Geology of Arabia Mountain, DeKalb County, Georgia
For the story of Arabia Mountain's landscape and natural history, click here.
Crop from: Higgins, M.W., T.J. Crawford, R.L. Atkins & R.F. Crawford. 2003. Geologic map of the Atlanta 30' x 60' quadrangle, Georgia. Scientific Investigations Map 2602. U.S. Geologic Survey, Reston, VA.
To understand the geology of Arabia Mountain, one must understand the metamorphic world of gneiss and migmatite and how they are similar yet differ dramatically from granite. All three forms occur on the mountain.
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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! While Arabia Mountain is mostly migmatitic gneiss, there are many pockets of granite. Pay attention to the pattern of the dark and light crystals as you wander the mountain and look for the small granite bodies. This photo of gneiss among the granites in the outwash plain of the Herbert Glacier in southeast Alaska illustrates just how easy it is to tell them apart. Random black and white crystals versus bands of black and white crystals. |
Gneiss among the granite
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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 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.
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.
Migmatite
Here is a thorough, if tortured, description of migmatite, with words not defined in the text asterisked and defined below:
"A migmatite is a banded, granular metamorphic rock that contains light coloured bands with evidence for partial melting. Migmatites comprise leucosome* bands, comprised largely of quartz and feldspar, which crystallised from partial melts or segregated metamorphic fluids, and mesosomes* and melanosomes* that represent the residues of partial melting and are often dominated by mafic components such as amphibole, pyroxene and biotite and are often finer-grained than leucosomes. The terms neosome and palaeosome are used to denote components of the migmatite that have experience significant and little partial melting respectively.
"Migmatites have usually experienced significant ductile deformation and exhibit incoherent folding with ptygmatic folds restricted to individual layers and extensive boudinage. Clast-like bodies can occur within migmatites and are composed of restite, solid blocks of partial melt residue, that do not undergo ductile deformation. Often leucosomes in migmatites have rims of mafic minerals known as selvages that form by reaction with the melt or a coexisting fluid. Migmatites are sometimes sub-divided into metatexites, that have experienced low degrees of partial melting, or diatexites, that have experienced high degrees. Metatexites can also often be termed hornfels or gniess and are prefixed with "migmatised".
"Migmatites form by high temperature regional and/or thermal metamorphism of protolith rocks where temperatures are sufficient to cause partial melting. They are often associated with granulites and gneisses, and often are spatially associated to granitic intrusions. The partial melting of crustal rocks (or anatexis), which results in migmatites, is responsible for the generation of many granite magmas. Partial melting within migmatites leads to the formation of a banded rock by segregation of viscous partial melts and rigid resistite due to differential stress." [Imperial College Rock Library]
*leucosomes are the lightest colored parts of a migmatite
*mesosomes are intermediate between the dark and light colored parts of a migmatite and usually an unmodified remnant of the protolith; synonymous with paleosome.
*melanosomes are the darkest parts of a migmatite
Finnish petrologist Jakob Johannes Sederholm (1863 – 1934) studied the strange mix of metamorphic and granitic rocks in the Precambrian Baltic Shield and coined the word from the Greek μιγμα migma, a mixture.
Migmatites form where the pressure and temperature regime tease with the melting point of the rock; sometimes above, sometimes below. The temperature must be at least 800°C but less than ~1,200°C. This simplifies a very complex world as all the minerals in the rock don't have the same melting point. Some will completely melt, others become plastic and yet others still completely solid and undergoing solid solution metamorphism. With viscous meeting solid and liquid, very strange patterns of minerals form, especially when the viscous are forced into the liquid or fractures in the solid and the resistance creates very odd shapes of ptygmatic folds and boudins.
