The consequences of fluid motion in volcanic conduits

When volcanoes are active, there are characteristic signs such as ground movement, sounds, heat and ejected material. Each of these signs is a result of, and hence an information source for, fluid motion in volcanic conduits. Here we briefly review some of the links between these signs and fluid flow processes and suggest future directions that should allow advancement of eruption forecasting as these links become understood more fully. Cross-fertilization between increasingly realistic numerical and experimental models, diverse geophysical data sources, and chemical and physical evidence in eruptive products can be achieved by simultaneously applying these approaches to well-studied volcanoes. Volcanic activity comprises a wide range of phenomena that result from complex non-linear interactions and feedback mechanisms. Processes that start in the volcanic plumbing system determine the nature of subsequent events and control eruption styles. On emerging from the vent, volcanic material enters the atmosphere and the ensuing interactions are key in determining the consequent transport of volcanic debris that defines the impact on the environment and on human lives and infrastructure. Our capability to mitigate volcanic hazards relies in large part on forecasting eruptive events, and this, in turn, requires a high degree of understanding about the physical and chemical processes operating during volcanism. The ability to interpret surface observations and measurements in terms of subsurface processes is a key step in eruption forecasting. The flow of magma in volcanic plumbing systems is a prerequisite for surface eruptive activity. For magma flow to take place, force must be applied and pressure gradients must exist. Forces and pressures in magma systems will change with time on a range of scales, resulting in motion of conduit walls that, if sufficiently large, will be measurable. Fluids in motion also exert drag on the walls of a conduit, variations of which will create ground motion (e.g. Green et al. 2006). Changes in force, pressure and drag may also couple into a range of resonant or other cyclical processes. This suggests that a major source mechanism of ground motion at active volcanoes is magma flow, and that this ground motion may, therefore, be interpreted in terms of fluid flow within the active volcanic conduit. Volcanoes are largely monitored by measurement of the consequences of fluid flow within conduit systems, including motion of the ground over a wide range of timescales (e.g. Chouet 2003; Chadwick et al. 2006), motion of the atmosphere in response to eruption (e.g. Matoza et al. 2007), thermal signatures (e.g. Ball & Pinkerton 2006), and the physicochemical nature of ejected material, for example gas chemistry (Burton et al. 2007), crystal zonation (Blundy et al. 2007), and pyroclast morphology (Lautze & Houghton 2007). Many of these natural signals have the same underlying source process, namely the motion of gases, liquids and solids within and beneath volcanic edifices. The linking of passive geo-physicochemical signals to the dynamics of fluids formed the focus of a Workshop entitled ‘The Physics of Fluid Oscillations in Volcanic Systems’, held at Lancaster University on 7 and 8 September 2006, and seeded the production of this Special Publication. Fluid flow in volcanoes is a difficult phenomenon to observe in action, especially with increasing volcanic explosivity index (VEI, Newhall & Self 1982; Pyle 2000). High-VEI eruptions do not happen very often, and the larger the explosion the less frequent it is, with, for example, repeat timescales of order 10 years for VEI 6 events such as the Krakatau 1883 eruption. This makes detailed syn-event measurements of large explosive episodes infrequent on the timescale of contemporary science; an event like Krakatau in 1883 has never been monitored by modern methods. Medium-VEI events such as vulcanian explosions frequently destroy near-field observation equipment (e.g. Voight et al. 1998), suggesting that proximal measurements during high-VEI eruptions are very difficult to obtain even when an event does occur. Processes that contribute to an explosive event are difficult to access directly; a prime example of this is how to obtain data about the nature of flow in the volcanic conduit system during explosive activity. Direct measurements are not possible, From: LANE, S. J. & GILBERT, J. S. (eds) Fluid Motions in Volcanic Conduits: A Source of Seismic and Acoustic Signals. Geological Society, London, Special Publications, 307, 1–10. DOI: 10.1144/SP307.1 0305-8719/08/$15.00 # The Geological Society of London 2008. and our main source of information is the deformation of the conduit wall created by flow that is unsteady on a wide range of spatial and temporal scales. The current level of understanding of high-VEI events is based on combining information from different volcanoes, which introduces further complexities. However, a small population