Variations in column height and magma discharge during the May 18, 1980 eruption of Mount St. Helens

Peak eruption column heights for the B1, B2, B3 and B4 units of the May 18, 1980 fall deposit from Mount St. Helens have been determined from pumice and lithic clast sizes and models of tephra dispersal. Column heights determined from the fall deposit agree well with those determined by radar measurements. B1 and B2 units were derived from plinian activity between 0900 and about 1215 hrs. B3 was formed by fallout of tephra from plumes that rose off pyroclastic flows from about 1215 to 1630 hrs. A brief return to plinian activity between 1630 and 1715 hrs was marked by a maximum in column height (19 km) during deposition of B4.Variations in magma discharge during the eruption have been reconstructed from modelling of column height during plinian discharge and mass-balance calculations based on the volume of pyroclastic flows and coignimbrite ash. Peak magma discharge occurred during the period 1215–1630 hrs, when pyroclastic flows were generated by collapse of low fountains through the crater breach. Pyroclastic flow deposits and the widely dispersed co-ignimbrite ash account for 77% of the total erupted mass, with only 23% derived from plinian discharge.A shift in eruptive style at noon on May 18 may have been associated with increase in magma discharge and the eruption of silicic andesite mingled with the dominant mafic dacite. Increasing abundance of the silicic andesite during the period of highest magma discharge is consistent with the draw-up and tapping of deeper levels in the magma reservoir, as predicted by theoretical models of magma withdrawal. Return to plinian activity late in the afternoon, when magma discharge decreased, is consistent with theoretical predictions of eruption column behavior. The dominant generation of pyroclastic flows during the May 18 eruption can be attributed to the low bulk volatile content of the magma and the increasing magma discharge that resulted in the transition from a stable, convective eruption column to a collapsing one.     

Physical properties of the 1980 Mount St. Helens cryptodome magma

 Physical properties of cryptodome and remelted samples of the Mount St. Helens grey dacite have been measured in the laboratory. The viscosity of cryptodome dacite measured by parallel–plate viscometry ranges from 10.82 to 9.94 log₁₀ η (Pa s) (T=900–982  °C), and shrinkage effects were dilatometrically observed at T>900  °C. The viscosity of remelted dacite samples measured by the micropenetration method is 10.60–9.25 log₁₀ η (Pa s) (T=736–802  °C) and viscosities measured by rotational viscometry are 3.22–1.66 log₁₀ η (Pa s) (T=1298–1594  °C). Comparison of the measured viscosity of cryptodome dacitic samples with the calculated viscosity of corresponding water-bearing melt demonstrates significant deviations between measured and calculated values. This difference reflects a combination of the effect of crystals and vesicles on the viscosity of dacite as well as the insufficient experimental basis for the calculation of crystal-bearing vesicular melt viscosities at low temperature. Assuming that the cryptodome magma of the 18 May 1980 Mount St. Helens eruption was residing at 900  °C with a phenocryst content of 30 vol.%, a vesicularity of 36 vol.% and a bulk water content of 0.6 wt.%, we estimate the magma viscosity to be 10^10.8 Pa s. Received: 25 August 1996 / Accepted: 19 July 1997     

Multiple levels of magma storage during the 1980 summer eruptions of Mount St. Helens, WA

