Archive for June, 2010

Yesterday after watching the greatest soccer match I have ever seen, Alan and I made way over to the Smithsonian’s Natural History Museum, specifically the geology exhibit. No bites on the private tour offers extended, but we had fun anyways. I wanted to share some of the pictures we saw there (some pretty extraordinary stuff):

Look at that psuedotachylite! I like to think of this as impact-breccia, as it formed during the impact of a meteorite. The pink rock is the original granite that broke off into melted rock (the black part) which melted from fracture friction.

Pretty sweet looking example of the Leesburg conglomerate. We have come across this formation during our excursions, but never got this great look of the interior all nicely polished. How much is it for a rock saw? It may be time to invest in one*.

*Vegas has the odds on me losing a finger at 4:1 (I take that as a compliment)

Here is Alan next to a cool display of columnar jointing. Gotta love those hexagonal shapes with their eye-catching 120 degree angles. Don’t look too long, buddy. Those are underage (~12 million years old).

I saved my favorite for last:

Holy garnite! This is the point where Alan and I jumped to the floor and pressed our faces against this rock (good thing there is a “Please Touch” sign), and some passer by quipped “You guys must be geologists”. Don’t be so snide, guy, we know you’re excited too. If you remember my recent posts on porphyroblasts, these ones dwarf my examples, about the size of a softball or grapefruit.

I highly suggest everyone go and check out the exhibit for themselves. There are also some great Burgess Shale fossils on the first floor, too.

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This was actually an accident. I meant to take a picture of this quartzite sandstone with a hand lens held up to my iPhone camera, but I took a video instead. I liked the outcome and here it is. Take notice of the blue quartz grains (about 1 mm in size). Alan and I found this beauty out in Fauquier County off of Cannonball Gate Road, possibly eroding away from the basement complex.
The other object in the video is a penny for scale.

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In three of my earlier posts I discussed the Billy Goat Trail at Great Falls National Park in Maryland. On Wednesday (quickly becoming Geology Day on my calendar) the team (Alan and I) trekked back to the BGT to further explore the geologic features. This time around the water level of the Potomac was considerably lower than previous, and exposed more outcrops for us to observe.

The bedrock of Great Falls was metamorphosed in multiple events, and evidence for these events is seen throughout the park. One piece of evidence is the presence of porphyroblasts found near the end of Mather Gorge in the heavily metamorphosed metagraywhacke. Porphyroblasts are large mineral crystals that form within a metamorphosed rock during periods of high temperature and pressure; one example is a garnet, the state mineral of Connecticut. When we first went through the BGT in April we came across a small sampling of porphyroblasts in the same vicinity. But, this time with more exposed rock we were in our own smorgasbord of porphyroblasts, boudinage, and kinematic porphyroclasts. Alan and I found five separate areas with the porphyroblasts, and could have found more if it weren’t for the blazing heat melting me to the rocks. One particularly good example is pictured below (pasty-pale hand for scale).

The question now arises: what are the porphyroblasts made of? Well, at this point they are composed of sericite, but only after they experienced retrograde metamorphism. In other words, after the intense pressure and temperature that created the porphyroblasts began to diminish the composition of them changed with the environment. Well then, what were they before they were sericite? Well, I don’t know, and I am not alone. There is some level of debate for this topic as some parties believe it to be kyanite and others sillimanite. The confusion arises out of both being polymorphs of each other along with andalusite. Polymorphs are minerals of the same chemical composition, but differing crystal structures depending on temperature and pressure levels. All three of our polymorphs discussed here have the chemical composition Al2SiO5, in relatively high pressure kyanite is formed while in high temperature sillimanite is formed. According to the USGS paper, “The River and the Rocks: the Geologic Story of Great Falls and the Potomac River Gorge” (1970) the porphyroblasts are sillimanite and suggests that they may have been kyanite and andalusite originally, but changed due to the heat from granite intrusions. George Fisher’s paper “The Piedmont crystalline rocks at Bear Island” from 1971, and discussed by Callan Bentley in his blog entry: “Crystal Ghosts” (April 2010), suggest evidence that all three polymorphs are found within the park, and that this could imply the conditions were near the aluminosillicate triple point in terms of pressure and temperature. I like this idea, and not just because it presents a more interesting scenario, but because there is solid evidence for both sides of the argument. With the triple point hypothesis both scenarios are explained.

I hope to have a better understanding once I complete the Igneous Metamorphic Petrology class, and will possibly explore the topic again then. For now I would have to consider the metamorphic environment of Great Falls during the Taconian Orogeny. Which would have been greater between temperature and pressure, or were both at the precise levels to generate all three polymorphs?

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On Sunday, the family and I headed towards the Blue Ridge with the intentions of stretching our legs along the Buzzard Rock North trail. According to hikingupward.com this is an easy trail with a few great vistas along the ridge. While they were spot on about the views, the word “easy” is relative and I don’t think it related to us on this excursion. Not that it was difficult, just that it wore out our poor 11 year old pooch who I had to carry at one point.

The trip was simply recreational and I had no plans of making anything geologic about it, though I did bring my rock hammer, hand lens, HCl, and chisel. Even still it is a hard skill to turn off and I noticed a few rocks that caught my attention. It should come as no surprise that Massanutten Mountain is comprised almost solely of the Massanutten sandstone (MSS). The MSS is a quartzite meaning that it is almost all quartz grains, therefore making it very durable. What I find very interesting, and what caught my eye along the hike, is the range in grain sizes of the quartz. I found rocks of well sorted sand sized grains all the way to large pebbles in a conglomeritic rock.

In all cases the larger pebbles of quartz were weathering away at a lesser rate than the smaller sand sized grains, making them pop out and be even more noticeable. In the rock pictured below the larger pebbles were almost ready to be plucked out of the rest of the rock.

Along the hike there were nice examples of cross bedding seen in a lot of the sandstone. Since it may be hard to tell from the picture below I added some nifty red lines to help in the visualization. While this example is not in situ, cross bedding was seen in the MSS to show it was not overturned.

The hike up to the first vista point is pretty straight forward, up and down a few hills, across a stream or two, but after that it gets more interesting. From this point the incline steepens, and your path is more likely to be MSS than the soft soil it was before. Once we reached the top (and took a desired breather) the trek along the ridge was full of great views of the Fort Valley and the other side of the basket. It was also visible that the beds of MSS were dipping towards the valley, some as steeply as 75°. Combined with the cross bedding orienting our paleo-up position this shows how Massanutten Mountain is part of a synclinorium. The entire valley is folded down in a syncline with parasitic folds as part of it, and the MSS is so durable it weathers away at a slower rate than the surrounding shale.  Much like the quartz pebbles in the conglomerate, Massanutten Mountain sticks out in high relief of the valley.

Along the ridge was where we set ourselves down and enjoyed some turkey sandwiches and fresh strawberries, and I can probably think of a million worse places to have a lunch than here:

We were also privy to some wildlife besides the vicious attack dog we brought with us. Up top we saw one of Virginia’s two vultures, the black vulture, which is distinguishable by the white tips to its otherwise black wings. And along the lower half of the trail we saw this little guy who I think is a gypsy moth, though I am admittedly not great at identifying my caterpillars.

The trip down was swifter and a little easier on the legs, though we were all pretty tired at this point. None more so than the dog who eventually just stopped walking and decided she had had enough. So let me ask you, who is the subservient one in this relationship then?

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