Another metamorphosed graded bed

Over the summer, when my blogging access was limited to my iPhone, I uploaded a photo (taken with the iPhone) of a metamorphosed graded bed on the summit of Mount Washington, New Hampshire.

Here’s another one that I saw, further down on the mountain, on the Auto Road (famous for its iconic bumper sticker):

Lens cap for scale. …And here’s the obligatory annotated copy:

Both of these images are enlargeable by clicking through (twice).

I think today’s photos are of better quality than the iPhone photo. This is the coolest freakin’ thing ever. What you have here are alternating beds of quartzite and andalusite schist. The boundaries between the two rock units are alternately crisp and gradational. Interpretation? Once upon a time, you had a turbidite sequence where the bigger, heavier grains (quartz sand) settled out first, followed by progressively finer and finer mud. The base of the graded bed is a crisp transition from mud to sand, but then as you go up through the graded bed, it grades from sand into mud.

Later, these distinctive primary structures were metamorphosed during the late Devonian-aged east coast mountain building episode called the Acadian Orogeny. The high temperatures and pressures cooked the rock. The sandy part, dominated by quartz, didn’t really change mineralogy much under the metamorphic conditions. The muddy part, on the other hand, was chock full of clay minerals which are not in equilibrium under elevated temperatures and pressures, so they reacted chemically. Their elements reorganized into new minerals: big honkin’ crystals of the mineral andalusite. They might just as well have reorganized into sillimanite or kyanite if conditions were slightly different, but temperature dominated over pressure, so andalusite was the mineral form that was stable (at equilibrium) under those conditions.

As a result, the “mud” was now coarser grained than the “sand.” The overall sense of grading had been flipped by the metamorphosis, yet the overall crisp/gradual pattern was preserved. This, my friends, is exquisite.

How to prep your pocket fold

“Pocket folds,” as my Rockies co-instructor Pete Berquist has defined them, are rock samples exhibiting folds that are small enough to stick in your pocket (and take back to your lab). Here’s a pocket fold that I found last week in the White Mountains of New Hampshire:

I brought it home, and today I unpacked it from the car, along with about 70 pounds of other samples. I turned on the hot water tap and took out a critical piece of sample prep equipment, the wire brush (suitable for cleaning a grill), and scrubbed off all the algae. You should only use wire brushes on relatively hard samples. Because the steel wires have a hardness of 5.5, they will scratch rocks like limestone, which is made of 3.0-hardness calcite. I also gave it a quick bath in 4.5 molar HCl, to kill off any remaining algae. A rinse, and then I set it in the sun to dry.

My White Mountain pocket fold is a quartzite, with some nice graded beds, which probably show up better when they are filled in with algae. Regardless, here’s the cleaned up sample, a gray ghost of a pocket fold:

If you collect field samples for teaching purposes or just because you find them beautiful, what steps do you take to prepare them?

Metamorphosed graded bed

This is the coolest thing I’ve seen this week: a graded bed metamorphosed via Acadian mountain building:

The graded bed starts at the Swiss army knife at left, where you see an abrupt transition between coarse grained metamorphic porphyroblasts (“pseudo-andalusites”) and finer grained quartzite. This was once a mud to sand transition when these were loose sediments in the Kronos Sea, but with elevated temperature and pressure, the clay minerals in the mud reacted to grow elegant porphyroblasts of andalusite and sillimanite. The sand was made of quartz: less reactive stuff, and all it did was fuse together when metamorphosed. I love this: the coarse to fine relationship in the original graded bed is flipped on it’s head by metamorphism. Follow the bed “up” (to the right), and you will see the quartzite grade into andalusite-dominated former mudstone. Pretty sweet, eh?

This example is from the summit area of Mount Washington, New Hampshire. More fun stuff when I get back to my home computer next Monday.

The routine

It’s that time of year for me… summer’s here, and I’m winding up my duties at NOVA in preparation for some travels. We leave Sunday night for two weeks in Turkey, followed by my regional field geology course in Montana (also two weeks), followed by some family time and mountain climbing in New Hampshire (three weeks), including hiking the Presidential Range.

This summer, as I have done for the past several summers, I’ll be subletting my home while I’m gone. Among the many disadvantages of living in DC (high taxes, high crime, lots of noise, all those politicians), this is a big advantage: you can sublet your apartment soooooo easily. It’s a cinch! The city’s government and nonprofit sectors draw in swarms of interns every summer, and I’ve been very fortunate to find great subletters via Craigslist to come pay my rent/mortgage and take care of my cat while I’m away.

The first summer I did this was the most extreme: I was gone for three months in 2006 on a road trip up to Alaska and back; but since then I’ve subletted for at least two months each summer, mainly while I was out west. The 2010 summer is the shortest sublet I’ve so far had: a mere seven weeks. Still, the routine each summer is roughly the same: stock up on cat food and litter, pack up my clothes and store them somewhere (mom’s attic; my office at NOVA), clean the place up, and then clear the heck out. The packing has been more complicated this year since I’m essentially packing for three trips with overlapping gear needs all at once. But it’s a nice annual tradition: right about the time that DC gets to be sweltering hot and humid, I can decamp for exotic locales and cooler climes. I feel very lucky not only to travel like this, but also to have my home and cat cared for in my absence, and bring in cash to pay the mortgage, too. It’s a sweet deal.

More immediately, I’ll be in an all-day workshop starting tomorrow night, and through Sunday. It’s a SERC workshop on the role two-year colleges like NOVA play in geoscience education. Between that and packing, this might be the last you hear from me for a while.

Blogging will likely be light around here for the next two months as I’m flitting about. I’ll do my best to log on and post some travelogues when I can, but I can’t promise too much. When I have phone service (will my iPhone work in Turkey?), I can offer a series of short posts to my Twitter account. Beyond that, you’re on your own!