Transitions in eruptive style—explosive to effusive, sustained to pulsatory—are a common aspect of volcanic activity and present a major challenge to volcano monitoring efforts. A classic example of such transitions is provided by the activity of Mount St. Helens, WA, during 1980, where a climactic Plinian event on May 18 was followed by subplinian and vulcanian eruptions that became increasing pulsatory with time throughout the summer, finally progressing to episodic growth of a lava dome. Here we use variations in the textures, glass compositions and volatile contents of melt inclusions preserved in pyroclasts produced by the summer 1980 eruptions to determine conditions of magma ascent and storage that may have led to observed changes in eruptive activity. Five different pyroclast types identified in pyroclastic flow and fall deposits produced by eruptions in June 12, July 22 and August 7, 1980, provide evidence for multiple levels of magma storage prior to each event. Highly vesicular clasts have H₂O-rich (4.5–5.5 wt%) melt inclusions and lack groundmass microlites or hornblende reaction rims, characteristics that require magma storage at P≥160 MPa until shortly prior to eruption. All other clast types have groundmass microlites; P_H20 estimated from both H₂O-bearing melt inclusions and textural constraints provided by decompression experiments suggest pre-eruptive storage pressures of ∼75, 40, and 10 MPa. The distribution of pyroclast types within and between eruptive deposits can be used to place important constraints on eruption mechanisms. Fall and flow deposits from June 12, 1980, lack highly vesicular, microlite-free pyroclasts. This eruption was also preceded by a shallow intrusion on June 3, as evidenced by a seismic crisis and enhanced SO₂ emissions. Our constraints suggest that magma intruded to a depth of ≤4 km beneath the crater floor fed the June eruption. In contrast, eruptions of July and August, although shorter in duration and smaller in volume, erupted deep volatile-rich magma. If modeled as a simple cylinder, these data require a step-wise decrease in effective conduit diameter from 40–50 m in May and June to 8–12 m in July and August. The abundance of vesicular (intermediate to deep) clast types in July and August further suggests that this change was effected by narrowing the shallower part of the conduit, perhaps in response to solidification of intruded magma remaining in the shallow system after the June eruption. Eruptions from July to October were distinctly pulsatory, transitioning between subplinian and vulcanian in character. As originally suggested by Scandone and Malone (1985), a growing mismatch between the rate of magma ascent and magma disruption explains the increasingly pulsatory nature of the eruptions through time. Recent fragmentation experiments Spieler et al. (2004) suggest this mismatch may have been aided by the multiple levels at which magma was stored (and degassed) prior to these events.Editorial responsibility: J Stix     

Magma supply,magma ascent and the style of volcanic eruptions

We propose a new way of looking at the sequence of events leading to different styles of silicic, volcanic eruptions. Small-to-medium sized eruptions, either explosive or effusive, are explained by the ascent of isolated magma batches from mid-crustal magma chambers. We separate magma ascent into four different zones: the Supply System, the Intermediate Storage System, the Transport System and the Eruptive System. Of primary importance is the concept that ascent from the Intermediate Storage System through the Transport System to the Eruptive System first requires the development of a fracture network. Initially, this fracture network allows the ascent of individual magma batches by opening and then closing after their passage. An increase in the complexity of the fracture network with time increases the connectivity of the fractures and hence the ease of upward magma movement. In this model, the dynamics of the ensuing eruptions are controlled entirely by the time spent in the Transport System. Large explosive eruptions require a full interconnectivity of the Transport System from the Intermediate Storage System to the Eruptive System. Moreover, we suggest that a fully connected conduit is rare, develops only under particular conditions, and typically generates catastrophic eruptions during formation. Here we examine two case histories that illustrate the interplay of these processes: Mt St. Helens, USA, between 1980 and 2004, and Mt. Pinatubo, Philippines, in 1991.     

Using amphibole phenocrysts to track vapor transfer during magma crystallization and transport: An example from Mount St. Helens,Washington

In order to evaluate and further constrain models for volatile movement and vapor enrichment of magma stored at shallow levels, amphibole phenocrysts from 2004–2005 Mount St. Helens dacite were analyzed for major and selected trace elements (Li, Cu, Zn, Mn, and REE) and Li isotopes. Several recent studies have examined fluid-mobile trace element abundances in phencryst phases and melt inclusions as a means of tracking volatile movement within subvolcanic magmatic systems, and high Li contents in plagioclase phenocrysts from 1980 and 2004 Mount St. Helens dacites have been interpreted as evidence that shallow magma was fluxed by a Li-bearing vapor phase prior to eruption.     

Simulation of the 1980 eruption of Mount St. Helens using the ash-tracking model PUFF

The dispersal of volcanic ash from the May 18, 1980 eruption of Mount St. Helens (MSH) has been simulated using the Lagrangian ash-tracking model PUFF. Previous applications of the model were limited to smaller, short-lived eruptions with ash dispersal occurring mainly within the troposphere. Two high-resolution atmospheric reanalysis datasets (ERA-40 and NCEP/NCAR-40) allowed MSH ash cloud dispersal to be simulated up to 30 km elevation. The 1980 eruption was divided into two distinct eruptive phases, (1) an initial, relatively short-lived blast/surge phase that injected ash up to 30 km and (2) a subsequent nine-hour plinian phase that maintained an average eruption column height of 16 km. Using PUFF, the two phases of the MSH eruption were modeled separately based on a range of individual input parameters and then combined to produce an integrated simulation of the entire eruption. The trajectory and areal extent of the modeled atmospheric ash cloud best match the actual distribution of MSH ash when input parameters are set to values inferred from satellite and radar data collected on May 18, 1980. The prevailing wind field exerts the strongest control on the advection and ultimate position of the modeled ash cloud, making the maximum column height and the vertical distribution of ash the most sensitive of the PUFF input parameters for this event. The results indicate that the PUFF model works well at simulating the dispersal of ash injected well into the lower stratosphere from a moderate, relatively long-lived eruption, such as MSH. However, attempts to use PUFF to recreate some granulometric aspects of the MSH fallout deposit, such as the maximum particle size as a function of distance from source, were not successful. PUFF consistently predicts much greater fallout distances for small ash particles (< 500 µm) than actually observed in the MSH deposit. The effective settling velocities used by the PUFF model appear to be too slow to accurately predict fallout distances of small ash particles. As a consequence the PUFF model may overestimate the duration of ash loading in the atmosphere associated with the distal fine ash component of explosive eruptions.     

