Introduction

The geology of the Canary Archipelago is almost entirely dominated by a succession of volcanic units and structures (Abdel-Monem et al. 1972; Carracedo et al. 2001; Jutzeler et al. 2010). The national territory features a unique landscape with diverse lava flows and pyroclastic deposits. Yet, at the regional level, there are significant variations in lithology, environment, scenery, and even meteorological conditions (Bechtel 2016). The national territory features a unique landscape with diverse lava flows and pyroclastic deposits. Yet, at the regional level, there are significant variations in lithology, environment, scenery, and even meteorological conditions. Numerous geomorphological features (large escarpments and deep ravines), which make the implementation and maintenance of infrastructures extremely difficult (Llanes et al. 2009).

In the Canary Islands, the lithology, structure and age of the rock formations directly or indirectly condition the geotechnical behaviour of the materials (González de Vallejo et al. 2006; Rodríguez-Losada et al. 2009) (see Fig. 1). Effusive volcanic activity in the Canary Islands has contributed to the formation and above-sea growth of the archipelago. Currently, the surface showcases either recently formed volcanic structures from the latest events or highly weathered volcanic shields and formations from the Miocene–Pliocene era, of which only a fraction of their original extent is observable today (García-Gil et al. 2023). This is the case for the oldest rock masses of the western and central islands (La Palma, La Gomera, Tenerife, Gran Canaria), whose age can be inferred from the deep, closed ravines excavated by continuous erosion and which, not having been filled by more recent volcanic emissions, present a morphology in the form of sharp ridges, very deep and narrow ravine bottoms and slopes with gradients that, on occasion, approach verticality. This abrupt landscape contrasts with the smooth relief of the older, more eroded eastern islands (Lanzarote, Fuerteventura) (Santana et al. 2006).

Fig. 1
figure 1

Classification of volcanic and sedimentary geotechnical units, and their geographic distribution in the Canary Islands. Data source: GEBCO_2023 Grid GEBCO Compilation Group (2023) GEBCO 2023 Grid (https://doi.org/10.5285/f98b053b-0cbc-6c23-e053-6c86abc0af7b)

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Although there is a prolific bibliography on the geology of the Canary Islands, in relation to the geotechnical properties of volcanic rocks, the information is not so extensive as the methods of analysis are designed for all types of rocks and not specifically for volcanic rocks. However, several studies have been carried out on the geomechanical properties of volcanic rocks in the Canary Islands, which have allowed a better understanding of the behaviour of both the rock matrix and the rock mass (González de Vallejo et al. 2007; Hernández-Gutiérrez 2014; Muñiz Menéndez and González-Gallego 2010; Perucho 2016; Rodríguez-Losada et al. 2007; Serrano et al. 2002a, b; Uriel and Serrano 1973). In addition, some specific studies have been carried out on the behaviour of volcanic rocks on slope stability in terms of risk and hazard in the Canary Islands (del Potro and Hürlimann 2008; González de Vallejo et al. 2020; Hürlimann et al. 2000; Leyva et al. 2023; Muñiz Menéndez et al. 2018; Rodríguez-Losada et al. 2009) and on other volcanic islands such as Ischia and Strombli in Italy (Massaro et al. 2023; De Falco et al. 2023; Forte et al. 2019; Romagnoli et al. 2009), Madeira in Portugal (Quartau et al. 2018) or Santorini in Greece which have allowed us to determine how these processes evolve on a local and regional scale and to establish the appropriate methodology for their analysis and prevention.

To provide basic geotechnical reference information for civil engineering and building projects, the different territorial surfaces of each of the seven islands were divided into geotechnical units based on a combination of lithological, structural and geomechanical criteria. As a whole, these units are sufficiently homogeneous for their cartographic delimitation and to establish minimum criteria for their geotechnical behaviour in civil engineering and building works. The result of this territorial zoning concluded with the publication of the Geotechnical Map of the Canary Islands (de Canarias 2011).

Based on the research team's three decades of experience in the study of the geotechnical characterisation of volcanic units (Rodríguez-Losada et al. 2007; Hernández-Gutiérrez 2014; Santamarta et al. 2023) and the problems of slope instability associated with them (Rodríguez-Losada et al. 2009; González de Vallejo et al. 2019, 2020; Leyva et al. 2023), it has been possible to establish which types of slope movements are the most frequent for each geotechnical unit. The main objective of this study is to present a classification of the types of slope movements characteristic of volcanic terrains and to assign to each of them the geotechnical units that have been previously defined in the Geotechnical Map of the Canary Islands. The aim of this classification is to offer technicians and authorities a useful tool for managing landslide risks and hazards in volcanic terrains, as well as for designing of mitigation measures. This tool is not only applicable to the Canary Islands, but can be extrapolated to other volcanic territories in the world.

