Change In Eruptive Style Research Articles

Petrogenetic evolution of late Cenozoic, post-collision volcanism in western Anatolia, Turkey

Following an Eocene continent-arc collision, the Western Anatolia region experienced a complete cycle of thickening and orogenic collapse. The early stage of collision-related volcanism, which was most evident during the Early Miocene (<21Ma), produced a considerable volume of lavas and pyroclastic deposits of basaltic andesite to rhyolite composition. The volcanic activity continued into the Middle Miocene with a gradual change in eruptive style and magma composition. The Middle Miocene activity formed in relation to localised extensional basins and was dominated by lava flows and dykes of basalt to andesite composition. Both the Early and Middle Miocene rocks exhibit calc-alkaline and shoshonitic character. The Late Miocene volcanism (<11Ma) was marked by alkali basalts and basanites erupted along the zones of localised extension.The Early–Middle Miocene volcanic rocks exhibit enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to the high field strength elements (HFSE) and have high 87Sr/86Sr (0.70757–0.70868) and low 143Nd/144Nd (0.51232–0.51246) ratios. Modelling of these characteristics indicates a mantle lithospheric source region carrying a subduction component inherited from a pre-collision subduction event. Perturbation of this subduction-metasomatised lithosphere by either delamination of the thermal boundary layer or slab detachment is the likely mechanism for the initiation of the post-collision magmatism.Petrographic characteristics and trace element systematics (e.g. phenocryst assemblages and relative depletion in MREE and heavy rare earth elements (HREE)) suggest that the Early–Middle Miocene magmas underwent hydrous crystallisation (dominated by plagioclase+pyroxene+pargasitic amphibole) in deep crustal magma chambers. Subsequent crystallisation in shallower magma chambers follows two different trends: (1) anhydrous (pyroxene+plagioclase-dominated); and (2) hydrous (edenitic amphibole+plagioclase+pyroxene dominated).AFC modelling shows that the Early–Middle Miocene magmas evolved through assimilation combined with fractional crystallisation, and that the effects of assimilation decreased gradually from the Early Miocene into the Middle Miocene. This may indicate a progressive crustal thinning related to the extensional tectonics that prevailed from the latest Early Miocene onwards.In contrast, the Late Miocene alkaline rocks are characterised by low 87Sr/86Sr (0.70311–0.70325) and high 143Nd/144Nd (0.51293–0.51298) ratios and have OIB-type like trace element patterns characterised by enrichment in LILE, HFSE, LREE and MREE, and a slight depletion in HREE, relative to average N-MORB. REE modelling indicates that these rocks formed by partial melting of a garnet-bearing lherzolite source. Trace element and isotope systematics are consistent with an origin by decompression melting of an enriched asthenospheric mantle source.

The change of eruption styles of Martian volcanoes and estimates of the water content of the Martian mantle

Estimated water contents in the Martian mantle range from 36 ppm to more than 1%. These values are based on the chemical analyses such as hydrous minerals in SNC meteorites and formation models of Mars. This study evaluates the water content of the Martian mantle using the change with time of volcanic eruption style on Mars as an observational constraint. Styles of volcanic activity depend on the volatile content of the magma and the atmospheric pressure. Because a low atmospheric pressure leads to a more explosive volcanic eruption, it has been believed that the volcanism on the current Martian environment would be very explosive. Our calculations, however, show that, under the current Martian atmospheric conditions, erupted magma cannot entrain the ambient air effectively, so the decrease in temperature of the magma during ascent is small. Consequently, the erupted magma may form a lava-like deposit when it falls back on the ground. This effusive-like style of eruption is a counterpart of clastogenic lava on Mars. On the other hand, numerical calculations under a thick CO 2 atmosphere, which may correspond to an ancient Martian atmosphere, reveal a rather explosive eruption style. Geological features of earlier stages of Martian history in the Noachian and Hesperian eras suggest that the volcanic eruptions on Mars were explosive then. Effusive eruptions, however, became dominant in more recent times. It has been widely accepted that Mars experienced a major climate change. In addition, the release factor of volatiles on Mars has been suggested to be as small as 0.017–0.112. This may imply that the volatile content has been almost constant throughout Martian history. Consequently, we assume that this change in eruption style was caused by the change in atmospheric pressure. For a given water content of magma, a major climatic change may lead to a transition in eruption style. If we know the atmospheric pressure at the time of this transition, we can calculate the possible range of the volatile content of the mantle using our numerical simulations. If the atmospheric pressure on Mars around late Hesperian era is about 1 bar, the estimated values for a typical Martian magma are 0.05–0.25 wt.%, which is within the range of the water content of typical terrestrial basaltic magmas.

