Eruption Of May Research Articles

Geodetic constraints for the mechanism of Anatahan eruption of May 2003

Anatahan Island is located at the southern end of the Mariana volcanic chain. On May 10, 2003, the eastern crater of the island erupted for the first time in recorded history. The Plinian eruption column reached an altitude as high as 13 km on May 11, and a thick layer of ash covered the island. The eruptive activity continued to June 2003, but most of the erupted material was expelled during the first week. The volcanic activity declined in the second half of 2003, but resumed in April 2004. In order to determine crustal deformation associated with the eruption, we conducted GPS measurements in July 2003 at a benchmark (ANAT) located approximately 7 km west–northwest of the active crater, where GPS campaign measurements had been repeated four times since 1992. In the period from January to July 2003 during the eruption, significant subsidence – as much as 21 cm – was detected, but horizontal movement was negligible. We began taking continuous GPS measurements at the same site in July 2003 to monitor the transient deformation that was probably associated with magma migration. To assess the spatial extent of the deformation more accurately, we established another permanent GPS site (ANA2) at a site approximately 3 km from the active crater in the northeastern part of the island in January 2004. The coordinates of this time series at ANAT probably show a change in trends at the beginning of 2004. Another subsidence of 2.8 cm and a westward motion of 2.1 cm were estimated to have occurred in the period from July to December 2003. This was followed by an uplift of 5.2 cm and movement in an eastward direction of 1.0 cm in the period from January to June 2004. We developed three preliminarily models of inflation/deflation sources for three different time periods. During the period from January to July 2003, a deformation source was located beneath the ANAT site and acted as a deflation source. Considering the gap in the GPS time series and errors in data (especially after July 2003), we expected that the deformation sources were located beneath the western part of Anatahan Island and not below the active crater.

Role of magma mixing in the petrogenesis of tephra erupted during the 1990–98 explosive activity of Nevado Sabancaya, southern Peru

The Nevado Sabancaya in southern Peru has exhibited a persistent eruptive activity over eight years following a violent eruption in May–June 1990. The explosive activity consisted of alternated vulcanian and phreatomagmatic events, followed by declining phreatic activity since late 1997. The mean production rate of magma has remained low (106–107 m3 per year).

Overview of the 1990–1995 eruption at Unzen Volcano

Following 198 years of dormancy, a small phreatic eruption started at the summit of Unzen Volcano (Mt. Fugen) in November 1990. A swarm of volcano-tectonic (VT) earthquakes had begun below the western flank of the volcano a year before this eruption, and isolated tremor occurred below the summit shortly before it. The focus of VT events had migrated eastward to the summit and became shallower. Following a period of phreatic activity, phreatomagmatic eruptions began in February 1991, became larger with time, and developed into a dacite dome eruption in May 1991 that lasted approximately 4 years. The emergence of the dome followed inflation, demagnetization and a swarm of high-frequency (HF) earthquakes in the crater area. After the dome appeared, activity of the VT earthquakes and the summit HF events was replaced largely by low-frequency (LF) earthquakes. Magma was discharged nearly continuously through the period of dome growth, and the rate decreased roughly with time. The lava dome grew in an unstable form on the shoulder of Mt. Fugen, with repeating partial collapses. The growth was exogenous when the lava effusion rate was high, and endogenous when low. A total of 13 lobes grew as a result of exogenous growth. Vigorous swarms of LF earthquakes occurred just prior to each lobe extrusion. Endogenous growth was accompanied by strong deformation of the crater floor and HF and LF earthquakes. By repeated exogenous and endogenous growth, a large dome was formed over the crater. Pyroclastic flows frequently descended to the northeast, east, and southeast, and their deposits extensively covered the eastern slope and flank of Mt. Fugen. Major pyroclastic flows took place when the lava effusion rate was high. Small vulcanian explosions were limited in the initial stage of dome growth. One of them occurred following collapse of the dome. The total volume of magma erupted was 2.1×10 8 m 3 (dense-rock-equivalent); about a half of this volume remained as a lava dome at the summit (1.2 km long, 0.8 km wide and 230–540 m high). The eruption finished with extrusion of a spine at the endogenous dome top. Several monitoring results convinced us that the eruption had come to an end: the minimal levels of both seismicity and rockfalls, no discharge of magma, the minimal SO 2 flux, and cessation of subsidence of the western flank of the volcano. The dome started slow deformation and cooling after the halt of magma effusion in February 1995.

