1. Introduction

Volcanic eruptions are one of the most powerful manifestations of the Earth’s internal energy. As demonstrated during the May 18, 1980, eruption of Mount St. Helens (Washington, USA), it can take minutes for a volcano to transform vast areas of its surroundings, with consequences lasting from decades to several thousand years (e.g., Driedger et al., 2020). The explosive Hunga Tonga-Hunga Ha’apai eruption that occurred on January 15, 2022, in the Kingdom of Tonga, southern Pacific, evidenced how oceanic and atmospheric perturbations triggered by large-magnitude volcanic eruptions can have impacts on a trans-oceanic or even global scale (Amores et al., 2022; Yuen et al., 2022; Le Mével et al., 2023; Purkis et al., 2023). Moreover, the September-December 2021 eruption of the Cumbre Vieja volcano (Canary Islands, Spain) highlights the devastating character that even smaller-scale eruptions may have in highly populated and exposed areas nearby the volcano (e.g., Houghton et al., 2021; Carracedo et al., 2022; Romero et al., 2022). In this way, the geological study of volcanic deposits, landforms, and structures is fundamental to the understanding of the formation and evolution of volcanoes, including the magnitude and recurrence of their eruptions, and therefore of the associated potential hazards. Also, geological studies based on mapping, sedimentology, and petrology, allow the identification, characterization, and assessment of geological heritage sites (geosites). Some of these sites are related to recent eruptions (e.g., Dóniz-Páez et al., 2024), and the study of their deposits may help to better understand the volcano’s most recent eruptive history, encouraging their preservation and providing an efficient means for the education of local communities and visitors about volcanic processes and their impacts. This is of particular interest for at-risk communities around active and hazardous volcanoes.

Geodiversity is the natural variety of geological features, including their relationships, interpretations, properties, and systems (Gray, 2004). On the other hand, geological heritage (geoheritage) corresponds to the set of geodiversity sites and elements (minerals, fossils, rocks, etc.) that are recognized for their scientific, cultural, and educative value (Carcavilla et al., 2008). Within these elements, geosites are defined as places that show in situ one or many characteristics, considered important in the geological history of a region (Ferreira, 2017). Mondéjar and del Ramo Jiménez (2004) argue that the study of geodiversity and geoheritage lies within a complex context because of the origin of the concept itself and the close relation with other disciplines, where the interest, in many cases, surpasses the geoscientific aspects. Geoheritage also represents an important didactic resource and has relevant cultural connotations. Geoconservation is thus developed by the need to protect and promote geoheritage (Gray, 2008).

Like many of the recent eruptions that dramatically impacted their surroundings, on April 22, 2015, Calbuco volcano (41.3° S, Southern Andes of Chile) sourced a moderate VEI 4 eruption with important social and environmental effects, the latter on a global scale (Manville et al., 2018; Pardini et al., 2018; Zhu et al., 2018; Hayes et al., 2019; Sangeetha et al., 2018). Calbuco is an ice-clad, 2,015 m high andesitic stratovolcano, currently ranked second in the Chilean volcanic risk ranking (Sernageomin, 2023)¹. During historical eruptions (>1750 CE), Calbuco has generated tephra fallout, pyroclastic density currents (PDCs), lava flows, and lahars. These products have been radially emitted from the summit down the river valleys (Sellés and Moreno, 2011), being the volcano’s north-northeast flank the most severely affected (Romero et al., 2021).

In this contribution, we present the first inventory of geological heritage sites of Calbuco volcano. Our research comprises the geological characterization of 25 potential geosites, which were assessed using qualitative and quantitative procedures. Based on the quantitative methodology, three of them are described in detail. These sites were selected due to either their high scientific value or their high score in touristic and educational use. Inhabited historically active volcanoes such as Calbuco represent challenging areas for volcanic risk management. Possible measures contributing to volcanic risk mitigation include the education of the population about volcanic processes and their potential impacts. The selected geosites can also be used as an effective tool to promote research divulgation and touristic activities in cooperation with local authorities and neighboring communities.

