Basaltic Plinian Eruptions Research Articles

Basaltic Plinian eruptions at Las Sierras-Masaya volcano driven by cool storage of crystal-rich magmas

Although rare, basaltic Plinian eruptions represent a considerable volcanic hazard. The low viscosity of crystal-poor basaltic magma inhibits magma fragmentation; however, Las Sierras-Masaya volcano, Nicaragua, has produced multiple basaltic Plinian eruptions. Here, we quantify the geochemistry and volatile concentrations of melt inclusions in samples of the Fontana Lapilli and Masaya Triple Layer eruptions to constrain pre-eruptive conditions. Combining thermometry and geochemical modelling, we show that magma cooled to ~1000 °C prior to eruption, crystallising a mush that was erupted and preserved in scoriae. We use these data in a numerical conduit model, which finds that conditions most conducive to Plinian eruptions are a pre-eruptive temperature <1100 °C and a total crystal content >30 vol.%. Cooling, crystal-rich, large-volume basaltic magma bodies may be hazardous due to their potential to erupt with Plinian magnitude. Rapid ascent rates mean there may only be some minutes between eruption triggering and Plinian activity at Masaya.

High-temperature oxidation of proximal basaltic pyroclasts, 1886 Tarawera, New Zealand

Microlite crystallization in erupting basalt can occur in the conduit, in flight, or in situ after deposition. Distinguishing the products of primary versus secondary (post-fragmentation) crystallization can be challenging in near-vent environments, but is vital for interpreting shallow conduit conditions from pyroclast textures. Here, we examine pyroclasts of the 1886 basaltic Plinian eruption of Tarawera volcano, New Zealand, to assess the roles of primary versus secondary crystallization of microlites. Lapilli and ash were selected from (a) an ultra-proximal section (T47), < 100 m from vent, which is dominated by pyroclasts derived from the Plinian jet and column margin, and (b) a medial fall deposit section (T43), 2.5 km from the fissure, which contains products from the umbrella cloud. Strong contrasts in pyroclast groundmass crystallinities exist between sections, from near-holocrystalline (90–97% void-free corrected; VFC) in T47 pyroclasts to highly crystalline (77–83% VFC) in T43 pyroclasts. Subhedral-euhedral Fe–Ti microlites (< 3 μm) are ubiquitous and abundant in T47 pyroclasts, whereas they are virtually absent in T43 pyroclasts. Olivine is present in both T47 and T43 pyroclasts but evidence of its subsolidus transformation is only seen in T47 clasts, whereas in T43 clasts olivine is fresh. Near-complete crystallization of the groundmass and subsolidus transformation of olivine in the ultraproximal T47 clasts are evidence that post-depositional modification of primary pyroclast textures occurred in ultra-proximal environments at Tarawera as a response to high residual temperatures and oxidizing conditions, aided by short transport times of relatively coarse ejecta at high accumulation rates, likely supplemented by intense vent-derived heat. Ultra-proximal basaltic eruption products can continue to crystallize after deposition and are thus unlikely to be faithful indictors of shallow conduit magmatic processes; we recommend the use of medial and distal products instead.

Pre- and syn-eruptive conditions of a basaltic Plinian eruption at Masaya Volcano, Nicaragua: The Masaya Triple Layer (2.1 ka)