Where does this sort of environment exist? Plate tectonics gives us the answer. When continents collide, especially large continents like Laurasia and Gondwanaland, massive amounts of pressure develop in many directions as the rocks pile on top of each other and fold from incredible lateral force. Migmatites form fairly deep in the earth under these gigantic collisions, usually at a depth of at least three kilometers, often much greater. But they can't be deep enough where the temperature and pressure will cause all the rock to melt. It is the strange world of "almost melting". Perhaps a poor analogy would be very slowly heating a wax candle until it is soft enough to bend, then pushing on it from several directions. If two different colors melt at different temperatures, one would melt and one would remain solid. Imagine what this would look like if you pushed on the mix from several directions at once.
The Arabia Mountain Migmatite
Here are three modern technical descriptions of the formation that includes the rocks of Arabia Mountain in chronological order.
"Although the name Arabia Mountain Migmatite was designated by Grant and others (1980) for a common variety of Lithonia Gneiss, in this report the rocks are informally named Arabia Mountain migmatite facies of Lithonia Gneiss. Rocks previously mapped as Norcross Gneiss are now thought to be a facies of Lithonia Gneiss; Norcross Gneiss is abandoned and its rocks assigned to Lithonia Gneiss. Recent geologic mapping, petrographic and geochemical studies, and some geochronological data of rocks called Lithonia Gneiss, Arabia Mountain migmatite facies within Lithonia Gneiss around Mount Arabia, "Odessadale Gneiss" in central GA (R.L. Atkins, unpub. mapping) and Farmville Metagranite of Opelika Complex in eastern AL have shown that 1) they are part of the same complex unit, 2) that several textural varieties are common to all of them in outcrop, 3) that they all occupy the same structural or stratigraphic interval, 4) that they have approximately the same mineral and chemical compositions, and 5) that their ages range between about 380 to 360 Ma (Middle to Late Devonian). Because of uncertainties in Pb-U zircon age data, because Rb-Sr and K-Ar ages could have been affected by metamorphism, and because overall sampling has been limited, all the gneisses are assigned an Ordovician to Devonian age although it is probably the age of the neosome; age of the paleosome of Lithonia Gneiss may be as old as Middle Proterozoic or as young as Middle Ordovician."
Crawford, T.J., M.W. Higgins, R.F. Crawford, R.L. Atkins, J.H. Medlin, & T.W. Stern. 1999. Revision of stratigraphic nomenclature in the Atlanta, Athens, and Cartersville 30- x 60-minute quadrangles, Georgia. Georgia Geological Survey Bulletin, no. 130, 45 p.
Higgins et al description of the Lithonia gneiss from their Atlanta quadrangle map:
"Dl Lithonia Gneiss, undivided (Devonian)—Lithonia Gneiss is a complex of metagranites and granitic gneisses. The most common rock type is a light-gray to grayish-white, medium-grained, poorly foliated metagranite that is cut by numerous pegmatite and aplite dikes and sills of several generations; dikes of different generations crosscut older dikes. This rock type probably forms about 80 to 90 percent of all rocks mapped as Lithonia Gneiss in the Athens quadrangle, whereas in the Atlanta and Griffin quadrangles, it may form only about 40 to 50 percent. The remainder of rocks mapped as Lithonia Gneiss are migmatite gneiss that belong to the Mount Arabia Migmatite of Grant and others (1980; also see Covert, 1986; and Size and Khairallah, 1989), which is included in this unit on this map. The Mount Arabia Migmatite of Grant and others (1980) is a light-gray to whitish-gray, medium-grained muscovite-biotite-microcline-oligoclase-quartz gneiss with well-defined, contorted, generally 3-mm- to 1-cm-thick gneissic layering. The migmatite gneiss is the prevalent rock type in Lithonia Gneiss near its edges, whereas the metagranite is far more prevalent away from the edges of most outcrop areas of Lithonia Gneiss. Garnet segregations in lenses as large as 2 m by 2 m are locally present in the migmatite gneiss. Scattered xenoliths, mainly of amphibolite, are present in the metagranite and the migmatite gneiss, but are probably more abundant in the migmatite gneiss. Pavement outcrops are characteristic of both the metagranite and the migmatite gneiss, and where deeply weathered both form light-whitish-yellow sandy soils. Both the metagranite and the migmatite gneiss are extensively quarried for crushed-stone aggregate and curbstone, and the migmatite gneiss is extensively quarried for monumental stone."
