Making a volcanic point: certain subduction zones worldwide accumulate highly hazardous, pointy stratovolcanoes

Volcanic landscapes are amongst the most breathtaking visual features on Earth. Volcano morphologies can be extremely varied, including negative-relief topographic depressions (e.g. calderas) as well as many different configurations of positive reliefs (e.g. shields or stratovolcanoes). These volcano morphologies provide information about magmatic and eruptive processes and, therefore, represent invaluable sources of data, especially for data-scarce volcanic systems. Volcano morphology also modulates volcanic hazard, for example, by providing potential energy (edifice height, mean flank slope, etc.) for propagation of volcanic mass flows (lava flows, pyroclastic density currents, lahars, etc.); and/or in relation to potential instability of the volcanic edifice which, upon gravitational collapse, can generate large-volume, long-runout debris avalanches and debris flows.

Here, we quantify volcano morphology at several hundred volcanic systems worldwide, using a metric derived from an innovative, data-driven method to search for analogue volcanoes (VOLCANS). The metric simplifies volcano morphology by combining: (1) edifice height, (2) mean flank slope, (3) crater diameter and (4) degree of truncation of the edifice (i.e. ratio between the width of the summit area divided by that of the whole edifice). This makes it possible to distinguish between high, steep, pointy volcanoes with small craters and low, gentle-slope, truncated volcanoes with large craters/depressions. The VOLCANS metric indicates that high, steep and pointy (i.e. non-truncated) stratovolcanoes (henceforth referred to as ‘pointy volcanoes’) do not occur at random. Instead, pointy volcanoes tend to accumulate within specific subduction zones worldwide. Some of the most striking examples of subduction segments with high proportions of pointy volcanoes include Guatemala, which hosts all the pointy volcanoes in the entire Central American region (e.g. Fuego, Agua, Atitlán, Santa María, Tacaná) and Kamchatka, Russia, which hosts around 20% of all the pointy volcanoes identified worldwide (e.g. Klyuchevskoy, Vilyuchik, Kronotsky, Koryaksky). Other pointy-rich subduction segments include: the Alaskan Peninsula and the Cascades, USA (e.g. Pavlof or Mt Baker), Ecuador (e.g. Sangay), Java, Indonesia (e.g. Semeru, Merapi) or Central and Southern Chile (e.g. Lanín, Osorno, Villarrica).

We postulate that, in order to build such extreme volcano morphologies, frequent eruptive activity of mildly-evolved magmas (with low-to-intermediate viscosities), plus a limited spatial variability in the location of the eruptive vent(s), are necessary to maintain vertical growth of the volcanic edifice. Moreover, sparsity of large-explosive eruptions safeguards the ‘pointiness’ of the volcano, avoiding truncation of the edifice and/or mantaining small craters. We acknowledge that volcano morphology represents just a snapshot in time within the geological evolution of any volcanic system. Interestingly, however, some pointy volcanoes have experienced gravitational collapse(s) of their edifices in the past (e.g. Acatenango-Fuego, Guatemala), and have managed to rebuild their pointy edifices through subsequent eruptive activity. Currently, we are exploring several datasets of: (i) subduction kinematics, (ii) magma geochemistry and (iii) eruptive fluxes, to try to tie our morphological observations to their possible causative processes. Such an analysis is extremely relevant, not only to improve our understanding of how volcanic systems operate but also to quantify volcanic hazard at subduction zones and their volcanic systems.