Thirty years of tephra erosion following the 1980 eruption of Mount St. Helens

Repeated measurement of tephra erosion near Mount St. Helens over a 30-year period at steel stakes, installed on 10 hillslopes in the months following the 1980 eruption, provides a unique long-term record of changing processes, controls and rates of erosion. Intensive monitoring in the first three post-eruption years showed erosion declined rapidly as processes shifted from sheetwash and rilling to rainsplash. To test predictions about changes to long-term rates and processes made based on the 3-year record, we remeasured sites in 1992, 2000 and 2010. Average annual erosion from 1983 to 1992 averaged 3.1 mm year⁻¹ and ranged from 1.4 to 5.9 mm year⁻¹, with the highest rate on moderately steep slopes. Stakes in rills in 1983 generally recorded deposition as the rills became rounded, filled and indistinct by 1992, indicating a continued shift in process dominance to rainsplash, frost action and bioturbation. Recovering plants, where present, also slowed erosion. However, in the second and third decades even unvegetated hillslopes ceased recording net measurable erosion; physical processes had stabilized surfaces from sheetwash and rill erosion in the first few years, and they appear to have later stabilized surfaces against rainsplash erosion in the following few decades. Comparison of erosion rates with suspended sediment flux indicates that within about 6 years post-eruption, suspended sediment yield from tephra-covered slopes was indistinguishable from that in forested basins. Thirty years after its deposition, on moderate and gentle hillslopes, most tephra remained; in well-vegetated areas, plant litter accumulated and soil developed, and where the surface remained barren, bioturbation and rainsplash redistributed and mixed tephra. These findings extend our understanding from shorter-term studies of the evolution of erosion processes on freshly created substrate, confirm earlier predictions about temporal changes to tephra erosion following eruptions, and provide insight into the conditions under which tephra layers are preserved. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.     

Emplacement of the 18 May 1980 lateral blast deposit ENE of Mount St. Helens,Washington

Facies variations east-northeast of Mount St. Helens preserve a record of depositional processes in the 18 May 1980 lateral blast cloud. This paper reports new field, grain-size and component data from the ENE sector of the timber-blowdown zone and presents a model for blast flow and sedimentation. The first-erupted ejecta was rich in juvenile components and extends to the distal blowdown limit. The last-erupted ejecta was rich in accidental lithics and reached no further than a few kilometres from the mountain due to waning discharge. The blast cloud was a turbulent stratified flow which transported and deposited sediment in the manner of a high-density turbidity current. The possibility that the blast was emplaced as a giant shearing fluidised bed is not favoured by compositional zoning patterns. Depositional conditions were strongly influenced by the rate of suspended-load fallout from the blast. Within about 8 km from vent rapid sedimentation caused deposition under moderate to high concentration conditions and formation of a basal hindered-settling zone able to detach gravitationally and drain into local depressions. The resulting proximal facies resembles a low-aspect-ratio ignimbrite. Fines depletion in the proximal facies is attributed to a combination of residual turbulence and rapid gas escape during particle settling and compaction through the hindered-settling zone. Component data suggest that the blast head played no significant role in the generation of fines depletion in the blast deposit as suggested by previous workers. With increasing distance from vent the rate of particle fallout declined and sedimentation took place under increasingly dilute and tractional conditions, building up antidune-like bedforms. Wavelengths of these bedforms range from 20 to <1 m, and decrease away from vent. There is a systematic relationship between antidune migration direction and depositional slope. The transition from proximal (ignimbrite-like) to distal (surge-like) facies suggests a possible gradation in transport and deposition processes between conventional pyroclastic surges and high-velocity pyroclastic flows.     

