Explosive Magma Water Interaction Research Articles

Ash aggregation processes in a basaltic tuff ring (Songaksan, Jeju Island, Korea): Controls of diatreme conditions and transport processes

Aggregation of ash generated by explosive volcanic eruptions controls the dispersal and residence time of ash in the atmosphere and, therefore, the hazard to aviation. Ash aggregation is particularly common in hydromagmatic volcanoes produced by explosive magma-water interactions. Nevertheless, few studies have been done on this process because of the lower risk of volcanic hazards posed by these volcanoes in comparison to classical stratovolcanoes. Observations of the microscopic textures of the tuff samples from a Holocene tuff ring in Jeju Island, Korea, show that the ash particles occur mainly in the form of coated particles and ash lumps, which are either rich or poor in clay minerals derived from the substrate. The clay-rich aggregates formed in the diatreme by cohesive and capillary bonding of clay-rich and water-undersaturated debris. When the diatreme was filled with clay-poor and water-saturated debris, particles were ejected mainly without ash coats and aggregation. Once emitted from the diatreme, the aggregates and uncoated/solitary particles could be further aggregated together or accreted on larger particles by liquid bonding, producing clay-poor aggregates, multiple-coated particles, and composite ash lumps. The ash aggregation occurred mainly in co-surge ash clouds and ash plumes but not in pyroclastic surges, in which the time available for ash aggregation was too short (a few tens of seconds or less). This study demonstrates that the substrate characteristics and environmental factors that can influence the material properties of the diatreme fill, such as the clay mineral and water contents, are the prime controls of ash aggregation in hydromagmatic volcanoes.

Hydrovolcanic activity on a continental shelf inferred from the subsurface diatreme- and crater-filling deposits of Jeju Island, Korea

Jeju Island comprises numerous tuff rings and tuff cones and their reworked deposits in the subsurface, which formed on the ca. 120-m-deep Yellow Sea continental shelf under the fluctuating Quaternary sea levels. Tens of meter-thick and massive deposits were found by chance during groundwater drilling at three sites. These deposits are interpreted as either syn-eruptive diatreme-filling deposits or post-eruptive crater-filling deposits, both of hydromagmatic volcanoes. The diatremes were cut into shelf sediment, 70 to 250 m thick, and developed generally within it below the Quaternary sea levels. Abundant external water was therefore available for explosive magma-water interactions at shallow levels. The diatreme deposit in one core shows some features attributable to extreme wetness or water saturation of the diatreme fill, such as the matrix support of larger clasts, meager vertical changes in matrix content, and an absence of features related to particle adhesion. Fluidally shaped clasts with delicate reentrant margins in the core suggest minimal particle abrasion and breakage in a water-saturated and highly fluid slurry of tephra and water that was probably filling a shallow bowl-like diatreme, which is distinguished from both phreatomagmatic and kimberlite diatremes. The diatreme deposits in other cores comprise blocky and angular clasts in a sideromelane ash matrix, suggesting phreatomagmatic explosions at a deeper level. One of the cores contains collapsed deposits of thinly stratified tuff emplaced by pyroclastic surges, indicating that the diatreme is associated with an emergent tuff ring. Both Surtseyan and phreatomagmatic eruptions are therefore interpreted to have occurred on the shelf under the controls of fluctuating Quaternary sea levels. The subsurface diatremes suggest that there can be a variety of diatremes with different sizes, shapes, and material characteristics beneath the craters of hydromagmatic volcanoes, including not only maars but also tuff rings and tuff cones.

Using the characteristics of rootless cone deposits to estimate the energetics of explosive lava–water interactions

During volcanic eruptions, the interaction of magma and groundwater can produce thermohydraulic explosions capable of significantly increasing the eruption energy. The most well-known mechanism by which explosive magma–water interactions occur, molten fuel–coolant interaction (MFCI), is a complex series of macro- and microscale processes which have been simulated using laboratory-scale experiments. As a natural analog for MFCI experiments, we utilize rootless cone beds formed by lava–water explosions to estimate explosion energy. The specific mechanical energy of the lava–water explosions studied here occurs over a broader range (4 to 178 kJ/kg) than MFCI experiments and includes estimates for the highest-energy lava–water explosions studied to date. Explosion energy is partitioned similarly over the two systems, with kinetic transport and fragmentation energy making up 25–40% and 42–80% of the mechanical energy, respectively, which overlap the ranges estimated for MFCI experiments. Our study of lava–water explosions therefore provides a field-scale analog of MFCI laboratory experiments for understanding the energetics, and therefore hazards, of MFCI in natural systems.

