Geology of Kennesaw Mountain, Marietta, Cobb County, Georgia
Metatrondhjemite gneiss boulders atop Little Kennesaw Mountain
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Important Note On How to Read These Pages
This accounts of the geology of Kennesaw Mountain is not traditional as it arises from my notes and they follow the discoveries I've made in more than three decades wandering the mountain. Learning is a process that takes place over time. While time is linear, learning isn't necessarily so. There are moments of great lucidity and little epiphanies when all sorts of oddball observations and learned stuff comes together and there is great understanding. I hope these pages illustrate that process and might encourage others to think—or better, write—about their own little discoveries. The next four pages take their outline from my "book learnin'". They begin with the straightforward telling of the story by many geologists and the maps they create, in chronological order. Then I add my own story of exploration that include my little epiphanies. Each story adds an important part in my understanding of the story the rocks tell. I can't read that story directly from the rocks, but with the help of these geologists and my own observations, the story becomes clearer. It is not yet clear. It will never be. Interpretations change, new evidence shows up, theories are modified. That's the way of science. Little is absolute. If you find something you disagree with in my ramblings, or that your interpretation is different, or you find something I say that is flat out wrong, I'd love for you to let me know. Send me an email at scott@scottranger.com. |
Nyssa, Annette and Scott on Kennesaw Mountain, January 16, 2011
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For the story of Kennesaw Mountain's landscape and history, click here.
My story of Kennesaw Mountain is long and complex. I first climbed Kennesaw Mountain the summer of 1974 when I was courting my wife, Annette. I'd driven by it on the old "Four Lane" of U.S. 41 as Interstate 75 had yet to be built from Marietta to north of Lake Allatoona, but had not climbed it until Annette took me there. I immediately recognized the rock as gneiss from the "gneissic banding" of alternating layers of light (feldspars and quartz) and dark (amphiboles) minerals. Exactly what I'm seeing and understanding it has taken three decades and comparing observation with the available literature. This has led to much confusion on my part as the two didn't seem to match, at least on some of the details.
Granite vs. Gneiss
Let's begin right away and clarify the difference between granite and gneiss. Granite is an igneous rock where the crystals form differentially upon cooling deep in the earth's crust and results in a rock with visible crystals that are randomly arranged. Gneiss is a metamorphic rock that shows obvious banding of light and dark minerals resulting from recrystallization of the original material due to high heat and pressure. If you can discern a pattern, it's not granite! There is no granite on Kennesaw Mountain.
Gneiss
Gneiss, however, is a "garbage can" of a word with a wide definition of that includes an astonishing assortment of rocks displaying this foliation that formed in a myriad of metamorphic environments. Gneiss can form at as low a temperature as 320°C with pressures of 10 GPa, but this occurs only in very deep—more than 30 km—in the mantle. More commonly it forms at ~600°C at 2 to 4 GPa, a realm that is common in orogenies (mountain building events) at depths of 8 to 12 km. Minerals that form in this environment include hornblende, pyroxene and garnet. The sheet silicates (micas like muscovite and biotite) are less stable in these environments and their molecules change as a solid solution into a form that is more stable in this high grade temperature and pressure environment. These new minerals form perpendicular to the pressure gradient and this is what makes the alternating light and dark lines.
My story of Kennesaw Mountain is long and complex. I first climbed Kennesaw Mountain the summer of 1974 when I was courting my wife, Annette. I'd driven by it on the old "Four Lane" of U.S. 41 as Interstate 75 had yet to be built from Marietta to north of Lake Allatoona, but had not climbed it until Annette took me there. I immediately recognized the rock as gneiss from the "gneissic banding" of alternating layers of light (feldspars and quartz) and dark (amphiboles) minerals. Exactly what I'm seeing and understanding it has taken three decades and comparing observation with the available literature. This has led to much confusion on my part as the two didn't seem to match, at least on some of the details.
Granite vs. Gneiss
Let's begin right away and clarify the difference between granite and gneiss. Granite is an igneous rock where the crystals form differentially upon cooling deep in the earth's crust and results in a rock with visible crystals that are randomly arranged. Gneiss is a metamorphic rock that shows obvious banding of light and dark minerals resulting from recrystallization of the original material due to high heat and pressure. If you can discern a pattern, it's not granite! There is no granite on Kennesaw Mountain.
Gneiss
Gneiss, however, is a "garbage can" of a word with a wide definition of that includes an astonishing assortment of rocks displaying this foliation that formed in a myriad of metamorphic environments. Gneiss can form at as low a temperature as 320°C with pressures of 10 GPa, but this occurs only in very deep—more than 30 km—in the mantle. More commonly it forms at ~600°C at 2 to 4 GPa, a realm that is common in orogenies (mountain building events) at depths of 8 to 12 km. Minerals that form in this environment include hornblende, pyroxene and garnet. The sheet silicates (micas like muscovite and biotite) are less stable in these environments and their molecules change as a solid solution into a form that is more stable in this high grade temperature and pressure environment. These new minerals form perpendicular to the pressure gradient and this is what makes the alternating light and dark lines.