2. Geological background

Between 33 and 46° S, the Southern Volcanic Zone (SVZ) of the Andes results from the oblique subduction of the Nazca Plate beneath the South American Plate at a rate of ~7-9 cm/year (DeMets et al., 2010). This zone includes at least, 60 historically and potentially active volcanoes in Chile and Argentina, as well as three silicic caldera systems and numerous minor eruptive centers (Stern, 2004). The central segment of the SVZ (CSVZ; 38-41.5° S) is characterized by the subduction of <18 Ma old oceanic lithosphere, and a narrow (~80 km wide) arc located in the boundary of the Central Valley and the western edge of the Main Andean Cordillera (Stern, 2004). Most of the CSVZ volcanoes display a compositional range between dominantly basaltic andesites and dacites (e.g., Vander Auwera et al., 2019). In this region, the volcanism is structurally controlled by the Liquiñe-Ofqui Fault Zone (LOFZ) (Fig. 1), a ~1,200 km long intra-arc feature represented as dextral transpressional ~N-S ductile-to-brittle shear zones (López-Escobar et al., 1995; Cembrano and Lara, 2009).

FIG. 1. Study area depicting the main volcanoes and lakes, and the main trace of the LOFZ (Liquiñe-Ofqui Fault Zone). Figure extracted from Cembrano and Lara (2009) and Orozco (2009).

Calbuco volcano lies over Miocene plutonic rocks of the North Patagonian Batholith and early Pleistocene volcanic and volcaniclastic rocks of the Hueñuhueñu strata (Munizaga et al., 1988; López-Escobar et al., 1992; Sellés and Moreno, 2011). Calbuco is at least ~100 kyr old, and its geological history has been grouped into four stages (Mixon et al., 2021). During the second stage, it experienced two lateral collapse events at ~17.5 and 9 ka, which affected the west and north of the current vent (Clavero et al., 2008; Sellés and Moreno, 2011; Zellmer et al., 2014; Mixon et al., 2021). Mixon et al. (2021) indicate that during the Holocene Calbuco has erupted remarkably homogeneous basaltic andesite to andesite products (54-58 wt% SiO₂) at eruption rates varying from 3.4 to 4.8 km³/kyr, higher than other CSVZ arc volcanoes. Similarly, Vander Auwera et al. (2021), through high-resolution geochemistry, show that no secular compositional change has occurred at Calbuco throughout its history, indicating a steady magmatic system beneath the volcano. Hence, Calbuco is one of the most productive volcanoes in the CSVZ and thus an active source of both landscape and environmental transformations.

Calbuco’s historical eruptive record extends back to ca. 230 years (~1790 CE), being summarized in detail by several authors (e.g., Petit-Breuilh and Moreno, 1997; Moreno, 1999; Petit-Breuilh, 1999; Sellés and Moreno, 2011). At least two types of eruptions have been described: moderate, mostly sub-Plinian events with repose intervals of 60±10 years; and smaller-scale eruptions (mostly Vulcanian, phreatic, phreatomagmatic, and dome-building effusive events) which seem to occur with a mean frequency of ~20 years (Romero et al., 2021). A map of the most relevant volcanic deposits in Calbuco’s northeastern flank is shown in figure 2.

FIG. 2. Geological map of the northern flank of the Calbuco volcano modified from Sellés and Moreno (2011), Mella et al. (2015) and Romero et al. (2021).

In 1792, the eruptive activity of Calbuco was accompanied by perceptible earthquakes (Petit-Breuilh and Moreno, 1997). In 1893-95 the volcano erupted again, with eruption columns that rose to a maximum height of ~12 km, accompanied by PDC generation and dome extrusion (Petit-Breuilh and Moreno, 1997; Sellés and Moreno, 2011). The 1893-95 eruption produced 0.54±0.13 km³ of basaltic andesite tephra (53-54 wt% SiO₂) and reached a magnitude and intensity of 4.8 and 10.5, respectively, thus being the volcano’s largest historical eruption (Romero et al., 2021). In 1917, a new eruption produced ash plumes dispersed to the east, together with lava flows and lahars in the volcano’s northeastern flank (Sellés and Moreno, 2011; Romero et al., 2021). Another explosive eruption in January 1929 produced a series of PDCs, affecting most of the volcano’s northeast flank by overbanking (Stone, 1930; Petit-Breuilh and Moreno, 1997; Sellés and Moreno, 2011; Romero et al., 2021).

