The key issues
Volcanoes can switch rapidly from dormancy to eruption, and from one eruptive regime to another, with potentially catastrophic consequences. In order to provide accurate real-time expertise for hazard mitigation, volcanologists need to understand not only the fundamental processes governing different eruption states, but also the signals or behavioural patterns indicative of impending regime transitions. The ability to use measurements and models to understand physically what is happening beneath the volcano, and hence to be able to anticipate changes in eruptive regime in near-real time, is one of the principal aims of modern volcanology.
Regime transitions are governed by processes occurring in the reservoir, conduit and plume, with complex feedbacks. The magma reservoir exerts a strong influence on eruption state through changes in pressure and in the physical properties of magmas extracted. There is a need for more accurate estimates of reservoir sizes, depths and pressures using surface geophysical methods. We also need more detailed measurements of the volatile contents, rheologies, and redox states of erupted magmas, and correlations of temporal variations of these parameters with geophysical and geochemical signals.
As magma ascends through the conduit, dissolved gases exsolve to form bubbles; the magma vesiculates and fragments explosively into a spray of gas-entrained particles. The gas phase is the motor of explosive volcanic activity, and its behaviour is central to understanding eruption dynamics. The mechanisms of bubble nucleation and growth are thought to affect eruption state and need to be better understood. The rate at which gas is able to segregate from ascending magma is a major factor governing the effusive-explosive transition. We need to better understand the mechanisms, depths and timescales of bubble coalescence, magma permeability development and gas segregation/migration. We need to link surface gas fluxes and compositions to physical processes in the conduit, with better estimates of volatile budgets. Outgassing raises magma viscosity and drives crystallization, forming rheologically stiffened plugs that can fail suddenly in transient explosions. We need to explore the mechanisms and timescales of plug generation, and the transitions between open- and closed-conduit conditions. We also need to better understand the mechanisms and energetics of fragmentation
Eruption plumes are advected far from the vent by high-level winds, the particles falling out to form tephra deposits. The physics of volcanic plumes are only partially understood, and there is a need for better information on the physical parameters governing their behaviour. Scaling laws for plume ascent are needed in order to reliably invert plume measurements and infer source parameters. We also need to understand better how plumes interact with mesoscale and global atmospheric motions in the far field, so as to improve models of plume dispersal relevant to aviation hazards. Plumes exhibit different states: stable, partially stable, and unstable, the causes of which are incompletely explored. The processes involved in the eruption, transport and crystallization of lavas need to be measured, and physically robust models generated.
The tools available
The increasing use of remote sensing technologies in volcanology has transformed our ability to make measurements on active volcanoes of the quality and number needed to test models. Many of these techniques have sampling frequencies of 1 Hz or less, allowing us to capture the details of even transient explosions, and in some cases the speed of data processing is approaching near-real-time. High-speed visual, infra-red and ultra-violet cameras provide images of eruption plumes from which temperatures, ascent speeds, mass fluxes and particle size distributions can be extracted. Doppler radar spectra provide measurements of ascent velocities, particle loadings and mass fluxes. Spectroscopic techniques such as Differential Optical Absorption Spectroscopy, Ultra-violet cameras and Open-Path Fourier Transform Infra-red Spectroscopy provide measurements of the compositions, ratios and fluxes of different gas species, and, indirectly, of the depth/dynamics of magma degassing. Infrasound microphones and microbarographs give information on bubble sizes, gas overpressures and magma/gas ratios in erupting conduits, and electrical measurements record the fragmentation processes. Remote sensing from satellites enables us to track plumes over large distances and extract heights, particle and gas loadings and geometric parameters for input into plume dispersion models. Volcano observatory monitoring networks have greatly increased in sophistication, with dense arrays of highly sensitive broadband seismometers, gravity and magnetic stations and deformation sensors.
Measurements of eruption parameters are also made by quantitative analysis of eruption products. The dispersal and grain size of tephra deposits are used to estimate plume heights and mass fluxes by model inversion. Cooling of particles through the liquid-glass transition ‘locks in’ a range of physical and chemical parameters that can be measured by microanalysis and used to probe conditions inherited from the reservoir and conduit. Sub-micron-resolution tomography allows 3D quantification of sample textures, and reconstruction of bubble and crystal nucleation/growth histories. Microbeam techniques such as electron, ion and nuclear probes, and vibrational and synchrotron X-Ray spectroscopies, allow analysis of volatiles, trace elements, isotopes and element redox states in glasses, crystals, glass inclusions, gas bubbles and nanoparticles, with quantification of eruption volatile budgets, degassing histories, quench pressures, magma decompression rates and particle cooling rates. Analysis of short-lived radioactive isotopes in volcanic gases constrains timescales of magma ascent and degassing.
High-temperature laboratory experiments allow measurements to be made on samples of natural magma, such those pertaining to magma rheology, vesiculation, crystallization and fragmentation. Phase-equilibria experiments are used, along with petrological characterisation of natural samples, to constrain pre-eruptive conditions of pressure, temperature and volatile content. Analogue experimentation allows us to derive phenomenological laws and scalings governing conduit flow and volcanic plumes. Application of such laws to remotely sensed eruption images allows extraction of first-order estimates of plume parameters – itself a type of remote sensing. Large-scale experimental facilities to explore the physics of magma fragmentation and volcanic plumes have been developed.
Mathematical and numerical models range from approximate theoretical analyses to highly complex numerical codes of multiphase flow in reservoirs, conduits, plumes and pyroclastic flows. Multiphase codes are now passing from 2D to 3D, opening up new possibilities in simulation capability, and models linking conduits to plumes, and plumes to large-scale atmospheric motions, are now being developed. Also, a new generation of models are appearing that link deep processes to surface measurables, such as by magma-rock coupling to predict geophysical signals resulting from reservoir recharge or a change in conduit flow, or by introducing experimentally constrained laws of chemical degassing of the major volatile species in order to compare the calculated compositions, fluxes and redox states of surface gases with remote sensing measurements.