*5.2. Inference from Historical Storms in the North Atlantic*

Given their geographic location, archipelagos located north of Cabo Verde off the northwest coast of Africa are likely to be impacted by high-energy storms [25]. The Azores Archipelago is struck by high-energy storms with a frequency every seven years [26], causing several shipwrecks in the harbor of Ponta Delgada on São Miguel (Figure 7a). More recently, the passage of Hurricane Lorenzo in October 2019 caused the destruction

of several piers among the islands, as well as the near disappearance of Lajes das Flores harbor (Figure 7b). The Canary Islands are no exception. The high-energy events that affect the islands have caused considerable damage [12] and even fatalities (Figure 7c,d).

**Figure 7.** Evidence of destruction caused by storms in the Azorean and Canary islands; (**a**) Shipwrecks in Ponta Delgada harbor (São Miguel, Azores) after the December 1996 storm (photo by João Brum); (**b**) Destruction of the pier of Lajes das Flores (Flores Island, Azores) after the passage of Hurricane Lorenzo in October 2019 (Flores Island, Azores) after the passage of Hurricane Lorenzo in October 2019 (*Journal Açores* 9, 2019); (**c**) Shipwreck beached in Gran Canaria after the November 1968 storm (*Efemérides Meteorlógicas de Canárias*, 2018); (**d**) destruction caused by the April 1970 storm that hit Gran Canari (*Efemérides Meteorlógicas de Canárias*, 2018).

Historical records (Table 3) are scarce in terms of available wave-height information. However, in some cases inferences can be made: the events of 21 February 1966 (10 to 12 m), 23 November 1968 (10 m), and 21 April 1970 (8 m). These inferences suggest that storm waves can reach considerable values in the Canary Islands. Empirical models for wave modulation about the island of Gran Canaria indicate average wave height values between 5.22 and 5.58 m [27,28]. Such wave heights are compatible with the maximum average values estimated for boulders summarized for location 6 (Table 2) with averages based on Equation (1) [17] of 5.5 m and Equation (2) [19] of 5.6 m. The location of El Confital beach within the bay also is pertinent. The development of the volcanic edifice of La Isleta (Figure 1c) played an important role in protecting the area. Although consideration of the effect and action of waves before the Last Interglacial Epoch is possible, the formation of a large sandy isthmus since that time connected the La Isleta to the larger island of Gran Canaria, making the area even more sheltered from storm events. No direct evidence is observable at El Confital, but it is necessary to remember that the Canary Islands are subject to tsunami waves resulting from earthquakes like the wider regional event of 1755 that destroyed Lisbon, as well as volcanic flank collapse on the home islands.


**Table 3.** Coastal disturbances between 1755 and 2009 that affected Gran Canaria and islands elsewhere in the Canary Archipelago.

#### *5.3. Comparison with Coastal Boulder Deposits Elsewhere*

Pleistocene and Holocene deposits formed by cobbles and boulders are widely distributed all around the world [3], but studies in coastal geomorphology seldom consider density as related to parent rock types when investigating the range of wave heights necessary for their development as eroded boulders. Application of mathematical formulae such as Equation (1) from Nott [17] to estimate storm wave height has been applied previously to coastal boulder deposits throughout Mexico's Gulf of California, including those formed by limestone, rhyolite, and andesite clasts [4–6]. Extension of this work to include Equation (2) as derived from Pepe et al. [19] also has been applied to coastal basalt deposits in the Azores [1]. The variation in mass among these rock types ranges from 1.86 gm/cm3 for limestone, 2.16 gm/cm3 for the rhyolite, 2.55 gm/cm<sup>3</sup> for andesite, and 3.0 gm/cm3 for basalt.

Among the largest Holocene boulders treated in these studies are those derived from rhyolite shores on the Gulf of California yielding mega-boulders with a calculated weight of 4.3 metric tons requiring an estimated wave height of 16.8 m to shift from the parent rocky shore [5]. In this particular case, the estimated wave height from storms in the Gulf of California is commensurate with the wave height formulated for the largest basalt boulder recorded anywhere on El Confital beach at study site 6 (Table 2). The recent history of hurricanes in the Gulf of California filmed under direct observation, confirms wave heights at least half that size against contemporary rhyolite sea cliffs [5].

The island of Santa Maria in the Azores has yielded a study comparing present-day and Upper Pleistocene basalt boulders [1] most applicable to the present study at El Confital beach in Gran Canaria. The largest Pleistocene boulder recorded in that study is smaller than many of the typical basalt boulders from El Confital, but still exemplified an estimated wave height of 8 m necessary for its emplacement. The largest boulder from a modern deposit on Santa Marine Island was calculated to require a wave height of 6.4 m for its emplacement [1].

Technically, hurricanes have a tropical to subtropical source dependent on high oceansurface water temperatures and excessive air moisture [29]. However, major storms of hurricane intensity also occur in Arctic latitudes, where contemporary and Holocene boulder deposits occur along the coast of Norway. Small boulders formed by low-grade chromite ore with a density of 3.32 g/cm3 are described from Holocene deposits on Norway's Leka Island that imply wave heights as much a 7 m for their emplacement [30].

#### *5.4. Notes on the Geoheritage of El Confital Beach*

El Confital beach takes its name from the former abundance of "confites" (candies in English) on the beach, a popular name given to rhodoliths throughout the Canary Islands for their white color and ball-like shape. Notably, the study area at El Conital beach is richly fossiliferous as known since the visit by Charles Lyell in 1854 to Gran Canaria island [31]. Historically, deposits with fossil rhodoliths, as well as other carbonates were exploited massively to manufacture lime, due to the lack of this resource in the Canary Islands. This industry has led to the loss of essential paleontological paleoecologial, and taphonomical information. The outcrop at El Confital was chosen as a Geosite (site of geological interest, acronym LIG in Spanish) [32,33] and is included in the Inventory of Geosites of the Canary Islands, carried out by project LIGCANARIAS [11,34], due to its high scientific value that represents an area where different types of geological heritage are combined. Stratigraphic sequence is the central feature around which others are related including paleontology, sedimentology, and geomorphology [34–37]. The volcano-sedimentary sequence at El Confital reaches a maximum level of 200 m above sea level in which characteristics of the geological evolution of Gran Canaria island are represented.

As shown in this study, the significance of El Confital is magnified as an example of an accumulation zone of basaltic boulders of different sizes that denote high-energy events essential to understanding the impact of storms and hurricanes in the island groups of the North Atlantic. Apart from the materials belonging to the last interglacial maximum (MIS5e) described in this paper, El Confital includes a range of other features represented by submarine and subaerial basaltic deposits and hyaloclastites (peperites) together with marine sands and conglomerates, aeolian sand dunes, and colluvial deposits [35].

For all its value as a Geosite with high scientific, educational, and touristic value [32,33,35,36], El Confital beach is extremely fragile and vulnerable to human impact and climate change. Therefore, it remains necessary to adopt a management plan that ensures its regulatory protection in the short- and intermediate-term [37]. Although adjacent to the Bahia del Confital Special Conservation Area, Community Interest Area, as well as La Isleta Marine Area, and the Protected Landscape of La Isleta, it is urgent that the Geosite attain an effective geoconservation plan sanctioned by the regional government of the Canary Islands.