Most recently, between January and March 1961, renewed activity produced PDCs and triggered lahars down the volcano’s north-northeastern and southeastern flanks, as well as two lava flows down the Tepu (La Poza) and Amarillo rivers (Klohn, 1963; Moreno et al., 2006; Sellés and Moreno, 2011; Romero et al., 2021). The paroxysmal stage consisted of a sub-Plinian eruptive column ~12 km high and a 0.1±0.01 km³ basaltic andesite (55-56 wt% SiO₂) fallout deposit dispersed east-northeast (Klohn, 1963; Daga et al., 2014; Romero et al., 2021). The 1961 eruption achieved a magnitude of 4.08 and an intensity of 9.74 (Romero et al., 2021). A short-lived ash emission event, probably phreatic in origin, was observed in August 1972 (Sellés and Moreno, 2011).

Finally, the latest eruption occurred on April 22, 2015, preceded by an apparent short period of unrest, consisting of ~140 volcano-tectonic events, roughly 3 hours before the eruption (Valderrama et al., 2016). The eruption had three explosive pulses, two of them sub-Plinian with eruptive columns up to 23 km high with a noticeable dispersion towards the northeast (Bertin et al., 2015; Castruccio et al., 2016; Romero et al., 2021). During this eruption, PDCs, tephra fallout, and lahars were generated, all of which damaged public and private infrastructure (Mella et al., 2015; Castruccio et al., 2016; Romero et al., 2023). The distribution of these products was constrained within the volcanic hazard areas identified by Moreno (1999) (Romero et al., 2016). The revised volume for the 2015 tephra deposit is 0.3±0.16 km³ (magnitude to 4.47 and intensity to 10.18); its composition was basaltic andesite (55-56 wt% SiO₂) with an andesite-to-dacite glass geochemistry (61-65 wt% SiO₂) (Romero et al., 2021).

The eruptive history of Calbuco implies a continuous transformation of the landscape through the rapid deposition of volcanic and volcaniclastic materials, especially on its north-northeastern flank. Additionally, the 2015 eruption showed striking aspects of a sudden initiation and short-lived precursors, thus revealing enhanced hazards. The triggering of this eruption has been explained by either a continuing crystallization inducing second boiling and an over-pressurization of the system (Arzilli et al., 2019), or localized heating of the sub-volcanic reservoir caused by an injection of hot magma (Morgado et al., 2019).

3. Conservation of natural heritage and protected areas

In the northern flank of Calbuco there is one public (Llanquihue Nacional Reserve) and two private (Valle Los Ulmos Park and Volcanes Park) protected areas (Fig. 3). The Valle Los Ulmos Park started in 1954 as an agricultural property, which in 2014, changed to a private park with the mission to preserve the volcanic ecosystem. The project integrates a community of 45 shareholders that value and encourage conservation through the purchase of stocks that give the rights and obligations for the protection of the volcanic ecosystem. The project design includes landscape restoration, the protection of 65 hectares of forest, and the possibility for the community to use the park areas for sustainable development, education, science, tourism, and human-nature coexistence. The Volcanes Park is a private project that seeks to preserve the ecosystem through the conservation and regeneration of the native forest. The park is currently inhabited and includes numerous houses. The park stands out by its large variety of native flora and fauna. Finally, the Llanquihue National Reserve is a public protected area created in 1912 with a surface of 33,972 hectares that is administrated by the National Forestry Corporation (Conaf). The objectives related to the management of natural resources of this reserve that are relevant for this study are (Conaf, 2014)²:

• To create and develop lines of action for scientific investigation.

• To protect ecosystems and living species of flora and fauna.

• To identify and manage lines of action for the protection of water resources.

The three protected areas listed above have different missions but do share a common objective: the conservation of nature and its use for local development.

FIG. 3. Northern flank of Calbuco volcano. Private protected areas: Valle Los Ulmos Park (red) and Volcanes Park (purple). Public protected area: Llanquihue National Reserve (blue).