Seismicity associated with the 1995–1996 eruptions of Ruapehu volcano, New Zealand: narrative and insights into physical processes

Seismicity associated with the 1995–1996 eruptions of Ruapehu reflects processes associated with the intrusion of magma to shallow depth in the volcanic edifice, its subsequent eruption, and changes in the volcanic plumbing system resulting from the eruptions. The sequence consisted of two distinct periods of eruptive activity, the first lasting from September 17, 1995 until late-October or early-November 1995 and the second from June 16, 1996 until late-July 1996. Immediately prior to and during the early stages of the 1995 eruptions, seismicity was similar to that recorded at Ruapehu during the previous 25 years, with the exception of the occurrence between September 17 and 25, 1995 of sub-1 Hz tremor, which is thought to represent magmatic intrusion. In early-October, a new pattern of seismicity was established with the disappearance of the 2 Hz tremor resonator source and a change to wideband (2–10 Hz) tremor and wideband volcanic earthquakes. This change coincided with the ejection of the last vestiges of Crater Lake and the change in eruptive style from phreatomagmatic to magmatic. Seismicity associated with the 1996 eruptions was very similar to that which accompanied the later part of the 1995 activity, implying the same eruption processes and that no significant changes had occurred in the volcanic plumbing system during the intervening period. Geochemical, geodetic, and seismic precursors to the 1995 eruptions were both minor and inconsistent, highlighting the difficulty in forecasting these eruptions. Furthermore, deep volcano-tectonic earthquakes were extremely rare throughout the 1995 and 1996 eruptive sequences, suggesting that stresses associated with magmatic intrusion were minor. This most likely resulted from the existence of either an open or a ductile pathway from the deep magma source to the surface prior to the eruptions and because the volume of magma intruded and subsequently erupted was relatively small (<0.05 km 3).

Thermochemical dynamics of magma chambers: A simple model

A magma chamber may be considered as an open, non adiabatic chemical reactor that continuously receives new liquid, crystallizes, and erupts lava. Heat is transferred to the walls, and crystals settle on the chamber floor. Considering, for simplicity, the chamber magma as a thermally and chemically homogeneous two‐component liquid in which the solidification rate is a bell‐shaped function of temperature and composition, the system is governed by two coupled differential equations for the evolution of temperature and composition with time. For constant external factors, such as fresh‐magma input flow or heat loss flux, the magma chamber tends to a steady state. For a certain domain of external factor values, there are two stable steady state solutions instead of one. When the external factors vary slowly, the magma chamber state also drifts continuously, except when the external factors leave the bistability domain. Then all characteristics of the volcanic system undergo a sudden jump, for example, from basic to felsic composition and from high to low lava output flow when the fresh‐magma input flow of a waning volcano declines. The jump takes place over a time of the order of the magma residence time and is thus short compared to the lifetime of the chamber. Such a magmatic catastrophe, which may occur for a small and continuous evolution of the external conditions, results in a radical change in eruption style and volcanic products and may trigger an exceptionally large or violent eruption. This kind of event may explain the widespread observation of compositional gaps in lava series (e.g., the Daly gap), a number of radical changes in eruptive style, and certain eruptions which appear unique in the lifetime of some composite volcanoes.

Overview of the eruption of Soufriere Hills Volcano, Montserrat, 18 July 1995 to December 1997

The onset of phreatic volcanic activity at the Soufriere Hills volcano, Montserrat on 18 July 1995 followed a three‐year period of heightened volcano‐seismic activity beneath the island. Phreatic explosions gave way to continuous eruption of juvenile andesitic magma in the form of a lava dome on or around 15 November 1995. Magma production rate has varied, leading to changes in eruptive style. An explosive eruption on 17 September 1996 followed a period of enhanced dome growth and large‐scale gravitational collapses from its eastern flank. Increasing dome volume led to stressing and overtopping of the confining crater walls to the southwest, north and west during early 1997. Sustained high magma production rate since June 1997 has led to three periods of major gravitational dome collapses followed by vulcanian explosive eruptions. Dome growth re‐started immediately after the cessation of the latest of these explosive phases in October 1997 and continues as of December 1997.