Higgins, M.W., T.J. Crawford, R.L. Atkins & R.F. Crawford. 2003. Geologic map of the Atlanta 30' x 60' quadrangle, Georgia. Scientific Investigations Map 2602. U.S. Geologic Survey, Reston, VA.
Three years after the publication of the map, Ralph Crawford mapped the Lawrenceville quadrangle and makes these comments on the Lithonia Gneiss and its Mount Arabia migmatite member:
"The Lithonia Gneiss, which underlies large areas in the quadrangle, has been mapped to the southwest into Alabama and into the Ordovician Farmville Granite (~477 Ma). The granitoid phase of the Lithonia is an Ordovician batholith that intruded the Stonewall Gneiss producing the Mount Arabia Migmatite. The Stonewall crops out around the edges of the Lithonia granitoid phase and in local roof pendants. The migmatite appears to occur only around the edges of the Lithonia and around roof pendants of Stonewall Gneiss, whereas the Lithonia away from any Stonewall is mostly metamorphosed granitoid. The aluminous schist and Chattahoochee Palisades Quartzite of the Sandy Springs Group structurally overlie the Lithonia, the Stonewall, and the Allatoona allochthon as well, on the Sandy Springs fault. The Lithonia, its country rock, the Stonewall Gneiss, and overlying Sandy Springs rocks constitute a mantled gneiss dome assemblage. The aluminous schist and quartzite are interpreted to have been deposited unconformably upon the Lithonia and the other units and were metamorphosed with the Lithonia and thrust across it. Sandy Springs Group rocks also border the Dacula shear zone on the northwest and probably dip beneath the zone and the Lithonia Gneiss."
Crawford, R.F. 2006. Geologic map of the Lawrenceville 7.5-minute quadrangle map, Georgia. The Geological Society of America Southeastern Section–55th Annual Meeting (23–24 March 2006) Paper No. 33-20.
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 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.
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.
Migmatite
Here is a thorough, if tortured, description of migmatite, with words not defined in the text asterisked and defined below:
"A migmatite is a banded, granular metamorphic rock that contains light coloured bands with evidence for partial melting. Migmatites comprise leucosome* bands, comprised largely of quartz and feldspar, which crystallised from partial melts or segregated metamorphic fluids, and mesosomes* and melanosomes* that represent the residues of partial melting and are often dominated by mafic components such as amphibole, pyroxene and biotite and are often finer-grained than leucosomes. The terms neosome and palaeosome are used to denote components of the migmatite that have experience significant and little partial melting respectively.
"Migmatites have usually experienced significant ductile deformation and exhibit incoherent folding with ptygmatic folds restricted to individual layers and extensive boudinage. Clast-like bodies can occur within migmatites and are composed of restite, solid blocks of partial melt residue, that do not undergo ductile deformation. Often leucosomes in migmatites have rims of mafic minerals known as selvages that form by reaction with the melt or a coexisting fluid. Migmatites are sometimes sub-divided into metatexites, that have experienced low degrees of partial melting, or diatexites, that have experienced high degrees. Metatexites can also often be termed hornfels or gniess and are prefixed with "migmatised".
"Migmatites form by high temperature regional and/or thermal metamorphism of protolith rocks where temperatures are sufficient to cause partial melting. They are often associated with granulites and gneisses, and often are spatially associated to granitic intrusions. The partial melting of crustal rocks (or anatexis), which results in migmatites, is responsible for the generation of many granite magmas. Partial melting within migmatites leads to the formation of a banded rock by segregation of viscous partial melts and rigid resistite due to differential stress." [Imperial College Rock Library]
*leucosomes are the lightest colored parts of a migmatite
*mesosomes are intermediate between the dark and light colored parts of a migmatite and usually an unmodified remnant of the protolith; synonymous with paleosome.
*melanosomes are the darkest parts of a migmatite
Finnish petrologist Jakob Johannes Sederholm (1863 – 1934) studied the strange mix of metamorphic and granitic rocks in the Precambrian Baltic Shield and coined the word from the Greek μιγμα migma, a mixture.
