Kenneth H. Wohletz

Mechanisms of hydrovolcanic pyroclast formation: Grain-size, scanning electron microscopy, and experimental studies

Pyroclasts produced by explosive magma/water interactions are of various sizes and shapes. Data from analysis of over 200 samples of hydrovolcanic ash are interpreted by comparison with experimentally produced ash. Grain size and scanning electron microscopy (SEM) reveal information on the formation of hydrovolcanic pyroclasts. Strombolian explosions result from limited water interaction with magma and the pyroclasts produced are dominantly centimeter-sized. With increasing water interaction, hydrovolcanism increases in explosivity to Surtseyan and Vulcanian activity. These eruptions produce millimeter- to micron-sized pyroclasts. The abundance of fine ash (<63 μm diameter) increases from 5 to over 30 percent as water interaction reaches an explosive maximum. This maximum occurs with interactions of virtually equal volumes of melt and water. Five dominant pyroclast shape-types, determined by SEM, result from hydrovolcanic fragmentation: (1) blocky and equant; (2) vesicular and irregular with smooth surfaces; (3) moss-like and convoluted; (4) spherical or drop-like; and (5) plate-like. Types 1 and 2 dominate pyroclasts greater than 100 μm in diameter. Types 3 and 4 are typical of fine ash. Type 5 pyroclasts characterize ash less than 100 μm in diameter resulting from hydrovolcanic fragmentation after strong vesiculation. Fragmentation mechanisms observed in experimental melt/water interactions result from vapor-film generation, expansion, and collapse. Fragments of congealed melt are products of several alternative mechanisms including stress-wave cavitation, detonation waves, and fluid instability mixing. All result in rapid heat transfer. These mechanisms can explain the five observed pyroclast shapes. Stress-wave fracturing (cavitation) of the melt results from high pressure and temperature gradients at the melt/water interface. Simultaneous brittle fracture and quenching produces Type 1 pyroclasts. Type 2 develops smooth fused surfaces due to turbulent mixing with water after fracture and before quenching. Fluid instabilities promote turbulent mixing of melt and water and produce fine ash. This kind of fragmentation occurs during high-energy explosions. The increased melt surface area due to fine fragmentation promotes high-efficiency heat exchange between the melt and water. Shapes of resulting pyroclasts are determined by maximum surface area (Type 3) and surface tension effects (Type 4). Type 5 pyroclasts result from nearly simultaneous vesicle burst and melt/water fragmentation.

Explosive magma-water interactions: Thermodynamics, explosion mechanisms, and field studies

Physical analysis of explosive, magma-water interaction is complicated by several important controls: (1) the initial geometry and location of the contact between magma and water; (2) the process by which thermal energy is transferred from the magma to the water; (3) the degree to and manner by which the magma and water become intermingled prior to eruption; (4) the thermodynamic equation of state for mixtures of magma fragments and water; (5) the dynamic metastability of superheated water; and (6) the propagation of shock waves through the system. All of these controls can be analyzed while addressing aspects of tephra emplacement from the eruptive column by fallout, surge, and flow processes. An ideal thermodynamic treatment, in which the magma and external water are allowed to come to thermal equilibrium before explosive expansion, shows that the maximum system pressure and entropy are determined by the mass ratio of water and magma interacting. Explosive (thermodynamic) efficiency, measured by the ratio of maximum work potential to thermal energy of the magma, depends upon heat transfer from the pyroclasts to the vapor during the expansion stage. The adiabatic case, in which steam immediately separates from the tephra during ejection, produces lower efficiencies than does the isothermal case, in which heat is continually transferred from tephra to steam as it expands. Mechanisms by which thermal equilibrium between water and magma can be obtained require intimate mixing of the two. Interface instabilities of the Landau and Taylor type have been documented by experiments to cause fine-scale mixing prior to vapor explosion. In these cases, water is heated rapidly to a metastable state of superheat where vapor explosion occurs by spontaneous nucleation when a temperature limit is exceeded. Mixing may also be promoted by shock wave propagation. If the shock is of sufficient strength to break the magma into small pieces, thermal equilibrium and vapor production in its wake may drive the shock as a thermal detonation. Because these mechanisms of magma fragmentation allow calculation of grain size, vapor temperature and pressure, and pressure rise times, detailed emplacement models can be developed by critical field and laboratory analysis of the resulting tephra deposits. Deposits left by dense flows of tephra and wet steam as opposed to those left by dilute flows of dry steam and tephra show contrasts in median grain size, dispersal area, grain shape, grain surface chemistry, and bed form.

