Vesicular Pumice Research Articles

Chrono-stratigraphy of the youngest (last 1500 years) rhyolitic eruptions of Lipari (Aeolian Islands, Southern Italy) and implications for distal tephra correlations

The youngest (last 1500 years) volcanic eruptions of Lipari, within the Aeolian Archipelago, produced the prominent pumice cone of Monte Pilato and the obsidian lava flows of Rocche Rosse and Forgia Vecchia, concentrated in the north-eastern sector of the island as well as highly dispersed white-coloured, fine-grained tephra layers of rhyolitic composition in terrestrial and marine settings on the regional scale. Here we describe in detail the stratigraphy of pyroclastic successions and lava flows erupted by different vents - Monte Pilato, Forgia Vecchia, Lami, and Rocche Rosse - combining field observations, sedimentological characteristics of the tephra deposits, and major and trace element compositions of the volcanic glass. All the pyroclastic materials consist of aphyric pumice lapilli and ash with a largely homogeneous rhyolitic composition. The Monte Pilato and Forgia Vecchia deposits primarily consist of highly vesicular pumice fragments and subordinate obsidian clasts, whilst Rocche Rosse and Lami are characterized by moderately vesicular juvenile fragments with a more significant fraction of obsidian. The Lami tephra also contains peculiar pumice clasts with a fibrous texture and breadcrust bombs.Stratigraphic relationships, and paleomagnetic and 14C ages of the lava and pyroclastic deposits are combined with the archaeological information and historical reports, enabling us to provide an accurate chrono-stratigraphic framework for the youngest eruptions of Lipari. Following the 8th century CE eruption of Monte Pilato, which produced a pumice cone and a obsidian lava flow, activity resumed in the second half of 13th century CE with the explosive eruption of Forgia Vecchia that culminated in the emission of a bilobate obsidian lava flow. This eruption was shortly followed by the explosive eruptions of Lami and Rocche Rosse, the latter concluded with the emission of the widely renowned obsidian lava flow.By integrating stratigraphy and geochemistry of tephra deposits with a new chronological scheme, our work facilitates the refinement of proximal-to-distal correlation of Lipari's rhyolitic tephra in continental marine environments of the central Mediterranean area in the last 1500 years. A fine-grained, rhyolitic ash found on Stromboli (~40 km NE from Lipari) has an origin from the Monte Pilato and thus, constrains tephra dispersion towards the NE. Very similar ash beds dispersed southwards and interlayered within the near-source deposits of La Fossa, Vulcano island (~10 km from Lipari) exhibit features that are consistent with the younger activities of the Rocche Rosse eruption. A possible link between previously identified rhyolitic ash layers identified in marine cores of the Ionian Sea and the Forgia Vecchia eruption are postulated, although the age and textural characteristics of these distal tephra are not univocal in indicating a correlation to either Monte Pilato or Forgia Vecchia.

The role of oxides in the shallow vesiculation of ascending magmas

Despite their generally low volume fraction, FeTi oxides have the potential to greatly influence the eruptive style because they lower the supersaturation pressure for heterogeneous bubble nucleation. Once nucleated, bubbles respond fast to pressure changes, fostering rapid expansion and explosive behavior. Yet, oxide microlite quantifications are often absent from data of explosive products. We used new, re-analyzed, and published data to build a compilation of oxide number densities (ONDs) and vesicle number densities (VNDs) of trachytic and calc-alkaline products. Four eruptive styles were selected: 1) Vulcanian explosions from Soufrière Hills volcano, Montserrat, Lascar volcano, Chile, and Kilian volcano, France, 2) blasts from Mt. Pelée volcano, Lesser Antilles, Mount St. Helens, USA, and Merapi volcano, Indonesia, 3) a sub-Plinian explosion from Merapi volcano, and 4) lava dome effusions with intermittent collapse from Soufrière Hills and Mt. Pelée volcanoes. Natural samples were separated into two groups according to the dominant texture of the products of each event: 1) vesicular pumice clasts from explosions with a strong vertical component and 2) dense clasts with diktytaxitic textures from dome collapse event and lateral blast. Group 1 samples are either distributed alongside the 1:1 trend between VND and OND that spans from 1015 to 1017 m−3, or have a constant VND of 1016–16.5 m−3 regardless of OND. A large proportion of oxides (55–100%) touch vesicles. A more variable proportion of vesicles (16–72%) are in contact with oxides because of syn-explosive growth and coalescence. Group 2 samples have ONDs in the same broad range as group 1 samples. We also used new and published data to build a compilation of ONDs and VNDs of five series of experimental decompression of rhyolitic and phonolitic melts. In samples with OND ≈ VND, most bubbles are in contact with more than one oxide and 64–88% of the oxides are in contact with bubbles. Such high levels of connectivity suggest that the role of oxides in controlling bubble nucleation has been underestimated. When VND ≥ OND, nucleation densities of experimental vesicles can be reproduced by heterogeneous nucleation models, which we used to calculate syn-explosive decompression rates from VNDs at Merapi, Soufrière Hills, and Kilian. These rates and textural evidence suggest that the decompression front accompanying these Vulcanian and sub-Plinian explosions is responsible for syn-explosive bubble nucleation. We calculated the average pre-explosive ascent rates necessary to yield the observed ONDs at Soufrière Hills and Merapi volcanoes. The resulting rates, 0.005–20 m/s, overlap considerably with the range of critical ascent rate inferred for the effusive–explosive transition, supporting the hypothesis that this transition is primarily controlled by oxide microlites in the conduit because oxides are a primer for explosive behavior when present in sufficiently high number densities. Focusing on the older eruption of Kilian, for which no observation is available, we infer that pre-explosive ascent rates of >7 × 10−3 m/s were necessary for explosive behavior to occur.