Lichen-covered metatrondhjemite gneiss boulder showing foliation planes atop Little Kennesaw Mountain
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A common misconception holds that granite is the original source of gneiss. While this can be true, almost any rock subjected to high-grade (high heat and pressure) regional metamorphism can form gneiss.
Gneiss derived from igneous rock (like granite) is termed orthogneiss. Gneiss derived from sedimentary rock is termed paragneiss. These words reflect the misconception as "ortho" means straight, upright, rectangular, regular; true, correct, proper; while "para" means alongside, beyond; altered; contrary; irregular, abnormal. Modifiers often precede the word to indicate the origin or a dominant mineral in the rock. Granite or diorite gneiss is metamorphosed from those rocks. Garnet or biotite gneiss are abundant in those minerals. As I've observed the rock at the summit of Kennesaw and Little Kennesaw mountains, the rock–-at least in places--looks like the granite-derived gneiss I was familiar with from the San Gabriel Mountains in southern California where I grew up. |
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Most of the rock on Kennesaw and Little Kennesaw mountains are covered with a vast flora of lichens and a few mosses as the boulder in the photo above illustrates. Since this is a National Park, I can't go around willy nilly breaking rocks to expose a fresh surface, so it requires wandering about to find a surface where the actual rock is visible, usually due to weathering.
At the summit of Kennesaw Mountain is an area the Park Service keeps clear of shrubs to maintain the view. Being a very popular place, many footsteps keep much of the rock bare. There are numerous historical grafitti where past visitors carved their name into the hard rock. Here I see two different banding patterns: small and large. The smallest are about 4 mm wide and the wide about 6 cm wide. Interpreting what they mean is the difficult part. The narrow bands are clearly the main metamorphic foliation grain. The larger seem to be veins from a very different rock. |
1887 grafitti atop Kennesaw Mountain
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Boulderfield on the southeast flank of Kennesaw Mountain
The Dark Rocks
Metabasalt garnet gneiss
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Metabasalt garnet gneiss
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Along the steep southeastern slope of Little Kennesaw Mountain, the trail wanders through a boulder field of mostly the light-colored gneiss. But occasionally there are far darker boulders about. Some of the boulders have a fascinating surface texture that can be seen from a distance of 10 or more meters distance away while hiking down the trail. Upon close inspection, they are 1 to 2 cm diameter garnets. This is clearly a very different rock than the light boulders. Early in my wanderings on this mountain I'd never encountered any regionally metamorphosed place like the Piedmont of Georgia and was utterly unfamiliar with Barrovian metamorphism, metamorphic facies and amphibolite.
Flatrock outcrop of Ropes Creek metabasalt migmatite with boudins
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The Camp Brumby Trail wanders along the southeast base of Kennesaw Mountain. In a few places the rock forms something of a pavement in outcrops very similar to the flat rock "granite" outcrops of the Piedmont. Here the rock is very dark, composed mostly of the minerals horblende, orthopyroxine and biotite. This makes them visually very different from the light-colored rock of the summit.
This was a new species of rock for me, and it took some basic study of the geology of the area and metamorphic rocks to learn what it is. . Metabasalt is an alternative name. Click here for the next adventure and a detailed explanation of amphibolite. |
Beware the dark rocks!
There will be much confusion if you just look for dark rocks. The rocks of Kennesaw Mountain have been subject to weathering for nearly 200 million years. That's a lot of time for the exposed surface of the rock to be modified chemically or biologically. There are probably a hundred or more species of lichen that grow on the rock, some are dark, some are light. The manganese in the rock is just soluble enough to come out of the rock and coat the surface with eons of wet and dry periods. This makes the rock very dark. The only way to really tell if the rock is light or dark is to break a piece and see an unweathered surface. Since this is a National Park, this is illegal, thus requiring more work at finding naturally exposed bare rock surfaces.
There will be much confusion if you just look for dark rocks. The rocks of Kennesaw Mountain have been subject to weathering for nearly 200 million years. That's a lot of time for the exposed surface of the rock to be modified chemically or biologically. There are probably a hundred or more species of lichen that grow on the rock, some are dark, some are light. The manganese in the rock is just soluble enough to come out of the rock and coat the surface with eons of wet and dry periods. This makes the rock very dark. The only way to really tell if the rock is light or dark is to break a piece and see an unweathered surface. Since this is a National Park, this is illegal, thus requiring more work at finding naturally exposed bare rock surfaces.
Managanese oxide coating metatrondhjemite gneiss making what is normally very light very dark
To continue the story with amphibolite, click here.