The Masaya Triple Layer tephra was deposited ~2100 years ago during a basaltic Plinian eruption of Masaya caldera, Nicaragua, and is one of few known examples of this extreme endmember of basaltic explosive volcanism. Masaya caldera is located approximately 25 km from Managua, the capital of Nicaragua, and a Plinian eruption presents a high potential risk to its population of 1 million people. Here we use geochemical and petrological tools to constrain the pre- and syn-eruptive physico-chemical magmatic conditions, in order to understand how low-viscosity basaltic magmas can erupt with Plinian style.By combining thermometric models with Rhyolite-MELTS simulations, we find that the Masaya Triple Layer magma was last stored between 1080 and 1100 °C and 21–42 MPa, with a pre-eruptive volatile content of ~2 wt% H2O. A small phenocryst volume fraction of 0.1 crystallised under water-saturated conditions within a shallow magma reservoir located at 0.8–1.6 km depth. During ascent, rapid microlite crystallisation occurred and lateral velocity gradients across the conduit produced heterogeneous inter-mingling regions of varying crystallinity, between 20 and 50 vol%. Crystal size distribution analysis shows substantial microlite crystallisation over a timescale of 1–5 min, which induced significant changes in viscosity and magma rheology during ascent, increasing the effective magma viscosity from ~10 Pa s to 106 Pa s and approaching the brittle fragmentation threshold. Comparable pre- and syn-eruptive conditions have been deduced for other examples of basaltic Plinian activity, indicating a common eruption mechanism. We find that the common requirements for a basaltic Plinian eruption are a low magmatic H2O concentration of ~2 wt%, storage at moderate temperatures of 1080–1100 °C, and rapid crystallisation during magma ascent. Ultimately, these conditions contributed to the production of this highly hazardous Plinian eruption of Masaya.

Magma fragmentation in highly explosive basaltic eruptions induced by rapid crystallization

Basaltic eruptions are the most common form of volcanism on Earth and planetary bodies. The low viscosity of basaltic magmas inhibits fragmentation, which favours effusive and lava-fountaining activity, yet highly explosive, hazardous basaltic eruptions occur. The processes that promote fragmentation of basaltic magma remain unclear and are subject to debate. Here we used a numerical conduit model to show that a rapid magma ascent during explosive eruptions produces a large undercooling. In situ experiments revealed that undercooling drives exceptionally rapid (in minutes) crystallization, which induces a step change in viscosity that triggers magma fragmentation. The experimentally produced textures are consistent with basaltic Plinian eruption products. We applied a numerical model to investigate basaltic magma fragmentation over a wide parameter space and found that all basaltic volcanoes have the potential to produce highly explosive eruptions. The critical requirements are initial magma temperatures lower than 1,100 °C to reach a syn-eruptive crystal content of over 30 vol%, and thus a magma viscosity around 105 Pa s, which our results suggest is the minimum viscosity required for the fragmentation of fast ascending basaltic magmas. These temperature, crystal content and viscosity requirements reveal how typically effusive basaltic volcanoes can produce unexpected highly explosive and hazardous eruptions. Volcanoes that typically erupt effusively can generate highly explosive eruptions of basaltic lava under specific temperature and viscosity conditions, suggest crystallization experiments combined with numerical models of magma fragmentation.

Melt inclusion evidence for long term steady-state volcanism at Las Sierras-Masaya volcano, Nicaragua

Las Sierras-Masaya volcanic system is a persistently active basaltic caldera complex in Nicaragua. While there has been almost no juvenile material erupted since 1772, Masaya volcano has been persistently degassing for >150 years. An additional unusual behaviour for the Las Sierras-Masaya volcanic complex is its ability to produce large caldera-forming basaltic Plinian eruptions with the most recent occurring about 1800 years ago.Here we present melt inclusion analyses that provide constraints on the magmatic system over time. Melt inclusions hosted in plagioclase and olivine crystals were analyzed for major, trace and volatile elements (S, Cl, F, H2O). The data supports a consistent parental magmatic source with restricted compositional variability explained by simple fractional crystallization of plagioclase, olivine, clinopyroxene and magnetite at a nearly constant temperature. This broadly agrees with previous whole rock geochemical studies showing that the overall chemical signature of volcanic products at Masaya has remained largely unchanged for ~60,000 years and that both shallow fractionation and degassing processes dominate the whole evolution of the magmatic series. Based on volatile element in melt inclusions and gas composition and flux measurements, we determine the magmatic flux to be ~0.19 km3 yr−1 implying that up to 47 km3 of magma may have degassed since the last effusive eruption. As at other persistently active basaltic volcanoes (e.g., Mt. Etna, Italy; Kilauea, Hawaii, USA), this magmatic flux must involve significant endogenous storage which is likely accommodated by extensional tectonics. However, Masaya volcano differs in its apparent simplicity with respect to its stable chemistry and its fully interconnected magmatic system.