Formation of tube-pumice structure under pure shear: Insights from extension tests of solidifying foam

Tube pumice, the bubbles of which are highly elongated in one direction, is a common product of explosive silicic eruptions, especially caldera-forming eruptions. To understand the conditions necessary to generate tube-like bubbles, we performed extensional experiments on polyurethane foam, which experiences vesiculation, deformation, and solidification at room temperature. The strain and strain rate of pure shear deformation were controlled by the pull length and pull rate of the extension, respectively. The shapes of bubbles in the solidified polyurethane foam were analyzed in three dimensions by X-ray computed tomography. The experimental results demonstrate that strain and capillary number control the degree of bubble deformation. Highly deformed (tube-like) bubbles were found in the experiments with large strain and large capillary number. In each experimental sample, there is scatter in the relation between the degrees of bubble deformation and expected capillary numbers, which we interpret to be the result of the inhomogeneous shear field around individual bubbles and bubble coalescence. Despite the large scatter, the average deformation degree is predicted well by the dynamics of a single bubble, where the viscous stress acting on the bubble surface is taken as the product of bulk strain rate and effective viscosity. Using this approach, the average deformation degree of experimental bubbles in each sample was approximately calculated from the deformation model of a single bubble at least in the range of pure shear strain smaller than 1.5. Small bubbles in the samples extended at low viscosities tend to be less elongated than those calculated from the deformation model, due to shape relaxation. The effect of shape relaxation is quantified by a new dimensionless number, i.e., the quench number, which represents the ratio of the timescale of increasing viscosity and shape relaxation. Our experiment suggests that bubble shapes in highly vesicular pumice can be used to analyze their viscosity and strain rate during the elongation process. An analysis of the published data was presented as a test case.

Stratigraphic and sedimentologic framework for tephras in the Wilson Creek Formation, Mono Basin, California, USA

Numerous tephra layers occur within the late Pleistocene Wilson Creek Formation, where they are interbedded with lacustrine deposits of Lake Russell, the ancestor of present-day Mono Lake. Most of the tephra layers are rhyolitic in composition, and were produced from the Mono Craters. We present detailed stratigraphy and sedimentology of the tephra layers, sampled at twelve outcrops near the shoreline of Mono Lake and the Mono Craters, and implement grain size, componentry, and surface morphology analysis to characterize their physical properties. Sub-unit correlation is proposed for certain tephra units. Noticeable features of the tephras, such as the occurrence of low-density rounded or highly vesicular pumice within certain sub-units, are highlighted. The abundant obsidian, lithics, and ostracods within many sub-units suggest that the associated eruption pulses involved water-magma interaction.Eruptions from the Mono Craters during the late Pleistocene were consistent neither in frequency nor volume. The Mono Craters were most active during the eruption of tephras in Sequences C (∼42.5–39.4 ka) and A (∼14.1–12.9 ka), and Pleistocene volcanic activity reached its peak during the eruption of tephra layer C11. Detailed interpretation of tephra layer B7 and tephras in Sequence A is given, which includes the number of eruption pulses, vent location, dispersal direction, occurrence of pyroclastic flow and surge, and other features and processes. Tephras in Sequence B (∼26.2–23.1 ka) may have had their vents located in the southern half of the Mono Craters, or are smaller in volume (except for B7), compared to the other tephras in the Wilson Creek Formation. Vents for tephra layers A4, A3, and A1 are located near the northern end of the Mono Craters. The stratigraphy of tephra layers B7 and A1 suggests an unstable depositional environment, which can be used to help constrain the water-level history of Lake Russell during the late Pleistocene.