Hydrovolcanism: Basic considerations and review

Abstract Hydrovolcanism refers to natural phenomena produced by the interaction of magma or magmatic heat with an external source of water, such as a surface body or an aquifer. Hydroexplosions range from relatively small single events to devastating explosive eruptive sequences. Fuel-coolant interaction (FCI) serves as a model for understanding similar natural explosive processes. This phenomena occurs with magmas of all compositions. Experiments have determined that the optimal mass mixing ratio of water to basaltic melt for efficient conversion of thermal energy into mechanical energy is in the range of 0.1 to 0.3. For experiments near this optimum mixture, the grain-size of explosion products is always fine (less than 50 μm). The particles generated are much larger (greater than 1–10 mm) for explosions at relatively low or high ratios. Both natural and experimental pyroclasts produced by hydroexplosions have characteristic morphologies and surface textures. SEM micrographs show that blocky, equant grain shapes dominate. Glassy clasts formed from fluid magma have low vesicularity, thick bubble walls, and drop-like form. Microcystalline essential clasts result from chilling of magma during or shortly following explosive mixing. Crystals commonly exhibit perfect faces with patches of adhering glass or large cleavage surfaces. Edge modification and rounding of pyroclasts is slight to moderate. Grain surface alteration (pitting and secondary mineral overgrowths) are a function of the initial water to melt ratio as well as age. Deposits are typically fine-grained and moderately sorted, having distinctive size distributions compared with those of fall and flow origin. Hydrovolcanic processes occur at volcanoes of all sizes ranging from small phreatic craters to huge calderas. The most common hydrovolcanic edifice is either a tuff ring or a tuff cone, depending on whether the surges were dry (superheated steam media) or wet (condensing steam media). Hydrovolcanic products are also a characteristic component of eruption cycles at polygenetic volcanoes. A repeated pattern of dry to wet products (Vesuvius) or wet to dry products (Vulcano) may typify eruption cycles at many other volcanoes. Reconstruction of eruption cycles in terms of water-melt mixing is extremely useful in modeling processes and evaluating risk at active volcanoes.

Derivation of the Weibull distribution based on physical principles and its connection to the Rosin–Rammler and lognormal distributions

We describe a physically based derivation of the Weibull distribution with respect to fragmentation processes. In this approach we consider the result of a single‐event fragmentation leading to a branching tree of cracks that show geometric scale invariance (fractal behavior). With this approach, because the Rosin–Rammler type distribution is just the integral form of the Weibull distribution, it, too, has a physical basis. In further consideration of mass distributions developed by fragmentation processes, we show that one particular mass distribution closely resembles the empirical lognormal distribution. This result suggests that the successful use of the lognormal distribution to describe fragmentation distributions may have been simply fortuitous.

Impact origin of sediments at the Opportunity landing site on Mars

Mars Exploration Rover Opportunity discovered sediments with layered structures thought to be unique to aqueous deposition and with minerals attributed to evaporation of an acidic salty sea. Remarkable iron-rich spherules were ascribed to later groundwater alteration, and the inferred abundance of water reinforced optimism that Mars was once habitable. The layered structures, however, are not unique to water deposition, and the scenario encounters difficulties in accounting for highly soluble salts admixed with less soluble salts, the lack of clay minerals from acid–rock reactions, high sphericity and near-uniform sizes of the spherules and the absence of a basin boundary. Here we present a simple alternative explanation involving deposition from a ground-hugging turbulent flow of rock fragments, salts, sulphides, brines and ice produced by meteorite impact. Subsequent weathering by intergranular water films can account for all of the features observed without invoking shallow seas, lakes or near-surface aquifers. Layered sequences observed elsewhere on heavily cratered Mars and attributed to wind, water or volcanism may well have formed similarly. If so, the search for past life on Mars should be reassessed accordingly.

Thermal evolution of the Phlegraean magmatic system

Abstract A series of 2-D conductive/convective numerical models show a rather limited range of possible magma chamber configurations that predict the present thermal regime at Campi Flegrei. These models are calculated by HEAT, which allows continuous adjustment of heterogeneous rock properties, magma injection/replenishment, and convective regimes. The basic test of each model is how well it reproduces the measured thermal gradients in boreholes at Licola, San Vito, and Mofete reported by AGIP in 1987. The initial and boundary conditions for each model consists of a general crustal structure determined by geology and geophysics and major magmatic events: (1) the 37 ka Campanian Ignimbrite; (2) smaller volume 37–16 ka eruptions; (3) the 12 ka Neapolitan Yellow Tuff; (4) recent magmatism (e.g., Minopoli at ∼10 ka and Monte Nuovo in 1538 AD). While magma chamber depth is well constrained, magma chamber diameter, shape, volume, and peripheral convective regimes are poorly known. Magma chamber volumes between 200 and 2000 km3 have been investigated with cylindrical, conical (funnel-shaped), and spheroidal shapes. For all reasonable models, a convective zone, developed above the magma chambers after caldera collapse, is necessary to achieve the high gradients seen today. These models should help us understand recent bradyseismic events and future unrest.