Tectonically controlled carbonate-seated maar-diatreme volcanoes: The case of the Volsci Volcanic Field, central Italy

Quaternary carbonate-seated maar-diatremes in the Volsci Range are one of the most intriguing products of the west-directed subduction of the Adriatic slab that drove the development of the Apennine mountain belt in Central Italy. The Volsci Volcanic Field is characterized by phreatomagmatic surge deposits, rich in accidental carbonate lithics, and subordinate Strombolian scoria fall deposits and lava flows, locally sourced from some tens of monogenetic eruptive centers (at least fifty tuff rings and scoria cones). We investigate the subsurface maar-diatreme processes in terms of relationships between faulting and explosive magma-water interaction, as well as the distribution pattern of the eruptive centers. With this aim, we present the following new data: i) description of the fold-and-thrust belt structure and associated eruptive centers, ii) componentry of volcanic rock-types, iii) determination of grain-size, degrees of whiteness and roundness of carbonate lithic inclusions, iv) micropaleontological analysis of carbonate lithics. We show that the clustering of eruptive centers is controlled by tectonic features. A first order control is tentatively related to crustal laceration and deep magma injection along a ENE-trending Quaternary lateral tear in the slab and to Mesozoic rift-related normal faults. A second-order control is provided by orogenic structures (mainly thrust and extensional faults). In particular, magma-water explosive interaction occurred at multiple levels (< 2.3 km depth), depending on the structural and hydrogeologic setting of the Albian-Cenomanian carbonates, which are intersected by high-angle faults. The progressive comminution, rounding and whitening of entrained carbonate lithics allow us to trace multistage diatreme processes. Finally, our study bears implications on volcanic hazard assessment in the region.

Fragmentation mechanisms associated with explosive lava–water interactions in a lacustrine environment

Rootless cones form when partially outgassed lava interacts explosively with external water. The explosions represent an end-member system that can elucidate mechanisms of explosive magma–water interactions in the absence of magmatic fragmentation induced by outgassing. The proportion of finely fragmented ejecta (i.e., ash), generated in rootless explosions, may contribute significantly to the energy of the explosion even if the ash volume is small relative to coarser ejecta. Laboratory experiments indicate that the degree of melt–water mixing and energy release are proportional to the abundance of blocky grains, fragmented by brittle disintegration, which effectively contribute thermal energy to the system. To constrain the mechanisms and dynamics of rootless explosive activity, we assess the nature and modes of fragmentation and ejecta characteristics through morphological, textural, and density analysis of rootless tephra associated with a pāhoehoe lava flow in a lacustrine (lake basin) environment. We observe strong correlations between the mean grain size and the mass percentage of both blocky (negative power law trend) and fluidal (positive logarithmic) tephra clasts of all sizes. We interpret these trends as scale-dependent fragmentation behavior due to the decreasing efficacy of hydrodynamic fragmentation as it occurs over finer scales, especially over the ash size range. Additionally, all analyzed beds contain fine ash-sized blocky and mossy clasts, which are thought to be diagnostic of a high transfer rate of thermal to mechanical energy, characteristic of molten fuel–coolant interactions. These results agree with a recent model of rootless cone formation, prior fragmentation theory, and scaled laboratory experiments and therefore provide a field-based analog for future experimental and modeling efforts.

Holocene phreatomagmatic eruptions alongside the densely populated northern shoreline of Lake Kivu, East African Rift: timing and hazard implications