Bimodal Density Distribution of Cryptodome Dacite from the 1980 Eruption of Mount St. Helens,Washington

The explosion of a cryptodome at Mount St. Helens in 1980 produced two juvenile rock types that are derived from the same source magma. Their differences-color, texture and density-are due only to vesicularity differences. The vesicular gray dacite comprises bout 72% of the juvenile material; the black dacite comprises the other 28%. The density of juvenile dacite is bimodally distributed, with peaks at 1.6 g cm^-3 (gray dacite) and 2.3 g cm^-3 (black dacite). Water contents, deuterium abundances, and the relationship of petrographic structures to vapor-phase crystals indicate both rock types underwent pre-explosion subsurface vesiculation and degassing. The gray dacite underwent a second vesiculation event, probably during the 18 May explosion. In the subsurface, gases probably escaped through interconnected vesicles into the permeable volcanic edifice. We suggest that nonuniform degassing of an initially homogeneous magma produced volatile gradients in the cryptodome and that these gradients were responsible for the density bimodality. That is, water contents less than about 0.2–0.4 wt% produced vesicle growth rates that were slow in comparison to the pyroclast cooling rates; greater water contents produced vesicle growth rates that were fast in comparison to cooling rates. In this scheme, the dacite densities are bimodally distributed simply because, following decompression on 18 May 1980, one clast population vesiculated while the other did not. For clasts that did vesiculate, vesicle growth continued until it was arrested by fragmentation.     

Summary of pre-1980 tephra-fall deposits erupted from Mount St. Helens,Washington State,USA

Mount St. Helens has been a prolific source of tephra-fall deposits for about 40 000 years. These tephra deposits (1) record numerous explosive eruptions, (2) form important regional time-stratigraphic marker beds, and (3) record repeated changes in composition within and between eruptive periods.Recognized tephra strata record more than 100 explosive eruptive events at Mount St. Helens; those tephra strata are classified as beds, layers, and sets. Tephra sets, each of which consists of a group of beds and layers, define in part the nine eruptive periods recognized at the volcano. Individual tephra sets are distinguished from stratigraphically adjacent sets by differences in composition or by evidence of clapsed time.Several tephra units from Mount St. Helens form important marker beds at distances of hundreds of kilometers downwind from the volcano. Cummingtonite phenocrysts, which are known in ejecta from only Mount St. Helens in the Pacific Northwest, characterize some marker beds and readily identify their source.The tephra sequence also records eruption of the mafic andesites that mark the appearance of the modern Mount St. Helens and numerous changes in composition among dacite, basalt, and andesite since that time.     

Leaching characteristics of ash from the May 18, 1980, eruption of Mount St. Helens volcano,Washington

Leaching of freshly erupted air-fall ash, unaffected by rain, from the May 18, 1980, eruption of Mount St. Helens volcano, Washington, shows that Ca²⁺, Na⁺, Mg²⁺, SO ₄ ^2? , and Cl^? are the predominant chemical species released on first exposure of the ash to water. Extremely high correlation of Ca with SO₄ and Na with Cl in water leachates suggests the presence of CaSO₄ and NaCl salts on the ash. The amount of water soluble material on ash increases with distance from source and with the weight fraction of small (less than 63 micrometers) ash particles of high-surface area. This suggests that surface reactions such as adsorption are responsible for concentrating the soluble material. CaSO₄, NaCl, and other salts are probably formed as microscopic crystals in the high-temperature core of the eruption column and are then adsorbed by silicate ash particles. The environmentally important elements Zn, Cu, Cd, F. Pb, and Ba are released by a water leach in concentrations which could pose short-term hazards to some forms of aquatic life. However, calculated concentrations are based on a water-to-ash ratio of 4:1 or less, which is probably an underestimation of the regionally operative ratio. A subsequent leach of ash by warm alkaline solution shows dramatic increases, in the amount of dissolved SiO₂, U, and V, which are probably caused by increased dissolution of the glassy component of ash. Glass dissolution by alkaline ground water is a mechanism for providing these three elements to sedimentary traps where they may coaccumulate as uraniferous silica or U-V minerals. Leaching characteristics of ash from Mount St. Helens are comparable to characteristics of ash of similar composition from volcanoes in Guatemala. Ashes from each locality show similar ions predominating for a given leachate and similar fractions of a particular element in the ash removed on contact with the leach solution.     