Migmatites form where the pressure and temperature regime tease with the melting point of the rock; sometimes above, sometimes below. The temperature must be at least 800°C but less than ~1,200°C. This simplifies a very complex world as all the minerals in the rock don't have the same melting point. Some will completely melt, others become plastic and yet others still completely solid and undergoing solid solution metamorphism. With viscous meeting solid and liquid, very strange patterns of minerals form, especially when the viscous are forced into the liquid or fractures in the solid and the resistance creates very odd shapes of ptygmatic folds and boudins.
Where does this sort of environment exist? Plate tectonics gives us the answer. When continents collide, especially large continents like Laurasia and Gondwanaland, massive amounts of pressure develop in many directions as the rocks pile on top of each other and fold from incredible lateral force. Migmatites form fairly deep in the earth under these gigantic collisions, usually at a depth of at least three kilometers, often much greater. But they can't be deep enough where the temperature and pressure will cause all the rock to melt. It is the strange world of "almost melting". Perhaps a poor analogy would be very slowly heating a wax candle until it is soft enough to bend, then pushing on it from several directions. If two different colors melt at different temperatures, one would melt and one would remain solid. Imagine what this would look like if you pushed on the mix from several directions at once.
The Arabia Mountain Migmatite
Here are three modern technical descriptions of the formation that includes the rocks of Arabia Mountain in chronological order.
"Although the name Arabia Mountain Migmatite was designated by Grant and others (1980) for a common variety of Lithonia Gneiss, in this report the rocks are informally named Arabia Mountain migmatite facies of Lithonia Gneiss. Rocks previously mapped as Norcross Gneiss are now thought to be a facies of Lithonia Gneiss; Norcross Gneiss is abandoned and its rocks assigned to Lithonia Gneiss. Recent geologic mapping, petrographic and geochemical studies, and some geochronological data of rocks called Lithonia Gneiss, Arabia Mountain migmatite facies within Lithonia Gneiss around Mount Arabia, "Odessadale Gneiss" in central GA (R.L. Atkins, unpub. mapping) and Farmville Metagranite of Opelika Complex in eastern AL have shown that 1) they are part of the same complex unit, 2) that several textural varieties are common to all of them in outcrop, 3) that they all occupy the same structural or stratigraphic interval, 4) that they have approximately the same mineral and chemical compositions, and 5) that their ages range between about 380 to 360 Ma (Middle to Late Devonian). Because of uncertainties in Pb-U zircon age data, because Rb-Sr and K-Ar ages could have been affected by metamorphism, and because overall sampling has been limited, all the gneisses are assigned an Ordovician to Devonian age although it is probably the age of the neosome; age of the paleosome of Lithonia Gneiss may be as old as Middle Proterozoic or as young as Middle Ordovician."
Crawford, T.J., M.W. Higgins, R.F. Crawford, R.L. Atkins, J.H. Medlin, & T.W. Stern. 1999. Revision of stratigraphic nomenclature in the Atlanta, Athens, and Cartersville 30- x 60-minute quadrangles, Georgia. Georgia Geological Survey Bulletin, no. 130, 45 p.