Fragmentation and Plinian eruption of crystallizing basaltic magma

Basalt is the most ubiquitous magma on Earth, erupting typically at intensities ranging from quiescently effusive to mildly explosive. The discovery of highly explosive Plinian eruptions of basaltic magma has therefore spurred debate about their cause. Silicic eruptions of similar style are a consequence of brittle fragmentation, as magma deformation becomes progressively more viscoelastic. Magma eventually crosses the glass transition and fragments due to a positive feedback between water exsolution, viscosity and decompression rate. In contrast to silicic eruptions, the viscosity of basaltic magmas is thought to be too low to reach conditions for brittle fragmentation. Pyroclasts from several basaltic Plinian eruptions, however, contain abundant micron-size crystals that can increase magma viscosity substantially. We therefore hypothesize that magma crystallization led to brittle fragmentation during these eruptions. Using combined oscillatory and extensional rheometry of concentrated particle-liquid suspensions that are dynamically similar to microcrystalline basaltic magma, we show that high volume fractions of particles and extension rates of about 1 s−1 or greater result in viscoelastic deformation and brittle fracture. We further show that for experimentally observed crystallization rate, basaltic magma can reach the empirical failure conditions when erupting at high discharge rates.

Conduit margin heating and deformation during the AD 1886 basaltic Plinian eruption at Tarawera volcano, New Zealand

During explosive eruptions, a suspension of gas and pyroclasts rises rapidly within a conduit. Here, we have analysed textures preserved in the walls of a pyroclastic feeder dyke of the AD 1886 Tarawera basaltic Plinian fissure eruption. The samples examined consist of basaltic ash and scoria plastered onto a conduit wall of a coherent rhyolite dome and a welded rhyolitic dome breccia. We examine the textural evidence for the response of the wall material, built of ∼75 vol.% glass and ∼25 vol.% crystals (pore-free equivalent), to mass movement in the adjacent conduit. In the rhyolitic wall material, we quantify the orientation and aspect ratio of biotite crystals as strain markers of simple shear deformation, and interpret juxtaposed regions of vesiculation and vesicle collapse as evidence of conduit wall heating. Systematic changes occur close to the margin: (1) porosity is highly variable, with areas locally vesiculated or densified, (2) biotite crystals are oriented with their long axis parallel to the margin, (3) the biotites have greater aspect ratios close to the margin and (4) the biotite crystals are fractured. We interpret the biotite phenocryst deformation to result from crystal fracture, rotation and cleavage-parallel bookcase translation. These textural observations are inferred to indicate mechanical coupling between the hot gas-ash jet and the conduit wall and reheating of wall rock rhyolite. We couple these observations with a simple 1D conductive heating model to show what minimum temperature the conduit wall needs to reach in order to achieve a temperature above the glass transition throughout the texturally-defined deformed zone. We propose that conduit wall heating and resulting deformation influences conduit margin outgassing and may enhance the intensity of such large basaltic eruptions.Electronic supplementary materialThe online version of this article (doi:10.1007/s00445-016-1006-7) contains supplementary material, which is available to authorized users.

Exploring the influence of vent location and eruption style on tephra fall hazard from the Okataina Volcanic Centre, New Zealand

