Large boulders have been found on marine cliffs of 24 study areas on Minorca, in the Balearic archipelago. These large imbricated boulders of up to 229 t are located on platforms that comprise the rocky coastline of Minorca, several tens of meters from the edge of the cliff, up to 15 m above the sea level and kilometers away from any inland escarpment. They are mostly located on the south-eastern coast of the island, and numerical models have identified this coastline as a zone with a high probability of tsunami impact. The age of the boulders of the studied localities range between 1574 AD and recent times, although most of them are concentrated around the year 1790 AD. Although some storm waves might play a role in their dislodging, the distribution of the boulder sites along the Balearic Islands, the degree and direction of imbrication and the run-up necessary for their placement suggest transport from northern African tsunami waves that hit the coastline of Minorca.
Although they are less frequent than those of the Pacific and Indian oceans, tsunamis in the Mediterranean Sea are well known from historical accounts (Soloviev, 1990). Large boulder accumulations observed and studied on various coastlines of the western Mediterranean have been associated with extreme wave events (tsunamis or storms): France (Shah-Hosseini et al., 2013), southern Italy (Barbano et al., 2010, 2011; Mastronuzzi et al., 2007; Mastronuzzi and Pignatelli, 2012; Pignatelli et al., 2009; Scicchitano et al., 2007, 2012) and Algeria (Maouche et al., 2009). Large boulders placed over coastal rocky cliffs on Minorca island have been found mainly on the south-eastern and western coastline (Roig-Munar, 2016) (Fig. 1). Some are positioned well above the maximum stand of any recorded storm wave (up to 27 m), many show imbricated boulder ridges, and all of them are located far from any high inland relief that might explain an origin from gravitational fall.
Boulder sites at the Balearic Islands (top). Location of the
sampled areas (middle):
The presence of large boulders on the rocky shores of the Balearic Islands has been analyzed by Bartel and Kelletat (2003), Schefers and Kelletat (2003) and Kelletat et al. (2005), but only on the island of Majorca. The authors linked the presence of large boulders on the coastal platform of Majorca with storm waves and/or tsunami processes, establishing a simple equation (transport figure) to discern those displaced by a storm wave or a tsunami event. In fact, in many areas of the western Mediterranean, imbricated, metric size boulders have been interpreted as remnants of the tsunamis that occurred in the last centuries (Pignatelli et al., 2009). Only on the Atlantic coast, which has much higher fetch, storm wave period and tidal range, are imbricated boulders at high altitudes tied to storm processes (Hansom and Hall, 2009; Etienne and Paris, 2010; Hall, 2011). However, the distinction between tsunami or storm boulders is not easy nor without controversy, though it is based on a set of sedimentological, morphological and chronological criteria to be treated in each case (Scheffers and Kinis, 2014). The main goal of this article is to demonstrate that some of the boulders located close to the coastal cliffs of Minorca were transported and deposited by tsunamis that occurred in the recent past and mostly originated from submarine earthquakes on the Algerian coast.
Both from a geological and geomorphological point of view, Minorca is divided into two parts by an imaginary WNW–ESE line that extends from Maó to Cala Morell (Fig. 1): (a) the Migjorn, which covers the southern half of Minorca, is formed by undeformed calcareous materials from the upper Miocene forming a nearly horizontal platform; and (b) the Tramuntana includes all the outcrops of Palaeozoic, Mesozoic and Oligocene age. These materials are faulted and folded by the alpine orogeny and constitute the northern half of the island is characterized by gentle hills and valleys.
The eight study sites of the southern sector (Fig. 1) and the eight study sites of the western sector are located on carbonated, horizontal, well-developed bedding, with Upper Miocene rocks forming a marine cliff with heights between 4.5 and 20 m. Five of the eight study sites of the northern sector correspond to outcrops of massive Jurassic limestones, forming sea cliffs between 2 and 20 m height. The other three study sites of the northern area are located on Plio-Quaternary eolianites: the Tirant and Tusqueta sites constitute a gentle ramp where cliffs are absent, while in Punta Grossa, eolianites comprise an 8 m high coastal cliff.
