Abstract The 2001 eruption of Etna volcano, prior to the flank eruption, was marked by 16 episodes separated from one another by few days of quiescence. Insights into fire fountain formation are provided by a close comparison of the sound produced by an episode solely involving a series of Strombolian explosions (4 July) and one also showing a transition to a fire fountain (12 July). The best fit between measured and synthetic waveforms gives the temporal evolution of the bubble length, 8–100 m, and overpressure, 0.2 MPa. Both episodes result from the coalescence of a foam layer trapped at the top of the reservoir. At the transition towards a fire fountain, the number of explosions and the bubble length increase simultaneously, suggesting that the foam destabilization is more efficient when a fire fountain is produced. Acoustic records give access to the gas volume trapped within the foam, called ‘active’ degassing, while the height of fire fountains also includes the gas from passive degassing. The small bubbles from passive degassing are carried to the surface via the wake of the slugs, coming from the depth of the reservoir. The proportion between active and total gas volume represents 38–48%.

Abstract The 2001 eruption of Etna, prior to the flank eruption, has shown an alternation between episodes rich in gas, composed with a series of Strombolian explosions sometimes leading to a fire fountain, and repose periods. The regular alternation results from the coalescence of a foam trapped in the reservoir and periodically rebuilt prior to each episode. The degassing of a magma reservoir depends on bubble diameter, gas volume fraction, surface area and height of the reservoir. These four parameters are deduced from the measured gas flux, the timescale over which the gas flux decreases and the foam dynamics. The dimensionless foam thickness, 0.76 for a purely Strombolian episode, increases to 0.89 for an episode leading to a fire fountain, indicating a more efficient foam coalescence. At Etna, the bubble diameter, gas volume fraction, surface area and height of the reservoir are estimated at 0.50–0.59 mm, 0.25–0.39%, >0.20 km 2 and 97–220 m, respectively. At Kilauea, the excellent agreement between the prediction of the foam model, 0.94–1.2 km 2 and that resulting from deformations, 1 km 2 , reinforces the validity of the foam model qualitatively and quantitatively. The thickness of the degassing reservoir, 16–12 m in 1959, is now 1.3–1.1 km.

On the Nuussuaq peninsula, Western Greenland sedimentary deposits of glass spherules also contain high Ir, Co, Ni, and Cu anomalies. The iron-rich silicate glass spherules (to ∼3 wt% NiO, ∼35 wt% FeO) are highly circular in cross section. They show surface dissolution, smectite replacement and calcite infilling of vesicles, though many glasses are optically unaltered. They are strikingly heterogeneous, with schlieren outlining counter-flowing convection cells. Their pronounced Fe-Ni correlation is unlike volcanic suites, but is explained by mixing between basaltic melt and an enriched iron-nickel source. Distinctive nickel-spinel (∼7–10 wt% NiO) contains very nickel-rich cores. Occasional glass spherules show compositional gradients toward resorbed silicates, (plagioclase, clinopyroxene); isotropic plagioclase has anomalous texture comparable to impact-melted lunar breccias. Their anomalously high copper and sulfur (to ∼1%) have lead to an explanation as products of fire-fountaining of exotic or picritic Disko lavas; they would be perhaps the only non-impact occurrence of Ni spinel. Since their discovery, better criteria for recognition of spherules ejected from large impacts have been established, and greater variations in meteorite chemistry as potential projectiles have been described. New mineralogical and petrographic textural data for the Nuussuaq spherules suggest they should be reinterpreted as impact ejecta; the highly oxidized Ni-spinel is a very characteristic signature of meteorite impact ejecta. Delicate preservation features rule out substantial sedimentary reworking, and spherule bed thicknesses imply a large source crater. Stratigraphically, the spherule beds are poorly constrained, but nannofossils and magnetostratigraphy place them close to onset of the West Greenland flood lavas (ca. 61–62 Ma). They share many characteristics with massive native iron localities in dykes and lavas up to >100 km away on Disko (Qeqertarsuaq) Island, but their precise relationship remains to be established.

Abstract Subglacial volcanic eruptions can generate large volumes of meltwater that is stored and transported beneath glaciers and released catastrophically in jökulhlaups. At typical basaltic dyke propagation speeds, the high strain rate at a dyke tip causes ice to behave as a brittle solid; dykes can overshoot a rock–ice interface to intrude through 20–30% of the thickness of the overlying ice. The very large surface area of the dyke sides causes rapid melting of ice and subsequent collapse of the dyke to form a basal rubble pile. Magma can also be intruded at the substrate–ice interface as a sill, spreading sideways more efficiently than a subaerial flow, and also producing efficient and widespread heat transfer. Both intrusion mechanisms may lead to the early abundance of meltwater sometimes observed in Icelandic subglacial eruptions. If meltwater is retained above a sill, continuous melting of adjacent and overlying ice by hot convecting meltwater occurs. At typical sill pressures under more than 300 m ice thickness, magmatic CO 2 gas bubbles form c. 25 vol% of the pressurized magma. If water drains and contact with the atmosphere is established, the pressure decreases dramatically unless the overlying ice subsides rapidly into the vacated space. If it does not, further CO 2 exsolution plus the onset of H 2 O exsolution has the potential to cause explosive fragmentation, i.e. a fire-fountain that forms at the dyke-sill connection, enhancing melting and creating another candidate pulse of meltwater. The now effectively subaerial magma body becomes thicker, narrower, and flows faster so that marginal meltwater drainage channels become available. If the ice overburden thickness is much less than c. 300m the entire sill injection process may involve explosive magma fragmentation. Thus, there should be major differences between subglacial eruptions under local or alpine glaciers compared with those under continental-scale glaciers.