From a geomorphological point of view, the plains predominate in Argentina; given the vastness of the territory, the mountainous areas are naturally also very extensive, corresponding to the eastern slope of the Andes, which is not steep and unitary, but rather dismembered and preceded by a mountainous system (pre-European system) or, better, by a large foreland with generally sweet and monotonous shapes: these are the oldest folds of the Andes in direct contact with the rigid and basal masses of the continent. These masses form the substratum of the Argentine plains as well as of Patagonia, the long southern appendage of the continent that extends as far as the Strait of Magellan beyond which is the mountainous Tierra del Fuego., of which the eastern section belongs to Argentina, a country of South America defined by militarynous, com. The crystalline base emerges, with its Precambrian and ancient Paleozoic rocks, only in a few areas of Argentina and precisely in the belt of the pre-Andean reliefs and on the mountainous ridges that precede, on the edge of the Pampa plains, the Patagonian plateaus, in the northern section of the Andes, where vertical dislocations of rigid plates have occurred that today form the plateaus (located at over 3000 m high) connected to the Chilean and Bolivian highlands. Elsewhere the archaic formations of the substrate are covered by Mesozoic, Cenozoic and recent sedimentations. The latter, consisting of both alluvial deposits and wind energy, cover all the plains: to the NE the so-calledArgentine Mesopotamia, the lowland bordered by the Uruguay and Paraná rivers; to the N the Gran Chaco, the wide piedmont plain between the Pilcomayo and Salado rivers crossed by the lazy rivers that flow down from the Andes; finally the Pampa, the immense flat region extending from the pre-coastal reliefs to the sea, between the Río de la Plata and the Paraná in the N up to the Río Colorado in the S, beyond which the Patagonian plateaus begin. Throughout the Argentine section of the Andes, morphologically very varied, there are extensive volcanic formations and intrusive, granite masses; the latter predominate in the extreme southern section of the chain, S del Tronador (3491 m), characterized by an orography rather irregular shaped by glacialism, with transverse valleys, tectonic depressions and numerous lake basins. The central section is the steepest and is dominated by Aconcagua (6959 m), the top of the continent, and by other very high mountains, such as the Mercedario (6769 m). There are few and difficult passes, including that of Cumbre or Bermejo. The northern section is characterized by majestic volcanic cones (Llullaillaco, 6739 m; Socompa, 6031; etc.), which rise above the plateaus, called Puna de Atacama, which can be accessed through the long longitudinal valleys.
From the hydrographic point of view, the Argentine territory is divided between the Río de la Plata-Paraná basin and, to a small extent, the autonomous basins of the rivers that descend directly from the Andes to the Atlantic. The Río de la Plata-Paraná basin is, after the Amazonian one, the largest in South America, covering over 3 million km². It is formed by the Paraná, which descends from the Brazilian highlands, by the Río Paraguay, which receives the waters of a large section between the Amazon watershed and the Bolivian Andes, and finally by the Andean rivers (Río Salado, etc.), which cross the Gran Chaco and the piedmont plains towards SSE, attracted to where the Platense lowland has its greatest depression: this is how that Río de la Plata originateswhich is at the same time an estuary and a continental recess of extraordinary breadth and which also reaches Uruguay, which marks the border of Argentina with the homonymous state, from which it collects the waters. Majestic and with a mature course, the Paraná is navigable and, with the Río Paraguay, forms a precious communication route for the country. Of the Andean rivers that directly flow to the Atlantic, the most important is the Río Colorado; follow the Patagonian rivers (Río Negro, Chubut, Deseado, etc.) which have an almost parallel course, from W to E, and affect the sedimentary plateaus of the region flowing into the sea with deep estuaries. However, there is no drainage to the sea in a vast area of the country corresponding to the Andean highlands and the more arid and difficult drainage areas of the Gran Chaco and the Pampa, where the streams are lost in the salinas (basins covered by salt encrustations), in the bañados (marshes) and in other transitional basins between lake and salt, such as the Mar Chiquita lagoon. Real lakes, some of which are also large and famous for their scenic beauty (such as Nahuel Huapi, Buenos Aires, Viedma, Argentino) are present in the Patagonian Andes and have typically alpine characteristics.