Introduction

There is a wide consensus around the geodiversity concept considered the variety of geological processes, forms, and materials that make up and shape the Earth. While relevant materials include minerals, rocks, sediments, fossils, soils, and water, forms correspond to any expression of morphology or relations between units of Earth material, like folds or faults (Gray 2004; Brilha 2014). Materials and forms result from geological processes that act upon, maintain, or modify either material or form. Some of them are long-term processes, like erosional ones, and others are instantaneous, like earthquakes, tsunamis, or volcanic eruptions.

Forms and materials displaying geoheritage contents (Pena dos Reis and Henriques 2009) can be framed within the geological heritage of the Earth as tangible objects, whether in situ like geosites, or ex situ like fossils (Henriques and Pena dos Reis 2015). The IUGS (2021, p. 2) has recently considered that “An IUGS Geological Heritage Site is a key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history”. But most of the geological processes correspond to an abstract concept, of dynamic nature, hardly to be represented by any concrete entity. In fact, all geosites and geoheritage elements are forms and materials resulting from geological processes, but these, in themselves, are essentially immaterial and intangible. Exceptions include earthquakes and other natural hazards, which can be directly experienced by anyone through the energy that they release in the form of seismic waves.

As so, the integration of geological processes within the geological heritage—e.g., processes of international scientific relevance—requires the emergence of a new geoheritage concept, whose definition and incorporation within Geoconservation will be argued in this paper: the intangible geoheritage.

The Concept of Intangible Geoheritage

Like any other science, geoconservation concepts and ideas are subject to permanent review and reformulation, giving rise to the emergence of new perspectives on aims and scope of the discipline. For instance, Brilha (2014) provided a new approach to this issue by distinguishing between geosites and geoheritage elements as the aim of geoconservation, and Henriques and Pena dos Reis (2015) enlarged the site-based geoconservation thinking to include fossils, rocks, and minerals which are currently housed in museums.

Both specimen-based and site-based approaches correspond to forms and materials with geoheritage contents that constitute and shape the Earth. But it is hard to assign such propriety to geological processes, whether long or instantaneous, due to its immaterial nature, despite the tangible character of their results. And some of them were and continue to be referred as representing “a landmark for the history of science, as well as a milestone for modern geology” (Vaccari and Lemon 2021, p. 19). The recent IUGS (2021) initiative “The first 100 IUGS Geological Heritage Sites” identifies 11 world class historical geosites which have “made history in the field of the geological sciences” (Vaccari and Lemon 2021, p. 19), as inspiring representations of intangible past phenomena, only materialized through its results.

As so, the geological heritage cannot be limited to geosites and geoheritage element identification, assessment, conservation, valuation, and monitoring. If some geological processes are to be study targets of geoconservation, they must be regarded as a distinct category from those referring to a form or material. They have attributes, of a non-tangible nature, that make them unique and decisive in the construction of geological thinking.

Regarding cultural heritage, according to the UNESCO (2022a), the “Cultural heritage does not end at monuments and collections of objects” (i.e., cultural materials), and “It also includes traditions or living expressions inherited from our ancestors and passed on to our descendants” (i.e., cultural processes). As so, the site-based approach that grounds the UNESCO World Heritage reasoning also evolved into a broader vision including “knowledge, knowhow, skills, practices and representations developed by communities by interacting with the natural environment” (UNESCO 2020). Such conceptual approach can be recalled when considering the perspective of integrating geological processes “with scientific relevance used as a reference, and/or with a substantial contribution to the development of geological sciences through history” (IUGS 2021) within the geological heritage of the planet. The coeval concept for this immaterial kind of geological heritage is the intangible geoheritage, which is closely related to the IUGS Geological Heritage Sites (2021) framed within the History of Geosciences type (Fig. 1).

Fig. 1
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Framing the concept of intangible geoheritage within the geological heritage and the UNESCO Cultural Heritage of the Earth

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The intangible geoheritage is mainly rooted in geological concepts and ideas rather than in objects. They refer to singular past phenomena, inferred from and/or supported by geosites and/or materials, “where the observation of some specific features determined new ideas, theories and interpretations which have changed human understanding of the geological phenomena and of the history of the Earth” (Vaccari and Lemon 2021, p. 19).