The Virunga Volcanic Province (VVP) represents the most active zone of volcanism in the western branch of the East African Rift System. While the VVP’s two historically active volcanoes, Nyamulagira and Nyiragongo, have built scoria cones and lava flows in the adjacent lava fields, several small phreatomagmatic eruptive centers lie along Lake Kivu’s northern shoreline, highlighting the potential for explosive magma-water interaction. Their presence in the densely urbanized Sake-Goma-Gisenyi area necessitates an assessment of their eruptive mechanisms and chronology. Some of these eruptive centers possess multiple vents, and depositional contacts suggest distinct eruptive phases within a single structure. Depositional facies range from polymict tuff breccia to tuff and loose lapilli, often impacted by blocks and volcanic bombs. Along with the presence of dilute pyroclastic density current (PDC) deposits, indicators of magma-water interaction include the presence of fine palagonitized ash, ash aggregates, cross-bedding, and ballistic impact sags. We estimate that at least 15 phreatomagmatic eruptions occurred in the Holocene, during which Lake Kivu rose to its current water level. Radiocarbon dates of five paleosols in the top of volcanic tuff deposits range between ∼2500 and ∼150 cal. year bp and suggest centennial- to millennial-scale recurrence of phreatomagmatic activity. A vast part of the currently urbanized zone on the northern shoreline of Lake Kivu was most likely impacted by products from phreatomagmatic activity, including PDC events, during the Late Holocene, highlighting the need to consider explosive magma-water interaction as a potential scenario in future risk assessments.

Transport and mixing dynamics from explosions in debris-filled volcanic conduits: Numerical results and implications for maar-diatreme volcanoes

Most volcanoes experience some degree of phreatomagmatism during their lifetime. However, the current understanding of such processes remains limited relative to their magmatic counterparts. Maar-diatremes are a common volcano type that form primarily from phreatomagmatic explosions and are an ideal candidate to further our knowledge of deposits and processes resulting from explosive magma–water interaction due to their abundance as well as their variable levels of field exposure, which allows for detailed mapping and componentry. Two conceptual models of maar-diatreme volcanoes explain the growth and evolution of the crater (maar) and subsurface vent (diatreme) through repeated explosions caused by the interaction of magma and groundwater. One model predicts progressively deepening explosions as water is used up by phreatomagmatic explosions while the other allows for explosions at any level in the diatreme, provided adequate hydrologic conditions are present. In the former, deep-seated lithics in the diatreme are directly ejected and their presence in tephra rings is often taken as a proxy for the depth at which that particular explosion occurred. In the latter, deep-seated lithics are incrementally transported toward the surface via upward directed debris jets. Here we present a novel application of multiphase numerical modeling to assess the controls on length scales of debris jets and their role in upward transport of intra-diatreme material to determine the validity of the two models. The volume of gas generated during a phreatomagmatic explosion is a first order control on the vertical distance a debris jet travels. Unless extremely large amounts of magma and water are involved, it is unlikely that most explosions deeper than ∼250 m breach the surface. Other factors such as pressure and temperature have lesser effects on the length scales assuming they are within realistic ranges. Redistribution of material within a diatreme is primarily driven by subsidence following debris jet passage. Natural debris jet deposits often contain wall rock material that originated above the explosion site; likely this material was first transported downward by subsidence associated with preceding explosions and debris jets. The length scales of debris jets decrease with increasing depth, all other factors staying equal. The characteristics of the domains formed from just one explosion along with the difficulty of erupting deep-seated material indicate that erupted material at maar-diatreme volcanoes has a complex history before it is erupted and is mostly sourced from the upper-most part of the diatreme.

Emplacement of the Cabezo María lamproite volcano (Miocene, SE Spain)

Lamproite volcanoes are uncommon in the geological record but are exceptionally well preserved in the Betic Cordilleras of SE Spain, where they erupted during the Late Miocene (Tortonian to Messinian stages). The parent melts are thought to have been channelled through major lithospheric faults to erupt at or near the faulted margins of Neogene sedimentary basins. Lamproite magmas are thought to be relatively CO2-poor (<1 wt %) and are typically characterised by an effusive eruption style and the development of lava lakes and scoria cones. Cabezo Maria is a relatively small (∼550 m diameter) lamproite volcano that was emplaced within the shallow-water marine-influenced Vera Basin. The lamproites are compositionally similar to those of the Roman Province and generally less potassic (K2O<5 wt%) than other (ultra-) potassic rocks in SE Spain (e.g. Cancarix, Fortuna). The initial eruption stages were dominated by explosive magma-water interactions and the formation of peperites. These are characterised by angular fragments of glassy lamproite lavas (and isolated lobes) incorporated in sediments, locally showing the effects of thermal metamorphism. Further, elutriation pipes and ‘jigsaw-fit’ textures are observed in the peperites. The lavas and peperites are overlain by outward-dipping well-stratified scoria deposits defining part of a cinder cone, which is inferred to have emerged above sea level. Steep internal contacts with inward-dipping, structureless breccias likely represent the inner wall deposits of a central conduit. The deposits are cross-cut by late-stage dykes, which supplied fissure eruptions of geochemically similar lavas capping the scoria cone. The transition from explosive to effusive behaviour may reflect the decreased availability of water, possibly due to downward migration of the feeder conduit below the level of water-saturated sediments.

