An inter-beach analysis and comparison of the petrographic results of the different Canary beaches under study was made, and the results were additionally compared with those from other hotspot-associated volcanic islands (Hawaii and Cape Verde).
The petrographic results of the Canary beaches reveal that the presence of certain sand grains and their abundance are related to geological and geographic elements and to natural and anthropic processes. The MR-fr volcaniclastic sand grains, with porphyritic texture and altered M-min, have their origin in polygenetic volcanoes (MSS), as can be observed for example in the beaches of Famara in Lanzarote. Likewise, the presence of FR-fr and F-min volcaniclastic sands, with trachytic and fluidal textures, is found in beaches close to outcrops of FDS materials as, for example, in the beaches of Gran Canaria (Las Canteras and San Bartolomé de Tirajana). The presence of MR-fr and M-min grains, with porphyritic and microporphyritic textures and low levels of alteration, is related to beaches close to MRS monogenetic volcanoes as, for example, in the beaches of La Graciosa, Corralejo, the S of Lanzarote, Telde and, to a lesser extent, the N de Lanzarote and NE and S of Gran Canaria. If a comparison is made with the beaches studied in the islands of Hawaii (Marsaglia 1993) and Cape Verde (Le Pera et al. 2021), it can be seen that altered volcaniclastic sands appear in the oldest islands, while glassy grains are fundamentally found in the younger islands. However, in the Canary Islands, the presence of these sands of altered lithoclasts is not related to island age, but rather to the existence of remains of polygenetic Miocene volcanoes close to the beaches. Moreover, the islands of Hawaii and Cape Verde have a high frequency of glassy grains generated in subaerial and submarine eruptions (hyaloclastites), whereas in the Canaries this type of grain is very rare or non-existent. Likewise, the M-min grains of Canary beaches are low in abundance except in some samples (Las Canteras, N of Telde and N of San Bartolomé), whereas in Cape Verde and Hawaii higher abundance values are found. In the analysis of volcanogenic grains in various parts of the world conducted by Affolter and Ingersoll (2019), it was found that lathwork textures predominate for grains of mafic composition, microlitic textures for felsic sands and granular textures for acidic sands. These lathwork textures are recorded for Hawaii and Cape Verde, but in the Canaries are very scarce as the polygenetic volcanoes have very few MSS feldspar basalts and FDS rhyolites are very rare. In general, discriminant models for the origin of sands in different geotectonic environments such as, for example, islands in subduction or continental collision areas (Dickingson 1985; Affolter and Ingersoll 2019), cannot be used in hotspot islands as the percentages of grains of quartz, feldspar or felsic and acidic rocks are low or non-existent (Marsaglia 1993; Le Pera et al. 2020).
The data with respect to calcareous bioclastic grains for the Canary beaches indicate a predomination of both flora and fauna. In some beaches, abundant mollusc fragments are found (N and S of Lanzarote, Corralejo, S of Gran Canaria), and in others coralline red algae (La Graciosa, Telde, Las Canteras). The latter are common today in subtidal and platform waters in the Canaries (Portillo 2008) and include, for example, Jania rubens, Liagora sp., Haliptilon virgatum, Hypnea spinella, Halopithys and Pterocladia capillacea. It is therefore easy to find algal grains in present day beach-dune systems and in beachrock and Quaternary eolianites. In Cape Verde beaches there is a notable predominance of flora over fauna grains (Johnsonn et al. 2013; Le Pera et al. 2021), whereas in Hawaii there is a greater abundance of bioclastic grains of foraminifera, followed by molluscs, algae and corals. The coralline grains have high abundance values in the older islands as the coral reefs are more developed in them (Marsaglia 1993). Coral sands have not been reported for either the Canary Islands or Cape Verde. Finally, the oldest islands of Hawaii and Cape Verde have higher bioclast values at the expense of a decrease in lithoclasts. In the Canaries, the bioclast values are not dependent on island age, but rather on geological and geographic aspects and on beach-related processes involving anthropogenic alteration In this regard, after analyzing for significant differences in the amount of bioclasts by beach type using a non-parametric Kruskal-Wallis test for independent samples (Fig. 6), while no differences were found between urban and semi-urban beaches, highly statistically significant differences were found between urban and natural beaches (p = 0.000) and between semi-urban and natural beaches (p = 0.007). This could be an indication that anthropogenic alteration of the beaches is significantly limiting the production of bioclasts in the subtidal and insular platform area of the urbanized environments, as in the case of the urban and semi-urban beaches as opposed to the natural beaches. In addition, other anthropic actions in the intertidal or supratidal zones, such as the periodic cleaning of urban beaches, the transit of vehicles, or the sand replacement tasks in beaches suffering erosion could also be influencing this pattern.