Extraordinary sediment delivery and rapid geomorphic response following the 2008–2009 eruption of Chaitén Volcano, Chile

AbstractThe 10 day explosive phase of the 2008–2009 eruption of Chaitén volcano, Chile, draped adjacent watersheds with a few cm to >1 m of tephra. Subsequent lava‐dome collapses generated pyroclastic flows that delivered additional sediment. During the waning phase of explosive activity, modest rainfall triggered an extraordinary sediment flush which swiftly aggraded multiple channels by many meters. Ten kilometer from the volcano, Chaitén River channel aggraded 7 m and the river avulsed through a coastal town. That aggradation and delta growth below the abandoned and avulsed channels allow estimates of postdisturbance traction‐load transport rate. On the basis of preeruption bathymetry and remotely sensed measurements of delta‐surface growth, we derived a time series of delta volume. The initial flush from 11 to 14 May 2008 deposited 0.5–1.5 × 106 m3 of sediment at the mouth of Chaitén River. By 26 May, after channel avulsion, a second delta amassed about 2 × 106 m3 of sediment; by late 2011 it amassed about 11 × 106 m3. Accumulated sediment consists of low‐density vesicular pumice and lithic rhyolite sand. Rates of channel aggradation and delta growth, channel width, and an assumed deposit bulk density of 1100–1500 kg m−3 indicate mean traction‐load transport rate just before and shortly after avulsion (∼14–15 May) was very high, possibly as great as several tens of kg s−1 m−1. From October 2008 to December 2011, mean traction‐load transport rate declined from about 7 to 0.4 kg−1 m−1. Despite extraordinary sediment delivery, disturbed channels recovered rapidly (a few years).

Origin of a zone of anomalously high porosity in the subduction inputs to Nankai Trough

Abstract One poorly understood feature of the subduction inputs to the Nankai Trough subduction zone (SW Japan) is a stratigraphic interval with an anomalously high porosity zone (HPZ), which is up to 240 m thick and located within the clay- and volcanic ash-rich Shikoku Basin facies. To investigate the origin of their peculiar physical properties, we integrated logging-while-drilling (LWD) data, shipboard density measurements, and visual descriptions of core samples recovered from four drill sites of the Ocean Drilling Program and Integrated Ocean Drilling Program. We combined those observations with scanning electron microscopy (SEM) and laboratory consolidation tests on both HPZ samples and artificial mixtures of ash (glass shards) + smectite and vesicular pumice + smectite. LWD data indicate that the HPZ mudstones have a large proportion of dispersed volcanic ash (~ 20–30%). The consolidation tests show that the rate of porosity loss with increasing effective stress (consolidation behavior) is consistent among HPZ specimens and matches artificial mixtures containing up to 60% volcanic material. However, absolute values of porosity remain higher for HPZ samples compared to artificial mixtures, so processes in addition to the mechanical effects of volcanic clasts must be contributing. We suggest that hydration and partial dissolution turns clusters of volcanic glass into aggregates with distinct microfabric. SEM images confirm the presence of strengthened grain-to-grain contacts, which probably inhibit the collapse of the intra-aggregate pore space. The aggregates behave like grains, so that cohesive strength of the bulk sediment and rate of porosity loss remain nearly unchanged during burial. The two-step diagenetic process of dissolution and precipitation depends critically on a threshold abundance of fine-grained dispersed volcanic ash/pumice. Older units in the Shikoku Basin with only traces of dispersed ash show no such effects. HPZs should be expected in other subduction zones with similar compositional and diagenetic prerequisites.

On the anatomy of magma chamber and caldera collapse: The example of trachy-phonolitic explosive eruptions of the Roman Province (central Italy)

