America's Volcanic Past
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| "Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994 |
Volcanic Highlights and Features:
| [NOTE: This list is just a sample of various Georgia features or events and is by no means inclusive. All information presented here was gathered from other online websites and each excerpt is attributed back to the original source. Please use those sources in referencing any information on this webpage, and please visit those websites for more information on the Geology of Georgia.] |
Georgia consists of four distinct geologic regions.
From northwest to southeast, those four regions are the Valley and Ridge, The Blue
Ridge, the Piedmont, and the Coastal Plain. All of these geologic regions extend into the surrounding states, but Georgia is the
only state south of Virginia to have all four regions.
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The Valley and Ridge: 3,5 The Valley and Ridge extends southwest into Alabama to around Birmingham and northeast in Tennessee from Chattanooga to Knoxville and beyond. In Georgia, the eastern and southern boundary of the Valley and Ridge is the Cartersville-Great Valley fault system, which runs south from Chatsworth to a point southeast of Cartersville and then west to the Polk-Haralson County line. The Valley and Ridge consists of Paleozoic sedimentary rocks that have been folded and faulted to cause long northeast-southwest-trending valleys and ridges that give the region its name. The rocks of the Valley and Ridge formed under a vast, shallow, inland sea that covered the area. Shells and other hard parts of ancient marine plants and animals accumulated to form limey deposits that later became limestone. The weathering of limestone, now exposed at the land surface, produces the fertile, lime-rich soils that are so prevalent in the Great Valley.
The Blue Ridge:3 Georgia is the southwest end of the Blue Ridge, which extends northeast to Virginia through Great Smoky Mountain and Shenandoah National Parks. The southern boundary of the Blue Ridge in Georgia depends on one's perspective. A purely topographic approach would limit the Blue Ridge to just a few ridges extending southwestward from North Carolina, so that the Piedmont would extend all the way to the Georgia-Tennessee state line. Some geologists, in contrast, would extend the Blue Ridge region all the way to the Brevard Fault zone, which runs through northwest Atlanta and Gainesville. One of the most commonly accepted boundaries, which is based on changes in rock types, would run just southeast of Canton, Dawsonville, Dahlonega, and Helen. The Blue Ridge region forms the North Georgia Mountains or the southern Appalachians. Some of the highest points are balds like Brasstown Bald. The topography is not as patterned as that in the Valley and Ridge, and drainage systems are generally dendritic, whereas they are linear in the Valley and Ridge. The Blue Ridge is a region of low-to-high-grade metamorphic rocks. Many of the rocks of the Blue Ridge appear to be the metamorphosed equivalents of Proterozoic and/or Paleozoic sedimentary rocks. Others are metamorphosed igneous rocks, such as the Corbin Metagranite, the Fort Mountain Gneiss, various mafic and ultramafic rocks, and the metavolcanic rocks of the Gold Belt.
The Piedmont:2,3 The Piedmont physiographic province of Georgia is composed of hard igneous and metamorphic rocks derived from the recrystallization of ancient (300 to 600 million year old) sediments that were once deeply buried and subjected to high temperatures and pressures. They were re-exposed during a collision between the African and North American Continents about 250 to 300 million years ago. The Piedmont is a region of moderate-to-high-grade metamorphic rocks, such as schists, amphibolites, gneisses and migmatites, and igneous rocks like granite. Topographically, the Piedmont mostly consists of rolling hills, although faulting has produced the impressive ridge of Pine Mountain near Warm Springs. Isolated granitic plutons also rise above the Piedmont landscape to give prominent features like Stone Mountain. Piedmont rocks have been extensively mined for a variety of different minerals and the local granite and marble are shipped worldwide as building stone.