The Mediterranean basin is characterized by a highly indented coastline that creates some small and well-defined subbasins, where wave energy is conditioned by wind speed and by limited fetches (Lionello et al., 2006). In the western Mediterranean, the most intense waves come from the NE (Sotillo et al., 2005), although the NW also generates strong waves between the Balearics, Corsica and Sardinia (Bertotti and Cavaleri, 2008).
The coast of Minorca island is subject to a maritime climate characterized in the last 50 years by a maximum wave height of 10 m from a NNE-dominant direction (Cañellas, 2010) (Fig. 1). The eastern coast of the island is characterized by a maximum wave height of 8.5 m with a dominant N component (Cañellas, 2010). At the northern sector of the island, the maximum wave height recorded since 1958 was 11 m height from a NNE direction. The Hs50 is estimated at 9.88 m (Cañellas, 2010). Monthly maximum periods calculated for WANA points around Minorca are between 11 and 14 s (Fig. 1). The tidal regime in Minorca is of very low amplitude (30 cm), almost negligible for this study.
Mediterranean hurricanes, called medicanes in the Mediterranean, are generated by
intense tropical cyclones and may cause a more likely extreme waveform on the
coast of Minorca. The remarkable medicane of 10–11 November 2001 was
associated with the seventh most intense cyclone around the Mediterranean,
in the period ERA-40 (1957–2002) and is the most intense of all
detected medicane in the westernmost Mediterranean, near the Balearic Islands
(Genovés et al., 2006). The wind exceeded 150 km h
According to Papadopoulos (2009), the major tsunamigenic source in the western Mediterranean is located north of Algeria (Fig. 2). The last tsunami registered was in 2003 and resulted in a large amount of damage to several marinas and entrances of the Balearic Islands, mostly due to harbor resonance (Vela et al., 2014). Roger and Hébert (2008) made a numerical simulation of this tsunami affecting the Balearic Islands (Fig. 3). Several seismic tsunamis have been recorded in the Balearic Islands; some of them have been described in chronicles as Fontseré (1918) (Table 1).
Instrumental seismicity of the western Mediterranean region (from ISC–GEM Global Instrumental Earthquake Catalogue) for depth interval 0–50 km. Modified from Vanucci et al. (2004). P refers to Palma, C refers to Campos and S refers to Santanyi.
Tsunami simulation, generated from a northern Algeria earthquake,
impacting the Balearic Islands. Accumulated maximum height 1.5 h after the
break of the fault, three segments at a time, with a deviation of 80
Historical tsunami phenomena impacting the Balearic Islands, modified from Roig-Munar (2016). Information sources (IS): (1) Fontseré (1918), (2) Martinez-Solares (2001) and Silva and Rodríguez Pascua (2014) (see Fig. 2, for location).
In this study, 3.144 boulders located in 24 areas of Minorca island (Fig. 1) have been analyzed. Boulder size was measured, as well as height above sea level, and the distance from the edge of the cliff. Orientation and imbrication were also considered, together with their geomorphological context (Fig. 4). The transport figure (TF; Scheffers and Kelletat, 2003) was used to assess the power needed to dislodge and transport each boulder. TF is calculated as the product of the height above sea level, distance from the edge of the cliff and weight. Scheffers and Kelletat (2003) consider boulders with TF > 250 to be indicative of tsunami boulders. In this paper we focus our study on boulders with TF > 1000 and on boulders found on cliffs well above the maximum storm wave height recorded in Minorca, which is 11 m (Cañellas, 2010).
Geomorphology map of the Alcaufar area (SE Minorca) White circles show boulder orientation for each site. Main circle shows mean wave directions recorded at Maó buoy. Yellow circle shows mean extreme wave direction.
Calculation of boulder weights requires a good estimation of density and
volume (Engel and May, 2012). In most cases the product of the three axis –
In addition to TF, different equations (Table 2) have been applied to all
the localities to calculate the water height required to dislodge and/or move
each boulder. Nott (2003) has pre-defined settings for transported boulders
(submerged, subaerial and joint-bounded boulders JBB), and for each boulder
type, a different equation for both tsunami and storm waves. Most
Minorcan boulders were dislodged from cliff edges (Fig. 6), so joint-bounded
and subaerial scenarios must be considered. Only nine boulders show features
(marine fauna or notch fragments) indicating that they were originally submerged.