4. Methodology

This research started with a literature review of geological heritage assessment methods, volcanic processes and hazards, and the Calbuco volcano eruptions record. The quantitative assessment procedure was based on Brilha (2016), which consisted of scoring each geosite using a 0-4 scale. Each criterion was supported by a set of parameters (Table A1 in the Appendix) from the absence of the attribute (0 points) to the excellence (4 points) regarding a specific criterion. A specific weight was considered for each criterion in accordance with its relative importance for the geological heritage value proposed by Brilha (2016). The sum of the scores results in the total score of each geosite, which can be also interpreted for each value or use.

A fieldwork was conducted on the northern flank of Calbuco to identify and characterize potential geosites. A simplified form based on Martínez (2010), Pantoja (2017), and Urrutia (2018) was used to characterize the geosites in the field. Diverse volcanic deposits were identified, including lahars, PDCs, lava flows, and tephra falls. We measured thicknesses, made schematic drawings accompanied by detailed photography, and elaborated stratigraphic sections of the deposits. Samples were collected for granulometric analyses and thin sections. Viewpoints were described differently, focusing on their observational properties and potential use for tourism.

A thematic geological map at 1:85,000 scale of the northern flank of Calbuco was made (Fig. 2) based on field information and previous works of Sellés and Moreno (2011), Mella et al. (2015) and Romero et al. (2021). Due to the low accessibility to some geosites, photointerpretation using satellite imagery (Google Earth ® ) was performed to complete the map, considering the morphology, colors, and spatial continuity of the known deposits.

Based on a selected list of criteria that consider four categories: scientific value, degradation risk, didactic use, and touristic use (Fig. 4), we ranked the potential geosites through the quantification methodology proposed by Brilha (2016). To avoid losing relevant information, we quantified the values of each category instead of only considering the total score of each geosite.

The selection of the three top geosites was based on the following two conditions:

• Highest scientific value: sites with the potential to be used in scientific research, given its novelty and unique occurrence in the study area.

• Highest didactic and touristic uses: sites with the capacity to be used for didactic and touristic purposes. These sites are relevant for scientific divulgation, educational (increasing knowledge in the communities), and touristic (major economic benefits for the communities) purposes.

Due to the purpose of the investigation, the degradation risk was not taken into account in the selection of the top three geosites. We chose geosites that can contribute to the understanding of geological/volcanological processes (education) and that can promote local economies through tourism and science.

 

FIG. 4. Criteria used in the quantitative assessment of potential geosites of the northern flank of the Calbuco volcano, separated in four values/uses: scientific, degradational risk, didactic use, and touristic use. Source: Brilha (2016).

5. Results

5.1. Identification and assessment of potential geological heritage sites

Twenty-five potential geosites were identified in the northern flank of Calbuco, most of them located within the Valle Los Ulmos Park (Fig. 5; Table A2 in the Appendix). Results for the scientific, didactic, and touristic uses of all geosites are listed in tables 1, 2, and 3, respectively (all the scores, including those for the degradation risk, are available in Table A3 in the Appendix). It is important to clarify that at the time of writing neither of these 25 geosites is used by schools, universities, or tour operators.

FIG. 5. Map showing the location of the 25 geosites identified in the present study (red dots). The striped, black area is the Valle Los Ulmos Park, where most of the geosites were accessed through.

In table 1 (scientific use) the highest score was 350, corresponding to geosite Los Volcanes Viewpoint (VC19-10). There are other geosites with scores close to that of geosite VC19-10, such as Blanco river volcanic deposits (VC19-23) with 340 or Lava front at La Poza (VC19-04) with 330.

In table 2 (didactic use) the highest score obtained is 365, corresponding to geosite Los Volcanes Viewpoint (VC19-10). This geosite has the highest potential in terms of education, so it can be used to increase knowledge in the communities. Since VC19-10 was already selected in the scientific use procedure, we added the second highest score (325), which corresponded to geosite Lava front at Blanco River (VC19-22).

In table 3 (touristic use) the highest scores were 320 and 310, corresponding to geosites Hueñu-Hueñu Viewpoint (VC19-24) and Los Volcanes Viewpoint (VC19-10), respectively. These sites show a high touristic potential, which is important for the development of local economies.

Based on the information provided above, three geosites were selected for a more detailed description (see following subsection): Los Volcanes Viewpoint (VC19-10), Lava front at Blanco River (VC19-22), and Hueñu-Hueñu Viewpoint (VC19-24).