Quaternary lahar stratigraphy of the western Ruapehu ring plain, New Zealand

One of the most widespread hazards associated with explosive activity at Mt Ruapehu is the production of potentially destructive lahars. Volcanic activity at this andesite stratovolcano has produced numerous lahars throughout Quaternary time, with the resulting deposits being preserved principally on the lower slopes of the composite cone and its ring plain. This paper describes the stratigraphy of volcaniclastic sequences on the western Ruapehu ring plain, in order to improve the record of known events. The lithologic record is principally one of lahar deposits interbedded with datable andesitic tephras, loesses, paleosols, and rhyolitic marker beds, allowing a chronological reconstruction of volcanic events in the region during the last 120 000 yr. The main laharic surfaces include the extensive Porewan‐aged (>65 000 yr B.P.) uplifted Erua plateau (Waimarino Formation, new) and voluminous Ratan‐Ohakean (10 000–65 000 yr B.P.) sequences of stacked diamictons east of the Waimarino Fault (Horopito Formation, new, and Te Heuheu Formation). A marked change in eruptive style occurred around 10 000 yr B.P. when pumiceous pyroclastic flows were emplaced to the east and the west of the Mt Ruapehu cone (Taurewa Eruptive Episode). At about this time, river entrenchment into the Ohakean surface began. Holocene lahar deposits are preserved only in the Mangaturuturu and Mangawhero catchments where they are confined to riparian strips. In the Mangawhero catchment a mid‐Holocene debris flow unit (Turoa diamicton) contains numerous jigsaw‐jointed bombs and vitric fragments, indicative of a coeval vulcanian eruption. In historical time, only lahars in 1975 left recognisable deposits in the Mangaturuturu catchment. Thus, the potential for long‐term preservation of modern, small‐size sandy diamictons is strictly limited on andesite stratovolcanoes such as Ruapehu.

Lacustrine Sedimentation Processes and Patterns During Effusive and Explosive Volcanism, Challis Volcanic Field, Idaho

ABSTRACT Lacustrine deposits in the Eocene Challis volcanic field accumulated in a deep lake bordered by steep ranges. Volcanic and nonvolcanic sediment was transported into the lake via a major dispersal system from the north. High-density turbidity currents transported this sediment throughout the lake system. Paleozoic rocks in the adjacent ranges were secondary sediment sources. This sediment was trapped along the shoreline, and only a small fraction was transported offshore by turbidity currents. The frequency of turbidity currents varied with changes in eruption style. Thus, turbidite accumulation patterns are a faithful record of changes in volcanic activity and associated landscape processes. Effusive volcanism that characterized the early activity in the Challis volcanic field produced little sand-size sediment and had little impact on lacustrine sedimentation patterns. Relatively slow settling from suspension characterized lake sedimentation during this time; turbidity currents were infrequent. The resulting accumulation pattern is a single sheet of turbidite sandstone encased in thick-bedded mudrock. Initial eruptions at the Van Horn Peak cauldron complex altered sedimentation patterns dramatically by increasing the frequency of turbidity currents. A turbidite ramp prograded into the study area, marking a major increase in sedimentation rate. Sediment composition and turbidite accumulation patterns record changes in landscape processes. Syneruption deposits are stacked turbidite sheets. Early turbidites contain mostly sediment derived from the fresh pyroclastic material. The proportion of sediment from pre-eruption sources increases upward through the stacked sheets, recording a change in sediment sources at the end of the syneruption period. Ramp growth continued through this part of the period, although at a much slower rate.

