ERUPTIVE STYLE TRANSITIONS AT LA FOSSA CONE, VULCANO ISLAND, ITALY

Vulcanian eruptions are short-lived explosive eruptions that can repeat at a single volcano, sometimes tens of times a day, persisting for decades. During the 16^th to 18^th centuries, La Fossa Cone on Vulcano Island, Italy, erupted repeatedly over a period of 230 years, referred to as the Pietre Cotte sequence. This explosive sequence is of interest because two eruptions in the middle of a vulcanian sequence were anomalously higher in intensity.

Magma mixing during the Pietre Cotte eruptions has been identified by several authors (e.g. Piochi et al. (2009) G3, v. 10; Bullock et al. (2019) JVGR, v. 372), and mixing between rising mafic magma and a static silicic body may have been sufficient to generate the more explosive, gas-rich eruptions in the middle of a long vulcanian sequence. In the present study, this hypothesis was investigated by comparing the eruptive units of the Pietre Cotte, whose compositions range from basaltic trachyandesite to rhyolite. This was done in part with textural studies on macroscopic to nanometer scales, encompassing sieving and componentry, petrographic microscope, and SEM imaging.

We find that the earliest vulcanian products were mostly low-density mafic juveniles. The anomalously explosive products were mostly low-density trachyte pumices that contain evidence of chemical mixing with varying amounts of mafic magma, either as isolated clots or as continuous bands. Both the earliest vulcanian products and the products of the most explosive phases also had vesicles that showed a wide range of nucleation and growth processes. Finally, the later vulcanian products were mostly moderate-density mafic lithics, whose vesicles primarily underwent a ripening process. In summary, we interpret the textural evidence to be in support of the magma mixing hypothesis. As the vulcanian sequence progresses, the products have textures interpreted to be caused by decreased gas content and denser, more mafic compositions.