Uncertainties in modelling volcanic hazards are often amplified in geographically large systems which have a diverse eruption history that comprises variable eruption styles from many different vent locations. The ~700 km2 Okataina Volcanic Centre (OVC) is a caldera complex in New Zealand which has displayed a range of eruption styles and compositions over its current phase of activity (26 ka–present), including one basaltic maar-forming eruption, one basaltic Plinian eruption and nine rhyolitic Plinian eruptions. All three of these eruption styles occurred within the past 3.5 ky, and any of these styles could occur in the event of a future eruption. The location of a future eruption is also unknown. Future vents could potentially open in one of three different areas which have been activated in the past 26 ky at the OVC: the Tarawera linear vent zone (LVZ) (five eruptions), the Haroharo LVZ (five eruptions) or outside of these LVZs (one eruption). A future rhyolitic or basaltic Plinian eruption from the OVC is likely to generate widespread tephra fall in loads that will cause significant disruption and have severe socio-economic impacts. Past OVC tephra hazard studies have focused on evaluating hazard from a rhyolitic Plinian eruption at select vent locations in the OVC’s Tarawera LVZ. Here, we expand upon past studies by evaluating tephra hazard for all possible OVC eruption vent areas and for both rhyolitic and basaltic Plinian eruption styles, and explore how these parameters influence tephra hazard forecasts. Probabilistic volcanic hazard model BET_VH and advection–diffusion model TEPHRA2 were used to assess the hazard of accumulating ≥10 kg m−2 of tephra from both basaltic Plinian and rhyolitic Plinian eruption styles, occurring from within the Tarawera LVZ, the Haroharo LVZ or other potential vent areas within the caldera. Our results highlight the importance of considering all the potential vent locations of a volcanic system, in order to capture the full eruption catalogue in analyses (e.g. 11 eruptions over 26 ky for the OVC versus only five eruptions over 26 ky for the Tarawera LVZ), as well as the full spatial distribution of tephra hazard. Although the Tarawera LVZ has been prominently discussed in studies of OVC hazard because of its recent activity (1886 and ~1315 ad), we find that in the event of a future eruption, the estimated likelihood of a vent opening within the Haroharo LVZ (last eruption 5.6 ka) is equivalent (<1 % difference) to that for the Tarawera LVZ (31.8 compared to 32.5 %). Including both the Haroharo LVZ and the Tarawera LVZ as possible source areas in the hazard analysis allows us to assess the full spatial extent of OVC tephra fall hazard. By considering both basaltic Plinian and rhyolitic Plinian eruption styles, as well as multiple vent location areas, we present a hazard assessment which aims to reduce bias through incorporating a greater range of eruption variables.

Relating vesicle shapes in pyroclasts to eruption styles

Vesicles in pyroclasts provide a direct record of conduit conditions during explosive volcanic eruptions. Although their numbers and sizes are used routinely to infer aspects of eruption dynamics, vesicle shape remains an underutilized parameter. We have quantified vesicle shapes in pyroclasts from fall deposits of seven explosive eruptions of different styles, using the dimensionless shape factor $\Omega $ , a measure of the degree of complexity of the bounding surface of an object. For each of the seven eruptions, we have also estimated the capillary number, Ca, from the magma expansion velocity through coupled diffusive bubble growth and conduit flow modeling. We find that $\Omega $ is smaller for eruptions with ${\rm{Ca}} \ll 1$ than for eruptions with Ca $\gg 1$ . Consistent with previous studies, we interpret these results as an expression of the relative importance of structural changes during magma decompression and bubble growth, such as coalescence and shape relaxation of bubbles by capillary stresses. Among the samples analyzed, Strombolian and Hawaiian fire-fountain eruptions have ${\rm{Ca}} \ll 1$ , in contrast to Vulcanian, Plinian, and ultraplinian eruptions. Interestingly, the basaltic Plinian eruptions of Tarawera volcano, New Zealand in 1886 and Mt. Etna, Italy in 122 BC, for which the cause of intense explosive activity has been controversial, are also characterized by ${\rm{Ca}} \gg 1$ and larger values of $\Omega $ than Strombolian and Hawaiian style (fire fountain) eruptions. We interpret this to be the consequence of syn-eruptive magma crystallization, resulting in high magma viscosity and reduced rates of bubble growth. Our model results indicate that during these basaltic Plinian eruptions, buildup of bubble overpressure resulted in brittle magma fragmentation.