Prolonged history of silicic peralkaline volcanism in the eastern Pacific Ocean

Socorro Island, Mexico, is an alkaline and peralkaline volcanic island located in the eastern Pacific Ocean on a mid‐ocean ridge spreading center that was abandoned at ∼3.5 Ma. Silicic peralkaline rocks comprise up to 80% of the surface of the island, rendering Socorro virtually unique in the Pacific Ocean. Precise, replicate 40Ar/39Ar ages of 21 peralkaline trachytes and rhyolites reveal a history of episodic volcanic activity from ∼540 to 370 ka that may have culminated with caldera formation; repose periods between these episodes may have had maximum duration of ∼30 kyr. After up to 200 kyr of quiescence, 40Ar/39Ar ages indicate that postcaldera silicic peralkaline activity commenced by 180 ka, forming the Cerro Evermann Formation. Postcaldera mafic alkaline lavas of the Lomas Coloradas Formation erupted dominantly between 70 and 150 ka based upon relative age relations. The dominant lithology of precaldera and syncaldera silicic peralkaline deposits on Socorro is nonfragmental and nonvesicular and lacks lithic fragments and fiamme; despite this, numerous lines of evidence including welding zonation, presence of a proximal ignimbrite or co‐ignimbrite deposit, association with a caldera, and compositional heterogeneity within eruptive units suggest that they are dominantly ash flow tuffs. A change in eruptive style, from predominantly explosive to predominantly effusive, followed caldera formation and suggests that a change in the efficacy of magma degassing may be linked to caldera formation. On the basis of the presence of a caldera, the magma chamber associated with Socorro Island is shallow and probably resides within the upper oceanic crust or the edifice. This together with a prolonged history of silicic magmatism indicates that intrusion of mafic magma maintained thermal viability of the magmatic plumbing system. The minimum calculated growth rate for the entire volcanic edifice (7 × 10−4 km3/yr) exceeds those of nonhotspot off‐axis volcanoes in the Pacific by almost an order of magnitude. Eruption rates for subaerial phases on Socorro may be several orders of magnitude smaller than this growth rate and are comparable to subaerial eruption rates of isolated ocean islands related to mantle plumes.

Submarine eruption near Socorro Island, Mexico: Geochemistry and scanning electron microscopy studies of floating scoria and reticulite

Products of an underwater eruption near Socorro Island in the NE Pacific were observed directly on January 29, 1993, ten days after precursors were first recorded by SOFAR (Sound Fixing and Ranging) hydrophones located in Hawaii and Tahiti. Eruptive activity was noticed from boats and ships as small steam plumes rising from the sea at an area centered at 18 °48′N, 111 °05′W, 2.4 km NW of Punta Tosca and 4.6 km SSW of Cape Henslow on Socorro Island. The observed steam was produced by 1–3-m-large blocks of hot, dark-grey, highly vesiculated basalt rising buoyantly to the surface from two submarine shallow vents at 210 and 30 m depth. Tens of blocks accompanied by bubbles could be observed rising to the surface in irregular pulses. These scoriaceous blocks remained floating at the surface until they would crack into smaller pieces by thermal contraction, emitting hissing noises from vapourizing seawater in contact with the hot interior of the blocks. Steam jets several metres in height were produced and occasionally blocks were propelled laterally by the steam jet. Depending on vesicularity and permeability, blocks remained floating and drifting with the surface current for 1–15 minutes before sinking back. Floating rocks covered an area of about 6000 m2. This intermittent activity has been observed ever since and has not stopped as of April 1994. Buoyant scoria and reticulite are indicative of volatile (mostly CO2) supersaturation and exsolution in the magma prior to rapid quenching, which inhibits loss of volatiles by bubble escape. A high-velocity ascent of low-viscosity magma in a relatively narrow conduit is also required to prevent substantial gas escape and allow formation of reticulite. The buoyant scoria is most probably ejected by intermittent lava fountaining at fixed vents as a result of changes in eruption velocities due to changes in the exsolved gas content of the lava. Between January and July 1993 floating blocks of scoria and reticulite were collected on several occasions from the surface of the sea for chemical and mineralogical analyses. Major- and minor-element analyses (including REE), as well as electron microprobe analyses of different phases revealed that the composition of the emitted lava has not changed through time. Blocks of basalt (SiO2 = 45–47%) are highly vitric and vesicular with tabular anorthite phenocrysts up to 3 mm in length and minor grains of forsteritic olivine. REE and trace-element composition of these rocks suggest an anomalous mantle source for the erupted alkali basalt lava. The ongoing eruption will either continue in a similar fashion as described and eventually cease or built up a mound that reaches the surface and forms an island with accompanying change in eruptive style.

