Geology of Kennesaw Mountain: a summary
There are basically only two rocks on Kennesaw Mountain: a metatrondhjemite gneiss, the Informal migmatite of Kennesaw Mountain; and, an amphibolite named the Ropes Creek metabasalt. Both are dated in a wide range from Middle Ordovician (472 million years ago) to Late Proterozoic (one billion years ago). This is amazingly more than half a billion years! To say we have a handle on when what happened here is a gross overstatement. The best that can be said about the age is these rocks were formed before the Taconic Orogeny of the Ordovician. We can reconstruct the order in which things happened much easier.
The Ropes Creek metabasalt here is part of much larger formation that formed in a spreading center in an ancient ocean.
Some time in this period the ancient supercontinent of Rodinia that was formed by the Grenville Orogeny broke apart and formed the Iapetus Ocean. This ocean grew wider and wider as the "halves" of Rodinia moved apart. This created a central ocean rift in the Iapetus where convection cells in the mantle were able to break through the very thin oceanic crust and erupt as undersea volcanoes. Since ocean sediments continued to accumulate on the ocean crust and newly formed basalts, an ophiolite suite formed.
The southern and northern land masses, now named Gondwanaland and Laurasia, reversed course and began to move toward each other creating a convergent continental margin. Three different orogenies (mountain building events) happened with this collision: the Taconic during the Ordovician, the Acadian during the Devonian and the Alleghenian during the Permian. As each collision happened, material from the previous was highly altered making reading the rocks very difficult. The Taconic is generally thought to be centered more on what is now New England, but there is growing evidence that it had a major effect here as well. The Acadian thrust the microcontinent Avalonia which lay between Laurasia and Gondwanaland upon Laurasia. The Alleghenian was "the big one" where "Africa" collides with "North America" to create the new supercontinent Pangaea.
At some fairly early point in the Taconic, a trondhjemite magma formed from mixing both mafic oceanic crust and felsic continental crust. This was injected into the country rock, here, the Ropes Creek metabasalt. This accounts for the xenoliths (literally, strange rocks) of amphibolite in the trondhjemite gniess.
The orogenies piled masses of rock on top of each other, many kilometers thick. This created the high temperature and pressure regime where high grade metamorphism occurs and the "light" rocks and "dark" rocks of Kennesaw Mountain were transformed into gneiss and migmatite.
Pangaea began to break up some 250 million years ago creating a diverging continental margin. The inexorable force of erosion now becomes the major geologic event and given this much time, much material is washed into the sea and the mountains reduced greatly in size and the deeply buried rocks created during the orogenies is now at the surface of the earth.
The Ropes Creek metabasalt here is part of much larger formation that formed in a spreading center in an ancient ocean.
Some time in this period the ancient supercontinent of Rodinia that was formed by the Grenville Orogeny broke apart and formed the Iapetus Ocean. This ocean grew wider and wider as the "halves" of Rodinia moved apart. This created a central ocean rift in the Iapetus where convection cells in the mantle were able to break through the very thin oceanic crust and erupt as undersea volcanoes. Since ocean sediments continued to accumulate on the ocean crust and newly formed basalts, an ophiolite suite formed.
The southern and northern land masses, now named Gondwanaland and Laurasia, reversed course and began to move toward each other creating a convergent continental margin. Three different orogenies (mountain building events) happened with this collision: the Taconic during the Ordovician, the Acadian during the Devonian and the Alleghenian during the Permian. As each collision happened, material from the previous was highly altered making reading the rocks very difficult. The Taconic is generally thought to be centered more on what is now New England, but there is growing evidence that it had a major effect here as well. The Acadian thrust the microcontinent Avalonia which lay between Laurasia and Gondwanaland upon Laurasia. The Alleghenian was "the big one" where "Africa" collides with "North America" to create the new supercontinent Pangaea.
At some fairly early point in the Taconic, a trondhjemite magma formed from mixing both mafic oceanic crust and felsic continental crust. This was injected into the country rock, here, the Ropes Creek metabasalt. This accounts for the xenoliths (literally, strange rocks) of amphibolite in the trondhjemite gniess.
The orogenies piled masses of rock on top of each other, many kilometers thick. This created the high temperature and pressure regime where high grade metamorphism occurs and the "light" rocks and "dark" rocks of Kennesaw Mountain were transformed into gneiss and migmatite.
Pangaea began to break up some 250 million years ago creating a diverging continental margin. The inexorable force of erosion now becomes the major geologic event and given this much time, much material is washed into the sea and the mountains reduced greatly in size and the deeply buried rocks created during the orogenies is now at the surface of the earth.
