ORIGEM DOS RELEVOS DO MACIÇO SIENÍTICO DO MENDANHA, RJ, COM BASE NAS ANÁLISES GEOMORFOLÓGICAS E SUA RELAÇÃO COM A HIPÓTESE DO VULCÃO DE NOVA IGUAÇU

This article presents the landform origin of the Mendanha Massif, State of Rio de Janeiro, Brazil, based on geomorphological analyses by summit-level map. The summit level maps show the presence of the Central Plateau and the lateral scarps of 700 m high. No geomorphologic indicators of the volcano have been observed. The Central Plateau is deeply eroded by drainages of low-angle slope. The scarps have short, shallow, and steep drainages. These characteristics are due to the high erosive resistance of the syenitic body based on a peculiar weathering phenomenon namely weathering passivity. That is, weathered surface constituted mainly by clay minerals makes difficult the surface water percolation into the rock body and consequent weathering advance. The linear scarps can be originated from a coffee filter-like three-dimensional form of the syenitic body.