lot as the sun dipped low in the sky. On the way back to campus, Honors students Ana and Hope fed us Swiss cookies and cheese & crackers. Altogether, it was a pretty great way to spend a November afternoon...

Labels: birdies, faults, field trips, granite, igneous, metamorphism, ordovician, piedmont, rivers, virginia

...from my apartment window!

Here's a series of photographs I've taken over the last month and a half to document the change in the leaves on the trees. I'm a big fan of repeat photography to document changes like these.





























































Maybe I'll do the reverse of this project in the spring to watch it greening up again...

Labels: dc, plants

A good post yesterday on Andrew's geology blog at About.com : "How many plates are there?" Some excellent points made, and made well.

All in favor of drawing the Somali Plate as its own entity on plate tectonic maps? Aye!

Labels: blogs, maps, plate tectonics

I've already posted some images from the VCCS Science Peer Conference a week and a half ago. Outside the offices of the Wintergreen Nature Foundation, they've arranged a series of large charismatic rock samples from the region. Some of them are from the Blue Ridge (where Wintergreen is located) and some are from adjacent physiographic provinces. These samples are from the Valley and Ridge province, showing some cool features sometimes found in sedimentary rocks.

First, some articulate brachiopod fossils in quartz sandstone (internal/external molds). This wasn't labelled as to its source formation, but it looks a lot like the Oriskany Sandstone, a major ridge-former in the Valley and Ridge. Quarter for scale.


Second, a breccia in limestone. (FYI, Andrew's Oakland Geology blog has another nice image of breccia today.) Perhaps a collapse breccia? Again, the sample wasn't labelled, so I have no idea which formation it was derived from. The white in-filling is calcite. Quarter for scale.

Labels: conferences, fossils, sediment, valley and ridge

Yesterday, there was an excellent post on Stratigraphy.net about what we are calling the various time-slices of the Cenozoic. As has already been noted, "Tertiary" and "Quaternary" mean "third" and "fourth," definitions which rankle those of us who think that there was a lot more time before them than two (or three) periods. So the International Commission on Stratigraphy (ICS) officially dropped Tertiary and is in the process of sorting out how best to drop Quaternary too.

Anyhow, trace the evolution of the nomenclature, as defined by the ICS.

Labels: blogs, geologic time, stratigraphy

Second in my "ductile flow in everyday objects" series... Ultimately the goal of posting these photos is to develop a repository of teaching images for familiar substances which flow when conditions of temperature and pressure are sufficient.

Here's a plastic cat-food dish (originally square) which deformed in a ductile fashion after going through the heat-dry cycle on a kitchen dishwasher:


Note how the dish has "sagged" around one of the dish rack's supporting bars, like a damp cloth draped over a stick.

Now that it has cooled, it can be removed and show how much it has deviated from its original shape (how much it has strained):

Labels: analogies, structure, teaching

This week, I'm taking some of my Honors students to Difficult Run, Virginia.

It's right across the Potomac River from my beloved Billy Goat Trail. Here's a map of the area:



Some discussion of the bedrock geology of Difficult Run can be found here, in an excellent field trip guide by Scott Southworth (USGS) and colleagues that's part of Excursions in Geology and History (Frank Pazzaglia, editor).

Here's a look at Difficult Run, looking upstream from below one of the several waterfalls there:



These outcrops were all relatively recently scoured (in 1972 by Hurricane Agnes), so there are some good exposures. We're going to look for a fault reported to be there, as well as the incision geomorphology of Difficult Run itself, and some nice exposures of granite pegmatites (keys for scale):





This field trip is less a guided tour, and more of an exploration, so I hope when we get back, I'll have some photos of new and interesting things to share.

Labels: faults, granite, igneous, metamorphism, ordovician, piedmont, rivers, virginia

Rocks flow when conditions are right. At the introductory level, many students exhibit an initial tendency to resist the idea of something they "know" is hard and brittle acting in any other way. Faulting, they get. Shear zones... not so much. I find analogies useful in communicating the behavior of rocks at depth, like mylonites. Often I invoke wax, which can be cold & brittle, hot & ductile, or molten.

But I reckon it's instructive to have other clear indications of ductile flow: everyday objects that have flowed under stress.