Materials and methods

The proposed classification was developed following a lengthy process, with a series of documentary materials obtained, on which the classification was based. Thus, in the first stage, studies were carried out to characterise the volcanic units of the Canary Islands and their spatial distribution, which culminated in the publication of the Geotechnical Map of the Canary Islands (de Canarias 2011). In a second phase, based on the experience accumulated in the study of landslides over the last 30 years, the types of slope movements among those defined by the USGS (Highland and Bobrowsky 2008) occurred in the archipelago. Finally, the relationship between these slope movements and the geotechnical units was found. The materials used to obtain the proposed classification are detailed below chronologically (Table 1).

Table 1 Chronology of documents and materials applied to the development of the proposed classification
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In 2002, the Regional Government of the Canary Islands initiated a project aimed at drafting a guide for the planning of geotechnical studies in the archipelago (Hernández et al. 2010), which resulted in the publication of the GETCAN-011 guide (de Canarias 2013). In the first stage, all the geological units of the Canary Islands were geotechnically characterised based on sampling at 330 locations and the execution of more than 9000 laboratory tests.

The information obtained from the tests and field surveys was projected onto the 1:25,000 scale geological map available for the archipelago (Barrera Morate and García Moral 2011). Based on the cartographic traces of the different geological units, a set of geotechnical units were established, merging within one or more geological units with similar lithological–geotechnical characteristics. This made it possible to establish, for each geotechnical unit, the minimum criteria to be considered in the GETCAN-011 guide. In this way, and depending on the characteristics of the construction and the geotechnical problems associated with each geotechnical unit, the minimum surveys required in each case would be proposed. This zoning resulted in seven volcanic geotechnical units, excluding sedimentary deposits and anthropic fills which have been represented in the Geotechnical Map of the Canary Islands (see Fig. 1). This resulted in a classification of geotechnical units of the Canary archipelago according to lithology, structure and their spatial distribution (Table 2). These geotechnical units of volcanic nature have been numbered according to the two main groups of geological units present in the Canary Islands:

  • Submarine volcanic units corresponding to the first stages of growth of the island edifice from the ocean floor to above sea level. They are represented by Unit I, basal complexes.

  • Subaerial volcanic units, which correspond to the volcanic products of the eruptions that took place in the growth stages of the islands once sea level had been surpassed. The following units have been differentiated: Unit II, salic lava flows and rock masses; Unit III, altered basaltic rock masses; Unit IV, fresh basaltic lava flows; Unit V, pyroclastic deposits; and Unit VI, volcanic breccias.

Table 2 Classification of volcanic geotechnical units of the Canary Islands
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Based on the types of slope movements defined by the USGS, United States Geological Survey (Highland and Bobrowsky 2008), distinguishing in general between rock masses (lava flows) and granular materials (pyroclastic deposits), it was deduced that the following movements are characteristic of volcanic units:

  • Rockfalls: these are sudden falls of blocks or masses consisting of non-interacting rocky blocks delimited by preexisting discontinuity planes (joints, stratification surfaces, tensile cracks, etc.). The material descends downslope mainly by free fall, bouncing or rolling. Preferentially affect solidified lava flows. Four types have been distinguished:

    1. o

      Falls of blocks and gravel occur in granular materials consisting of a fine, soft matrix encompassing larger, hard fragments (gravel, bombs and blocks). Erosion of the matrix causes the detachment and occasional fall of the larger fragments. These are characteristic of granular deposits (sedimentary or pyroclastic).

    2. o

      Falls by undercutting: the differential erosion that occurs in the stacking of layers of hard materials (rocks) resting on soft levels (slags and pyroclasts) causes the rocks to lose their support and fall. The cooling joints affecting basaltic lavas favours the fall of prismatic blocks.

    3. o

      Topples: these occur mainly on rock faces with slender vertical prisms that under the action of gravity, rotate around their base and fall. They are frequent in lavas with a mainly basaltic composition.

    4. o

      Collapses: this peculiar movement is characteristic of "pahoehoe" basaltic flows tubes or cavities that are intersected by slope surfaces. When the cavity roofs collapse, slope instability occurs.