Pignatelli (2009) defined a new equation to obtain the minimum tsunami
height (HT) that can move a joint-bounded boulder (JBB). The Nott-derived
equation differs from the original in relation to the
Equations used in the analysis of Minorca boulders.
The ages of the boulders were determined using two different methods: (a) radiocarbon
dating of marine incrusting fauna, and (b) dating surface
post-transport features. Most of the boulders show unconformable
post-depositional solution pans on the surface, related to karstic
dissolutions after the transport of the boulder. Some (Fig. 5b) of these
post-depositional solution pans intersect pre-existing ones that developed
conformably with stratification. Karstic dissolution rate of these pans was
estimated at an average of 0.3 mm yr
Other qualitative observations were taken into account: (a) relation of the boulders with their source area and presence of fractures that can promote detachment of the boulders, (b) the presence of incrusting marine fauna indicating the origin of the boulder before its displacement, (c) the presence of pre-detachment and post-detachment solution pans which have been used as date indicators of boulder emplacement, (d) the degree of rounding of the boulders, presence or absence of other types of sediment as well as presence of abrasion surfaces due to boulder quarrying and transport, and (e) the presence of flow-outs, which are areas with denudated beds forming channels over the cliff favoring the entry and acceleration of the water flows and leaving a boulder ridge in its front.
The 24 areas analyzed (Fig. 1) have been grouped into three sectors: SE, W and N. All the boulders were processed, but those with a TF lower than 1000 were excluded from the final analysis. Therefore, results are based on the analysis of 720 boulders.
Although 1.766 boulders have been analyzed in eight areas of the SE sector
(Figs. 1 and 7), only 274 (16 %) had a TF > 1000. These
boulders have an average size of 3.1 m along their longest axis (
Engel and May (2012) formulations show that the boulders with a TF > 1000 from this sector require a column of water between 8.8 m (subaerial) and 14.4 m (JBB) to explain storm wave run-ups, and between 7.3 and 8.7 m for the tsunami run-ups.
Comparison for JBB with TF>1000 of calculated
storm run-up, tsunami run-up, transport figure and maximum wave height.
Location and main characteristics of SE Minorca boulders. Picture corresponds to an imbricate ridge of boulders in Sant Esteve. Geomorphological sketch shows boulder distribution at Alcaufar.
We calculated that 33 % of the TF > 1000 boulders are in areas above the maximum stand of the waves registered (7.5 m), and many of them show imbrication patterns. Due to these two reasons, we interpreted these boulder deposits as produced by tsunami events. However, 79 % of all the boulders are positioned at a height at which they can be reworked by storm waves.
The boulder setting of this sector can be characterized by the presence of several ridges of imbricate boulders (five of the eight sites show this setting) (Fig. 7), as well as subrounded boulders (5 of 8), and isolate groups of imbricate boulders (4 of 8). Although the cliff altitude of this sector is quite low (6.8 m, average), and many sites show subrounded blocks (5 of 8), there is no clear relationship between these characters. As an example, some of the lower cliffs do not show any ridges, while some higher cliffs do have ridges.
Along the cliffs of the western area (Figs. 1 and 8) 1.043 boulders were
measured, and 232 boulders (22 %) showed a TF > 1000. These
boulders have an average size of 2.38 m along the longest axis (
Formulations from Engel and May (2012) show that the boulders with a TF > 1000 require a column of water between 13.7 m (subaerial) and 18.6 m (JBB) to explain storm wave run-ups, and between 12.4 and 13.6 m for the tsunami run-ups. Almost all the TF > 1000 boulders are positioned above the maximum stand for waves registered along the western coast of Minorca (8 m). Only 16 % of all the boulders are positioned at a height at which they can be reworked by storm waves. The storm run-up heights for these boulders are out of the reach of storm waves.
The boulder setting of the western sector of Minorca is characterized by higher cliff altitudes and imbricate boulder ridges at half of the sites analyzed (4 of 8). Only two of the sites show subrounded boulders – the lower sites – and just one has isolated groups of imbricate boulders.
