Passive margin

A continental margin between oceanic and continental crust that is not an active plate boundary and has little tectonic activity. It commonly develops as a continent rifts apart and new oceanic crust forms.

Continental rifting: The stretching and thinning of continental lithosphere that can split a continent apart. Rifting initiates the transition from continental crust to a passive ocean basin.
Continental crust: The relatively thick, low-density crust that underlies continents and continental shelves. It forms the landward side of the boundary across a passive margin.
Active margin: A continental margin that coincides with an active plate boundary and experiences substantial tectonic activity. It contrasts with passive margins, which lie within plates rather than at their boundaries.
U.S. Atlantic margin: The passive continental margin along the eastern United States, extending from the shoreline to the deep Atlantic basin. It illustrates a sediment-rich margin formed as the Atlantic opened.
Seafloor spreading: The creation of oceanic crust as magma rises at a divergent plate boundary and moves outward. It forms oceanic crust after rifting separates the continental plates.
Oceanic crust: The relatively thin, dense crust that forms the floor of most ocean basins. It forms the seaward side of the continental-oceanic transition.
Subduction: The descent of one tectonic plate beneath another into the mantle. Subduction commonly drives the earthquakes and volcanism absent from quiet passive margins.
Brazilian margin: The passive continental margin along Brazil's Atlantic coast, developed during South Atlantic opening. Its basins preserve the history of rifting between South America and Africa.
Thermal subsidence: The gradual sinking of lithosphere as it cools, contracts, and becomes denser. Cooling after breakup helps deepen the margin and create space for sediment.
Lithosphere: Earth's rigid outer shell, comprising the crust and the uppermost mantle. Its deformation and cooling govern margin formation and long-term subsidence.

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