The initial giant umbrella cloud of the May 18th, 1980, explosive eruption of Mount St. Helens

The initial eruption column of May 18th, 1980 reached nearly 30 km altitude and released 10¹⁷ joules of thermal energy into the atmosphere in only a few minutes. Ascent of the cloud resulted in forced intrusion of a giant umbrella-shaped cloud between altitudes of 10 and 20 km at radial horizontal velocities initially in excess of 50 m/s. The mushroom cloud expanded 15 km upwind, forming a stagnation point where the radial expansion velocity and wind velocity were equal. The cloud was initiated when the pyroclastic blast flow became buoyant. The flow reduced its density as it moved away from the volcano by decompression, by sedimentation, and by mixing with and heating the surrounding air. Observations indicate that much of the flow, covering an area of 600 km², became buoyant within 1.5 minutes and abruptly ascended to form the giant cloud. Calculations are presented for the amount of air that must have been entrained into the flow to make it buoyant. Assuming an initial temperature of 450°C and a magmatic origin for the explosion, these calculations indicate that the flow became buoyant when its temperature was approximately 150°C and the flow consisted of a mixture of 3.25 × 10¹¹ kg of pyroclasts and 5.0 × 10¹¹ kg of air. If sedimentation is considered, these figures reduce to 1.1 × 10¹¹ kg of pyroclasts and 1.0 × 10¹¹ kg of air.     

Temporal geochemical variations in volatile emissions from Mount St. Helens, USA, 1980–1994

Fumarole discharges (95–560°C) collected from the dacite dome inside Mount St. Helens crater show temporal changes in their isotopic and chemical compositions. A δD vs. δ¹⁸O plot shows that condensed waters from the gases are mixtures of meteoric and magmatic components, but that the apparent magmatic end-member in 1994 was depleted by about 7‰ in δD relative to the apparent end-member in 1980. Based on δD modeling, approximately 63% of shallow, post-1980 magma has yet to degas. Surprisingly, Cl and F contents in the 1994 samples were only 0.47 and 3.8%, respectively, of the concentrations determined for end-member magmatic fluid in 1980. The data indicate that Cl (and F and B) is degassed from magma relatively quickly compared to water and/or that most of the Cl degassed in later years is dissolved into the shallow Mount St. Helens hydrothermal system. Because metals are often transported in magmatic and hydrothermal fluids as Cl complexes, rapid changes in surface volatile compositions may have implications for the timing and location of metals transport and deposition in some volcanoes.     

Post-eruption erosion and deposition in the 1980 crater of Mount St Helens,Washington, determined from digital maps

The crater of Mount St Helens shows one of the world's highest known rates of mass wasting. On many summer days, rockfall is almost continuous, and many large rock and dirty-snow avalanches have travelled several kilometres from their sources on the crater walls. Since formation of the crater on 18 May 1980, talus cones exceeding 100 m in thickness have formed at the base of the unstable 600 m high crater walls. To estimate rates of erosion and deposition, a series of digitized topographic maps made from aerial photographs taken of the crater in 1980, 1981, 1983, 1986 and 1988 were analysed using a geographic information system. Between 1980 and 1988, 30 × 10⁶ m³ of rock were eroded from the crater wall, representing a mean retreat rate of 2.1 m yr^?1. To account for the volume increase that occurs when bedrock is transformed into scree, this volume is multiplied by 4/3; this provides an estimate of the rock-debris volume supplied to the crater floor of 40 × 10⁶ m³. The actual volume of deposits that accumulated during this 8 year period, however, is 68 × 10⁶ m³. The difference of 28 × 10⁶ m³ is presumably the volume of snow intercalated between insulating layers of rock debris. Similar calculations for each of four time intervals between 1980 and 1988 suggest that wall erosion and thus talus accumulation rates are declining, but that rates will probably remain high for decades to come.     

Crystallization of Mount St. Helens 1980–1986 dacite: A quantitative textural approach