GOES weather satellite observations and measurements of the May 18, 1980, Mount St. Helens eruption

We demonstrate the use of Geostationary Operational Environmental Satellite (GOES) images of the May 18, 1980, Mount St. Helens volcanic plume in providing details of the dynamics and changing character of this major explosive eruption. Visible and thermal infrared (IR) data from a sequence of images at 30‐min intervals from 0850 to 1720 Local Time (LT) give information on dispersal and plume top temperature. Initial visible and IR images at 0850 show the top of a spreading co‐ignimbrite‐like umbrella plume and an overshooting column emerging from it, both rising off the ground‐hugging pyroclastic gravity flow generated by the opening directed blast. The overshooting column had a minimum temperature significantly colder than local ambient atmosphere, indicating substantial undercooling, and a maximum altitude of 31±2 km at 0920. This large plume system then formed a high‐velocity, radially spreading, gravitationally driven current before becoming advected in the wind field at an average downwind velocity of 29 m/s. Within 30 to 60 min the undercooled plume adjusted to ambient temperatures. After this initial event, the eruption column fluctuated over the next 8 hours between stable Plinian column rise and unstable conditions favoring column collapse, leading to pyroclastic flows and further development of co‐ignimbrite ash columns. Over this period of observation, profiles of plume top temperatures derived from IR data versus distance along the downwind axis have distinctive patterns, with steep slopes associated with co‐ignimbrite plumes and shallower slopes associated with Plinian plumes. Moreover, reflectance values from visible GOES data change from lower to higher during periods of transition from darker toned Plinian to lighter toned coignimbrite plumes indicating that in this case satellite data resolved changes in eruptive style from plumes with a coarser to a finer dominant particle size.

Geochemistry of Pliocene to Quaternary alkali basalts from the Huri Hills, northern Kenya

Pliocene to Quaternary basaltic lavas from the Huri Hills show a continuous change in eruptive style from earlier fissure-type eruptions producing extensive lava flows to later central-type activity, resulting in cinder cones and associated small lava flows. This change in eruptive style was accompanied by systematic variations in chemical composition and isotopic signature. With decreasing age, rock compositions change from alkali basalt to basanite (CIPW-normative nepheline increases from 3% to 22%). Whereas chondrite-normalised La abundances increase from ∼ 40 to 200, Yb stays almost constant at ∼ 10× chondritic. Concurrently, samples with Mg#>64 exhibit an increase in chondrite-normalised Tb/Yb and Zr/Y, as well as a decrease in Sc and Ti/Zr. Since no correlation between any of these parameters and the CaO Al 2 O 3 ratio or Mg# is observed, the systematic temporal variations in trace-element ratios of slightly fractionated magmas can best be explained by an increasing amount of garnet in the residue of the melts. With decreasing age and increasing CIPW-normative nepheline, Huri Hills lavas show decreasing 87 Sr 86 Sr ratios (0.7033-0.703) and increasing 143 Nd 144 Nd (0.5129-0.51295) and 206 Pb 204 Pb (18.7–19.3) ratios. Concurrently, ratios such as Ba/Th, K/La and Sr/Nd decrease. These covariations suggest binary mixing of two chemically and isotopically distinct end-member compositions. The first end-member, with high 206 Pb 204 Pb ratios, is most probably derived from a plume source with HIMU affinities. The second end-member, showing low 206 Pb 204 Pb , and high 87 Sr 86 Sr , Ba/Nb and Sr/Nd ratios, is tentatively attributed to the lithospheric mantle but could also be a second plume component.

Some remarks on volcanic vent evolution during plinian eruptions

The magma mass discharge rate governs the eruption magnitude of plinian eruptions, and depends strongly on conduit geometry. The presence of lithic clasts in pumice deposits results largely from conduit erosion, and the magma discharge rate and the lithic abundance in the resulting deposit are parameters linked by the overall vent widening rate. Many pumice fall deposits have lithic-enriched basal and top sections; pumice flow deposits tend to carry more lithics than the preceding fall layer. Theoretical models of vent evolution for a cylindrical conduit with radius R indicate an R 4 dependency of the magma discharge rate, which would lead to rapidly decreasing lithic abundances from bottom to top in plinian fall deposits, and from fall to flow deposits, contrary to observations. To simulate observed lithic abundance patterns, a model of conduit evolution with a stepped geometry was developed, in which the lower conduit section remains at approximately constant radius, while the upper conduit section widens. The mass discharge rate during an eruption then increases with R 2 top , and predicted lithic abundance patterns agree qualitatively with field data. Field evidence for conduit widening in the upper conduit section only is presented from plinian pumice beds from Nisyros, Greece. The change in eruption style from plinian fall to surge and ash flow is sometimes accompanied by a dramatic change in lithic abundance and lithic type. In these cases, this transition in deposition style is not the result of a gradual increase in mass discharge rate because of overall vent widening, but is caused by catastrophic rupture of the lower vent section during incipient caldera collapse, with resulting changes in magma discharge rates, lithic abundance and possibly water/magma interaction.