3-D reconstruction of ash vesicularity: Insights into the origin of ash-rich explosive eruptions

Explosive volcanic eruptions are characterized by highly variable degrees of magma fragmentation, even during a single eruptive event. The increasing amount of fine pyroclasts is often uncritically related to explosive magma–water interaction (i.e., hydromagmatic fragmentation). Here we report examination of two examples of major explosive eruptions from the Quaternary Vulsini Volcanic District (central Italy), in which the fine-grained nature of deposits, even in near-vent settings, indicates negligible effect of transport and implies the eruption of highly fragmented magmas. SEM morphoscopy of the juvenile products rules out extensive ash production due to hydromagmatic fragmentation. We apply a recently developed Stereo-Scanning Electron Microscopy (SSEM) technique (Proussevitch et al., 2011) to determine vesicularity features (e.g., bubble size distribution and bubble number density; hereafter BSD and BND, respectively) of ash particles. SSEM analysis provides new insights into magma vesiculation history and fragmentation mechanism leading to major ash-rich eruptions. We conclude that extensive ash production was related to essentially magmatic processes involving high degrees of decompression in shallow magma reservoirs.

First evidence of hydromagmatism at Colima volcano (Mexico)

Here we report for the first time evidences of hydromagmatism at Colima volcano (western sector of Trans-Mexican Volcanic Belt). The studied location exposes 9m of fine ash deposits dated at around 15calkyrBP by 14C measurement on organic matter collected at the very base of the stratigraphic succession. The ash deposits rest directly on the top of a debris avalanche deposit. The morphology and grain size distribution of ash samples from the basal part of the stratigraphic succession testify for an explosive magma–water interaction, which produced finely bedded layers with blocky fragments characterized by adhering particles, quenching cracks and chemical pitting. The availability of ground water was hypothesized to be in close relationship with the amelioration of climate conditions at the end of the Last Glacial period, with rising temperatures that induced melting of glaciers and snow previously accumulated on the volcano summit slopes. This recognition demonstrates how, even a “dry” volcano like Colima, can experience hydromagmatism under adequate climatic conditions.

“Poseidic” explosive eruptions at Loihi Seamount, Hawaii

Much remains unknown about submarine explosive eruptions. Their deposits are found to great depths in all the world’s oceans, but eruptions are typically described by analogy to a subaerial nomenclature that ignores the substantial and inevitable infl uences of hydrostatic pressure and magma-water interaction at submerged edifi ces. Here we explore magmatic volatile exsolution and magma-water interaction for a pyroclastic cone-forming eruption at ~1 km depth on Loihi Seamount, Hawaii. We examine vesicle textures in lapilli—the physical manifestation of degassing; dissolved volatiles in matrix glasses and olivine-hosted glass inclusions—the geochemical record of ascent and volatile exsolution; and fi ne ash morphology—the evidence for if and how external water assisted in fragmentation. This approach allows a submarine explosive eruption style to be defi ned: the magma achieved ~40% vesicularity through almost perfectly closed-system volatile exsolution from ~3 km below the vent, which accelerated and weakened the melt, allowing it to be fragmented by explosive magma-water interaction. We introduce the name “Poseidic” for this end-member style of submarine basalt explosivity. Poseidic eruptions are identifi able from measurable features in pyroclasts, and are possible at all subaqueous basaltic volcanoes.

