**5. Evolution of Mazo eruption and causes of the flank collapse**

On January 20th 1731, after an intense seismic crisis, Mazo eruption started being the fourth eruptive fissure of Timanfaya. The initial activity was of hawaian type, documented in the outcrops of agglutinates of scoria and clastogenic lavas in the flank of the preserved edifice, and also by the presence of the same type of material integrated in toreva blocks and in some hummocks of the DAD. Later the style of the eruption shifted to strombolian type with emission of scoria (**Figure 8C**).

The rapid growth of the volcanic cone and a high emission rate may have been determining factors of the flank collapse that took place the same day as the eruption began. In this way, part of the collapse could be favored by accumulation processes in the cone that made it grow extremely quickly, exceeding its stability limit. Thus, once this limit is exceeded, small mass additions can generate debris avalanches [31, 32]. Also, the presence of a huge amount of lava in the crater or at the base of the cone could have favored the collapse [14, 33–35].

Doming process does not seem to be the trigger for the collapse, as the faults and fractures affecting the cone are practically parallel and do not follow the fracturing patterns associated with the intrusion and inflation processes [36, 37]. Even so, the geometry of the fractures can vary substantially depending on whether the intrusion is located within, below, or outside a volcanic edifice, and may vary according to the local geology and cause very different consequences [38]. The doming process cannot be ruled out because the original fracture pattern has been obliterated during displacement.

The existence of fractures that generate a graben structure arranged perpendicular to the direction of collapse, together with the presence of a higher and proximal toreva domain and a hummock domain at the bottom of the collapsed flank, could be related to the existence of basal layers with low viscosity and ductile behavior on the substrate of the volcanic cone, located during movement under the hummock domain [39]. The presence of a basal layer with these characteristics is evidenced in the lava injection processes and squeeze-ups formations in the avalanche sectors subject to compression. In stratovolcanoes, this hypothetical low-viscosity layer belongs to the initial stratigraphic sequence of the stratovolcano and may originally be composed of weak material such as poorly consolidated proximal pyroclasts, coarse-grained tephra sequences, pyroclastic flows, or even blocky lava flows [39].

In monogenetic mafic volcanoes, the existence of basal spatter layers emitted during the initial stages of the eruption and subject to charging processes by accumulation of pyroclasts, has been used to explain rafting processes of volcanic cones. In the case of the flank collapse of a monogenetic edifice like Mazo, this layer may correspond to the spatter emitted during the initial phases, configuring the base of the stratigraphic sequence so that as the height of the volcano increases its weight and so their plasticity increases, thus causing the collapse. Any case, analogue models realized by [2], shows that the deformation of the base is needed for the formation of deep collapses that affect the central area of the cone, as well processes linked to the fracturing of the basement, both through horizontal, oblique or vertical motions.

In Mazo, the existence of a well-defined fault in the cone, parallel to a normal fault affecting recent deposits of Timanfaya [40] (**Figure 9**) with a fault displacement of at least 45 m, suggests a structural control. These faults are also parallel to the main eruptive fissure of Timanfaya. A change in the stress field during Mazo eruption is evident if we consider that Mazo is located at the end of the Timanfaya first NW-SE alignment and that Mazo fault is trending parallel to the second ESE-WSW Timanfaya fissure where the volcanic activity was concentrated after Mazo eruption. The intense seismicity previous to Mazo eruption could also be connected

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

sources: 1) the description of the eruption made in 1744 by the priest of Yaiza in his diary (hereinafter CY) (referred in [29]); 2) the data contained in a manuscript with the dossier promoted by the Royal Court of the Canary Islands, which is currently preserved in the General Archive of Simancas (hereinafter MsS) [9, 16]; and