Higgins et al description of the Lithonia gneiss from their Atlanta quadrangle map:
"Dl Lithonia Gneiss, undivided (Devonian)—Lithonia Gneiss is a complex of metagranites and granitic gneisses. The most common rock type is a light-gray to grayish-white, medium-grained, poorly foliated metagranite that is cut by numerous pegmatite and aplite dikes and sills of several generations; dikes of different generations crosscut older dikes. This rock type probably forms about 80 to 90 percent of all rocks mapped as Lithonia Gneiss in the Athens quadrangle, whereas in the Atlanta and Griffin quadrangles, it may form only about 40 to 50 percent. The remainder of rocks mapped as Lithonia Gneiss are migmatite gneiss that belong to the Mount Arabia Migmatite of Grant and others (1980; also see Covert, 1986; and Size and Khairallah, 1989), which is included in this unit on this map. The Mount Arabia Migmatite of Grant and others (1980) is a light-gray to whitish-gray, medium-grained muscovite-biotite-microcline-oligoclase-quartz gneiss with well-defined, contorted, generally 3-mm- to 1-cm-thick gneissic layering. The migmatite gneiss is the prevalent rock type in Lithonia Gneiss near its edges, whereas the metagranite is far more prevalent away from the edges of most outcrop areas of Lithonia Gneiss. Garnet segregations in lenses as large as 2 m by 2 m are locally present in the migmatite gneiss. Scattered xenoliths, mainly of amphibolite, are present in the metagranite and the migmatite gneiss, but are probably more abundant in the migmatite gneiss. Pavement outcrops are characteristic of both the metagranite and the migmatite gneiss, and where deeply weathered both form light-whitish-yellow sandy soils. Both the metagranite and the migmatite gneiss are extensively quarried for crushed-stone aggregate and curbstone, and the migmatite gneiss is extensively quarried for monumental stone."
Higgins, M.W., T.J. Crawford, R.L. Atkins & R.F. Crawford. 2003. Geologic map of the Atlanta 30' x 60' quadrangle, Georgia. Scientific Investigations Map 2602. U.S. Geologic Survey, Reston, VA.
Three years after the publication of the map, Ralph Crawford mapped the Lawrenceville quadrangle and makes these comments on the Lithonia Gneiss and its Mount Arabia migmatite member:
"The Lithonia Gneiss, which underlies large areas in the quadrangle, has been mapped to the southwest into Alabama and into the Ordovician Farmville Granite (~477 Ma). The granitoid phase of the Lithonia is an Ordovician batholith that intruded the Stonewall Gneiss producing the Mount Arabia Migmatite. The Stonewall crops out around the edges of the Lithonia granitoid phase and in local roof pendants. The migmatite appears to occur only around the edges of the Lithonia and around roof pendants of Stonewall Gneiss, whereas the Lithonia away from any Stonewall is mostly metamorphosed granitoid. The aluminous schist and Chattahoochee Palisades Quartzite of the Sandy Springs Group structurally overlie the Lithonia, the Stonewall, and the Allatoona allochthon as well, on the Sandy Springs fault. The Lithonia, its country rock, the Stonewall Gneiss, and overlying Sandy Springs rocks constitute a mantled gneiss dome assemblage. The aluminous schist and quartzite are interpreted to have been deposited unconformably upon the Lithonia and the other units and were metamorphosed with the Lithonia and thrust across it. Sandy Springs Group rocks also border the Dacula shear zone on the northwest and probably dip beneath the zone and the Lithonia Gneiss."
Crawford, R.F. 2006. Geologic map of the Lawrenceville 7.5-minute quadrangle map, Georgia. The Geological Society of America Southeastern Section–55th Annual Meeting (23–24 March 2006) Paper No. 33-20.
