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 Kentucky volcanic features or events and is by no means inclusive.]
The Geologic Map of Kentucky shows the distribution of sedimentary strata totaling as much as 15,000 feet in thickness and ranging in age from Middle Ordovician to Holocene, with minor amounts of Permian intrusive rocks.
Kentucky lies mostly within the "central stable region"
of the North American Continent. The State includes parts
of four major structural provinces of the
eastern midcontinent -- the Illinois and Appalachian basins with the
intervening Cincinnati arch, and the Mississippi Embayment
in the west. These structural provinces, apparently
initiated before the mid-Paleozoic, are broad, shallow,
crustal warps generally measuring 200 or more miles across and,
except for the Appalachian basin, with only a mile or
two of structural relief. These regional structures locally have been
deformed into smaller scale gentle folds and have been
cut by high-angle faults and grabens of small displacement.
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The Appalachians:3
The Interior Plains:3 The Interior Plains is a vast region that spreads across the stable core (craton) of North America. This area had formed when several small continents collided and welded together well over a billion years ago, during the Precambrian. Precambrian metamorphic and igneous rocks now form the basement of the Interior Plains and make up the stable nucleus of North America. With the exception of the Black Hills of South Dakota, the entire region has low relief, reflecting more than 500 million years of relative tectonic stability. The Atlantic Plain:3 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 Atlantic 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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| Kentucky's Precambrian |
Kentucky's Precambrian:1
Known Precambrian history in this area
began with the
emplacement of a vast, horizontal,
7-mile-thick layered sheet of granite
(coarse-grained igneous rock formed at depth) and rhyolite
(fine-grained volcanic equivalent of granite formed near the surface)
beneath western Ohio and neighboring states
to the west.
This emplacement has been attributed to an
uprising in the Earth's mantle, known as a superswell.
Radioisotopic dating suggests that this event took
place between about 1.4 and 1.5
billion years ago, forming what geologists call the
Granite-Rhyolite Province.
Continued continental doming of the superswell caused
the crust beneath western Ohio, Indiana,
and Kentucky to extend and split (rifting), resulting
in major faulting and consequent downdropping
to form a complex rift basin, now known as the
East Continent Rift Basin. Molten
basalt flowed
upward as erosion began to fill the basin with
clastic sediment, perhaps as much as 20,000 feet
thick in some places. This extensive deposit is
known as the Middle Run Formation. About 1 billion
years ago, doming ceased and the rift became a
failed or aborted rift. Rifting, volcanic activity, and
basin filling also ceased.
| Kentucky's Volcanic Rocks |
Kentucky Rocks:2
Most of the rocks found in Kentucky are sedimentary.
Sedimentary rocks are formed
from (1) the weathering and transport of pre-existing rocks and (2) the chemical precipitation of
sediments. Examples of sedimentary rocks are limestones, sandstones, and shales.
Igneous rocks
result from the cooling of molten rock or magma to create rocks like
granites, basalts, and rhyolites.
Metamorphic rocks
have been physically and mineralogically changed by heat and
pressure to form another type of rock; for example, the sedimentary rock limestone will become
the metamorphic rock marble; the sedimentary rock shale will become the metamorphic rock
slate; and the igneous rock granite will become the metamorphic rock gneiss (pronounced nice).
Kentucky's Igneous Rocks:2
Igneous and metamorphic rocks
are not common in Kentucky
but have been observed in glacial
drift in northern Kentucky, and have been
found as constituents in sandstones in eastern Kentucky
and in very deep wells drilled throughout the State.
Igneous rocks
are very rare in surface exposures in Kentucky.
They are formed by the
cooling and solidification of molten rock (magma) that originated deep within the earth.
The two main types of igneous rocks are intrusive, which formed when magma cooled slowly and
hardened beneath the earth's surface, and extrusive,
which formed when magma solidified after it
reached the surface.
Kentucky's Oldest Rocks:
The oldest rocks at the surface in the State
occur in central Kentucky, because older
rock strata is pushed upward along a broad bulge,
called the Cincinnati Arch, which in Kentucky,
stretches from Covington in the north, to just west of
Dale Hollow Lake in the south.
Below the sedimentary rocks are igneous and
metamorphic rocks.
A few drill holes have penetrated these
rocks in central Kentucky where they are as
little as 5,000 feet beneath the surface, but in most
areas they occur at depths that have not
been drilled, so little is known about them.
Other igneous rocks such as
granites, basalts, gneisses, and amphibolites occur in the
deep subsurface of Kentucky.
Most of these
rocks occur at depths of 5,000 to 15,000 feet
below the surface, and samples of these rocks
are encountered when oil or mineral well tests are
drilled.
Kentucky's Glacial Deposits:2
Igneous rocks
have been found in glacial deposits in northern Kentucky.
Kentucky's Peridotite:2
In Kentucky,
a dark-colored
igneous rock, peridotite,
occurs in sills and dikes
(intrusions) in Elliott County in
northeastern Kentucky and Caldwell, Crittenden, and Livingston
Counties in western Kentucky.
Peridotites formed very deep in the earth near the mantle under
high temperature and pressure,
and were thrust explosively toward the surface where they
intruded into the host rocks.
Peridotite consists in part of the minerals olivine and pyroxene.
Excerpts from:
1) Michael C. Hansen, The Geology of Ohio -- The Precambrian,
GeoFacts No. 13,
Ohio Department of Natural Resources, Division of Geological Survey,
Ohio Geological Survey Website, July 2001
2) Kentucky Geological Survey Website, July 2001
3) USGS/NPS Geology in the Parks Website, August 2001
4) Robert C. McDowell (ed.), 2001,
The Geology of Kentucky -- A Text to Accompany the Geologic Map of
Kentucky: USGS Professional Paper 1151-H
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