Obsidian Lava Research Articles

A LA-ICP-MS study of minerals in the Rocche Rosse magmatic enclaves: Evidence of a mafic input triggering the latest silicic eruption of Lipari Island (Aeolian Arc, Italy)

The volcanic products of Lipari Island (Aeolian Arc, Italy) younger than 10 ka are mostly aphyric rhyolitic pumices and obsidians emitted during unusual strombolian-type eruptions, which ended with the emplacement of lava flows. The last volcanic activity on the island dates back to 1230 ± 40 AD, with the extrusion of Rocche Rosse (RR) obsidian lava flow. Recently, mafic enclaves of latitic to trachytic composition have been identified and an evolution process between these enclaves and the rhyolitic magma has been documented in detail [Davì, M., 2007. The Rocche Rosse rhyolitic lava flow (Lipari, Aeolian Islands): magmatological and volcanological aspects. Plinius, supplement to the European Journal of Mineralogy 33, 1–8]. In this work textural and trace-element investigation of mineral phases of the RR enclaves, such as crystals of clinopyroxene, olivine, plagioclase, alkali-feldspar and biotite, was carried out to delineate the most recent feeding system of the island, since such a reconstruction could be significant in terms of hazard forecasting. The results indicate that most of the mineral phases are reversely or oscillatory zoned with respect to both major and trace elements, suggesting an early crystallization under low fO 2 conditions from melts of intermediate composition, followed by a later growth from a more mafic (presumably shoshonitic–basaltic) magma than that from which their cores crystallized. Crystals of magnesium-rich pyroxene and forsteritic-rich olivine are indicative of the presence of this shoshonitic basaltic magma. Based on microanalytical data, it is suggested here that the feeding system of recent Lipari volcanic activity was characterized by a shoshonitic–basaltic magma originating from a deep reservoir, which may have evolved and stopped in the crust, generating zoned magma chambers at different depths, in which latitic and rhyolitic magmas reside. The sudden arrival of a new input of mafic melt may have interacted with these resident fractionated magmas and triggered the eruption. A plumbing system of this type has been shown to be active in the southern sector of the Aeolian Islands since the last 42 ka [Gioncada, A., Mazzuoli, R., Milton, A., 2005. Magma mixing at Lipari (Aeolian Islands, Italy): Insights from textural and compositional features of phenocrysts. J. Volcanol. Geotherm. Res. 145, 97–118; Peccerillo, A., Frezzotti, M.L., De Astis, G., Ventura, G., 2006. Modeling the magma plumbing system of Vulcano (Aeolian Islands, Italy) by integrated fluid-inclusion geobarometry, petrology and geophysics. Geology 34, 17–20]. It is concluded that the hazard assessment of Lipari Island should take into account the arrival of deep and never erupted mafic melts as eruption triggers of more evolved shallower magma bodies.

An explosive–intrusive subglacial rhyolite eruption at Dalakvísl, Torfajökull, Iceland

This paper describes unusual rhyolitic deposits at Dalakvisl, Torfajokull, Iceland that were emplaced during a Quaternary subglacial eruption. Despite its small volume (<0.2 km3), the eruption mechanisms were highly variable and involved both explosive and intrusive phases. The explosive phase involved vesiculation-driven magma fragmentation at the glacier base and generated a pumiceous pyroclastic deposit containing deformed sheets of dense obsidian. Textures suggest that the obsidian was generated by the collapse of partly fragmented foam that was intruding the deposit and water contents indicate quenching at elevated pressures. In contrast, the intrusive phase of the eruption generated vesicle-poor quench hyaloclastites associated with a variety of peperitic lava bodies. The presence of juvenile-rich fluvio-lacustrine sediments is the first documented evidence that meltwater may pond close to the vent during subglacial rhyolite eruptions if the bedrock topography is favourable. In order to explain the variable eruption mechanisms, a conceptual model is presented in which the transition from an explosive to an intrusive eruption was controlled by the space available for fragmentation within the subglacial cavity melted above the vent. When the cavity became completely filled by volcanic deposits, the vent became blocked and rising magma was forced to intrude through poorly consolidated debris. This led to arrested fragmentation and welding of foam domains to form vesicle-poor obsidian lava; the transition to an intrusive eruption has taken place. Although this vent-blocking mechanism is particularly relevant to subglacial eruptions, it may also apply to subaerial rhyolitic eruptions, where patterns of explosive and effusive activity cannot be explained by shallow degassing processes alone. Meanwhile, the variable style of a small-volume subglacial rhyolite eruption further highlights the complex processes that mediate volcano-ice interactions.

