In the early 1990’s, the Historic Gold Hill and Mines Foundation, Inc., was building the rock wall which parallels St. Stephens Church Road here. Elderly members of the community knew the approximate location of the old jail, and when the wall was built in this area, the workers discovered the foundation of the old building. The building was re-constructed with rocks that had come out of the various mines in the area, plus a few notable ones from a little further north.
The wall you are facing is a smorgasbord for someone who is interested in rocks and geology. It is obvious, just casually looking at this wall, that there are many types of rocks. Even if there were hundreds of different types of rocks represented in this wall, however, all of them could be fitted into the three basic categories of rocks: Igneous, sedimentary and metamorphic.
Igneous Rock: Look at the large, rather odd-colored, kind of rectangular-shaped, pinkish to whitish rock about half-way up the left edge of the wall. This is an igneous rock, one of the three main rock types. Igneous rocks are born deep within the earth where pressure and temperature are intense; so intense, in fact, that some rocks present will actually melt, and become a sort of semi-fluid mass called magma. Magma, like blobs in a lava lamp, will rise towards the surface, and as it does so, temperature and pressure are reduced. Eventually, as this happens, the magma begins to solidify, meaning that elements in the magma lock together to form crystals. (This is the same thing that happens as liquid water cools to become ice.) As the crystals begin to bond together with other crystals, an igneous rock is created.
If the magma never reaches the surface, the crystals may have tens of thousands or even hundreds of thousands of years to gradually grow as the mass cools. Now, we are ready to look at our first rock. If you look closely, you can see pink blobs (feldspar crystals), white blobs (quartz crystals), and black specks (iron- and magnesium-rich mica and other minerals). This rock is not a “native” of the Gold Hill area, as it came from the (appropriately named) town of Granite Quarry, about 10 miles north of where you are standing. The relatively small amount of iron/magnesium, the abundance of the pink feldspar, and the relatively large size of the crystals (you can see them with the naked eye), identify this rock as granite. Granite can only form well inside the earth, where gradual cooling allows the formation of large crystals; therefore, granite is referred to as an “intrusive” igneous rock.
If the magma does reach the surface, the result is often an explosive eruption. To see what rocks with the same chemical composition as granite look like when they are blasted out in a volcano (an extrusive igneous rock), go to the Morrow Mountain’s Special Rock EarthCache, which is here.
Sedimentary Rock. If igneous rocks are born of fire, sedimentary rocks are born of sleep. The sediments that are the building blocks of these rocks range from microscopic to boulder-sized. They may be composed of clay, sand, silt, cobblestones, remains of calcium-shelled organisms, and many, many other types of sediments. They may accumulate in the desert (sandstone), the bottom of a shallow sea or lake (siltstone), or way down beneath the deep ocean (shale). Sedimentary rocks have to sleep a long time to develop, as millions of tons must accumulate over them to generate the heat and pressure necessary to turn those sediments into consolidated rock.
Look at a few examples of sedimentary rocks: From the granite on the left wall of the jail, go up four rocks and over one. This light to dark gray rock has the orderliness of sedimentary rocks; if you lock closely, you can see different layers, exhibiting such differences as color and apparent compactness of the sediments (the individual sediments are far too small to see without a microscope).
From the chunk of granite on the left side of the wall, look across the wall to the right about ¾’s of the way, and you’ll see two large, tear-drop shaped white, milky rocks, which almost look like mirror images of each other. Of those two rocks, look at the one on the right, and go up two rocks, to look at a flat, slightly reddish-tan rock. This is another example of a sedimentary rock, and if you have a magnifying glass, you might even be able to see a little granular structure in the rock.
Why do these two rocks appear to look so different? Different minerals/mineral combinations, different depositional environments (deep ocean, stream, etc), and any number of other environmental conditions – but they’re both rocks born of sediments, and a long, sleepy period of accumulation.
Metamorphic Rock. The third type of rock that makes up the Earth’s crust is called “metamorphic”, which, translated from its Greek root words, means “to change the shape”. Metamorphic rocks can be created from any other type of rock: Igneous, sedimentary, or even other metamorphic rocks. It is primarily a change in temperature and pressure that changes the original rocks, but it can also be done underground via very hot circulating fluids, or even atmospheric changes.
A metamorphic rock doesn’t just look different than its parent rock – it is fundamentally different, as the chemical and structural make-up of the rock body has been changed. It is fair to say that understanding metamorphic rocks is complicated – even for professional geologists.
Two rocks above and to the right of the second sedimentary rock we looked at, there is a large, bluish-gray, triangular-shaped rock (like a triangle with the points cut off), which is a fine specimen of a mudstone (a simple sedimentary rock which was laid down here millions of years ago), which has been subjected to substantial heat and pressure over a long period of time. If you were to microscopically compare the difference between this “metamudstone” to a sample of the sedimentary mudstone, you would see distinct differences. Just how much of a difference would depend on how much temperature and pressure, hydrothermal alteration, and perhaps other processes the original rock had been subjected to. This rock has been changed, but not greatly so. Indeed, if the temperature and pressure it had been subjected to were much higher, it would be called by another name, and would look like (and probably be composed of) a much different rock.
So, are you now able to venture forth and break down every rock you see from now to the end of your life into the one of the three basic categories? Probably not, but you are at the point where you can appreciate the following:
“To ask an intelligent question, you need to know at least 75% of the answer.” We’re probably not up to 75% yet, but we’re definitely getting there.
To see other EarthCaches in the Gold Hill series, go here.
To receive credit for this EarthCache, please send me an e-mail with the following information:
Make the first line: “Gold Hill: Classifying Rocks”
How many were in your party?
1. Look for another example of a sedimentary rock in the wall. Describe the following:
Describe the rock, including color, your estimate of the size of the sedimentary grains in the rock, and how hard you think the rock is (please don’t bang on the rock! – just scrape it with your fingernail).
What are the visual differences between this rock and the first sedimentary rock described in the text above?
2. Look for a rock that has a lot of variation in its color. Such a rock is probably a metamorphic rock, changed by hydrothermal alteration. Describe the rock you have selected, including colors, apparent hardness/softness, and solidity (i.e., does it look like it’s about to fall apart, or is it quite solid?
3. Post a photo of you and your party with the wall behind you.
The author thanks Vivian Hopkins, Vice President, The Historic Gold Hill and Mines Foundation, Inc., and Chair of the Foundation's History Committee. She has been a tour guide, source of knowledge, and careful fact checker for the author.