Bowen Lecture: Bubbles, crystals, and melts -" the three-phase world of volcanic eruptions

A wonderful diversity of volcanic activity is created by variations in the relative rates of magma and gas flux, combined with the evolving material properties of magma ascending from crustal storage regions to the Earth's surface. Here I place field-based perspectives of volcanic eruption styles within the framework provided by the past decade of analytical and experimental advances that allow us to track the movement of magmas (melt + crystals + gases) through volcanic systems. An important control on eruption style is the evolution of magma as it ascends and decompresses. During decompression, hydrous melts lose water, causing (1) bubble nucleation and growth (vesiculation) and, commonly, (2) crystallization of anhydrous phases. The extent of degassing-induced crystallization is inversely proportional to magma ascent rate and melt viscosity, and thus has the greatest influence on moderate eruptions of mafic to intermediate compositions. Mafic pyroclasts also show a wide range of bubble sizes resulting from variations in both magma ascent rates and post-fragmentation cooling histories. In contrast, large kinetic delays to both crystal and bubble nucleation in viscous silicic melts generate finely vesicular pumice clasts for a wide range of eruption styles. The combined ascent and vesiculation history of a magma also determines the relative rate of magma migration and volatile (gas) flux. When magma flux is approximately equal to volatile flux, bubbles travel with the melt. Under these conditions, low magma rise rates produce bubbly lava flows, while high rates of magma rise, and simultaneous rapid bubble expansion, may create dispersed gas-particle flows if the magma expansion rate is sufficient to fragment the magma. Commonly, however, volatile flux greatly exceeds magma flux, either through two-phase flow (bubble-liquid separation) or gas flow through permeable networks. Two-phase flow is common in mafic systems where typical low melt viscosities and large bubble sizes allow extensive volatile segregation. The rich complexity of two-phase flow behavior can be used to explain the diversity of eruption styles associated with basaltic volcanism, as well as the simultaneous explosive and effusive activity that characterize Hawaiian and violent Strombolian eruptive styles. High volatile fluxes may also be achieved by flow through permeable networks. Permeable flow is evidenced by high gas fluxes from non-erupting volcanoes, particularly immediately following explosive eruptions. Permeable flow, followed by compaction of permeable networks, is also responsible for the formation of both degassed and dense (vesicle- poor) lava domes and plugs. Challenges for the future include developing methods of tracking the evolution of bubbles, crystals, and melts through subvolcanic systems in real-time to establish viable methods of predicting styles, as well as timing, of volcanic eruptions.