Along the northern coast of Minorca, 338 boulders have been measured (Figs. 1 and
9), and 214 (63 %) showed a TF>1000. The boulders have an
average size of 2.56 m along the longest axis (
Formulations from Engel and May (2012) show that the boulders with TF > 1000 require a column of water between 9.8 m (subaerial) and 21.6 m (JBB) to explain storm wave run-ups, and between 8.3 and 11.3 m for the tsunami run-ups. Most of the TF > 1000 boulders (74 %) are positioned above the maximum wave height registered along the northern coast of Minorca (9 m). In addition, 24 % of the boulders are positioned at a height at which they can be reworked by storm waves. The storm run-up heights for boulders of this sector are out of the reach of storm waves.
Few imbricate ridges (just two of the eight sites), only one site with isolated imbricate groups of boulders and a greater presence of subrounded blocks (6 of 8) characterize the setting of the northern boulders.
The results for each area indicate the average size and weight for all the boulders with a TF > 1000, but we will consider some of our findings on the largest boulders of each area. The largest boulders of the SE area of Minorca are located on Illa de l'Aire (Fig. 7), just 960 m off the SE coastal tip of Minorca. The largest boulders of this area weigh 228, 154 and 114 t. The Engel and May (2012) equations provide storm run-up estimations of 32, 23 and 22 m respectively, while for a tsunami run-up they required 12, 9 and 9 m.
Locations and main characteristics of W Minorca boulders. Picture corresponds to isolated boulders from Punta Nati (31 m a.s.l.). Geomorphological sketch shows boulder distribution at Sa Caleta.
The largest boulders on the western area of Minorca weigh 21.9, 18.2 and 16.8 t, but they are located higher up and more inland than those of the SE coast. The results of the Engel and May (2012) equations of this area show storm run-ups of 20.2, 16.4 and 16.5 m and tsunami run-ups of 9.9, 10.5 and 10.5 m.
The largest boulders on the northern coast weigh 128.3, 56.5 and 53.7 t. They are found on the small islet of Illa des Porros (Fig. 9), just 426 m off the northern tip of Minorca (Fig. 9). According to the equations from Engel and May (2012), storm run-ups of 46.3, 45.4 and 37.7 m are required to transport these boulders, and heights of 19.8, 22.6 and 16.6 m are required for a tsunami run-up.
Five of the analyzed boulders show marine fauna, indicating that they have
been dislodged from the submerged area and deposited above the cliff. Two of
these boulders have been sampled for
Some of the boulders in the spray areas show post-depositional dissolution
pans (Fig. 5b). Although dissolution rate for these pans is not uniform (it
increases near the cliff edge), we have considered an average of
0.3 mm yr
Radiocarbon dating and estimating dates using dissolution ratios provided a range of ages for 12 locations between 1574 and 1813 AD, although 8 of the 12 dates are around the year 1790 AD (Fig. 10).
Location and main characteristics of N Minorca boulders. Picture corresponds to caballería boulders. Geomorphological sketch shows boulder distribution at Illot d'Addaia.
Chronology of the post-depositional dissolution pans found on the
surface of southern Minorca boulders: the ages, in years AD, correspond to the
post depositional dissolution pans measured on the boulders of the sampled
localities. The blue dots indicate the average age of each locality. The bar
indicates the range of dispersion of calculated ages, and the numbers in
parentheses show the number of measured pans in each area. The left column
displays the earthquakes that have occurred with intensity >
These results place the processes that lead to the deposition of blocks within a few hundred years, discarding geologically older events. In all likelihood, there were previous events that either were obscured by the youngest and most intense events or have not yet been possible to identify.