Intangible Geoheritage Examples

Despite representing past witnesses of Earth´s history, not any outcrop, mineral, rock, or fossil can be considered displaying heritage value. The documentary record of past catastrophic phenomena like earthquakes is an important source to define the concept of intangible geoheritage, but not any geological hazard can be seen as immaterial geological heritage. It will be necessary to define requirements that make it possible to differentiate, due to its uniqueness, an earthquake from among the enormous number of earthquakes that occur at all times on Earth.

Examples of intangible geoheritage concept include the 1755 Lisbon earthquake and tsunami, one of the two or three most studied earthquakes in history. With an estimated magnitude of 8.7 (Zonno et al. 2009), this event has long held a place among the greatest in the modern world (Brillinger and Bolt 2009). It powerfully trembled not only Lisbon and a sizable surrounding geographic region (Oliveira 2008), but also the era’s intellectual ideas, being considered by some as the birth of modern seismology (Fuchs 2009; Muir-Wood and Mignan 2009) and crisis management and of what are now named as risk management (Almeida 2009). But the worldwide importance of this earthquake is not limited to the scientific and technical communities but it also affected many other disciplines of humankind related to the effects and consequences of the event (Fig. 2). It inspired philosophers, theologians, historians, poets, writers, and artists, and “the related documentation may be considered a monument of Portuguese culture, one that challenges and defies inter-disciplinarity even today” (Barata 2009, p. 41). To mark more than 250 years after the Lisbon´s earthquake, the newly opened Lisbon Earthquake Center provides an immersive earthquake experience enabling to take an incredible journey back in time to 1755 and relive Lisbon’s earthquake through the use of simulators, video-mapping, and interactive 4D technology (Quake 2022; Fig. 3).

Fig. 2
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Copper engraving representing Lisbon during the great earthquake of 1st November 1755. The engraving shows the city in ruins and in flames and tsunamis rushing upon the shore, destroying the wharfs. It also shows highly disturbed water in the harbor, which sank many ships. Passengers in the left foreground show signs of panic. Original in Museu da Cidade, Lisbon (Portugal)

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Fig. 3
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The Lisbon Earthquake Center (Lisbon), where visitors can have an immersive earthquake experience enabling to relive the 1755 Lisbon’s earthquake through the use of simulators, video-mapping, and interactive 4D technology. Photo: courtesy of Quake–Centro do Terramoto de Lisboa

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Other analogous past phenomena can be assigned to this concept like the impact of a large Earth-crossing asteroid about 65 million years ago that supports the Cretaceous-Tertiary mass extinctions (Alvarez et al. 1980; Ganapathy 1980). The impact hypothesis of Alvarez et al. (1980) strongly challenged the Lyellian paradigm of gradualism/uniformitarianism, and it can be also regarded as one of the two most significant shifts in scientific thinking in Earth Sciences of the twentieth century, along with the theory of plate tectonics (Montanari and Coccioni 2019).

Discussion and Conclusion

The intangible geoheritage concept is not immune to the controversies inherent in the emergence of any new concept or idea. By being related to phenomena rather to a thing may be debatable whether it is a scientific concept. However, as pointed by Henriques et al. (2011), the three levels of entities and corresponding components distinguishable in any science, including geoconservation, are the conceptual, linguistic, and physical levels and the last one refers not only to things, but also to facts and proprieties. In this sense, the concept of intangible geoheritage fits into the body of knowledge that constitutes geoconservation, and can be related to the Anthropocene (Ruban 2019).

Tangible geoheritage includes geosites and elements of geoheritage (Brilha 2014). If they contain superlative natural phenomena or areas of exceptional natural beauty and esthetic importance, or if they are outstanding examples representing major stages of Earth’s history, including the record of life, significant on-going geological processes in the development of landforms, or significant geomorphic or physiographic features, they can be included as natural or mixed sites, along with cultural sites, on the UNESCO World Heritage List (UNESCO 2022b). Like the intangible cultural heritage, which refers to social processes, the intangible geoheritage refers to geological processes that condition and have always conditioned human life, being particularly attractive for both geoscientists and historians, among others. In this sense, its study provides special opportunities to overcome the traditional gap between the natural and the human and social sciences (Harrison 2015; Werlen et al. 2016).