Today, I offer the first in what I hope will eventually build into a longer series: everyday examples of ductile flow. We begin with a cassette tape left in a hot car (viewed through the back window, which is why the photo is so lousy):

Even the relative moderate stress of leaning on the seat cushion was sufficient to bend this cassette tape, provided it had attained the right temperature (which it's easy to do in the Virginia summer time in a closed automobile).

Anyone else have examples of everyday examples of ductile flow?

Labels: analogies, structure, teaching

It's that time of the semester, when the field trips are over, and the field trip essays start rolling in. These papers I assign are intended to be syntheses of the field trips I take my students on. I want them to interpret the landscape as a geologist would, and support each claim about geologic events in the past with supporting evidence observed or discussed on the trip.

I offer my students the opportunity to submit a rough draft of their field trip paper, and then I give them feedback about both content and formatting/writing style, so they have a chance to revise before submitting a final draft. Each semester, about a quarter of the students avail themselves of this opportunity for feedback before the "real" paper is due. Giving them quality feedback is a time-consuming process, but I feel it's important both to cement geologic concepts in their minds, and to guide them in developing their writing skills.

Accordingly, it's been a slow week for posting on this blog. I've been too busy with work. However, this morning it occurred to me that I could capitalize on my grading efforts by sharing a student essay with you all, edits and all. Why do I think you'll be interested in such a thing? (A) I think it gives some insight into the practice of teaching geology at the introductory college level, and (B) I think this is an excellent rough draft for an essay about Washington, DC's geologic history. The student's name, of course, has been redacted:

Labels: cretaceous, dc, geology, nova, ordovician, teaching

The Virginia Community College System (VCCS) organizes conferences occasionally where faculty in different disciplines can get together. This weekend was the "peer conference" for the natural and physical sciences. It was held at the lovely mountain resort called Wintergreen, in central Virginia's Blue Ridge Mountains.

Here's a map of the area:

That's the Shenandoah Valley on the left (part of the Valley & Ridge province), the Blue Ridge in the middle (running from NE to SW), and the Piedmont province on the far right. Wintergreen is a bit SW of Charlottesville.

The conference was fruitful and interesting. I enjoyed getting to meet a bunch of the other VCCS geology faculty and discussing what we want to do in the future in terms of supporting one another and professional development. I gave a talk about new technologies in geology instruction, which included information about the geoblogosphere and other sundry web resources I use. My colleague Erik Burtis at NOVA-Woodbridge led us on a cool "field trip" to Glacial Lake Missoula, via Google Earth.

I spent a lot of time talking with Pete Berquist, from Thomas Nelson Community College, discussing next summer's Regional Field Geology of the Northern Rocky Mountains course. We laid out a series of goals for the students, and created a tentative itinerary. Pete and I took a great hike at the end of the first day, poking around in the rocks and watching the sun set over those gorgeous mountains. Friday evening, there was a cool astronomy session, where Ed Murphy from UVA showed us the Ring Nebula, the Andromeda Galaxy, and assorted other stuff in outer space. He had a great laser pointer that extended a green laser line up about 80 feet into the sky... Very useful for pointing things out. Low light levels in the forested mountains meant excellent stargazing. Saturday morning, Bill Warren of Lord Fairfax Community College gave a good talk about the global energy crisis, and potential solutions. I picked up a few good resources there that I'll use next semester in teaching Environmental Geology. And then when the conference concluded, there was a geology "hike" out to look over the landscape. By driving us to a couple of different overlooks, Doug Coleman of the Wintergreen Nature Foundation showed us spots where we were able to look east into the Piedmont, and west into the Valley & Ridge. Pretty cool, though we didn't look too closely at the actual rocks exposed there. Fortunately, I have an inclination to do that on my own... as you'll see below:

Catoctin Formation greenstone (meta-basalt), showing chlorite-rich portions (left) and epidote-rich portions (right). Quarter for scale.


More Catoctin, the volcanic breccia layer. Lots o' epidote. Quarter for scale.


Is this a quartz vein or a granite dike?
At first glance, it appears to be your standard hydrothermal quartz vein full of milky quartz, but then you'll notice that it's not just quartz. There are also two crystals of orthoclase feldspar in there. (The dark shapes are just empty holes & shadow, not mafic minerals.) I pointed this phenomenon out before, but I'll state it again: I think that hydrothermal quartz veins and granite dikes are not separate phenomena, but points along a spectrum of composition. Quarter for scale.