Cooling rate correction of paleointensity determination for volcanic glasses by relaxation geospeedometry

The influence of the cooling rate on paleointensity estimates was investigated for samples from a vertical profile across a 600 A.D. obsidian lava flow ramp from Lipari, Italy. The natural cooling rates at the glass transition, which were previously determined for the seven investigated samples by relaxation geospeedometry, vary by a factor of more than 4. Rock magnetic investigations indicate a magnetic microlite fraction in the single-domain grain size range and strong magnetic anisotropy. The thermoremanence anisotropy tensor was determined for each specimen to correct the paleointensity results for this anisotropy. The cooling rate dependency of the thermoremanence was determined experimentally. Extrapolation to natural cooling rates indicate an overestimate of the paleointensity by 13% to 20% during experiments with typical laboratory cooling rates. Correcting for the different cooling rates and the cooling rate dependencies within the vertical profile, significantly reduces the standard deviation of the average flow paleointensity. The average paleointensity for the 543 ± 19 A.D. flow ramp results in 52.4 ± 1.1 μT, corresponding to a virtual axial dipole moment of 9.2 ± 0.2 Am2. Uncertainties, introduced by anisotropy correction and cooling rate extrapolation, are considered by error propagation.

Dynamics of obsidian flows inferred from microstructures: insights from microlite preferred orientations

The flow of obsidian lava leads to crystal alignments that reflect both the accumulated strain and the type of flow across the surface. Microlite preferred orientations are used to investigate the emplacement dynamics, strain history, and structural evolution of Obsidian Dome, eastern California. Measurements of three-dimensional microlite trend and plunge in samples from the near-vent region, distal flow, and flow front show: (1) the flow directions along the dome margins, (2) the deformation style (e.g., pure versus simple shear) at the dome margins, and (3) the variation in strain as a function of position within the flow. Microlites form well-developed lineations in the plane of flow banding in all samples. Stereographic projections indicate that lineations trend normal to the western flow front and plunge shallowly away from the margin. The radial flow pattern indicated by measurements made along the western margin suggests that extrusion was from a roughly elliptical vent. These results highlight a strong correlation between microlite trend and the bulk flow direction inferred from the geometry of the flow. Along most of the eastern periphery, lineations trend parallel to the margin and likely reflect the local flow direction as influenced by compression against the thickening flow crust, marginal talus piles, and topography. Orientation distributions imply that radial spreading accompanied by flattening was the dominant mechanism for flow emplacement. Comparisons of measured orientation distributions with theoretical predictions suggest that microlite fabrics in flow front and near-vent samples developed in a pure shear flow. Microstructures in a sample from near the distal flow base records a component of simple shear. Variance in microlite trend provides a measure of the amount of strain acquired during flow. Standard deviation in trend decreases from the near-vent region to the flow margins, reflecting progressive alignment of microlites during transport. Pure shear strain inferred from orientation distributions increases from approximately 0.3 near the vent to about 1.1 at the flow front. The difference between these strains (0.8) is an estimate of the strain associated with flow emplacement. Such strain is similar in size to that estimated from mesoscopic structures (∼1) and for horizontal spreading of a fluid whose volume is equal to that of Obsidian Dome (∼1.6). This suggests that flow on the surface was sufficient to produce observed microlite preferred orientations in the flow front. These techniques can be applied to interpret older dissected lavas where erosion has erased much of the original flow front or where larger-scale structures indicative of flow directions are poorly preserved.

Natural gamma-radiation in the Aeolian volcanic arc

Pulse-height distributions of γ-rays, obtained with a field NaI(Tl) scintillation spectrometer in numerous sites of the Lipari and Vulcano islands (Aeolian volcanic arc, Italy), were measured to determine the U, Th and K concentrations of the bedrock and the relative values of the air absorbed dose rate. U is spatially related to both Th and K and the Th/U ratio is on average 3.1–3.5. The magmatic evolution is reflected by the concentration of the three radioelements, as they are more abundant within the more felsic units of the volcanic series. The higher values of U (15.7–20.0ppm) coincide with higher Th (48.3–65.9ppm) and K (4.9–6.1%) concentrations associated with rhyolitic rocks of the third cycle (<50ky). The air absorbed dose rate varies from 20 to 470nGyh−1. The highest values (>350nGyh−1) are observed on outcrops of rhyolitic obsidian lava flows. The cosmic-ray contribution is also evaluated to estimate the total background radiation dose rate.

