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:
[This list is just a sample of
various Appalachian Mountains volcanic features or events and is by no means inclusive.]
The Appalachians are old. A look at rocks exposed in today's Appalachian
mountains reveals elongate belts of folded and thrust faulted marine
sedimentary rocks, volcanic rocks
and slivers of ancient ocean floor. Strong evidence that these rocks
were deformed during plate collision. The birth of the
Appalachian ranges,
some 480 million years ago, marks the first of several mountain
building plate collisions that culminated in the
construction of the supercontinent Pangea with the
Appalachians near the center.
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The Earth is approximately 4.5 billion years old. In the Southern Appalachians, the history of the last billion years is recorded in the rocks. By reading the rocks, we can trace their incredible history. BREAK UP OF A SUPERCONTINENT: The rocks at the core of the Appalachian Mountains formed more than a billion years ago. At that time,all of the continents were joined together in a single supercontinent surrounded by a single ocean. About 750 million years ago, the crust of the supercontinent began to thin and pull apart. As the crust expanded, a deep basin -- the Ocoee -- formed in what is now the western Carolinas, eastern Tennessee, and northern Georgia. Seawater filled the basin. Sediments formed by the weathering of surrounding hills were transported by water and deposited in layers on the floor of the basin. Over a long period of time, a great thickness of sediments accumulated. These sediments now form the bedrock of the Great Smoky Mountains. Within these sediments, minerals like pyrite and metals like copper were deposited. At the same time that the sediments were being laid down volcanoes were erupting in present-day Virginia, the Carolinas, and Georgia. Lava from some volcanoes flowed in slow moving sheets, but some eruptions were explosive. Then, about 540 million years ago, the supercontinent split into pieces that drifted away from each other. Seawater spread into low areas between crustal plates and,in time, formed new oceans. A shallow sea covered most of what is now the United States. CONTINENTAL COLLISION:
About 470 million years ago, the motion of the crustal plates
changed, and the continents began to move toward each other.
Eventually, about 270 million years ago, the continents ancestral to North
America and Africa collided. Huge masses of rock were pushed west-ward
along the margin of North America and piled up to form the
mountains that we know as the Appalachians.
As blocks of continental crust rode across one another, some rocks
became so hot that they melted. Some of the molten rock remained
deep below ground. There, it cooled slowly and crystallized to form
bodies of rock that are called plutons. Granite is an example of an
plutonic rock. Some molten rock cooled very slowly and formed
coarse-grained veins called pegmatites. Pegmatites have been the
source of high-purity minerals (such as feldspar,quartz,and mica)
and gemstones (such as emeralds and beryl).
When continental masses collided with the edge of ancestral North
America, rocks were subjected to intense pressure and heat. Where the
temperature was high but below the melting point of the rocks, the rocks
deformed and recrystallized to become metamorphic rocks.
The components separated into bands, and some flowed with a consistency like that
of toothpaste. During metamorphism, temperatures and pressures can
vary. In less extreme conditions, original rock layers may be partly
retained. As a result, some minerals recrystallized in sheets, forming
rocks (slate or schist) that split easily into thin, smooth layers.
The collision of continental plates is also expressed in the rocks by folds (bends)and faults (breaks). Earthquakes happen because of slippage along a fault. Although earthquakes are now rare in the Southern Appalachians, during the time of continental collision, earthquakes were a common occurrence. ANOTHER CONTINENTAL BREAK UP:
Although a collision of continents caused the formation of the
Appalachian Mountains, the present-day margin of North America is
the result of a reversal in crustal plate movement. After the continents
collided, the continental mass began to pull apart. About 240 million
years ago, at the beginning of the age of the dinosaurs, a new ocean
basin began to form -- the present-day Atlantic. The widening of the
Atlantic Ocean at the mid-Atlantic Ridge is evidence of this continuing
movement.
CARVING THE MOUNTAINS: At the time they formed, the Appalachians were much higher than they are today -- more like the present-day Rocky Mountains. While the Atlantic Ocean was still in its infancy, the Appalachians were already being attacked by erosion. For the last 100 million years, erosion has carved away the mountains, leaving only their cores standing. Erosion continues today and is constantly altering the landscape of the Southern Appalachians.
Four times during the past 2 million to 3 million years, great
sheets of ice advanced steadily southward from the polar region. The
glaciers did not extend as far south as the Southern Appalachians, but
they triggered a change in climate that can be seen today in both the
rocks and the life of the region.
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| Appalachian National Scenic Trail |
Appalachian National Scenic Trail:4
The Appalachian National Scenic Trail is a
2,167-mile (3,488 kilometers) footpath along the ridge
crests and across the major valleys of the
Appalachian Mountains from Katahdin in Maine to
Springer Mountain in north Georgia. The trail
traverses Maine, New Hampshire, Vermont,
Massachusetts, Connecticut, New York, New
Jersey, Pennsylvania, Maryland, West Virginia,
Virginia, Tennessee, North Carolina and Georgia.
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Blue Ridge Mountains -
Blue Ridge National Parkway |
Blue Ridge Mountains:6
Most of the rocks that form the Blue Ridge are
ancient granitic and metamorphosed volcanic formations,
some exceeding
one billion years in age. By comparison, humans have been
associated with this land only about 9,000 years.