The Fall Line:3 The Fall Line is the boundary between the Piedmont and the Coastal Plain. Its name arises from the occurrence of waterfalls and rapids that are the inland barriers to navigation on Georgia's major rivers. Thus the cities of Columbus, Macon, Milledgeville, and Augusta developed where boats had to be unloaded on the Chatahoochee, Ocmulgee, Oconee, and Savannah Rivers, respectively. Those waterfalls and rapids occur where the rivers drop off the hard crystalline rocks of the Piedmont onto the more readily eroded sedimentary rocks of the Coastal Plain. The Coastal Plain (also known as the Atlantic Plain):1,2 Savannah is located in the Coastal Plain physiographic province of Georgia. The Coastal Plain is a wedge of poorly consolidated sand, clay, and limestone that thickens in a coastward direction from the midpoint of the State (the Fall Line). The sediments of the Coastal Plain were deposited by ancient seas and river deltas that periodically dominated the landmass over the last 120 million years. At Savannah, this sedimentary wedge is approximately 4,000 feet (or 1,200 meters) thick, and is an important economic resource for the region. The Coastal Plain (also called the Atlantic Plain) is the flattest of the provinces. It stretches over 2,200 miles in length from Cape Cod to the Mexican border and southward another 1000 miles to the Yucatan Peninsula. The plain slopes gently seaward from the inland highlands in a series of terraces. This gentle slope continues far into the Atlantic and Gulf of Mexico, forming the continental shelf. This region was born during the breakup of the supercontinent Pangea in the early Mesozoic Era.
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| Georgia's Early Volcanoes |
Sediment and Volcanoes:4
During the early part of the expansion of the
continental crust (about 750 million years ago),
a deep basin, known as the Ocoee basin,
formed on the margin of the supercontinent
what is now the western Carolinas,
eastern Tennessee, and
Georgia. Seawater filled
the basin. Rivers from the surrounding
countryside carried clay,
silt, sand, and gravel to the basin,
much as rivers today carry
sediment from the mid-continent
to the Gulf of Mexico.
The sediment spread out in layers on
basin floor. The basin continued
to subside, and over a long
of time, probably millions of
years, a great thickness of sediment accumulated.
The sediments of the Ocoee basin
now form the bedrock of the Great
Smoky, Unicoi, and Plott Balsam
Mountains.
At the time that sediments were being
deposited and mineral deposits were
forming in the Ocoee basin,
volcanoes
were erupting in areas that are now
Virginia, the Carolinas, and Georgia.
Lava from some volcanoes flowed in
slow-moving sheets like lava from the
Hawaiian volcanoes, but other
eruptions were explosive, like Mount St. Helens.
Although volcanic activity
ended hundreds of millions of years ago,
rocks that formed from these ancient
volcanoes are still visible.
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| Granitic Plutons |
Granitic Plutons:6
Granitic plutonic rocks underlie large areas in the crystalline terrane of Georgia and Alabama. At present, the number of radiometric data coupled with geologic data indicate three moajor groups of granitic plutons: 1) Cambrian (and Early Ordovician?) plutons; 2) Silurian-Devonian plutons; and 3)
Carboniferous plutons. The Cambrian plutons appear to have been the result of island-arc or ocean-ridge volcanism, whereas the younger plutons appear to have been derived through anatexis (melting of pre-existing rock) of lower thrust sheets.
| Stone Mountain |
Stone Mountain:3
The Piedmont is a region of moderate-to-high-grade metamorphic rocks,
such as schists, amphibolites, gneisses and
migmatites, and igneous rocks like granite.
Topographically, the Piedmont mostly consists of rolling hills, although faulting has
produced the impressive ridge of Pine Mountain near Warm Springs.
Isolated granitic plutons also rise above the Piedmont
landscape to give prominent features like Stone Mountain.
Excerpts from:
1) USGS/NPS Geology in the Parks Website, 2001
2) USGS Georgia Water Resources Website, 2003
3) University of Georgia Geology Department Website, 2003
4) Clark, et.al., Birth of the Mountains: The Geologic Story of the
Southern Appalachian Mountains: USGS General Interest Publication
5) Clark, et.al., 2001, Teacher's Guide for the U.S. Geological Survey Video -- The Southern Appalachians, A Changing World.
6) Higgins, M.W., Atkins, R.L., Crawford, T.J., Crawford, III, R.F., Brooks, R., and Cook, R.B., 1988, The Structure, Stratigraphy, Tectonostratigraphy, and Evolution of the Southernmost Part of the Appalachian Orogen: U.S. Geological Survey Professional Paper 1475.
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