  • Spreads: the rupture takes place along flat surfaces of weakness or preexisting discontinuities (bedding, cooling joints, fault planes, etc.). They are not usually very deep, although they can have a large extension and reach great distances. Can affect any type of volcanic units. The rock masses or pyroclastites usually slide as parallelepiped blocks previously separated by discontinuities or tension cracks.

  • Rotational landslide: a slide in which the breaking surface is curved concave upwards and the movement of the sliding mass is approximately rotational around an axis parallel to the ground surface and transverse to the slide. Can affect any type of volcanic units.

  • Debris: are very rapid processes of falling of large masses of highly diaclased rocks or with a high degree of weathering, so that when they fall they generate debris that is disintegrated and pulverised during the fall, giving rise to deposits with a chaotic distribution of blocks, with very different sizes (fine and coarse) without structure and with low porosity. They occur in altered lava rock mass.

  • Rock avalanches: are very rapid processes of falling of large masses of hard, moderately diaclased, poorly weathered rock mass, so that when they break off they generate a mass of medium-to-large rock fragments, without fine crushed material. They are deposits with a large number of voids. They occur in healthy lava rock mass (fresh).

  • Debris flows: affecting loose or poorly cemented granular deposits. These are complex movements involving rock fragments, boulders, cobbles and gravels in a fine matrix of sands, silts and clay mobilised by water saturation.

It should be borne in mind that lithological (singularities of the volcanic units) and structural (shrinkage diaclases) control factors act differently, first in triggering the detachment of the rock slope, and subsequently in the propagation of the landslide of the mass of materials (rocks and pyroclasts) generated by the detachment. For this reason, movements classified as landslides are not as dependent on the volcanic nature of the materials involved as are landslides.

Description and properties of volcanic geotechnical units

In order to study the mechanical behaviour of volcanic geotechnical units, geomechanical classifications were applied. From rock mass descriptors, indices are obtained and used in mathematical formulations for geotechnical design. The most widely used geomechanical classifications are currently three: Q-system (Barton 1976), RMR (Bieniawski 1989), and GSI (Hoek and Brown 2019). The Q-system implies a rigorous analysis of the state of the joints of the rock mass, while the RMR and GSI are simpler and quicker to determine, as these are descriptive methods based primarily on the observation of the state of the massif and its appearance. In the volcanic terrains of the Canary Islands, the Q-system is applied almost exclusively in tunnels, while RMR and GSI are used for slope stability. By its part, RMRb is the basic RMR index, without penalising for the orientation of the discontinuities with respect to the work to be undertaken.

Recently, specific geomechanical classifications for volcanic rock masses have been developed by adjusting or modifying existing ones (Miranda et al. 2018; Muñiz Menéndez and González-Gallego 2018; Singh and Connolly 2003), but their application is still limited and they have not already replaced the original RMR and GSI.

Some authors have used the GSI index for the study of Canary Island volcanic formations with a high degree of heterogeneity, disintegration and agglomeration (del Potro and Hürlimann 2008), while the RMR is the most commonly used for more massive and homogeneous rock masses (Santamarta et al. 2023). In this study, the RMR was chosen for the description of the volcanic geotechnical units. In some cases, the GSI index was used to classify those geotechnical units with a higher degree of alteration, and in these cases the correlation with the RMR index was obtained through the expression given by Hoek and Brown (1997) (Eq. 1):

$${\text{GSI }} = {\text{ 5xRMR for RMR }} > { 23}{\text{.}}$$
(1)

Results and discussion

The description of the volcanic geotechnical units of the Canary Islands and their characteristic RMRb values are presented below:

Unit I basal complexes: The basal complexes of the Canary Islands are represented by Cretaceous sediments, submarine lavas (Fig. 2) and plutonic rocks (gabbros and syenites). This complex is crisscrossed by a multitude of dikes with such a high frequency that they often leave no trace of the host rock. They often present a high degree of alteration, so rocky materials are generally crumbly and difficult to recognise. All this gives them the characteristics of weak, fractured rock, generally with RMRb values (Bieniawski 1989) lower than 40. They occur in areas of abrupt and tectonised relief of great heterogeneity, where debris avalanches are frequent.