Textural and compositional features of pyroclastic products erupted during caldera-forming events often reveal the tapping of different portions of variably zoned magma chambers due to changing geometries of the conduit/vent systems. Here we report on ultrapotassic trachytic–phonolitic explosive eruptions of the Roman Province (central Italy), which show remarkable changes of textural features and glass compositions in the juvenile material, even if the bulk chemical composition is essentially constant. In each example, the lower eruption sequence contains whitish, crystal-poor (leucite-free), highly vesicular pumice, emplaced by early Plinian fallout and/or pyroclastic currents; upsection, the eruption sequence contains black, low porphyritic (sanidine+leucite-bearing), moderately vesicular, scoria or spatter, emplaced by major pyroclastic flows (red tuff with black scoria) and associated co-ignimbrite, coarse lithic-rich breccias. This suggests a shift from a central feeder conduit, tapping the central part of the magma chamber, to a ring fracture vent system, tapping the peripheral portions of the magma chamber, during caldera collapse. Key features of these evacuating magma chambers are the thermal and volatile concentration (Xvol) gradients that produce the observed textural and compositional spectrum of trachy-phonolitic rock types. In particular, the degrees of freedom during the crystallization of these ultrapotassic magmas are increased by the variation of the leucite stability field at different PH2O conditions. Both leucite-free and leucite-bearing differentiated ultrapotassic rock types can be produced in the course of individual eruptions, as a result of pre-eruptive conditions in the feeder magma, with no need to invoke different differentiation suites related to mantle source heterogeneities of parental magmas.

Highly vesicular pumice generated by buoyant detachment of magma in subaqueous volcanism

Many submarine caldera volcanoes are blanketed with deposits of highly vesicular pumice, typically attributed to vigorous explosive activity1, 2, 3, 4. However, it is challenging to relate volcanic products to specific eruptive styles in submarine volcanism5, 6. Here we document vesicularity and textural characteristics of pumice clasts dredged from the submarine Macauley volcano in the Kermadec arc, southwest Pacific Ocean. We find that clasts show a bimodal distribution, with corresponding differences in vesicle abundances and shapes. Specifically, we find a sharp mode at 91% vesicularity and a broad mode at 65–80%. Subordinate clasts show gradients in vesicularity. We attribute the bimodality to a previously undocumented eruptive style that is neither effusive nor explosive. The eruption rate is insufficient to cause magma to fragment explosively, yet too high to passively feed a lava dome. Instead, the magma foam buoyantly detaches at the vent and rises as discrete magma parcels, or blebs, while continuing to vesiculate internally. The blebs are widely distributed by ocean currents before they disintegrate or become waterlogged. This disintegration creates individual clasts from interior and rim fragments, yielding the bimodal vesicularity characteristics. We conclude that the generation and widespread dispersal of highly vesicular pumice in the marine environment does not require highly explosive activity.

Grain-size distribution of volcaniclastic rocks 1: A new technique based on functional stereology

The power of explosive volcanic eruptions is reflected in the grain size distribution and dispersal of their pyroclastic deposits. Grain size also forms part of lithofacies characteristics that are necessary to determine transport and depositional mechanisms responsible for producing pyroclastic deposits. However, the common process of welding and rock lithification prevents quantification of grain size by traditional sieving methods for deposits in the rock record. Here we show that functional stereology can be used to obtain actual 3D volume fractions of clast populations from 2D cross-sectional images. Tests made on artificially consolidated rocks demonstrate successful correlations with traditional sieving method. We show that the true grain size distribution is finer grained than its representation on a random 2D section. Our method allows the original size of vesicular pumice clasts to be estimated from their compacted shapes. We anticipate that the original grain-size distribution of welded ignimbrites can also be characterized by this method. Our method using functional stereology can be universally applied to any type of consolidated, weakly to non-deformed clastic material, regardless of grain size or age and therefore has a wide application in geology.

Dynamics, stratigraphy and proximal dispersal of supraglacial tephra during the ice-confined 2004 eruption at Grímsvötn Volcano, Iceland

The basaltic, phreatomagmatic eruption of Grimsvotn volcano, Iceland, in November 2004 (G2004) lasted for 5 days, during which time two separate vents were active. Significant deposition of tephra occurred in the first 45 h only. We have subdivided the deposit into seven units (A–G) on the basis of differences in texture, grain size and componentry, and the presence of sharp contacts between the layers. The distribution of tephra lobes was used to infer the vent of origin for each unit. The G2004 deposit is poorly sorted overall and consists of non-vesicular to highly vesicular juvenile components. Units A and B comprise almost exclusively non- to poorly vesicular glass fragments, whereas units C–G contain at least 30 vol.% highly vesicular pumice. The proportion of non-juvenile fragments increases significantly in the final unit (unit F) of the main phase; non-juvenile fragments are restricted to the coarse (>0 Φ) fraction of the deposit. Main phase units C and E account for 80% of the total deposit volume, including the entire distal portion, and are interpreted to represent a mixture of (1) a widely dispersed component that fell from the upper margins of a strongly inclined (∼45°), 6–10 km high plume and (2) a locally dispersed (<3 km from source) component originating from pyroclastic density currents and minor tephra jets.

