Natural Conditioning Factors in the Landscape Modeling of the Ilhabela Archipelago, Brazil

The Ilhabela archipelago, located off the northern coast of São Paulo State, Brazil, is renowned for its remarkable biodiversity. The integration of geospatial data with geological, soil, and vegetation maps has enabled the identification of three distinct regions with notable geomorphological characteristics: the northern, eastern, and southern sectors. Each sector displays a well-defined relief, characterized by dissected, heterogeneous terrain, significant elevations, and steep slopes. These features contribute to the formation of deep valleys and abrupt escarpments. The local geology comprises crystalline basement rocks intruded by several Cretaceous alkaline bodies, which exhibit varying resistance to erosion. These lithological differences have been pivotal in shaping prominent escarpments and sharply defined ridges. The predominantly dendritic surface drainage pattern reflects the interplay between the geological framework and erosion processes, with the main watercourses aligned in NW-SE and NE-SW orientations. The dense Atlantic Forest cover plays a crucial role in protecting soil and stabilizing slopes, acting as a natural barrier against erosion. However, the combination of steep slopes, less erosion- resistant geological units, and a thick weathering layer creates conditions prone to landslides, particularly during heavy rainfall. Identifying highly vulnerable areas under such conditions is essential for effective spatial planning and management. Consequently, continuous monitoring strategies, especially in steeper gradient areas, are strongly recommended to mitigate geological hazards. Furthermore, integrated efforts involving environmental protection, sustainable land management, and community awareness initiatives are vital for reducing the impact of natural disasters while preserving Ilhabela’s rich natural heritage.

Introduction

According to the IBGE website [1], the Ilhabela Archipelago, located on the northern coast of the state of São Paulo (Fig. 1), was discovered by Amerigo Vespucci on January 20, 1502, and was originally named São Sebastião Island. It was granted city status in the late 18th century. In 1805, the region, encompassing several islands, was renamed Villa Bella da Princesa. However, in 1934, it was demoted to the status of Paz District and incorporated into the municipality of São Sebastião. In 1940, its status as a municipality was reinstated, and it became known as Ilhabela [1]. Given the existence of various historical names—some of which persist in recent publications—the current official name, Ilhabela, has been adopted for this work.

Fig. 1. Ilhabela’s geographical position on the northern coastal of São Paulo State.

With the establishment of Ilhabela State Park in 1977, more than 80% of the archipelago’s total area—approximately 27,000 hectares—was designated as a protected area. This measure aimed to preserve environmental quality and regulate land use, as well as other activities that could pose environmental risks [2]. According to Guimarães [3] and Noffs [4], Ilhabela retained a predominantly rural character until the 1960s. However, from that decade onward, population growth and economic development driven by tourism have had a profound impact on the archipelago’s natural environment, resulting in urban expansion, infrastructure development, and increased demand for natural resources. These changes have upset the ecological balance and impacted formerly protected areas. The increase in tourists and new residents has made it more difficult to preserve the local ecosystem, manage waste and reduce pollution. This adverse scenario, which is rapidly progressing, requires careful environmental management to mitigate negative impacts, protect biodiversity and promote sustainable development that respects the archipelago’s natural heritage.

Recent studies conducted in Ilhabela have centered on identifying and classifying its geological and geomorphological features. Prochoroff [5] has made significant contributions to geoconservation by promoting the sustainable management of Ilhabela’s geological resources and underscoring the importance of protecting its geologically significant areas. Similarly, Rodrigues [6] offers a comprehensive analysis of the geodiversity in the island’s northern region, emphasizing the value of these geological features and advocating for the integration of scientific knowledge into environmental protection initiatives.

Considering the aforementioned aspects, this study aims to integrate remote sensing products with geological maps to achieve a more comprehensive and accurate understanding of the region’s morphological characteristics. Specifically, the study aims to identify and describe various relief elements, such as surface drainage features, slopes, valleys, plains and estuaries, considering their formation, origin and occurrence. Such analysis is key to understanding the region’s geological stress history, environmental risks, and local degradation, especially in remote or poorly studied areas. Moreover, it offers a solid scientific basis for developing public policies focused on effective land-use management and planning.