Fig. 2
figure 2

Detail of a pillow lavas from the Unit I basal complexes

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Unit II salic lava flows and salic rock mass: This unit consists of highly resistant rocky materials of a salic nature, such as trachytes and phonolites (see Fig. 3), manifesting in two types of outcrops: (1) thick lava flows, typically horizontally arranged or forming substantial tabular packages. These create moderately sloped expanses with extensive horizontal coverage. Occasionally, these packages may comprise very compact breccias composed of fragments with a similar salic nature. (2) Domes take the form of massive rock formations with vertically substantial blocks anchored in the subsoil and limited horizontal extension. Despite these differences, the geotechnical characteristics of both outcrop types are considered practically similar and are classified as the same geotechnical unit. These formations consist of massive bodies of trachytic or phonolitic composition, generally exhibiting high bearing capacity and Rock Mass Rating (RMRb) values ranging between 75 and 90.

Fig. 3
figure 3

Detail of a massive and compact phonolitic lava flow from the Unit II flows and salic rock masses

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Unit III altered basaltic rock masses: Formed by relatively thin basaltic flows (1 m or less) and moderate-to-high alteration (Fig. 4). The outstanding peculiarity of basaltic flows is that they appear as a vertical alternation of hard levels of compact basalt and soft levels of slag (granular material), which generally appear in the form of autobreccia due to the degree of alteration. These altered rock masses are also interspersed with pyroclastic covers and frequent rubefacted areas, known as "almagres", which sometimes correspond to palaeosoil levels that have been calcined by the heat of the overlying lava flow. Generally, the layers present gentle dips that can vary between 10° and 30°, but due to their age, they form steep reliefs with steep slopes, which give rise to multiple problems of slope stability. They usually have RMR at the slope surface values between 40 and 60.

Fig. 4
figure 4

Altered basaltic lava flow bounded above by pyroclasts and below by scoria. Unit III altered basaltic rock mass

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Unit IV fresh basaltic lava flows: This unit includes basaltic lava flows that retain their original structure due to their low state of alteration so that the "pahoehoe" and "aa" types can be distinguished. The "aa" lavas formed from magmas somewhat more viscous than the "pahoehoe" lavas, flow more slowly and have a completely different appearance. The lava advances like the chains of a backhoe, so that the cooled scoriaceous surface collapses in front of the front of the moving lava flow and is covered by the still molten interior of the advancing lava. Thus, the vertical section of an "aa" lava consists of a central band of dense rock furrowed by a network of cleavages or fissures formed by shrinkage as the melt cools and solidifies, bounded below and above by two irregular scoriaceous bands.

Depending on the effusive rate, magma viscosity, chemical composition, gas content, slope of the terrain and distance travelled, these basaltic flows present great variability, so they have been divided into two subunits:

  • Subunit IVa "aa" lava flows little scoriaceous (Fig. 5): Basaltic lava flows of type "aa" that present thicknesses of basaltic compact (rock) equal to or greater than 2 m, generally conserving their lateral continuity, with scoriaceous levels less than 0.5 m thick and absence of cavities. They have RMRb values higher than 80.

  • Subunit IVb "Pahoehoe" lava flows and "aa" lava flows very scoriaceous: "pahoehoe" basaltic lava flows and "aa" lava flows with basaltic compact (rock) thicknesses less than 2 m, intercalated scoriaceous levels and/or presence of cavities. Typical RMRb values range between 60 and 80.

Fig. 5
figure 5

a Subunit IVa “aa” basaltic flow of great thickness of rock compact and thin levels of scoria. b Subunit IVb “aa” thin blocky basaltic lava flow and a large amount of scoria

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In both subunits, instabilities of natural or excavated slopes are frequent because loose scoriaceous levels can produce collapses of the massive levels by differential erosion.

Unit V pyroclastic deposits: According to their degree of consistency, which influences their geotechnical behaviour, they have been divided into two groups:

  • Subunit Va ignimbrites and tuffs: these are moderate or high strength rocks (ISRM 1981) (Fig. 6). They correspond to very compact pyroclastic pumiceous or cinereous pyroclastic deposits, such as ignimbrites with or without eutaxitic texture or compact cinerites. This type of material forms when a mass of pyroclastic products is transported as a gas dispersion with high or moderate particle density; the result is a material with hard rock characteristics, with a variable degree of consistency and/or cementation. They have surface RMRb values between 60 and 75.

  • Subunit Vb loose or weakly cemented pyroclastic deposits: these are soil-like granular deposits. They are usually loosely packed and easily collapsible. They form when magma fragments fall and are deposited in the vicinity of the eruptive centre. They are low-density pyroclastic deposits, with average dry bulk specific gravities typically between 7 and 13 kN/m3.