Reconstruction of the Sibinal Pumice, an andesitic Plinian eruption at Tacaná Volcanic Complex, Mexico–Guatemala

The Tacaná Volcanic Complex, located on the Mexico–Guatemala border, has had numerous small historical explosions, the latest being a phreatic explosion in 1986. The stratigraphic record, however, suggests that much more voluminous eruptions have occurred in the past. Here, we document one such eruption, the Sibinal Pumice deposit, which occurred ca. 23,540years B.P. The deposit consists of two pumice-rich units: 1) a lower stratified member (SM) that consists of at least seven, normally graded fall layers, interbedded with pyroclastic wet surge layers; and 2) an upper massive member (MM), made up of a single fall deposit. Both members can reach up to 2.5m in thickness and are separated by a single massive, indurated, yellowish, pumice-rich reworked layer. SM was dispersed to the northeast (N70°E), with its 6-cm isopach covering an area of 275km2, whereas MM was dispersed to the north (N22°E), with its 70-cm isopach covering an area of ca. 330km2. Yellow, vesicular pumice fragments, light-gray lithics, and hydrothermally altered (red and black) dense lithics are abundant in both members, although altered lithics are more abundant at the base of each, reaching 15vol.%. Pumice compositions range from basaltic to andesitic (48–61wt.% SiO2, anhydrous basis), but are highly altered. They contain plagioclase (andesine–labradorite), augite, hypersthene, Fe–Ti oxides, and rare amphibole. No compositional or mineralogical differences occur between the units indicating a common magma source during the same eruption. The Sibinal Pumice eruption started with a weak, pulsating column that reached at most 19km in height, ejecting 2.9km3 of tephra (1.1km3 DRE) at an average mass discharge rate of 4.7×107kg/s, with repeated hydromagmatic explosions that generated wet surges down the slopes of the volcano. The eruption ceased for a while during which rainfall generated a widespread lahar that eroded the top of SM. The eruption began anew with a sustained and stable Plinian column that ejected 4.6km3 (1.9km3 DRE) of tephra at a mass discharge rate of 8.1×107kg/s. The lack of any compositional differences in the magma suggests that the change in eruptive style was driven by influxes of external water provoking hydromagmatic explosions that cleared the conduit, then, the system changed to dry system and the eruption style changed from pulsating to stable Plinian. As the conduit became wider, the mass discharge rate also increased.

Explosive rhyolite tuya formation: classic examples from Kerlingarfjöll, Iceland

Rhyolite eruptions in Iceland mostly take place at long-lived central volcanoes, examples of which are found associated with each of the present-day rift-zone ice caps. Subglacial eruptions at Kerlingarfjöll central volcano produced rhyolite tuyas that are notable for their exposures of early-erupted pyroclastic material. Observations from a number of these edifices are synthesised into a general model for explosive rhyolite tuya formation. Eruptions begin with violent phreatomagmatic explosions that generate massive tuff (mT), but the influence of water quickly declines, leading to the formation of massive lapilli-tuffs (mLT) containing magmatically-fragmented vesicular pumice and ash. These are deposited rapidly near the vent, probably by moist pyroclastic density currents, confined by ice but not within a meltwater lake. The explosive-effusive transition is controlled by the ascent rate and gas content of the magma. An unusual obsidian-rich massive lapilli-tuff lithofacies (omLT) is identified and interpreted as pyroclastic material that was intruded into gas-fluidised deposits at the explosive-effusive transition. The effusive phase of eruption involves the emplacement of intrusions and lava caps. Intrusions of lava into the early-erupted phreatomagmatic deposits are characterised by peperitic margins and the formation of hyaloclastite. Intrusions into stratigraphically higher levels of the pyroclastic material show more limited interaction with the host tephra and have microcrystalline cores. Large lava bodies with columnar-jointed margins cap the tuyas and have intrusive basal contacts with the tephras. The main influence of the ice is to confine the rhyolite eruptive products to immediately above the vent region. This is in contrast to subglacial basaltic tuya-forming eruptions, which are characterised by the formation of meltwater lakes, phreatomagmatic fragmentation and subaqueous deposition. The lack of meltwater storage may reduce the potential for large jökulhlaups.