Inclusions in Mount St. Helens dacite erupted from 1980 through 1983

Inclusions of plutonic, metavolcanic and volcanic rocks are abundant in dacite pumice and lava from the 1980–1986 eruption sequence at Mount St. Helens. Point counts of inclusions exposed in talus blocks from the dome from 1980 through 1983 show that inclusions form approximately 3.5 vol% of the lava. Eighty-five percent of the inclusions are medium-grained gabbros with an average diameter of 6 cm. Additional rock types include quartz diorite, hornfelsic basalt, dacite, andesite and vein quartz. Disaggregated inclusions are common and define shear planes within the dome. These fragmented inclusions may significantly contaminate analyses of the dacite. The gabbroic inclusions are of four distinct types, all with mineral assemblages consistent with crystallization pressures of less than 9 kb. Textures and major-element compositions indicate that most of the gabbroic inclusions are cumulates. The most abundant inclusion type is laminated gabbronorite, which contains up to 9% interstitial glass, derived from partial melting. The presence of quartz veins and hornblende-bearing veins within sheared zones in the laminated gabbronorite indicates that the source of these inclusions was holocrystalline rock that had been penetrated by water-rich fluids. The gabbronorite contained sufficient water to be susceptible to partial melting when the magma that fed the 1980–1986 eruption sequence was emplaced nearby. Various types of gabbroic inclusions, including the laminated gabbronorite, are common in Mount St. Helens lavas of approximately the last 3000 years. This coincides with the interval in which Mount St. Helens first erupted basalt and basaltic andesite lavas. These observations, together with the fact that the gabbroic inclusions are compositionally unlike any of the Tertiary intrusive rocks in the Mount St. Helens area, strongly suggest that the inclusions are related to the introduction of basalt to the Mount St. Helens magmatic system. The source of the gabbros could be a layered mafic pluton formed through crystal accumulation from multiple batches of basaltic magma emplaced at mid-to upper-crustal depths beneath the volcano. The prevalence of explosive eruptions at Mount St. Helens may play a part in bringing the inclusions to the surface. The eruptive products of the cataclysmic eruption of May 18,1980 contain notably fewer inclusions than the pyroclastic flows and dome lavas erupted subsequently. This suggests that the May 18 eruption shattered conduit wall rock that was subsequently stoped into the magma and carried to the surface later in the eruption series.

Seismicity at Mount St. Helens, 1987–1992: Evidence for repressurization of an active magmatic system

Several changes have occurred in seismicity associated with Mount St. Helens since it last erupted in October 1986. Perhaps the most significant is the recurrence of earthquakes deeper than 3 km, previously observed only in the months following the cataclysmic eruption of May 18, 1980, and briefly in March 1982. Events located below 6.5 km define a circular aseismic zone, similar in location to one observed after May 18, 1980. Focal mechanisms are computed for two depth ranges, 4–6.5 km (Intermediate) and 6.5–10 km (Deep), using both polarity and amplitude ratio data. Mechanisms for Intermediate events are mostly strike‐slip, and many P and T axes point in directions ∼80° offset from the regional stress field, indicating that earthquakes in this zone are caused by magmatic as well as tectonic forces. Mechanisms for Deep events are also mostly strike‐slip, and P and T axes form a pattern suggestive of a pressure increase within the aseismic zone. The observed Deep patterns are compared with theoretical stress fields generated by a numerical model of a pressurized hole in an elastic plate. Results from modeling support the pressure increase hypothesis for 1987–1992 seismicity, in contrast to a pressure drop for post‐May 18, 1980, earthquakes. This apparent repressurization is proposed to be a result of the sealing of the shallow conduit system.

