Michael F. Sheridan

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.

The petrogenesis of topaz rhyolites from the western United States

AbstractHigh-silica topaz-bearing rhyolites of Cenozoic age are widely distributed across the western USA and Mexico. They are characteristically enriched in fluorine (>0.2 wt.%) and incompatible lithophile elements (e.g. Li, Rb, Cs, U, Th, Be). In addition to topaz, the rhyolites contain garnet, bixbyite, pseudobrookite, hematite and fluorite in cavities or in their devitrified groundmasses. Magmatic phases include sanidine, quartz, oligoclase and Fe-rich biotite. Allanite, fluorite, zircon, apatite and magnetite occur in most; pyroxene, hornblende, ilmenite and titanite occur in some. The rhyolites crystallized over a wide temperature interval (850° to 600° C) at

Compaction of the Bishop Tuff, California

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Fuarmolic Mounds and Ridges of the Bishop Tuff, California

that ranges from QFM to NNO. The REE patterns of most topaz rhyolites are almost flat (La/YbN=1 to 3) and have deep Eu anomalies (Eu/Eu*=0.01 to 0.02). Both parameters decrease with differentiation. Titanite-bearing rhyolites have prominent middle REE depletions. Topaz rhyolites appear to have evolved from partial melts of a residual granulitic source in the Precambrian lower crust. According to the proposed model, the passage of hot mafic magmas through the crust produced partial melts as a result of the decomposition of F-rich biotite or amphibole. An extensional tectonic setting allowed these small batches of magma to rise without substantial mixing with contemporaneous mafic magmas.Some of the compositional differences between topaz rhyolites and peralkaline rhyolites may be attributed to the accumulation of fluorine and fluorphile elements (Al, Be, Li, Rb, U, Th, HREE) in melts which give rise to topaz rhyolites and chlorine and chlorophile elements (Ti, Fe, Mn, Zn, Zr, Nb and LREE) in melts which yield peralkaline rhyolites. Hence the F/Cl ratio of the melt or its source may determine the alumina saturation of the magma series. Topaz rhyolites are distinguishable from calc-alkaline rhyolites by lower Sr, Ba, Eu and higher F, Rb, U and Th. The usually low La/Yb ratios of topaz rhyolites distinguish them from both peralkaline and calc-alkaline rhyolite suites.

Discrimination of grain-size subpopulations in pyroclastic deposits

Abstract In decreasing order of abundance, the principal emplacement mechanisms for deposits of the Fossa edifice of Vulcano include: dry-surge, wet-surge, pyroclastic-fall, lahar, lava-flow, pyroclastic-flow and epiclastic processes. Most samples from pyroclastic beds on Fossa contain two grain-size populations. The coarser mode at 0.5 - 1.5 φ resulted from modified ballistic transport whereas the finer mode at 2.5 – 3.5 φ is related to surge transport. Reconstruction of the entire stratigraphy of the cone permits the beds to be grouped into five principal eruptive cycles, all of which postdate the lower Pilato ash of Lipari (dated at 11,000 to 8,500 y.b.p): Pte. Nere cycle, Palizzi cycle, Commenda, cycle, Pietre Cotte cycle, and the modern cycle. Additional cycles may be present, but exposures on the cone are insufficient for their adequate definition. All cycles follow a similar stochastic pattern starting with surge eruptions and ending with effusion of lava from the crater rim. The frequency of pyroclastic-fall events increases with time throughout each cycle. This progressive decrease in the efficiency of water/melt interaction through the duration of each cycle may be due to a gradual rise of each new magma column coupled with an increase in magmatic volatiles. Tectonism and magma mixing both play a role in triggering eruptions on Fossa. The present crater lies at the intersection of two prominent tectonic trends. A NNW fracture system parallels the main eruptive centers and an ENE set aligns with the migration of minor vents. The silicic lavas of Fossa contain a mixture of two components. Xenocrysts of plagioclase, augite, and olivine, surrounded by a dark glass are intimately dispersed into an evolved rhyolitic matrix.