The Lami pyroclastic succession (Lipari, Aeolian Islands): A clue for unravelling the eruptive dynamics of the Monte Pilato rhyolitic pumice cone

The Lami succession piled up during the explosive activity of the Monte Pilato on Lipari Island (Central Aeolian Arc, Italy). It comprises a repetitive alternation of fall and pyroclastic density current beds, which define 13 Units separated by fine ash beds. Despite the homogeneous rhyolitic composition the juveniles fragments show strong textural heterogeneities from highly vesicular pumice to dense black obsidians. The pumice types are termed: i) “tube”, which shows elongated tubular vesicles, ii) “frothy”, highly vesicular with large, sub-rounded vesicles, and iii) agglomerated and welded. Two types of obsidian pyroclasts are recognized: i) aphyric black obsidian, and ii) grey obsidian showing spherulitic texture. All these pyroclastic types occur throughout the stratigraphic succession, and mixed in the same bed. They account for changing physical conditions along the conduit, which is hypothesised to be filled by a melt plug during the eruption of 12 out of 13 Units. The strong textural heterogeneities of the juvenile fragments testify for the existence of differential shear rates, which can be explained though two shear mechanism: i) simple shear, producing horizontal zonation of texture in the conduit with black obsidians along the conduit walls passing to tube and frothy pumice to the centre, and ii) pure shear, producing a vertical zonation in the conduit, with black obsidians at the top overlying frothy pumice. Agglomerated and welded pumice of Unit 13 is thought to represent the portion of magma heated by a batch of hotter melt that entered the magma chamber and triggered the eruption. The deposits of Unit 13 represent a unique case of spatter accretion due to accumulation of hot, low-viscous rhyolitic pumice fragments, emplaced from pyroclastic fountaining.

Effects of water depth on pumice formation in submarine domes at Sumisu, Izu-Bonin arc, western Pacific

Domes of the Sumisu volcanic complex (western Pacific), having summits at ocean depths of 1100, 600, 245, and 95 m, are mantled with compositionally identical rhyolitic pumice that has similar vesicularity, but that varies systematically in distribution, size, and surface texture—suggesting that facies and morphology can serve as useful indicators of eruption depth. At depths >500 m, the pumice formed a thick carapace on dense rhyolite and disintegrated by quench fracture and mechanical failure into a jumble of giant (meters to tens of meters) polyhedral blocks with smooth curviplanar surfaces that display a single quenched margin. Vesiculation was arrested on eruption in seawater in all but the interior of the thickest carapace. At <500 m depths, the pumice occurs as an apron of blocky giant and smaller rough-textured clasts enclosed by quenched margins and pockmarked by coarse (cm) vesicles. No carapace pumice occurs, and the summit is composed of craggy dense dome rock. These shallower water pumice clasts resemble those spalled from historic submarine dome-forming eruptions that buoyed to the sea surface. We interpret spalling to result from vent-derived, weak volatile-driven explosions that take place at water depths <500 m. Our study shows that an increase in hydrostatic pressures over a range of 12 MPa reduces volatile-driven explosivity for subaqueous, rhyolitic, dome-forming eruptions, but does not affect vesicularity. We conclude that meter-size, highly vesicular pumice is diagnostic of subaqueous dome eruptions in water depths of at least 1300 m, and its morphology can be used to distinguish between explosive and effusive origins

Quenching of steam-charged pumice: Implications for submarine pyroclastic volcanism

Huge quantities of silicic pumice have been deposited in intra-oceanic convergent margin settings throughout Earth's history. The association of submarine silicic calderas with thick proximal accumulations of pumice lapilli suggests that these pyroclasts were deposited as a direct result of submarine eruptions. Yet when first erupted, these highly vesicular, gas-filled clasts had densities significantly less than seawater. Experiments carried out 1-atm on heated pumice samples whose vesicles were charged with steam, the dominant component of magmatic volatiles show that buoyancy of freshly erupted submarine pumice is transient. Upon quenching, the phase change of steam-to-liquid water creates strong negative pore pressures within the pumice vesicles that accelerate the absorption of surrounding water, generating high-density pumice and promoting rapid clast sinking. Variations in the physical properties of steam with temperature and pressure have important implications for submarine pyroclastic eruptions. Firstly, highly vesicular pumice can be deposited on the seafloor at temperatures elevated significantly above ambient if they are erupted at sufficient depths to remain wholly submarine (> ∼ 200 m) and either the fluid in which they cool contains heated water and/or they only absorb sufficient water to sink. Secondly, the rapid increase in density of the eruption column caused by condensation and the transition from buoyant (gas-filled) to denser (water-saturated) pumice lapilli, together with turbulent mixing with the surrounding seawater favour collapse and transport of pyroclasts in water-supported gravity currents. Finally, this mixing of the ejecta with seawater and the ease of water ingestion into permeable pumice clasts suggest that water-supported transport mechanisms can operate as primary dispersal processes in explosive submarine eruptions.

