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Welcome to Stone Mountain, Georgia! (Not to be confused with any of the other Stone Mountains in the Southeast).
Stone Mountain is located in the Piedmont (but closer to the Blue Ridge) and is an intrusive monadnock, meaning it's a mountain that stands away from any mountain range.
In physical geology, you learn about intrusive versus extrusive igneous rocks. Another name for extrusive rocks, is volcanic (if a magma makes it to the surface some sort of volcanic process had to be involved), while another name for intrusive rocks is plutonic.
These mountains that we call plutons are not like other mountains. They are often made of entirely one type of stone. They also are not a part of a mountain belt. Thus, these are standalone mountains, earning the name monadnock — meaning an isolated hill of bedrock, standing conspicuously above the general level of the surrounding topography.
The top exposed "granite" occupies a small portion of an area underlain by the same "granite." Plutons form from a magma chamber that cooled within the earth. With the help of weathering (and their typically light density) they make it to the surface and are exposed, rather than forming structurally, as seen in mountain belts.
Once the pressure of the overburden is removed, the granite then expands in all directions, leading to sheets of the granite breaking off (termed exfoliation) and creating the characteristic dome.
Above: Observe the mass wasting occurring as slabs are exfoliated from the top of the dome.
Now, let's take a look at some features specific to Stone Mountain!
Stonemountain1.mp4Stone Mountain 2.mp4Stone mountain 1.5.mp4Stone Mountain(last).mp4In the above explanation, there was a hint that this is a granite, but this is not totally accurate. The reason for this is that while these plutons are granitic bodies (meaning they are felsic intrusive rocks), calling something a "granite" is a direct reflection of the minerals present. We are only focused on modal mineralogy now, and not the more precise geochemical measurements. Thus, is we would be remiss to call Stone Mountain granite.
The diagram to the right is called a QAP diagram, one way to classify and name plutonic igneous rocks.
When we look at a rock, you can see a lot of minerals that jump out at you. For example, let’s look at this rock below:
Left: We can see a pink mineral (potassium feldspar), some white minerals (quartz and plagioclase), as well as dark minerals (we can call it biotite, but it could be amphibole). To classify a rock using the QAP diagram, we first come up with a list of all the minerals you can see and estimate their percent abundance. If you have a Rite-in-the-Rain field book, in the back they have a cheat sheet to help you estimate the amounts you see, such as the chart pictured below.
Things to keep in mind when estimating percentages:
Your eyes tend to always want to overestimate the amount of dark minerals present.
You should end up with a total of 100%.
This takes practice. As you do more of this (and you will in classes), you will get better.
Let’s try this out together on this picture from Enchanted Rock (right):
K-spar: 45%. The potassium feldspar (called K-spar) is the pink mineral.
Quartz: 25%. This is the most difficult to see in the picture. It is the gray (and nearly purple) mineral.
Plagioclase: 15%. This is the white mineral.
Amphibole: 5%. This is a portion of the small black grains.
Biotite: 10%. This is the remain small black grains. Note that you can't differentiate between biotite and amphibole in this photo, and that's okay.
Now that we've made our estimates for the whole rock, the QAP diagram above only has 3 sides, and thus is only defined by three minerals: quartz, plagioclase and K-spar (an alkali feldspar). We have to normalize our percent estimates, as if the rock we are looking at has only those 3 minerals, in order to plot it on the diagram. In other words, make it so that the relative portions of these three minerals stay the same, but they all add up to 100%:
We originally had 45% K-spar, 25% quartz, and 15 % plagioclase:
45+25+15=85 45/85=53% 25/85=30% 15/85=17%
So, we now have 53% K-spar, 30% quartz, and 17% plagioclase. We're now ready to plot this on the QAP diagram!
Take a deep breath and try not to get overwhelmed — the hardest part is over! Each corner of the diagram is home to one of the three minerals. Since we are plotting percentages, the colored lines in the below images are percentage lines in increments of 10%. The points of the triangle associated with a mineral represents 100% of that mineral and every line leading away from a point is 10% less until you get to the base of the triangle opposite of the associated mineral, which is 0%. Let's plot one mineral at a time.
Shown here in red are the QAP's quartz percentage lines. If we had 50% quartz, it would fall somewhere along the 50% quartz line. If we had 20% quartz, it would fall somewhere on the 20% quartz line, and so on.
Shown in green are the K-spar lines on the QAP diagram. If we had 30% K-spar, it would fall on the 30% K-spar line. If we had 0%, it would fall on the 0% K-spar line.
Shown here in blue are the lines for plagioclase. If we had 70% plagioclase, it would fall on the 70% plagioclase line. If we had 100%, it would be in the 100% plagioclase corner.
Using our above numbers of 53% K-spar, 30% quartz, and 17% plag, plotting it would look something like this:
Where all lines intersect is the name of the associated rock type. We can see that this is, in fact, a granite!
If you are having trouble, try comparing the colored lines on the QAP diagram on the right with the three diagrams above. Remember, each base of the triangle is 0% of the mineral at the opposing point. Try counting from up from 0% through each percentage line, until you get to the desired number (i.e. 30% quartz is the third line from the quartz's opposing base).
You now have an introduction to intrusive igneous rocks (don't worry, you'll get to practice the QAP diagram and other ternary diagrams later). On to the next stop!
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