of accessible, persistently active, low-VEI volcanoes with relatively uncomplicated magma rheology, such as Erebus (Antarctica) and Stromboli (Italy), produce weakly explosive and highly repeatable eruptions on timescales of hours and, therefore, lend themselves to detailed study. Kilauea Volcano in Hawaii is heavily instrumented and continuously active. Dome-building and vulcanian eruptions, such as the Soufrière Hills, West Indies, and Sakurajima, Japan, are currently undergoing intensive study. It is from such lowerVEI volcanoes that the linking of magma flow to measurable effects is most likely. In future, a unified process-based interpretation of passive geo-physicochemical signals honed on low-VEI volcanoes should be applied in order to increase understanding and forecasting of high-VEI events. The approach taken to gain insight into flow in volcanic conduits relies on field observations and measurements of events, their products and consequences, combined with theoretical analysis and models of processes, and laboratory experimental modelling of materials and mechanisms. Laboratory and numerical approaches may predict currently uninterrupted and undetected volcanic signals, as well as providing knowledge of system behaviour (e.g. Longo et al.; Kurzon et al.; Rust et al.; James et al., 2008). A range of types of field observations of both volcanic activity and post-eruption volcanic rocks provides the ‘ground truth’ for numerical and experimental modelling, as well as providing vital historical data. However, using one or two types of field observation in isolation makes it difficult to attribute those observations to a specific fluid-flow source process in a volcanic conduit. A combined interpretation of many types of observation and measurement at individual volcanoes is likely to be a powerful way forward. Flow of low-viscosity magma Eruptions that typically occur at Stromboli and Hawaii involve low-viscosity basalt magma. Strombolian eruptions are discrete, gas-rich events that result from the large-scale separation of water vapour from magma. Hawaiian eruptions are longer-lived and relatively continuous events where water vapour and magma are erupted with less separation. See Parfitt & Wilson (2008) for further details and the relationship between eruptive styles and magma viscosity. Ohminato et al. (1998) linked a particular ground deformation signal measured at Kilauea Volcano, Hawaii, to a conceptual fluid-flow source process (Fig. 1). The key components of this source process were conduit geometry and the presence of two phases of significantly different compressibility, viscosity and density, namely water vapour and silicate melt. The interaction of conduit geometry with gas-liquid flows, where gas bubbles are of similar dimension to the conduit radius, provides a rich vein of source processes with which to generate seismic and acoustic signals. Inversion of highly repeatable seismic measurements at Stromboli (e.g. Auger et al. 2006; Chouet et al. 2003) suggests that magma motion generates a downward-directed force of about 10 N, together with an expansion of the source region (Fig. 2). Inversion also reveals that the conduit is an inclined dyke. This has important consequences for low-VEI systems where gravity often plays a major role in magma flow. Magma generally contains 1–5 wt% water; however, Strombolian eruptions at Stromboli are much more water rich (Chouet et al. 1974; Blackburn et al. 1976). This strongly suggests that water vapour is separating from the parent magma to form the erupted material. The chemistry of gases erupted during the Strombolian activity also suggests rapid separation of magma and volatile (on diffusion timescales) from depths between 900 m and 2.7 km (Burton et al. 2007), supporting the previous observations. Intermittent gas-rich eruptions at Stromboli suggest the periodic rise and burst of large overpressured bubbles of water vapour separated by relatively bubble-poor magma. In order to understand how Strombolian eruptions are created, it is critical to study the processes that transform a bubbly magma deep in the volcano to the burst of one large bubble at the surface. Water-gas systems, extensively studied for industrial purposes (e.g. Mudde 2005), show that large bubbles form by coalescence of many small bubbles when the overall gas volume fraction exceeds about 0.25 (Clift et al. 1978). However, geometric considerations indicate that the time intervals between large bubble bursts would be similar to burst duration with such a high gas volume fraction. Stromboli erupts for tens of seconds, separated by gaps of tens of minutes (Chouet et al. 2003), suggesting that other mechanisms operate to promote the formation of gas-rich regions some 1 km or more below the top of the magma column. One approach to gain understanding of volcanic processes is to carry out small-scale laboratory experiments using liquids and gases analogous to magma. Jaupart & Vergniolle (1988, 1989) injected J. S. GILBERT & S. J. LANE 2