Groundmass crystallization of Mount St. Helens dacite, 1980?1986: a tool for interpreting shallow magmatic processes

The 1980–1986 eruption of Mount St. Helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale. Groundmass plagioclase size measurements are coupled with measured changes in matrix glass, plagioclase and Fe−Ti oxide chemistry to document increasing groundmass crystallinity, and thus to better constrain proposed physical models of the post-May 18, 1980 magmatic reservoir. Measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth (approximately 10-9 cm/s) of groundmass plagioclase occurred immediately subsequent to May 18. Relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10-11 cm/s; plagioclase growth rates then decreased to <1x10-11 cm/s through 1986. Changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst-rim chemistry, although the matrix glass composition appears to have remained approximately constant from 1981–1986 after a rapid compositional change from May 18 until the end of 1980. Plagioclase microphenocrysts show increasingly more complex zoning patterns with time; microphenocryst-core compositions are commonly positively correlated with crystal size. Both of these observations indicate continuous groundmass plagioclase growth through 1986. Magmatic temperatures estimated from Fe−Ti oxide pairs are approximately constant through 1981 at eruption temperatures of ∼ 930°C and at log fO2 of -10.8; by 1985–1986 oxide temperatures decreased to ∼ 870°C. Chemical and textural changes can be explained by: (1) rapid degassing and crystallization in response to the intrusion of magma into a shallow (<4.5 km) reservoir toward the end of the May 18, 1980 eruption; (2) continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir. Growth rates (and consequent chemical changes) appear to decrease at the end of 1980—this is coincident with the change in eruption style from explosive eruptions, sometimes followed by dome growth, to solely extrusive (dome-building) events, and can be explained by the expected viscosity increase of both degassing and increasing crystallinity. The model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies (Casadevall et al. 1983; Gerlach and Casadevall 1986 a, b), patterns of crater deformation (Chadwkck et al. 1988) and post-1980 seismicity (Endo et al. 1990), although it does not explain the experimental data of Hill and Rutherford (1989) on the growth rate of amphibole reaction rims. Textural measurements on Mount St. Helens dacite can also be used to evaluate crystallization kinetics in silicic magmas, systems for which experimental data is almost non-existent. Plagioclase growth rates are 5–10 times slower than estimated plagioclase growth rates in basaltic systems, a result consistent with the higher viscosity of a more silicic melt. Furthermore, patterns of textural change (both average crystal size and number density) are similar to those observed during the 1984 Mauna Loa eruption by Lipman and Banks (1987), suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates, such that the rate of crystallization scales with the melt viscosity.

The influence of geometry on the ascent of magma in open fissures

During steady eruption, the flow conditions (emitted mass flux, exit velocity and exit pressure) depend on the geometry of the conduit in which the eruption occurs. This dependence is examined for the onedimensional, isothermal ascent of a homogeneous basaltic magma with an aqueous volatile phase and newtonian rheology. By fixing the geometry of the feeding fissure, the mass flux flowing in steady conditions can be determined at any depth, as well the magma pressure and vertical velocity. Flow behaviour is analysed for three fissure shapes: constant width, slowly upward narrowing and lenticular. In all the cases examined the magma arrives at the earth's surface with a pressure greater than atmospheric. The results are compared with those obtained when a lithostatic pressure gradient is assumed for the magma column. Some speculations are made, moreover, about the change in eruption style, if conduit geometry varies during a non-steady phase.