Looking southeast towards the Piedmont:


Looking northwest towards the Valley & Ridge:

Labels: appalachians, blue ridge, conferences, igneous, metamorphism, piedmont, valley and ridge

Labels: ediacaran, fossils, psw

A couple of weeks ago, I mentioned AGI's Image Bank, and to illustrate it I picked three photos by Marli Miller (at the University of Oregon). Dr. Miller has written me and shared a link to a website she has put together to share high-resolution images of geological features and processes. Educators have permission to download the photos for teaching (non-commercial) use. And everyone can benefit from visiting to check out the many gorgeous images there.

Labels: art, teaching, websites

Four years ago, this website gave me some solace in looking at the breakdown between "red" and "blue" states. Now the author, Mark Newman, has performed some similar cartography on the 2008 presidential election results.

Labels: maps, politics, websites

The author Michael Crichton died yesterday. He gave us Jurassic Park; He also wrote a novel based on the idea that global warming was a hoax (State of Fear). A mixed bag, but he'll be remembered for Jurassic Park. I've read everything he's written except for the global warming book. My favorite was non-fiction: Travels. Check it out.

NY Times obituary.

Labels: books, dinosaurs, global warming

8:30am: Just got back from voting. As with 4 years ago, the line stretched around the block: awesome to see so many of my neighbors and fellow citizens participating. It gave me the warm fuzzies.

Today's the day, people! Let's make it happen.

Labels: dc, politics

Labels: art, geologic time, oceans, plate tectonics

...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

Last week was field trip week for me. I led trips to the Billy Goat Trail on Tuesday and Thursday, and to Washington, DC, on Saturday.

On the Physical Geology field trip to the Billy Goat Trail, we saw rocks like amphibolite, metagraywacke, and migmatite:







Hope and Ana checking out the migmatite:


The group poses with the migmatite, to show how close anatexis is to their hearts...


Jane examines lamprophyre in a weathered-out dike:


Noting the characteristics of metagraywacke:




Traversing 'Pothole Alley'... Joel looks chilly...


Our lunch spot... Alex pretends to dive into the Potomac River...


Traversing 'The Traverse':


On the Historical Geology field trip to DC on Saturday, we were amused to find a jack-o-lantern that had facial hair resembling mine...



But that's not all! We also saw some geology. While you can get a more complete picture at my "DC Rocks" webpage, I'll post a few new photos of new outcrops here...

Here's a nice slab of granite (very angular) set in metagraywacke matrix (metamorphosed accretionary wedge complex)...


Here's two members of the Georgetown Intrusive Suite, showing the (earlier) gabbro stoping xenoliths into the (later) granite:


I love field trips. I love seeing my students light up at being outside, at getting a handle on the stuff we talk about all semester in class. I think field trips are super duper important.

Labels: dc, field trips, granite, igneous, maryland, metamorphism, nova, piedmont, teaching, xenoliths

My favorites from a recent raft of geoblogosphere posts:

• Field work on sedimentary rocks in the Dead Sea region of Jordan (from Nologic)
• Anomalocaris fossils (from Ediacaran)
• Charming but obsolete mining terms (from Hypocentre)
• Other cool stuff in a similar conglomeration from BrianR (at Clastic Detritus)

Labels: blogs, field trips, fossils, geology, mining

As a geologist, I love most rocks I encounter. But yesterday morning, I went out to my car and saw this:



Can you tell what's missing? Hint: here it is, all over the front seats:



My car had been broken into over night. Bummer. I live in the city, so this happens sometimes in the city, but I've learned never to leave anything of value in the car, and certainly never to leave anything of value in plain view. The people who broke into my car last night must therefore be classed as vandals, not thieves, since they didn't steal anything. Grrr...

Then, in the course of calling the insurance company, I opened up the driver's side door, and saw this:



See the rock there, between the driver's seat and the driver's door? That rock came through the passenger window, from the other side of the car, with enough force to slam into the other side of the vehicle. And it left its mark on the panel on the interior of the driver's door:



So, the poor little Prius is now in the shop, getting a new door panel and a new window. Major bummer. And now I know that there is at least one rock in this world that I do not like.

Labels: dc, prius