Quantitative measurements of crystal size distributions (CSDs) have been used to obtain kinetic information on crystallization of industrial compounds (Randolph and Larson 1971) and more recently on Hawaiian basalts (Cashman and Marsh 1988). The technique is based on a population balance resulting in a differential equation relating the population densityn of crystals to crystal sizeL, i.e., at steady staten =n ^o exp(–L/itG), wheren ^o is nucleation density,G is the average crystal growth rate, is the average growth time, and the nucleation rateJ =n ^o G. CSD (Inn vsL) plots of plagioclase phenocrysts in 12 samples of Mount St. Helens blast dacite and 14 samples of dacite from the 1980–1986 Mount St. Helens dome are similar and averageG = 9.6 (± 1.1) × 10^–3 cm andn ^o = 1–2 × 10⁶ cm^–4. Reproducibility of the measurements was tested by measuring CSDs of 12 sections cut from a single sample in three mutually perpendicular directions; precision of the size distributions is good in terms of relative, but not necessarily absolute values (± 10%). Growth and nucleation rates for plagioclase have been calculated from these measurements using time brackets of = 30–150 years; growth ratesG are 3–10 × 10^–12cm/s, and nucleation ratesJ are 5–21 × 10^–6/cm³ s.G andJ for Fe-Ti oxides calculated from CSD data areG = 2–13 ± 10^–13 cm/sec andJ = 7–33 × 10^–5/cm³ s, respectively. The higher nucleation rate and lower growth rate of oxides resulted in a smaller average crystal size than for plagioclase. Sizes of plagioclase microlites (<0.01 mm) in the blast dacite groundmass have been measured from backscatter SEM photographs. Nucleation of these microlites was probably triggered by intrusion of material into the cone of Mount St. Helens in spring 1980. This residence time of 52 days gives minimum crystallization estimates ofG = 1–3 × 10^–11 cm/s andJ = 9–16 × 1O³/cm³ s. The skeletal form of the microlites provides evidence for nucleation and growth at high values of undercooling (T) relative to the phenocrysts. A comparison of nucleation and growth rates for the two crystal populations (phenocrysts vs microlites) suggests that while growth rate seems to be only slightly affected by changes inT, nucleation rate is a very strong function of undercooling. A comparison of plagioclase nucleation and growth rates measured in the Mount St. Helens dacite and in basalt from Makaopuhi lava lake in Hawaii suggests that plagioclase nucleation rates are not as dependent on composition. Groundmass textures suggest that plagioclase microphenocrysts crystallized at depth rather than in the conduit, in the dome, or after extrusion onto the surface. Most of this crystallization appears to be in the form of crystal growth (coarsening) of groundmass microphenocrysts at small degrees of undercooling rather than extensive nucleation of new crystals. This continuous crystallization in a shallow magmatic reservoir may provide the overpressurization needed to drive the continuing periodic domebuilding extrusions, which have been the pattern of activity at Mount St. Helens since December 1980.     

Mount St. Helens a decade after the 1980 eruptions: magmatic models,chemical cycles,and a revised hazards assessment

Available geophysical and geologic data provide a simplified model of the current magmatic plumbing system of Mount St. Helens (MSH). This model and new geochemical data are the basis for the revised hazards assessment presented here. The assessment is weighted by the style of eruptions and the chemistry of magmas erupted during the past 500 years, the interval for which the most detailed stratigraphic and geochemical data are available. This interval includes the Kalama (A. D. 1480–1770s?), Goat Rocks (A.D. 1800–1857), and current eruptive periods. In each of these periods, silica content decreased, then increased. The Kalama is a large amplitude chemical cycle (SiO₂: 57%–67%), produced by mixing of arc dacite, which is depleted in high field-strength and incompatible elements, with enriched (OIB-like) basalt. The Goat Rocks and current cycles are of small amplitude (SiO₂: 61%–64% and 62%–65%) and are related to the fluid dynamics of magma withdrawal from a zoned reservoir. The cyclic behavior is used to forecast future activity. The 1980–1986 chemical cycle, and consequently the current eruptive period, appears to be virtually complete. This inference is supported by the progressively decreasing volumes and volatile contents of magma erupted since 1980, both changes that suggest a decreasing potential for a major explosive eruption in the near future. However, recent changes in seismicity and a series of small gas-release explosions (beginning in late 1989 and accompanied by eruption of a minor fraction of relatively low-silica tephra on 6 January and 5 November 1990) suggest that the current eruptive period may continue to produce small explosions and that a small amount of magma may still be present within the conduit. The gas-release explosions occur without warning and pose a continuing hazard, especially in the crater area. An eruption as large or larger than that of 18 May 1980 (0.5 km³ dense-rock equivalent) probably will occur only if magma rises from an inferred deep (7 km), relative large (5–7 km³) reservoir. A conservative approach to hazard assessment is to assume that this deep magma is rich in volatiles and capable of erupting explosively to produce voluminous fall deposits and pyroclastic flows. Warning of such an eruption is expectable, however, because magma ascent would probably be accompanied by shallow seismicity that could be detected by the existing seismic-monitoring system. A future large-volume eruption (0.1 km³) is virtually certain; the eruptive history of the past 500 years indicates the probability of a large explosive eruption is at least 1% annually. Intervals between large eruptions at Mount St. Helens have varied widely; consequently, we cannot confidently forecast whether the next large eruption will be years decades, or farther in the future. However, we can forecast the types of hazards, and the areas that will be most affected by future large-volume eruptions, as well as hazards associated with the approaching end of the current eruptive period.     

