Explosive-effusive volcanic eruption transitions caused by pyroclast sintering

This project will address a first-order challenge in volcanology - understanding the controls on transitions in eruption style and intensity during the most hazardous eruptions. Most silicic eruptions begin with a high-energy, high-hazard explosive phase, then either wane and stop, or transition to hybrid and effusive behaviour that produces relatively short-range lava flows with much lower hazard potential. Understanding the timing of these transitions, and of the end of an eruption, is a major challenge that impacts hazard assessment and eruption response. Existing models assume that a transition from explosive to effusive behaviour is driven from below by a change in either the magma ascent rate or by the permeable release of pressurised gas, effectively 'defusing' the explosive potential. However, these bottom-up models fail to explain two fundamental aspects of silicic volcanism: (i) simultaneous explosive-effusive behaviour that was witnessed directly during the 2011-12 eruption of Cordon Caulle (Chile) and subsequently inferred elsewhere, and (ii) widely documented evidence for in-conduit pyroclast sintering preserved in the deposits from all phases of these eruption types. Members of the project team have used this evidence to develop a new paradigm for explosive-effusive transitions in silicic eruptions (Wadsworth et al., 2020) in which transitions are driven from above by shallow welding of fragmented magma and occlusion of the shallow conduit. In this 'cryptic fragmentation' paradigm, all silicic eruptions are explosive at depth, even when apparently effusive at the surface. This new idea demands a wholesale re-evaluation of silicic volcanic systems.

Our new model proposes that apparently effusive lava is generated directly from explosive volcanism, assembled by the viscous amalgamation - sintering - of hot volcanic ash and pumice in the volcanic conduit in the shallowest parts of the Earth's crust (see CfS). The cryptic fragmentation model was developed in response to evidence from crystal-poor silicic systems. In this new study we go further, and propose that the model also applies to crystal-rich intermediate systems, which are much more common, and pose a global hazard. This hypothesis is based on abundant evidence from crystal-rich systems, similar to that summarized above.

This project will deliver:

(1) New analysis of dome-forming and crystal-rich lavas worldwide using existing samples from multiple laboratories. We will constrain the textures in the groundmass - with a focus on pore-textures indicative of sintering petrogenesis - and macro-scale textures associated with breaking and sintering, such as fractures will with partially sintered particles. This new textural work, coupled with analytical and petrophysical measurements, will underpin our extension of the cryptic fragmentation model to crystal-bearing magma systems.

(2) A comprehensive suite of new experimental volcanology measurements of sintering rates with multiphase magmatic particles - glass with crystals. Relying on the PI's large body of experimental and theoretical sintering work, we will develop new experimentally-validated models for sintering rates with crystals in systems under elevated pressures, in the presence of magmatic volatiles, and under shear stresses. For the first time, this will push sintering theory to magmatic conditions and allow the first quantitative test of sintering rates at volcanoes.

(3) We will apply these sintering rate equations to active crystal-bearing volcanic eruptions of the past at the same sites from which the sample suites were collected, with a focus on Colima volcano (Mexico) via engagement with stakeholders at volcano observatories.

Cryptic fragmentation model reference:
Wadsworth, F.B., Llewellin, E.W., Vasseur, J., Gardner, J.E. and Tuffen, H., 2020. Explosive-effusive volcanic eruption transitions caused by sintering. Science advances, 6(39), p.eaba7940