Methods and Procedures

The IKONOS images, with a spatial resolution of 1 m, were provided by IPA-SEMIL-SP. Similarly, the digital elevation data, with a resolution of 30 m, were obtained from the Shuttle Radar Topography Mission and are accessible through the United States Geological Survey website. The technical procedures for data collection, organization, integration, and thematic map creation were carried out using Global Mapper 22, Golden Surfer 19, and Inkscape 1.2. The identification and classification of geomorphological features followed the principles detailed in The Earth’s Land Surface: Landforms and Processes in Geomorphology by Gregory [7] and Key Concepts in Geomorphology by Bierman and Montgomery [8]. The techniques applied in geomorphological mapping, particularly those focused on landscape interpretation and environmental planning, were based on the methodologies presented in Geomorphological Mapping: Methods and Applications by Smith et al. [9].

Geological Setting

The earliest geological processes that contributed to the formation of Ilhabela are associated with the Precambrian period (Fig. 2a) and are referenced in academic literature as part of the Ribeira Belt, which is located in the central region of the Mantiqueira Province [10]. This period was marked by the amalgamation of the crystalline basement through intense tectonic deformation and regional metamorphism caused by the collision of continental masses, culminating in the formation of the supercontinent Gondwana. During this process, metamorphic rocks of the archipelago, including gneisses and migmatites, were formed [11]. These older units constitute the structural framework of the region, a characteristic shared by many crystalline basement rocks along Brazil’s southeastern coast [11].

Fig. 2. The maps provide a summarized representation of the geological and geomorphological aspects of Ilhabela, along with the types of soil and vegetation present in the region. The geological map (a) was compiled from Barreto [12] and Bellieni et al. [23]; the digital terrain model map (b) was created from NASA SRTM data; the soil (c) and vegetation cover (d) maps were extracted from the Management Plan of Ilhabela State Park, SMA [20].

In the later stages of the Brazilian cycle, between 600 and 540 million years ago, the region was influenced by post-collisional acidic magmatic processes that occurred in response to the relaxation of regional tectonic stresses [11]. The granitic rocks found across several areas of Ilhabela are evidence of this extensive magmatic activity and represent an important phase in the regional tectonic evolution [12].

During the Mesozoic, an extensional tectonic regime led to the opening of the Atlantic Ocean, the formation of sedimentary basins along the Brazilian continental margin, and significant extrusive basic magmatic activity [13], [14]. This tectonomagmatic event also facilitated the intrusion of numerous basic dikes and Early Cretaceous alkaline rocks into the crystalline basement of the archipelago [14], [15]. The presence of these lithological varieties highlights the tectonic processes that were shaping the region.

The most recent geological event contributing to the formation of a rift system along the Brazilian Atlantic margin, and also shaping the morphology of Ilhabela, is associated with Cenozoic tectonic activity [16]. Since the Eocene, the reactivation of normal faults and the uplift of the Serra do Mar have exposed the ancient rocks of Ilhabela [17]. This rifting process has not only shaped the current morphology of the archipelago but has also facilitated the uplift of crustal blocks along normal faults—a phenomenon that continues to influence the coastal relief of southeastern Brazil [16], [17].

At present, external dynamic processes such as weathering, erosion, and sedimentation actively shape the landscape of the archipelago. The dissected terrain, characterized by high hills and deep valleys, results from the differential erosion of crystalline basement rocks and alkaline intrusions, which undergo progressive weathering under tropical climatic conditions. Furthermore, fluctuations in sea level have contributed to the formation of beaches, coastal plains, and recent sedimentary deposits.

Geomorphologic Aspect

From a geomorphological perspective, Ilhabela lies within the Serra do Mar, where imposing steep slopes, supported by granites and granite gneisses, descend abruptly towards the sea. Located in the Coastal Province of São Paulo State, Ilhabela features a complex morphology and geology, dominated by metamorphic and igneous rocks, as well as dike intrusions, which characterize the dissected relief [18].

Isolated from the continent by the subsequent flooding of the extensive valley that formed the São Sebastião Channel, the island’s current geomorphological configuration (Fig. 2b) arose as a result of both tectonic activity and erosional processes [18]. The highest areas of Ilhabela are composed of Cretaceous rocks, including alkaline intrusions and basic dikes, which reinforce its structure and distinguish it from the surrounding Precambrian terrain [15]. These intrusions significantly contribute to the archipelago’s prominent relief, characterized by youthful escarpments, linear profiles, and deep ravines [18], [19].

The evolution of Ilhabela’s coastal landscape is closely tied to differential erosion and the unusual lithological resistance of its geological formations. This combination has led to the gradual retreat of cliffs, driven by the intense erosive forces prevalent in the Serra do Mar region. Moreover, Ilhabela’s position on the continental shelf structurally links it to the Juqueriquerê and Parati mountain ranges, thus illustrating the rejuvenation of the landscape surface driven by erosion processes [18].