5.2. Characterization of selected geosites

5.2.1. VC19-10: Los Volcanoes Viewpoint

This geosite is located at the Valle Los Ulmos Park (703741 E; 5427081 N), and allows the observation of four volcanoes of the region (Calbuco, Osorno, La Picada, and Puntiagudo) (Fig. 6; see Fig. 1 for a location map). From this site, it is possible to interpret the structural control of these volcanic centers due to the LOFZ, where ~N-S alignments prevail. Contrary to other sites, this viewpoint offers the visitor large-scale geodiversity elements. In addition, it has the potential to provide comprehensive explanations about long-term (i.e., thousands of years) landscape evolution and geological processes. Thus, this geosite can be part of geotourism and educational initiatives. The accessibility criterion was positively assessed, as the geosite can be accessed either by vehicle or foot from the entrance of the Valle Los Ulmos Park. Since the site is a viewpoint, a set of initiatives and tools regarding the interpretation of geoheritage, volcanic processes, and volcanic hazards could be implemented (e.g., infographic panels). Moreover, the implementation of geotouristic routes within the park could include this geosite as a starting point.

FIG. 6. Calbuco, Osorno, La Picada, and Puntiagudo volcanoes, as observed from geosite VC19-10.

5.2.2. VC19-22: Lava front at Blanco River

This geosite is located next to the Blanco River (702,838 E; 5,423,863 N), and shows the ~30 m-thick blocky lava flow front emplaced during the eruption of 1961 (Fig. 7A). The lava is an andesite with pyroxene, plagioclase, and amphibole crystals showing diverse textures, where pyroxenes and plagioclases form cumulates (Fig. 7B). Unlike the 1961 lava front at La Poza River (site VC19-04), the one at the Blanco River is less prominent and does not show its base, only the central, massive part of the lava is visible. The surface of the lava is mostly covered by tephra of the 2015 eruption, smoothing its blocky surface. The 1961 lava flow stopped at ~1.6 km from the volcano in the La Poza River valley (VC19-04), and at ~0.6 km in the Blanco River valley (VC19-22). The accessibility criterion was also positively assessed, because the site can be reached by car, and then a walk of less than 1 km. Along this path, it is possible to visit other geosites assessed in this work, such as the Degassing Pipes and rotational movements at Blanco River (VC19-19), the Degassing pipes at Blanco River (VC19-20), and the Volcanic sequence of the Blanco River (VC19-21).

FIG. 7. A. The 1961 lava front along the Blanco River corresponds to the geosite VC19-22. The gray material that covers the surface is tephra fall from the 2015 Calbuco eruption, which caused the death of small trees. B. Thin section images of the 1961 pyroxene andesites showing sieve and porphyritic textures. Left image: crossed polars; right image: plane-polarized light. Cpx: Clinopyroxene, Ox: Oxides, Opx: Ortopyroxene, Plg: Plagioclase.

5.2.3. VC19-24: Hueñu-Hueñu Viewpoint

This geosite, located at the confluence of the Hueñu-Hueñu and Blanco rivers (711,267 E; 5,429,409 N), is very favorable in terms of accessibility. Around the site there is a bridge with a sidewalk, visited frequently due to its panoramic view (Fig. 8A). Laharic deposits up the Blanco River are of volcanological and geomorphological interest, and can be used to explain depositional and erosive processes in active fluvial systems influenced by volcanic processes as well as their interaction with the forest (Romero et al., 2023) (Fig. 8B). The Hueñu-Hueñu (Fig. 8C) and Blanco (Fig. 8B) rivers are born on the northeastern flank of Calbuco and both are anastomosed. The Blanco River has a wider flood plain, and the deposition of volcanic material has generated sand and gravel bars (Carrizo, 2019) (Fig. 8B). During the 1893-95 eruption, lahars descended through the Blanco and Hueñu-Hueñu rivers, being described as “a wall with a black superior part and a reddish inferior part, covering big part of the oriental horizon.” (Petit-Breuilh and Moreno, 1997). This observation probably refers to hot lahars.

FIG. 8. A. The confluence of Hueñu-Hueñu and Blanco Rivers. B. Erosion and depositional zones in the Blanco River. C. Hueñu-Hueñu River view.

