left - the two-faced Janus; right - diagram of the conduit system that fed the two crater areas of Stromboli during the May 2019 eruptions (SCA: Southwestern Crater Area; and NCA: Northeastern Crater Area).
The two faces of two-faced Janus are side by side, but their gaze opens onto two different perspectives. One face sees only the future, while the other sees only the past. Among the ancient Romans, Janus was the god of "passage". Even the "mouths" of volcanoes, like Janus, represent a passage. The passage between the interior of the earth and its surface, between the inside and the outside. Through the "mouth" of the volcano, what originated and took shape in depth comes to light, and we volcanologists climb to the summit of volcanoes precisely to carefully collect these “precious products” that the eruption has brought to light. Precious because the internal structure of these eruptive products (i.e. the lavas) tells us the story of the magma (an example here), which for us, eager to understand how the volcano "works", is a fundamental element.
In the work to which we refer in this note (full paper here), the team composed of Alessio Pontesilli, Elisabetta Del Bello, Piergiorgio Scarlato, Silvio Mollo, Ben Ellis, Daniele Andronico, Jacopo Taddeucci and Manuela Nazzari shows us that during the explosive eruptions of May 11th 2019, the Stromboli volcano, like two-faced Janus, showed two distinct faces. For years we have known that on the summit of this volcano we could roughly distinguish two adjacent crater areas (SCA and NCA in the figure), but this meticulous study reveals, for the first time, that these two vents (the two crater areas) erupt at the same time magmas that tell us two different stories. What does this mean? It means that the distinction between the two crater areas is not just a superficial expression, but instead has a deeper root. The study revealed that the two "vents" emit magmas that have different temperatures and rise at different velocities. This evidence can only be explained if we imagine that the two vents are fed by a branched conduit, in which the two magmas can follow two independent paths, at least in the last hundreds of meters before reaching the surface.