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.
Logging Requirements:
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.