A case history of paroxysmal explosion at Stromboli: Timing and dynamics of the April 5, 2003 event

On April 5, 2003, Stromboli volcano (Italy) produced the most violent explosion of the past 50 years. The event was exceptionally well documented thanks to the presence on the island of several scientists and a large number of instruments deployed over the preceding months to monitor the effusive eruption that began in December 2002. Integration of visual documentation, deposit features and geophysical data allowed an accurate reconstruction of the explosive event and its dynamics. The eruption consisted of a 8-min long explosive event which evolved through four phases whose timing was precisely recorded by an infrared thermometer located about 450 m from the summit crater. Phases 2 and 3 lasted 39 and 42 s, respectively. Both had an impulsive character, were responsible for ejecting almost the entire mass of the pyroclastic products. Phases 1 and 4 represented, respectively, a short-lived precursory event and a waning tale. During Phase 2, meter-sized ballistic blocks were launched with velocities of 170 m/s to altitudes of up to 1400 m above the craters. These fell on the volcano flanks and on the village of Ginostra, about 2 km distant from the vent. A vertical jet rose above the craters which developed to feed a convective plume that reached a height of up to 4 km. The calculated mass of the Phase 2 fallout deposit and mass discharge rate were 1.1–1.4 × 10 8 kg and 2.8–3.6 × 10 6 kg/s, respectively. During Phase 3 a scoria flow deposit, with an estimated volume of 0.9–1.1 × 10 4 m 3, was erupted from the same vent that fed the ongoing sustained lava flow. The average mass discharge rate for this phase was 2.5–3.1 × 10 5 kg/s. Products emitted during Phases 2 and 3 consisted of lithic and fresh magmatic material in similar proportions. The juvenile fraction consisted of a deep-originated, almost aphyric, highly vesicular pumice mingled with a shallow-derived, crystal-rich, moderately vesicular scoria. Similarities with the eruption dynamics of other historical paroxysms at Stromboli makes the April 5, 2003 explosion representative of these highly energetic events that constitute the most hazardous volcanic phenomena at Stromboli volcano.

Fragmentation phenomena in populations of magmatic crystals

Fragmentation of crystals is an important mechanism, and a component of particle dynamics in igneous and metamorphic rocks that has received surprisingly little attention. Recent advances in textural analysis, extraction techniques, digital imaging, and computer-assisted measurements enable rapid accumulation of 3D data on particle shapes and size distributions. This paper reviews fragment size distributions (FSD) that result from fragmentation: lognormal, fractal, loggamma, and Weibull; discusses their genesis mechanisms; and presents relevant examples of fragmentation from experimental physics. Next, the paper considers FSDs of feldspars on digitized images of thin sections and on published images, and quartz extracted from vesicular pumice by acid from eight well-known large-volume eruptive units. The acid solution of pumice enables examination of volume abundance, 3D shapes, proportions of fragmented crystals, and measurements of their CSDs and FSDs. FSDs were also measured in samples of welded tuff and a granite disaggregated by electric pulse. Products of syneruptive shock wave fragmentation, and fragmentation by an electric pulse are found to be fractal with large breakage probabilities, branching ratios, and fractal dimensions of 2 to 3. In contrast, most quartz fragments in pumice obey a lognormal distribution and fragmentation is driven by a melt inclusion decrepitation mechanism, which results in low breakage probability and small number (2–3) of fragments per breakage cycle. These results are consistent with one atmosphere heating experiments of quartz phenocrysts that led to melt inclusion decrepitation and caused quartz to break up into several smaller pieces collectively having lognormal FSD. Measured melt inclusion size distributions suggest decrepitation of outermost melt inclusions, and low survival rate for large inclusions, and inclusions with large radius/crystal size ratio. The modeling of periodic fragmentation of crystals with melt inclusions due to overheating and/or decompression, which may occur many times during the lifetime of a long-lived magma body, may explain concave-down, lognormal CSDs abundant in igneous rocks. The genesis of lognormality can be explained by the fragmentation algorithm of Kolmogorov (1941). Other algorithms may generate lognormal-like loggamma distributions. Fragmentation serves as an important size limiting factor, a nucleation aid, and it facilitates isotopic and trace elemental exchange.

