A new model for the evolution of the Scotia Arc is presented. During the Late Jurassic-Early Cretaceous, contemporaneous with the initial separation of eastern and western Gondwanaland, a volcanic arc founded on continental crust broke away from the “south-western” margin of the supercontinent, allowing the intrusion of mafic magma directly behind the active arc (back-arc spreading). This led to the development of the mafic crust of an ensialic marginal basin. This basin extended from southern South America where ophiolites representing the relics of the uplifted floor of the basin have been recognized along the Andean Cordillera as far north as 51°S, through the central part of the Scotia Sea, which is shown to have a geophysical characteristics comparable to some of the western Pacific marginal basins, and probably southward into the Weddel Sea. The Falkland (Malvinas) Trough may represent the first manifestation of the back-arc extension. The Antarctic Peninsula, the southernmost, bathymetrically elevated, portion of the central Scotia Sea, and the Patagonian batholith along the Pacific margin of South America, represent the frontal volcanic arc.The eastern and western parts of the Scotia Sea floor were generated during the Cenozoic as South America and Antarctic separated from Africa and by differential motion of the South American and Antarctic plates.The model, together with new geologic data, leads to a new reconstruction of south-western Gondwanaland in which the Paleozoic/Early Mesozoic rocks of southern South America and the Antarctic Peninsula represent the arc—trench tectonic environment of the “Samfrau geosyncline”.