In interpreting the cause of extreme wave events, there are two feasible
hypotheses, namely tsunami waves or storm waves. The former are long period
waves (up to 10
Small recent tsunamis have affected the island of Minorca as stated by local
newspapers (Diario de Menorca, 2003, 22 and 23 May). The tsunamigenic
source is the Algerian coast, which, according to the historical and
instrumental seismicity, is exposed to relevant seismic hazards and risks
(Papadopoulos, 2009). The last tsunami that affected Minorca island was
generated by the Zemmouri (Algeria) earthquake that took place on 21 May 2003,
with a magnitude of 6.9
Thus, there is currently seismic activity at the bottom of the Algerian basin that gives rise to tsunamis affecting the coast of Minorca. In the recent past, in the last 500 years, there have been tsunamis affecting the Balearic Islands (Table 1). There are also historical tsunami records reporting a flood event with a run-in of to 2 km inland in Santanyí (location on Fig. 2), on the eastern coast of Majorca in 1756 (Fontseré, 1918). Numerical models of tsunami simulation from submarine earthquakes at the northern African coast (i.e. Álvarez-Gómez et al., 2011; Roger and Hébert, 2008) show that the south-eastern and western areas of Minorca would be the most affected by the tsunami impacts. The fetch length for the southern coast of Minorca is relatively low: 300 km in the S direction and 500 km in the easterly direction. Thus, in the last 50 years the maximum extremal wave height detected in an offshore buoy was 11 m high at the 2001 medicane (Jansà, 2013).
According to Papadopoulos (2009), the major tsunamigenic source in the western Mediterranean is located north of Algeria (Fig. 2), although the Alborán region has to be taken into account too. In other areas as the Liguro-Provençal basin and the Valencia trough (Fig. 2), the seismicity is too low for it to be classified as a tsunamigenic area. The seismicity of the northern region of Algeria is dominated by thrust focal mechanisms to the west and central part of this area and by strike-slip faults to the east (e.g., Bezzegoud et al., 2014). The Alboran region is dominated by strike-slip and extensional focal mechanisms where the largest magnitudes are usually low to moderate (Vanucci et al., 2004).
If we focus on northern Algeria, since 1716 there have been seven seismic events
(Fig. 10) with intensities greater than
The geographical distribution of boulder sites (Figs. 1 and 3) in the Balearic Islands gives clear indications of their tsunamitic origins. Boulder sites in Majorca are distributed along the eastern and southern coasts and the same is found in Ibiza. Only in Minorca did we find boulder sites on the northern coast, despite most of the boulder settings being located on the southern coast of the island. In Fig. 3 we show a perfect correspondence between the expected locations where a northern-Africa-generated tsunami should hit the Balearic Islands (from numerical model simulation) and the sites where boulder accumulations are. Storm waves have larger fetch in the northern coast of the Balearic Islands, but almost no large boulders have been found on the western and northern coasts of Majorca, nor on the northern coast of Ibiza.
Although we are aware that hydrodynamic equations need to be reviewed (Cox et al.,
2018) and they are not a definitive approach for discerning tsunami boulders from
storm boulders, we used the Engel and May (2012), and Nott (2003) and Pignatelli et al. (2009) equations. The
Engel and May equation calculates the wave height needed to transport
boulders located at sea level. The boulder heights are not
contemplated in this equation. In Table 3 we present the average results for
joint-bounded boulders (JBB) of the three sectors studied.
According to the setting of the boulders and the results of the hydrodynamic equations, it seems clear than large boulders cannot be transported by a single storm event, nor by a series of storms. However, hydrodynamic equations require run-ups of the tsunami wave that multiply the heights that models forecast for tsunami waves in the open sea between 2 and 10 times. First of all, the run-up of tsunamis on vertical cliffs is several times higher than that occurring in low coastal areas (Bryan, 2001). Run-up is also enhanced due to several factors (Lekkas et al., 2011): (1) the distance from the tsunami generation area (of only 300 km in our case), (2) the narrowness of the continental shelf (as in Minorca), (3) the fact than the tsunami propagation vector is almost perpendicular to the main shoreline direction, and (4) by land morphology, characterized by vertical cliffs with entrances (inlets). For these reasons, run-ups heights on Minorca should be several times higher than tsunami wave heights. However, as they shoal, wave heights increase run-up heights to a lesser extent and thus, it is impossible to reach the run-up values obtained from the hydrodynamic equations.
Recent examples in the Balearic Islands confirm the last statement: the
tsunami of 2003 had an offshore wave height of 30–40 cm (according to
simulations) and reaches the western part of Ibiza with a run-up of 3 m, which
means a multiplying factor of
Regarding the dating of the boulders, although only two blocks with embedded
marine fauna (and located only 1 m above the sea level) have been
radiocarbon dated, dates serve as a reference to the second dating
method used. Our
The second dating method is based on an average dissolution rate of dissolution pans. This requires identifying post-depositional dissolution pans, that is, those that have been formed after the movement of the boulders. They can be formed on the same boulder once transported or on the denudation surface that results from the quarry of the boulder. A margin of error can be established based on the variability of the dissolution rate, which is not very high because the boulders are located away from the cliff edge, where the dissolution rate is more variable. However, in no way do the resulting values (age values) match with marine levels that are different from the current one. Other similar boulders dated by Kelletat et al. (2005) on the neighboring island of Majorca have ages between 565 and 1508 AD.