Sub-surface dynamics and eruptive styles of maars in the Colli Albani Volcanic District, Central Italy

Eruptive scenarios associated with the possible reactivation of maar-forming events in the Quaternary, ultrapotassic Colli Albani Volcanic District (CAVD) provides implications for volcanic hazard assessment in the densely populated area near Rome. Based on detailed stratigraphy, grain size, componentry, ash morphoscopy and petro-chemical analyses of maar eruption products, along with textural analysis of cored juvenile clasts, we attempt to reconstruct the eruptive dynamics of the Prata Porci and Albano maars, as related to pre- and syn-eruptive interactions between trachybasaltic to K-foiditic feeder magmas and carbonate–silicoclastic and subvolcanic country rocks. Magma volumes in the order of 0.5–3.1 × 10 8 m 3 were erupted during the monogenetic Prata Porci maar activity and the three eruptive cycles of the Albano multiple maar, originating loose to strongly lithified, wet and dry pyroclastic surge deposits, Strombolian scoria fall horizons and lithic-rich explosion breccias. These deposits contain a wide range of accessory and accidental lithic clasts, with significant vertical stratigraphic variations in the lithic types and abundances. The two maar study cases hold a record of repeated transitions between magmatic (i.e, Strombolian fallout) and hydromagmatic (wet and dry pyroclastic surges) activity styles. Evidence of phreatic explosions, a common precursor of explosive volcanic activity, is only found at the base of the Prata Porci eruptive succession. The quantitative evaluation of the proportions of the different eruptive styles in the stratigraphic record of the two maars, based on magma vs. lithic volume estimates, reveals a prevailing magmatic character in terms of erupted magma volumes despite the hydromagmatic footprint. Different degrees of explosive magma–water interaction were apparently controlled by the different hydrogeological and geological–structural settings. In the Prata Porci case, shifts in the depth of magma fragmentation are proposed to have accompanied eruption style changes. In the Albano case, a deeply dissected geothermal aquifer in peri-caldera setting and variable mass eruption rates were the main controlling factors of repeated shifts in the eruptive style. Finally, textural evidence from cored juvenile clasts and analytical modeling of melt–solid heat transfer indicate that the interacting substrate in the Prata Porci case was at low, uniform temperature (~ 100 °C) as compared to the highly variable temperatures (up to 700–800 °C) inferred for the geothermal system beneath Albano.

Will subglacial rhyolite eruptions be explosive or intrusive? Some insights from analytical models

AbstractSimple analytical models of subglacial eruptions are presented, which simulate evolving subglacial cavities and volcanic edifices during rhyolitic eruptions beneath temperate glaciers. They show that the relative sizes of cavity and edifice may strongly influence the eruption mechanisms. Intrusive eruptions will occur if the edifice fills the cavity, with rising magma quenched within the edifice and slow melting of ice. Explosive magma–water interaction may occur if a water- or steam-filled gap develops above the edifice. Meltwater is assumed to drain away continuously, but any gap above the edifice will be filled by meltwater or steam. Ductile roof closure will occur if the glacier weight exceeds the cavity pressure and is modelled here using Nye’s law. The results show that the effusion rate is an important control on the eruption style, with explosive eruptions favoured by large effusion rates. The models are used to explain contrasting eruption mechanisms during various Quaternary subglacial rhyolite eruptions at Torfajökull, Iceland. Although the models are simplistic, they are first attempts to unravel the complex feedbacks between subglacial eruption mechanisms and glacier response that can lead to a variety of eruptive scenarios and associated hazards.

Field characteristics of deposits from spatter-rich pyroclastic density currents at Summer Coon volcano, Colorado

The Oligocene, deeply eroded Summer Coon composite volcano contains mafic andesite deposits that are massive to poorly bedded, have abundant flattened and deformed spatter clasts, have varying proportions of dense lithic clasts, and are supported mostly by a coarse-ash matrix. Although superficially these deposits resemble typical facies from Strombolian eruptions (emplaced ballistically, by fallout, and by rolling and local grain-avalanches down steep cone slopes), there are several lines of evidence that lead to an interpretation that the deposits were emplaced by pyroclastic density currents. These include local coarse-tail grading, deformation of spatter clasts in a down-flow direction, incorporation of matrix ash and lapilli into flattened spatter clasts, imbrication of large clasts, plastering of spatter on stoss sides of large lithic blocks and lenses of lithic-rich material on lee sides, deposition on angles less than the angle of repose, and a paucity of clast shapes associated with Strombolian mechanisms. The deposit characteristics are consistent with rapid sedimentation from a low-particle-concentration, turbulent flow onto an aggrading bed. We infer two potential mechanisms for generating these density currents: (1) explosive magma–water interaction involving lithic debris and relatively unfragmented melt; and (2) collapse of oversteepened upper cone slopes due to rapid accumulation of spatter from voluminous Strombolian eruptions.