Constraints on eruption dynamics of basaltic explosive activity derived from chemical and microtextural study: The example of the Fontana Lapilli Plinian eruption, Nicaragua

The Fontana Lapilli deposit is one of very few examples of basaltic Plinian eruptions discovered so far. Juvenile clasts have uniform chemical composition and moderate ranges of density and bulk vesicularity. However, clast populations include two textural varieties which are microlite-poor and microlite-rich respectively. These two clast types have the same clast density range, making a distinction impossible on that base alone. The high bubble number density (∼ 10 7 cm − 3 ) and small bubble population of the Fontana clasts suggest that the magma underwent coupled degassing following rapid decompression and fast ascent rate, leading to non-equilibrium degassing with continuous nucleation as it is common for silicic analogues. The Fontana products have lower microlite contents (10–60 vol.%) with respect to the other documented basaltic Plinian eruptions suggesting that the brittle fragmentation, implied for the other basaltic Plinian deposits, does not apply to the Fontana products and another fragmentation mechanism led the basaltic magma to erupt in a Plinian fashion.

The Masaya Triple Layer: A 2100 year old basaltic multi-episodic Plinian eruption from the Masaya Caldera Complex (Nicaragua)

The Masaya Caldera Complex has been the site of three highly explosive basaltic eruptions within the last six thousand years. A Plinian eruption ca. 2 ka ago formed the widespread deposits of the Masaya Triple Layer. We distinguish two facies within the Masaya Triple Layer from each other: La Concepción facies to the south and Managua facies to the northwest. These two facies were previously treated as two separated deposits (La Concepción Tephra and the Masaya Triple Layer of Pérez and Freundt, 2006) because of their distinct regional distribution and internal architectures. However, chemical compositions of bulk rock, matrix and inclusion glasses and mineral phases demonstrate that they are the product of a single basaltic magma batch. Additionally, a marker bed containing fluidal-shaped vesicular lapilli allowed us to make a plausible correlation between the two facies, also supported by consistent lateral changes in lithologic structure and composition, thickness and grain size. We distinguish 10 main subunits of the Masaya Triple Layer (I to X), with bulk volumes ranging between 0.02 and 0.22 km 3, adding up to 0.86 km 3 (0.4 km 3 DRE) for the entire deposit. Distal deposits identified in two cores drilled offshore Nicaragua, at a distance of ∼ 170 km from the Masaya Caldera Complex, increase the total tephra volume to 3.4 km 3 or ∼ 1.8 km 3 DRE of erupted basaltic magma. Isopleth data of five major fallout subunits indicate mass discharges of 10 6 to 10 8 kg/s and eruption columns of 21 to 32 km height, affected by wind speeds of < 2 m/s to ∼ 20 m/s which increased during the course of the multi-episodic eruption. Magmatic Plinian events alternated with phreatoplinian eruptions and phreatomagmatic explosions generating surges that typically preceded breaks in activity. While single eruptive episodes lasted for few hours, the entire eruption probable lasted weeks to months. This is indicated by changes in atmospheric conditions and ash-layer surfaces that had become modified during the breaks in activity. The Masaya Triple Layer has allowed to reconstruct in detail how a basaltic Plinian eruption develops in terms of duration, episodicity, and variable access of external water to the conduit, with implications for volcanic hazard assessment.

New physical characterization of the Fontana Lapilli basaltic Plinian eruption, Nicaragua

The Fontana Lapilli deposit was erupted in the late Pleistocene from a vent, or multiple vents, located near Masaya volcano (Nicaragua) and is the product of one of the largest basaltic Plinian eruptions studied so far. This eruption evolved from an initial sequence of fluctuating fountain-like events and moderately explosive pulses to a sustained Plinian episode depositing fall beds of highly vesicular basaltic-andesite scoria (SiO2 > 53 wt%). Samples show unimodal grain size distribution and a moderate sorting that are uniform in time. The juvenile component predominates (> 96 wt%) and consists of vesicular clasts with both sub-angular and fluidal, elongated shapes. We obtain a maximum plume height of 32 km and an associated mass eruption rate of 1.4 × 108 kg s−1 for the Plinian phase. Estimates of erupted volume are strongly sensitive to the technique used for the calculation and to the distribution of field data. Our best estimate for the erupted volume of the majority of the climactic Plinian phase is between 2.9 and 3.8 km3 and was obtained by applying a power-law fitting technique with different integration limits. The estimated eruption duration varies between 4 and 6 h. Marine-core data confirm that the tephra thinning is better fitted by a power-law than by an exponential trend.

