THE 122 BC PLINIAN ERUPTION OF MT. ETNA, SICILY: INSIGHTS INTO EXPLOSIVE ERUPTIVE BEHAVIOR IN BASALTIC SYSTEMS

Though Jon Davidson was perhaps best known for his petrological work in the Lesser Antilles and the Andes, he also made a huge impact on Italian magmatic systems. Here, we honor the contributions of Jon Davidson to the understanding Earth’s magmatism with a study of the 122 BC eruption of Mt. Etna in Sicily. Mt. Etna is a persistently active volcano that remarkably accounts for 5-10% of global emissions of CO₂ and SO₂ gases into Earth’s atmosphere. Moreover, Etna’s renewed activity in February 2017 serves to remind that Etna warrants intensive study as Europe’s largest and most threatening volcano. Etna typically erupts effusive lava flows, yet Mt. Etna can surprise with episodic explosive activity. This anomalous behavior was reinforced in the last 15 years when vigorous eruptions in 2001-2003 were followed by passive eruptions in 2004-2007. Whereas this recent volcanic activity at Etna draws attention, it pales in comparison to violent eruptions in the more distant past. In fact, the 122 BC Plinian eruption of Etna is one of the five largest eruptions of a basaltic volcano known on Earth. We examined section 26, which is 80-cm thick and located about 20 km southeast of the vent in Cratere del Piano caldera. Moreover, we took samples from the thinner section 22 and an ash sample from the explosive eruption of April 2012. The 2 mm size fraction of the scoria samples were cleaned and washed to aid the handpicking of fresh, vesiculated juvenile scoria fragments for XRF analysis. The 122 BC eruption scoria plot as hawaiite and exhibit small variations in major elements such as SiO₂ ranging from 50.51-51.46%, MgO from 2.78-2.98%, Na₂O from 4.32-4.52%, and K₂O from 1.73-1.97%. Noteworthy, the April 2012 paroxysm of Mt. Etna had much higher MgO at 4.69% wt%. CaO/Al₂O₃ ratios range from 0.41 to 0.45 compared to 0.57 in the 2012 eruption indicating greater pyroxene and plagioclase fractionation in the 122 BC Plinian eruption. Previous glass inclusion studies on rare olivine and common plagioclase crystals show more primitive compositions and H₂O concentrations that range from 1.02-3.13 wt% in olivine and 0.98-2.77 wt% in plagioclase. Thus, moderate H₂O contents likely did not drive the 122 BC Plinian eruption column, instead the pre-eruptive storage conditions may have had more control on the eruptive style by influencing magma viscosities.