Answer:
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 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, 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.
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.
Excerpt and Graphic from:
USGS/NPS Geology in the Parks Website, Atlantic Plain Province, August 2001
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