Explosive volcanic eruptions on Mars: Tephra and accretionary lapilli formation, dispersal and recognition in the geologic record

Explosive volcanic eruptions, potentially involving large amounts of magmatic and entrained water, are thought to have been very important in the Noachian and Hesperian periods earlier in Mars history, and basaltic plinian eruptions are likely to have occurred throughout martian history. Previous treatments of explosive volcanic plumes on Mars have simply extrapolated plume models for the Earth's atmosphere to Mars. Taking account of known limitations to the applicability of this approach, which suggest that convective plumes may be capable of reaching only ∼ 20 km height on Mars, we introduce the concept of inertial plumes, capable of carrying clasts to much greater heights and dropping them into the lower atmosphere over a wide area. Atmospheric circulation patterns guarantee wide dispersal and thick deposits of tephra resulting from both types of eruption. The presence of large amounts of water in convecting explosive eruption plumes can also lead to condensation of water on small particles and the consequent accretion of other particles as the smaller particles fall through the plume, producing accretionary lapilli. Formation of accretionary lapilli significantly alters the spatial distribution and grain sizes of pyroclastic fall deposits from those involving discrete juvenile clasts with negligible clast interactions. We model the eruption and dispersal of tephra, and the formation of accretionary lapilli on Mars under current atmospheric conditions and explore the consequences of this for the geometry and grain size of deposits formed from explosive eruption plumes. We show that explosive eruptions can produce thick widespread deposits of ash and lapilli similar to those thought to have produced mantling deposits in several regions of Mars. We develop a detailed example that shows that local hydrovolcanic explosive eruptions and solely magmatic eruptions originating from the nearby Apollinaris Patera could have emplaced tephra and accretionary lapilli in the Columbia Hills region of the Mars Exploration Rover Gusev site. If the atmospheric pressure was higher early in Mars' history than now, eruptions would have led to somewhat more extensive pyroclast dispersal than under current conditions.

Contrasting grain size and componentry in complex proximal deposits of the 1886 Tarawera basaltic Plinian eruption

The 1886 Plinian eruption of Tarawera, New Zealand, is a unique basaltic fissure-fed eruption with exceptionally well preserved fall deposits to within 200 meters of the source vents. These proximal deposits form a series of spatter/cinder half-cones along the northeastern 8-km-long segment of the 1886 fissure. Here we examine these deposits using grain size and clast componentry techniques. We contrast the products of the phreatomagmatic (phases I and III) and Plinian (Phase II) stages of the eruption and examine deposit variability as a function of contrasting eruptive intensity within the climactic phase (II) of the eruption. The opening phreatomagmatic phase I of the eruption involved gas-rich magma interacting with water and fragmenting at least 300 meters below the surface. The deposits of the climactic phase that followed have relatively uniform grain size but marked contrasts in the relative abundance of juvenile and wall rock (lithic) clasts. Deposits linked to vents associated with the high Plinian plume are more uniform than those characterized by a weaker cone-forming eruption style. During the third, and closing, phase of the eruption, magma withdrawal accompanied the onset of decoupling of the exsolved gas phase, leading to fragmentation at increasingly greater depths and significant wall rock collapse into the erupting vents. Variability in eruptive style during phase II along the fissure appears to be a function of shallow seated controls, in particular the variable extent of incorporation of lithic wall rock into the erupting jet, as a consequence of vent wall collapse. Widely dispersed beds centralized around Plinian sources along the fissure have very low lithic content; cone-forming beds at other craters that contain very high lithic contents. This incorporation led to a significant reduction of the velocity and stability of the jet at these latter steep-walled craters, and induced episodicity in the form of vent-clearing explosions. The result is a large reduction of the physical and thermal ability of these vents to contribute to a stable high eruptive plume. Instead large volumes of ejecta were sedimented prematurely from shallow heights at rates an order of magnitude greater than for historical Strombolian, Hawaiian and subPlinian eruptions. This study illustrates that sustained powerful Plinian eruptions can be accompanied by heterogeneities and instabilities of the eruptive jet. At Tarawera, the record of complex proximal transport and deposition processes in the eruptive jet cannot be inferred from the eruption products at distances greater than 400 m from the eruptive fissure. We suggest that study of ultraproximal deposits, as seen at Mt Tarawera, provides the only opportunity to document the complex, dynamic behavior of the jet region of Plinian eruptions.