It is generally accepted that Timanfaya multiple eruption began at Caldera de Los Cuervos volcano on September 1th, 1730 and lasted until mid-September [9, 11, 19, 21]. After a short rest, on October 10th, 1730 two new eruptive fissures were opened at Caldera de la Rilla and Pico Partido volcanoes forming a NW-SE alignment with Caldera de Los Cuervos (**Figure 1**). Activity in these fissures ended on November 1730 and January 16th, 1731, respectively [30]. On 20th January 1731 a new volcano erupted. Although, some authors assume that this new volcano was Caldera de la Rilla [19], the chronicles say it was located half a quarter of a league (2.4 km) from the previous eruption of Pico Partido [30] and at the destroyed village of Mazo (MsS, letter of February 19th 1731), which was burned and covered by lava flows from Caldera de Los Cuervos volcano on September 11th 1730 (MsS, letter of 17th October 1731 [29], previously to Mazo eruption. There is only one eruptive complex that meets all the conditions, namely: location in the place where the burned village of Mazo was located and distance from the Pico Partido complex of about 2.4 km. That volcano is undoubtedly Mazo. In addition, it is in the continuation of the first volcanic alignment of Timanfaya (**Figure 1**) and have a similar composition (basanitic) of those volcanoes on the NW-SE alignment [18].

Assuming Mazo is the fourth eruptive fissure of Timanfaya some information about the eruption can be extracted from the chronicles. However, it is interesting that there is no reference to this volcanic episode of January 20th in the CY manuscript, which is one of the main sources of information on the eruption. In turn, there is a mention to an eruption starting the 10th of the same month that does not appear in the rest of the consulted documentary sources. Some errors regarding the start dates of some volcanic episodes of the eruption are relatively common in this chronicle [11, 26], probably due to the great spatio-temporal extension of the eruption, to the lack of a continuous monitoring of the eruptive vents, and also to the fact that this manuscript was written 8 years after the ending of the eruption [11], or even to transcription and translation errors of the original document [26]. Even so, if the specific dates are ignored, the sequence of events is similar in all the chronicles consulted. In fact, eruptive activity developed in January 1731, whatever the source consulted or the specific dates, is characterized by the cessation of activity of the volcano opened in the sector of Pico Partido on October 10th, followed by the occurrence of a seismic crisis of considerable intensity whose effects were felt in Gran Canaria Island [9], more than 190 km away, and by the beginning

The narration realized by the Priest of Yaiza for the eruption of January 20th also says: "On the 10th [in place of 20th] of January a mountain raised that the same day crumbled with and incredible crash inside its own crater, and covered the island with stones and ashes. Incandescent currents of lava collapsed onto the malpais up to the sea" [29]. Evidently, CY is describing the sudden collapse of Mazo volcanic cone and the formation of incandescent currents that reached the sea, with a total length of 6 km. The concatenation of later phenomena described in the documentary sources put in evidence that this process gave place to the formation of a high eruptive column that dispersed the pyroclasts over the whole island of Lanzarote (CY, MsS) and part of Fuerteventura (MsS). In mid-February the documentary sources (letter from Ambrosio Cayetano de Ayala; MsS) cite a score of villages in the central part of Lanzarote affected by ash fall [16, 30] (**Figure 8B**). The eruptive event of Mazo volcano lasted only seven

days, as CY mentions that this eruption ended on January 27th, 1731.

3) notarial and religious data contemporary with the eruptions [25, 30].

**184**

of a new eruption [16, 25, 30].

with the modification of the stress regime that conditioned latter magma intrusion. The regional extension that facilitates the ascent of magma is accommodated by the formation of normal faults [41]. This orientation of extensional stress field in Timanfaya area is also confirmed by studies of fault population analysis [42]. Tectovolcanic processes affecting the basement are also supported by the large distal megablock included in the DAD. The generation of the collapse in the northern sector of the building reveals the influence of the stress regime within the volcano motivated by regional tectonic stresses in the first phase of Timanfaya or by the geometry of contact with the substrate, as it has been observed in central volcanic building collapses [43–45].

The flank collapse produced a volcanic debris avalanche that affected most of the volcanic edifice, including the summit area and part of the basement. The characteristics of the Mazo DAD are equivalent to those observed in stratovolcanoes, with a proximal area characterized by the presence of block facies through which more fractured material was emplaced forming flows at high slope and relatively short paths, while the most disaggregated material due to friction between blocks and fluidized by the presence of molten lava reached a longer distance producing more dispersed hummocks. The collapse formed a 500 m long amphitheater on the southern flank of the cone, and a DAD that, according to chronicles, reached the sea on the coast more than 6 km away. The volume of slipped cone and DAD is impossible to calculate as they are partially covered by lavas from subsequent eruptions.