Pre-emergent construction of a lacustrine basaltic volcano, Pahvant Butte, Utah (USA)

The subaqueous phases of an eruption initiated approximately 85 m beneath the surface of Pleistocene Lake Bonneville produced a broad mound of tephra. A variety of distinctive lithofacies allows reconstruction of the eruptive and depositional processes active prior to emergence of the volcano above lake level. At the base of the volcano and very near inferred vent sites are fines-poor, well-bedded, broadly scoured beds of sideromelane tephra having local very low-angle cross-stratification (M1 lithofacies). These beds grade upward into lithofacies M3, which shows progressively better developed dunes and cross-stratification upsection to its uppermost exposure approximately 10 m below syneruptive lake level. Both lithofacies were emplaced largely by traction from relatively dilute sediment gravity flows generated during eruption. Intercalated lithofacies are weakly bedded tuff and breccia (M2), and nearly structureless units with coarse basal layers above strongly erosional contacts (M4). The former combines products of deposition from direct fall and moderate concentration sediment gravity flows, and the latter from progressively aggrading high-concentration sediment gravity flows. Early in the eruption subaqueous tephra jetting from phreatomagmatic explosions discontinuously fed inhomogeneous, unsteady, dilute density currents which produced the M1 lithofacies near the vent. Dunes and crossbeds which are better developed upward in M3 resulted from interaction between sediment gravity flows and surface waves triggered as the explosion-generated pressure waves and eruption jets impinged upon and occasionally breached the surface. Intermingling of (a) tephra emplaced after brief transport by tephra jets within a gaseous milieu and (b) laterally flowing tephra formed lithofacies M2 along vent margins during parts of the eruption in which episodes of continuous uprush produced localized water-exclusion zones above a vent. M4 comprises mass flow deposits formed by disruption and remobilization of mound tephra. Intermittent, explosive magma–water interactions occurred from the outset of the Pahvant eruption, with condensation, entrainment of water and lateral flow marking the transformation from eruptive to "sedimentary" processes leading to deposition of the mound lithofacies.

Hydrovolcanic processes forming basaltic tuff rings and cones on Cheju Island, Korea

Tuff rings and tuff cones are small volcanoes produced by explosive magma-water interactions and have been regarded as resulting from relatively dry and wet eruptions, respectively, which are related to low and high mixing ratios of water to magma. However, comparative work on four Pleistocene basaltic tuff rings and cones on Cheju Island, Korea, shows that there are dry and wet types in both tuff rings and tuff cones, and their variations are not satisfactorily explained by the prevailing model. Instead, it is inferred that the morphological variations are directly caused by depositional processes (pyroclastic surge–dominated in tuff rings and fallout-dominated in tuff cones), irrespective of water-magma mixing ratios. The depositional processes are interpreted to be in turn controlled by a number of fundamental controls, which include depositional settings, type, level, and lithology of aquifers, strength of country rocks, ground-water behavior, and properties and behavior of magma. These controls determine the explosion depth, conduit geometry, mode of magma-water interaction, magnitude of explosion, eruption-column behavior, and subsequent depositional processes. The Suwolbong and Songaksan tuff rings, which formed almost entirely on land above fragile and permeable sediments and granites with some aquiclude beds, were produced by contact-surface steam explosivity at depth because of the fragility of country rocks, insufficient and inhibited supply of shallow-level external water into the vents, and interaction of nonvesiculated magma with interstitial water. These conditions led to generation of buoyancy-dominated eruption columns and pyroclastic surges, resulting in tuff rings. On the other hand, the Ilchulbong and Udo tuff cones formed in shallow seas above extremely permeable but rigid basalt lavas. The explosions occurred at shallow depths mainly by bulk-interaction steam explosivity because of the rigidity of country rocks, sustained supply of shallow-level external water into the vents, and interaction of vesiculated magma with free water. This process resulted in the generation of dense, inertia-dominated jets and the formation of tuff cones mainly by fallout processes. It is thought that the morphological and sedimentological variations of these volcanoes are more successfully explained by the fundamental controls rather than solely by the water-magma ratio. It is suggested that the water-magma ratio can explain the evolution of a single volcano or a group of volcanoes under otherwise identical conditions, but cannot explain the variability of tuff rings and cones in different hydrogeologic settings because the nature of hydroeruptions is governed by a number of fundamental controls.

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.