The Maciço de Gericinó-Mendanha stands out in the landscape of Rio de Janeiro's west zone as a distinct morphostructural unit, both for its altitude and for the singular nature of its geological substrate. Unlike the crystalline massifs formed by gneisses and granites typical of the Rio de Janeiro basement, the Gericinó-Mendanha is structured on alkaline rocks, the product of magmatic intrusions that broke through the continental crust during the Cretaceous period, around 80 to 100 million years ago.
Image description: This is an aerial photograph of a city. On the left side, we see green areas with dense vegetation. In the center, there is a large concentration of residential buildings, varying in size and color. On the right, the city stretches to the horizon, where distant mountains stand out under a clear blue sky.The imposing Mendanha overlooking Rio de Janeiro's northern lowlands. In the foreground on the left, the Camboatá Forest, the subject of the controversial racetrack construction project.
It is, therefore, an intrusive alkaline massif, whose predominant rocks include syenites and others rich in minerals typical of alkaline-potassic magmatism. This characteristic gives the massif a distinctive geochemical composition and greater resistance to erosion, which is directly reflected in the landforms.
Its elevations rise above 800 meters, with Pico do Guandu being the highest at 974m. Its mountain ranges basically form three large ridges: Serra de Madureira, with Pico do Guandu, Morro do Gericinó, Pico da Furna da Andorinha; the Serra do Mendanha with Morro do Salvador, Morro do Mariano and Morro do Guandu; and the small Serra do Marapicu, with Morro do Marapicu and Morro do Vieira.
Image description: This is an aerial photograph of a mountain massif covered by dense vegetation. On the left, we see a large green hill, while on the right, there is a flatter area with green fields and some scattered buildings. On the horizon, in the background, is a distant city, with buildings and streets visible. The sky is light blue with some white clouds.Serra do Mendanha, the first spur aligned parallel to the Serra de Madureira, in the Maciço do Gericinó-Mendanha. Forest remnants preserved under the Parque Municipal Natural do Mendanha.
Its shapes are like domes and ridges that have resisted dissection. Over time, chemical weathering processes acted on these alkaline rocks, forming weathered mantles with shallow soils and residual boulders that break loose from the slopes. Of course, susceptibility to mass movements is part of its geomorphological evolution process. However, its slopes are protected by state and municipal conservation units, but even so, in its surroundings, especially in areas occupied by urban expansion, the need for geotechnical risk prevention is unquestionable.
Image description: This is an aerial photograph of a mountain range. From left to right, we see a series of green hills covered in dense vegetation. In the center, there is a more pronounced elevation with a triangular shape. Moving further to the right, we observe urbanized areas with houses and buildings scattered across the base of the hills. In the background, we see an extensive city, with many buildings and streets. Further in the distance, other mountains can be seen. The image is in vivid colors, with green tones predominating on the hills and light blue in the sky.Serra do Quitungo, a small ridge associated with the Gericinó-Mendanha Massif, running parallel to the Serra do Mendanha. Remaining agropastoral activities keep the area deforested, with Baía de Sepetiba in the background.
The compartmentalized morphology of this relief, marked by linear valleys and steep slopes, reveals the structural control exerted by fracture and/or fault zones that follow the preferential directions of the tectonic processes the massif underwent. The alignment of the ridges and rivers naturally follows the Northeast-Southwest direction, typical of the regional geology. Rivers such as the Sarapuí, the Cabuçu, and tributaries of the Guandu river have their headwaters set into the slopes of the massif, flowing along zones of structural weakness and contributing to the erosive shaping of these slopes.
Image description: This is an aerial photograph of a city surrounded by mountains. From left to right, we see densely urbanized areas with many buildings close together. In the center, there are green areas with dense vegetation, while in the background, mountains covered with vegetation form a distant horizon. The sky is light blue with a few scattered clouds. There is no visible text in the image.Foothills of Serra do Quitungo, in the Gericinó-Mendanha Massif, which suffers from urban expansion over what remains of agriculture in the West Zone. In the middle of the photo, the alignment of Serra do Retiro, one of the isolated ranges in the northern lowlands, and, in the background, the slopes of the Maciço da Pedra Branca.
In the transitions between the massif and the adjacent lowlands — such as those of the Realengo, Campo Grande, and Bangu neighborhoods — the rocky bedrock is partially covered by slope deposits and immediately gives way to recent fluvial plains, which mark the areas of sedimentary accumulation at the lowest elevations. This interface between morphological domains is also a territory of forest and urban tension, where occupation pressures frequently meet the geotechnical risks of altered slopes and deforestation.
Image description: This is a photograph of a mountain range under a clear blue sky. On the left side, we see a densely populated city with buildings and streets. Moving toward the right, the terrain gradually rises, revealing green areas covered by dense vegetation. In the center-right, there is a more prominent mountain, its summit covered with trees. The horizon is filled with other distant mountains, forming a continuous line. The sky is blue, with few clouds visible.Alignment of the Serra de Madureira mountain range in the Gericinó-Mendanha Massif, ending with Morro do Marapicu.
The Maciço de Gericinó-Mendanha, due to its unique geological makeup and its strategic position within the physical compartmentalization of Rio's territory, functions as a witness block of the magmatic history of Southeastern Brazil, revealing events related to the opening of the Atlantic Ocean. Through its relief, its very existence bears witness to the peri-urban expansion and rural remnants of a municipality under full real-estate speculation and environmental urgency.