Dynamics of carbon dioxide emissions,crystallization, and magma ascent: hypotheses,theory, and applications to volcano monitoring at Mount St. Helens

 Measurements of CO₂ fluxes from open-vent volcanos are rare, yet may offer special capabilities for monitoring volcanos and forecasting activity. The measured fluxes of CO₂ and SO₂ from Mount St. Helens decreased from July through November 1980, but the record includes variations of CO₂/SO₂ in the emitted gas and episodes of greatly increased fluxes of CO₂. We propose that the CO₂ flux variations reflect two gas components: (a) a component whose flux decreased in proportion to 1/ √t with a CO₂/SO₂ mass ratio of 1.7, and (b) a residual flux of CO₂ consisting of short-lived, large peaks with a CO₂/SO₂ mass ratio of 15. We propose two hypotheses: (a) the 1/ √t dependence was generated by crystallization in a deep magma body at rates governed by diffusion-limited heat transfer, and (b) the gas component with the higher CO₂/SO₂ was released from ascending magma, which replenished the same magma body. The separation of the total CO₂ flux into contributions from known processes permits quantitative inferences about the replenishment and crystallization rates of open-system magma bodies beneath volcanos. The flux separations obtained by using two gas sources with distinct CO₂/SO₂ ratios and a peak minus background approach to obtain the CO₂ contributions from an intermittent source and a continuously emitting source are similar. The flux separation results support the hypothesis that the second component was generated by episodic magma ascent and replenishment of the magma body. The diffusion-limited crystallization hypothesis is supported by the decay of minimum CO₂ and SO₂ fluxes with 1/ √t after 1 July 1980. We infer that the magma body at Mount St. Helens was replenished at an average rate (2.8×10⁶m³d^–1) which varied by less than 5% during July, August, and September 1980. The magma body volume (2.4–3.0 km³) in early 1982 was estimated by integrating a crystallization rate function inferred from CO₂ fluxes to maximum times (20±4 years) estimated from the increase of sample crystallinity with time. These new volcanic gas flux separation methods and the existence of relations among the CO₂ flux, crystallization rates, and magma body replenishment rates yield new information about the dynamics of an open-vent, replenished magma body. Received: 15 February 1995 / Accepted: 30 March 1996     

Interrelations among pyroclastic surge,pyroclastic flow,and lahars in Smith Creek valley during first minutes of 18 May 1980 eruption of Mount St. Helens,USA

A devastating pyroclastic surge and resultant lahars at Mount St. Helens on 18 May 1980 produced several catastrophic flowages into tributaries on the northeast volcano flank. The tributaries channeled the flows to Smith Creek valley, which lies within the area devastated by the surge but was unaffected by the great debris avalanche on the north flank. Stratigraphy shows that the pyroclastic surge preceded the lahars; there is no notable “wet” character to the surge deposits. Therefore the lahars must have originated as snowmelt, not as ejected water-saturated debris that segregated from the pyroclastic surge as has been inferred for other flanks of the volcano. In stratigraphic order the Smith Creek valley-floor materials comprise (1) a complex valley-bottom facies of the pyroclastic surge and a related pyroclastic flow, (2) an unusual hummocky diamict caused by complex mixing of lahars with the dry pyroclastic debris, and (3) deposits of secondary pyroclastic flows. These units are capped by silt containing accretionary lapilli, which began falling from a rapidly expanding mushroom-shaped cloud 20 minutes after the eruption's onset. The Smith Creek valley-bottom pyroclastic facies consists of (a) a weakly graded basal bed of fines-poor granular sand, the deposit of a low-concentration lithic pyroclastic surge, and (b) a bed of very poorly sorted pebble to cobble gravel inversely graded near its base, the deposit of a high-concentration lithic pyroclastic flow. The surge apparently segregated while crossing the steep headwater tributaries of Smith Creek; large fragments that settled from the turbulent surge formed a dense pyroclastic flow along the valley floor that lagged behind the front of the overland surge. The unusual hummocky diamict as thick as 15 m contains large lithic clasts supported by a tough, brown muddy sand matrix like that of lahar deposits upvalley. This unit contains irregular friable lenses and pods meters in diameter, blocks incorporated from the underlying dry and hot pyroclastic material that had been deposited only moments earlier. The hummocky unit is the deposit of a high-viscosity debris flow which formed when lahars mingled with the pyroclastic materials on Smith Creek valley floor. Overlying the debris flow are voluminous pyroclastic deposits of pebbly sand cut by fines-poor gas-escape pipes and containing charred wood. The deposits are thickest in topographic lows along margins of the hummocky diamict. Emplaced several minutes after the hot surge had passed, this is the deposit of numerous secondary pyroclastic flows derived from surge material deposited unstably on steep valley sides.     