As for the intraclasts, these sand grains have low abundance values in all the Canary samples (< 15%), with the highest found in the beaches of Corralejo, La Graciosa and the C sector of Las Canteras. This could be attributable to the presence of nearby Quaternary substrates of beachrock, eolianites and paleosols. In Cape Verde, moderate values of sparite-cemented intraclasts have been recorded, whereas in the Canaries and Hawaii they are scarce or non-existent (Marsaglia 1993; Le Pera et al. 2021).
It has been possible to relate the high total values of abundance for volcaniclastic grains in some beaches, along with a decrease in total values of bioclasts, to geomorphological aspects such as the location of beaches at ravine mouths or their proximity to cliffs and ravine slopes (Tables 1 and 2) which contribute sediments. This tendency has been observed in the beaches of the S of La Graciosa (samples 4–5), NE of Lanzarote (samples 8–9), S of Las Canteras (samples 33–37), Telde (samples 39–40 and 42), and the N and W of San Bartolomé de Tirajana (samples 44–45, 49–50). In contrast, the beaches where the greatest abundance of bioclasts is observed have no coastal area ravines, cliffs or slopes as, for example, in the N of La Graciosa, in Lanzarote and in Fuerteventura (Corralejo). The influence of landforms was not considered in the consulted published sand provenance analyses for Cape Verde and Hawaii. The analysis of the petrographic data of volcanic bioclasts, intraclasts and lithoclasts in kilometer-long beaches (as with samples 26–37 in the case of Las Canteras) suggests that coastal dynamics could be acting as a homogenizing factor in terms of types of component and that the small variations are due to the influence of geological and geographic factors. Thus, it was found that there is an increase in intraclasts in the C sector of this beach due to the presence of beachrock, eolianite and paleosol substrates, whereas in the S sector there is a higher presence of volcanic lithoclasts (MR-fr and M-min) as a consequence of sediment contribution from the mouth of La Ballena ravine. These variations in components are observed if different beaches on the same island are compared, as for example with the beaches of La Graciosa, Lanzarote or Fuerteventura. Thus, the high bioclast values of El Reducto (Lanzarote) are due to beach regeneration works with nearby eolian sediments. However, sometimes there are variations in one or other of the different percentages of abundance which may be attributable to a variety of reasons. By way of example, in the case of the E coast of Gran Canaria (Telde), it is possible to identify the influence of longshore drift from N to S beaches with the contribution of felsic grains (FR-fr), whereas there are no outcrops of this type in the littoral of the beaches under study in this area.
Finally, in the beaches of Corralejo (Fuerteventura) it was possible to corroborate the conceptual model of Hernandez-Calvento et al. (2017) of an island-encapsulating beach-dune system for a circum-island route of sandy grains in hotspot-associated volcanic islands involving successive littoral beaches and terrestrial dunes. However, in the other beaches studied in the eastern islands, the petrographic results seem to indicate that, though the beaches may have been interconnected at some point in the past, many currently behave as independent coastal sandy systems disconnected from the rest. In these cases, local factors (geographic and geological), as well as the degree of anthropogenic alteration, are the variables that best explain the differences observed in the petrographic analyses.