Changing conditions of magma ascent and fragmentation during the Etna 122 BC basaltic Plinian eruption: Evidence from clast microtextures

The Etna 122 BC basaltic eruption had two Plinian phases, each preceded and followed by weak phreatic and phreatomagmatic activity. This study infers changing eruption dynamics from density, grain size, and microtextural data from the erupted pyroclasts. The Plinian clasts show no evidence for quenching by external water; instead, all clasts are microvesicular and have high bubble number densities relative to the products of weaker basaltic explosive eruptions, suggesting that the 122 BC magma underwent coupled degassing linked to rapid ascent and decompression. This coupled degassing was probably enhanced by crystallization of abundant microlites, which increased the magma's effective viscosity during conduit ascent. Detailed measurements of vesicles and microlites show wide variations in number densities, size distributions, and shapes among clasts collected over narrow stratigraphic intervals. For such a diversity of clasts to be expelled together, portions of melt with contrasting ascent and degassing histories must have arrived at the fragmentation surface at essentially the same time. We suggest that a parabolic velocity profile across the conduit ensured that magma near the conduit walls ascended more slowly than magma along the axis, leading to a longer residence time and more advanced degrees of outgassing and crystallization in the marginal magma. In our model, accumulation of this outgassed, viscous magma along conduit walls reduced the effective radius of the shallow conduit and led to blockages that ended the Plinian phases.

The influence of conduit processes on changes in style of basaltic Plinian eruptions: Tarawera 1886 and Etna 122 BC

Basaltic volcanism is most typically thought to produce effusion of lava, with the most explosive manifestations ranging from mild Strombolian activity to more energetic fire fountain eruptions. However, some basaltic eruptions are now recognized as extremely violent, i.e., generating widespread phreatomagmatic, subplinian and Plinian fall deposits. We focus here on the influence of conduit processes, especially partial open-system degassing, in triggering abrupt changes in style and intensity that occurred during two examples of basaltic Plinian volcanism. We use the 1886 eruption of Tarawera, New Zealand, the youngest known basaltic Plinian eruption and the only one for which there are detailed written eyewitness accounts, and the well-documented 122 BC eruption of Mount Etna, Italy, and present new grain size and vesicularity data from the proximal deposits. These data show that even during extremely powerful basaltic eruptions, conduit processes play a critical role in modifying the form of the eruptions. Even with very high discharge, and presumably ascent, rates, partial open-system behaviour of basaltic melts becomes a critical factor that leads to development of domains of largely stagnant and outgassed melt that restricts the effective radius of the conduit. The exact path taken in the waning stages of the eruptions varied, in response to factors which included conduit geometry, efficiency and extent of outgassing and availability of ground water, but a relatively abrupt cessation to sustained high-intensity discharge was an inevitable consequence of the degassing processes.

Discovery of a Plinian basaltic eruption of Roman age at Etna volcano, Italy

Basaltic Plinian eruptions are rare and poorly known volcanic phenomena. Etna is an active basaltic volcano, the activity of which is dominated by effusive eruptions that represent a continuous threat to a large populated area. We report on a Plinian eruption of basaltic composition that occurred in 122 b.c. Lapilli fallout caused extensive damage to the southern flank of the volcano. The discovery of this large explosive eruption raises important issues for previous hazard assessment at Etna and other basaltic volcanoes: An effusive basaltic volcano, generally nonhazardous for humans, can become very dangerous.

