Chemical Weathering of Building Materials EarthCache

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egroeg: I figured EarthCache Day would be the right time to check on the progress of construction here. They are still working, but I could get close enough to see that they have removed all those wonderful stalactites. Oh well, now I have to look around town to see if I can find another good example of this phenomenon.
Thanks to everyone who visited this EC. I hope you enjoyed it as much as I did.

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Hidden : 1/22/2013
Difficulty:
1.5 out of 5
Terrain:
1.5 out of 5

Size: Size: other (other)

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Geocache Description:

This is an EarthCache, so there is no cache container hidden at these coordinates. An EarthCache is meant to provide an earth science lesson by having you make observations and reporting these to the cache owner. For more information about EarthCaches, visit EarthCache.org

Chemical weathering of rocks occurs when water (and substances dissolved in the water) interacts with the rocks to change their chemical composition. It is usually a gradual process, and affects different rocks to different degrees. Rocks that have been mined or quarried, then used in construction, are not immune to this chemical weathering, and this Earthcache examines some of the effects seen in weathering of man-made structures.

BACKGROUND Limestone is a common rock all over the world, and it has been put to use in construction since ancient times. (Both the Sphinx and the Pyramids are mostly limestone.) Products derived from grinding and treating limestone are major components of cement and concrete. Millions of tons of limestone are used every year for construction.
What defines a rock as limestone is that its major component is the chemical calcium carbonate (CaCO3).  This calcium carbonate is soluble in water (H2O) that is even weakly acidic.  Rain or groundwater that has some carbon dioxide (CO2) dissolved in it is weakly acidic, and when the water contacts limestone, it will slowly dissolve the limestone.

CaCO3 + H2O + CO2 → Ca(HCO3)2 (in solution)


If this acidic water is seeping through cracks in a natural bed of limestone, then the limestone on the surface of the cracks is slowly dissolved. After centuries of such seepage, the crack is greatly expanded, and a cave can be formed. But if this water is attacking limestone used in a structure, the stone is very slowly erodoed away. Some signs of this might be pitting on a block's surface, cracks or crumbling in concrete, or the smoothing over of carved details, like the lettering on tombstones or the face of the Sphinx.

What happens to all this dissolved limestone? Much of it remains in solution in the water, where it eventually reaches the sea, maybe to become part of a clam's shell. If it ends up used as drinking water, it will be seen as the "hardness" of the water. Hard water can demonstrate one other final resting place for the dissolved limestone: when the water evaporates, it leaves behind the scale we see coating shower walls or around pipes that have a slow leak. Remember the chemical equation shown above? This is a reversible reaction, and the limestone may come out of solution if some water is removed by evaporation, or if some of the CO2 flows out of solution. The calcium carbonate forms microscopic crystals which can build up into the visible deposits.

This scale can be considered a form of mineral called "dripstone". Dripstone is a catch-all name for minerals deposited from solution, and includes scale deposits as well as the formations found in caves, such as stalactites, stalagmites, and flowstone, which is just a larger deposit of scale.
Have you ever seen a whitish deposit coming from the joints in a block wall? This is dripstone! The lime used in the mortar is being chemically weathered by rainwater seeping through the joints, and the CaCO3 is being deposited when the solution meets the air.

Another form of dripstone that most people are familiar is found in caves. If the cave becomes large enough, some of the water seepage may start dripping directly from the ceiling instead of flowing down the walls.  If the dripping is slow enough, the chemical composition of the drop will change - maybe some of the water evaporates, maybe some of the carbon dioxide leaves the solution.  Either of these events will cause some of the calcium carbonate to fall out of solution to form a microscopic calcite crystal.  After many such drips, a cylinder of calcite forms - this is the origin of the stalactites seen on the ceilings of many caves.  The rest of the drop falls to the floor, where more calcite may form, resulting in the growth of a stalagmite on the floor of the cave. Given enough time, the stalactite and stalagmite may grow toward each other and meet, forming a solid column. The average growth rate of stalactites is about 0.005 inches per year, but can be as high as 0.1 inches per year where water flow is higher and the water is more acidic.


EARTHCACHE You will visit several sites where the results of chemical weathering can be seen. The only thing you might need to bring is a small ruler.

The first location is the railroad bridge over Merchant Street. A cast iron box truss bridge was built here around 1851 to carry the Pittsburgh, Fort Wayne & Chicago Railroad, but the bridge was replaced around 1860 with the riveted wrought iron bridge seen today. As you follow the sidewalk, look up at the underside of the bridge. Stalactites!! These examples of dripstone form mostly around the drainage holes in the iron trusses. The limestone gravel used as ballast to support the tracks is the source of the dissolved calcium carbonate.
Requirement 1: 1A. Locate what you think is the longest stalactite. Estimate its length, then calculate the growth rate, in inches per year, assuming it started in 1860. 1B. Why do you think this is so different from the growth rate seen in typical caves (which is 0.005 in/year)? 1C. If the iron bridge pier at the south end is Pier #1, tell me where your stalactite is found. (For example: "half-way between Piers 3 and 4")
(A small stalgmite can be seen on the sidewalk near pier 1. Foot traffic probably keeps more of these from being formed.)

Make your way to the second waypoint, the bridge over Federal Street. There are far fewer stalactites here, but some long ones can be seen on the west side, near the center.
Requirement 2: 2A. Estimate the length of the longest stalactite, and calculate its growth rate. 2B. Why do you think there are so few stalactites at this location?

2 Sep 15: Due to construction at this location I am temporarily suspending Requirement 2.


7 Jan 17: I haven't noticed any new stalactites yet. If you pass this location walking from Site 1 to Site 3, look around for any stalactites. Bonus points to the first cacher who spots the chemical weathering process beginning again!!



The final location is the overpass at Sandusky Street (the continuation of East Commons). Again, there are fewer stalactites, but you will not be investigating them. About 75 feet west are some large flowstone deposits emanating from cracks in the concrete facing. Look at the ones above the gray painted area.
Requirement 3: 3A. Estimate the depth (thickness) of the thickest portion of the flowstone here. 3B. Do you think this flowstone has been growing for as long a time as the stalactites?

I can't give you much advice on parking, especially if you go on a workday. Parking is always at a premium around here. You might find on-street parking along Merchant Street, north of the bridge, depending upon when you go.
Please email me the answers to all questions when you log the cache. Feel free to post phots that don't include spoilers. As always, cache safely and have fun!

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