Sugarloaf Mountain Tephra — A Pleistocene Rhyolitic Deposit of Base-Surge Origin in Northern Arizona

The foliation in welded tuffs is defined by planar alignment of glass shards, platy crystals, and flattened pumice fragments. The textural features and interrelationships of these elements are clearly the result of the deformation of ash-flow material. The strain involved in developing the alignment can be determined by measuring deformed objects of known original shape. The orientation and shape of the two-dimensional finite strain ellipse, computed from measured bubbles and Y-shaped shards, demonstrates that the long axis parallels the foliation and that there is a close correspondence between the ratio of the principal strains and the bulk density. The measured strain is inhomogeneous on at least two scales. Single cooling units show a regular and continuous vertical variation in deformation: the upper and lower portions are nearly undeformed, whereas the middle portion of the sheet is strongly deformed. On a small scale, the strain varies systematically around rigid lithic fragments and crystals and reaches high ratios at the tops and bottoms of these objects, while pressure shadow zones develop at the sides, which may have complex strain histories. All the lines of evidence point to compaction as the only mechanism involved in the production of the observed characteristic features of the Bishop Tuff, a Pleistocene ash-flow sheet in eastern California. Deformation in the tuff is defined by (1 + e 1 ) = 1.0. During flow and the earliest stages of compaction, pumice lapilli behave as rigid bodies. At about 50 percent porosity, the pumice collapses with the matrix, but at a more rapid rate, and the final forms are flattened in the plane of the foliation. Final shape ratios may reach 25 by simple volume loss; further flattening by essentially volume constant deformation may account for the relatively high mean ratios in fully compacted tuff, and for large ratios reported by others. A comparison of these results with selected specimens of other tuff units and published data strongly suggests that compaction is the dominant mechanism in producing the strong parallel alignment of textural components in all these welded tuffs. In cases where late or postcompactional deformation has also taken place, its effects are superimposed on the earlier compactional features; even so, the results appear to be more closely related to processes in the compactional stage. Extensive flow at or near final tuff densities does not explain most of these features.

Volcanic hazards at Fossa of Vulcano: data from the last 6,000 years

Abstract A previously developed computer-assisted model has been applied to several pyroclastic-surge eruptions at three active volcanoes in Italy. Model hazard maps created for various vent locations, eruption types, and mass production rates reasonably reproduced pyroclastic-surge deposits from several recent eruptions on Vulcano, Lipari, and Vesuvius. Small-scale phreatic eruptions on the island of Vulcano (e.g. the 1727 explosion of Forgia Vecchia) pose a limited but serious threat to the village of Porto. The most dangerous zone affected by this type of eruption follows a NNW fissure system between Fossa and Vulcanello. Moderate-sized eruptions on Vulcano, such as those associated with the present Fossa Crater are a much more serious threat to Porto as well as the entire area within the caldera surrounding the cone. The less frequent surge eruptions on Lipari have been even more violent. The extreme mobility of surges like those produced from Monte Guardia (approx. 20,000 y.b.p.) and Monte Pilato would not only threaten the entire island of Lipari, but also the northern part of neighboring Vulcano. Eruptions at Vesuvius with energy and efficiency similar to that of the May 18, 1980 blast of Mount St. Helens would be still more destructive because of the great initial elevation of the summit vent. In addition, surge eruptions at Vesuvius are generally part of more complex eruption cycles that involve several other types of volcanic phenomena including Plinian fall and pyroclastic flows.

Age and petrology of the Late-Pleistocene brown tuffs on Lipari, Italy

Fumarolic mounds and ridges are prominent surface features of the southern part of the Bishop Tuff and are characterized by their greater induration relative to the surrounding ash or tuff. They stand 0.5 to 15 m above the surrounding terrain and are of two main types: (1) domical mounds up to 60 m in diameter, and (2) straight or curved vertical joint ridges 1.5 to 5 m high and up to 600 m long. Fumaroles were formed in greatest numbers where crystallization within the sheet was intense and are absent from areas where the sheet was thick and densely welded but remained vitric. Early conjugate joint sets, which followed welding but predated escape of fumarolic vapors, controlled the distribution of fumarolic mounds and ridges. Later random orthogonal joints, which postdate vapor-phase activity, resulted from release of thermal stress stored in the cooling sheet. Welding, conjugate fumarolic jointing, fumarolic activity, and random orthogonal jointing represent successive discrete stages in the cooling history of the Bishop Tuff. The general mineralogy of the fumarolic areas and the vapor-phase zone are similar, but hydro-biotite and marialite are identified in the inner alteration zone. Likewise, the chemical composition of the fumarolic mounds is similar to that of the vapor-phase zone. The inner zone around some fumarolic vents shows significant changes through a decrease in SiO 2 and an increase in A1 2 O 3 , K 2 O, and H 2 O.