Small Mammal Recolonization on the Mount St. Helens Volcano: 1980-1987

-In this paper we summarize small mammal studies on Mount St. Helens since its catastrophic eruption in May 1980. Species surviving the initial eruption (residuals) have been described by Andersen and MacMahon (1985a). Herein we document recolonization of small mammals [Rodentia, Insectivora, Lagomorpha and Carnivora (Mustelidae)] into representative sites of four broadly defined montane habitats (forests, clearcuts, subalpine meadows and riparian ravines) that were subjected to increasing degrees of volcanic disturbance (undisturbed, tephrafall, mud flow, tree blowdown and pyroclastic/ debris flow). We live-trapped small mammals on 19 sites (with extensive population studies on 12 sites) from 1980-1987. By 1987, of a hypothetical list of 32 resident small mammal species, six species were captured or observed in the pyroclastic/debris flow zone, 15 species in the tree blowdown zone, and 22 species in the tephrafall zone. Small mammal communities in forest and clear-cut habitats, and across different disturbance zones, exhibited little species similarity among sites, even after 7 years of posteruption recovery. Small mammal species composition in several subalpine meadows converged through time to that found on a clearcut site in a tephrafall zone. Pre-eruption conditions, such as the patchiness of habitats and small mammal populations, and stochastic events, such as (1) the initial survival of small islands of vegetation and resident mammals; (2) localized habitat alteration by posteruption erosional processes, and (3) an unusually wet summer (1983) followed by 3 yr of summer drought (1984-1986), had a major influence on small mammal recolonization in this complex mosaic of habitats. We argue that observed differences in species recolonization and establishment success were more attributable to the availability of requisite food and shelter resources than to differences in species-specific dispersal capabilities. INTRODUCTION The explosive eruption of Mount St. Helens on 18 May 1980 resulted in a large-scale destruction of montane/subalpine forest and meadow habitats and caused the local extirpation of many small mammal species (Franklin et al., 1985). At least 14 species of small mammals survived the initial eruption and concomitant habitat alterations (Andersen, 1982; Andersen and MacMahon, 1985a). Individuals of many residual species survived the volcanic blast by virtue of their fossorial habits; being in underground burrows at the time of the early morning (0832 PDT) eruption shielded them from the rapidly moving hot gasses and tephra. Although the blast virtually annihilated all exposed plants and arthropods in nearby areas N of the volcano, the existence of underground plant parts and invertebrates, combined with a limited rain of recolonizing arthropods onto the disturbed areas (Edwards et al., 1986), provided an adequate food supply for some of the surviving mammals. Localized 'Present address: Department of Biology, University of New Mexico, Albuquerque 87131 2 Present address: World Wildlife Fund, 1250 24th St. NW, Washington, D.C. 20037. 3 Present address: Mount St. Helens National Volcanic Monument Headquarters, Route 1, Box 369, Amboy, Washington 98601.

An analysis of the earthquake time series at Mount St. Helens, Washington, in the framework of recent volcanic activity

The analysis of the earthquake time distribution at Mount St. Helens reveals a good correlation between the physical state of the volcano and statistical parameters of earthquake sequence. There are three main seismic phases in the whole 1980–1986 period. The first one precedes the main eruption of May 18, 1980. It begins with a sudden increase of the seismicity level in late March and continues with an Utsu (1961) type decay of the seismic occurrence rate, characterized by a small value of the decay coefficient, β. The second phase lasts from the cataclysmic eruption on May 18, 1980 until the continuous dome building episode in 1983 and is characterized by a very slow exponential increase of the background level of seismicity. The third phase covers the remaining part of the sample and is characterized by a stationary earthquake clustering process episodically interrupted by peaks of activity related to eruptions. The trends in seismic occurrence rate within each phase, as well as the statistical parameter variations at each transition, are analyzed and discussed in the framework of volcanic activity. This leads to the conclusion that statistical techniques may give a significant contribution in understanding changes in volcanic processes such as those at Mount St. Helens.

Dynamics of herbaceous vegetation recovery on Mount St. Helens, Washington, USA, after a volcanic eruption

Recovery of herbaceous vegetation on Mount St. Helens was studied annually after the massive lateral eruption of May 18, 1980. Measures such as species richness, cover, and diversity were combined with detrended correspondence analysis to describe vegetation recovery rates under different combinations of initial impact intensity and degree of isolation from recolonization sources. A major key to recovery is whether any plants survived the devastation. Survival of even a few individuals markedly accelerated recovery. Where no plants survived, the degree of isolation becomes paramount. New, barren substrates, a few meters from undisturbed sites, have begun to develop some vegetation, while more isolated sites have scarcely any subalpine plants present. On any site, plant-mediated processes that improve conditions for growth and the invasion of other species predominate in the early stages, but as vegetation develops, biotic inhibition and establishment of seedlings from adults already in the habitat gain importance. The rate at which this conversion occurs is a function of the size and intensity of the initial impact.