The decompression caused immediately after the debris avalanche generated a blast cloud and ballistic projectiles composed of heavy blocks and bombs that were deposited in proximal areas and as far as 500 m from the vent. The blast cloud was a driven-gravity flow, probably divided in two parts [46]: 1) a coarse-grained basal flow of rock fragments; and (2) a fine-grained turbulent upper flow that originated the blast surge covering all the previous deposits. The blast deposit has been found at distances up to 6 km from the vent, and based on the historical chronicles, the fine-grained fragments affected the whole central area of Lanzarote. This deposit covered the DAD but now is only preserved in the areas where they either 1) suffered hydrothermal alteration due to its location onto hot toreva blocks or hummocks, or 2) overlaid by pyroclasts from the last phase. Hydrothermal alteration in hummocks and squeeze-ups in the DAD also support this was a syn-eruptive collapse.

The presence of oncoids in an inter-toreva depression located in the proximal area indicates that hydrothermal activity related to degassing along fractures was generated after the collapse. Mazo oncoids were then formed under boiling water in a degassing-phase related to a fracture close to the crater vent. Oncoids, travertine and sulfur laminated mound-type deposits have been described in other volcanic environments related to hydrothermal activity, warm and hot springs and geyser deposits [47–49].

After the blast, the eruption went on with a last strombolian phase finishing six days later. The lapilli emitted in this stage completely covered the topography burying smaller irregularities of the surface and homogenizing the geological landscape of the whole area of Mazo volcano.

### **6. Implications for volcanic risk assessment**

Recent studies point out that monogenetic eruptions, usually characterized by hawaian-strombolian eruptive episodes, can also include sudden and more violent episodes that imply a higher risk for the population [6, 12, 14]. The identification of a syn-eruptive flank collapse and the associated blast of Mazo during the 1730–36

**187**

*Syn-Eruptive Lateral Collapse of Monogenetic Volcanoes: The Case of Mazo Volcano…*

Timanfaya eruption provide evidence of a new hazard to be considered. It is also important to emphasize that this is not an isolated phenomenon since the historical chronicles refer to other collapse phases during the month of April 1731 that affected more than one volcanic edifice at a time [29]. However, there is no detailed description of the features of these processes except for some mentions to fractures,

A flank collapse like that occurred during the eruption of Mazo volcano, besides

The probability of flank collapses development during future mafic eruptions rises the potential risk for the population, even more when it is considered that the population has increased from 5000 inhabitants in Lanzarote in 1730 (as stated in MsS) to 205,910 nowadays plus 3,065,575 visitors [54]. An eruption with similar characteristics to that of Mazo at present times, not only would cover with ashes the whole island of Lanzarote and part of Fuerteventura but would also cause the closure of the two islands airports and ports. This would in turn cause serious damage to air and maritime transport of the islands, which are key aspects of the current economic system of both islands based on tourism and totally dependent on

In fact, the intensity of Mazo eruption and the syn-eruptive flank collapse show

The development of debris avalanches and the generation of eruptive columns with high altitude associated to a collapse during a mafic monogenetic eruption oblige us to change the perception of hazards linked to the growth of such volcanic cones. All this indicates that we should pay more attention to this kind of processes and shows the need for detailed studies to identify and characterize them, more when the studied DAD had been previously described as lava flows [26, 27]. This will allow to obtain a deeper knowledge of the triggering factors and causes of this type of processes in order to carry out effective volcanic hazard assessment policies.