Estimations using dissolution rates of surface pans are consistent with the
two macro-fauna radiocarbon
Finally, settings of the boulders depend on local physiography and the characteristics of the flow that transported them. Most of the imbricate ridges are found along the SE sector, with lower cliffs and a bigger impact from potential tsunamis. Up to 62 % of the boulders along the SE coastline are subrounded, indicating some reworking by storm waves. Boulders along the western sites are positioned higher, and only 25 % are subrounded, overlapping with the presence of flow-out morphologies. Most of the boulders in this sector have been detached and transported by tsunami flows, but storm waves have moved some boulders several centimeters, reworking them locally. The position of the boulders along the northern coast sector shows evidence of both tsunami and storm wave flows: 75 % of the sites have subrounded blocks and just 25 % of the sites have imbricate ridges. The weight, distance inland and height of some boulders cannot be explained by storm waves. The tsunamis hitting the northern coast of Minorca could be caused by a refraction of a tsunami wave originating from the northern African coast but we do not exclude submarine landslides occurring off the Catalan platform or at the Liguro-Provençal basin platform (Fig. 3).
More than 3000 large boulders have been analyzed on the coastal platforms of Minorca, 720 of which (the ones with larger transport figure values) have been selected for this study. Weight, height above sea level and distance from the edge of the cliff indicate that they have been dislodged and positioned by the action of tsunami waves, although some of these boulders have also been reworked by storm waves.
Boulder sites in the Balearic Islands are mainly located in the southern and eastern parts of the islands. This fact demonstrates that they have been transported by tsunamis and not by storms: whereas the prevailing and strongest wind comes from the north, the main tsunamigenic area is the Algerian coast, located S–SE of the Balearic Islands.
Tsunamis generated off the Algerian coast are quite well known. What was little known is the potential impact of these waves on the coastline of the Balearic Islands, including Minorca. Tsunami simulation models have confirmed the high probability of tsunami wave impact along the coast of the Balearic Islands. The historical chronicles of tsunami events hitting the islands have supported these models. The last 2003 tsunami episode caused a large amount of damage to some harbors of the Balearic Islands.
Despite the location of the boulders being very important, further information obtained from boulder orientations and the presence of imbricated ridges and/or isolated groups of imbricated boulders is evidence of a continuous flow which can only originate from a tsunami. Distance from local escarpments can exclude any of the analyzed boulders that originated from a rockfall.
Hydrodynamic equations applied to these boulders give wave run-up values that are very far from the reach of the waves recorded in the last 50 years, a clear indication that a tsunami wave was the cause of their dislodgement, transport and setting. Weights up to 228 t (Illa de l'Aire, Fig. 7), altitudes reaching 31 m (Punta Nati, Fig. 8) above sea level, and distances from the cliff edge of up to 136 m (Illa de l'Aire) confirm the results obtained in our calculations. Historical data on storm waves, or even medicane (11 m) events, cannot explain the size and positioning of the boulders.
Dating by
Most of the data come from Roig-Munar (2016).
Most of the field data were obtained by FXRM in his phD thesis. Processing, mapping and GIS was done by JAMP. AR and JMV were phD directors. All authors have agreed on the final version and have made substantial contributions to conception and design, acquisition of data, or analysis and interpretation of data, drafting the article or critically revising the intellectual content.
The authors declare that they have no conflict of interest.
This study was supported by the projects CGL2013-48441-P, the CGL2016-79246-P (AEI/FEDER, UE), the CHARMA project (MINECO, ref. CGL2013-40828-R), the PROMONTEC project (MINEICO, ref. 444CGL2017-84720-R) and the CSO20015-64468-P (MINECO/FEDER) project. Edited by: Mauricio Gonzalez Reviewed by: two anonymous referees