Constraints on rheology of obsidian lavas based on mesoscopic folds

The geometry of mesoscopic single and multiple layer folds in rhyolitic obsidian flows is investigated. Folds are composed of obsidian embedded in a matrix of pumice. Folds form by buckling processes as indicated by discontinuous deformation between obsidian and pumice layers and by the geometries of wavetrains. Buckling occurs through a continuum of styles controlled largely by the thickness ratio of pumice to obsidian ( N). Styles of folds include chevron, harmonic, polyharmonic, disharmonic and single-layer assemblages. Harmonic and chevron folds are observed for small values of N. For large values of N, folds buckle independently of one another and form disharmonic and single-layer assemblages. Wavelength-to-thickness ratios of single-layer folds are compared to theoretically predicted ratios for Newtonian and power law fluids as a means of estimating shear viscosity ratios of obsidian and pumice. While all folds indicate that bubble-free rhyolite is more viscous during flow than bubbly rhyolite, estimates of shear viscosity ratio based on Newtonian theory (∼10–500), may exceed estimates based on power law rheologies by more than an order of magnitude. Newtonian buckling theory involves a number of simplifications and does not account for the possibly complex rheology of bubble-bearing rhyolite.

Plinian eruptions at Glacier Peak and Newberry volcanoes, United States: Implications for volcanic hazards in the Cascade Range

Several fall deposits from Glacier Peak and Newberry volcanoes, both located in the Cascade Range, United States, have been studied to determine eruptive column heights, intensities, and volumes. The late Pleistocene eruptions of Glacier Peak ranged from small phreatic explosions to two Plinian eruptions that each erupted more than 1 km 3 of magma at intensities >10 8 kg/s, generating plumes with heights >30 km. At Newberry volcano, the last Plinian eruption (ca. 1300 14 C yr B.P.) had an intensity of 2.8 × 10 7 kg/s and a plume height of 18 to 21 km. About 0.1 km 3 of magma was erupted in the Plinian phase, followed by eruption of a pyroclastic flow and an obsidian lava flow. Combined with similar data from Mount St. Helens, Mount Rainier, and Mount Mazama (Crater Lake), these eruptions define the range of Plinian events that have occurred in the Cascade volcanic arc in the past 12 k.y. During this period there have been Plinian eruptions with plume heights between 11 and 55 km, intensities between 10 6 and 10 9 kg/s, and volumes between 0.01 and >5 km 3 of magma. All eruptions with intensities greater than or equal to 10 8 kg/s also produced large-volume pyroclastic flows and surges. Monitoring column height (intensity) during eruptions could help mitigate hazards because it may indicate pending generation of pyroclastic flows. In the Cascade Range, there have been at least 12 eruptions of >1 km 3 of tephra in the past 12 k.y., suggesting that eruptions of such magnitude occur about once every 1 k.y., although such frequencies vary greatly at each volcano. The volume of magma erupted in each event correlates with both column height (intensity) and magma composition, suggesting that eruptions of these volcanoes relate to the accumulation rate of magma in their reservoirs.

Fragmentation versus transportation mechanisms in the pyroclastic sequence of Monte Pilato-Rocche Rosse (Lipari, Italy)

Monte Pilato is a rhyolitic pumice cone formed about 1400 years ago during the last volcanic activity which occurred at Lipari. Its activity consisted of numerous explosive episodes, ending with the extrusion of the Rocche Rosse obsidian lava flow. We have identified nine different pyroclastic lithofacies, of which four main lithofacies represent 90% of the erupted products; they include: (1) deposits consisting of decimeters- to meters-thick well-sorted, graded beds of pumice lapilli, which are of fall origin; (2) massive deposits consisting of decimeters-thick beds of pumice lapilli in an ash matrix, interpreted as being of pyroclastic flow origin; (3) deposits of centimeters-thick ash beds of surge origin; (4) decimeter-thick fine ash layers with scattered subcentimetric accretionary lapilli of turbulent “flow” origin. The complex grain size distributions of the main lithofacies were distinguished using a multivariate statistic analysis ( R-mode factor analysis). Variations of the grain size characteristics in relation to the distance from the vent area have been investigated in order to discriminate the roles played by fragmentation and transportation processes. In the lithofacies 1 fall deposits, the grain size distribution is always unimodal and varies from a Rosin-type distribution near the vent to a Gaussian one in the distal outcrops. This is caused by the transport process and the different particle trajectories. In lithofacies 2, 3 and 4, the grain size distributions are always polymodal. The polymodality is not caused by preferential processes occurring during transportation, but seems, rather, to be connected with the original fragmentation of the pyroclastic material. The pumice lapilli present both in lithofacies 1 and 2 have mean vesicularity index values always in excess of 65%, and the value is homogeneous throughout the sample. These data suggest that for these two lithofacies, the pumice lapilli originated from exsolution of magmatic gases. In contrast, the fine ash particles in lithofacies 2, and also the main mode of lithofacies 3 and 4 deposits, show typical features of hydromagmatic fragmentation processes. The grain size population is always centered at 4–5 ϕ (0.032–0.064 mm) and, interestingly, the same grain size has been obtained recently (Zimanowski et al., 1991) for fragments produced by experimental magma/water interaction. Field and laboratory data suggest that during Monte Pilato-Rocche Rosse eruptions magmatic and hydromagmatic mechanisms were frequently operative at the same time.