Arabia Mountain Migmatite Mineralogy
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Fresh exposure of Arabia Mountain Migmatite
The alternating bands of dark and light minerals are very obvious in this close up photo. So what is different here from plain old gneiss? It is the folding of the bands that makes the Arabia Mountain migmatite so fabulous! There are swirls of bands, sharp bends of bands and an assortment of very complicated geometric shapes. These are the signature feature of migmatites, all formed at the bizarre world that exists where some of the rock is solid, some liquid, but most of it in that strange place where it's neither solid nor liquid but is ductile. This might be sort of like the contents of a very old tube of toothpaste: some of it has hardened and flows en masse under great pressure and might be deformed as it pushes against the tube, but there is much that still flows relatively easily under pressure. Imagine if it is one of those brands that has different components with different textures and colors and you've now created a mental picture of this very strange place. |
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Exposed and weathered surface with boudinage
The orange-brown color is created by the weathering of iron-rich hornblende [Ca2 Fe5 (Al,Si)8 O2 2(OH)2]. It results in several oxides of iron including the earthy red hematite [Fe2 O3] and yellow limonite [FeO(OH)·nH2O]. Here it can be seen where the normally black bands of horneblende has turned color. What caught my eye here is the pattern the weathering highlights, a boudin. This area of sqarrish rock was relatively solid when the material around it was more fluid. The pressure squeezed it into this shape as the crystals around it flowed, albeit very slowly. Here the flow was from the top left to the bottom right where something of a tail developed as the boudin itself was heated to nearly the same temperature as the hotter and more plastic rock around it. |
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Chaotic Folding
As rock approaches the melting point it becomes more ductile. Since the minerals are a mix of both ductile and rigid, flow patterns become chaotic to the eye as I look down on the rock. Years ago, I began a survey of the mountain trying to discern a regional pattern to the folding but gave up as it was well beyond my ability to understand. I'm sure a structural mineralogist could reconstruct the forces that created this amazing mix of folds, swirls, tucks that today please my eye, but I can't. This world is just too bizarre when compared to the world that I exist on. It is very "other-worldly"! Chaotic folding is the hallmark of migmatites. If you see it, you have a migmatite! |
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Nested chevrons
Chevron folding is more easy to understand. It is created when a band of alternating minerals is compressed from the sides causing the bands to buckle perpendicular to the direction of the stress. In the left photo it is seems the material on the bottom left was relatively solid and was pressing upon a more ductile material creating the chevrons. The boundary between the two is obvious and represents where the solid almost became as ductile as the chevrons. In the right photo the pressure was more stable in that it pressed on the bands from a single direction so the chevrons are more regular and not twisted. A liberal sprinkling of lichen can be seen growing on the surface of the rock.
Chevron folding is more easy to understand. It is created when a band of alternating minerals is compressed from the sides causing the bands to buckle perpendicular to the direction of the stress. In the left photo it is seems the material on the bottom left was relatively solid and was pressing upon a more ductile material creating the chevrons. The boundary between the two is obvious and represents where the solid almost became as ductile as the chevrons. In the right photo the pressure was more stable in that it pressed on the bands from a single direction so the chevrons are more regular and not twisted. A liberal sprinkling of lichen can be seen growing on the surface of the rock.
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This exposure illustrates both sharp and gently curved chevrons. The sharply curved bands were more solid than the curved when the pressure created them. The upper left material was less ductile then the bottom right where it was more fluid and formed a tight arc of folds rather than a sharp chevron.
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Arabia Mountain Structural Geology:
Jointing
Jointing
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Joints are a fracture in a rock where there is little or no movement along the trace, created when forces greater than the brittle strength of the rock cause it to break. Unloading joints form when a substantial overburden is removed allowing the compressive force allowing the rock to expand laterally exceeding the cohesive strength of the rock. Nearly all hard crystalline rock surfaces are littered with joints of many sizes. These joints have been exploited by both weathering forces and humans. The two photographs on the left illustrate a relatively young joint with sharp edges and a much older joint greatly expanded by the weathering force of water. The photograph on the right shows a small joint being exploited by miners using metal pins to enlarge the joint so as to remove pieces of the rock.
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Joints that drop down the sides of the rock provide the path for water to flow. As the water flows it continues to open the joint while at the same time round off its sides. Where the slope or jointing is uneven, the gradient decreases and a pool forms. Since there is virtually no absorption of water by the rock, when rains come hard, water flows amazingly fast carrying with it pieces of the rock. The greater the flow, the large the pieces can be. When these pieces hit the low gradient places they can themselves become part of the erosive force as water propels them against the country rock forming potholes. Here a stream of water actively flows down a joint with small waterfalls as it enters each of the potholes.
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Arabia Mountain Structural Geology:
Exfoliation
Exfoliation
Exfoliation is a special form of unloading joints. Exfoliation is most simply defined as the shedding of layers of rock along joints parallel to the surface of the rock. It is commonly compared to peeling off the layers of an onion. Here two relatively thin slabs of migmatite have spalled off along a joint and expose fresh rock below.