The volcanic ash problem

Abstract Explosive volcanic eruptions are the result of intensive magma and rock fragmentation, and they produce volcanic ash, which consists of fragments

Martian rampart crater ejecta: experiments and analysis of Melt-Water interaction

Abstract Viking images of Martian craters with rampart-bordered ejecta deposits reveal distinct impact ejecta morphology when compared to that associated with similar-sized craters on the Moon and Mercury. Topographic control of distribution, lobate and terraced margins, cross-cutting relationships, and multiple stratigraphic units are evidence for ejecta emplacement by surface flowage. It is suggested that target water explosively vaporized during impact alters initial ballistic trajectories of ejecta and produces surging flow emplacement. The dispersal of particulates during a series of controlled steam explosions generated by interaction of a thermite melt with water has been experimentally modeled. Preliminary results indicate that the mass ratio of water to melt and confining pressure control the degree of melt fragmentation (ejecta particle size) and the energy and mode of melt-ejecta dispersal. Study of terrestrial, lobate, volcanic ejecta produced by steam-blast explosions reveals that particle size and vapor to clast volume ratio are primary parameters characterizing the emplacement mechanism and deposit morphology. Martian crater ramparts are formed when ejecta surges lose fluidizing vapors and transported particles are deposited en masse. This deposition results from flow yield strength increasing above shear stress due to interparticle friction.

Eruptive mechanisms of the Neapolitan Yellow Tuff interpreted from stratigraphie, chemical, and granulometric data

Abstract The Neapolitan Yellow Tuff (12 ka) is the second largest pyroclastic deposit of the Campanian Volcanic Area covering at least 1000 km 2 with conservative estimates of volume placed at 40 km 3 . Previous studies showed that this mainly trachytic deposit, composed of two members, was erupted by (1) a central-vent, mostly phreatoplinian phase (Lower Member) that generated pyroclastic surges and fallout reaching 34 km from the vent, followed by (2) a multiple-vent, phreatomagmatic and magmatic phase (Upper Member) associated with onset of caldera collapse that produced surges extending 14 km from the vent. The Lower Member is well bedded and comprises 13 subunits that alternate between phreatoplinian surges/flows and Plinian pumice-and-ash fallout. The Upper Member is relatively lithic-rich and more massive in character. For both members, magma compositions vary from alkali trachyte through trachyte to latite, which does not fit a simple inversion of magma chamber gradients. Calculations based on magma chemistry show an increase in magma density, a decrease followed by an increase in viscosity, and a general decrease of gas fraction during the course of both eruptive phases. After migrating to a depth of about 400 m, calculated fragmentation depths gradually rise during each phase. Application of sequential fragmentation/transport analysis to granulometric data shows for the lower member an average ratio of phreatomagmatic to magmatic components of 70:30 while the upper member shows an average ratio of 80:20. However, considering the tephra volume represented by samples, computed water/magma interaction ratios ( R ) are shown to fluctuate, but generally decrease from about 0.65 to 0.05 during eruption of the Lower Member while the Upper Member shows R fairly constant at about 0.1. Furthermore, surge/flow runout distances and estimates of eruptive velocities from R values show that column collapse heights were extremely high (6 to 7 km) during the first phase and were substantially lower during the second phase (2 to 3 km). Vent radii required for calculated eruption velocities of 180 to 370 m/s are between 70 and 300 m, suggesting a cumulative eruption duration of over 10 hours, perhaps spanning of one to several days.

Water/magma interaction: some theory and experiments on peperite formation

Experiments, using molten thermite as a magma analog, produce peperite when the melt interacts with wet sand. These experiments also show explosive behavior, developing Strombolian- and Surtseyan-like bursts. The results demonstrate that the application of fuel–coolant interaction (FCI) theory is appropriate for interpretation of peperites. The theory described includes discussion of the importance of mass interaction ratios of wet sediment and magma (Rs), which determine thermal equilibrium temperature limits and contact interface dynamics. The dynamics of the interface between magma and wet sediments involves heat transfer over a wide range of rates from passive quenching to explosive fragmentation. A vapor film layer develops at the interface and acts both as an insulating barrier, promoting passive quenching, as well as a potential energy reservoir that can cause magma fragmentation, mingling of the magma with wet sediments, and explosive quenching when the vapor film becomes unstable. An important parameter in determining the behavior of the vapor film is the value of Rs, which controls whether heat can be convectively removed from the layer as more is being added from its contact with magma. If Rs>1 for fully saturated sediments, there is enough water in the sediments to make convective heat flow effective in quenching the magma, but below that value, there is the potential that the vapor film will be unstable, producing highly dynamic phenomena, including explosive fragmentation. At values of Rs<0.1 there is insufficient water to allow the escalation of explosive fragmentation.