Based on detailed field work on Mazo volcano and the exhaustive review of historical documents we have been able to propose a new eruptive sequence for the first months of 1730–36 Timanfaya eruption. The inclusion of Mazo volcano as the fourth eruptive fissure of Timanfaya and the formation of a tectonic controlled

a high impact at a regional scale which exceeded the capacity of the insular and regional authorities. It was this fourth eruption that prompted the Royal Court of Canary Islands to carry out a dossier file to request the intervention of the King of Spain. This file is presently archived in the General Archive of Simancas in Valladolid province (Spain) and constitutes one of most complete documentary sources of the eruption. Fortunately, the fact that the eruption occurred in an area already devastated by the first episodes of Timanfaya eruption reduced its risk

the DAD and the associated blast, may originate hydromagmatic and violent strombolian episodes due to the post-collapse depressurization that significantly increases the eruption energy and form eruptive columns of great height and wide dispersion. Nevertheless, the studies on volcanic hazards in monogenetic volcanic fields are mainly concerned with the analysis of volcanic susceptibility and with the development of scenarios of lava flows, pyroclastic density currents (PDC), and pyroclastic ballistics and fallout [51–53]. Volcanic hazard assessment is rarely multi-hazard and is normally focused on lava flows invasion. In this context, is has not been considered eruptive scenarios including instability processes of volcanic edifices, ranging from less violent processes like rafting to flank collapses like this

*DOI: http://dx.doi.org/10.5772/intechopen.93882*

described for Mazo volcano.

the outside.

in 1730.

**7. Conclusions**

probably associated to semicircular collapses [9, 19, 50].

### *Syn-Eruptive Lateral Collapse of Monogenetic Volcanoes: The Case of Mazo Volcano… DOI: http://dx.doi.org/10.5772/intechopen.93882*

Timanfaya eruption provide evidence of a new hazard to be considered. It is also important to emphasize that this is not an isolated phenomenon since the historical chronicles refer to other collapse phases during the month of April 1731 that affected more than one volcanic edifice at a time [29]. However, there is no detailed description of the features of these processes except for some mentions to fractures, probably associated to semicircular collapses [9, 19, 50].

A flank collapse like that occurred during the eruption of Mazo volcano, besides the DAD and the associated blast, may originate hydromagmatic and violent strombolian episodes due to the post-collapse depressurization that significantly increases the eruption energy and form eruptive columns of great height and wide dispersion. Nevertheless, the studies on volcanic hazards in monogenetic volcanic fields are mainly concerned with the analysis of volcanic susceptibility and with the development of scenarios of lava flows, pyroclastic density currents (PDC), and pyroclastic ballistics and fallout [51–53]. Volcanic hazard assessment is rarely multi-hazard and is normally focused on lava flows invasion. In this context, is has not been considered eruptive scenarios including instability processes of volcanic edifices, ranging from less violent processes like rafting to flank collapses like this described for Mazo volcano.

The probability of flank collapses development during future mafic eruptions rises the potential risk for the population, even more when it is considered that the population has increased from 5000 inhabitants in Lanzarote in 1730 (as stated in MsS) to 205,910 nowadays plus 3,065,575 visitors [54]. An eruption with similar characteristics to that of Mazo at present times, not only would cover with ashes the whole island of Lanzarote and part of Fuerteventura but would also cause the closure of the two islands airports and ports. This would in turn cause serious damage to air and maritime transport of the islands, which are key aspects of the current economic system of both islands based on tourism and totally dependent on the outside.

In fact, the intensity of Mazo eruption and the syn-eruptive flank collapse show a high impact at a regional scale which exceeded the capacity of the insular and regional authorities. It was this fourth eruption that prompted the Royal Court of Canary Islands to carry out a dossier file to request the intervention of the King of Spain. This file is presently archived in the General Archive of Simancas in Valladolid province (Spain) and constitutes one of most complete documentary sources of the eruption. Fortunately, the fact that the eruption occurred in an area already devastated by the first episodes of Timanfaya eruption reduced its risk in 1730.

The development of debris avalanches and the generation of eruptive columns with high altitude associated to a collapse during a mafic monogenetic eruption oblige us to change the perception of hazards linked to the growth of such volcanic cones. All this indicates that we should pay more attention to this kind of processes and shows the need for detailed studies to identify and characterize them, more when the studied DAD had been previously described as lava flows [26, 27]. This will allow to obtain a deeper knowledge of the triggering factors and causes of this type of processes in order to carry out effective volcanic hazard assessment policies.