Overburden and Rebound
Grove Karl Gilbert (1843–1918) pioneered the study of geomorphology, the study of landforms and the processes that create them. He is credited with the widely-accepted theory of exfoliation based upon the removal of overburden and subsequent rebound of the material. What this means to the Mount Arabia migmatite is that it was deeply buried under the Pangaean mountains that created the other-wordly place where the migmatite and granite formed. Under some 10 to 20 kilometers of crust, the pressure was extreme. Over the last 200 million years that overburden has been eroded away and now forms the Coastal Plain and reduced the pressure to essentially nothing. Since the rock was compressed, as it decompresses it creates a new stress pattern that expands radially from the base of the structure. As these stress forces reach the surface, it creates fracture zones parallel to the surface. While Gilbert's model is very appealing (I've understood it as the reason for most of my adult life!), it is, at best, incomplete.
Compressive stress and extensional fracture
Rather than having the stress created by overburden compression, it now appears that most of it comes from lateral compression, parallel to the surface of the land.* These forces create two kinds of fracture joints, one parallel to the pressure and one perpendicular to it. Complicating the picture is the opposite force of extension. Current thinking has lateral compression initializing the fracture zones and as that pressure is reduced or eliminated, extension pulls them apart.
Goodman, R.E. 1993. Engineering Geology. John Wiley and Sons, NY.
Overburden and Rebound
Grove Karl Gilbert (1843–1918) pioneered the study of geomorphology, the study of landforms and the processes that create them. He is credited with the widely-accepted theory of exfoliation based upon the removal of overburden and subsequent rebound of the material. What this means to the Mount Arabia migmatite is that it was deeply buried under the Pangaean mountains that created the other-wordly place where the migmatite and granite formed. Under some 10 to 20 kilometers of crust, the pressure was extreme. Over the last 200 million years that overburden has been eroded away and now forms the Coastal Plain and reduced the pressure to essentially nothing. Since the rock was compressed, as it decompresses it creates a new stress pattern that expands radially from the base of the structure. As these stress forces reach the surface, it creates fracture zones parallel to the surface. While Gilbert's model is very appealing (I've understood it as the reason for most of my adult life!), it is, at best, incomplete.
Compressive stress and extensional fracture
Rather than having the stress created by overburden compression, it now appears that most of it comes from lateral compression, parallel to the surface of the land.* These forces create two kinds of fracture joints, one parallel to the pressure and one perpendicular to it. Complicating the picture is the opposite force of extension. Current thinking has lateral compression initializing the fracture zones and as that pressure is reduced or eliminated, extension pulls them apart.
Goodman, R.E. 1993. Engineering Geology. John Wiley and Sons, NY.
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Small scale exfoliation
These two photos illustrate, what by my experience wandering the mountain, the most common form of exfoliation. These areas are small, mostly less than a square meter, and shallow, usually less than 3 cm thick. In both these instances the rock above the surface joint is missing and exposes fresh material that was below. This leads to yet another complication to Gilbert's theory as chemical weathering surely plays a great role here. As soon as the parallel surface joint is created, it provides an entry point for water. One of the slow ways water can erode is by the expansion of minerals as they hydrate, creating yet another stress on the rock. This process appears, to my eye at least, to be at work in each of the spalls. It cannon explain the large-scale joints that appear here as well.
These two photos illustrate, what by my experience wandering the mountain, the most common form of exfoliation. These areas are small, mostly less than a square meter, and shallow, usually less than 3 cm thick. In both these instances the rock above the surface joint is missing and exposes fresh material that was below. This leads to yet another complication to Gilbert's theory as chemical weathering surely plays a great role here. As soon as the parallel surface joint is created, it provides an entry point for water. One of the slow ways water can erode is by the expansion of minerals as they hydrate, creating yet another stress on the rock. This process appears, to my eye at least, to be at work in each of the spalls. It cannon explain the large-scale joints that appear here as well.