Fig. 6
figure 6

a Trachytic ignimbrite quarry from the Subunit Va welded ignimbrite. b Basaltic and salic lapilli-sized pyroclastic deposits from subunit Vb loose or weakly cemented pyroclastic deposits

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Unit VI volcanic breccias: This unit is associated with violent eruptive episodes highly explosive, sometimes related to caldera collapses or giant landslides of the volcano slopes phenomena. The end results a BIM-rock (block-in-matrix rocks) a mixture of rocks, composed of geotechnically significant blocks within a bonded matrix of finer texture (Medley and Zekkos 2011). The chaotic and brecciated mass formed by blocks of different natures, generally very angular, with a great variation in size, surrounded by a fine matrix that is more or less cemented and occasionally gives rise to rocks of high strength (ISRM 1981) (see Fig. 7). They form thick packages (up to hundreds of metres thick) and present steep slopes of compact and chaotic breccias of mono- or polymict nature. They can exhibit high strength and, in some cases, moderate strength s rock characteristics. They have RMRb values between 60 and 75.

Fig. 7
figure 7

Detail of a debris avalanche deposit from the Unit VI volcanic breccias

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Based on the study of hundreds of slope movements that have occurred in the Canary Islands and in other volcanic archipelagos, such as the Azores and Madeira (González de Vallejo et al. 2019), it has been established which types of instabilities are characteristic or common to each volcanic geotechnical unit to determine the type of slope movement that could affect the slope or hillside in question.

This classification of slope movement types versus volcanic geotechnical unit is presented in Table 3A and B where:

  • In the left column (Type of instability) the movement and its denomination are described, accompanied by a graphical visual description that allows a simple identification of the type of movement

  • The central column indicates the geotechnical units, previously defined in the Geotechnical Map of the Canary Islands (Government of the Canary Islands 2011), that are affected by the type of movement in question.

  • The right-hand column presents some observations on the characteristics of the detached materials and other singularities that characterise the type of instability.

Table 3 Classification of instability types versus volcanic geotechnical units described in the Canary Islands
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Using Table 3A and B, we can make a preliminary estimate of the type of instability that may affect a particular volcanic geotechnical unit. Table 3A represents rockfalls, which usually manifest themselves as local falls of rocks, while Table 3B represents instabilities consisting of movements of large masses of rocks or granular pyroclasts.

The case of "aa" basaltic lava flows is particularly significant because experience in the Canary Islands has shown that the majority of slope movements involve these materials. This is due to the uniqueness of the structure of these lavas. The slope resulting from the clearing of a basaltic lava of type "aa" always shows a vertical section with a central band of dense rock furrowed by a network of cooling joints or fissures, bounded at top and bottom by two irregular scoriaceous bands (granular material). The cooling joints isolate prismatic. The much higher erosion of the lower scoriaceous level at the bottom against the overlying basaltic blocky level generates ledges.

Conclusions

This paper presents the most characteristic RMR values of the defined volcanic geotechnical units, which can be used as a reference for geotechnical calculations. In fact, as expected, there is no relationship between the RMR value characterising each volcanic geotechnical unit and the type of associated movement. Nevertheless, the RMR value remains essential for the assessment of the quality of the volcanic rock mass and for the geotechnical calculation necessary for studies aimed at stabilising the unstable slope.

The physical and resistant properties of each type of material, together with the presence of water, govern the geotechnical behaviour of volcanic slopes, but also the lithological and structural characteristics are closely related to the type of instability that can occur.

Conversely, there is an unequivocal relationship between the type of geotechnical unit and the type of associated slope movement, which has allowed instabilities on volcanic slopes to be classified according to the materials involved in the processes, generally distinguishing between rocks, debris and soils, and by the mechanisms and types of rupture.

Knowledge of the types of slope instability that may affect each volcanic geotechnical unit allows technicians to anticipate future hazards that may negatively affect public infrastructures or spaces frequented by people. Moreover, the classification proposed in Table 3A and B is an interesting planning aid for civil engineering and building works. For the 8 types of volcanic geotechnical units described in the Canary Islands, 8 types of slope movements have been described. Some instabilities are exclusive to one type of geotechnical unit, such as Unit IVb collapses, but others, such as rotational landslides, can affect all 8 geotechnical units.

In terms of the occurrence of movement types, rockfalls in the form of isolated blocks are the most frequent, with mass movements being the least frequent. This is because basaltic flows are the dominant units in the Canary Islands, and rockfalls affect both subunits IVa and IVb.

As the volcanic units of the Canary Islands are characteristic of most volcanic regions of the planet, the proposed classification is perfectly applicable to any volcanic territory.