Soils and Vegetation Cover

The intense weathering processes that have affected the lithological units of the archipelago, characteristic of humid tropical climate, have favored the development of lateritic soils rich in iron and aluminum oxides. These soils are characterized by their acidic nature and sandy texture [2], [20] and include varieties such as cambisols and latosols (Fig. 2c). Despite their limited nutrient-holding capacity, cambisols play a critical role in sustaining native vegetation on steep slopes and in erosion-prone areas due to their good drainage properties [2], [20]. Latosols, on the other hand, are deep, highly weathered soils commonly found in areas with gentle, stable relief. Characterized by excellent drainage and high water- and organic matter-holding capacity, these soils support dense forests. In the coastal and low-lying areas of the archipelago, alluvial soils are formed by the deposition of sediments from waterways. These soils are generally less acidic and richer in organic matter, promoting the development of coastal sandbank vegetation [2], [20].

The vegetation of Ilhabela is dominated by the Atlantic Forest. This dense rainforest is distributed according to the topographic gradient. In highest areas and on steep slopes, the forest consists of large trees, many of which exceed heights greater than 20 m. Common species include jequitibá-rosa (Cariniana legalis) and jacaranda (Machaerium villosum), while epiphytes such as bromeliads (Bromeliaceae) and orchids (Orchidaceae) contribute to the structural biodiversity of this environment [2], [20]. In the coastal plain, certain species, such as Calophyllum brasiliense and Cecropia pachystachya, are adapted to sandy and saline conditions and play an important role in stabilizing the coastline against erosion [2], [20].

In addition, recent studies highlight the presence of secondary forest formations in areas previously degraded by urban expansion. These areas are undergoing regeneration and are being colonized by medium-sized native species, which facilitate the restoration of original biodiversity and the recovery of ecosystem services, such as hydrological cycle maintenance and erosion prevention [2], [20].

Morphological Features of Ilhabela

As proposed by Montini and Velázquez [21], the diversity of the abiotic and biotic components in the Ilhabela Archipelago—evidenced by its lithological variation, morphological configuration, and surface drainage patterns—allows its division into three sectors: central-north, central-east, and central-south (Fig. 3). This division reflects not only the physical characteristics of the archipelago but also the interaction between natural factors and human occupation, highlighting the importance of sustainable management.

Fig. 3. The Ilhabela area is divided into three distinct sectors: A) central-north, B) central-east, and C) central-south.

Central-North Sector

The central-north sector is characterized by a dissected and heterogeneous landscape, marked by distinctly sharp topography. This region is dominated by hills rising to elevations of approximately 1,025 m, with Pico do Baepi standing out as a prominent topographic feature (Fig. 4a). The relief includes escarpments, narrow valleys, rounded ridges, and limited coastal plains. The largest valleys in the region display a parallel arrangement, predominantly oriented in a NE-SW direction (Figs. 4a and 4d). The smallest valleys, which intersect the larger ones at oblique angles, follow two principal orientations: NW-SE and NE-SW, with the former being more common. The rounded ridges generally exhibit a linear orientation, though some adopt a well-defined curvilinear shape, resulting in a slightly oval and concentric appearance. At the center of this feature is a circular depression with steeply sloped edges converging towards its center (Fig. 4b). This topographic configuration increases the area’s vulnerability to landslides, particularly during periods of heavy rainfall.

Fig. 4. The central-north sector of the archipelago exhibits variations in elevation (a), slope orientation (b), surface drainage flow direction (c), and topographic profile (d). The solid white line with red tips indicates the profile location, with the vertical scale magnified threefold. See the text for further details.

The steep slopes are a striking feature, particularly around the highest hills and along the coast, where elevation changes abruptly from sea level to upland areas. The slope ramps vary considerably in length, with most facing the western quadrant, including orientations toward the NW and SW (Fig. 4b). These slopes can extend for several hundred meters, connecting the coastal areas to the higher continental region. Along their length, the morphology alternates between concave and convex profiles (Fig. 4d), enhancing the relief’s diversity.

Surface drainage in the region is strongly influenced by the topography and the dense vegetation of the Atlantic Forest. The drainage network is predominantly dendritic, with large rivers and streams branching into smaller tributaries that descend steep slopes (Fig. 4c). These waterways typically originate from the highest altitudes and flow directly toward the coastline. The high drainage density results from heavy rainfall, steep slopes, and the terrain’s low infiltration capacity, which increase surface runoff. The region also contains several small river basins draining directly into the ocean. The hydrological regime of these rivers and streams is generally intermittent, with higher flows during the rainy season, leading to significant seasonal fluctuations in water volume.

