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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
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Map, Location of Louisiana

Volcanic Highlights and Features:
[NOTE: This list is just a sample of various Louisiana 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 Louisiana.]

  • Louisiana
  • Louisiana Regions
  • Louisiana's Volcanic Rocks in South America


The surface of Louisiana is underlain by geologically young sedimentary sequences that were deposited in or adjacent to rivers and deltas in a coastal plain setting. These deposits and those in other states flanking the Mississippi valley indicate that a major river system corresponding to the Mississippi has persisted at least since the Gulf of Mexico began to form by the separation of North America from South America. In general, the entire suite of fluvial, deltaic, and coastal deposits has advanced farther into the Gulf through time, and continues now to fill it with sediment. Evaporation of the shallow sea that was the early Gulf of Mexico produced thick salt deposits, now deeply buried, from which salt domes intrude the overlying strata. Most surface exposures in Louisiana consist of Quaternary sediment.

Louisiana's Oldest Rocks
The oldest exposed rocks in Louisiana are small patches of Late Cretaceous marine rocks that outcrop along the edges of the Prothro and Rayburns salt domes in Bienville Parish. At both salt domes, pits excavated for the extraction of agricultural lime exposed these rocks. Cretaceous fossils have reportedly been collected from the vicinity of the King salt dome in Bienville Parish. However, the outcrops that produced these fossils have been lost. Only the Late Cretaceous strata at the Rayburns salt dome has been studied in detail. Where exposed in a borrow pit, these often highly fossiliferous strata consist of blue-gray marls and chalks, hard white chalk, greensand, gray chalky marl, and olive-gray marl. The lithology of these strata and the fossils found in them indicate that they consist of formations that also outcrop in Arkansas. About 70 to 82 million years ago these strata accumulated at the bottom of a shallow sea that covered Louisiana. The next oldest strata consist of a small outcrop in Caddo Parish, and consist of marine mudstones exposed along the southeast edge of Caddo Lake. They accumulated about 55 to 66 million years ago.

Excerpts from: Louisiana Geological Survey Website, 2001

Louisiana Regions

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.


Louisiana's Volcanic Rocks in South America

Louisiana's Volcanic Rocks in South America:2
Exposed in the Andes of Argentina are ancient rocks that, hundreds of millions of years ago, once underlayed a part of Louisiana. These rocks consist of a block of granitic continental crust over 120 miles (200 kilometers) wide, capped by 3,000 to 4,900 feet (900-1500 meters) of 470 to 570 million year-old sedimentary strata. These strata are composed of fossiliferous limestone and dolomite overlying shale and sandstone. This block of continental crust called the "Precordillera Terrane," once was part of North America. However, opening of the Iapetus Ocean, a predecessor to the Atlantic Ocean, rifted the Precordillera Terrane away from North America, leaving oceanic crust where Louisiana now lies. Then, plate tectonics caused the block to drift across the ancient Iapetus Ocean. Eventually, this feature collided with and was welded onto a continental land mass that became South America.

Excerpts from:
1) Louisiana Geological Survey Website, July 2001
2) Louisiana Geological Survey Website, Louisiana GeoFacts, July 2001
3) USGS/NPS Geology in the Parks Website, September 2001

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01/28/03, Lyn Topinka