The 1974 west flank eruption of Mount Etna : a data-driven model for a low elevation effusive event

2020

Article référencé dans le Web of Science WOS:000604605800001

Lormand C., Harris A. J. L., Chevrel Magdalena Oryaelle, Calvari S., Gurioli L., Favalli M., Fornaciai A., Nannipieri L.

Frontiers in Earth Science, 2020, 8, p. 590411 [23 p.]

Low elevation flank eruptions represent highly hazardous events due to their location near, or in, communities. Their potentially high effusion rates can feed fast moving lava flows that enter populated areas with little time for warning or evacuation, as was the case at Nyiragongo in 1977. The January-March 1974 eruption on the western flank of Mount Etna, Italy, was a low elevation effusive event, but with low effusion rates. It consisted of two eruptive phases, separated by 23 days of quiescence, and produced two lava flow fields. We describe the different properties of the two lava flow fields through structural and morphological analyses using UAV-based photogrammetry, plus textural and rheological analyses of samples. Phase I produced lower density (similar to 2,210 kg m(-3)) and crystallinity (similar to 37%) lavas at higher eruption temperatures (similar to 1,080 degrees C), forming thinner (2-3 m) flow units with less-well-developed channels than Phase II. Although Phase II involved an identical source magma, it had higher densities (similar to 2,425 kg m(-3)) and crystallinities (similar to 40%), and lower eruption temperatures (similar to 1,030 degrees C), forming thicker (5 m) flow units with well-formed channels. These contrasting properties were associated with distinct rheologies, Phase I lavas having lower viscosities (similar to 10(3) Pa s) than Phase II (similar to 10(5) Pa s). Effusion rates were higher during Phase I (>= 5 m(3)/s), but the episodic, short-lived nature of each lava flow emplacement event meant that flows were volume-limited and short (<= 1.5 km). Phase II effusion rates were lower (<= 4 m(3)/s), but sustained effusion led to flow units that could still extend 1.3 km, although volume limits resulted from levee failure and flow avulsion to form new channels high in the lava flow system. We present a petrologically-based model whereby a similar magma fed both phases, but slower ascent during Phase II may have led to greater degrees of degassing resulting in higher cooling-induced densities and crystallinities, as well as lower temperatures. We thus define a low effusion rate end-member scenario for low elevation effusive events, revealing that such events are not necessarily of high effusion rate and velocity, as in the catastrophic event scenarios of Etna 1669 or Kilauea 2018.

Sciences fondamentales / Techniques d'analyse et de recherche [020] ; Géophysique interne [066]

ITALIE ; ETNA MONT

Fonds IRD [F B010080574]

fdi:010080574