Central-East Sector

The central-eastern sector is less dissected, with average elevations ranging from 250 m to 550 m, and features a gradual transition from sea level to higher elevations (Figs. 5a and 5d). The landscape comprises hills with elongated ridges, narrow valleys, and coastal plains. These ridges form parallel alignments predominantly oriented NE–SW (Fig. 5a) and are typically separated by narrow, deep valleys that extend over long distances, serving as natural channels for rainwater runoff. The valleys generally exhibit a well-defined parallel alignment, with some branching into secondary tributaries. They typically display “V”-shaped cross-sectional profiles (Fig. 5d), indicating significant fluvial erosion. The combination of elongated valleys and high ridges forms steep slopes that are often prone to erosion and landslides.

Fig. 5. Central-east sector of the archipelago. The morphological features, legend and symbols are as shown in Fig. 4.

The slopes are gentle, with a gradual transition from sea level to higher elevations. They are predominantly oriented towards W, with slight deviations towards SW (Fig. 5b), and present minimal variation in length. Although relatively short, these slopes are characterized by gently undulating surfaces, alternating between concave and convex profiles along their extent (Fig. 5d).

The surface drainage pattern is characterized by a dendritic network of small rivers flowing predominantly from east to west (Fig. 5c) toward the sea. The low drainage density is largely due to the short lengths of the slopes and valleys, which promote rapid surface runoff. As a result, river channels tend to be shorter and follow more linear trajectories.

Central-South Sector

The central-southern sector exhibits a varied relief, characterized by dissected topography and significant elevations reaching up to 1300 m (Fig. 6a), exemplified by Pico de São Sebastião. The dominant morphological features include long, steep ridges, open “V”-shaped valleys, and coastal lowlands. These elongated ridges run parallel in a NW-SE direction and are separated by wide valleys with open “V”-shaped cross sections (Fig. 6d). Although valley widths vary, they are generally narrow, accentuating the perception of depth and verticality in the landscape. This geomorphic configuration contributes to the terrain’s complexity and supports a diverse range of ecosystems in the region.

Fig. 6. Central-south sector of the archipelago. The morphological features, legend and symbols are as shown in Fig. 4.

The slopes of the hills exhibit a distinct pattern, with steep gradients at higher elevations that gradually transition into gentler slopes at lower elevations. The dominant orientation of the slopes is in the western quadrant (Fig. 6b), with variations to NW and SW. Shorter slopes are predominantly convex, while longer slopes exhibit a combination of convex and concave profiles. This variation in relief, along with slope orientation, plays a crucial role in the distribution of accumulative and dispersive flow areas. Convex slopes tend to promote the dispersion of surface runoff, whereas concave slopes are more likely to accumulate flow, thereby influencing the region’s erosion and sedimentation processes. These topographic variations significantly contribute to the terrain’s complexity, shaping both hydrological dynamics and patterns of soil and sediment transport.

The dendritic drainage pattern is closely related to the dominant geomorphic features, including elongated ridges, deep valleys, and steep slopes (Fig. 6c). Rivers and streams in the region have an intermittent flow regime, with runoff increasing significantly during the rainy season. This variation is largely driven by the humid climate and intense seasonal rainfall, followed by periods of reduced flow. The longitudinal profiles of the rivers reflect topographic variations. In higher-elevation areas, near ridges and steep slopes, rivers exhibit steep gradients and rapid flows, intensifying erosion and carving deep channels with steep banks. Conversely, as rivers approach the coast, the gradient decreases, slowing the flow and promoting the development of shallower channels with reduced erosive power.

Discussion and Final Considerations

The diverse relief of Ilhabela, along with its varied soil types and vegetation, is the result of a complex interplay between lithology, tectonic activity, and weathering processes. The lithological units consist of crystalline basement rocks [11], [12], eruptive and alkaline plutonic rocks [22], [23], and, to a lesser extent, basic dykes [20]. The remarkable mineralogical diversity presents in these lithological units, combined with the region’s prolonged weathering processes, has led to the development of diverse soils and distinct morphological features. In addition, the abundant occurrence of brittle structures [15], [17], such as fractures and faults, significantly contributes to the development of linear surface drainage patterns, which, in turn, influence the evolution of the archipelago’s relief. The interaction among these elements not only fosters the development of diverse ecosystems but also influences the distribution of human settlements. Therefore, Ilhabela represents an exceptional natural laboratory for examining the relationships among lithology, soil characteristics, and landscape evolution.