The May 1915 eruptions of Lassen Peak, II: May 22 volcanic blast effects, sedimentology and stratigraphy of deposits, and characteristics of the blast cloud

Abstract The May 22, 1915 eruptions of Lassen Peak involved a volcanic blast and the emplacement of three geographically and temporally distinct lahar deposits. The volcanic blast occurred when a Vulcanian explosion at the summit unroofed a shallow magma source, generating an eruption cloud that rose to an estimated height of 9 km above sea level. The blast cloud was probably caused by the collapse of a small portion of the eruption column; absence of a flank vent associated with these eruptions argues against it originating as an explosion that has been directed by vent geometry or location. The volcanic blast devasted ∼7 km2 of the northeast flank of the volcano, and emplaced a deposit of juvenile tephra and accidental lithic and mineral fragments. Decrease in blast deposit thickness and median grain size with increasing distance from the vent suggests that the blast cloud lost transport competence as it crossed the devastated area. Scanning electron microscope examination of pyroclasts from the blast deposit indicates that the blast cloud was a dry, turbulent suspension that emplaced a thin deposit which cooled rapidly after deposition. Lahar deposits were emplaced primarily in Lost Creek, with minor lahars flowing down gullies on the west, northwest and north flanks of the volcano. The initial lahar was apparently triggered early in the eruption when the blast cloud melted the residual snowpack as it moved down the northeast flank of the peak. The event that triggered the later lahars is enigmatic; the presence of approximately five times more juvenile dacite bombs on the surface of the later lahars suggests that they may have been triggered by a change in eruption style or dynamics.

The entrainment of high-viscosity magma into low-viscosity magma in eruption conduits

We report experiments on the flow of two fluids of contrasting viscosity through a pipe in which low-viscosity fluid occupies the center of the pipe. The volume flux of the low-viscosity fluid in the pipe increased during an experiment but did not reach 100% in most cases. The transition from high- to low-viscosity-dominated outflow involved a drop in pressure gradient and an increase in flow rate due to reduced viscous resistance in the pipe. Initially, the central flow was thin and parallel-sided, but as its diameter increased the flow became unstable. A sequence of instabilities was observed during the course of each experiment, both in time and as a function of height in the pipe. In the most commonly observed instability the central flow adopted a helical geometry. The transition from parallel-sided to unstable flow first appeared at the top of the pipe and propagated downwards against the flow. Axisymmetric instabilities originating at the pipe entrance were also observed. All forms of instability exhibited entrainment of viscous fluid into the faster moving central flow. Entrainment was extensive early in the existence of the central flow, but later on the volume flux of lower-viscosity fluid in the central flow rose more rapidly than the rate of entrainment and the proportion of lower-viscosity fluid increased with time. These compositional changes determined the viscosity of the central flow which was found to control its diameter and velocity. In banded pumice deposits, silicic pumice without mafic component is commonly erupted alongside banded pumice blocks. We infer that banded pumice may correspond to the central flow in our experiments, i. e., that viscous magma has been incorporated into less viscous melt, and that pure acid pumice is derived from the outer flow. Changes in eruption style may be caused by variations in pressure gradient and flow rate due to changes in the viscosity of the melt in the conduit. Varied mafic/silicic proportions and degree of mixing in magmatic associations are controlled by the bulk volume erupted, discharge rate, initial temperature difference and aspect ratio of the conduit.

Physical character of archean felsic volcanism in the vicinity of the Helen iron Mine, Wawa, Ontario, Canada

Archean felsic volcanic rocks form a 2000 m thick succession stratigraphically below the Helen Iron Formation in the vicinity of the Helen Mine, Wawa, Ontario. Based on relict textures and structures, lateral and vertical facies changes, and fragment type, size and distribution, the felsic volcanic rocks have been subdivided into (a) lava flows and domes (b) hyalotuffs, (c) bedded pyroclastic flows, (d) massive pyroclastic flows, and (e) block and ash flows. Lava flows and domes are flow-banded, massive, and/or brecciated and occur throughout the stratigraphic succession. Dome/flow complexes are believed to mark the end of explosive eruptive cycles. Deposits interpreted as hyalotuffs are finely bedded and composed dominantly of ash-size material and accretionary lapilli. These deposits are interlayered with bedded pyroclastic flow deposits and probably formed from phreatomagmatic eruptions in a shallow subaqueous environment. Such eruptions led to the formation of tuff cones or rings. If these structures emerged they may have restricted the access of seawater to the eruptive vent(s), thus causing a change in eruptive style from short, explosive pulses to the establishment of an eruption column. Collapse of this column would lead to the accumulation of pyroclastic material within and on the flanks of the cone/ring structure, and to flows which move down the structure and into the sea. Bedded pyroclastic deposits in the Wawa area are thought to have formed in this manner, and are now composed of a thicker, more massive basal unit which is overlain by one or more finely bedded ash units. Based on bed thickness, fragment and crystal size, type and abundance, these deposits are further subdivided into central, proximal and distal facies. Central facies units consist of poorly graded, thick (30–80 m) basal beds composed of 23–60% lithic and 1–8% juvenile fragments. These are overlain by 1–4 thinner ash beds (2–25 cm). Proximal facies basal beds range from 2–35 m in thickness and are composed of 15–35% lithic and 4–16% juvenile fragments. Typically, lithic components are normally graded, whereas juvenile fragments are inversely graded. These basal beds are overlain by ash beds (2–14 in number) which range from 12 cm to 6 m in thickness. Distal basal beds, where present, are thin (1–2 m), and composed of 2–8% lithic and 6–21% juvenile fragments. Overlying ash beds range up to 40 in number. The climax of pyroclastic activity is represented by a thick (1000 m) sequence of massive, poorly sorted, pyroclastic flow deposits which are composed of 5–15% lithic fragments and abundant pumice. These deposits are similar to subaerial ash flows and appear to mark the rapid eruption of large volumes of material. They are overlain by felsic lavas and/or domes. Periodic collapse of the growing domes produced abundant coarse volcanic breccia. The overall volcanic environment is suggestive of caldera formation and late stage dome extrusion.