The El Cajete Series, Valles Caldera, New Mexico

Three rhyolite eruptive units totaling 3 km3 dense rock equivalent volume are the youngest products from the Valles caldera, New Mexico. These pyroclastic and effusive units, herein called the El Cajete Series, were erupted over an appreciable time period with Plinian and ignimbrite‐forming activity preceding an effusive phase by a lengthy but indeterminable interval. New U‐Th disequilibrium and published fission track dates support an age for the event between 170 and 130 ka. The explosive phases formed the El Cajete Plinian deposit (about 1.3 km3), minor dry pyroclastic surges, and the Battleship Rock tuff (about 1.0 km3), a valley‐confined welded ignimbrite. The Plinian eruption column is estimated to have been 28 km high during deposition of the most widely dispersed pumice fall unit. Slow effusion of the Banco Bonito obsidian lava flow (&lt;1.0 km3) onto a dissected surface cut into the El Cajete and Battleship Rock pyroclastic deposits, accompanied by minor explosive activity, terminated the event. Twenty‐four bulk‐rock samples from all three eruptive stages exhibit little significant compositional variation, and the three units are petrographically identical apart from differences arising from contrasting eruption styles. Very few phenocrysts appear to have been in equilibrium with their enclosing high‐silica rhyolitic liquid. Plagioclase grains are typically strongly resorbed, while biotite and hornblende frequently occur in aggregate grains in which textures characteristic of plutonic rocks can be discerned. These features result from partial melting of preexisting crustal igneous rock, probably of dioritic to granodioritic composition, and subsequent eruption of the resulting liquid plus restite crystals. Rapid generation and eruption of rhyolitic magma during this most recent phase of activity in the Jemez Mountains may imply that the Valles magma system is presently in a state where small magma bodies are transient phenomena.

Volcanic history of Lipari (Aeolian Islands, Italy) during the last 10,000 years

Abstract Examination of the volcanic stratigraphy of deposits younger than 10,000 years on Lipari indicates four principal periods of volcanic activity related to specific centers. The products from each different volcanic center are defined as volcano-stratigraphic unit (VSU). From the oldest these are: the Canneto Dentro, Gabellotto-Fiume Bianco, Forgia Vecchia and Monte Pilato-Rocche Rosse VSUs. The study of textures and dispersal of the deposits permitted the vents to be localized and the recent volcanic history of Lipari to be reconstructed. The oldest event formed a small explosion breccia cone with a final obsidian lava in the Canneto Dentro area. Immediately afterward, a complex series of explosions produced the widespread dry-surge deposits of the Gabellotto-Fiume Bianco sequence. This activity ended with the extrusion of a domical lava flow. The renewal of activity occurred in the Pirrera area with an explosive eruption that produced explosion breccia deposits. The last eruptions from this vent were coeval with the first eruption of M. Pilato. The rim of the explosion breccia cone was partially destroyed by the Forgia Vecchia lava flow. M. Pilato cone grew in a very short period of time due to a continuous swarm of explosive events. After a short repose time, a series of more energetic and superficial explosions occurred through a vent slightly to the south. The extrusion of the Rocche Rosse lava flow (about 729 A.D.) ended this cycle of activity. All the volcanic centers follow a quite similar stochastic pattern starting with a fall or surge eruption and ending with effusion of viscous rhyolitic lavas. The four centers are aligned along either NW-SE or NE-SW fault systems according to the structural pattern of the island. They can be placed into two groups: the Canneto Dentro-Gabellotto centers and the Forgia Vecchia-M. Pilato centers. A long quiescence ( ⋍ 3,500 years ) separates the activity of these two groups while inside of each the activities were nearly contemporaneous or in rapid succession.