Variability of plinian fall deposits: examples from Okataina Volcanic Centre, New Zealand

Plinian eruptions are intense, sustained, open-vent volcanic outbursts that produce extensive beds of generally well-sorted and generally highly vesicular pumice fallen from eruption plumes up to 55km high. Their deposits tend to be almost homogeneous throughout their thickness, although inverse grading commonly occurs. Here, we detail departures from this near homogeneity that we observed among the Mangaone Subgroup, a group of 12 plinian pumice-fall deposits from Okataina Volcanic Centre (one of the dormant rhyolitic volcanoes in the Taupo Volcanic Zone, New Zealand). We propose that plinian eruptions may pass through seven states, ranging from inception (commencement of activity, or renewed activity following a shutdown), waxing, steady discharge, climax, and waning leading to column collapse or shutdown (decrease to zero intensity). Some deposits include intercalated water-flushed ash-rich beds having a bimodal grain size. Other examples of non-homogeneity include phreatoplinian ashes that may alternate with the plinian pumice as though modulated by a delicate balance in water flux, and still other eruptions may switch completely from dry to wet conditions when the column wanes towards the latest stages of the eruptions. Pink pumice occurs in some deposits and is totally absent in others. We infer that it oxidized during temporary storage in or around the vent. Lithics are ubiquitous, and concentrations of them may indicate vent-wall collapses and explosive ejection of some of the resulting debris. Some lithic-rich horizons neither coincide with apparent variations in the plinian discharge nor interrupt it, as is the case with some rain-flushed bimodal beds. The events that produced these incongruent beds proceeded synchronously with, but independent of, the main plinian discharge, in an isolated portion of the vent system (such as a subsidiary vent). Other variations of density, vesicularity and crystallinity among the eruption products record the pre- and syn-eruptive history of the magma. A local inverse relationship between pumice density and crystallinity of one eruptive unit is inferred to reflect a control by crystals on vesiculation and fragmentation.

Estimating silicic lava vesicularity with thermal remote sensing: a new technique for volcanic mapping and monitoring

Remote monitoring of active lava domes pro- vides insights into the duration of continued lava extru- sion and detection of potentially associated explosive activity. On inactive flows, variations in surface texture ranging from dense glass to highly vesicular pumice can be related to emplacement time, volatile content, and internal structure. Pumiceous surface textures also pro- duce changes in thermal emission spectra that are clear- ly distinguishable using remote sensing. Spectrally, the textures describe a continuum consisting of two pure end members, obsidian and vesicles. The distinct spec- tral features of obsidian are commonly muted in pu- mice due to overprinting by the vesicles, which mimic spectrally neutral blackbody emitters. Assuming that this energy combines linearly in direct proportion to the percentage of vesicles, the surface vesicularity can be estimated by modeling the pumice spectrum as a lin- ear combination of the glass and blackbody spectra. Based on this discovery, a linear retrieval model using a least-squares fitting approach was applied to airborne thermal infrared data of the Little Glass Mountain and Crater Glass rhyolite flows at Medicine Lake Volcano (California) as a case study. The model produced a ve- sicularity image of the flow with values from 0 to F70%, which can be grouped into three broad textural classes: dense obsidian, finely vesicular pumice, and coarsely vesicular pumice. Values extracted from the image compare well with those derived from SEM anal- ysis of collected samples as well as with previously re- ported results. This technique provides the means to accurately map the areal distributions of these textures, resulting in significantly different values from those de- rived using aerial photographs. If applied to actively deforming domes, this technique will provide volcano- logists with an opportunity to monitor dome-wide de- gassing and eruptive potential in near-real-time. In July 1999 such an effort will be possible for the first time when repetitive, global, multispectral thermal infrared data become available with the launch of the Advanced Spaceborne Thermal Emission and Reflectance Ra- diometer (ASTER) instrument aboard the Earth Ob- serving System satellite.

Physical volcanology and emplacement history of the Ben Lomond rhyolite lava flow, Taupo Volcanic Centre, New Zealand

Abstract Currently, there are two contrasting models concerning the state in which rhyolite lava reaches the surface: (1) the permeable foam model, where the lava reaches the surface as an expanded foam and collapses into dense obsidian during lava flow; and (2) the traditional model, where decreased load pressure leads to vesiculation of the upper surface of the dome. In order to test these models, textural parameters were documented on outcrops of the Ben Lomond rhyolite dome, which has two flow lobes comprising an eroded dome (c. 100 ka old) cut by the Whangamata Fault. A composite stratigraphy of the upper 60 m of the flow, assembled from three sections (roadcut, airstrip and fault scarp), comprises, irom the flow top: finely vesicular pumice, black aphyric obsidian, and a spherulitic transition zone above a central crystalline rhyolite core. An explosion breccia occurs as a pod at a depth of c. 35 m and cross‐cuts the upper two units us an inverted cone‐shaped deposit—an infilled explosion pit. Eight...