**1. Introduction**

The origin and characteristics of flank collapses in stratovolcanoes and volcanic islands have been recognized and described around the world (eg. [1–5]). The resulting Volcanic Debris Avalanche (VDA) deposits are composed essentially of rock fragments of the affected edifice and usually show a hummocky topography around the collapsed original volcanic landform. The magnitude of these collapses and the huge volume of involved materials make these processes the most

catastrophic events in the evolution of polygenetic volcanic structures. Factors inducing or triggering volcanic flank collapses include the violence of the eruption, high eruptive rates, hydrothermal alteration, existence of relatively steep slopes, presence and reactivation of faults, magma intrusion, high saturation of volcanic rocks in water, presence of lava plugs during the active period, structural heterogeneities and geotechnical differences between volcanic edifices and their basement, seismicity, caldera collapse or even climatic fluctuations ([2] and reference therein).

In contrast, instability processes in monogenetic volcanoes have been much less documented and have often received less attention given its less volume and less potential hazard (eg. [6–11]). Nevertheless, it should be taken into account that mafic monogenetic volcanic systems are the most frequent and widespread magmatism on Earth, usually located very close to population centers [7]. The most documented instability process in monogenetic cones are those related to the partial collapse and passive transport of fragments of the edifice during the emission of lava flows, process known as rafting (eg. [6–11]). The clearest evidence of rafting processes in monogenetic edifices is the existence of huge blocks on the surface of lava flows composed of agglutinated materials coming from the cone [12]. Rafting has been related to lava flows and sill emplacement at the base of the cone, changes in eruptive style or the existence of previous cones or topographical constrains ([13] and reference therein). Although flank collapses forming VDA with liquified non-turbulent granular flows are usually linked to stratovolcanoes [14] and rafting processes are the common result of instability in monogenetic cones, in this work we demonstrate VDA also happen in small volcanic cones, such as in the historical volcanic cone of Mazo (Lanzarote, Canary Islands). Here, we present the first detailed description of a syn-eruptive volcanic flank collapse in a monogenetic volcanic cone and describe the associated debris avalanche and blast deposits; as well as the conditioning and triggering factors of the collapse, and the implications for the volcanic eruption development. The finding of this volcanic flank collapse during a mafic fissure eruption has local implications in the interpretation, timing and reconstruction of the Timanfaya eruption and global implications in the understanding of hazards in monogenetic volcanic fields.