6. Discussion

6.1. Methodology and limitations

In this study, we followed the proposal of Brilha (2016) to incorporate a quantitative methodology to inventory and assess geosites. This methodology provides a quantitative assessment of each geosite respect to a particular value or use. This is relevant as in case a single score is calculated for each geosite, we are not able to identify the usefulness of each geosite in geoheritage or geoconservation strategies. Another important aspect of the Brilha (2016) methodology lies in the global use it brings, different from other methodologies, for example, Serrano and González-Trueba (2005) or Santos et al. (2020), that can mostly be used for landforms or rural landscapes. Brilha (2016) provides a useful methodology that can be used from landforms to deposits or even to more specific geological elements.

The methodology used in this study has some subjectivity when it comes to quantifying the geosite values and criteria, as the scores are assigned by the person or group in charge of the evaluation. By introducing a personal bias in the assessment, the results are influenced by subjective opinions and appreciations, even within the strict framework of the same evaluation criteria valid for all sites. This drawback could be overcome with additional assessments conducted by other experts, using the same methodology.

6.2. Geoconservation and geoeducation initiatives

After selecting 25 potential geosites, they were scored accordingly. The top-three ranked geosites with the highest scientific, didactic, or touristic values were described in detail. These geosites are representative of the different processes of Calbuco volcano. Also, they hold high potential not only for scientific and educative purposes, but also for conservation and tourism. The latter is relevant as, by conserving geosites, their value can be preserved for future generations (Gray, 2008; Henriques et al., 2011; Hose and Vasiljevic, 2012). Geoconservation initiatives should be framed within the scope of conservation strategies of the protected areas. Likewise, valorization initiatives are needed, aiming to promote conservation and local socioeconomic development through geotourism and educational tools.

Azman et al. (2010), Henriques et al. (2011), and Sánchez (2011) state the importance of using geoheritage in the education of local communities, especially those that are more exposed to volcanic processes. Calbuco volcano represents a serious geological hazard to the population of Ensenada. A recent study conducted by Alegría and Vergara-Pinto (2024) shows that people living in Ensenada are eager to participate in future emergency management planning and adopt preventive attitudes at local, household, and individual levels. The implementation of tailored educative and touristic initiatives can be improved with interactive information about volcanic processes and hazards. The sustainable use of the selected geosites for education about Earth and environmental sciences can therefore contribute to volcanic risk reduction in the region.

7. Conclusions

The geological heritage sites studied in this contribution can be used to explain active geological processes in the Calbuco volcano area. The geosites contain diverse volcanic deposits, landforms, and lithologies that can be used for educational purposes.

The inventory of 25 geosites allows not only the compilation of information under the same criteria, but represents a systematic way to provide comparisons and the selection of the most relevant sites. This methodological approach can be implemented in future studies in other similar volcanic regions. Regarding the Calbuco volcano area, the addition of further geosites to the inventory provided here is encouraged for a more detailed characterization and the gradual inclusion of geoconservation and geotourism initiatives.

The systematic assessment of geosites to increase scientific knowledge and awareness of volcanic processes and hazards can benefit the local population through social and economic development. Geotourism activities in a specific area can be an incentive for local economic development and a way to educate the population about geodiversity, geoheritage, and geological hazard topics. Geoconservation and geotourism strategies will require the participation of local entities, such as local government, universities, local associations, and research and nature conservation institutions.

Acknowledgments
The authors are grateful to R. Rivera, P. Martínez, and one anonymous reviewer for their critical reviews that contributed to improve this manuscript. The editorial handling of D. Bertin, together with constructive ideas and suggestions, is also acknowledged.

¹ Sernageomin. 2023. Ranking de riesgo específico de volcanes activos en Chile (available at https://rnvv.sernageomin.cl/que-es-ranking-de-riesgo/)

² Conaf. 2014. Plan de Manejo Reserva Nacional Llanquihue. Corporación Nacional Forestal: 166 p.

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Appendix

Table A1. Detail of each criterion with its respective parameter and scores assigned. Extracted from Brilha (2016).

Table A2. Summarized information of the 25 potential geosites on the northern flank of Calbuco.

Table A3. Quantification procedure for the selection of the top-three geosites to be characterized in detail.