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.