Air pressure waves from Mount St. Helens eruptions

Weather station barograph records as well as infrasonic recordings of the pressure wave from the Mount St. Helens eruption of May 18, 1980, have been used to estimate an equivalent explosion airblast yield for this event. Pressure amplitude versus distance patterns in various directions compared with patterns from other large explosions, such as atmospheric nuclear tests, the Krakatoa eruption, and the Tunguska comet impact, indicate that the wave came from an explosion equivalent of a few megatons of TNT. The extent of tree blowdown is considerably greater than could be expected from such an explosion, and the observed forest damage is attributed to outflow of volcanic material. The pressure‐time signature obtained at Toledo, Washington, showed a long, 13‐min duration negative phase as well as a second, hour‐long compression phase, both probably caused by ejecta dynamics rather than standard explosion wave phenomenology. The peculiar audibility pattern, with the blast being heard only at ranges beyond about 100 km, is explicable by finite amplitude propagation effects. Near the source, compression was slow, taking more than a second but probably less than 5 s, so that it went unnoticed by human ears and susceptible buildings were not damaged. There was no damage at Toledo (54 km), where the recorded amplitude would have broken windows with a fast compression. An explanation is that wave emissions at high elevation angles traveled to the upper stratosphere, where low ambient air pressures caused this energetic pressure oscillation to form a shock wave with rapid, nearly instantaneous compression. Atmospheric refraction then returned part of this wave to ground level at long ranges, where the fast compressions were clearly audible.

Equisetum and the cycling of mercury at Mount St. Helens: Plant-soil relations, 1980?1984

The Hg content ofEquisetum shoots at six stations near Mount St. Helens rose to a peak value in July 1982, 26 mo after the great eruption of May 1981.

Growth of Native Plant Species on Recent Volcanic Substrates from Mount St. Helens

Subalpine vegetation on Mount St. Helens is recovering from the volcanic eruptions of May 1980. The recovery rate depends on the nature and intensity of destruction suffered by the vegetation at a particular site. New volcanic substrates were collected annually from 1980 through 1983 and bioassays were used to analyze their ability to support plant growth. The importance of soil structure and fertility was also investigated. Lack of soil nutrients appears to be a major factor limiting recovery. Soil weathering, the influx of airborne nutrients and the burrowing of small mammals enhance fertility. Some species now grow as well on these substrates as they do on an infertile loam. Despite the ability of some species to grow on these substrates, many areas of the mountain were devoid of vegetation as of October 1984. Immigration, a function of isolation, rather than soil development, may be the key to revegetation of totally devastated habitats.

Source mechanism of the May 18, 1980, Mount St. Helens eruption from regional surface waves

A reference source equalization method using regional surface waves from the Mount St. Helens eruption of May 18, 1980, and the Elk Lake event of February 14, 1981, was used to determine the St. Helens eruption source time history. The time domain deconvolution signals display a complex series of pulses lasting 250 s. An initial pulse was isolated and was found to have a smoothly varying two‐lobed radiation pattern for both Love and Rayleigh waves, suggesting a near‐horizontal point force directed south consistent with previous results. The initial southward force corresponds to the acceleration of the landslide in a northern direction about 5–10 s after the triggering earthquake. Lateral forces were isolated 70–140 s later, consistent with the occurrence of the large lateral blast and the slide changing its direction down the Toutle River Valley. There also appears to be significant source activity between 140 and 250 s after the initial pulse. This later activity exhibits forces in the northwest‐southeast azimuth and may correspond to the final deceleration of the landslide. Several vertical pulses were located between 10–60 and 100–120 s after the eruption began. Much of the vertical force represents the two sets of explosions which occurred during the eruption.

Proximal bedded deposits related to pyroclastic flows of May 18, 1980, Mount St. Helens, Washington

Thin-bedded, dacitic, pumiceous pyroclastic-flow deposits partly cover the steep northern flank of Mount St. Helens volcano, Washington. They are termed proximal bedded pyroclastic-flow (PBPF) deposits and were formed during the eruption of May 18, 1980. These unconsolidated deposits, as much as 20 m thick, are characterized by well-defined, chiefly plane-parallel bedding sets separated by erosion surfaces. Most beds dip generally parallel to the slope of the underlying volcano flank, which averages 15° to the north but locally is as much as 30°. Individual beds range in thickness from 2 mm to >1 m. Cross-bedding, in which bedding sets dip gently to the north or locally to the south, is abundant. Cross-bedding is generally associated with large longitudinal dunes, apparently antidunes. Some cross-bedding sets, however, constitute small longitudinal anti-dunes (chute-and-pool structures) containing stoss-side beds that migrated south and dip steeply south toward the crater source. We suggest that PBPF deposits, base-surge deposits, and ignimbrite-veneer deposits are products of a broad spectrum of high-flow–regime pyroclastic processes. PBPF deposits at Mount St. Helens formed during deposition of high-velocity, partly turbulent, dry, fluidized pyroclastic flows that poured profusely down the volcano flank on May 18. Some beds of the PBPF deposits probably were deposited from typical high-concentration laminar pyroclastic flows. Other beds, especially those that display evidence of high-angle stoss-side accumulation, were probably deposited from low-concentration turbulent flow under high velocity as pyroclastic surges. Most beds in the PBPF deposits, however, may have formed during transitional flow conditions. Pyroclastic-surge and transitional flow-surge conditions may have resulted when the flows exceeded the threshold velocity or threshold minimum particle concentration for pyroclastic flows while lofting over obstacles or depressions or after collapse from Plinian ash columns.