Mars: Review and analysis of volcanic eruption theory and relationships to observed landforms

We present a theoretical treatment of the ascent, emplacement, and eruption of magma on Mars. Because of the lower gravity, fluid convective motions and crystal settling processes driven by positive and negative buoyancy forces, as well as overall diapiric ascent rates, will be slower on Mars than on Earth, permitting larger diapirs to ascend to shallower depths. This factor also favors a systematic increase in dike widths on Mars by a factor of 2 and, consequently, higher effusion rates by a factor of 5. As a result of the differences in lithospheric bulk density profile, which in turn depend on differences in both gravity and surface atmospheric pressure, magma reservoirs are expected to be deeper on Mars than on Earth, by a factor of about 4. The combination of the lower Martian gravity and lower atmospheric pressure ensures that both nucleation and disruption of magma occur at systematically greater depths than on Earth. Although lava flow heat loss processes are such that no major differences between Mars and Earth are to be expected in terms of flow cooling rates and surface textures, the lower gravity causes cooling‐limited flows to be longer and dikes and vents to be wider and characterized by higher effusion rates. Taken together, these factors imply that we might expect compositionally similar cooling‐limited lava flows to be about 6 times longer on Mars than on Earth. For example, a Laki type flow would have a typical length of 200–350 km on Mars; this would permit the construction of very large volcanoes of the order of 500–700 km in diameter. For strombolian eruptions on Mars the main difference is that while the large particles will remain near the vent, the finer material will be more broadly dispersed and the finest material will be carried up into a convecting cloud over the vent. This means that there would be a tendency for broader deposits of fine tephra surrounding spatter cones on Mars than on Earth. On Mars, strombolian eruption deposits should consist of cones that are slightly broader and lower relative to those on Earth, with a surrounding deposit of finer material. Martian hawaiian cones should have diameters that are about a factor of 2 larger and heights that are, correspondingly, about a factor of 4 smaller than on Earth; central craters in these edifices should also be broader on Earth by a factor of up to at least 5. Grain sizes in Martian hawaiian edifices should be at least 1 order of magnitude finer than in terrestrial equivalents because of the enhanced magma fragmentation on Mars. Differences in the atmospheric pressure and temperature structure cause Martian plinian eruption clouds to rise about 5 times higher than terrestrial clouds for the same eruption rate. Essentially the same relative shapes of eruption clouds are expected on Mars as on Earth, and so the cloud‐height/deposit‐width relationship should also be similar. This implies that Martian fall deposits may be recognized as areas of mantled topography with widths in the range of several tens to a few hundred kilometers. A consequence of the lower atmospheric pressure is that Martian plinian deposits of any magma composition will be systematically finer grained than those on Earth by a factor of about 100, almost entirely subcentimeter in size. Basaltic plinian eruptions, rare on Earth, should be relatively common on Mars. The production of large‐scale plinian deposits may not signal the presence of more silicic compositions, but rather may be linked to the enhanced fragmentation of basaltic magma in the Martian environment or to the interaction of basaltic magma with groundwater. The occurrence of steep‐sided domes, potentially formed by viscous, more silicic magma, may be largely precluded by enhanced magma fragmentation. Pyroclastic flow formation is clearly inherently more likely to occur on Mars than on Earth, since eruption cloud instability occurs at a lower mass eruption rate for a given magma volatile content. For a given initial magma volatile content, eruption speeds are a factor of at least 1.5 higher on Mars, and so the fountains feeding pyroclastic flows will be more than twice as high as on Earth. Pyroclastic flow travel distances may be a factor of about 3 greater, leading to values up to at least a few hundred kilometers. Martian environmental conditions thus operate to modulate the various eruption styles and the morphology and morphometry of resulting landforms, providing new insight into several volcanological problems.