About the viscosity of Vesuvius and Erebus volcanoes!

The viscosity of a magma is an important parameter that influence how it erupts at the Earth surface. For instance, basaltic melts, poor in silica, typically present low viscosities, and hence, flow easily. They erupt in those effusive eruptions, where nice lava flows and lava fountains are observed. Contrary to that, magmas rich in silica, with rhyolitic for instance, are viscous and do not flow at eruptive temperatures. This can result in forming instable magmatic foams in volcanic conduits, their fragmentation leading to the so-called explosive eruptions.

The Vesuvius volcano is famous for the Pompeii eruption, that took place in 79 AD. This volcano produced numerous explosive eruptions that involved a phonolitic magma, containing around 54 wt% silica oxide. Interestingly, the southern volcano in the world, a.k.a the Erebus volcano located in Antarctica, also erupts such kind of lava. However, at Erebus, lava gently flows and forms a lava lake.

Viscosity of the Erebus and Vesuvius magmas are very similar. We investigated them in the lab and we made a model that allows us to calculate how viscosity of phonolitic melts change with temperature, water and crystals contents. In the paper, we focused the modeling on the Erebus case mostly, because we wanted to try explaining the surprizing occurrence of the lava lake at the summit of the volcano.

The paper we published in Earth and Planetary Science Letter emphasizes how magma viscosity can be modelled. It also highlights the various difficulties that still exist and that prevent precise modeling, despite the apparent simplicity of the Erebus case (low water and crystal contents, the dry viscosity versus temperature relationship is known...). Finally, our work also points the difference of volatile contents between the Erebus and Vesuvius magmas at depth. Indeed, the Vesuvius magma can contain up to 6 wt% water in the magma chamber, whereas that of Erebus is mostly dehydrated with a mean water content of 0.2 wt%. For Vesuvius, the degassing of such a high amount of water together with crystallisation of microleucite crystals drive a strong increase of the magma viscosity that probably promoted the explosivity of the Vesuvius magma during the 79 AD Pompeii eruption.

Le Losq C., Neuville D. R., Moretti R., Kyle P. R., Oppenheimer C. (2015) Rheology of phonolitic magmas – the case of the Erebus lava lake. Earth and Planetary Science Letters 411(1):53-61.