Magma-water interaction in volcanic eruptions
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In the course of work in the Hopi Buttes volcanic field, it began to become apparent that the deposits of these maar volcanoes contained a great abundance of disseminated, fine-grained, country-"rock" debris. This material, comprising smectitic mud with quartzofeldspathic silt, was derived from playa muds at the time of eruption, and dispersed virtually throughout many of the surge deposits surrounding the maar craters. I inferred from their ubiquity in primary pyroclastic deposits that the playa muds were present at the sites of phreatomagmatic explosions, and began to consider how such material would affect phreatomagmatic explosion processes.
Sediment and phreatomagmatism
Phreatomagmatism in the strict sense is a variety of hydrovolcanism in which groundwater interacts with magma to modify the style of the resulting eruption (A.G.I. dictionary), the most common products of which are mafic maar volcanoes, diatremes, tuff rings and tuff cones. In many settings groundwater is limited, and eruptions with ongoing steam explosivity involve influx of sediment laden water and/or significant commingling of water with recycled clastic eruptive debris (+/- water). Because of this, mixing of magma with water-saturated sediment (clastic debris of any origin) is likely to be a universal process in the throats of volcanoes during phreatomagmatism. Intimate mixtures of magma and wet sediment have been described from various non-Surtseyan settings, and such mixtures are important in sustaining phreatomagmatic eruptions. Even if a phreatomagmatic vent is initially open and clean, fallback alone causes subsequent interactions to take place in the presence of recycled clasts returning to the interaction site. Recycled same-eruption clasts make up more than 2/3 of the pyroclasts in some, relatively "dry", phreatomagmatic deposits, and studies of "wet" composite cone craters and diatremes similarly illustrate the debris-filled nature of phreatomagmatic vents, in which recycled ash, fragmented country rock and disaggregated sediment make up a large part of the clast population.
In natural phreatomagmatic systems, bodies of pure water are rarely, if ever, available at the sites of fuel-coolant interactions in which the fuel is magma. If the coolant as a whole is considered to be a fluid with which a fuel intimately mixes and to which it transfers heat, at least four types of coolant can be distinguished: pure coolants (a single, wholly vaporizable fluid), and three types of impure, or complex, ones including those in which; (1) the coolant fluid contains dissolved compounds; (2) the coolant fluid contains particles (such as sediment grains), and; (3) the fluid contains dissolved compoundsand particles. The latter two types range from fluid-dominant, in which particles are widely separated and have negligible effect on the properties of the fluid, to particle-dominant, in which particles are in contact with one another and fluid occupies interstitial space (liquefied sediment, dense suspension). A fifth type, in which the coolant locally contains an immiscible fluid, may be involved in eruptions through crater lakes with basal pools of molten sulfur (e.g. the 1995 eruption of Ruapehu, New Zealand). The coolant most plausibly present in any volcanic vent is an impure one containing solid particles and dissolved compounds. The compounds and particles are not vaporizable, but the mixture as a whole is (albeit with a residual component). It is animpure coolant. Ionic or molecular compounds dissolved in a coolant can be precipitated during vaporization (e.g. salts from seawater), but cannot be physically separated from liquid coolant. Effects of solid particles on the FCI process, andvice versa, depend on particle properties, their concentration in the coolant, and the size of the particles relative to the scale of FCI. Impure coolants affect the initial "coarse mixing" phase of an interaction in two ways. First, they alter the relative densities, viscosities, wetting angles, and surface tensions of magma and coolant. Second, they change the ability of the mixture to explode, thereby shortening or lengthening the time in which coarse mixing can take place. These effects are important whether coarse mixing takes place by film instability processes or under the impetus of magmatic injection as is likely in most eruptions. The increased density, viscosity, and surface tension of sediment-water coolants should all act to damp the explosivity of volcanic FCI's as compared with experimental examples. They do not, however, prevent the mixing of fuel and coolant from proceeding, and the damping of explosivity may in fact lead to much more effective intermingling of magma with impure, geologically plausible, phreatomagmatic coolants.
Some peperites illustrate styles of coarse mixing between magma and sediment-laden impure coolants in phreatomagmatic settings. Development of some fluidal peperites has been attributed to (1) repeated damped steam explosions and (2) mixing as a result of contact instabilities between fluids. Peperite and swirly dikes demonstrate that contact geometries in unconsolidated or poorly consolidated sediment-water mixtures can be very complex, with contact surface areas much greater than typical where magma is simply extruded into water. In this way, sediment-water coolants can greatly enhance the degree of pre-detonation mixing and intermingling relative to pure water, thereby greatly increasing the volume of the coarsely intermingled magma-coolant system available to be detonated. Although peperites and related magma-sediment mixtures demonstrate the ability of magma to mix coarsely with impure sediment laden coolants, it is not clear whether this mixing is the same as that involved in FCI explosions. Because peperites are preserved and thus can represent onlyfrozen, or incomplete FCI's, they cannot provide direct evidence of mixing geometries in successful FCI's. It is clear, however, that peperite-style mixing prior to development of FCI explosions can greatly increase the magma-coolant contact area and allow magma to partially or completely engulf local volumes of coolant.
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Phreatomagmatic eruptions are driven by fuel-coolant interaction processes, but the coolant is invariably impure, and almost invariably particle-laden. Particle laden water interacts with magma where the latter enters unconsolidated sediment, and in volcanic vents containing vent-wall debris and pyroclastic-fallback tephra. Sediment-laden coolants differ from pure water in density, viscosity, availability of bubble nucleation sites, bubble wetting angles, and other fluid properties. Some aspects of sediment laden coolants, such as the increased availability of nucleation sites, may enhance FCI explosivity directly. Other properties, such as increased coolant density and viscosity, should inhibit explosivity to a degree, but favour increased pre-explosive coarse mixing of fuel and coolant, thereby setting the stage for larger individual interactions, which must be vigorously triggered. Triggers for these larger interactions are most likely volcanogenic seismicity, impacts from vent wall collapse, or small superheat explosions. During continuous eruption, phreatomagmatic FCI's in the magma-tephra-water vent-bottom slurry are discontinuously initiated by fallback impacts, vent-debris collapse, and wall collapse (Figure). In the complex vent-base region of an erupting phreatomagmatic volcano, the size and location of impacts, state of mixing between magma and wet tephra, and the tephra's water content, at moments of impact, and degrees of heating and local fluidization in the tephra probably exert more influence on resulting FCI-driven eruption pulses than does the mass-ratio of water to magma present during any given time at the FCI site.
For a more complete story see the article from which this information is drawn: White, J. D. L. (1996) Impure coolants and interaction dynamics of phreatomagmatic eruptions.J. volcanol. geotherm. Res.,74, 155-170.
Ongoing experimental studies of phreatomagmatic explosion processes are underway with Bernd Zimanowski and Ralf Buettner at the laboratory of Volker Lorenz in Wuerzburg, Germany.
A field study, the dissertation work of Jean-Baptiste Rosseel, is designed to determine the specific sites and mechanisms of phreatomagmatic explosions during the 1886 eruption of Rotomahana, on the North Island of New Zealand. Additional information on this topic will soon be published in a thematic issue of the Journal of Volcanology and Geothermal Research entitled Peperites: products and processes of magma-sediment mingling.