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!

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* 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

I led my "History Before History: The Geologic Saga of Washington, DC" tour twice this weekend as part of the twice-annual "Walkingtown, DC" weekend. The folks attending the tour both days were really cool, and were full of good questions. We covered the sedimentary origins of DC's rocks at the bottom of the Iapetus Ocean, their metamorphosis during Taconian mountain building, the intrusion of plutonic rocks, the erosion of those ancient mountains, the deposition of river gravels during the Cretaceous (together producing an unconformity), and the faulting of that unconformity sometime post-Cretaceous (probably Miocene). I'm kind of tired after all that geologic history, especially repeated twice in two days!



The photo above is of a boulder in Rock Creek Park showing all three members of the Georgetown Intrusive Suite, a series of igneous plutons that were intruded into the crust during late-Ordovician mountain-building. I like this boulder because it illustrates well two of the principles of relative dating: the gabbro must be older than the diorite, because there are xenoliths of the gabbro in the diorite (inclusions). You can't break off a piece of gabbro unless it already exists. The granite dike must be younger than the diorite, because it cuts across the diorite (cross-cutting relationships). You can't crack open diorite unless it already exists.

Just thought I'd share an informative little outcrop like this. Please ignore the white graffiti that mars the central part of the exposure. A pen at the top is circled to give a sense of scale.

I hope everyone had a relaxing weekend!

Labels: dc, granite, igneous, plutons, tourism, xenoliths

Cleaning up my hard drive today, before switching over to the laptop for my summer travels. Thought I would share a few annotated photos from my "Geology of Glacier National Park and surrounding areas" class that I took last summer.

Here's Chief Mountain:


On the trail to Firebrand Pass, here's the contact between the Altyn Formation (lowest of the Belt Supergroup exposed at Glacier) and the overlying Appekunny Formation:


The Purcell Sill is a readily recognizable feature high on the glacially-carved walls of Glacier National Park. This shot is from the trail on the way up to Grinnell Glacier:


Here's a shot from Sun River Canyon, showing one of the many imbricate thrust faults there, with some glacial till thrown in as a bonus feature:


Just outside of Sun River Canyon, we saw some nice recumbent drag folds on some thrust faults in the Cretaceous rocks:


This one was from early in the trip, on the road from Helena up north towards Glacier. Specifically, we stopped in Little Prickly Pear Canyon, near Wolf Creek, and saw these chevron folds in the Cretaceous rocks there:


Along those same lines (folded Cretaceous strata), here's a gorgeous fold just outside the park's boundary, on the road leading north from Two Medicine towards Many Glacier:


No annotations on this one, but I wanted to share it anyhow: a blind thrust / drag fold complex, in the Grinnell Formation (exposed on the trail up to Grinnell Glacier):


Lastly, some snow photos. I took this shot on my way up the trail to Grinnell Glacier, because the holes in the snow reminded me of the scary mask face from the Scream movies. But then on the way down, I realized I had the opportunity to document how much snowmelt occurs in six hours of Glacier NP summer weather. Hence, the bottom "after" shot:


That's it for today... Enjoy!

Labels: field trips, montana, msse, plutons, proterozoic, sediment, structure

On Saturday, I was due to take my Audubon class to the Billy Goat Trail. But, since we had so much rain earlier in the week, the Potomac was running higher than normal, and parts of the trail were flooded, so the Park Service closed it. I was ticked off that I couldn't take my students even on the non-flooded portions of the trail to show them pre-Taconian relict graded beds or Acadian lamprophyre dikes. But the NPS are sticklers for the rules, and there was one hyperenthusiastic volunteer standing guard to make sure we didn't venture onto the trail. So we didn't.

We walked another trail, the Berma Road, instead, which parallels the Washington Aqueduct from Great Falls down to the Old Anglers Inn. Along the way we saw plenty of metagraywacke, migmatite, and granite intrusions. Here's me pretending to 'hold up' up a massive metagraywacke xenolith in the Bear Island Granite:

We also saw a lot of structure, including boudinage, folds, and faults. While we didn't get to see some of the more striking features of the Billy Goat Trail proper, we made it work okay. And everybody in the class had a great sense of humor in regards to being kept off the BGT itself.

I thought this was a funny form of political protest:

Both photos by Audubon student Paula. Thanks, Paula!

Labels: granite, national parks, plutons, politics