Underwater detection system

Products of an underwater eruption near Socorro Island in the NE Pacific were observed directly on January 29, 1993, ten days after precursors were first recorded by SOFAR (Sound Fixing and Ranging) hydrophones located in Hawaii and Tahiti. Eruptive activity was noticed from boats and ships as small steam plumes rising from the sea at an area centered at 18 °48′N, 111 °05′W, 2.4 km NW of Punta Tosca and 4.6 km SSW of Cape Henslow on Socorro Island. The observed steam was produced by 1–3-m-large blocks of hot, dark-grey, highly vesiculated basalt rising buoyantly to the surface from two submarine shallow vents at 210 and 30 m depth. Tens of blocks accompanied by bubbles could be observed rising to the surface in irregular pulses. These scoriaceous blocks remained floating at the surface until they would crack into smaller pieces by thermal contraction, emitting hissing noises from vapourizing seawater in contact with the hot interior of the blocks. Steam jets several metres in height were produced and occasionally blocks were propelled laterally by the steam jet. Depending on vesicularity and permeability, blocks remained floating and drifting with the surface current for 1–15 minutes before sinking back. Floating rocks covered an area of about 6000 m2. This intermittent activity has been observed ever since and has not stopped as of April 1994. Buoyant scoria and reticulite are indicative of volatile (mostly CO2) supersaturation and exsolution in the magma prior to rapid quenching, which inhibits loss of volatiles by bubble escape. A high-velocity ascent of low-viscosity magma in a relatively narrow conduit is also required to prevent substantial gas escape and allow formation of reticulite. The buoyant scoria is most probably ejected by intermittent lava fountaining at fixed vents as a result of changes in eruption velocities due to changes in the exsolved gas content of the lava. Between January and July 1993 floating blocks of scoria and reticulite were collected on several occasions from the surface of the sea for chemical and mineralogical analyses. Major- and minor-element analyses (including REE), as well as electron microprobe analyses of different phases revealed that the composition of the emitted lava has not changed through time. Blocks of basalt (SiO2 = 45–47%) are highly vitric and vesicular with tabular anorthite phenocrysts up to 3 mm in length and minor grains of forsteritic olivine. REE and trace-element composition of these rocks suggest an anomalous mantle source for the erupted alkali basalt lava. The ongoing eruption will either continue in a similar fashion as described and eventually cease or built up a mound that reaches the surface and forms an island with accompanying change in eruptive style.

The ejection of protons from nitrogen nuclei, photographed by the Wilson Method : P. M. S. Blackett. ( Proc. Royal Soc., A 742.)

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. 23,540 years 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 2.5 m 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 275 km2, whereas MM was dispersed to the north (N22°E), with its 70-cm isopach covering an area of ca. 330 km2. 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 15 vol.%. Pumice compositions range from basaltic to andesitic (48–61 wt.% 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 19 km in height, ejecting 2.9 km3 of tephra (1.1 km3 DRE) at an average mass discharge rate of 4.7 × 107 kg/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 4.6 km3 (1.9 km3 DRE) of tephra at a mass discharge rate of 8.1 × 107 kg/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.