Bridge mystery revealed!
Yesterday I noted that there's an interesting pattern to be seen as one crosses DC's Duke Ellington Bridge:
After sharing these photos yesterday, I posed question for you: What's up with the coloration of these exposures? Why are they black on top and white on bottom? It's the same rock (Indiana limestone), so why the difference in color?
The answer has two parts. First, the calcite (calcium carbonate) which comprises the limestone is sitting out there in the air, and is subject to rain and what-not. Some of that rain has sulfuric acid in it, and that dilute sulfuric acid reacts with the calcite, producing a thin layer of gypsum (calcium sulfate). Those itty-bitty crystals of gypsum have bladed habits, and those bladed crystals are really good at trapping soot and dust. So while the calcite underneath isn't as effective as a soot-trap, the thin layer of chemically-altered gypsum on the surface of the blocks rapidly accumulates dark-colored particulate matter.
So that explains the dark color, but what about the lighter-colored lower portions? Is it simply that they aren't exposed to as much acid rain? Perhaps because they're further down on the "outcrop"? Nope... though that's clearly a consideration (note the thin white vertical lines below some of the stars), it wouldn't explain the abrupt transition from dark colored above to light-colored below. So: what gives?
It's here that context plays an important role. This is an urban location, an outcrop in the city. Like many flat surfaces in the city, it's subject to being tagged with graffiti. Periodically, the City sends along a crew to power-wash the bridge's graffitied surfaces. When they do this, they strip away not only the spray-paint, but also the gypsum and its trapped soot! Because graffiti artists can only reach so high, the city only power-washes so high, and the upper portion of the bridge "outcrop" is both unmolested by graffiti and uncleaned by the City. It records a continual accumulation of gypsum and soot, but the lower portion has its proverbial slate cyclically wiped clean!
I'm on a field trip this weekend (I wrote this post on Thursday and set it to publish while I was away), so I don't know who won the prize (a "GEOLOGY ROCKS" bumper sticker!) but as soon as I get back, I'll settle up with the clever winner. In advance, I'll congratulate you: Nice job!
After sharing these photos yesterday, I posed question for you: What's up with the coloration of these exposures? Why are they black on top and white on bottom? It's the same rock (Indiana limestone), so why the difference in color?
The answer has two parts. First, the calcite (calcium carbonate) which comprises the limestone is sitting out there in the air, and is subject to rain and what-not. Some of that rain has sulfuric acid in it, and that dilute sulfuric acid reacts with the calcite, producing a thin layer of gypsum (calcium sulfate). Those itty-bitty crystals of gypsum have bladed habits, and those bladed crystals are really good at trapping soot and dust. So while the calcite underneath isn't as effective as a soot-trap, the thin layer of chemically-altered gypsum on the surface of the blocks rapidly accumulates dark-colored particulate matter.
So that explains the dark color, but what about the lighter-colored lower portions? Is it simply that they aren't exposed to as much acid rain? Perhaps because they're further down on the "outcrop"? Nope... though that's clearly a consideration (note the thin white vertical lines below some of the stars), it wouldn't explain the abrupt transition from dark colored above to light-colored below. So: what gives?
It's here that context plays an important role. This is an urban location, an outcrop in the city. Like many flat surfaces in the city, it's subject to being tagged with graffiti. Periodically, the City sends along a crew to power-wash the bridge's graffitied surfaces. When they do this, they strip away not only the spray-paint, but also the gypsum and its trapped soot! Because graffiti artists can only reach so high, the city only power-washes so high, and the upper portion of the bridge "outcrop" is both unmolested by graffiti and uncleaned by the City. It records a continual accumulation of gypsum and soot, but the lower portion has its proverbial slate cyclically wiped clean!
I'm on a field trip this weekend (I wrote this post on Thursday and set it to publish while I was away), so I don't know who won the prize (a "GEOLOGY ROCKS" bumper sticker!) but as soon as I get back, I'll settle up with the clever winner. In advance, I'll congratulate you: Nice job!
10 Comments:
This still seems incomplete unless I'm missing something... In the second picture, ( http://www.flickr.com/photos/31233452@N03/2926880944/ ) what is the deal with the edges of the pillar. The color isn't the same as the power-washed areas or the sooty portion and the band is narrow at the top and wider at the bottom.
No graffiti there... now. Looks like that area may have been pressure-washed since the bridge was built, but not as recently as the graffitied portions of the rock.
Does that clear it up?
C
Sorry... not trying to be a pain but in that case, shouldn't the portion of the vertical band that overlaps with the powerwashed horizontal band be the same color as the (presumably later washed) horizontal band rather than the color of the vertical band?
Sorry...
Different interpretations of "edge" are playing into this, I think. I think you meant "edge of the upper part" whereas I thought that "edge" meant "edge of the lower, bulkier/wider portion." You're correct to note that the edges of the upper portion are also white, which means they were also pressure-washed.
I don't know if you've ever pressure-washed anything, but basically the way it works is you get this "wand" which has a "blade" of high-pressure water that comes out of it, and if you twist it this way and that, it scours off whatever is in its path.
It's nothing for a hourly-wage DC city worker to tilt his pressure-spraker up along the edge of the facade; it takes three seconds. I think that's what's happened, based on the pattern here and my own observations of the graffiti-removal crews at work.
CB
Thanks.. Maybe I'm just seeing a color difference that isn't there.
To sum up:
YES, there is a difference on the other part (ignore the first comment I posted here saying "no graffiti there," as I was referring to the LOWER edge). That difference in color that you correctly note is due to pressure washing. It's just probably NOT due to actual graffiti on that edge location, and more likely due to the utter fun of power washing, where you can run that stream of high-pressure water along the edges of your 'canvas'...
CB
Oh dear, I fear what I have begun. I'm sorry.
What I was noting is that the vertical edge has a color different from the powerwashed horizontal band. It has a yellowy tone to it which seems like it may have a different cause or timing. Do you see what I mean?
-R
I see it, sure. Not sure about its significance, though. Anyone else able to illuminate the yellowy color?
-CB
I think you can see a similar yellowish or brownish band or area in the second photo, low down above the top of the lowest pressure-washed area. That color seems to be associated with the edges of pressure-washed areas, which might occur along the edges (including the vertical edges of the blocks) because it is the boundary area between the white and the black areas. In other words, it may simply be the result of incomplete or less vigorous washing.
Possibly, however, this will require a field trip!
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