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Large scale exfoliation
This photo illustrates the "onion layer" form of exfoliation. The joint parallel to the surface here is about 1 meter below the current surface of Arabia Mountain. While chemical weathering can assist in the expansion of the joint, it had nothing to do with the creation of the joint. What seems to have happened here is the Arabia migmatite, harder and more resistant than the surrounding Lithonia gneiss, was compressed in geologic time laterally by the country rock around it. This created these fracture zones parallel to the surface of the earth. As the mountain appeared through eons of erosion, these pre-existing joints switched to extensional forces and became subject to the surface forces of weathering. A question remains in my mind to explain the shattering joints of the spalled surface. How much of this is due to paraglacial events during the Pleistocene? Nearby, at Panola Mountain, many granite boulders can be found that clearly were split by frost heaving that could only have occurred during the much colder Pleistocene environment. |
Arabia Mountain Structural Geology:
Solution Pits
Solution Pits
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For many, the most interesting structure on Arabia Mountain are the solution pits. There has been remarkably little research on these common structures. The classic study by Fairbridge has this short, but accurate description:
"Solution pits and pans are small-scale, surface weathering features found in many parts of the world and in most climatic belts in such diverse rock types as granite, basalt, limestone and quartzitic sandstone. The pits are small pockmarks, from a few millimeters to some centimeters in diameter and depth; the pans develop from pits by lateral extension in all directions but for the most part maintain a perfectly flat floor. The larger pans (50 cm to 2 meters diameter) usually have undercut rims, reaching a few centimeters under the lip of the pan. The floor of each pan is usually indurated and smooth, while the undercut is hackly in texture and shows every sign of chemical etching. Joint planes are further etched out, showing the solvent effects also of overflow waters. The pits and pans are thus evidently related to solution in standing water."
Fairbridge, R.W. 1968. Solution pits and pans. Geomorphology, January 1968, p. 1033-1036.
The top of the main summit of Arabia Mountain is legendary for its large solution pits, by far the largest I've ever seen. The largest spans nearly a quarter of an acre! The summit of the mountain is fairly flat with only a gentle slope. With this fact, one must rule out flowing water as a force to create the pit. The term "solution" here is a key to its formation. When carbon dioxide is dissolved in water, carbonic acid [H2CO3] forms. This occurs both physically and biologically. The myriad of lichens that cover all but the most recently exposed surface of the rock form and hold carbonic acid in place. While a rather weak acid, there is enough--over time--to chemically react with the feldspars in the rock and form carbonates that dissolve out of the rock.
"Solution pits and pans are small-scale, surface weathering features found in many parts of the world and in most climatic belts in such diverse rock types as granite, basalt, limestone and quartzitic sandstone. The pits are small pockmarks, from a few millimeters to some centimeters in diameter and depth; the pans develop from pits by lateral extension in all directions but for the most part maintain a perfectly flat floor. The larger pans (50 cm to 2 meters diameter) usually have undercut rims, reaching a few centimeters under the lip of the pan. The floor of each pan is usually indurated and smooth, while the undercut is hackly in texture and shows every sign of chemical etching. Joint planes are further etched out, showing the solvent effects also of overflow waters. The pits and pans are thus evidently related to solution in standing water."
Fairbridge, R.W. 1968. Solution pits and pans. Geomorphology, January 1968, p. 1033-1036.
The top of the main summit of Arabia Mountain is legendary for its large solution pits, by far the largest I've ever seen. The largest spans nearly a quarter of an acre! The summit of the mountain is fairly flat with only a gentle slope. With this fact, one must rule out flowing water as a force to create the pit. The term "solution" here is a key to its formation. When carbon dioxide is dissolved in water, carbonic acid [H2CO3] forms. This occurs both physically and biologically. The myriad of lichens that cover all but the most recently exposed surface of the rock form and hold carbonic acid in place. While a rather weak acid, there is enough--over time--to chemically react with the feldspars in the rock and form carbonates that dissolve out of the rock.
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Solution pits of many sizes can be found on the mountain, all in the relatively flat places. Those that are deep enough to hold water for extended periods often support a healthy population of several interesting plants. For a general discussion of this habitat, click here. For all the red stuff, click here.