Geology Tour of Washington, D.C.

Image 1: A marble sample with a black stylolite in the right-hand corner, caused by metamorphic stress to the rock. This sample was found in a bathroom in a café in D.C. Finger for scale.

Sarah here-

Recently, I went to the Washington D.C. area to visit the Smithsonian Museum of Natural History (which you can read about here) and to attend a workshop on best practices for new faculty members. But while I was there, I spied some excellent geology right in the city! I already showed you some of those while I was in the museum itself, so I’ll show you some of the other amazing pieces of Earth history that I saw!

I want to remind you that looking at amazing geology doesn’t have to wait for you to be on vacation or in a faraway destination-you can see these sites anywhere, if you’re paying attention! If you want to read more of these types of posts, check out my post from last year on the geology of bathrooms.

Image 2: This is a staircase in the Union Station in D.C.! This is another type of marble, but clearly a very different type of marble than the one we saw earlier.

This first image is of a beautiful stylolite in a marble countertop in the bathroom of a café in the center of Washington D.C. A stylolite is caused when rock, most commonly carbonate rocks like limestone (which we call marble when they are metamorphosed), are put under extreme pressure and the individual grains will compress and leak fluid, leaving behind a squiggly line, like what you see in image 1. Just beautiful!

Image 3: Here’s a granite sample I found on the wall of a building outside. The large crystals indicate that the sample cooled slowly. What I found interesting about this sample is the large presence of the darker colored mineral (amphibole) in the sample! Finger for scale.

Our next stop brings us to Union Station in Washington D.C., where I found this magnificent staircase completely by accident (image 2). I was visiting Gallaudet University and the first signing Starbucks, when I got turned around and ended up at a different Metro station than I had originally intended. Well, serendipitously, I found this absolute beauty, making the detour more than worth it. This rock, just like the image before, is a type of marble, though it has very different colors. The red color in this marble can be attributed to chemical impurities- red is typically what we’d see if iron and feldspar was present in the marble sample. You can also see veins filled with calcite and look like quartz all throughout the staircase! I was intrigued about where this marble came from, so I did a little research. There wasn’t a lot of information, but it seems that this marble likely came from Vermont (See this blog here: https://blogs.agu.org/magmacumlaude/2014/06/13/building-dc-union-station-just-the-floors/), which was created over 400 million years ago, when limestone produced from a shallow sea collided with a volcanic arc and metamorphosed in an orogeny, or a tectonic collision. This is a fairly common scenario with how we get a lot of our marble from the Paleozoic in North America.

Image 4: Here we can see phyllite, a low-grade metamorphic rock as a decorative feature of a wall. Phyllite is easily recognizable by its slight banding (which looks more like waviness when you’re looking at this particular rock) and the glittery sheen to it, given by the muscovite mica which develops during the metamorphic process.

Our tour continues to just outside of Washington D.C., to Arlington, VA, where I was visiting a friend in the area. As we were walking to breakfast, I was treated to a spectacular number of rocks featured in the buildings’ walls along the way. First, is a beautiful granite (image 3). The pink mineral is potassium feldspar (K-spar, for short), intermixed with the milky white mineral (quartz) and a lot of amphibole, the black colored mineral that’s heavily present on the left side of the block. Granite also usually contains biotite, a black mica.  If you take a look at this granite, you’ll see that the individual crystals are quite large, which tells us a lot about its formation. It’s telling us that it was formed intrusively; meaning, it was formed in an area not exposed to Earth’s surface and it cooled slowly, giving the crystals time to grow. I stopped to take a photo of this because the amphibole (there are many varieties of amphibole-hornblende is the most common in granite) because the heavy presence of the swirling amphibole isn’t something I usually see in most granite samples. Second, I saw these gorgeous phyllite samples on the outer wall of a building (image 4). Phyllite is a low-grade metamorphic rock, which means it’s not exposed to extremely high amounts of heat and pressure, but it has undergone significant changes from its protolith (otherwise known as its parent rock). In the case of phyllite, its protolith was a shale (compacted mud). You can recognize phyllite by a few different characteristics. During the metamorphic process, muscovite (a soft mineral in the mica family) develops, giving phyllite a really lovely shiny appearance (you can think of mica as being like nature’s glitter; just like glitter, mica is nearly impossible to completely get rid of if you accidentally get it everywhere!). You can also recognize phyllite by the gentle bands that form. Many metamorphic rocks are foliated, which we can think of as banding across a rock. The more pronounced the banding usually indicates a higher amount of metamorphism applied to the rock.  Phyllite has subtle banding, which indicates that lower amount of metamorphism.

So, this next image (image 5) isn’t in D.C., but it was found during this trip in College Park, Maryland on the University of Maryland’s campus. It’s another gorgeous example of granite, this time in a fountain. Sometimes it can be really hard to recognize rocks when you’re used to seeing them beautifully polished and sealed (like the granite in image 3, but you can definitely do it with practice!) Just like in image 3, if you look closely at this fountain, you’ll see large crystals, because it’s an intrusive rock, and the same types of minerals- our pink K-spar, milky quartz, and black amphiboles. An intrusive magmatic event from millions of years ago had to form and cool, and then that granite had to be exhumed (brought to the surface) for someone to make that fountain. So cool!

Image 5: A fountain on the University of Maryland’s campus made of unpolished granite. You can tell its granite by the types of minerals in the rock (quartz, K-spar, and amphibole) and the larger crystal grains that make up the fountain!

Last, but certainly not least, let’s look at the marble here in the Ronald Reagan airport (image 6). This gorgeous marble makes up part of a seafood restaurant right near the entrance to the airport, before you go through the security line. Sorry that the image is kind of far away, but this was the closest I was able to get before having to get through the security line! One of my favorite things about marble is how different it can look from sample to sample. This marble shows completely different features than the ones I showed in images 1 and 2-remember, the color of marble is driven by chemical impurities. You can see large scale veins of what is likely calcite all over the rock itself as well as some dissolution features on the left side.

Image 6: Marble being used as the wall around the elevator shaft in the Ronald Reagan Airport. This marble shows large veins and dissolution features that we didn’t see as much in images 1 and 2!
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