Analysis of seismic body waves excited by the Mount St. Helens eruption of May 18, 1980

Seismic body waves which were excited by the May 18, 1980, eruption of Mount St. Helens and recorded by the Global Digital Seismographic Network stations are analyzed to determine the nature and the time sequence of the events associated with the eruption. The polarity of teleseismic p waves (period ∼20s) is identical at six stations, which are distributed over a wide azimuthal range. This observation, together with a very small S to P amplitude ratio (at 20 s), suggests that the source is a nearly vertical single force. A simple model shows that for seismic radiation a volcanic eruption can be represented by a single,force applied in the direction opposite to the blast direction. The time history of the vertical force suggests two distinct groups of events, about 2 min apart, each consisting of several subevents with a duration of about 25 s. The magnitude of the force is approximately 2.6×1012 N. This vertical force is in contrast with the long‐period (∼150 s) southward horizontal single force which has been determined by a previous study and interpreted to be due to the massive landslide. An Ms = 5.2 earthquake initiated the eruption sequence. The direction of the P wave first motion of this event observed at two nearby stations is consistent with the radiation pattern expected for the landslide and suggests that it represents the onset of the landslide. The ground motions observed at station LON (Δ = 67 km) are dominated by Rayleigh waves (i.e., Lamb pulse) and provide tight constraints on the time sequence of the events.

An outbreak of waterborne giardiasis associated with heavy water runoff due to warm weather and volcanic ashfall.

From mid-June through early August 1980, an outbreak of gastrointestinal illness in Red Lodge, Montana affected approximately 780 persons, as estimated from attack rates of 33 per cent and 15 per cent in urban and rural residents, respectively. Giardia lamblia was identified in stool specimens from 51 per cent of 47 persons with a history of untreated gastrointestinal illness and in 13 per cent of 24 specimens from asymptomatic persons (p = .00045, Fisher's Exact Test). The epidemic curve was bimodal with peaks in mid-June and mid-July. Each peak occurred about three weeks after an episode of very heavy water runoff resulting from warm sunny weather and snow darkened by ashfall from the Mt. St. Helens volcanic eruption of May 18, 1980. Unfiltered and inadequately chlorinated surface water was supplied by the city water system, which was implicated as the vehicle of transmission in the outbreak. Water systems providing unfiltered surface water are more likely to become contaminated during periods of heavy water runoff.

QUALITATIVE AND QUANTITATIVE OBSERVATIONS ON AQUATIC ALGAL COMMUNITIES AND RECOLONIZATION WITHIN THE BLAST ZONE OF MT. ST. HELENS, 1980 AND 19811

ABSTRACTDespite the destruction and total rearrangement of much of the area north of Mt St. Helens, many different species of algae became established in the aquatic systems located in the blast zone within 3–4 months after the eruption of May 18, 1980. Initial sites of intense algal activity were found in benthic regions associated with warm springs and in the littoral and phytoplanktonic communities of some small lakes. In the summers of 1980–81, overall phytoplankton numbers and activity were low in the large lakes closest to the crater (e.g. Spirit Lake). However, diatoms, blue‐green and green algal isolates from these lakes were obtained in laboratory cultures on a reduced inorganic medium used to enumerate metal and sulfur oxidizing bacteria. Species of Nodularia, Pseudanabaena, Anabaena, Oscillatoria, Nitzschia, Tribonema, Chlamydomonas, Chlorella, and microflagellates (predominantly cryptomonads) were among the more common forms found in preserved samples or isolated in cultures.

Petrologic Monitoring of 1981 and 1982 Eruptive Products from Mount St. Helens

New material from the dacite lava dome of Mount St. Helens, collected soon after the start of each successive extrusion, is subjected to rapid chemical and petrologic analysis. The crystallinity of the dacite lava produced in 1981 and 1982 is 38 to 42 percent, about 10 percent higher than for products of the explosive 1980 eruptions. This increase in crystallinity accompanies a decrease in the ratio of hornblende to hornblende plus orthopyroxene, which suggests that the volatile-rich, crystal-poor material explosively erupted in 1980 came from the top of a zoned magma chamber and that a lower, volatile-poor and crystal-rich region is now being tapped. The major-element chemistry of the dacite lava has remained essentially constant (62 to 63 percent silica) since August 1980, ending a trend of decreasing silica seen in the products of the explosive eruptions of May through August 1980.