鹿児島県大口白土鉱山におけるハロイサイトの産状とその成因

The Ookuchi Clay Mine is situated at northwest part of Ookuchi-city, northwestern area of Kagoshima Prefecture, southern part of Kyushu, southern Japan. The mine has been worked for white clay, which is mainly composed of halloysite of spherical shape under the electron microscope. The purified clay, by means of water-washing methods, has been used for mixing clay material of coating clay for paper manufacturing industry. The geological formations consist mainly of andesitic lava flows and their pyroclastic materials, and the deposit is observed in an obsidian lava flow, which is intercalated in the andesitic formation (Fig. 2). The halloysitization in this mine is reactions of volcanic gases and waters to the glassy rock and it is also observed in andesite of upper horizon of the obsidian lava flow in small scale. Formation mechanisms of halloysite are investigated by means of chemical analyses, thermal analyses and X-ray powder methods, as shown in Tables 1 and 2 and Fig. 3.

Structural geologic constraints on the rheology of rhyolitic obsidian

Laboratory measurements of the rheology of volcanic glasses may be supplemented with estimates obtained from field measurements of structures on natural obsidian lava flows. These viscosity values reflect the textural heterogeneity of volcanic glasses, which are often overlooked in laboratory determinations. By mapping the distribution of different pumiceous and glassy textures, patterns may be observed that can be interpreted using a variety of deformational models: large scale surface folds indicate the ratio of surface to interior viscosities; the regular spacing of pumice diapirs reflects the relative viscosities of glassy and pumiceous zones; and the depth and form of fractures are controlled by thermal and elastic properties of the lava. Small scale structures such as folds and boudinage indicate that obsidian layers have a pseudoplastic, power law rheology, a finding confirmed by recent laboratory studies.

Paleomagnetism, Geochemistry, and Potassium-Argon Ages of the Rio Grande de Santiago Volcanics, Central Mexico

Seventy-two oriented cores of 2.5 cm diameter have been drilled from seven bodies comprising part of a sequence of ignimbrite sheets interdigitated with basalts and basaltic andesites, in the Grand Canyon of the Rio Grande de Santiago of the Mexican Cordilleran Volcanic Province. Field observations and whole-rock analyses reveal two groups: an olivine basalt-high alumina basalt-basaltic andesite series, and a sequence of rhyolitic ignimbrite sheets, with some rhyolitic and obsidian lava flows. Crystal separation, partial melting of oceanic tholeiite, and simple mixing of basalt and oceanic sediments or granodiorite cannot acceptably explain the observed fractionation trends. The favored origin is a partial melting of a second or third stage product, such as a basalt or amphibolite enriched in K, Rb, Th, and Pb, relative to oceanic tholeiite. The observed fractionation trends are parallel to those of the andesite-dacites of the Mexico City region, but the Rio Grande de Santiago rocks have a systematically higher K 2 O/SiO 2 ratio, which can be explained by a greater magma generation depth. The paleomagnetic polarity sequence with decreasing age is R,R,N,R,R,N,N, providing easily measurable markers of high potential stratigraphic value. The mean virtual geomagnetic pole is not significantly different from the present spin axis. The potassium-argon analyses show that the sampled section contains two distinct volcanic episodes, from approximately t = 4.6 to 5.5. m.y. and from approximately t = 8.7 to 9.5 m.y. The polarities recorded by the youngest volcanic cycle are consistent with the predicted polarity time scale of Heirtzler and others (1968), while the older cycle polarities can be explained by minor modification of the predicted polarity scale, to include a short period of reversed polarity between 9.0 and 9.5 m.y., within the normal polarity anomaly 5 (t = 8.79 to 9.94 m.y.) of the time scale.

On the Lithophyses in the Obsidian of the Rocche Rosse, Lipari

The Rocche Rosse lava-stream, partly filling the pumice crater of Monte Pelato, Lipari, and breaching it on the north-eastern side, was described by Prof. Judd in 1875. As recorded by him and by Cortese we find in this locality all varieties of obsidian lava, from compact black glass to the pumice of commerce, or even a honeycombed material passing into ‘thread-lace scoria’ A person who, arriving at the south-western or landward side of the crater, begins walking over the hummocky surface of the Rocche Rosse along the centre of that part which lies within the crater, cannot fail to be struck by the fact, emphasized by Cortese, that much of the lava here, as at the Forgia Vecchia, has a texture intermediate between that of pumice and compact glass. In fact, he might walk along the middle of perhaps the first third of the Rocche Rosse without finding anything that he would call obsidian. However, if he turn to the right and make his way towards the southern margin of the lava, he will find portions of it in which black glass predominates. From one of the numerous outstanding hummocks of this part of the Rocche Rosse were obtained the specimens of black glass with vesicles, lithophyses, and spherulites, which have led to the writing of this paper, and from which the sections figured in Plate XII. have been taken. It must not be supposed, however, that the rock here differed in any essential from the lithophyse-bearing obsidian found elsewhere