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Geologic Provinces of the United States: Atlantic Plain Province

clickable province index map Atlantic Coastal Plain Pacific Colorado Plateau Ozark/Ouachita Interior Highlands Appalachian Highlands Laurentian Upland Columbia Plateau Interior Plains Basin and Range Rocky Mountains
Atlantic Plain

Atlantic Plain Province

This is the flattest of the provinces. It stretches over 2200 miles in length from Cape Cod to the Mexican border and southward another 1000 miles to the Yucatan Peninsula.

List of National Parks exhibiting Atlantic Plain Geology

The Atlantic plain slopes gently seaward from the Inland Highlands in a series of terraces, continuing far into the Atlantic and Gulf of Mexico; it forms the continental shelf. The relief at the land-sea interface is so low that the boundary between them is often blurry and indistinct, especially along stretches of the Louisiana bayous and the Florida Everglades. This means that if you looked at a shaded relief map of the region without sea-level displayed it would be difficult to identify the exact location of the coast itself. For an example of a coastal profile, click here.

Mangroves Mangrove swamps obscure the shoreline. Everglades National Park. Photo by Marli Miller

The break up of a supercontinent

This region was born during the breakup of the supercontinent Pangea in the early Mesozoic Era. From about 280-230 million years ago, (Late Paleozoic Era until the Late Triassic) the continent we now know as North America was continuous with Africa, South America, and Europe.

Pangea before break-up
Pangea before break up.

Pangea first began to be torn apart when a three-pronged fissure grew between Africa, South America, and North America. Rifting began as magma welled up through the weakness in the crust, creating a volcanic rift zone. Volcanic eruptions spewed ash and volcanic debris across the landscape as these severed continent-sized fragments of Pangea diverged.

The gash between the spreading continents gradually grew to form a new ocean basin, the Atlantic. The rift zone known as the mid-Atlantic ridge continued to provide the raw volcanic materials for the expanding ocean basin.

Meanwhile, back at the edge...

North America was slowly pulled westward away from the rift zone. The thick continental crust that made up the new east coast collapsed into a series of down-dropped fault blocks that roughly parallel today's coastline. At first, the hot, faulted edge of the continent was high and buoyant relative to the new ocean basin. As the edge of North America moved away from the hot rift zone, it began to cool and subside beneath the new Atlantic Ocean. This once-active divergent plate boundary became the passive, trailing edge of westward moving North America. In plate tectonic terms, the Atlantic Plain is known as a classic example of a passive continental margin.

Pangea before break-up
Present configuration of the seafloor and continents.Click here to view unlabeled map. Image adapted from NOAA.
Pangea before break-up
Present configuration of the seafloor and continents.Click here to view unlabeled map. Image adapted from NOAA.

Sediments eroded from the Appalachian and other inland highlands were carried east and southward by streams and gradually covered the faulted continental margin, burying it under a wedge, thousands of feet thick, of layered sedimentary and volcanic debris.

Today most Mesozoic and Cenozoic sedimentary rock layers that lie beneath much of the coastal plain and fringing continental shelf remain nearly horizontal or tilt gently toward the sea.

Mid-Atlanic Ridge labeled
Present configuration of the seafloor and continents. Click here to view unlabeled map. Image adapted from NOAA.

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