**7. Conclusions**

Based on detailed field work on Mazo volcano and the exhaustive review of historical documents we have been able to propose a new eruptive sequence for the first months of 1730–36 Timanfaya eruption.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

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

building collapses [43–45].

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

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

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

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

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

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collapse.

deposits [47–49].

of the whole area of Mazo volcano.

**6. Implications for volcanic risk assessment**

collapse previous to an important change in the eruptive dynamics is quite significant since it occurred at a time of important stress changes that notably affected the Timanfaya eruption and led to the formation of a large (>13 km) eruptive fissure along which the eruption developed from that moment on. The existence of faults affecting Timanfaya volcanic products demonstrate that there was an important structural control during the eruption. Thus, the eruptive processes produced during the first six months of this eruption, which is the best recorded in contemporary documentation, show that the previously established geological history constitutes a simplification of the events that took place in this area and that a reinterpretation of the historical chronicles and new field work should be carried out to clarify the evolution of the whole Timanfaya eruption, the largest historical eruption of the Canary Islands, and one of the most important in recent times in the world.

The example of Mazo illustrates that flank collapses are not processes uniquely linked to stratovolcanoes. Mazo is an example that during the construction of a scoria cone volcano-tectonic process might trigger a flank collapse as well, although the size of the amphitheater and the avalanche deposits are significantly smaller than those developed in stratovolcanoes. Mazo deposits display features and morphologies similar to those described to characterize volcanic instability processes generated in large volcanic structures, being the main difference the scale. This research emphasizes that mafic monogenetic volcanic eruptions can result in rafting or flank collapse. In both processes, morphology and structures in the cone can be similar, being the main difference the impact of the phenomena: while rafting is a relatively quiet emission of lavas with rafts, during a flank collapse occurs a sudden dramatic formation of an avalanche debris and a blast.

Understanding the causes of syn-eruptive collapses in monogenetic mafic eruptions is essential to correctly interpret the signs of active volcanoes during risk management for land planning and risk reduction in this type of eruptions. In addition to its implications for the Timanfaya eruption comprehension, the morphology of Mazo volcano, and its well exposed DAD deposits make it an ideal case study to characterize flank collapses and formation of DAD in monogenetic edifice, reason why it has been proposed as a geosite in the Canary Islands geoheritage inventory, being suitable to be proposed as a Global Geosite of international relevance for Spain.
