America's Volcanic Past
|"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 Kentucky 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 Kentucky.]|
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.
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.
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|
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.
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.
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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