Overview:
Have a look around at the two massive shapes in front of you.
As they slowly move down mountain valleys under the pressure of prehistoric glaciers like giant ball bearings, these huge rocks have been rounded into more or less spherical shapes.
In ROCKshadow, artist Peter Shelton used an 85,000 pound white granite glacial till boulder measuring roughly fourteen feet tall and nine feet around as a pattern to build its shadow in black bronze.
Notes from the artist:
It is common in my work to contrast and compare seemingly opposite values like inside and outside, big and small, heavy and light, transparent and opaque, space and object.
In this work I am fascinated with the idea of using the boulder as a dressmaker would use a mannequin to create a coat and stand the original boulder and its ersatz bronze cloak together in a Seattle plaza. I am curious about how having them together would reveal their relative sense of weight and solidity or lack thereof. Would the white boulder, ROCK, feel lighter than its black bronze shadow even though we know that it is many times heavier? Would the boulder feel obdurately solid or spongy like a marshmallow? Would the black “skin” of the bronze clinging to the interior ribbed support structure make the shadow feel hollow like a tent? Would this skin feel taught and tensile like a drum head next to impossibly soft white granite? Would it be clear that the ROCK was used as a pattern to fashion its shadow. I am interested in how the original form would feel standing next to what is essentially a mold of it that now becomes a once-removed and materially distinct replica. Would we ask which came first?
I stood a glacial boulder on end and not unlike a contour map, drew with chalk on it surface lines that revealed and even exaggerated the shapes of the ROCK. Over these lines we laid preheated ½” round bars of wax and joined them with crossing wax lines. Over this cage-like grid we heated sheets of wax and rubbed them onto the cage such that the grid beneath was revealed as it resisted the softened sheet wax. Then the wax was removed in many pieces and was run through a lost-wax casting process. Reassembled with a black patina, these bronze parts became what I call the shadow.
Finally, both the ROCK and its shadow were installed in the plaza to become ROCKshadow.
Description:
Granites are some of the most distinguishable rocks out there, and even amateur earthcachers usually have no problem in identifying them.
Granites are igneous rocks. This means that they're not formed as a result of a volcanic eruption, but rather through the slow, gradual cooling of magma. They tend to have pretty big, coarse crystals (visible with the naked eye). As a rule of thumb, the bigger the crystals, the slower the cooling. For instance, in other eruptive rocks which cooled extremely fast, you can't see crystals with the naked eye, or there aren't any crystals at all, because they've had no time to form. But with granites, named for their granular structure, you don't have this problem, you just see the crystals with relative ease, no equipment needed.
The two main minerals in a granite are quartz and feldspar. In an extreme case, you could have a granite comprising of just those two minerals, though that almost never happens.
So, the first two minerals (quartz and feldspar) are the white ones, while the other ones (generally amphiboles, pyroxenes, biotite) are black or very dark.
Minerals:
Quartz is a very common mineral in the Earth's crust and is almost always well represented in granites. According to the generally accepted definition, a granite must have at least 20% quartz.
Quartz is typically light-grey and kind of looks like an opaque window. If you look at the crystals closely, they should stand out from the surrounding rocks. Quartz is pretty sturdy and you can't scratch it with your fingernail (unlike other minerals).
Feldspar is often the most abundant rock in a granite -- this is why the rock looks white with dark spots, and not dark with white spots. Feldspar pops up pretty much everywhere in geology, in igneous to sedimentary rocks. It makes up some 40% of the Earth's crust, and knowing how to recognize it is key for any earthcacher -- amateur or expert. Feldspars are hard minerals, almost always white (sometimes with a pink hue), typically sporting a glassy luster. A big difference between feldspar and quartz, for example, is how it breaks. While quartz breaks irregularly, often in curvy shapes, feldspar breaks readily along flat faces, a property called cleavage. Feldspar tends to look quite blocky as a result, as opposed to other minerals whose individual crystals might stand out.
We'll group amphiboles and pyroxenes together because of their many similarities. They're both black, and if you could look at them closely, they look like prisms rising up. The main visual difference is that amphiboles are generally taller than pyroxenes, though this distinction is extremely hard to make for a non-specialist.
Biotite is black mica. Amateur earthcachers often have a rough time distinguishing between biotite and other black minerals, but there are a few things which can help you. First of all, biotite is basically a series of planar sheets, unlike amphiboles and pyroxenes which are prisms. So biotite is flat, the others are tall. Because of its sheet-like structure, it often starts to split up at the ends, much like an old book (can be hard to see with a naked eye). Biotite is also easily scratched with a fingernail, that can be a good tell.
Overall, it's not any one of these minerals that make a granite a granite, but rather the way they all fit together.
White granite is a granite that is composed primarily of quartz (milky white) and feldspar (opaque white) minerals and weighs approximately 170lbs per cubic foot.
Bronze is an alloy primarily consisting of copper with 12% tin typically. In the case of black bronze like you see in the Shadow in front of you, both silver and gold are added to the mix. Bronze weighs approximately 541lbs per cubic foot.
To log this Earthcache:
Please answer the following questions and then proceed with your log. Anyone who doesn't submit answers to the questions in a timely manner will have their logs deleted without notice.
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Was this rock formed as a result of a volcanic eruption? Why or why not?
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Examine the rock closely. Using the mineral features visible to you, estimate the percentage of this rock that is made up of quartz. Does it qualify as a granite?
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Which is heavier? Granite or bronze?
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Which is heavier? ROCK or shadow? Why?
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