The linear morphological features of the central-northern region predominantly exhibit a NW orientation, corresponding to the direction of the main brittle structures. In contrast, the curvilinear features align with the contact zone between the alkaline massif and the surrounding basement rocks (Figs. 2a, 2b). The relief is characterized by steep slopes, deep valleys, and gently rounded summits. The highest altitude areas in the region are composed of alkaline rocks, which support soils that are typically clay-rich, shallow, and nutrient-poor (Figs. 2b, 2c). Dense vegetation covers this sector, consisting of ombrophilous forest adapted to the poorly developed soils, transitioning from primary vegetation in higher areas to degraded primary vegetation near the coast (Fig. 2d). This fragile ecological environment is particularly susceptible to erosion due to steep slopes and dense dendritic surface drainage.

The central-eastern sector is characterized by less pronounced geomorphological features, low elevations, and marked linear erosion that follows the orientation of the fractures. The slopes of the larger “V”-shaped valleys, formed over the alkaline massif, are cut obliquely by smaller channels that increase the erosion. The lithological combination of alkaline rocks and crystalline basement units promotes the development of deeper soil layers with enhanced water and nutrient retention capacities (Fig. 2c). This edaphic environment supports a denser and more diverse vegetation cover (Fig. 2d), sustaining a well-developed primary ombrophilous rainforest that contributes to slope stability.

The main morphological feature of the central-southern sector is an extensive valley developed on alkaline rocks, about 2 km wide and 4 km long. Its parallel linear margins are oriented NW-SE, with steep slopes towards NE and SW (Figs. 6a, 6d). The inner part of the valley is filled with weathered rock material, resulting in fertile, well-drained, deep-layered soils (Fig. 2c). In some areas, the accumulation of plant debris favors the formation of soil layers rich in organic matter. However, these areas may contain sand layers with low fertility and limited water retention capacity. In higher areas, where erosion is dominant, the soils tend to be shallower. The vegetation cover of the central-southern sector is notable for its exuberance, featuring a dense ombrophilous forest that ranges from areas of primary vegetation at higher altitudes to secondary vegetation at lower elevations (Fig. 2d). This vegetation, characteristic of the Atlantic Forest, plays a crucial role in stabilizing the soil and reducing erosion on sloping terrain.

For effective territorial planning aimed at maximizing the prevention of geological risks in Ilhabela, an integrated understanding of the lithological, pedological, vegetation, and geomorphological characteristics of the region is essential. Lithological variations directly influence soil types, fertility, and susceptibility to erosion, while vegetation cover plays a key role in slope stability. At the current stage of landform evolution, the morphological characteristics of each sector determine the pattern and flow of surface drainage, susceptibility to erosion, and vegetation distribution. This underscores the importance of maintaining active initiatives and regulatory measures to safeguard the environment.

Therefore, it is recommended that detailed mapping be conducted for each sector of Ilhabela, similar to the approach used by Rodrigues et al. [24], taking into account its lithological, geomorphological, and structural characteristics. This should facilitate targeted management practices in areas of potential risk, aiming to improve environmental protection and mitigate natural disasters in more densely populated regions. For instance, in coastal areas, the preservation of coastal sandbank vegetation and mangroves is crucial to mitigate the effects of tides and erosion. On steep slopes, effective surface drainage should be implemented to prevent water accumulation in soil layers and reduce the risk of landslides, particularly in areas with clay soils that are highly susceptible to erosion. In these instances, maintaining native vegetation plays a critical role in soil stabilization. In plateaus and exposed rocky regions, such as the Pico do Baepi massif and other highly frequented sites, monitoring faults, fractures, and the stability of rock formations along the slopes is crucial to prevent accidents caused by mass movements after heavy rainfall. In areas heavily visited by tourists, the installation of warning signs is essential, not only to restrict access to hazardous zones and ensure the safety of visitors but also to protect sites where geological units serve as significant tourist and educational attractions. Excessive visitation can alter and degrade the characteristics of the area; therefore, measures must be taken to minimize the environmental impact while promoting safety and protection.

In this and similar cases, environmental education has proven effective in raising awareness about geological risks associated with rock formations and the importance of preserving vegetation for both the local population and tourists. This approach not only promotes accident prevention but also supports the conservation of Ilhabela’s natural environment. Ultimately, these integrated strategies aim to protect valuable natural resources, mitigate geological risks, and foster a safe and sustainable environment for the entire community.