Blue Ridge National Parkway:5,8
The Blue Ridge Parkway extends 469 miles along the crests of the Southern Appalachians and links
two eastern national parks: Shenandoah and Great Smoky Mountains.
In many places along the Blue Ridge Parkway, there are metamorphic
rocks (gneiss) with folded bands of light-and dark-colored minerals,
which sometimes look like the folds and swirls in a marble cake.
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Great Smoky Mountains -
Great Smoky Mountains National Park |
Great Smoky Mountains:2
Great Smoky Mountains National Park lies within the Appalachian Blue Ridge
geologic and physiographic province. The highest mountains in eastern
North America occur in the Blue Ridge province, and some of the
highest peaks in this province are in the Great Smoky Mountains National Park.
The oldest rocks in the Blue Ridge province are at least 1 billion years
old and consist of metamorphosed sedimentary and
igneous rocks.
These Proterozoic rocks form the core of the ancient Appalachian Mountains.
Sediments deposited over these older rocks form the majority of rocks in the
Great Smoky Mountains National Park.
Great Smoky Mountains National Park Rocks:2
Most of the rocks in Great Smoky Mountains National Park
are metamorphosed sedimentary rocks. These sedimentary rocks
were formed from approximately 800 to 450 million years ago
from deposits of clay, silt, sand, gravel, and lime or calcium carbonate.
The oldest sedimentary rocks consist of vast amounts of
clastic material that washed down into lowland
basins from adjacent highlands.
Rocks of the ancient highlands are similar to the ancient granite,
gneiss, and schist found in the eastern parts of the
Great Smoky Mountains National Park.
Approximately 450 million years ago, the rocks were metamorphosed
or "changed" by heat and pressure. For example, sandstone was
changed to quartzite and shale to slate.
Last Phase:2
About 200 to 300 million years ago, the
last phase of Appalachian Mountain building,
(the Alleghenian orogeny) occurred when the North American continent
collided with the African and European continents,
closing the early Atlantic Ocean.
This process, part of a continuous mountain
building cycle known as "plate tectonics,"
ended sedimentation in the Appalachian region and uplifted the entire
Appalachian mountain chain from Newfoundland,
Canada to Georgia.
These mountains may have been much higher than they are today,
with elevations perhaps similar to the present-day Rocky Mountains.
As the continents collided, the original horizontal layers were bent
or folded and broken by fractures and faults.
Tremendous forces caused huge
masses of older rocks to be pushed westward, up and over younger rocks,
along nearly flat-lying faults. Rocks in the
Great Smoky Mountains National Park
moved westward on the Great Smoky Mountain fault.
Oldest Rocks in the Park:3
Granitic gneiss with foliation defined by bands of black biotite mica
and white plagioclase feldspar is exposed near Cherokee (Cades Cove).
At 1 billion years old they are the oldest rocks in the park.
| Shenandoah National Park |
Shenandoah National Park:6
Shenandoah National Park lies astride a beautiful
section of the Blue Ridge Mountains, which form the
eastern rampart of the Appalachian Mountains
between Pennsylvania and Georgia. The Shenandoah
River flows through the valley to the west,
with Massanutten Mountain, 40 miles long, standing
between the river's north and south forks.
The rolling Piedmont country lies to the east of the park.
Skyline Drive, a 105-mile road that winds
along the crest of the mountains through the length of the
park, provides vistas of the spectacular
landscape to east and west. The oldest rocks in Shenandoah
National Park were formed between
1 and 1.2 billion years ago. These granitic rocks can be seen at
Old Rag Mountain and Mary's Rock Tunnel.
Two other major rock types you can see in the park
include basalts, made from individual lava flows,
each 30 to 90 feet deep, formed about 570 million
years ago; and sedimentary rocks (including sandstone,
quartzite and phyllite)
formed later.
Columnar jointing in an ancient lava flow
in the Blue Ridge Mountains,
Shenandoah National Park, Virginia:7
The flow that contains the columns
of an extensive series of lava flows, each averaging about 200 feet thick,
poured over the land more than 570 million years ago. Columns formed as
cooling or shrinkage joints when a hot lava flow cools quickly; the columns form
perpendicular to the cooling surface.
Old Rag Granite:1
Old Rag
Mountain, Shenandoah
National Park, in the Blue
Ridge province of Virginia.
The craggy rock peak is exposed billion year old granitic gneiss called
the Old Rag Granite.
Blue
Ridge to the west is underlain by Late Proterozoic Catoctin Formation
which is comprised of massive 575 million year old basalt
metamorphosed to greenstone in the Middle Paleozoic. Blue Ridge
forms the provincial boundary with the Great Valley of the Valley and
Ridge province.
| White Top Mountain |
White Top Mountain:8
Although volcanic activity ended hundreds of millions of years ago,
rocks that formed from ancient volcanoes can still be seen at White Top
Mountain in southern Virginia. Some rocks in this area contain angular
fragments that had cooled and solidified, then later broke up, and were
engulfed in lava flow. Others contain small, irregular, mineral-filled
holes that formed as gases slowly bubbled up through hot lava.
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