Dynamics of co-ignimbrite plumes generated from pyroclastic flows of Mount St. Helens (7 August 1980)

 Four co-ignimbrite plumes were generated along the flow path of the pyroclastic flow of 7 August 1980 at Mount St. Helens. Three of the plumes were generated in discrete pulses which can be linked to changes in slope along the channel. One plume was generated at the mouth of the channel where the flow decelerated markedly as it moved onto the lower slopes of the pumice plain. Plume generation here may be triggered by enhanced mixing due to a hydraulic jump associated with an abrupt slope change. Measurements of plume ascent velocity and width show that the co-ignimbrite plumes increased in velocity with height. The plumes have initial velocities of 1–2 m/s. Two of the plumes reached a velocity maximum (4.6 and 8.8 m/s, respectively, at heights of 270 and 315 m above the flow) and thereafter decelerated. The other plumes reached velocities of 6.2 and 13 m/s. The four plumes become systematically less energetic downstream as measured by their ascent rates, which can be interpreted as a consequence of decreasing interaction of the pyroclastic flow front with the atmosphere. Theoretical models of both co-ignimbrite plumes and discrete co-ignimbrite clouds assume that there is no initial momentum, and both are able to predict the observed acceleration stage. The rising plumes mix with and heat air and sediment out particles causing their buoyancy to increase. Theoretical models agree well with observations and suggest that the initial motion of the ascending material is best described as a discrete thermal cloud which expands as it entrains air, whereas the subsequent motion of the head may become influenced by material supplied from the following plume. The models agree well with observations for an initial temperature of the ash and air mixture in the range of 500–600 K, which is in turn consistent with the measured initial ash temperature of around 920 K. Ash masses of 3.4×10⁵ to 1.8×10⁶ kg are estimated. Received: 11 January 1996 / Accepted: 7 October 1996     

The isotopic and chemical evolution of Mount St. Helens

Isotopic and major and trace element analysis of nine samples of eruptive products spanning the history of the Mt. St. Helens volcano suggest three different episodes; (1) 40,000–2500 years ago: eruptions of dacite with ε_Nd = +5, ε_Sr = ?10, variable δ¹⁸O,²⁰⁶Pb/²⁰⁴Pb ～ 18.76, Ca/Sr ～ 60, Rb/Ba ～ 0.1, La/Yb ～ 18, (2) 2500-1000 years ago: eruptions of basalt, andesite and dacite with ε_Nd = +4 to +8, ε_Sr = ?7 to ?22, variable δ¹⁸O (thought to represent melting of differing mantle-crust reservoirs), ²⁰⁶Pb/²⁰⁴Pb= 18.81?18.87, variable Ca/Sr, Rb/Ba, La/Yb and high Zr, (3) 1000 years ago to present day: eruptions of andesite and dacite with ε_Nd = +6, ε_Sr = ?13, δ¹⁸O～6‰, variable²⁰⁶Pb/²⁰⁴Pb, Ca/Sr ～ 77, Rb/Ba= 0.1, La/Yb ～ 11. None of the products exhibit Eu anomalies and all are LREE enriched. There is a strong correlation between⁸⁷Sr/⁸⁶Sr and differentiation indices. These data are interpreted in terms of a mantle heat source melting young crust bearing zircon and garnet, but not feldspar, followed by intrusion of this crustal reservoir by mantle-derived magma which caused further crustal melting and contaminated the crustal magma system with mafic components. Since 1000 years ago all the eruptions have been from the same reservoir which has displayed a much more gradual re-equilibration of Pb isotopic compositions than other components suggesting that Pb is being transported via a fluid phase. The Nd and Sr isotopic compositions lie along the mantle array and suggest that the mantle underneath Mt. St. Helens is not as depleted as MORB sources. There is no indication of seawater involvement in the source region.