Particles and gases in the emissions from the 1980–1981 volcanic eruptions of Mt. St. Helens

Detailed airborne measurements of particles and gases were made in the volcanic effluents from Mt. St. Helens from March 1980 through August 1981. The results of these measurements are summarized and are used to deduce emission fluxes of particles and gases from the volcano. Measurements on the periphery of the plume of effluents produced by the violent paroxysmal eruption of May 18, 1980, showed number concentrations of particles <2 μm and >2 μm in diameter up to 1500 and 20,000 times greater, respectively, than in the ambient air. Mass concentrations of particles in the periphery of the plume on May 18 were ∼7000 μg m−3 with particles >2 μm making the dominant contribution. However, the percentage of the total mass in particles <2 μm increased with time after May 18 and was ∼100% of the emissions after June 18, 1980. The fluxes of particles from Mt. St. Helens ranged from 1020 s−1 during the May 18 eruption to 2×1015 s−1 in phreatic emissions. The particulate mass fluxes ranged from 6×104 to 10−2 kg s−1. The total input to the atmosphere of particulate mass from Mt. St. Helens from March 28, 1980, to March 28, 1981, is estimated to be ∼2 Tg. A sometimes substantial, but highly variable, percentage (0–91) of the mass of material in the plume of effluents consisted of soluble halides and sulfates. Fluxes of cloud condensation nuclei (active at 1% supersaturation) in the plume from the volcano ranged from 1016 to 2×1017 s−1. Gases in the effluents included H2O, SO2, H2S, CO2, N2, NO, CO, CH3, Cl, COS, and CS2. An unusually high percentage (35–100) of the sulfur was H2S, particularly during the paroxysmal and intraeruptive periods. Ozone was depressed below ambient values in the plume. SO2‐to‐SO4 = and NO2‐to‐NO3− conversions were observed in the plume at rates of ∼0.1–1% h−1. About 1% of the total material in the tropopause plume from Mt. St. Helens on May 18, 1980, was estimated to have reached the stratosphere. This yields estimates for the stratospheric injections on May 18, 1980, of 3×109 (and 1.5×108) kg of H20, 1.3×105 (and 6×104) kg of particles <2 μm, 1×1018 (and 1×1018) Aitken nuclei, 6×104 (and 3×104) kg of SO2, 3×104 (and 3×104) kg of H2S, 4×109 (and 1×109) kg of CO2 and 3×107 (and 3×107) kg of CO where, in each case, the first number is the total injection (i.e., volcanic plus entrained tropospheric material) and the number in parentheses is the material that derived from the volcano itself.

Compression of the Venusian ionosphere on May 10, 1979, by the interplanetary shock generated by the solar eruption of May 8, 1979

An interplanetary shock wave that was produced by a solar eruption and its associated coronal transient on May 8, 1979, has been ‘tracked’ through interplanetary space to a rendezvous 2 days later with Venus. The interaction of the shock wave with the ionospheric obstacle at Venus produced a significant compression of the dayside ionosphere. It is believed that the tracking, as it were, was accomplished for the first time via the diagnostic observations provided by Hα and white light imagery near the sun and the plasma and field measurements of two, nearly radially aligned, spacecraft.

Magmatic model for the Mount St. Helens blast of May 18, 1980

Analytical and numerical solutions to the hydrodynamic equations of motion, constrained by physical properties of juvenile ejecta in the Mount St. Helens blast deposit, were used to investigate magmatic conditions required to produce the initial devastating blast phase of the eruption of May 18, 1980. Evidence that the blast was magmatic includes equivalence in volume of juvenile blast ejecta to preemption inflation of the cone, substantial vesicularity of this ejecta, and continued vesiculation of large juvenile clasts after eruption. Observed or inferred ejecta velocities of 100 to 250 m/s are shown to require 0.2 to 0.7 wt % water vapor preexisting in magma unloaded by a landslide 200 to 900 m thick. These conditions imply total magmatic water contents of 0.7 to 1.7 wt %, respectively. Such low required water content suggests that volcanic blasts may be regarded as a normal consequence of magma intrusion into an unstable edifice.